![Seed Production and Germination Characteristics of Woolly Cupgrass (Eriochloa Villosa [Thumb] Kunth) Iliya A](http://data.docslib.org/img/3b5dcffad23e4a22cf3999568f4f0700-1.webp)
Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations
1988 Seed production and germination characteristics of woolly cupgrass (Eriochloa villosa [Thumb] Kunth) Iliya A. Bello Iowa State University
Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Agricultural Science Commons, Agriculture Commons, and the Agronomy and Crop Sciences Commons
Recommended Citation Bello, Iliya A., "Seed production and germination characteristics of woolly cupgrass (Eriochloa villosa [Thumb] Kunth) " (1988). Retrospective Theses and Dissertations. 8825. https://lib.dr.iastate.edu/rtd/8825
This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. INFORMATION TO USERS The most advanced technology has been used to photo graph and reproduce this manuscript from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are re produced by sectioning the original, beginning at the upper left-hand corner and continuing from left to right in equal sections with small overlaps. Each original is also photographed in one exposure and is included in reduced form at the back of the book. These are also available as one exposure on a standard 35mm slide or as a 17" x 23" black and white photographic print for an additional charge. Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6" x 9" black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order.
University Microfilms International A Bell & Howell Information Company 300 North Zeeb Road, Ann Arbor, Ml 48106-1346 USA 313/761-4700 800/521-0600
Order Number 8909126
Seed production and germination characteristics of woolly cupgrass {Eriochloa villosa [Thumb.] Kunth.)
Bello, Iliya A,, Ph.D.
Iowa State University, 1988
UMI 300N.ZeebRd. Ann Arbor, MI 48106
Seed production and germination characteristics of woolly cupgrass (Erioçhloa villosa [Thumb.] Kunth.)
By
Iliya A. Bello
A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY
Department: Agronomy Major: Crop Production and Physiology
Approved:
Signature was redacted for privacy.
In Charge of Major Work
Signature was redacted for privacy.
Signature was redacted for privacy.
Iowa State University Ames, Iowa 1988 ii
TABLE OF CONTENTS Page
DEDICATION V
INTRODUCTION 1 LITERATURE REVIEW 5 Weed Seed Production and Fate 6
Concept of Seed Dormancy 12 Classes of Seed Dormancy 17
Factors Causing Seed Dormancy 20 Termination of Seed Dormancy 26 Seed Germination 32 CHAPTER 1. EFFECT OF PLANTING DATE ON SEED PRODUCTION OF WOOLLY CUPGRASS (Erioçhloa villosa (Thumb.) Kunth.) 39
Introduction 39 Materials and Methods 40 Results and Discussion 42 Conclusions 46
References Cited 49 CHAPTER 2. STUDIES ON DORMANCY FACTORS IN WOOLLY CUPGRASS SEEDS (Eriochloa villosa [Thumb.] Kunth.) . 50 Introduction 50 Materials and Methods 52 Seed source 52
Effect of seed hulls on water imbibition 54 iii
Effects of hulls, oxygen, and temperature on germination of dormant woolly cupgrass seeds .. 55
Respiration rate experiment 56 Leaching experiment 57
Embryo excision study 58
Effects of prechilling and oxygen on seed dormancy of woolly cupgrass 59 Effect of prechilling on seed dormancy of woolly cupgrass 61 Effects of field after-ripening and soil depth on seed dormancy of woolly cupgrass .... 61
Results and Discussion 63 Effect of seed hulls on water imbibition 63 Effects of hulls, oxygen, and temperature on seed germination of woolly cupgrass 66
Respiration rate experiment 70 Leaching experiment 73 Embryo excision study 76 Effects of prechilling and oxygen on seed dormancy of woolly cupgrass 76
Effect of prechilling on seed dormancy of woolly cupgrass 80 Effects of field after-ripening and soil depth on seed dormancy of woolly cupgrass 83 Conclusions 91
References Cited 93
CHAPTER 3. GERMINATION AND EMERGENCE CHARACTERISTICS OF Eriochloa villosa [Thumb.] Kunth. 97 iv
Introduction 97
Materials and Methods 99 Effect of light on germination of woolly cupgrass seeds 100
Effect of temperature on germination of nondormant woolly cupgrass seeds 101
Effect of oxygen on germination of nondormant woolly cupgrass seeds 102 Effect of soil depth on seed germination and seedling emergence of woolly cupgrass 103 Results and Discussion 104
Effect of light on germination of woolly cupgrass seeds 104
Effect of temperature on germination of nondormant woolly cupgrass seeds 105 Effect of oxygen on germination of nondormant woolly cupgrass 108
Effect of soil depth on seed germination and seedling emergence of woolly cupgrass Ill Conclusions 115
References Cited 116
REFERENCES CITED 119
ACKNOWLEDGEMENTS 136
APPENDIX TABLES 139 V
DEDICATION
I arrived at Iowa State University from Nigeria, West Africa, on September 12, 1983 for my M.S. program. I was the last graduate student of Dr. David W. Staniforth.
I had the opportunity of knowing Dr. staniforth for 5 months before he passed away. He was dedicated to all of
his graduate students. He visited with us in our offices to talk with us regularly. In one of his regular visits to our office on November 11, 1983, Dr. Staniforth
apologized to me that he was very sorry to bring me to the US because he may not see me through my M.S. program. I asked whether he was going to retire. He said, "not exactly, but I have just a few months to struggle with life. I was diagnosed with Cancer" smiling! I was very sad. He looked at me and said, "Mr. Bello, do not feel sad for me. You have your life to live for. I will
arrange for a major Professor for you." He then left. About a week before he passed away on March 27, 1984, Dr. Staniforth stopped by my office and said to me "I have arranged for a new major Professor for you. Dr. Mike Owen was one of my graduate students. He will take good care of you during your program here."
I admire Dr. Staniforth for his distinguished vi
Scientific achievements and for his dedication to work and to his graduate students. The choice of my Ph.D. research problem in the area of biology and ecology is the inspiration that I got from the excellent research work of Dr. D. W. Staniforth. With respect and great pride, I dedicate my Ph.D. dissertation to the late Dr. David W. Staniforth. 1
INTRODUCTION
Woolly cupgrass (Erioçhloa villosa [Thumb.] Kunth.) is a plant native to East Asia and is believed to have entered the United States in ship ballast near Portland,
Oregon, during the 1940s (Hitchock, 1950). It is an annual grass weed approximately 0.6 to 1.5 m tall at maturity. The grass is a member of the Millet tribe and reproduces sexually.
Woolly cupgrass was first collected and identified in Iowa on August 8, 1957 (Pohl, 1959). Since then, it has been found and reported in Illinois, Minnesota, Pennsylvania, and Wisconsin. Woolly cupgrass has a diverse habitat. It was found growing on coal spoil piles, filled dirt at a park, wet woods, and in cultivated corn (Zea mays L.) and soybean (Glycine max L.) fields^. This grass has been reported by Strand and Miller (1980) as a weed threat in the Midwestern agroecosystems.
Increasing awareness of this grass as a potential weed threat in the Midwest led to herbicide control trials by the Universities of Minnesota, Wisconsin, Illinois, and Iowa State University.
^Ada Hayden Herbarium, Dept. Bot., Bessey Hall, Iowa State Univ., Ames, lA 50011. 2
During the early herbicide control trials, woolly cupgrass was found to be tolerant to atrazine (Strand and Miller, 1980). Also, woolly cupgrass is highly competi tive in com (Owen, 1987) and is spreading relatively quickly in Iowa. In Iowa, it was first reported in
Ringgold County about 30 years ago. Presently woolly cupgrass has been identified in 56 Counties (Figure l). Thus woolly cupgrass appears to be successfully adapting to the agroecosystem of the Midwestern States.
Some of the major characteristics that allow annual weeds to become adapted to an agroecosystem are aggressive growth habit, prolific seed production, and the ability for the seeds to remain dormant in the soil seedbank. Information about these characteristics in woolly cupgrass is lacking. Germination tests conducted immediately after harvesting physiologically mature woolly cupgrass seeds in the falls of 1986 and 1987 suggested that the seeds were innately dormant. Innate dormancy is the failure of a viable seed to germinate when conditions are favorable for germination and the establishment of the seedlings. However, the seeds that were collected during the months of February and March of 1987 from the soil surface in the same field, germinated under laboratory conditions. It becomes obvious then, that seed dormancy in woolly 3
^ COUNTIES INFESTED •COUNTIES NOT INFESTED
IVM onou
nm funo cum# •imn
UIHMIK
UM nonr
nu HUSUIMC
WIU CUM UICM wmnc
Figure 1. The distribution of woolly cupgrass infestation in Iowa. (Courtesy of Ada Hayden Herbarium, Bot. Dept., and Integrated Pest Management, Iowa State Extension Service, Iowa State Univ. Ames, lA 50011). 4 cupgrass is a likely survival mechanism. Therefore, to ensure the continuation of the species, germination occurs only when conditions are sqitable for seedling establish ment and growth.
Research studies were undertaken to evaluate the growth and seed production characteristics of woolly cupgrass in the field as affected by planting dates during the growing season. Experiments were conducted to study and identify the factor(s), and the mechanisms underlying the maintenance and termination of seed dormancy in this grass species. Finally, studies investigating the effects of temperature, light, oxygen, and soil depth on the germination and emergence of woolly cupgrass were initiated. It is hoped that the information obtained from these studies will provide a solid foundation for designing effective and successful management strategies allowing the control of this weed. 5
LITERATURE REVIEW
Weeds constitute an essential component of any agro- ecosystem (Brenchley, 1917; Aldrich, 1984). Conceptually, weeds have been viewed differently in the scientific literature. Moolani and Singh (1968) described weeds as unwanted and undesirable species of plants that often persist in crops and interfere with agricultural operations, increase costs of labor, and cause yield and quality reduction of crops. Edwards (1981) viewed weeds simply as plants growing out of place, noxious, poisonous, and without economic value. Based on weed-crop association, Aldrich (1984) defined a weed as "a plant that originated under a natural environment and, in response to imposed and natural environments, evolved and continues to do so, as an interfering associate with our crops and activities". In the agroecosystem, weed-crop association is an inevitable phenomenon as expressed by Brenchley (1917) when he wrote that "it is impossible to sow a crop without the certainty that other crops (weeds) will appear".
The significance of crop-weed association to man and his agriculture have been evaluated and recognized.
Between 1973 and 1977, Chandler (1980) estimated crop 6 losses in the U.S. due to annual weeds to be nearly $8 billion. The economic losses for soybean and corn production in the U.S. caused by velvetleaf interference alone was estimated to be over $340 million annually (Spencer, 1986). Recognizing the fact that weeds are a part of the dynamic ecosystem, most of research efforts in weed management have been focused on how best to prevent crop losses from them. Inspite of the continuous battle against weeds, they continue to persist in the agroeco- system. Their persistence in the agroecosystem may be attributed, in part, to their ability to produce large numbers of seeds per plant annually (Chandler and Dale, 1974; Carvers and Harper, 1964; Evans and Young, 1974; Stevens, 1932) and the ability of the seeds to remain viable and dormant in the soil seedbank for several years (Darlington and Steinbauer, 1961; Kivilaan and Bandurski, 1981; Livingston and Allessio, 1968; Lueschen and Anderson, 1980; Costing and Humphreys, 1940; Toole and Brown, 1946).
Weed Seed Production and Fate
In the ecosystem, weeds have two principal methods of reproduction; asexual reproduction is by the means of vegetative organs and sexual reproduction is by the 7 seeds. A seed was defined by Aldrich (1984) as "a fertilized, matured ovule having an embryonic plant, stored food material (rarely missing), and a protective coat or coats".
Weed seeds play some vital roles in the life cycle of plants. Aldrich (1984) outlined four advantages of seeds (sexual reproduction) in the life history of a plant. First, the seeds play a role in dispersal which helps to disseminate the seeds from their mother plants to other places. Second, seeds offer protection during conditions unfavorable for germination and seedling establishment.
Third, seeds are a temporary food source for the embryo. Last, they are the vehicle for the transfer and recombination of genetic factors. Characteristically, - some weeds can only reproduce sexually as the means by which to ensure the continuation of the species. Unfortunately, plants that reproduce in this manner face a number of predispersal hazards. Fenner (1985) indicated that pollination failure, resource deficiency, predators and lethal gene combinations are some of the predispersal hazards that can cause significant weed seed losses. Also, unfavorable environmental conditions following seed dispersal can cause weed seed losses. Therefore, the ability of weeds to persist in the agroecosystem must be 8 due to a combination of several factors.
One of these factors is the ability to potentially produce large numbers of viable seeds per plant annually. The seed production potentials of some of the troublesome annual weeds in row crop production have been demonstrated
(Carvers and Harper, 1964; Chandler and Dale, 1974; Dickerson arid Sweet, 1971; Evans and Young, 1974; Gomes et al., 1978; Shaw et al., 1962; Stevens, 1932). These numbers range between 2,400 and 500,000 seeds per plant for each reproductive cycle.
At the end of each growing season, weed seeds that were produced and have survived predispersal hazards are disseminated from the mother plants onto the soil surface where they are incorporated into the soil naturally or mechanically (Radosevich and Holt, 1984; Roberts, 1970).
Harper (1977) and Fenner (1985) have visualized and described the proportion of the weed seeds that have been successfully dispersed onto and are incorporated into the soil as part of a "store" or a "seedbank" from which both deposits and withdrawals are made intermittently over time. According to Harper (1977), seeds deposited into the soil seedbank are the results of seeds produced in the immediate area at the end of each life cycle and by means 9 of those dispersed from elsewhere. On the other hand, seeds withdrawals from the soil seedbank occur by germination, death, and prédation (Harper, 1977; Sarukhan; 1974). Aldrich (1984) described a seedbank as that which contains seeds of different ages, some of which are dormant, nonviable, or nondormant seeds that can germinate and emerge under favorable conditions to interfere with our use of land. Harper (1959) stressed the ecological significance of the soil seedbank in weed management. Since the seeds in the soil seedbank vary in their degree of dormancy. Harper (1959) maintained that the soil seedbank serves as a buffer which continues to provide a nonsynchronous source of weed seedlings during the growing season. The consequence of this is the spread of weed seedling flushes over a long period of time thus causing repeated weed problems in the agroecosystem. Harper (1977) summarized the fate of weed seeds in a soil seedbank in the diagram shown in Figure 2.
Recognizing the fact that the amount of yield loss in a particular field is determined by the number of weed seeds that germinated and survived control efforts, attempts have been made to evaluate seed numbers and the longevity of seed viability in arable soils (Brenchley and 10
Warington, 1930; Khedir and Roeth, 1981; Kropac, 1966;
Roberts, 1981; Warwick, 1984) and in undisturbed forest soil seedbanks (Frank and Safford, 1970; Van der Valk and Davis, 1978; Wipple, 1978).
IN SITU SEED PRODUCTION (SEASONAL) SEED INFLUX FROM OUTSIDE BY DISPERSAL AGENTS
DORMANT SEEDBANK | ACTIVE SEEDBANK with dormant seeds with nondormant seeds stimulation
PREDATION GERMINATION INTO SEEDLINGS
Figure 2. The fate of weed seeds in the soil seedbank (after Harper, 1977)
Also, techniques to extract soil and to estimate their weed seed populations in arable soil seedbanks have been extensively reviewed (Robinson et al., 1967; 11
Schweizer and Frey, 1974; Standifer, 1980). Evidence from these studies have shown that the weed seed numbers in the top 20-25 cm of the soil seedbank ranged from 3.6 to 88 million weed seeds/ha. Longevity of weed seeds has also been a sub]ect of great concern to ecologists. Results from the seed longevity studies have shown that weed seeds can retain viability between 2.2 and 100 years in the soil seedbank
(Egley and Chandler, 1983; Kivilaan and Bandurski, 1981; Livingston and Allessio, 1968; Roberts and Feast, 1973; Toole and Brown, 1946).
Although, arable soils have high weed seed numbers per unit area, Roberts (1981) indicated that weeds emerging in a particular locality represent approximately 1 to 15% of the seed population in the soil seedbank. Perhaps this may be due to the fact that a large proportion of the seeds in the soil seedbank are dormant. Our understanding of weed seed numbers, the species, and the longevity in the soil seedbank is very important in weed management. This knowledge will no doubt enable us to predict future weed pressures in a given locality. 12
Concept of Seed Dormancy
Seed dormancy is one of the basic problems of seed physiology today. Dormancy is also one of the factors that enables weeds to persist in the agroecosystem. The inherent ability of seeds to retain viability for a period of time without germinating is seed dormancy. This allows seeds to survive in the soil seedbank for many years. The classic seed viability experiments initiated by Seal (1905) in 1879 and by Duvel (1924) in 1902 were the earliest experiments on seed dormancy and its ecological implications. The ecological significance of dormancy is demonstrated by the ability of the seeds of Moth mullein fVerbascum blattaria L.), common mullein fVerbascum thapsus L.) and dwarf mallow fMalva rotundifolia L.) to retain viability and germinate after 100 years of burial
(Kivilaan and Bandurski, 1981). Although much has been published on the phenomenon of seed dormancy, little is known about the likely selective pressures that resulted in seed dormancy.
Ross (1984) speculated that seed dormancy may have originated and evolved in nature as a mechanism to suppress vivipary while the seed continues to accumulate dry weight until physiological maturity, Kozlowski and
Gunn (1972) defined vivipary as "the development of the 13
premature embryo into a seedling without a resting stage,
while still within the seed and fruit and while still attached to the parent plant." The significance of
physiological seed maturity (maximum dry matter
accumulation) during seed development was demonstrated by
Blackman (1919). He stated that "for the highest production of vegetative material by the plant, two factors are necessary: (1) large seed and (2) a high economy of working represented by a large efficiency
index". This suggests that the level at which plant
growth begins is determined by the seed size until the seedling becomes autotrophic.
Over the last two decades, two major concepts have dominated our approach to the study and the understanding
of seed dormancy. First, Tuan and Bonner (1964) hypothesized that seed dormancy may be controlled by gene repression and derepression. They suggested that seeds are dormant because of a direct repression of some part of
the genome responsible for the production of messenger RNA (mRNA) that is translated into proteins and initiates seed germination. Although this hypothesis is attractive, it has yet to be supported scientifically.
The second concept is that seed dormancy is under the control of hormonal promoter-inhibitor concentrations 14
(Amen, 1968; Bewley and Black, 1978; Villiers and Wareing, 1964). According to this hypothesis, the initiation and maintenance of seed dormancy is as a result of high level of the growth inhibitor hormone compared to the promoter hormone. In contrast, the termination of dormancy is due to the deterioration of the growth inhibitor hormone and the subsequent synthesis and increased level of the growth promoter hormone. This suggests that growth inhibitor level predominates in the dormant seed, then in response to some environmental stimuli, dormancy is terminated following synthesis of growth promoting hormone. This hypothesis has been criticized by Trewavas (1981). He argued that the dose response for exogenously applied growth promoters was linear. Further, the scientific literature provides no supportive data on what cell or tissues respond to the changes in inhibitor-promoter growth hormones in the seed.
The phenomenon of dormancy was long recognized before a definition was established. Dormancy is essentially a survival mechanism that enables seeds to retain viability for a period of time and allows the germination, establishment and continuation of the species in the agroecosystem (Amen, 1968; Chepil, 1946; Egley and
Chandler, 1983; Lewis, 1973). 15
Amen (1968) defined dormancy as the failure of a seed to germinate under conditions that are considered favorable for the germination and growth of the seedling. Dormancy was defined by Villiers (1972) as "the state of arrested development whereby the organ or organism, by virtue of its structure or chemical composition, may possess one or more mechanisms preventing its own germination." Seed dormancy was further defined as the "the failure of seeds to germinate owing to factors associated with the seed coat or embryo" (Anderson, 1983). The definition of dormancy by Villiers (1972) and Andersen (1968) classified dormancy into two distinct types; the dormancy imposed by the physical structures and that associated with the biochemistry and physiology of the seed or species.
