INTRASPECIFIC VARIATION IN GERMINATION OF SCOTCH THISTLE (ONOPORDUM ACANTMUM L.) CYPSELAS
by Miwais Mauj Qaderi
Department of Plant Sciences
Submitted in partial fulfillment of the requirements for the degree of Master of Science
Faculty of Graduate Studies The University of Western Ontario London, Ontario August 1998
cb Mirwais Mauj Qaderi 1998 National Library Biiliothbque nationale du Canada Acquisiîiis and Acquisitions et Bibliographie Services seMcas bibliographiques
The author has granted a non- L'auteur a accordé une licence non exclusive licence ailowing the exclusive permettant à la National Li'brary of Canada to Bibliothèque nationale du Canada de reproduce, loan, distri'bute or sell reproduire, prêter, disûi'buer ou copies of this thesis m microfom, vendre des copies de cette thèse sous paper or electronic formats. la forme de microfiche/fiLa de reproduction sur papier ou sur format élecîronique.
The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fiom it Ni la thèse ni des extraits substantiels may be printed or otherwise de celleci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son pexmission. autorisation. Cypselas (seeds) of Scotch thistle, Onopordum acanthium, range from non- dormant to strongly dormant. The effects of environmental factors during tipening and on dispersed but ungerrninated cypselas are not well understood. Through three sets of experiments I investigated how domancy in Scotch thistle cypselas could be affeded. First, conditions under which cypselas matured on the mother plant had a great impact on the onset of dorrnancy. The warmer the conditions the more germinable were the cypselas. Second, cypselas treated with gibberellic acid and potassium nitrate germinated to higher percentages while those treated with sodium bicarbonate germinated to the same or lower percentages. Third, incubation conditions strongly affected germination patterns.
Light had no effect on percent germination at 35/20°C but strongly promoted germination at 25/10°C. Thus, both pre- and postdispersal conditions were involved in the germinability of Scotch thistle cypselas. In al1 experiments stn'king differences among locally collected populations were rewrded.
Keywords: Scotch thistle, Onopordum acanthium, weed, cypsela, dorrnancy,
seed maturation, mother plant, collection time, gibberellic acid,
potassium nitrate, sodium bicarbonate, Iight, temperature. CO-AUTHORSHIP
The following thesis contains material from research performed by Minvais
Qaderi. The papers will be coauthored with my supervisor, Dr. Paul B. Caven.
Chapter 5 has been submitted to the Canadian Journal of Botany. Chapters 3 and 4 will be submitted to the Canadian Journal of Botany and Seed Science
Research, respectively. DEDICATION
To my children Bizhan, Shahnad and Homan. ACKNOWLEOGEMENTS
It is my pleasure to acknowledge the following people for sharing their knowledge and love with me and bringing this thesis to completion.
I would like to decfare rny great thanks to my supervisor, Dr. Paul B.
Caven, who took me to the unlimited land of weed seed ewlogy. Many thanks are also extended to my committee advisors, Dr. Norman P. A. Huner and Dr. M.
Anwar Maun for their constructive comments and helpful suggestions thmugh the entire time of rny study.
I would like to thank several members of the Department of Plant
Sciences for their advice and technical support: Dr. James Phipps, Dr. Jane
Bowles, Dr. Jianhua Zhang, Dr. Allan Hamill, Dr. Sheila Macfie, Marguerite Kane,
Peter Duenk, Caroline Rasenberg, Daphne Boyce, Magdalena van Hal, Vicky
Lightfoot, Erika Mueller, Monika von Dehn, Ron Smith, Alan Noon, lan Craig,
Don Yakobchuk, Donna Cheshuk, Stefani Tichbourne and Doreen Beres.
I am very grateful to the members of the Department of Zoology, Dr.
Roger Green, Dr. Robert Bailey and Dr. Gary Umphrey, for their helpful suggestions with statistical analyses of data and Dr. Terence Laverty and Dr.
Stan Caveney for their help with identifying sorne pollinating insects.
I am indebted to the following colleagues for their friendship and help:
Michael Downs, Randy Manku, AnneMarie Coulombe, Hua Chen, Amy Tsang,
Prashant Patil, James Macklin, Kellie White and undergraduate helpers, especially Alexa Seal and Shelley Kilby. Many thanks to Jeff Goossens, Park Programs and Services supervisor at the Upper Thames River Conservation Authority and Gord Tanton, manager of the Stebbins Paving and Construction Limited, for allowing me to use th& properties for cypsela collecting from thistles growing in those areas.
Finally, I would like to thank my brother, Hashim Qaderi, for his help in taking thistle pictures and assisting in amputer skills and applications and my wife, Zakera, for her great patience, help and kindness - without her support it would not have been possible to complete this project.
vii TABLE OF CONTENTS
Page
CERTlFlCATE OF EXAMINATION ii
ABSTRACT iii
CO-AUTHORSHIP iv
ACKNOWLEDGEMENTS vi
TABLE OF CONTENTS viii
LIST OF TABLES xi
LIST OF FIGURES xiv
LIST OF APPENDICES xv
CHAPTER 1 GENERAL INTRODUCTION 1
3.1 VVhat is a weed? 1.2 Life cycles and role of seeds 1.3 Seed dormancy 1.3.7 Domancy mechanisms 1.3.1.1 Embryo dortnancy 7.3.1.2 Coat-imposed dormancy 1-4 Germination requirements 1.5 Seed banks 1.6 Cypsela germination and seedling emergence in Scotch thistle 1.7 Thesis objectives 1.8 References
CHAPTER 2 DESCRIPTION OF ONOPORDUM ACANTWIUM L. 13
2.1 Geographical distribution 2.2 Botanical description 2.3 Life cycle
viii 2.4 Economic importance 2.4.1 Detrimental 2.4.2 Beneficial 2.5 Methods of control 2.5.1 Mechanical control 2.5.2 Chemical control 2.5.3 Grazing management 2.5.4 Other biological control 2.6 References
CHAPTER 3 INTERPOPULATION VARIANCE IN GERMINATION RESPONSES OF CYPSELAS OF SCOTCH THISTLE, ONOPORDUM ACANTHIUM L., MATURED UNDER GREENHOUSE AND FIELD CONDITIONS
3.1 lntroduction
3.2 Materials and methods 3.2.1 Cypsela collection 3.2.1.1 1996 (greenhouse and field) 3.2.7.2 1997 (greenhouse and Md) 3.2.2 Germination tests 3.2.3 Statistical analyses
3.3 Results 3.3.1 Cypsela maturation (greenhouse and field, 1996) 3.3.2 Cypsela maturation (greenhouse and field, 7 99 7)
3.4 Discussion 3.4.1 The mle of temperature during maturation 3.4.2 Seasonal msponses - early versus late 3.4.3 Differences among populations 3.4.4 The germination window
3.5 References
CHAPTER 4 INTERPOPULATION VARIANCE IN GERMINATION RESPONSES OF SCOTCH THISTLE, ONOPORDUM ACANTWM L., TO VARIOUS CONCENTRATIONS OF GA,, KNO3 AND NaHCO3
4.1 Introduction
4.2 Materials and methods 4.2.7 Cypsela collecfion 4.2.2 Germination tests 4.2.3 Statistical analyses
4.3 Results 4.3. i The effects of gibberellic acid 4.3.2 The efkcfs of potassium nitrate 4.3.3 The eHects of sodium bicarbonate
4.4 Discussion 4.4.7 Gibberellic acid and cypsela getminafion 4.4.2 Potassium nitrate and cypsela germination 4.4.3 Sodium bicarbonate and cypsela germination 4.4.4 Ecological implications 4.4.5 Variation in response to chernicals among populations
4.5 References
CHAPTER 5 INTERPOPULATION VARIANCE IN GERMINATION RESPONSES OF SCOTCH THISTLE, ONOPORDUM ACANTHlUM L., UNDER CONTRASTING LlGHT AND TEMPERATURE REGIMES
5.1 Introduction
5.2 Materials and methods 5.2.7 Cypsela aollection 5.2.2 Germhafion tests 5.2.3 Statistical analyses
5.3 Results 5.3.1 Temperature efects 5.3.2 Light-temperature interactions 5.3.3 Different populations
5.4 Discussion 5.4. f The impact of different temperature regimes 5.4.2 The role of light 5.4.3 Four populations from one local area
5.5 References CHAPTER 6 GENERAL DISCUSSION