In the scientific literature, dormancy has been viewed and defined differently by various authors. In view of the diversity that exists in our understanding of seed dormancy, the concept of seed dormancy must, therefore, be interpreted with great caution. For example, the fact that a viable seed fails to germinate does not necessarily imply that the seed is categorically dormant. Bewley and Black (1982) described such a seed as being quiescent due to the fact that it simply lacked one 16 or more of the environmental factors for germination. In the literature, attempts have been made to differentiate dormancy from quiescence. Amen (1963) and Mayer and
Poljakoff-Mayber (1963) described quiescence as the state of arrested development imposed by the absence of one or more of the environmental factors necessary for germination. Unlike dormant seeds, quiescent seeds will germinate upon exposure to the proper conditions for germination and seedling growth. In contrast to quiescence, dormant seeds will not germinate under the conditions optimum for germination. However, and as a rule of thumb, dormant seeds require a period of time during which some physical and or physiological changes must occur before germination can occur. Amen (1968), in his model to explain the phenomenon of seed dormancy, speculated that dormancy is under the control of four distinct stages. These are described as an inductive, maintenance, trigger, and germination stages. He speculated that the trigger stage represents the time of sensitivity of the seed to specific environmental factors and is manifested by an increase in growth promoter level and enzyme activities that result in germination. In the scientific literature, several hypotheses on the phenomenon of seed dormancy have been 17 proposed. However, information on the exact time for the trigger stage and the exact receptor site for the signal within a dormant seed which initiates the cascade of events that lead to the termination of dormancy are unknown in the scientific literature. Thus, most of the discussions on seed dormancy are based on speculations.
Classes of Seed Dormancy Although the existence of the seed dormancy phenomenon in nature has been established and recognized, the terminologies which describe it are variable. The discrepancies in these terms undoubtedly are due to our poor understanding of dormancy. In the scientific literature, several classes of seed dormancy have been described. While Crocker (1916) classified dormancy into primary and secondary types.
Harper (1977) recognized three types; innate, enforced, and induced. According to Crocker (1916), primary dormancy describes the inability of the seed to germinate while on, or immediately after dispersal from, the mother plant. Unlike the primary dormancy, secondary dormancy is imposed on the seed by unfavorable environmental conditions. Harper (1977), in his classification of seed dormancy, stated that some seed are "born dormant, some 18 achieve dormancy, and others have it thrust upon them".
The three are synonymous to "innate", "induced", and "enforced" dormancy, respectively. Cook (1980) described innate dormancy as the failure of a seed to germinate while on the mother plant and during dispersal. Aldrich
(1984) explained the mechanisms of innate dormancy hypothesizing that innate dormancy may be the result of immature embryo, seed coat impermeability to moisture, a seed coat that prevents gaseous exchange, or that contains growth inhibitors. Contrary to innate dormancy. Harper
(1977) viewed induced dormancy as the inability of a nondormant seed to germinate after it is released from the parent plant due to its exposure to unfavorable external conditions. Unlike induced dormancy. Harper described enforced dormancy as the failure of a nondormant seed to germinate due to the absence of one or more of environ mental factors necessary for germination such as oxygen, light, temperature, or moisture.
In another classification of seed dormancy, Nikolaeva (1969) envisioned embryo dormancy and seed coat-imposed dormancy. According to him, embryo dormancy is caused and maintained by factors within the embryo, while coat-imposed dormancy is maintained by the physical structures such as lemma and palea and seed coat encasing 19 the seed. Recently, Radosevich and Holt (1984) classified seed dormancy into seasonal and opportunistic categories. According to them, the maintenance and termination of seed dormancy is mostly influenced by the rhythm of daylength. They suggested that seasonal dormancy can only be terminated when the critical daylength period is reached. On the other hand, the termination of dormancy in response to some events in the ecosystem is classified as opportunistic dormancy.
Examples of this category of dormancy include the following; a seed may remain dormant under dense canopy (shade) but will germinate after the removal of the canopy. Also, a seed may be dormant when buried deep in the soil but will germinate when moved near or on the soil surface following deep cultivation.
Karssen (1982) indicated that weed seeds in the field may lose and acquire dormancy several times while in the soil seedbank. This suggests that nondormant seeds and seeds that have had innate dormancy terminated may undergo secondary dormancy if germination is prevented by certain unrequired or unfavorable environmental factors. From the above, it becomes obvious that seasonal and opportu- istic dormancy classifications can be categorized as secondary dormancy previously reported by Crocker (1916). 20
Factors Causing Seed Dormancy
The factors causing seed dormancy and the conditions which are conducive to the termination of dormancy and germination of seeds are diverse. In the scientific literature, seed dormancy has been implicated to be a result of one or the combination of the following mechanisms: (1) seed coat or hull impermeability to moisture or gaseous exchange (2) mechanical resistant of seed structures to embryo growth (3) the presence of growth inhibitors (4) embryos requiring an after-ripening period (Amen, 1968; Bewley and Black, 1982; Crocker, 1916; Egley and Duke, 1985; Egley et al., 1983; Hamly, 1932; Marbach and Mayer, 1974; Quinlivin, 1971; Villiers, 1975; Werker, 1980/81).
Impermeability of seed covers to water
Rolston (1978) wrote on the significace of seed covers impermeability to water. He hypothesized that the impermeability of seed covers to water help to sustain the seed under harsh conditions of harvest and storage, particularly under high relative humidity. The mechanism by which seed dormancy in some plant species is imposed and maintained by seed covers was unknown until the work of Crocker (1916). Seed covers 21 refer to the hulls (lemmae and palea) of the gramineae, the pericarp, testa, perisperm, and endosperm surrounding the seed embryo (Bewley and Black, 1982; Egley and Duke, 1985; Evenari, 1949; Mayer and Poljakoff-Mayber, 1963; Roberts, 1961).
Seed dormancy imposed by the inhibition of imbibition has been reported in many families such as Chenopodiaceae, Convolvulaceae, Leguminosae, Gramineae, Malvaceae, and Solanaceae (Bewley and Black, 1982; Harrington, 1923; Lacroix and Staniforth, 1964; Werker, 1980/81). Several hypotheses have been suggested to explain the mechanisms by which seed covers prevent imbibition in seeds. The mechanisms may be categorized as; (1) Prevention of germination due to the chemical properties of the seed covers and (2) prevention of germination due to the physical properties of the seed covers.
Several researchers (Aitken, 1939; Slattery et al., 1982; Werker, 1980/81) have suggested that seed coat impermeability to water is due to the polymerization of hydrophobic substances such as cutin, lignin, pectin, suberin, and hemicellulose in the seed coat. While Ballard (1973) implicated waxy coatings of seed covers as the inhibitor of imbibition, Marbach and Mayer (1974) 22 suggested that the pigments in the seed coat are respon sible. In the literature, some authors (Corner, 1951;
LaCroix and Staniforth, 1964; Raleigh, 1930) viewed seed cover impermeability to water as a purely mechanical problem.
Ballard (1973) demonstrated that seed imbibition does not involve the entire seed cover but occurs at specific areas of the seed. These specific areas have been identified as strophiolar plug in legumes, chalaza region in velvetleaf and in prickly sida (Sida spinosa L.), and through a hinged flap-like germination lid at the base of the lemma in gramineae (Egley and Paul, 1981; LaCroix and Staniforth, 1964; Rost, 1975).
Seed covers impermeable to gaseous exchange
It has been established in some weed species that innate dormancy is maintained by the seed coat restricting gaseous exchange (Atwood, 1914; Bewley and Black, 1982; Coumans et al., 1976; Crocker, 1906; Johnson, 1935;
McDonald and Khan, 1977; Werker, 1980/81; Witztum et al., 1969; Weisner and Kinch, 1964). Working with the seeds of wild oats (Avena fatua L.), Atwood (1914) found that germination of intact dormant seeds increased with various seed treatments and by increased oxygen pressures. 23
He therefore concluded that the seeds of wild oats are dormant at physiological maturity because of the restriction of oxygen. Roberts (1961) reported that the seed husks of redrice (Orvzspsis hvmenoides L.) limited oxygen availability to the embryo, thus preventing germination. Research on the role of the seed coat on oxygen diffusion revealed that integumentary mucilage of seed coat can curtail oxygen diffusion to the embryo (Werker, 1980/81; Witztum et al., 1969). Although not conclusively demonstrated. Come and Tissaoui (1973) and Coumans et al. (1976) suggested that phenolic compounds within the seed covers of some species might limit oxygen from reaching the embryo. They explained further that the direct oxidation of the phenolic compounds at the expense of the metabolism of the embryo can be a factor that limits oxygen from reaching the embryo. This suggests that germination can only occur after the phenolic compounds have been oxidized to a critical level before oxygen becomes available to the embryo.
Conceptually, any seed coat that restricts the influx of oxygen would obviously prevent the outward diffusion of carbon dioxide from the metabolic embryo. Consequen tly, carbon dioxide build up within the embryo tissue 24
might inhibit germination (Villiers, 1975). However, the
fact that Thornton (1944) was able to terminate the innate dormancy of the seeds of Xanthium species with 10% carbon dioxide, makes this concept untenable.
Seed covers preventing embrvo growth
Evidence implicating the mechanical restriction of the seed coat to embryo growth as a mechanism influencing innate dormancy was demonstrated by Crocker (1906) and
Crocker and Davis (1914) in the seeds of redroot pigweed fAmaranthus retroflexus L.). Ikuma and Thimann (1963), Pavlista and Haber (1970), and Egley (1972) were able to stimulate the germination of lettuce (Laçtuça sativa L.) seeds and witch weed (Striga asiatica L.) seeds by piercing the seed coat at the embryo. They found that the intact seeds or the seeds with seed coats pierced elsewhere other than at the embryo showed no germination response. They concluded that the germination was due to the release of a mechanical restriction to growth. These species do not have any evidence of endogenous inhibitors within the embryo or the seed coat. According to Gardner et al. (1985) seeds that maintain dormancy by seed coats which mechanically inhibit embryo growth readily imbibe water but resist swelling and embryo growth. 25
Endogenous inhibitors in the seed covers/embrvo
In the literature, many investigators (Black, 1959; Black and Naylor, 1959; Cox et al., 1945; Elliot and Leopold, 1953; Evenari, 1949; Kommedhal et al., 1958;
Yearn et al., 1988) have reported the presence of water soluble growth inhibitors in the seed covers and in the embryos of various plant species. They have associated seed dormancy in these species with the presence of the endogenous growth inhibitors.
Several researchers have reported the involvement of growth inhibitors in the maintenance of seed dormancy (Amen, 1968; Kelly, 1969). Abscisic acid (ABA) has been shown to be one of the principal growth inhibitors that prevents seed germination. Studies have shown that external application of abscisic acid to nondormant seeds inhibited germination (Bradbeer, 1968; Khan, 1967;
Rudnicki, 1969; Sondheimer et al., 1968; Sumner and Lyod, 1967). The mechanism by which abscisic acid imposes and maintains dormancy in seeds has been demonstrated (Ho and Varner, 1976; Jarvis and Shannon, 1981; Karssen, 1982; Tuan and Bonner, 1964; Slater and Bryant, 1982; Walbot et al., 1975; Wood and Bradbeer, 1967). In these studies, it was found that the stratification which terminates seed dormancy results in the accumulation of RNA in the 26
embryonic axis. Therefore, it was concluded that ABA
affects dormancy by suppressing or inhibiting the
synthesis of RNA in a an imbibing dormant seed. This conclusion was based on the evidence that ABA level
decreases during the termination of dormancy (Martin et
al., 1969; Rudnicki, 1969; Sondheimer et al., 1968).
Embrvo requirements
In some plant species seeds are dispersed from the mother plants with immature embryos (Asakawa, 1955; Steinbauer, 1937; Villiers, 1975). In other plant species, the seeds are dispersed with fully developed embryos but a period of time called after-ripening is required to complete physiological development (Bewley and
Black, 1982; Villiers and Wareing, 1964). According to Gardner et al. (1985), after-ripening in some species requires the exposure of imbibed seeds to a cold treatment or alternating warmer temperatures.
Termination of Seed Dormancy It was a common practice at one time to subject the seeds of some cultivated plants to some special treatments in order to break dormancy prior to sowing. Presowing treatments were considered necessary in order to achieve 27 uniform seed germination and seedling establishment. As a result of selection and breeding, Egley and Duke (1985) and Gardner et al. (1985) believed that many cultivated plant species have lost seed dormancy characteristics. However, seed dormancy in weeds is a unique property which enables them to survive unfavorable conditions (Amen,
1968). The importance of seed dormancy in weeds with respect to weed management is stressed by Egley (1982) and Jordan et al. (1982a). They maintained that, apart from soil sterilization, dormant seeds in the soil seedbank are difficult to kill with the currently available control techniques. Although, Fenner (1985) believed that the termination of seed dormancy was only a prerequisite for germination, Harper (1957), Hall et al. (1978), and Jordan et al. (1982a) suggested that it could be used as a critical weak point for a successful weed control strategy.
There are a variety of ways to break seed dormancy.
In the laboratory, mechanical seed treatments such as scarification (abrasion and puncturing the seed coat) and the use of boiling water are highly effective in breaking seed dormancy of many plant species. These plant species included alfalfa fMedicaao sativa L.), Indian ricegrass 28
(Orvzspsis hvmenoides L.), velvetleaf, and smallflower morningglory fJaccruemontia tamnifolia L.) (Bewley and Black, 1982; Dexter, 1955; Jordan et al., 1982b; McDonald and Khan, 1977; Eastin, 1983). They believed that scarification allows impermeable seed covers to become permeable to moisture and oxygen.
Similarly, various researchers (Egley, 1972; Ikuma and Thimann, 1963; Pavlista and Harber, 1970) have demonstrated that scarification in lettuce and witchweed is an effective method to overcome dormancy imposed by seed covers that inhibit embryo growth. LaCroix and Staniforth (1964) conducted laboratory studies to determine effective methods for breaking velvetleaf seed dormancy and suggested that leaching seeds with running water may be effective in breaking seed dormancy imposed by endogenous water soluble growth inhibitors in the seed covers or in the embryo.
Stratification or prechilling is the exposure of imbibed dormant seeds to temperatures slightly above freezing (1 to 10 C) for a period of time. Stratification was found to promote the germination of dormant seeds (Ho and Varner, 1976; Jarvis and Shannon, 1981; Villiers and
Wareing, 1964).
Seed dormancy imposed by embryos that requires an 29 after-ripening period may be overcome by exposing seeds containing about 8 to 10% moisture to low temperature between 0 and 10 C (Afanasiev, 1937; Egley, 1982; Stoller and Wax, 1974) or to temperatures as high as 50 C (Taylorson and Brown, 1977). The term "after-ripening" is defined by Afanasiev (1937) as "those physical and chemical changes in a mature, viable seed which take place between the time of seed maturity and germination, and without which the seed is unable to germinate". The nature of the changes that occur during the after-ripening period has not been identified. However, several authors (Atwood, 1914; Eckerson, 1913; Pack, 1921) have demonstrated some physiological changes that are involved with the after-ripening process. These physiological changes included increased acidity in the seed (Atwood, 1914; Eckerson, 1913; Pack, 1921), changes in catalase and peroxidase activity in the seed (Eckerson,
1913; Flemion, 1934; Jones, 1920), and the mobilization of food reserve from the unavailable to the available form (Evans, 1933).
In the scientific literature, Thompson et al. (1977) believed that alternating temperatures (high/low) can also overcome seed dormancy imposed by the mechanical restriction of seed covers to embryo growth and the 30 impermeability of seed covers. In an attempt to explain the mechanism by which alternating temperatures may break seed dormancy, Bewley and Black (1982) hypothesized that it may be through a change in membrane integrity and metabolic activities.
Hormones such as gibberellic acid (GA3), ethylene, and cytokinin have been used as effective agents in terminating seed dormancy (Corns, 1960; Egley, 1982;
Eplee, 1975; Kallio and Pironinen, 1959; Kollman and
Staniforth, 1972; Paul et al., 1976; Taylorson, 1979).
The mode of action of these hormones is believed to be through the increase of membrane permeability, the stimulation of mRNA synthesis, the stimulation of alpha- amylase synthesis in the aleurone cells, cytokinesis, and enzyme activation (Chandra and Varner, 1965; Chrispeels and Varner, 1967; Paleg, 1961; Skoog et al., 1965; Varner and Chandra, 1964). These lead to the breakdown of food reserves and protein synthesis.
Interestingly, some respiratory inhibitors such as azide, cyanide, carbon monoxide, and hydrogen sulphide are effective in breaking seed dormancy in gramineae (Cohn and Hughes, 1986; Major and Roberts, 1968; Taylorson and Hendricks, 1973). The mechanism mode of action of these materials is not known. 31
Nitrate is an example of nonhormonal compounds that
can break seed dormancy (Henson, 1970; Fawcett and Slife,
1978; Schimpf and Palmblad, 1980). Hendricks and Taylorson (1975), Taylorson and Hendricks (1976), and Yentur and Leopold (1976) accounted for the mode of action of nitrate on the termination of dormant weed seeds. They
reported that nitrate prevents the activity of catalase, thus allowing respiration in dormant seeds to shift to the pentose phosphate pathway (PPP), which presumably, is the prerequisite for seed germination.
Data on oxidants such as hypochlorite (Hsiao, 1979; 1980; Okwonkwo and Nwoke, 1975) and some anesthetics such as ethanol and acetone (Taylorson and Hendricks, 1979) have demonstrated that they are effective in breaking seed dormancy. These compounds are likely cell membrane effectors which act to promote the permeability cell membranes to nutrients and other metabolic intermediates (Taylorson and Hendricks, 1979).
From the foregoing, it becomes obvious that the
results of the laboratory studies on the various methods of breaking dormancy may enable us to speculate the natural ways by which seed dormancy in the soil seedbank
is likely terminated in the field. From the scientific literature, it is now clear that seed dormancy is a 32 complex phenomenon and may involve seed covers, hormones, nonhormonal compounds, environmental factors, and the possible interactions.
Seed Germination
Seed germination and seedling establishment in the field is the result of the termination of seed dormancy. Howell (1960) defined germination as "the emergence and development from the seed embryo of those essential structures capable of producing a normal plant under favorable conditions." Seed germination may be regarded as a physiological process involving the resumption of growth by the embryo and identified by the appearance of the radicle outside the seed coat. Seed germination is critical in the establishment of weeds that are propagated only by seeds. Thus, the time of germination and seedling establishment will influence the growth habit and the potential competitive ability of a weed in the agroecosystem (Anderson, 1968).
Roberts (1972) and Harper (1977) believed that in situ germination is the major fate of weed seeds in any soil seedbank. In the absence of reseeding, Roberts and Dawkins (1967) and Roberts and Neilson (1981) found that as a result of germination, weed seeds in a disturbed. 33 arable soil seedbank were depleted in number by 25% on the average, annually. The significance of weed seed germination to a weed control program is stressed by Jordan et al. (1982a). They indicated that unlike dormant seeds that are often immune to weed control measures, germinating seeds are often vulnerable to most weed control techniques. Thus, knowledge on the factors that favor seed germination in the ecosystem is vital to weed control programs.
Staniforth and Wiese (1985) viewed seed germination as an expression of the interaction of environmental factors such as moisture, temperature, oxygen, and the presence or the absence of light on a previously dormant seed. The effects of these factors on seed germination have been studied and evaluated. Several workers (Bewley and Black, 1982; Hand et al., 1982; Roberts, 1981; Taylorson and Hendricks, 1972; Warington, 1936) reported that the presence of only 1 of these 4 factors for germination would not promote germination. They explained that germination is a result of the interaction of these factors. The process of seed germination involves a number of complex processes which include imbibition, respiration, and other metabolic activities. 34
Imbibition and respiration
The process of seed germination starts with the uptake of moisture, a process called imbibition (Mayer and Pol]akoff-Mayber, 1963). Though the process of imbibition is a prerequisite for germination, it occurs in dormant, nondormant, and in dead seeds (Bewley and Black, 1978; Egley and Duke, 1985). According to Bewley and Black (1978), the process of water uptake by a nondormant seed is triphasic, being influenced by the properties of the seed and temperature. During imbibition, the initial phase, (phase 1) is a physical process occurring rapidly and is said to be dependent on the osmotic potential of the seed (Shull, 1920). This phase occurs regardless of whether the seed is dormant, nondormant, viable or dead. Phase 2 is termed the lag period of imbibition.
Marcus et al. (1966) have suggested that the phase 2 be called the germination phase. Unlike nondormant seeds, dormant and nonviable seeds maintain imbibition curves that are only typical to phases 1 and 2. Phase 3 of imbibition is another burst of water uptake and has been associated with rapid embryo growth and radicle protrusion.
The water imbibtion in nondormant seeds is believed to be a triggering factor for the initiation of many 35
physiological events such as membrane organization, a rise in ATP and mRNA levels, enzyme activation, and protein synthesis (Come and Tissaoui, 1973; Linko and
Milner, 1959; Marcus et al., 1966). Bewley and Black (1978) and Mayer and Poljakoff-Mayber (1975) pointed out that respiration (oxygen uptake) and imbibition are similar in kinetics but with respiration initially lagging behind imbibition. Laboratory studies have shown that these two processes are temperature dependent (Fayemi, 1957; Meyers et al., 1984). They found that the rate of imbibition and germination in alfalfa fMedicaao sativa L.), red clover fTrifolium pratense L.), alsike clover (Ta_ hvbridum L.), ladino clover (T^ repens L.), and grain sorghum (Sorahum bicolor (L.) Moench) was better at 25 C compared with 6.7 C.
Temperature effect on seed germination
Different weed seeds have different maximum temperature requirements. Increasing temperature up to the maximum requirement stimulates and increases the germination rate of nondormant seeds. The optimum temperature range of 20 to 38 C for many weed seeds has been reported (Erasmus and Van, 1986; Roberts, 1981;
Roberts and Boddrell, 1985; Taylorson and Hendricks, 1972; 36
Toole, 1976). Warington (1936) studied the effect of constant and fluctuating temperatures on the germination of the weed seeds. She found that most weed seeds germinate better when exposed to alternating temperatures. Sutcliffe (1977) speculated that the better seed germina tion at alternating temperatures compared to constant temperatures may be due to the increased membrane permea bility to water, oxygen, and metabolites associated with alternating temperatures. Also, Toole et al. (1956) suggested that the effect of the alternating temperatures may be on metabolites. They speculated that metabolites are more readily available during the high temperature cycle and are utilized at the low temperature part of the cycle for the synthesis of proteins and radicle growth.
Though this concept is attractive, there is no supportive data in the scientific literature.
Light effect
Many plant species are influenced by the presence or absence of light in seed germination process (Hand et al., 1982; Taylorson and Hendricks, 1972; Toole, 1976; Vincent and Roberts, 1977). A majority of weed seeds require light to germinate and such seeds are said to be photoblastic (Bewley and Black, 1982). This, in part, 37 explains weed seedling flushes after soil disturbances.
Effect of seed depth on aermination/seedlina emergence
The germination and emergence depths of many weed species have been studied and reported (Banting, 1979;
Dawson and Bruns, 1975; Hawton and Drennan, 1980; Fenner, 1985; Wesson and Wareing, 1969a). Weed seeds produced each year are dispersed onto the soil surface where they are incorporated into the soil at various depths. In arable soils, for example, the depth at which tillage is performed, affects the depth at which the weed seeds are placed and consequently, influences the germination of seeds and the emergence of seedlings. Studies on the emergence depths of annual weeds have shown that most of the seedlings emerged from shallow depths between 0 and 8 cm usually associated with soil disturbance during seed bed preparations (Banting, 1979; Brecke and Duke, 1980; Dawson, and Bruns, 1975; Marrow et al., 1982). According to De la Cruz (1974), 75-80% of annual weed seedlings emerged from the upper 4 cm of the soil profile in cultivated fields. He also found that in the no-till agroecosystem, about 50% of the seedlings emerged from the upper 1 cm of the soil. These studies have shown that seed germination and seedling emergence of annual weeds 38
declined with increasing soil depth. From the foregoing, it becomes obvious that our better understanding of the depths of weed seeds germination and emergence will better enhance the effectiveness of our weed control management strategies. 39
CHAPTER 1. EFFECT OF PLANTING DATE ON SEED PRODUCTION OF WOOLLY CUP6RASS fEriochloa villosa [Thumb.] Kunth.)
Introduction Woolly cupgrass is relatively a new annual grass weed in the Midwest. This weed was found to be highly
competitive with corn (Owen, 1987), and it appears to be spreading and adapting relatively quickly in the agroeco- systems of the Midwest.
According to the National Academy of Sciences (1968), successful annual weeds depend largely on prolific seed
production for dissemination and survival in any agroeco- system. The seed production potentials of some of the troublesome annual weeds in row crop production have been demonstrated (Carvers and Harper, 1964; Chandler and
Dale, 1974; Dickerson and Sweet, 1971; Evans and Young,
1974; Gomes et al., 1978; Shaw et al., 1962; Stevens, 1932). These numbers range between 2,400 and 500,000 seeds per plant for each reproductive cycle.
Gardner et al. (1985) described seed production (yield) as the product of a number of yield components such as the number of reproductive units (e.g., panicles) per unit area, the number of grains per reproductive unit,
and the average weight per grain. They explained further that yield is the result of the interaction of genotype 40
(which sets the potential for tillering, flower number, and others), environment, and management.
Information on the seed production potentials of woolly cupgrass is lacking. Knowledge on the approximate number of seeds that a weed plant contributes into the
soil seedbank at the end of each life cycle is important to weed scientists. This knowledge may be useful in predicting weed seed populations of a soil seedbank and may also facilitate the prediction of future weed pressures in an agroecosystem. This study was designed to investigate the seed production potentials of woolly cupgrass.
Materials and Methods
The seeds used for this study were collected from the soil surface in a cultivated corn field at Stratford,
Iowa, in February of 1987. The seeds were dried at 30 C for 72 hours and then cleaned with an air separator (Model
A No. 42, Ames Powercount Co., Brookings, South Dakota).
The cleaned seeds were stored in plastic bags in plastic boxes at 5 C until used. Germination tests of this seed lot under laboratory conditions demonstrated 100%
germination within 6 days of evaluation. Viability of 41 the seed lot as determined by tetrazolium (TZ) test was
100%. Henceforth, unless otherwise stated, the term "seed" refers to the entire dispersal unit made up of the lemma, palea, and the caryopses. A two-year field study was conducted in the summers of 1987 and 1988 to evaluate the seed production capability of this grass species as affected by planting date. The experiments were conducted on a Browton silty clay loam (Mesic Typic Haplaquolls) soil with a pH of 7.98 and 5.23% organic matter. The studies consisted of 5 planting dates (treatments) replicated 3 times and were arranged in a randomized complete block design. Nondormant woolly cupgrass seeds were planted biweekly. The seedlings for the initial and the last planting dates emerged on May 12 and July 7, in 1987 and 1988, respectively. An experimental unit was a 2 m^ plot containing 9 seedlings which were later thinned to 5 seedlings at the 6-8 leaf-stage. Preliminary studies have shown that 5 woolly cupgrass plants in a plot of 2 m^ did not result in the overlapping of aerial parts thus, was regarded a noncompetitive environment. Plots were kept weed free throughout the entire period of the study. From each plot, observations and data were recorded from 3 randomly selected plants. The parameters measured 42 included the number of tillers/plant, panicle numbers/plant, the number of racemes/plant, seed numbers/raceme, and the number of seeds/plant. At harvest, the 3 plants from each plot were cut at ground level and dried to a constant weight at 60 C for dry weight determination. An analysis of variance was conducted for all the parameters studied. Differences among planting dates were compared using Least
Significant Differences (LSD) test at the 5% level of significance.
Results and Discussion
The effect of the date of planting on woolly cupgrass seed production is summarized in Table 1. Although the results from 1987 and 1988 demonstrated similar trends, the effect of planting date on all the yield parameters studied was more severe in 1988 than in 1987. Variation between years may be partially due to the differences in rainfall distribution during the growing season. Rainfall total from the months of May through August 1987 was 62.08 cm compared to 28.32 cm for 1988. The rainfall total for the 1987 growing season was 54.4% more than the 1988 growing season. Generally, planting date significantly 43
(P<0.01) affected all the seed parameters studied in both years.
Number of Tillers/Plant
The number of tillers/plant at harvest declined significantly (P<0.01) as the planting date was delayed during the growing season (Table 7 in the Appendix). In 1987, there were no significant differences in the number of tillers/plant between the woolly cupgrass plants from the first two planting dates (May 12 and 26) and between those plants from the last two planting dates (June 23 and July 7). Woolly cupgrass plants that were planted any time in the month of May produced 59 tillers/plant compared to 32, 18, and 12 tillers/plant for the plants that were planted on June 9, 23, and July 7, respectively.
In 1988, the number of tillers/plant differed significantly (P<0.01) between planting dates. The woolly cupgrass plants planted on May 12, 1988 produced 31 tillers/plant compared to 21, 12, and 4 tillers/plant for those plants that were planted on May 26, June 9, and 23, respectively. 44
Plant Dry Weight
Planting date significantly (P<0.01) affected the dry weight production of woolly cupgrass in both years (Table 1). In 1987, there was no significant (P=0.05) difference in the production of plant dry weight between woolly cupgrass plants from the first 3 planting dates (May 12 through June 9) and between plants from the last 2 planting dates (June 23 and July 7). The accumulation of plant dry weight from the last planting date (July 7) was significantly (P<0.05) reduced by 92% compared to the dry weight of the plants planted on May 12.
In 1988, woolly cupgrass dry weight production also declined significantly (P<0.01) with delayed planting date (Table 8 in the Appendix). Although planting date did not significantly affect the dry weight production of woolly cupgrass comparing to the first 2 planting dates (May 12 and 26), the effect was significant (P<0.05) between plants that were planted on June 9 and 26.
Seed Production
The seed production parameters evaluated in these studies included the number of panicles/plant, the number of seeds/panicle, the number of racemes/panicle, the number of seeds/raceme, and the approximate total number 45 of seeds/ plant. All of the seed yield parameters declined significantly (P<0.01) with delayed planting date for both years (Table 1 and Tables 9 through 13 in the Appendix). In 1987, the production of panicles/plant, the number of seeds/plant, the number of racemes/panicle, and the seed numbers/raceme from the plants in the last planting date (July 7) were reduced by 94, 73.7, 33, and 64.7%, respectively, compared to the plants that were planted any time in May (Table 1). A woolly cupgrass plant that emerges any time in the month of May is capable of producing a total of approximately 164,169 seeds/plant. The late planting date (July 7) reduced woolly cupgrass seed production by 98.4% compared to the plants that were planted any time in May (Table 1).
In 1988, the effect of planting date on all the yield parameters studied was highly significant (P<0.01) and the trend was similar to the 1987 results. The seeds that were planted at the last planting date in 1988 did not germinate because of the drought. Consequently, there is no yield data for this planting date. However, the number of panicles/plant, the number of seeds/panicle, racemes/panicle, seed numbers/raceme, and the approximate total number of seeds/plant for the plants that were planted on June 23 were reduced significantly (P<0.05) by 46
60.6, 51.8, 33, 44, and 81%, respectively compared to the plants that were planted any time in May. The results of these studies have shown that all the yield parameters studied were significantly affected by date of planting. Similar results with velvetleaf (Bello, 1986; Oliver,
1979) and with giant foxtail fSetaria faberii Herrm.) (Schreiber, 1964) have been reported. They found that delayed planting of these weed species greatly reduced the growth and seed production capabilities. According to Oliver (1979), the reduction effect of planting dates on velvetleaf plant dry weight and the yield components was largely due to photoperiodic response.
Conclusions
The results of these studies demonstrated that delayed planting greatly reduced all of woolly cupgrass yield parameters. Woolly cupgrass that emerges any time in May is capable of producing and returning up to 164,169 seeds into the soil seedbank under a noncompetitive environment. Woolly cupgrass that emerged during the first week of July was able to produce over 2,600 seeds/pant. Planting date affected the yield parameters of a woolly cupgrass plant through a reduction in the production of tillers, panicle numbers, seed 47 numbers/panicle, and plant dry weight. The number of racemes/panicle was relatively constant regardless of planting date and was the least affected yield component of woolly cupgrass.
These findings have agronomic and ecological implications. Dormant seeds are difficult to kill with the currently available control techniques. Since woolly cupgrass seeds are dormant at physiological maturity, the major control strategies should include the following: First, woolly cupgrass seed production must be reduced.
Second, proper crop production management strategies such as early planting and proper row spacing may enhance crop canopy development thus increasing crop competition which suppresses the growth and seed production potential of this weed. Results from the field after-ripening studies have shown that dormancy of woolly cupgrass seeds on the soil surface was terminated by the first week of December.
Also, growth chamber and field studies indicated that the optimum seedling emergence depth of woolly cupgrass was from the top 4 cm. Since germinating seeds and young seedlings are vulnerable to most control measures, fall tillage which may incorporate woolly cupgrass seeds deep into the soil profile must be limited or avoided. Table 1. Effect of planting date on various woolly cupgrass yield parameters
Parameters Measured^
Date No. of Plant No. of No. of No. of No. of No. of Planted Tillers/ weight Panicles/ Seeds/ Racemes/ Seeds/ Seeds/ (Biweekly) Plant (g) Plant Panicle Panicle Raceme Plant 1987
May 12 56® 381.3® 802® 152® 6® 26® 123,456® May 26 59® 369.0® 897® 186® 6® 34® 164,169® June 9 32^ 245.4® 348% 165® 5b 34® 57,860® June 23 18° 76.2® 125b 7 lb 5b 15b 8,908b° July 7 12° 30.9b 55b 49b 4° 12b 2,649c
LSD (P=0.05) 12 155.4 312.2 38.3 0.77 8.2 50,808
1988
May 12 31® 117.1® 203® 139® 6® 25® 28,221® May 26 21^ 111.6a 169b 77b 5b 16b 13,056b June 9 12° 58.4b 120° 75b 5b 15b 8,996° June 23 4d 36.3° 80<^ 67° 4° 14b 5,359°
July 7^ ...... • • • LSD (P=0.05) 2.2 17.7 24 5.5 0.4 1.8 1,695
lvalues within each parameter are the means of three replicates of three plants/plot. Values sharing the same letter within each parameter are not significantly different at the 5% level, according to the LSD test.
^Data not recorded because of lack of seed germination due to drought. 49
References Cited
Bello, A. I. 1986. Effects of shade and Glycine max L. competition on the growth and dormancy of Abutilon theophrasti Medic. M.S. Thesis. Iowa State University, Ames, Iowa. 125 pp.
Carvers, P. B., and J. L. Harper. 1964. Biological flora of the British Isles, Rvtmex obtus if olius L. and Rumex cripus L. J. Ecol. 52: 737-766.
Chandler, J. M., and J. E. Dale. 1974. Comparative growth of four malvaceus species. Proc. Southern Weed Sci Soc. 27:116-117. Dickerson, Jr., C. T., and R.D. Sweet. 1971. Common rapweed ecotypes. Weed Sci. 19:64-66.
Evans, R. A., and J. A. Young. 1974. Today's weed. Russian thistle. Weeds Today 5(4):18. Gomes, F. L., J. M. Chandler, and E. E. Vaughan. 1978. Aspects of germination, emergence, and seed production of three Ipomoea taxa. Weed Sci. 26:245- 248.
National Academy of Sciences. 1968. Weed Control. Vol. 2. Principles of plant and animal pest control. National Academy of Sciences Publ. 1596:6-35. Oliver, L. R. 1979. Influence of soybean (Glycine max) planting date on velvetleaf fAbutilon theophrasti^ competition. Weed Sci. 27:183-188. Owen, M. D. K. 1987. Control of woolly cupgrass fEriochloa villosa (Thumb.) Kunth.) in corn with three acetamide herbicides. Proc. Weed Sci. Soc. Am. 27:4.
Schreiber, M. M. 1964. Effect of date of planting and stage of cutting on seed production of giant foxtail. Weeds 60:62-68.
Shaw, W. C., D. R. Shepherd, E. L. Robison, and P. F. Sand. 1962. Advances in witchweed control. Weeds 10:182-192.
Stevens, O. A. 1932. The number and weight of seeds produced by weeds. Am. J. Bot. 19:784-794. 50
ŒAPTER 2. STUDIES ON DORMANCY FACTORS IN WOOLLY CUPGRASS (Erioghloa villosa [Thumb.] Kunth.) SEEDS
Introduction Woolly cupgrass fEriochloa villosa [Thumb.] Kunth.) is an annual plant native to East Asia and was believed to have entered the United States in ship ballast near Portland, Oregon, in the 1940s (Hitchock, 1950). In Iowa, woolly cupgrass was first reported in Ringgold County about 31 years ago. Presently, it has been identified in 56
Counties (Figure 1) and thus, it appears to be successfully adapting to the agroecosystem of the Midwestern States.
This grass was first collected and identified in Iowa on August 8, 1957 (Pohl, 1957). Since then, woolly cupgrass has been found in Illinois, Minnesota, Pennsylvania, and Wisconsin^.
Woolly cupgrass was reported by Strand and Miller (1980) as a weed threat in the Midwestern agroecosystems and found to be tolerant to atrazine (2-Chloro-4-
(ethylamino)-6-(isopropylamino)-s-triazine). Also, woolly cupgrass is highly competitive in corn fZea mavs L.) (Owen, 1987).
One of the major characteristics of annual weeds that
^Ada Hayden Herbarium, Dept. Bot., Bessey Hall, Iowa State Univ., Ames, lA 50011. 51 enables them to adapt and persist in any agroecosystem is the ability to remain dormant and viable in the soil seedbank for a relatively long period of time (Darlington and Steinbauer, 1961; Kivilaan and Bandurski, 1981;
Livingston and Allessio, 1968; Lueschen and Anderson, 1980; Costing and Humphreys, 1940; Toole and Brown, 1946).
Villiers (1972) defined dormancy as the "state of arrested development whereby the organ or organism, by virtue of its structure or chemical composition, may possess one or more mechanisms preventing its own germination." According to
Kollman and Staniforth (1972), seed dormancy is a survival mechanism that prevents the fall germination of species that are not winter-hardy.
In the scientific literature, seed dormancy in the Graminae is thought to be the result of one or more of the following mechanisms: (1) seed coat and or hull impermeability to moisture (Bewley and Black, 1982; Harrington and Crocker, 1923; Hinton, 1955 Werker,
1980/81), (2) seed coat and or hull restriction of oxygen from reaching the embryo (Atwood, 1914, Harrington and Crocker, 1923; Johnson, 1935; McDonald, 1977; Navasero et al., 1975; Weisner and Kinch, 1964), and (3) the presence of growth inhibitors in the seed coat, hulls, or embryo
(Atwood, 1914; Bewley and Black, 1982; Elliot and Leopold, 52
1953; Kommedhal et al., 1958; Yearn et al., 1988).
The seeds of woolly cupgrass at physiological maturity in the fall are dormant. However, the seeds collected from the soil surface in the month of February 1987, germinated under laboratory conditions. This suggests that seed dormancy in woolly cupgrass is a survival mechanism that allows the continuation of the species.
Information on seed dormancy factors in woolly cupgrass is lacking. Thus, studies were conducted in the laboratory to determine the effect of seed hulls on water imbibition and oxygen uptake. Also, seed leaching and bioassay studies on the germination of dormant seeds were done to determine the effect of seed leachate and to evaluate the presence of water soluble growth inhibitor(s). The effect of embryo excision on germination of woolly cupgrass seeds was also evaluated.