6.1 The higher the maturation temperature the wider the germination window 6.2 Changes in germination window by chernicals 6.3 A wider germination window at higher germination temperatures 6.4 Concluding remarks 6.5 References
APPENDICES
VlTA LIST OF TABLES
Page
Table 3.1 Total germination percentages of freshly-harvested cypselas of Onopordum acanthium, from three populations (ESW, FCA, GP) grown under greenhouse and field conditions in London, Ontario. Cypselas were collected from Aug. 05 to Oct. 04, 1996 and inwbated at 25OC for 14h light, and 10°C for 1Oh dark. 33
Table 3.2 Coefficient of germination rates of freshly-harvested cypselas of O. acanthium from three populations (ESW, FCA, GP) grown under greenhouse and field conditions and incubated at 25OC for 14h light, and lO0C for 10h dark. 35
Table 3.3 Total germination percentages of freshlyharvested cypselas of 0. acanthium, from three populations (ESW, QI Ola) grown under greenhouse and field conditions in London, Ontario. Cypselas from al1 three populations were collected from July 06 to July 23, 1997 in the greenhouse and from Aug. 09 to Sept. 21, 1997 in the field and incubated at 25OC for 14h light, and 1O°C for 1Oh dark. 37
Table 3.4 Coefficient of germination rates of freshly-harvested cypselas of O. acanthium from three populations (ESW, Q, Ola) grown under greenhouse and field conditions and incubated at 25OC for 14h light, and 1O°C for 10h dark. 38
Table 4.1 The pH of solutions used as germination media for cypselas of O. acanthium collected in London, Ontario in 1996197. 53
Table 4.2 Total germination percentages of cypselas of 0.acanthium, collected from four populations (ESW, Q, Ola, GP) in London, Ontario in Sept. 1996 and Sept. 1997. Cypselas were subjected to various concentrations of gibberellic acid (Gk)and incubated at 25OC for 14h light, and 10°C for 10h dark from Dec. 22, 1996 to Feb. 05, 1997 (stored) and from Sept. 1 1 to Oct. 15, 1997 (fresh). 60
Table 4.3 Coefficient of germination rates of cypselas of O. acanthium from four populations (ESW, Q, Ola, GP) subjected to various concentrations of GA3 and incubated at 25OC for 14h light, and 10°C for 1Oh da& 62
xii Table 4.4 Total germination percentages of cypselas of 0. acanthium, collected from four populations (ESW, Q, Ola, GP) in London, Ontario in Sept. 1996 and Sept. 1997. Cypselas were subjected to various concentrations of potassium nitrate (IWO3) and incubated at 25OC for 14h Iight, and 1O0C for 1Oh dark frorn Dec. 22, 1996 to Feb. 05, 1997 (stored) and from Sept. 11 to Od. 15, 1997 (fresh). 64
Table 4.5 Coefficient of germination rates of cypselas of O. acanthium from four populations (ESW, Q, Ola, GP) subjected to various concentrations of potassium nitrate (KNOa) and incubated at 25OC for 14h light, and 1O°C for 1Oh dark. 65
Table 4.6 Total germination percentages of cypselas of 0. acanthium, colleded from four populations (ESW, Q, Ola, GP) in London, Ontario in Sept. 1996 and Sept. 1997. Cypselas were subjected to various concentrations of sodium bicarbonate (NaHC03) and incubated at 25OC for 14h light, and 1O°C for 1Oh dark from Dec. 22, 1996 to Feb. 05, 1997 (stored) and from Sept. 11 to Oct. 15, 1997 (fresh). 69
Table 4.7 Coefficient of germination rate of cypselas of O. acanthium from four populations (ESW, Q, OIa, GP) subjected to various concentrations of sodium bicarbonate (NaHC03) and incubated at 25OC for 14h light, and 1O0C for 1Oh dark. 71
Table 4.8 The effects of solutions of different pH on total percent germination of cypselas of O. acanthium collected from the ESW population in London, Ontario in Sept. 1997 and incubated at 25OC for 14h light, and 1O0C for 1Oh dark. 73