Materials and Methods Seed source
The seeds used for these studies were harvested from native woolly cupgrass populations in a cultivated corn field at Stratford, Iowa, in September of 1986 and 1987. 53
The seeds at physiological maturity were tannish-brown in color. The seeds were dried at 30 C for 72 hours and then cleaned with an air column blower or separator (Model A No. 42, Ames Power-count Co., Brookings, South Dakota). The cleaned seeds were stored in plastic bags in plastic boxes at 5 C until used. The mean weight of 100 dormant seeds during storage was 920 mg. Henceforth, unless otherwise stated, the term "seed" refers to the entire dispersal unit made up of the lemma, palea, and the caryopses. Germination tests on the freshly harvested seeds of woolly cupgrass during the Fall of 1986 and 1987 demonstrated that the seeds are innately dormant.
During the months of February and March, 1987, seeds were collected from the soil surface in the same field as those collected in the Fall. Germination tests on this seed lot under laboratory conditions demonstrated 100% germination within 6 days. The seeds from this seed lot were considered nondormant, and they were stored under the same conditions as the dormant seeds until used. Viability of the seeds was determined by a tetrazolium
(TZ) test. Three groups of 50 seeds were presoaked in distilled water for 5 hours at 25 C. Each seed was then cut longitudinally into two halves with a razor blade. The seed halves were placed in 0.1% tetrazolium solution at 54
35 C for 12 hours to stain. Viable and nonviable embryos stained red and colorless, respectively, in a TZ solution. The result of the TZ test demonstrated 100% viability of the seed lot.
Effect of seed hulls on Water imbibition
Intact dormant and nondormant woolly cupgrass seeds were used for this study. The seeds for,each treatment were placed between two moist circular disks of blotter paper (Ropaco blue blotter paper, Rochester, MI 48063) and imbibition was evaluated at 25 C for 36 hours. An experimental unit was a petri dish with intact dormant or intact nondormant woolly cupgrass seeds, and treatments were replicated three times. The initial weight (mg) of seeds for each treatment was recorded prior to the onset of the imbibition. During imbibition the weight of the seeds was recorded at the intervals of 0.5, 1, 3, 6, 12, 24, and
36 hours. Preliminary germination tests of nondormant woolly cupgrass at 25 C prior to the imbibition experiment have shown that the first visible signs of germination of nondormant seeds occur at 36 hours. Therefore, the imbibition data reported here were collected before germination occurs (pregerminative). 55
Effects of hulls, oxygen, and temperature on germination of dormant woolly cupgrass seeds
The experimental design for this experiment was factorial with three replications arranged in a complete block design. There were 60 treatments, which consisted of
4 temperature regimes as the main plots, 5 oxygen levels as the subplots, and 3 seed types as the sub-subplots. The 4 temperature regimes were 2 constant temperatures (20 and 25
C) and 2 alternating temperatures (15/25 C and 20/30 C). The oxygen levels were 8, 16, 21, 35, and 45%. The 3 seed types were intact dormant seeds, intact dormant seeds with 1/4 distal end dehulled, and dehulled dormant caryopses. Dehulling was accomplished by using tweezers and razor blades.
Prior to the experiment, all the seeds were surface sterilized in a solution of 1% hypochlorite solution (Iowa Prison Industries, Anamosa, lA 52205) for 5 minutes. The seeds were then rinsed 5 times with double deionized water and dried at 30 C for 30 minutes. For each treatment, a group of 25 seeds or caryopses was placed on 36 g of sterilized fine sand (Hallett Construction Co., Boone, lA 50036), moistened with 8 ml of double deionized water in a
125 ml serum bottle^. After placing the seeds or
^Fisher Scientific, 1600 West Glenlake Ave., Itasca, Illinois 60143, USA. 56
caryopses in the serum bottles, the bottles were sealed
with a 13 X 20 mm tear-off aluminum seal^ using a manual crimpers^. Each serum bottle was flushed and filled with the desired gas mixture made from pressurized gas cylinders of oxygen, nitrogen, and 2% carbon dioxide (v/v). The
oxygen level of the inlet gas mixture was measured with an oxygen electrode (Yellow Springs Instrument, Yellow Springs, OH 45387). Germination was evaluated for 10 days at the temperature regimes.
Respiration rates experiment
Respiration rates of intact dormant, intact nondormant, and dehulled dormant woolly cupgrass seeds were evaluated in the laboratory using the Gilson
differential respirometer (Model G-20, Gilson Medical
Electronics, Box 27, Middleton, WI 535662). Four replications of a group of 25 intact dormant, intact nondormant, and dehulled dormant woolly cupgrass seeds
which had imbibed water for 12 hours at 25 C were placed in a 12 ml respirometer flask and moistened with 2 ml double deionized water. A single paper wick (a layer of corrugated filter paper) was placed in the center well of each flask and saturated with 0.3 ml of 20% KOH (w/v)
^Fisher Scientific, 1600 West Glenlake Ave., Itasca, Illinois 60143, USA. 57
solution to absorb carbon dioxide. The respirometer flasks were incubated in a water bath at 25 C for the entire experimental period. Prior to the measurements of oxygen uptake, air free of carbon dioxide (by passing the air through a solution of
KOH) was used to purge and to fill the system for 15 minutes. The system was allowed to equilibrate for 10 minutes prior to readings. Oxygen uptake measurements were made at the intervals of 1 hr for 6 hrs.
Leaching experiment
Leaching (continuous washing with water) was employed to determine the role of water soluble inhibitors located in the hulls on seed germination. The leaching technique described by LaCroix and Staniforth (1964) was used, and the leaching experiment was repeated 3 times. Seeds were leached for 14 days at 4 C. For each leaching experiment, 40 g of surface sterilized, intact dormant seeds were leached with 400 ml of double deionized water. After 14 days of leaching, the germination of the seeds was evaluated at 25 C daily for 10 days. An experimental unit was a petri dish with 50 dormant seeds and each treatment was replicated 3 times.
A bioassy experiment using the leachate from the 58 dormant seeds as the source of moisture for nondormant seeds was also conducted. For this experiment, 5 replicates of a group of 25 nondormant seeds were placed on 36 g of moistened sterilized fine sand in standard petri dishes. Germination was evaluated at 8 temperature regimes for 6 days. Five constant and 3 alternating (night/day) temperature regimes were used. The temperature regimes were 15, 20, 25, 30, and 35, and 15/25, 15/30, and 20/30 C, respectively.
In another bioassay experiment, sterilized crushed hulls of 1000 dormant seeds were soaked in 100 ml of double deionized water for 14 days at 10 C. The leachate, including the crushed hulls, was used as the source of moisture for nondormant seeds. Four groups of 50 nondormant seeds were placed on 36 g of sterilized fine sand and moistened with 8 ml of the leachate in standard petri dishes. In all the leachate experiments, control treatments were established by using distilled water as the source of moisture. Germination was evaluated at 25 C daily for 6 days.
Embryo excision study Embryos were carefully isolated from dehulled, dormant woolly cupgrass seeds that had imbibed water for 12 hours 59 at 30 C. The isolation was done with sterilized scalpels, forceps, dissecting needles, and razor blades under a dissecting microscope at 50X magnification. The isolated embryos were placed on two 9 cm moist circular disks of sterilized, blue blotter papers in sterilized standard petri dishes. Embryo growth was evaluated at alternating (night/day) temperature of 20/30 C for 10 days. An experimental unit was a petri dish with 15 embryos placed on two circular moist blotter papers replicated 3 times. An analysis of variance was conducted, and differences between treatments were compared using the Least
Significant Difference (LSD) at the 5% level of significance.
Effects of prechilling and oxygen on seed dormancy of woolly cupgrass
An experiment was conducted in the laboratory to study effects of prechilling, temperature, and oxygen on germination of dormant woolly cupgrass seeds. Intact woolly cupgrass seeds were prechilled between two moistened
15 X 22.5 cm blue blotter papers in 27.5 x 16.25 x 4.38 cm clear styrene boxes at 5 C. The seeds were surface sterilized with 1% hypochlorite solution prior to prechilling.
During prechilling, seeds were removed biweekly for 60
germination tests. The experiment design was factorial with three replications arranged in a randomized complete block design. There were 40 treatments consisted of 4 temperature regimes as the main plots, 5 oxygen levels as the subplots, and 2 prechilling periods as the sub- subplots. The two prechilled periods were 2 and 4 weeks. The temperature regimes were 20, 25, 15/25, and 20/30 C. The oxygen levels were 8, 16, 21, 35, and 45%.
Prior to the onset of the experiment, the seeds were surface sterilized in 1% hypochlorite solution for 5 minutes. The seeds were then rinsed 5 times with double deionized water and dried at 30 C for 30 minutes. For each treatment, a group of 20 seeds was placed on 36 g of sterilized fine sand moistened with 8 ml of double deionized water in a 125 ml serum bottle. After the seeds were placed, each serum bottle was sealed with a 13 x 20 mm red rubber stopper and tightly secured by a 20 mm tear-off aluminum seal using a manual crimpers. Each serum bottle was flushed and filled with the desired gas mixture made from pressurized gas cylinders of oxygen, nitrogen, and 2% carbon dioxide (v/v). The oxygen level of the inlet gas mixture was measured with an oxygen electrode. Germination was evaluated for 8 days.
An analysis of variance was conducted for effects of 61 prechilling, temperature, oxygen, and the interaction of these factors on germination. Differences between treatments were compared using the LSD test at the 5% level of significance.
Effect of prechilling on seed dormancy of woolly cupgrass
A prechilling experiment was conducted in the laboratory to determine the effect of prechilling requirement on the termination of woolly cupgrass seed dormancy. Intact and dehulled dormant woolly cupgrass seeds were prechilled between two moistened 15 x 22.5 cm blue blotter papers in 27.5 x 16.25 x 4.38 cm clear styrene boxes at 5 C for 12 weeks. The seeds were surface sterilized with a 1% hypochlorite solution prior to prechilling.
During the prechilling period, 3 replicates of a group of 50 seeds were evaluated for germination at 6 temperature regimes biweekly for 12 weeks. The temperature regimes used were 20, 25, 30, 15/25, 15/30, and 20/30 C.
Effects of field after-ripening and soil depth on seed dormancy of woolly cupgrass
In a separate study, the effect of seed depth and field after-ripening on the dormancy of woolly cupgrass seeds was evaluated in the field from the month of October 62
through the month of December, 1988. For this experiment,
3 wooden boxes were constructed. Intact dormant seeds were placed on the soil surface, at 2 cm, and at 4 cm depths within the boxes. The seeds for the 2 and 4 cm depth treatments were placed on fine mesh screens and then covered with soil. The 3 boxes were covered with an aluminum screen to exclude birds and rodents. At weekly intervals for 9 weeks, seeds from each treatment were removed, rinsed and surface sterilized in a 1% hypochlorite solution prior to germination tests. In the laboratory, 3 replications of a group of 25 seeds from each treatment were placed on 36 g of fine sand moistened with 8 ml of distilled water in standard petri-dishes. Germination was evaluated at 7 different temperatures. The temperature regimes were 20, 25, 30, and 40 C as constant temperatures and 15/25, 15/30, and 20/30 C as alternating (night/day) temperatures. 63
Results and Discussion
Effect of seed hulls on water imbibition Figure 3 shows the rate of imbibition for dormant and nondormant seeds for 36 hrs. The analysis of variance demonstrates that there was no significant (P=0.05) difference in water uptake between the dormant and nondormant seeds throughout the experimental period (Table 14 in the Appendix). The rates of water uptake for dormant and nondormant woolly cupgrass seeds were the same and rapid during the first 0.5 hr of exposure to moisture. These results are similar to the imbibition characteris tics of the Graminae previously reported (Black, 1959; Roberts, 1961). During this period, dormant and nondormant woolly cupgrass seeds gained an average of
0.47 mg/min. The imbibition rate after the first 0.5 hr was slower, with treatments gaining only 0.01 mg/min of fresh weight on the average. These two imbibition rates are synonymous to the phase 1 and the phase 2 of a typical imbibition curve previously reported (Bewley and Black, 1982; Shull, 1920). Although the rate of water imbibition for dormant and nondormant woolly cupgrass seeds was the same during the first 3 hrs, a slight variation between the two treatments 64
290
A INTACT DORMANT SEEDS O INTACT NONDORMANT SEEDS
275
E t/t o 260 wLU
1 s 245 1
230
215
0 0.5 1 3 6 12 24 36 TIME (HOURS)
Figure 3. Rate of water imbibition in dormant and nondormant woolly cupgrass seeds at 25 C. Data are the means of three replications (LSD=50.0). 65
occurred after 3 hrs (Figure 3). After this period, the curves started to diverge suggesting that the nondormant seeds imbibed more water in phase 2 of imbibition compared to the dormant seeds.
Unlike the nondormant seeds, dormant seeds attained maximum water uptake about 24 hrs after exposure to moisture. This curve is characteristically unique for dormant or nonviable seeds (Marcus et al., 1966). The failure of fully imbibed dormant seeds to germinate suggests that the attainment of maximum imbibition is not a factor for the germination of dormant woolly cupgrass seeds. Therefore, some physiological factors other than water imbibition must be responsible for the dormancy of this grass species. The imbibition curve of the nondormant seeds may demonstrate phase 3 of the typical imbibition curve for nondormant seeds if the imbibition data were taken beyond the 36 hrs. Phase 3 of the imbibition curve, as previously explained (Bewley and Black, 1978), demonstrates another burst of water uptake and is usually associated with the onset of embryo growth and the radicle protrusion. 66
Effects of hulls, oxygen, and temperature on seed germination of woolly cupgrass
It was previously hypothesized that dormancy in the grass family may be imposed by the hull acting as a barrier to oxygen diffusion to the embryo during imbibition
(Harrington, 1923; Navasero et al., 1975; Roberts, 1961). This study was conducted to establish whether or not the hull of dormant woolly cupgrass seeds acts as a barrier to oxygen diffusion during germination.
Figures 4 and 5 are a summary of the germination percentage of intact, partly dehulled, and dehulled woolly cupgrass seeds in response to various temperature regimes and oxygen levels after 10 days. Intact dormant woolly cupgrass seeds did not respond to any oxygen or temperature regimes when compared to the partly dehulled seeds or the caryopses (Figure 5). Partial dehulling promoted the germination of dormant woolly cupgrass seeds by over 80% when exposed to any of the temperature regimes used. The germination of dehulled seeds was better (96.5%) at 20 C compared to 94, 94.7, and 92% for 25, 15/25, and 20/30 c, respectively. The effect of temperature on the germination of dehulled seeds was significantly (P<0.05) greater than the partly dehulled dormant woolly cupgrass (Figure 4).
Oxygen levels significantly (P<0.01) affected the 67
SEED TREATMENT
•i INTACT SEEDS O INTACT SEEDS (1/4 DISTAL END DEHULLED) ^ DEHULLED SEEDS
100
90
80
70
60
I 50 Oi «lU 40
30 -
20
10
0 20 25 15-25 20-30 TEMPERATURE (°C)I
Figure 4. Germination of dormant woolly cupgrass seeds as affected by temperature. Data are the means of 5 oxygen concentrations (LSD=2.5). 68
SEED TREATMENT •I INTACT SEEDS •INTACT SEEDS (1/4 DISTAL END DEHULLED) ^ DEHULLED SEEDS
100
90
80
70-
60.
50.
40 .
30
20 10- Î 0 . 8 16 21! 35 45! 1OXYGEN LEVEL (%) 1 5. Effect of oxygen concentrations on germination of dormant woolly cupgrass seeds as affected by seed type. Data are the means of 4 temperature regimes (LSD=3.8). 69 germination of dehulled and partly dehulled dormant woolly cupgrass seeds (Table 15 in the Appendix). The germination response of the dehulled dormant seeds to oxygen levels was significantly (P<0.01) greater than the partly dehulled seeds. These results were consistent with the findings of
Roberts (1961). He found out that the removal of the husk some distance away from the embryo was not as effective in breaking the dormancy of redrice as the removal of part of the husk immediately over the embryo.
The variation in the germination percentage between the dehulled and the partly dehulled woolly cupgrass seeds may be likely due to the following possible reasons. First, the seeds in the partly dehulled treatment had approximately 1/4 of the distal end dehulled and this was away from the embryo. Therefore, the embryo was completely covered by the hull. Also, the fact that the caryopses swell following imbibition may cause a tight seal between the hull and the caryopses. If this concept is correct, then oxygen diffusion and the outflux of carbon dioxide may likely be reduced. Since germination is oxygen dependent, the results of this study suggest that the hulls of woolly cupgrass seeds play some role in the dormancy of this grass species. This is likely by preventing oxygen from reaching the embryo. 70
In this study, there was no interaction of temperature and
oxygen on the germination of woolly cupgrass. However, there was an interaction of seed treatment and temperature or seed treatment and oxygen on the germination of dehulled or partly dehulled dormant woolly cupgrass seeds. The results of these studies imply that the termination of seed
dormancy and germination of woolly cupgrass seeds was temperature and oxygen dependent.
Respiration rate experiment
In the previous experiment, intact dormant seeds of woolly cupgrass did not respond to oxygen levels above atmospheric concentration. However, dehulled and partly dehulled seeds responded (80% germination) to oxygen level
as low as 8%. In a follow up experiment to further substantiate the concept that dormancy in this grass weed is likely caused by the hulls limiting oxygen from reaching the embryo, measurements on respiration rates for intact dormant, dehulled, and intact nondormant seeds were made in the laboratory. The measurements of the oxygen uptake were pregerminative. The results of this experiment are summarized in Figure 6. Seed type significantly (P<0.01) affected the rate of oxygen uptake (Table 16 in the
Appendix). Over the 6 hours, oxygen uptake by dehulled 71
dormant woolly cupgrass was greater than either the intact
dormant or intact nondormant seeds (Figure 6). During the first 1 hour, the rate of oxygen uptake by the dehulled dormant seeds was 0.83 ^1/min compared to 0.78 and 0.67 ^1/min for the intact nondormant and the intact dormant seeds, respectively. At the 6 hour of oxygen uptake
measurements, the rate of the oxygen uptake by the dehulled, intact nondormant, and intact dormant woolly cupgrass seeds were 0.91, 0.71, and 0.59 pl/min, respectively.
Dehulling increased the oxygen uptake of dormant seeds by 22% and 35% compared to the intact nondormant and the intact dormant seeds, respectively. Similar results have been reported in wild oats (Chen and Varner, 1969) and in Galium aparine L. seeds (Hilton and Thomas, 1987). They
found that the rate of oxygen uptake by dormant seeds was less than in nondormant seeds. The results of these studies demonstrate that the hulls of woolly cupgrass seeds restrict oxygen diffusion. The results of the two
oxygen experiments support the concept that dormancy in this weed species is likely associated with the seed hulls which limit oxygen diffusion to the embryo. 72
330
SEED TYPE O DEHULLED DORMANT SEEDS • INTACT DORMANT SEEDS A INTACT NONDORMANT SEEDS
250
a 200
< 150
100
1 2 3 4 5 6 TIME (HOURS)
Figure 6. Respiration rates in intact dormant, intact nondormant, and dehulled dormant woolly cupgrass seeds at 25 C. Data are the means four replications (LSD=18.9). 73
Leaching experiment
Research by Elliot and Leopold (1953) and Nieto-Hatem (1963) supports the hypothesis that dormancy in some plant species is the result of water soluble inhibitors located in the seed hulls. The intact dormant woolly cupgrass seeds were leached to investigate the presence of water soluble inhibitors in the seed hulls and the possible role on the dormancy of this species.
Table 2 shows that leaching was not effective in promoting the germination of intact dormant woolly cupgrass seeds. The rate of germination of intact nondormant seeds exposed to the leachate and the control was the same. Therefore, the leachate from the intact dormant seeds did not inhibit the germination of nondormant woolly cupgrass seeds (Figure 7 and Table 17 in the Appendix). Also, the leachate of crushed hulls of dormant seeds was not effective in preventing the germination of intact nondormant woolly cupgrass (Table 2). Nieto-Hatem (1963) obtained similar results and found that leaching promoted the germination of intact Setaria lutescens L. seeds but the leachate was ineffective in inhibiting the germination of nondormant seeds. Evidence from these studies ruled out the likely presence of water soluble inhibitors in the hulls of woolly cupgrass seeds. 74
Table 2. Effect of seed leaching and leachate from the dormant seeds on germination of intact nondormant woolly cupgrass seeds at 25 C for 6 days
Treatment % Germination®
lb 1 2C 100 3d 100
4® 100
^Means are the average of three replications of 50 seeds ^Leached intact dormant seeds after 14 days of leaching with water at 4 C.
^Nondormant seeds + the leachate of treatment 1. ^Nondormant seeds + leachate of crushed hulls from 1000 dormant seeds.
^Control (nondormant seeds + distilled water). 75
TREATMENT A LEACHATE O DISTILLED WATER 100
75
50
25
0 1 2 3 4 5 6 DAYS
Figure 7. Germination of nondormant woolly cupgrass seeds as affected by leachate of dormant seeds and temperature. Data are the means of of 8 temperature regimes (LSD=1.0). 76
Embryo excision study
The embryo culture study was conducted to evaluate embryo dormancy and to establish if the dormancy in woolly cupgrass seeds is imposed and maintained by the embryo.
The isolated embryos were 91% germinated after 10 days at
20/30 C. The ungerminated embryos looked wrinkled but were uncontaminated. The inability of the ungerminated embryos to grow may be due to some damage inflicted upon them during the process of the excision. It is possible that water soluble inhibitors, if any, can diffuse out of the embryos during water imbibition thus, allowing the embryos to grow. The result of this study demonstrated that the dormancy characteristics of woolly cupgrass seeds may be caused and maintained by factors associated with the seed covers (hulls, seed coat, or the endosperm) other than the embryo. The role of the seed coat or the endosperm in the dormancy of this grass species was not investigated..
Effect of prechilling and oxygen on seed dormancy of woolly cupgrass
Temperature regimes and oxygen levels significantly (P<0.01) promoted seed germination of intact dormant woolly cupgrass prechilled for 4 weeks (Table 18 in the appendix).
Germination was 87% at alternating temperature of 15/25 c compared to 77.9% for 20 C. Germination of woolly 77 cupgrass seeds increased with increasing oxygen levels.
Woolly cupgrass seed germination at either 35% or 45% oxygen levels was 88% compared to 85% and 69% for 21% and 8% oxygen levels, respectively.
Germination of prechilled intact dormant woolly cupgrass seeds was better under the interaction of temperature and oxygen, temperature and prechilling, or prechilling and oxygen than under either temperature or oxygen levels alone. After 8 days of evaluation, germination of prechilled dormant seeds under all the oxygen levels increased with increasing temperature regimes. The interaction of temperature and oxygen promoted germination of prechilled dormant woolly cupgrass seeds by 92% after 8 days (Figure 8).
The interaction of temperature and prechilling period on germination of dormant woolly cupgrass seeds was highly significant (P<0.01) (Table 18 in the Appendix). Germination of dormant woolly cupgrass seeds increased with increasing prechilling period and with increasing temperature. Germination of prechilled dormant woolly cupgrass seeds was better with alternating temperatures compared to the constant temperatures (Figure 9). The interaction of temperature and prechilling promoted germination of dormant woolly cupgrass seeds by 95.4% 78
OXYGEN LEVEL /o 35% [=• 16% [Z] 45% 21% IOOt
X - X \ M X K X > X V X / X / 80 •- s X / V X \ X X X X X / / \ \ X X \ X / X X X X \ X / X / s X / X \ X X X s X X / / s X X \ X / X X 60" X X \ X / X / X X s K / X X / X s X s X / < X X X / X 7^ M / X V X X / s X / X X X X 40" s X X / / X X 0: \ X / s X s X X UJ s X X / X / o s X / X X X X s X X / / X X \ X / X X X 20" \ X X / / \ X / X X X X s M / X / s X / X X X X \ X X / / X X > / X X W1 X / X 0 1 20 25 15/25 20/30
TEMPERATURE (C)
Figure 8. Effect of prechilling on germination of woolly cupgrass seeds as affected by the interaction of temperature and oxygen. Data are the means of 2 prechilling periods. This interaction is highly significant (P<0.01). 79
PRECHILLING TIME •i 2 WEEKS CD 4 WEEKS lOOx
< T q: LU c
20 25 15/25 20/30
TEMPERATURE (C)
Figure 9. Germination of dormant woolly cupgrass seeds as affected by prechilling and temperature. Data are the means of 5 oxygen concentrations. This interaction is highly significant (P<0.01). 80 after 8 days (Figure 9).
Germination of dormant woolly cupgrass seeds under all the oxygen levels increased significantly (P<0.01) with increasing prechilling period (Table 18). This interaction terminated the dormancy of woolly cupgrass seeds by 98.4% after 8 days of evaluation (Figure 10).
Effect of prechilling on seed dormancy of woolly cupgrass In fall, the seeds of woolly cupgrass are dormant. However, the seeds that were recovered from the field in the months of February and March, 1987 germinated under laboratory conditions. This suggests that dormancy in this weed species is a mechanism to ensure that the seeds produced in the fall do not germinate during the winter when growing conditions are unfavorable. Nevertheless, the low temperatures of winter are required during which dormancy is terminated allowing germination to occur in early spring when conditions become favorable.
These studies were conducted to investigate the effect of prechilling on the germination of dormant woolly cupgrass seed. Figure 11 shows that prechilling significantly (P<0.01) promoted the germination of dormant woolly cupgrass seeds (Table 19 in the Appendix). Two weeks of prechilling at 5 C terminated the dormancy of 81
PRECHILLING TIME Wm 2 WEEKS CD 4 WEEKS lOOx
zg
CK w o
8 16 21 35 45
OXYGEN LEVEL (%)
Figure 10. Germination of dormant woolly cupgrass seeds as affected by the interaction of prechilling and oxygen. Data are the means of 4 tempera ture regimes. This interaction is highly significant (P<0.01). 82
100
80
60
40
20
0 0 2 4 6 8 PRECHILL TIME (WEEKS)
Figure 11. Effect of prechilling on germination of intact dormant woolly cupgrass seeds. Data are the means of three replications of 6 temperature regimes (LSD=3.8). 83
dehulled dormant seeds by 98% when exposed to the alternating temperatures of 15/30 or 20/30 c, compared to 19% for the intact dormant seeds (Figure 12). The germination of prechilled intact dormant seeds of woolly cupgrass at all the temperatures, increased with
prechilling period. Six weeks of prechilling at 5 C completely terminated the dormancy of intact dormant woolly cupgrass seeds germinated at 15/30 or 20/30 C. Prechilled dormant woolly cupgrass seeds germinated better at
alternating temperatures than at the constant temperatures
(Figure 13). These results are similar to the findings of several investigators (Steinbauer and Grigsby, 1957; Vincent and Roberts, 1977; Warington, 193 6). The mechanism/s by which seed germination, under alternating temperatures, is superior, compared to constant temperatures, is not known. The interaction of prechilling and temperature regime on germination of dormant woolly cupgrass was highly (P<0.01) significant (Figure 14 and Table 19 in the Appendix).
Effects of field after-ripening and soil depth on seed dormancy of woolly cupgrass
The depth of seed planting in the field significantly
(P<0.01) affected the germination of after-ripened dormant woolly cupgrass seeds (Figure 15 and Table 20 in the 84
TREATMENT E INTACT DORMANT SEEDS O DEHULLED DORMANT SEEDS 100 I 80
«4
0 12 60 1UJ o 40
20
2 4 6
PRECHILL TIME (WEEKS)
Figure 12. Germination of intact dormant and dehulled dormant woolly cupgrass seeds at 15/30 and 20/30 C as affected by prechilling (LSD=14.5). 85
100
80
20
20 ' 25 ' 30 ^ 15-25 ' 15-30 ' 20-30 TEMPERATURE (°C)
Figure 13. Effect of temperature regimes on germination of intact dormant woolly cupgrass seeds. Seeds were prechilled at 5 C for 8 weeks. Data are the means of 8 weeks of prechilling (LSD=6.4). 86
PRECHILL TIME (WEEKS)
• NO PRECHILL • 2 WEEKS OF PRECHILL ^ 4 WEEKS OF PRECHILL B 6 WEEKS OF PRECHILL mn 8 WEEKS OF PRECHILL 100
ë i i 100
20 -
15-25 15-30 20-30 TEMPERATURE (°C)
Figure 14. Germination of prechilled intact dormant woolly cupgrass seeds as affected by temperature. Data are the means of three replications and all the prechilling periods. This interaction is highly significant (P<0.01). 87
75
m L_ 0 2 4 SEED DEPTH IN THE FIELD (cm)
Figure 15. Germination of intact dormant woolly cupgrass seeds as affected by field after-ripening and soil depth. Data are the means of 7 tempera ture regimes and 9 weeks of prechilling (LSD=3.4). 88
Appendix). The germination of the seeds that were after- ripened on the soil surface was significantly (P<0.05) greater than the seeds after-ripened at the 2 cm or 4 cm depth. There was no significant variation on the germination percentage between the seeds after-ripened at 2 cm and at 4 cm depth. The germination of intact dormant woolly cupgrass seeds in the field increased with decreasing temperatures during the after-ripening period
(Figure 16 and Table 3). By December 2, 1987, the dormancy of woolly cupgrass seeds on the soil surface in the field was terminated by 91% compared to 63% and 51% for the seeds at 2 and 4 cm depth, respectively. 89
80
60
0 1 40 i (3LU
20
2 4 5 6 7 8 AFTER RIPENING PERIOD (WEEKS)
Figure 16. Germination of dormant woolly cupgrass seeds as affected by field after-ripening. Data are the means of three replications of 7 tempera ture regimes (LSD=5.2). 90
Table 3. Maximum and minimum soil temperature in Agronomy Weed Garden, Ames, Iowa from October 29th to December 30th, 1987
Mean temperature (C) Soil Surface At 4cm depth Date Max. Min. Max Min.
October 29 18.33 0.60 18.33 3.90
November 11 11.00 —6.00 8.33 2.78 November 25 3.89 0.00 4.44 3.89 December 2 2.22 -6.10 2.80 2.22 December 9 8.90 1.11 8.33 5.00 December 16 —5.00 -11.70 2.22 1.67 December 23 3.33 -2.22 0.56 0.56
December 30 2.22 -7.22 1.11 1.11 91
Conclusions
The rate of water imbibition between dormant and nondormant seeds was found to be the same, suggesting that water uptake was not a factor of dormancy in this species. Intact dormant seeds did not respond to any temperature regime or to oxygen levels above atmospheric concentration. Dehulling promoted and increased the rate of oxygen uptake in dormant woolly cupgrass seeds. Although intact nondormant seeds significantly (P<0.05) consumed more oxygen than intact dormant seeds, the rate of oxygen uptake in dehulled dormant seeds was substantially (P<0.05) higher than the intact dormant seeds. This implies that the hulls of woolly cupgrass seeds limit the diffusion of oxygen to the embryo, and this may in part be responsible for the dormancy characteristics of this grass weed.
The results of the leaching and the bioassay studies demonstrated that the presence of water soluble growth inhibitors in the hulls and the possible role in the seed dormancy of woolly cupgrass seeds was negligible.
Excised embryos from dormant seeds germinated under laboratory conditions. This suggested that seed dormancy in woolly cupgrass was imposed and maintained by the seed coverings such as the seed hulls, seed coat, or the 92 endosperm.
The results of these investigations have demonstrated that prechilling period is required for the termination of dormancy in imbibed dormant woolly cupgrass seeds. The effect of prechilling on the termination of seed dormancy in woolly cupgrass was temperature and oxygen dependent. The effect of prechilling on seed dormancy of woolly cupgrass may be by modifying the seed covering structures and thus, allowing oxygen diffusion to the embryo. Dormancy of woolly cupgrass seeds can be terminated completely when the seeds are prechilled at 5 C for 4 weeks and exposed to a minimum alternating temperatures (night/day) of 15/25 C and to oxygen levels above atmospheric concentration.
The field experiment demonstrated that the termination of dormancy in woolly cupgrass seeds in the field depended on the length of exposure to low temperatures. It may be very difficult to determine the exact length of time and the temperature of prechilling requirements that can terminate seed dormancy. 93
References Cited
Atwood, W. M. 1914. A physiological study of the germination of Avena fatua. Bot. Gaz. 57:386-414. Bewley, J. D., and M. Black. 1978. Physiology and biochemistry of seeds in relation to germination. Vol. 1: Development, germination and growth. Springer-Verlag, New York. 306 pp.
Bewley, J. D., and M. Black. 1982. Physiology and biochemistry of seeds in relation to germination. Vol. 2: Viability, dormancy and environmental control. Springer-Verlag, New York. 375 pp. Black, M. 1959. Dormancy studies in seeds of Avena fatua. I. The possible role of germination inhibitors. Can. J. Bot. 37:393-402.
Chen, S. S. C., and E. J. Varner. 1970. Respiration and protein synthesis in dormant and after-ripened seeds of Avena fatua L. Plant Physiol. 46:108-112. Darlington, H. T., and G. P..Steinbauer. 1961. The eighty-year period for Dr. Beal's seed viability experiment. Am. J. Bot. 48:321-325. Elliot, B. B., and C. A. Leopold. 1953. An inhibitor of germination and amylase activity in oats seeds. Physiol. PIantarum 6:65-77.
Harrington, G. T., and W. Crocker. 1923. Structure, physical characteristics, and composition of the pericarp and integument of Johnson grass seed in relation to its physiology. J. Agric. Res. 23:193- 222. Hilton, R. J., and J. A. Thomas. 1987. Changes in repiratory potential of dormant and nondormant Galium asparine L. (leavers) seeds during dry storage. J. Expt. Bot. 38:1484-1490.
Hinton, J. J. C. 1955. Resistance of the testa to entry of water into the wheat kernel. Cereal Chem. 32:296- 306.
Hitchock, A. S. 1950. Manual of the grasses of the United States. U. S. Dept. Agriculture Pub, No. 200. 94
Johnson, L. P. V. 1935. General preliminary studies on the physiology of delayed germination in Avena fatua. Can. J. Res. Sect. Contri. Bot. Sci. 13:283- 300.
Kivilaan, A., and R. S. Bandurski. 1981. The one hundred- year period for Dr. Beal's seed viability experiment. Am. J. Bot 68:1290-1292. Kollman, G. E., and D. W. Staniforth. 1972. Hormonal aspects of seed dormancy in yellow foxtail. Weed Sci. 20:472-477. Kommedhal, J. E., E. J. DeWay, and M. C. Christensen. 1958. Factors affecting dormancy and seedling developement in wild oats. Weeds 6:12-18.
LaCroix, L. J., and D. W. Staniforth. 1964. Seed dormancy in velvetleaf. Weeds 12:171-174.
Livingston, R. B., and M. L. Allessio. 1968. Buried viable seeds in successional field and forest stands. Harvard Forest, Massachusetts. Bull. Torrey Bot. Club 95:58-69. Lueschen, W. E., and R. N. Anderson. 1980. Longevity of velvetleaf (Abutilon theophrasti) seeds in soil under agricultural practices. Weed Sci. 28:341-346.
Marcus, A., J. Freely, and T. Volcani. 1966. Protein synthesis in imbibed seeds. III. Kinetics of amino acid incorporation ribosome activation, and polysome formation. Plant Physiol. 41:1167-1172. McDonald, M. B., Jr., and A. A. Khan. 1977. Factors determining germination of indian ricegrass seeds. Agron. J. 69:558-563.
Navasero, P. E., L. C. Baun, and B. o. Juliano. 1975. Grain dormancy, peroxidase activity and oxygen uptake in Orvza sativa. Phytochemistry. 14:1899-1902. Nieto-Hatem, J. 1963. Seed dormancy in Setaria lutescens. Ph.D. Dissertation. Iowa State University, Ames, Iowa. 95
Costing, H. J., and M. E. Humphreys. 1940. Buried viable seeds a successional series of old field and forest soils. Bull. Torrey Bot. Club 67:253-273. Owen, M. D. K. 1987. Control of woolly cupgrass fEriochloa villosa [Thumb.] Kunth.) in corn with three acetamide herbicides. Proc. Weed Sci. Soc. Am. 27:4. Pohl, R. W. 1957. Introduced grasses in Iowa. Proc. Iowa Academy of Sci. 66:160-162.
Roberts, H. E. 1961. Dormancy in rice seed II. The influence of covering structures. J. Expt. Bot. 12:430-445.
Shull, C. A. 1920. Temperature and rate of moisture intake in seeds. Bot. Gaz. 69:361-390.
Steinbauer, G. P., and B. Grisgsby. 1957. Interactions of temperature, light, and moisturing agent in the germination of weed seeds. Weeds 5:681-688. Toole, E. H., and E. Brown. 1946. Final results of the Duvel buried seed experiment. J. Agric. Res. 72:201-210. Villiers, T. A. 1972. Seed dormancy. Pages 219-276 in T. T. Kozlowski (ed.). Seed Biology. Vol. 2. Academic Press Inc., New York.
Vincent, E. M., and E. H. Roberts. 1977. Interaction of light, NOg, and alternating temperature in promoting the germination of dormant seeds of weed species. Seed Sci, and Technol. 5:659-670.
Warington, K. 1936. The effect of constant and fluctuating temperature on the germination of the weed seeds in arable soil. J. Ecol. 24:185-204. Weisner, E. L., and R. C. Kinch. 1964. Seed dormancy in green needlegrass. Agron. J. 56:371-373.
Werker, E. 1980/81. Seed dormancy as explained by the anatomy of embryo envelopes. Israel J. Bot. 29:22- 44. 96
Yearn, Y., D. B. Mandava, P.H. Terry, J. J. Murray, and H. L. Portz. 1988. Identification and quantification of Abscicic Acid in Zoysiagrass seeds and its inhibitory effect on germination. Crop Sci. 28:317- 321. 97
CHAPTER 3. GERMINATION AND EMERGENCE CHARACTERISTICS OF (Eriochloa villosa [Thumb.] Kunth.)
Introduction Weed seeds are one of the major devices for the invasion of weeds into a new area (Crafts, 1956).
According to Roberts (1970), the problems that weeds constitute to crop production occur only after seed germination and seedling emergence and establishment in the field.
Seed germination and seedling establishment in the field is the result of the termination of seed dormancy. Seed germination is critical in the establishment of weeds that are propagated only by seeds. Staniforth and Wiese (1985) viewed seed germination as an expression of the interaction of environmental factors such as moisture, temperature, oxygen, and the presence or the absence of light on a previously dormant seed. The effects of these factors on seed germination have been studied and evaluated. Several workers (Bewley and Black, 1982; Hand et al., 1982; Roberts, 1981; Taylorson and Hendricks, 1972; Warington, 1936) reported that the presence of only 1 of these 4 factors for germination would not promote germination. They explained that germination is a result 98
of the interaction of these factors.
Many plant species are influenced by the presence or absence of light during the seed germination process (Hand et al., 1982; Taylorson and Hendricks, 1972; Toole, 1976; Vincent and Roberts, 1977). A majority of weed seeds require light to germinate; such seeds are said to be photoblastic (Bewley and Black, 1982). Red light stimulates germination, while far-red inhibits the germination of photoblastic seeds (Black, 1969). Different weed seeds have different maximum temperature requirements for germination. Increasing temperature up to the maximum requirement stimulates and increases the germination rate of nondormant seeds. The optimum temperature range of 20 to 38 C for many weed seeds has been reported (Erasmus and Van, 1986; Roberts, 1981;
Roberts and Boddrell, 1985; Taylorson and Hendricks, 1972;
Toole, 1976). Warington (1936) studied the effect of constant and fluctuating temperatures on the germination of the weed seeds. She found that most weed seeds germinate better when exposed to alternating temperatures. Weed seeds produced each year are dispersed onto the soil surface where they are incorporated into the soil at various depths. In arable soils, for example, the depth at which tillage is performed affects the depth at which 99 the weed seeds are placed and consequently, influences the germination of seeds and the emergence of seedlings.