Table 5.1 Total germination percentages of cypselas of 0. acanthium, collected frorn four populations (ESW, Q, Ola, GP) in London, Ontario in Sept. 1996. Cypselas were incubated at four temperature regimes (35120, 2511 O, 1O/S°C; 14h light, 1Oh dark and constant dark, and 20°C; constant dark) from Oct. 24,1996 to Feb. 12,1997. 94
Table 5.2 Total germination rates of cypselas of O. acanthium from four populations (ESW, Q, Ola and GP) over 111 days incubation in 35/20, 25/10 and 1015°C; 14h light, 1Oh dark treatments. 95
xiii LIST OF FIGURES
Page
Figure 3.1 Cumulative percent germination of cypselas of Onopordum acanthium from the (A) first, (0)second, (C) third, and (D) fourth colledion of the ESW population matured under greenhouse and field conditions in 1996 and 1997 and incubated under 25OC for 14h light, and 1O°C for 1Oh dark. 44
Figure 4.1 Cumulative percent germination of cypselas of O. acanthium from the ESW, Q, Ola, and GP populations collected in 1997. For al1 populations the KN03 treatments with 0.05, 0.025 and 0% (control) concentrations, were incubated under 25OC for 14h light, and 10°C for 1Oh dark. 67
Figure 5.1 Cumulative percent germination of cypselas of O. acanthium from (A) the ESW, (B) the Q, (C) the Ola, and (D) the GP populations collected in 1996 and incubated under 35/20°C and 25/1O0C; 14h light, 1Oh dark 98
xiv LIST OF APPENDICES
Page
Appendix I Description of collection sites of populations of Onopordum acanthium in London, Ontario. 113
Appendix II Plant species growing with the populations of Scotch thistle in three natural habitats in or near London, Ontario collected on July 9 and 11, 1996. 114
Appendix 111 lnsects collected from sites of two Scotch thistle populations (UWO campus and ESW) in or near London, Ontario from June 28 to September 2, 1996. 117
Appendix IV Mean daily air temperature and total rainfall in London, Ontario for four months in each of 1996 and 1997. 118 CHAPTER 1. GENERAL INTRODUCTION
1.1 What is a weed?
Weeds are "undesirable species*, 'unwanted plantsa,or " plants out of placen and the worst of them are called 'noxiousn (Parsons, 1973; Weed Science Society of
America, 1984). Weeds may change the scenery of lawns and gardens, interfere with crop production, reduce crop yield and quality, lower feed palatability, poison livestock, or cause long lasting human allergy by contact or by their pollen (Alex and Switzer, 1985).
Despite strong chernical and biological control programs mounted against thern, weeds are able to occupy cultivated areas successfully and cause darnage to agricultural products. The success of a weed may be related either to its resistance to herbicides or to its life cycle strategy (Chancellor, 1982).
1.2 Life cycles and role of seeds
Weeds rnay be annual, biennial, or perennial (Aiex and Switzer, 1985). In their
Iife cycle, weeds can propagate either sexually (by seeds) or vegetatively (clonal growth), or by both means (Fenner, 1985; Silvertown and Lovett Doust, 1993).
Even though clonal growth is an important rnechanism for plant propagation, many successful weed species continue their generation by seeds through sexual reproduction. There are differences behnreen the two forms of propagation. Seeds are small, produced in large numbers and are easily dispersed to new areas. They also can tolerate harsh environmentai conditions such as drought, while vegetatively produced offspring cannot. Therefore, seeds have important roles in multiplication, dispersal and stress tolerance (Fenner,
1985). Dormancy is one of the main survival strategies of seeds of many weed species (Bradbeer, 1988; Fenner, 1991).