Studies on the emergence depths of annual weeds have shown that most of the seedlings emerged from shallow depths between 0 and 8 cm usually associated with soil disturbance during seed bed preparations (Banting, 1979; Brecke and
Duke, 1980; Dawson and Bruns, 1975; Marrow et al., 1982). According to De la Cruz (1974), 75-80% of annual weed seedlings emerged from the upper 4 cm of the soil profile in cultivated fields. He also found that in the no-till agroecosystem about 50% of the seedlings emerged from the upper 1 cm of the soil.
Materials and Methods
The seeds used for these studies were the nondormant seeds previously reported in the materials and methods of Chapter 1.
The problems that weeds constitute to crop production occur only after seed germination and seedling emergence and establishment in the field. Field observations in 1987 and 1988 have shown that the germination and emergence of woolly cupgrass in the field can occur in early spring and continues throughout the growing season. 100
A series of experiments were conducted in the laboratory growth chamber and in the field to evaluate the effects of environmental factors such as light, temperature, oxygen, and the depth of planting on the germination and emergence of woolly cupgrass seeds.
Effect of light on germination of woolly cupgrass seeds The objective of this study was to determine if woolly cupgrass seeds are photoblastic. A group of 50 intact nondormant woolly cupgrass seeds was imbibed and evaluated for germination under continuous darkness at 25 C. Dark conditions were created by wrapping the petri-dishes with two layers of alumimum foil. An experimental unit was a petri dish with 50 nondormant woolly cupgrass seeds on two moistened blue blotter papers (Anchor Paper Co., Box 3648,
St. Paul, MN 55165). Treatments were replicated 4 times. Germination was evaluated after 6 days.
In another experiment, 3 replications of groups of 50 nondormant woolly cupgrass seeds were imbibed for 24 hours on two moistened blue blotter papers in standard petri dishes in the dark. The imbibed seeds were then exposed to red light (600-600 nm) and far-red light (700-760 nm) for 10, 15, and 20 min. The source of both light treatments were tungsten lamps. 101
The seeds were exposed to the red and the far-red light treatments by filtering white light through 1 layer and 2 layers of 3 mm plexiglass, respectively. The source of the light treatments was 25 cm above the seeds.
Germination was evaluated under continuous darkness after 6 days.
Effect of temperature on germination of nondormant woolly cupgrass seeds
The germination rate of intact nondormant woolly cupgrass seeds as affected by temperature was investi gated with a two-way thermogradient plate (Agricultral Research Service, U.S. Department of Agriculture). The thermogradient used for these studies was made up of 25 thermocouples. The generated temperature gradient was monitored by the thermocouples.
Intact nondormant woolly cupgrass seeds were placed on two layers of moist blue blotter papers. An experimental unit was a thermocouple with an area of 5 square inches containing 25 seeds. Germination was evaluated for 6 days. There was a problem of drought in the area of the thermocouples with temperatures above 25 C. Consequently, these areas were kept moist at intervals of 10 hours throughout the experimental period. The experiment was repeated in time. Germination values for 10 and 45 C were 102 zero and thus, were excluded in the analysis of variance.
The germination percentage for some of the thermocouples was the same. Consequently, only the germination values of temperatures from 15, 25, 30, 35, 38, and 40 C were used for the analysis. Differences between temperatures were compared using the LSD at the 5% level of significance.
Effect of oxygen on germination of nondormant woolly cupgrass seeds
Three groups of 25 intact nondormant woolly cupgrass seeds were placed on 36 g of sterilized fine sand, moistened with 8 ml of distilled water in a 125 ml serums- bottle, and exposed to 8 oxygen levels. The oxygen levels used were 0, 2, 4, 8, 16, 21, 30, and 45%. After the the seeds have been placed, each serum bottle was sealed with a
13 X 20 mm red rubber stopper^ and tightly secured in place by a 20 mm tear-off aluminum seal^ using manual crimpers^. Each serum bottle containing the seeds was flushed and filled with the gas mixture made from pressured gas cylinders of oxygen, nitrogen, and 2% carbon dioxide (v/v).
The oxygen level of the gas mixture was measured with an oxygen electrode (Yellow Springs Instrument, Yellow Springs OH 45387, USA). Germination was evaluated at 25 C for 6
^Fisher Scientific, 1600 West Glenlake Ave., Itasca, Illinois 60143, USA. 103 days. An analysis of variance was conducted and differences between oxygen levels were compared by the
Least Significant Difference (LSD) at the 5% level of significance.
Effect of soil depth on seed germination and seedling emergence of woolly cupgrass
Germination and seedling emergence of woolly cupgrass as affected by soil depth was studied in the growth chamber and in the field. In the growth chamber studies, a group of 50 intact nondormant woolly cupgrass seeds was planted in sterilized sandy soil at the depths of 0, 1, 2, 4, 6, 8,
10, 15, and 20 cm in 30 cm clay pots.
The pots were arranged in a randomized complete block design and treatments were replicated 5 times. The temperature of the growth chamber was maintained at alternating (night/day) temperatures of 20/30 C during the study period. Emergence was evaluated every 2 days for 10 days. An experimental unit was a clay pot containing 50 intact nondormant seeds. The experiment was repeated three times.
In the field, the depth of seedling emergence of woolly cupgrass was evaluated. An experimental unit was a plot of 3 X 7.5 m and treatments were replicated 5 times.
The experimental design was a randomized complete block 104 design. From each plot, 50 woolly cupgrass seedlings at the 4-6 leaf stage were randomly selected and excavated with a hand trowel. Seedlings with the seeds still attached were used to evaluate the depth of seedling emergence by measuring the length (cm) of the mesocotyl from the seed up to the ground level. Two sampling dates were used (May and June).
In both studies, an analysis of variance was conducted. Differences between depths were compared by the LSD test at the 5% level of significance.
Results and Discussion
Effect of light on germination of woolly cupgrass seeds Light is known as one of the environmental factors that can affect the seed germination of some plant species. The response of seeds to light is determined by the wave length, duration of exposure, intensity, and the temperature before and after the exposure (Toole, 1973).
Red light stimulates germination, while far-red inhibits the germination of photoblastic seeds (Black, 1969). In this study, all the light treatments were ineffective in preventing the germination of woolly cupgrass seeds. The 100% germination of woolly cupgrass 105 seeds under continuous darkness suggests that the seeds of woolly cupgrass are not photoblastic. However, it is possible that the duration of the seed exposure to the light treatments was not long enough to affect the germination of woolly cupgrass seeds.
Effect of temperature on germination of nondormant woolly cupgrass seeds
Temperature regimes significantly (P<0.05) affected the germination percentage of intact nondormant woolly cupgrass seeds (Figure 17 and Table 21 in the Appendix). During the 6 days of evaluation, nondormant woolly cupgrass seeds did not germinate at 10 and 45 C. However, the germination percentage of the seeds increased with increasing temperature until 30 C after which there was a gradual decline (Figure 17).
After 6 days of evaluation, the minimum and the maximum temperature requirements for the germination of woolly cupgrass seeds were 15 and 40 C, respectively. Woolly cupgrass has a broad optimum range for germination between 20 and 38 C. Within this temperature range, the germination percentage was 93% or better.
The interaction of temperature and time on woolly cupgrass seed germination was highly (P<0.01) significant
(Table 21 in the Appendix). The rate of germination was 106
TEMPERATURE (°C)
Figure 17. Germination of nondormant woolly cupgrass seeds as affected by temperature. Germination was evaluated for 6 days (LSD=26). 107 temperature dependent, being greatest at 30 C compared to the other temperatures used in this study. At 30 c, germination was completed within 2 days compared to 4 and 6 days for 25 and 15 C, respectively (Table 4).
Table 4. The interaction of temperature and time on germination of intact nondormant woolly cupgrass seeds. Germination was evaluated for 6 days Temperature Germination time (days)^ regime 2 4 6 (C) % 10 0 0 0 15 33.00 33.00 100.00 20 84.00 96.70 98.70 25 94.70 100.00 100.00 30 100.00 100.00 100.00 35 97.00 100.00 100.00 38 92.00 100.00 100.00 40 64.00 84.00 85.00 45 0 0 0
^Values are the means of three replications of 25 seeds/temperature. This interaction is significant at the 5% level. 108
Effect of oxygen on germination of nondormant woolly cupgrass seeds
The effect of oxygen levels on the germination percentage of intact nondormant woolly cupgrass seeds was highly (P<0.01) significant (Table 22 in the Appendix). Intact nondormant seeds did not respond to 0% oxygen.
However, the germination percentage increased with increasing oxygen levels and was maximum at atmospheric oxygen concentration (Table 5). After 6 days of evaluation, there was no significant (P=0.05) variation on germination percentage of nondormant woolly cupgrass seeds between oxygen levels above atmospheric concentration. Also, the interaction of oxygen levels and time (days) on the germination of nondormant woolly cupgrass seeds was highly (P<0.01) significant (Table 6 and Table 22 in the Appendix).
During the 6 days of the evaluation, germination at 25 C was completed in 8, 16, and 21% or greater oxygen levels at 6, 4, and 2 days, respectively. This suggested that the rate of germination significantly (P<0.01) increased with increasing oxygen levels. Edwards (1973) in her discussions on the external oxygen concentration requirement for germination, wrote that "the resistance to the penetration of oxygen depends partly on the covering structures and partly on the size of the seed and the 109
Table 5. Germination of intact nondormant woolly cupgrass seeds as affected by oxygen levels. Germination was evaluated at 25 C for 6 days
Oxygen level Percent % Germination^
0 O.OOe
2 35.60d
4 56.20c 8 91.10b 16 97.80ab
21 100.00a
30 100.00a 45 100.00a (P=0.05) 7.40
lvalues are the means of three replications of 25 seeds per treatment. Values sharing the same letter within each parameter are not significantly different at the 5% level, according to LSD test. 110
Table 6. Germination of intact nondormant woolly cupgrass seeds as affected by the interaction of oxygen and time. Germination was evaluated at 25 C for 6 days
Oxygen level Germination time (days)^ % 2 4 6 %
0 0 0 0 2 20.00 35.60 51.00 4 35.50 46.50 86.70 8 75.50 97.80 100.00 16 93.30 100.00 100.00
21 100.00 100.00 100.00 30 100.00 100.00 100.00 45 100.00 100.00 100.00 ^Values are the means of three replications of 25 seeds per treatment. This interaction was highly significant (P>0.01). Ill position of the meristems. Barley (Hordeuin vulaare L.) grains are larger than charlock fsinanis arvensis L.) seeds and the meristems are situated deeper in the tissues, therefore they may well require an external oxygen concentration of more than 4% for germination."
In this study, 87% of intact nondormant woolly cupgrass seeds germinated at 4% oxygen in 6 days. The reduction in the germination percentage of woolly cupgrass seeds with decreasing oxygen level may be due to reduced oxygen diffusion to the embryo.
Effect of soil depth on seed gemination and seedling emergence of woolly cupgrass
The results presented on Figure 18 demonstrated the effect of soil depth on the germination and seedling emergence of woolly cupgrass under growth chamber conditions. Seedling emergence started 2 days after planting. The depth of planting significantly (P<0.01) affected the germination and percentage of seedling emergence of woolly cupgrass (Table 23 in the Appendix).
The optimum soil depth for woolly cupgrass seedling emergence under growth chamber conditions was between 1 cm and 4 cm. The percentage of seedling emergence of woolly cupgrass seeds placed on the surface was significantly
(P<0.05) reduced (64%) compared to 98% for the seeds 112
100
ca
CO
0 1 2 4 6 8 10 15 20 DEPTH OF SEEDLING EMERGENCE (cm)
Figure 18. Germination and emergence of woolly cupgrass as affected by the depth of planting under growth chamber conditions. Data are the means of 50 seeds for each treatment (LSD=4.98). 113 planted at the depth of 1 cm. Depths greater than 4 cm significantly (P<0.05) reduced seedling emergence.
Seedling emergence was completely inhibited at 20 cm soil depth.
The results from the field study also showed that greater depths significantly (P<0.01) reduced the percentage of woolly cupgrass seedling emergence (Table 24 in the Appendix). In the field, the optimum depth for woolly cupgrass emergence was 2 cm, after which there was a gradual decline with increasing soil depth (Figure 19).
The maximum depth for the emergence of woolly cupgrass in the field was 9 cm. The results of these studies are consistent to the findings of several researchers (Brecke and Duke, 1980; Dawson and Bruns, 1975; De la Cruz, 1974; Gomes et al., 1978; Peters, 1986). 114
DEPTH OF SEEDLING EMERGENCE (cm) Figure 19. Depth of seedling emergence of woolly cupgrass in the field. Data are the means of the emergence frequencies for five replicates of 50 seedlings (IiSD=2.99). 115
Conclusions
The results of these studies demonstrated that light was not a requirement for the germination of nondormant woolly cupgrass seeds. The results from the growth chamber and field studies have shown that the germination and emergence of woolly cupgrass declined with increasing depth of planting. The results further suggested that the occurrence of secondary dormancy and the effect of soil depth on the germination and emergence of woolly cupgrass may be imposed by factors other than a light requirement.
This may explain why woolly cupgrass is able to germinate under dense corn and soybean canopies during the growing season (Field observations).
The results from the temperature study have shown that the germination of woolly cupgrass is temperature dependent. Also, woolly cupgrass enjoys a broad optimum temperature range for germination between 20 and 38 C. After 6 days of evaluation, the minimum and the maximum temperature requirements for the germination of nondormant woolly cupgrass seeds were 15 and 40 C, respectively. These results may explain why the germination and emergence of woolly cupgrass in the field occurs early in spring and continuously throughout the growing season.
The results of the oxygen studies clearly 116 demonstrated that the germination of intact nondormant seeds is highly oxygen dependent. Also, the rate of germination is influenced by the oxygen levels of the seed microenvironment. One of the ecological implications of these results is that the germination of woolly cupgrass seeds in the field will be reduced greatly under heavy soil compaction, heavy soil texture saturated with water and with increasing soil depths. The results of the soil depth studies demonstrated that a majority of the seedlings of woolly cupgrass that emerged came from the top 4 cm of the soil. The results of these studies suggest that the depth of Preplanted Incorporated (PPI) herbicides for the effective control of woolly cupgrass in the field is the top 6-8 cm of the soil.
References Cited
Banting, J. D. 1979. Germination, emergence and persistence of foxtail barley. Can. J. Plant Sci. 59:35-41.
Bewley, J. D., and M. Black. 1982. Physiology and biochemistry of seeds in relation to germination. Vol 2: Viability, dormancy and environmental control. Springer-Verlag, New York. 375 pp.
Black, M. 1969. Light-controlled germination of seeds. Soc. Expt. Biol. Symp. 23:193.
Brecke, B. J., and W. B. Duke. 1980. Dormancy, germination and emergence characteristics of fall panicum fPanicum dichotomiflorum) seed. Weed Sci. 28:683-685. 117
Crafts, A. S. 1956. Weed control: Applied Botany. Am. J. Bot. 43:548-556.
Dawson, J. H., and V. F. Bruns. 1975. Longevity of barnyardgrass, green foxtail, and yellow foxtail seeds in soil. Weed Sci. 23:437-440. De la Cruz, R. 1974. Weed seedling emergence depths under field conditions. Ph.D. Dissertation. Iowa State University, Ames, Iowa. 115 pp.
Erasmus, D. J., and J. Van. 1986. Germination of Chromolaema ordorata (L.) K and R. achenes: Effect of temperature inhibition and light. Weed Res. 26:75-81. Gomes, F. L., J. M. Chandler, and E. E. Vaughan. 1978. Aspects of germination, emergence, and seed production of three Ipomoea taxa. Weed Sci. 26:245- 248.
Hand, D. J., G. Craig, M. Takaki, and R. E. Kendricks. 1982. Interaction of light and temperature on seed germination of Rumex obtusifolius L. Planta 156:457-460.
Marrow, L. A., L. Y. Frank, and G. F. Duane. 1982. Seed germination and seedling emergence of jointed goat- grass fAeailops cylindrical. Weed Sci. 30:395-398.
Peters, B. C. N. 1986. Factors affecting seedling emergence of different strains of Avena fatua L. Weed Res. 26:29-38.
Roberts, H. A. 1970. Viable weed seeds in cultivated soils. National Veg. Res. Stn. Ann. Rep. 1969: 25-38.
Roberts, H. A. 1981. Seed banks in soil. Adv. Appl. Biol. 6:1—55.
Roberts, H. E., and J. E. Boddrell. 1985. Temperature requirements of buried seeds of Acthusa cvnapium L. Weed Res. 25:267-274.
Staniforth, D. W., and A. F. Wiese. 1985. Weed biology and its relationship to weed control in Limited- tillage systems. In Weed Control in Limited-Tillage Systems. Weed Sci. Soc. Am., Champaign, Illinois. 118
Taylorson, R. B., and S. B. Hendricks. 1972. Interactions of light and temperature shift on seed germination. Plant Physiol. 49:127-130.
Toole V. K. 1973. Effects of light, temperature, and their interactions on the germination of seeds. Seed Sci. Technol. 1:339-396. Toole, V. K. 1976. Light and temperature control of germination in Aaropvron smithii seeds. Plant and Cell Physiol. 17: 1263-1272.
Vincent, E. M., and E. H. Roberts. 1977. Interaction of light, NOg, and alternating temperature in promoting the germination of dormant seeds of weed species. Seed Sci. and Technol. 5:659-670.
Warington, K. 1936. The effect of constant and fluctuating temperature on the germination of the weed seeds in arable soil. J. Ecol. 24:185-204. 119
REFERENCES CITED
Afanasiev, M. 1937. A physiological study of dormancy in seed of Magnolia acuminata L. Cornell Univ. Agric. Expt. Station, Ithaca, New York.
Aitken, Y. 1939. The problem of hard seeds in subterranean clover. Proc. Roy. Soc. Vict. (N.S.) 51:187-212.
Aldrich, R. J. 1984. Weed-Crop Ecology; Principles in weed management. Breton Publishers, North Scituate, Massachusettes. Amen, R. D. 1963. The concept of seed dormancy. Am. Sci. 51:408-424. Amen, R. D. 1968. A model of seed dormancy. Bot. Rev. 34:1-31.
Andersen, R. N. 1968. Germination and establishment of weeds for experimental purposes. Weed Sci. Soc. Am., Urbana, II 236 pp.
Anderson, W. P. 1983. Weed Science: Principles. 2nd ed. West Publishing, St. Paul. 655 p.
Asakawa, S. 1955. Hastening the germination of Pinus koraiensis. J. Pap. Forest. Soc. 37: 127.
Atwood, W. M. 1914. A physiological study of the germination of Avena fatua. Bot. Gaz. 57: 386-414. Ballard, L. A. T. 1958. Studies of dormancy in the seeds of subterranean clover. I. Breaking of dormancy by carbon dioxide and activated carbon. Austr. J. Biol. Sci. 11:246-260.
Ballard, L. A. T. 1973. Physical barriers to germination. Seed Sci. Technol. 1:285-303. Banting, J. D. 1979. Germination, emergence and persistence of foxtail barley. Can. J. Plant Sci. 59:35-41.
Beal, W. J. 1905. The vitality of seeds. Bot. Gaz. 38:140-143. 120
Bello, A. I. 1986. Effects of shade and Glycine max L. competition on the growth and dormancy of Abutilon theophrasti Medic. M.S. Thesis. Iowa State University, Ames, Iowa. 125 pp. Bewley, J. D., and M. Black. 1978. Physiology and biochemistry of seeds in relation to germination. Vol.1: Development, germination and growth. Springer-Verlag, New York. 306 pp.
Bewley, J. D., and M. Black. 1982. Physiology and biochemistry of seeds in relation to germination. Vol 2: Viability, dormancy and environmental control. Springer-Verlag, New York. 375 pp. Black, M. 1959. Dormancy studies in seeds of Avena fatua. I. The possible role of germination inhibitors. Can. J. Bot. 37:393-402.
Black, M., and M. J. Naylor. 1959. Prevention of onset of seed dormancy by gibberellic acid. Nature 184:468-469. Blackman, H. V. 1919. The compound interest law and plant growth. Ann. Bot. 33:353-360. Bradbeer, J. W. 1968. Studies in seed dormancy IV. The role of endogenous inhibitors and gibberellin in the dormancy and germination of Corylus avellana L. seeds. Planta 78: 266-276.
Brecke, B. J., and W. B. Duke. 1980. Dormancy, germination and emergence characteristics of fall panicum fPanicum dichotomiflorum) seed. Weed Sci. 28:683-685.
Brenchley, W. E. 1917. The effects of weeds upon cereal crops. New Phytol. 16:53-76.
Brenchley, W. E., and K. Warington. 1930. The weed seed population of arable soil. II. Influence of crop, soil and methods of cultivation upon the relative abundance of viable seeds. J. Ecol. 21:103-127.
Carvers, P. B., and J. L. Harper. 1964. Biological flora of the British Isles, Rumex obtusifolius L. and Rumex cripus L. J. Ecol. 52: 737-766. 121
Chandler, J. M. 1980. Assessing losses caused by weeds. In Proceedings of E.G. Stakman Commemorative Symposium, Miscellaneous Publication No. 7. Agricultural Experiment station, University of Minnesota, St. Paul.
Chandler, J. M., and J. E. Dale. 1974. Comparative growth of four malvaceus species. Proc. Southern Weed Sci. Soc. 27:116-117.
Chandra, R. M., and J. E. Varner. 1965. Gibberellic acid controlled metabolism of RNA in aleurone cells of barley. Biochem. Biophys. Acta 108:583-592. Chen, S. S. C., and E. J. Varner. 1970. Respiration and protein synthesis in dormant and after-ripened seeds of Avena fatua L. Plant Physiol. 46:108-112. Chepil, W. S. 1946. Germination of weed seeds. I. Longevity, periodicity of germination and vitality of seeds in cultivated soil. Sci. Agric. 26:307. Crafts, A. S. 1956. Weed control: Applied Botany. Am. J. Bot. 43:548-556.
Chrispeels, M. J., and J. E. Varner. 1967. Hormonal control of enzyme synthesis. On the mode of action of gibberellic acid and abscission in aleurone layers of barley. Plant Physiol. 42:1008-1016.
Cook, R. 1980. The biology of seeds in the soil. In O. T. Solbrig (ed.). Demography and evolution in Plant populations. Univ. California Press, Berkeley. Cohn, M. A., and J. A. Hughes. 1986. Seed dormancy in redrice V. Response to azide, hydroxylamine, and cyanide. Plant Physiol. 80:531-533.
Come, D., and T. Tissaoui. 1973. Interrelated effects of imbibition, temperature, and oxygen on seed germination. In W. Heydecker (ed.). Seed Ecology. Butterworths, London.
Corner, E. J. H. 1951. The leguminous seed. Phytomorphology. 1:117-150.
Corns W. G. 1960. Effects of gibberellin treatments on germination of various species of weed seeds. Can. J. Plant Sci. 40:47-51. 122
Coumans, M., D. Dome, and T. Gasper. 1976. Stabilized dormancy in sugar beet fruits. I. Seed coats as a physiological barrier to oxygen. Bot. Gaz. 137:274- 278. Cox, L. G., H. M. Hunger, and E. A. Smith. 1945. A germination inhibitor in the seed coats of certain varieties of cabbage. Plant Physiol. 20:289-293. Crocker, W. 1906. The role of seed coats in delayed germination. Bot. Gaz. 42:265-291.
Crocker, W. 1916. Mechanisms of dormancy in seeds. Am. J. Bot. 3:99-120.
Crocker, W., and W. E. Davis. 1914. Delayed germination in seeds of Alisma plantaao L. Bot. Gaz. 58:285-321. Darlington, H. T., and G. P. Steinbauer. 1961. The eighty- year period for Dr. Beal's seed viability experiment. Am. J. Bot. 48:321-325.
Dawson, J. H., and V. F. Bruns. 1975. Longevity of barnyardgrass, green foxtail, and yellow foxtail seeds in soil. Weed Sci. 23:437-440. De la Cruz, R. 1974. Weed seedling emergence depths under field conditions. Ph.D. Dissertation. Iowa State University, Ames, Iowa. 115 pp.
Dexter, S. T. 1955. Alfalfa seedling emergence from seed lots varying in origin and hard seed content. Agron. J. 47:357-361.
Dickerson, Jr., C. T., and R. D. Sweet. 1971. Common rapweed ecotypes. Weed Sci. 19:64-66. Duvel, J. W. T. 1924. The vitality of buried seeds. U.S. Bur. Plant Indus. Bull. 83. Eastin, E. F. 1983. Smallflower morningglory (Jaccmemontia tamnifolia) germination as influenced by scarification, temperature, and seedling depth. Weed Sci. 31:727-730. Eckerson, S. 1913. A physiological and chemical study of after-ripening. Bot. Gaz. 55:286-299. 123
Edwards, D. E. 1981. Control weeds without chemicals. The New Farm. A Rodale Press, Inc., Publ., Emmaus, PA. 38 pp.
Edwards, M. M. 1973. Seed dormancy and seed environment- internal oxygen relationships. Pages 169-188 in w. Heydecker (ed.). Seed Ecology. Proc. 19th Easter School in Agric. Sci., University of Nottingham. The Pennsylvania State University Press, London. Egley, G. H. 1972. Influence of seed envelope and growth regulators upon seed dormancy in witch weed (Striga lutea Lour.). Ann. Bot. 36:755-770. Egley, G. H. 1982. Ethylene stimulation of weed seed germination. Agric. and For. Bull. (Univ. Alberta) 5:13-18.
Egley, G. H., and J. M. Chandler. 1983. Longevity of weed seeds after 5.5 years in the Stoneville 50-yèars buried seed study. Weed Sci. 31:264-270.
Egley, G. H., and S. O. Duke. 1985. Physiology of weed seed dormancy and germination. Pages 27-64 In S. O. Duke (ed.). Weed Physiology. Vol. I. CRC Press, Boca Raton, Florida. Egley, G. H., and R. N. Paul Jr. 1981. Morphological observations on the early imbibition of water by Sida _ spinosa (Malvaceae) seeds. Am. J. Bot. 68:1056-1065.
Egley, G. H., R. N. Paul, Jr., C. K. Vaughn, and S. O. Duke. 1983. Role of peroxidase in the developement of water-impermeable seed coats in Sida spinosa L. Planta 157:224-232.
Elliot, B. B., and C. A. Leopold. 1953. An inhibitor of germination and amylase activity in oat seeds. Physiol. Plantarum 6:65-77.
Eplee, R. E. 1975. Ethylene: A witchweed germination stimulant. Weed Sci. 23:433-436. Erasmus, D. J., and J. Van. 1986. Germination of Chromolaema ordorata (L.) K and R. achenes: Effect of temperature inhibition and light. Weed Res. 26:75- 81. 124
Evans, C. R. 1933. Germination behavior of Magnolia arandiflora L. Bot. Gaz. 94:729-754.
Evans, R. A., and J. A. Young. 1974. Today's Weed. Russian thistle. Weeds Today 5(4):18.
Evenari, M. 1949. Germination inhibitors. Bot. Rev. 15:153-194.
Fawcett, S. R., and F. W. Slife. 1978. Effects of field application of nitrate on weed seed germination and dormancy. Weed Sci. 26:594-596. Fayemi, A. A. 1957. Effect of temperature on the rate of seed swelling and germination of legume seeds. Agron. J. 49:75-76.
Fenner, M. 1985. Seed Ecology. Chapman and Hall, New York. 151 p.
Flemion, F. 1931. After-ripening, germination, and vitality of seeds of Sorbus aucuoaria L. Boyce Thompson Inst. Plant Res. Contr. 3:413-440.
Frank, R. M., and O. L. Safford. 1970. Lack of viable seeds on the forest floor after clearcutting. J. Forestry 68:776-778.
Gardner, P. F., R. B. Pearce, and R. L. Mitchell. 1985. Physiology of crop plants. Iowa State Univ. Press, Ames.
Gomes, F. L., J. M. Chandler, and E. E. Vaughan. 1978. Aspects of Germination, Emergence, and Seed Production of Three Ipomoea taxa. Weed Sci. 26:245- 248.
Goss, W. L. 1924. The vitality of buried seeds. J. Agric. Res. 29:349-362.
Hall, M. A., I. C. Cantrell, J. F. Jones, and S. T. Olatoye. 1978. Seed dormancy in relation to weed control. Proc. Br. Weed Control Conf. 14:997-1003.
Hamly, D. H. 1932. Softening of the seeds of Melilotus alba Oesrousseaux. Bot. Gaz. 39:345-375. 125
Hand, D. J., G. Craig, M. Takaki, and R. E. Kendricks. 1982. Interaction of light and temperature on seed germination of Rumex obtusifolius L. Planta 156:457- 460.
Harper, J. L. 1959. The ecological significance of dormancy and its importance in weed control. Proc. 4th Int. Congr. Plant Protection 1:415-420. Harper, J. L. 1977. The population biology of plants. Academic Press, London.
Harrington, G. T. 1923. Forcing the germination of wheat and other cereals. J. Agric. Res. 23:79. Hawton, D., and D. S. H. Drennan. 1980. Studies on the longevity and germination of seeds of Eleusine indica and Crotolaria aoreensis. Weed Res. 20:217-223.
Hendricks, S. B., R. B. Taylorsdn. 1975. Breaking of dormancy by catalase inhibition. Proc. Natl. Acad, sci. USA 72:306-306. Henson, E. L. 1970. The effects of light, potassium nitrate and temperature on the germination of Chenopodium album L. Weed Res. 10:27-39.
Hilton, R. J., and J. A. Thomas. 1987. Changes in respira tory potential of dormant and nondormant Galium aparine L. (Cleavers) seeds during dry storage. J. Expt. Bot. 38:1484-1490.
Hitchock, A. S. 1950. Manual of the grasses of the United States. U. S. Dept. of Agriculture Publ. No. 200. Ho, D. T., and J. E. Varner. 1976. Response of barley aleurone layers to abscisic acid. Plant physiol. 57:175-178.
Howell, P., and P. Piroinen. 1959. Effect of GA on the termination of dormancy in some seeds. Nature 183:1830-1831.
Howell, R. W. 1960. Physiology of soybean. Adv. Agron. 12:265-310.
Hsiao, A. I. 1979. The effect of sodium hypochlorite and gibberellic acid on seed dormancy and germination of wild oats fAvena fatual. Can. J. Bot. 57:1927. 126
Hsiao, A. I. 1980. The effect of sodium hypochlorite, gibberellic acid, and light on seed dormancy and germination of stinkweed and wild mustard. Can. J. Plant Sci. 60:643-649.
Ikuma, H., and V. K. Thimann. 1963. The role of seed coats in germination of photosensitive lettuce seeds. Plant Cell Physiol. 4:169-185.
Jarvis, B. C., and H. R. P. Shannon. 1981. Changes in Poly (A) RNA metabolism in relation to storage and dormancy breaking of Hazel seeds. New Phytol. 88:31- 40. Johnson, L. P. V. 1935. General preliminary studies on the physiology of delayed germination in Avena fatua. Can. J. Res. Sect. Contr. Bot. Sci. 13:283-300. Jones, H. A. 1920. Physiological study of maple seeds. Bot. Gaz. 69:127-152. Jordan, L. J., D. W. Staniforth, and C. M. Jordan. 1982a. Parental stress and prechilling effects on Pennsylvania smart weed fPolygonum pensvlvanicum) achenes. Weed Sci. 30:243-248. Jordan, L. J., L. S. Jordan, and C. M. Jordan. 1982b. Effect of boiling water on velvetleaf seed germination. Res. Progr. Report. Western Soc. Weed Sci., 4 p.
Kallio, P., and P. Pironinen. 1959. Effect of GA on the termination of dormancy in some seeds. Nature 183:1830-1831.
Karssen, C. M. 1982. Seasonal patterns of dormancy in weed seed. ^ A. A. Khan (ed.). The physiology and biochemistry of seed development, dormancy and germination. Elsevier, New York. 243 pp.
Kelly, R. J. 1969. Abscisic acid and gibberellic acid regulation of seed germination and dormancy. Biologist 51:91-99.
Khan, A. A. 1967. Antagonism between dormin and kinetin in seed germination and dormancy. Am. J. Bot. 54:639 127
Khedir, D. K., and F. W. Roeth. 1981. Velvetleaf lAbutilon rheophrasti) sefâd populations in six continuous corn fZea mavs) fields. Weed Sci. 29:485- 490. Kivilaan, A., and R. S. Bandurski. 1981. The one-hundred- year period for Dr. Seal's seed viability experiment. Am. J. Bot. 68:1290-1292. Kollman, G. E., and D. W. Staniforth. 1972. Hormonal aspects of seed dormancy in Yellow foxtail. Weed Sci. 20:472-477. Kommedhal, J.E., E. J. DeVay, and M. C. Christensen. 1958. Factors affecting dormancy and seedling developement in wild oats. Weeds 6:12.
Kozlowski, T. T., and C. R. Gunn. 1972. Importance and characteristics of seeds. In T. T. Kozlowski (ed.). Seed biology. Vol. 1. Academic Press, New York.
Kropac, Z. 1966. Estimation of weed seeds in arable soil. Pedobiologia 6:105-128.
LaCroix, L. J., and D. W. Staniforth. 1964. Seed dormancy in velvetleaf. Weeds 12:171-174. Lewis, J. 1973. Longevity of crop and weed seeds: Survival after 20 years in soil. Weed Res. 12:179.
Linko, P., and M. Milner. 1959. Gas exchange in dry wheat embryos induced by wetting. Cereal Chem. 36:274-279.
Livingston, R. B., and M. L. Allessio. 1968. Buried viable seed in successional field and forest stands, Harvard Forest, Massachusetts. Bull. Torrey Bot. Club 95:58- 69.
Lueschen, W. E., and R. N. Anderson. 1980. Longevity of velvetleaf fAbutilon theophrastil seeds in soil under agricultural practices. Weed Sci. 28:341-346. Major, W., and E. H. Roberts. 1968. Dormancy in cereal seeds. I. The effects of oxygen and respiratory inhibitors. J. Expt. Bot. 19:77-89.
Marbach, I., and M. A. Mayer. 1974. Permeability of seed coats to water as related to drying conditions and metabolism of phenolics. Plant Physiol. 54:817-820. 128
Marcus, A., J. Freely, and T. Volcani. 1966. Protein synthesis in imbibed seeds. III. Kinetics of amino acid incorporation ribosome activation, and polysome formation. Plant Physiol. 41:1167-1172. Marrow, L. A., L. Y. Frank, and G. F. Duane. 1982. Seed germination and seedling emergence of jointed goatgrass fAeailops cylindrical Weed Sci. 30:395-398.
Martin, G. C., M. I. R. Mason, and H. I. Forde. 1969. Changes in endogenous growth substances in the embryos of Jualana reaia during stratification. J. Amer. Soc. Hort. Sci. 94:13-17. Mayer, A. M., and A. Poljakoff-Mayber. 1975. The germination of seeds. Inter series of Monographs on Pure and Appl. Biol. Plant Physiol. Div. Pergamon Press, Oxford.
Meyers, P. S., C. J. Nelson, and R. D. Horrocks. 1984. Temperature effects on imbibition, germination and respiration of grain sorghum seeds. Field Crops Res. 8:135-142.
McDonald, M. B., Jr., and A. A. Khan. 1977. Factors determining germination of Indian ricegrass seeds. Agron. J. 69:558-563.
Moolani, M. K. and R. P. Singh. 1968. Nature of weed-crop competition. Weed control in field crops. Punjab Agric. Univ., Hissar.
Nieto-Hatem, J. 1963. Seed dormancy in Setaria lutescens. Ph.D. Dissertation. Iowa State University, Ames, Iowa.
Nikolaeva, M. 6. 1969. Physiology of deep dormancy in seeds. National Science Foundation, Washington, D.C. Okwonkwo, S. N. C., and O. I. F. Nwoke. 1975. Bleach- induced germination and breakage of dormancy of seeds of Alectra voaelii. Physiol. Plantarum 35:175-180. Oliver, L. R. 1979. Influence of soybeans (Glycine max) planting date on velvetleaf (Abutilon theophrasti) competition. Weed Sci. 27:183-188. 129
Oosting, H. J., and M. E. Humphreys. 1940. Buried viable seeds a successional series of old field and forest soils. Bull. Torrey Bot. Club 67:253-273. Owen, M. D. K. 1987. Control of woolly cupgrass fEriochloa villosa (Thumb.) Kunth.) in corn with three acetamide herbicides. Proc. Weed Sci. Soc. Am. 27:4.
Pack, D. A. 1921. After-ripening and germination of juniperus seeds. Bot. Gaz. 71:32-60.
Paleg, L. 6. 1961. Physiological effects of gibberellic acid. III. Observation on its mode of action on barley endosperm. Plant Physiol. 36:829-837. Paul, K. B., L. J. Crayton, and K. P. Biswas. 1976. Germination behavior of Florida pusley seeds II. Effects of germination-stimulating chemicals. Weed Sci. 24:349-352.
Pavlista, A. D., and H. A. Haber. 1970. Embryo expansion without protrusion in lettuce seeds. Plant Physiol. 46:636—637. Peters, B. C. N. 1986. Factors affecting seedling emergence of different strains of Avena fatua L. Weed Res. 26:29-38.
Pohl, R. W. 1959. Introduced grasses in Iowa. Proc. Iowa Academy of Sci. 66:160-162.
Pollack, J. R. A. 1959. Studies in barley and malt XV. Growth substances and other compounds in relations to dormancy in barley. J. Inst. Brew. 65:334 Quinlivin, B. J. 1971. Seed coat impermeability in legumes. J. Aust. Inst. Agric. Sci. 37:283-295.
Radosevich, R. S., and J. S. Holt. 1984. Weed Ecology: Implications for vegetation management. John Wiley and Sons, Inc., New York. 225 pp
Raleigh, G. J. 1930. Chemical conditions in maturation, dormancy, and germination of seeds of Gvmnoclaus dioica. Bot. Gaz. 89:273-294. 130
Roberts, H. A. 1970. Viable weed seeds in cultivated soils. National Veg. Res. Stn. Ann. Rep. 1969: 25-38.
Roberts, H. A. 1981. Seed banks in soil. Adv. Appl. Biol. 6:1-55.
Roberts, H. A., and A. P. Dawkins. 1967. Effect of cultivation on the numbers of viable weed seeds in soil. Weed Res. 7:290-301. Roberts, H. A., and P. M. Feast. 1973. Emergence and longevity of seeds of annual weeds in cultivated and undisturbed soil. J. Appl. Ecol. 10:133-143. Roberts, H. A., and E. J. Neilson. 1981. Changes in the soil seedbank of four long term crop/herbicide experiments. J. Appl. Ecol. 18:661-668.
Roberts, E.H. 1972. Dormancy: a factor affecting seed survival in the soil. Pages 321-359 in E. H. Roberts (ed.). Viability of Seeds. Syracuse Univ. Press, London.
Roberts, H. E. 1961. Dormancy in Rice Seed II. The influence of covering structures. J. Expt. Bot. 12:430-445.
Roberts, H. E. 1981. The interaction of environmental factors controlling loss of dormancy in seeds. Ann. Appl. Biol. 98:552-555.
Roberts, H. E., and J. E. Boddrell. 1985. Temperature requirements of buried seeds of Acthusa cvnanium L. Weed Res. 25:267-274.
Robinson, E. L., O. B. Wooten, and F. E. Fulgham. 1967. A new method of extracting soil samples. Weeds 15:368- 370.
Rolston, M. P. 1978. Water impermeable seed dormancy. Bot. Rev. 44:365-396.
Ross, D. J. 1984. Metabolic Aspects of Dormancy. Pages 45-75. In D. R. Murray (ed.). Seed physiology Vol. 2. Germination and Reserve Mobilization. Academic Press, Australia. 131
Rost, L. T. 1975. The morphology of germination in Setaria lutescens (Gramineae): The effects of covering structures and chemical inhibitors on dormant and nondormant florets. Ann. Bot. 39:21-30. Rudnicki, R. 1969. Studies on abscisic acid in apple seeds. Planta 86:63-68.
Sarukhan, J. 1974. Studies on plant demography: Ranunculus repens L., g. bulbosus L., and R. acris L. II. Reproductive Strategies and seed population dynamics. J. Ecol. 62: 151-177. Schimpf, J. D., and I. G. Palmblad. 1980. Germination response of weed seeds to soil nitrate and ammonium with and without stimulated overwintering. Weed Sci. 28:190-193.
Schreiber, M. M. 1964. Effect of date of planting and stage of cutting on seed production of giant foxtail. Weeds 60:62-68.
Schweizer, E. E., and C. R. Frey. 1974. Collecting soil cores in plastic liners. Weed Sci. 22:4-5. Shaw, W. C., D. R. Shepherd., E. L. Robison, and P. F. Sand. 1962. Advances in witchweed control. Weeds 10:182- 192.
Shull, C. A. 1920. Temperature and rate of moisture intake in seeds. Bot. Gaz. 69:361-390.
Skoog, F., and C. D. Miller. 1957. Chemical regulation of growth and organ formation in plant tissue cultures in-vitro. Symp. Ext. Biol. 11:118-131. Skoog, F., F. D. Strong, and C. D. Miller. 1965. Cytokinins. Sci. 148:532-533.
Slater, J. R., and J. A. Bryant. 1982. RNA metabolism during breakage of seed dormancy by low temperature treatment of fruits of Acer platanoides (Norway Maple). Ann. Bot. 50:141-149.
Slattery, H., and J. B. Atwell, and J. Kuo. 1982. Relationship between color, phenolic content and impermeability in the seed of various Trifolium subterraneum L. genotypes. Ann. Bot. 50:373-378. 132
Sondheimer, E., D. S. Tzou, and E. C. Galson. 1968. Abscisic acid levels and seed dormancy. Plant Physiol. 43:1443-1447.
Spencer, N. R. 1986. Velvetleaf fAbutiIon theophrasti^ (Malvaceae^, history and economic impact in the United States. Econ. Bot. 38:407-416. Standifer, L. C. 1980. A technique for estimating weed seed populations in cultivated soil. Weed Sci. 28:134- 138.
Staniforth, D. W., and A. F. Wiese. 1985. Weed biology and its relationship to weed control in limited- tillage systems. In Weed Control in Limited-Tillage Systems. Weed Sci. Soc. Am., Champaign, Illinois. Steinbauer, G. P. 1937. Dormancy and germination of Fraxinus seeds. Plant Physiol. 12:813.
Steinbauer, G. P., and B. Grisgsby. 1957. Interactions of temperature, light, and moisturing agent in the germination of weed seeds. Weeds 5:681-688. Stevens, 0. A. 1932. The number and weight of seeds produced by weeds. Am. J. Bot. 19:784-794. Stoller, E. W., and L. M. Wax. 1974. Dormancy changes and fate of some annual weed seeds in the soil. Weed Sci. 22:151-155.
Strand, O. E., and G. R. Miller. 1980. Woolly cupgrass a new weed threat in the Midwest. Weeds Today 16 p. Sumner, D. C, and J. L. Lyod. 1967. Effects on (+-) abscisic II on seed germination in four species of grasses. Planta 75:28-32.
Sutcliffe, J. 1977. Plants and temperature. The Institute of Studies in Biology No. 86. Edward Arnold Publishers Ltd., London. 57 pp.
Taylorson, R. B. 1979. Response of weed seeds to ethylene and related hydrocarbons. Weed Sci. 27:7-10.
Taylorson, R. B., and M. M. Brown. 1977. Accelerated after-ripening for overcoming seed dormancy in grass weeds. Weed Sci. 25:473-476. 133
Taylorson, R. B., and S. B. Hendricks. 1972. Interactions of light and temperature shift on seed germination. Plant Physiol. 49:127-130.
Taylorson, R. B., and S. B.Hendricks. 1973. Promotion of seed germination by cyanide. Plant physiol. 52:23- 27. Taylorson, R. B., and S. B. Hendricks. 1976. Aspects of dormancy in vascular plants. Bioscience 26:95-101.
Taylorson, R. B., and S. B. Hendricks. 1979. Overcoming dormancy in seeds with ethanol and other anesthetics. Planta 145:507-510.
Thompson, K., J. P. Grime, and G. Mason. 1977. Seed germination in response to diurnal fluctuations of temperature. Nature (London) 267:147-149. Thornton, N. C. 1944. Carbon dioxide storage XII. Germination of seeds in the presence of carbon dioxide. Contr. Boyce Thompson Inst. 13:355-360.
Toole, E. H., and E. Brown. 1946. Final results of the Duvel buried seed experiment. J. Agric. Res. 72:201- 210.
Toole, E. H., S. B. Hendricks, H. A. Bortwick, and V. K. Toole. 1956. Physiology of seed germination. Ann. Rev. Plant Physiol. 7:229-324.
Toole, V. K. 1973. Effects of light, temperature, and their interactions on the germination of seeds. Seed Sci. Technol. 1:339-396.
Toole, V. K. 1976. Light and temperature control of germination in Acrropvron smithii seeds. Plant and cell Physiol. 17: 1263-1272. Trewavas, A. 1981. How do plant growth substances work? Plant Cell Environment 4:203-228.
Tuan, D. Y. H., and J. Bonner. 1964. Dormancy associated with repression of genetic activity. Plant Physiol. 39:768-772.
Van der Valk, A. G., and B. C. Davis. 1978. The role of seed banks in the vegetation dynamics of prairie glacial marshes. Ecolology. 59:322-3 35. 134
Varner, J. E., and G. R. Chandra. 1964. Hormonal control of enzyme synthesis in barley endosperm. Proc. Nat. Acad. Sci. 52:100-106. Villiers, T. A. 1972. Seed dormancy. Pages 219-276 in T. T. Kozlowski (ed.). Seed Biology. Vol. 2. Academic Press Inc., New York. Villiers, T. A. 1975. Dormancy and the survival of plants. Edward Arnold Publication Ltd., London.
Villiers, T. A., and P. F. Wareing. 1964. Dormancy in fruits of Fraxinus excelsior L. J. Expt. Bot. 15; 359- 367.
Vincent, E. M., and E. H. Roberts. 1977. Interaction of light, NO3, and alternating temperature in promoting the germination of dormant seeds of weed species. Seed Sci. and Technol. 5:659-670. Walbot, v., M. Clutter, and I. Sussex. 1975. Effects of abscisic acid on growth, RNA metabolism, and repspiration in germinating bean axis. Plant Physiol. 56:570-574.
Warington, K. 1936. The effect of constant and fluctuating temperature on the germination of the weed seeds in arable soil. J. Ecol. 24:185-204.
Warwick, M. A. 1984. Buried seeds in arable soils in Scotland. Weed Res. 24:261-268. Weisner, E. L., and R. C. Kinch. 1964. Seed dormancy in green needlegrass. Agron. J. 56:371-373.
Werker, E. 1980/81. Seed dormancy as explained by the anatomy of embryo envelopes. Israel J. Bot. 29:22-44.
Wesson, G., and P. F. Wareing. 1969a. The role of light in the germination of naturally occurring populations of buried weed seeds in arable soil. J. Ecol. 24:185-204. Williams, M. P., J. D. Ross, and J. W. Bradbeer. 1973. Studies in seed dormancy. VII. The abscisic acid content of the seeds and fruits of Corvlus avellana L. Planta 110:303-310. 135
Wipple, S. A. 1978. The relationship of buried, germinating seeds to vegetation in an old growth Colorado subalpine forest. Can. J. Bot. 56:1505-1509. Witztum, A., Y. Gutterman, and M. Evenari. 1969. Integumentary mucilage as an oxygen barrier during germination of Blepharis persica (Burm.) Kuntze. Bot. Gaz. 130:238-241.
Wood, A., and W. J. Bradbeer. 1967. Studies on seed dormancy. II. The nucleic acid metabolism of the cotyledons of Corvlus avellana L. seeds. New Phytol. 66:17—26.
Yeam, Y. D., N. B. Mandava, P. H. Terry, J. J. Murray, and H. L. Portz. 1988. Identification and quantification of abscisic acid in zoysiagrass seeds and its inhibitory effect on germination. Crop Sci. 28:317- 321.
Yentur, S., and A. C. Leopold. 1976. Respiratory transition during seed germination. Plant Physiol. 57:274-276. 136
ACKNOWLEDGEMENTS
The author is indebted to Dr. Micheal D. K. Owen, the Committee Chair of my graduate program for suggesting the weed species for these studies. I sincerely appreciate his friendship, understanding, and his valuable and tireless guidance in the preparation of this manuscript. I am thankful to Dr. Allen D. Knapp at the Seed Center for the space, the facilities for these studies, and his continued expertise advice throughout the period of these studies. I will like to express my gratitude to Dr. J. B.
Peterson and Dr. C. R. Steward of the Botany Department and to Dr. J. S. Burris at the Seed Center for the advice, space, and the facilities for all the oxygen experiments I conducted.
My acknowledgment to Dr. P. M. Hinz for his help with the statistical analyses. My appreciation to all the members of my program of study committee, Drs. E. J. Braun,
J. S. Burris, R. S. Fawcett, A. D. Knapp, and L. P. Pedigo for their interest, support, and academic guidance. My wife, Joanna contributed enormously to the success of these studies. She helped directly in weeding, seed counting, and dehulling. Her love, understanding, continued support, sacrifice, and her encouragement 137 inspired me during the period of my Ph.D. program. I thank our children, Richard, and Khaddy for their patience and understanding. I sincerely appreciate the financial support that I got from Dr. B. T. Thielen, Vice President Students Affairs and Dr. M. D. K. Owen during a period of some financial difficulties.
My sincere appreciation to Dr. I. A. Musah for his help. The help of the summer crew members is highly appreciated. I enjoyed the useful discussion and the encouragement of the Weed Science graduate students. I am grateful to Mr. Bill McCormick and Mr. James Lux for their assistance.
I sincerely acknowledge the support of my parents during my educational career. I am very thankful to the University of Maiduguri, Borno State, Nigeria for the financial support during the period of my graduate program.
In Ames Iowa, I met a few individuals that on many occasions made me feel at home. That helped me greatly.
For that, I am grateful to Charles V. Owen, his wife, Joanna, and their entire family members for their friendship and the nice time I had with them at their home during Thanksgiving, Christmas, and New Year celebrations during my five years at Iowa State University. Also, Mr. 138 and Mrs. Emilio Oryazabal and their kids have been very friendly and helpful. I really cherish their friendship. The difficult experiences I had with a few individuals during my academic struggles have broadened my idiosyncrasy and my perspectives about life in general. "With God, all things are possible" (Matthew 19:26).
I am most thankful to God for His guidance and for making my dreams for higher degrees come true.
While accepting the credits that may be associated with the objectives and findings of this research, the author concurs to accept the shortcomings of the same. 139
APPENDIX TABLES 140
Table 7. Analysis of variance for the total number of tillers per woolly cupgrass plant
Source of df Mean square F value P > F variation
1987
Replication 2 30.87 Planting date 4 1404.83 34.87** 0.0001 Error 8 40.28 Total 14 428.81 1988
Replication 2 0.19 Planting date 3 410.89 329.40** 0.0001 Error 6 1.25 Total 11 112.78
** P<0.01 141
Table 8. Analysis of variance of plant dry weight of woolly cupgrass
Source of df Mean square F value P > F variation
1987 Replication 2 5441.84 Planting date 4 78993.41 11.60** 0.0021 Error 8 6807.59 Total 14 27237.00 1988 Replication 2 31.35 Planting date 3 4751.88 60.41** 0.0001 Error 6 78.66 Total 11 1344.57
**P<0.01 142
Table 9. Analysis of variance for the total number of panicles/plant as affected by planting date
Source of df Mean square F value P > F variation
1987 Replication 2 11,662.47 Planting date 4 447,003.22 16.26** 0.0007 Error 8 27,497.22 Total 14 145,094.12 1988 Replication 2 69.25 Planting date 3 8,751.56 60.72** 0.0001 Error 6 144.14 Total 11 2,478.00
**P<0.01 143
Table 10. Analysis of variance for the total number of seeds per woolly cupgrass panicle as affected by planting date
Source of variation df Mean square F value P > F
1987
Replication 2 351.20 Planting date 4 11113.57 26.90** 0.0001 Error 8 413.12 Total 14 3461.54
1988
Replication 2 52.80 Planting date 3 3358.03 439.09** 0.0001 Error 6 7.65 Total 11 929.60
**P<0.01 144
Table 11. Analysis of variance for the number of racemes per panicle for woolly cupgrass plant.
Source of variation df Mean square F value P > F
1987 Replication 2 0.47 Planting date 4 12.24 13.46** 0.0013 Error 8 0.17 Total 14 0.80 1988 Replication 2 0.04 Planting date 3 1.00 28.67** 0.0006 Error 6 0.03 Total 11 0.30
**P<0.01 145
Table 12. Analysis of variance for the total number of seeds per raceme for woolly cupgrass plant
Source of Mean variation df square F value P >F
1987
Replication 2 39.69 Planting date 4 312.10 16.49** 0.0006 Error 8 18.93 Total 14 105.65 1988
Replication 2 3.45 Planting date 3 77.64 92.09** 0.0001 Error 6 0.84 Total 11 22.26
**P<0.01 146
Table 13. Analysis of variance for the number of seeds per woolly cupgrass plant as affected by planting date
Source of variation df Mean square F value P > F
1987
Replication 2 403,546,186.40 Planting date 4 15,098,510,415.23 20.73** 0.0003 Error 8 728,167,437.48 Total 14 4,787,605,252.43
1988
Replication 2 2,997,727.08 Planting 3 302,808,135.60 420.49** 0.0001 Error 6 720,128.64 Total 11 835,521,875.72
**P<0.01 147
Table 14. Analysis of variance for the effect of hulls on woolly cupgrass seed water imbibition.
Source of variation df Mean squares F value P > F
Replication 2 69.25 Seed type 1 82.69 0.20 0.69 Rep X seed type 2 412.75 Time 7 1199.02 104.72** 0.0001 Seed Type X time 7 11.45 1.74 0.1791 Error 14 6.60 Total 47 205.73
**P<0.01 148
Table 15. Analysis of variance for the effects of hull, oxygen, and temperature on dormant woolly cupgrass seed germination
Source of Mean variation df square F value P>F
Replication 2 44.40 Temperature 3 84.09 5.27* 0.0405 Error (a) 6 15.96 0.77 0.6002 Oxygen 4 184.84 4.46** 0.0078 Temp. X Oxygen 12 18.76 0.45 0.9232 Error (b) 24 41.46 1.99 0.0262 Seed Type 1 37.09 37.09** 0.0089 Temp. X Seed Type 3 66.49 3.20* 0.0335 Oxy. X Seed Type 4 77.47 3.72** 0.0114 Temp. X Oxy. X Seed Type 12 28.04 1.35 0.2310 Error (C) 40 20.80 Total 119 58.58
*P<0.05 **P<0.01 149
Table 16. Analysis of variance for the respiration rates of dormant and nondormant woolly cupgrass seeds
Source of Mean variation df square F value P>F
Replication 3 1052.40 Seed type 2 24995.45 69.78** 0.0001 Error (a) 6 358.19 5.23 Time 5 82043.84 1197.50** 0.0001 Seed type X Time 10 1655.90 24.17** 0.0001 Error (b) 30 68.51 Total 71 6838.09
** P<0.01 150
Table 17. Analysis of variance for the effect of leaching and leachate on the germination of dormant and nondormant woolly cupgrass seeds
Source of Mean variation df square F value P>F
Replication 2 0.66 Temperature 7 802.29 202.51** 0.0001 Error (a) 14 3.96 1.58 Leachate 1 3.56 1.41 0.2362 Temp X Leachate 7 6.98 1.85 0.1557 Error (b) 14 3.78 1.5o Time 5 3635.58 1445.40** 0.0001 Leachate X Time 5 6.14 2.44* 0.0366 Temp X Time 35 110.17 43.80** 0.0001 Temp X Time Leachate 35 6.92 2.75** 0.0001 Error (c) 160 2.52 2.52
Total 287 221.15 »
* P<0.05 ** P<0.01 151
Table 18. Analysis of variance for the effect of prechilling, temperature, and oxygen on seed dormancy of woolly cupgrass seeds
Source of Mean variation df square F value P>F
Replication 2 3.96 Temperature 3 18.96 7.06** 0.0001 Error (a) 6 2.05 Oxygen 4 63.56 23.68** 0.0001 Temp X Oxygen 12 9.08 3.39** 0.0018 Error (b) 24 2.82 Prechill 1 ' ' ' 496.13 184.89** 0.0001 Temp X Prechilling 3 2.47 0.92 0.4403 Oxygen X Prechilling 4 10.03 3.74** 0.0100 Temp X Oxygen X Prechill 12 4.42 1.65 0.1174 Error (c) 40 2.68 Total 119 10.22
** P<0.01 152
Table 19. Analysis of variance for the effect of prechilling on the germination of intact dormant woolly cupgrass seeds
Source of Mean variation df square F value P>F
Replication 2 29.17 Temperature 5 1503.47 30.54** 0.0001 Error (a) 10 49.23 1.56 Prechlll 3 24498.59 775.18** 0.0001 Temp X Prechlll 15 244.33 7.73** 0.0001 Error (b) 30 31.60 Total 71 1216.54
** P<0.01 153
Table 20. Analysis of variance for the effect of field after-ripening, soil depth, and temperature on germination of intact dormant woolly cupgrass seeds Source of Mean variation df square F value P>F
Temperature 6 3954.58 220.95* 0.0001 Depth 2 6042.87 33.87** 0.0001 Error (a) 12 303.94 4.96 Week 5 9461.17 154.52** 0.0001 Depth X Week 10 740.98 Temp. X Week 30 879.19 14.36** 0.0001 Depth X Temp X Week 60 178.97 2.92** 0.0001 Error (c) 252 61.23 Total 377 955.58
** P<0.01 154
Table 21. Analysis of variance for the effect of temperature regimes on the germination of intact nondormant woolly cupgrass seeds in the laboratory
Source of Mean variation df squares F values P>F
Replication 2 647.88 Temp 6 2467.22 14.04** 0.0001 Error (a) 12 641.95 3.65 Time 2 1519.49 8.64** 0.0015 Temp X Time 12 611.34 3.48** 0.0045 Error (b) 24 175.79 Total 62 629.76
**P<0.01 155
Table 22. Analysis of variance for the effect of oxygen concentration on the germination of intact nondormant woolly cupgrass seeds in the laboratory
Source of Mean variation df square F Value P>F
Replication 2 30.92 Oxygen 6 6175.14 118.57** 0.0001 Error (a) 12 52.08 8.42 Time 2 1376.41 222.48** 0.0001 Oxy X Time 12 352.33 56.95** 0.0001 Error (b) 24 6.19 total 62 724.04
** P<0.01 156
Table 23. Analysis of variance for the effect of the depth of planting on the germination and emergence of woolly cupgrass under growth chamber conditions Source of Mean Variation df squares F value P>F
Replication 4 15.96 Depth 7 1678.63 113.71** 0.0001 Error 28 14.76 Total 39 313.53
** P<0.01 157
Table 24. Analysis of variance for the effect of soil depth on the emergence of woolly cupgrass in the field
Source of Mean variation df square F value P>F
Replication 4 0.15 Depth 9 70.71 13.05** 0.0001 Error 36 5.42 Total 49
**P<0.01