1.3 Seed dormancy
When shed, seeds of some species are immediately capable of germination if
provided with water and oxygen at an appropriate temperature. For various
reasons, however, the seeds of some other species show dormancy (Street and
Opik, 1984). Hilhorst (1995) defined seed domancy as: ' the absence of
germination of an intact, viable seed under germination favoring conditions within
a specified time lapse". Dormancy is a state in which an intact viable seed fails to
germinate when supplied with conditions nomally favorable for germination
(Huxley et al., 1992; Bewley, 1997). Pre- and postdispersal factors are involved
in seed domancy. The degree of dormancy specifies the germination phenotype
and is affected by genetic and environmental factors during seed maturation (Li
and Foley, 1997). Crocker (1916,1948) who described seven kinds of seed
dormancy based ptimarily on treatments to overcome them, formulated a
historically early system of categories for seed dormancy. Subsequently,
Nikolaeva (1 977) defined a system based predominantly upon physiological
controls of dormancy. Atwar (1980) showed that morphological characteristics,
including both seed morphology and types of seed covering and taxonornic characteristics of plant families could be associated with domancy categories.
Harper (1959) said: ' some seeds are bom donnant, some acquire dormancy and some have dormancy thnist upon them" and called those three categories
'innate", 'induced" and 'enforced" dormancy. According to Harper (1977) innate dormancy occurs when seeds are in a dormant state on release from the parent plant, whereas induced domancy is used to describe the situation in which domancy develops in response to some experience after release from the parent plant. Enforced dormancy is an inability to germinate due to an environmental restraint such as shortage of water, low temperature or poor aeration.
3 Dormancy mechanisms
Dormancy in many cases develops only towards the end of seed maturation
(Bewley and Black, 1982). Seed domancy is imposed by different mechanisms in different species. These mechanisrns may be grouped into two broad categories: (a) embryo domancy, where the control of dormancy resides within the embryo itself and (b) coatjmposed dormancy, in which donnancy is maintained by the structures enclosing the embryo, such as the seed mat
(Bryant, 1985).
1.311 Embryo domancy
Embryo dormancy is an extremely complex phenomenon that has not been explained satisfactorily despite the very large number of studies devoted to it (Côme and Thevenot, 1982). Embryo domancy is recognized by the failure of the viable, mature embryo to germinate even when it is isolated from the seed.
Dormancy is marked not only by the inability of the isolated embryo to germinate, which is clearly due to a deficiency in the axis, but also by metabolic blocks within the cotyledons (Bewley and Black, 1982). Metabolic deficiencies exist in the cotyledons as well as in the axes of the dormant embryo. In embryo dormancy the following factors are recognized as control mechanisms: (a) an undifferentiated and underdeveloped embryo, (b) blocks to nucleic acid and protein synthesis, (c) failure to mobilize food reserves for the embryo, (d) a deficiency of plant growth hormones, and (e) the presenœ of inhibitors in the embryo (Bradbeer, 1988).
f.3.1.2 Coat-imposed domancy
In many species seed donnancy is imposed by the structures surrounding the embryo, which are often referred to as the seed mat. The structures responsible for imposing and maintaining domancy Vary from species to species. They include the glumes, paleas and lemmas, the pericarp, testa, perisperrn and endosperm. In coatimposed dormancy, the embryo in the intact seed is dormant but when isolated from the enclosing structures it is not (Bewley and Black,
1982). The mechanism by which the seed coat imposes donnancy is poorly understood but evidence points to a number of possibilities. These include: (a) restriction of water uptake, (b) restriction of gas exchange, (c) prevention of the escape of inhibitors from the embryo, (d) an increase in the amounts of inhibitors supplied to the embryo, (e) the embryo covering serving as a Iight filter, and (f) a mechanical barrier to embryo growth (Bradbeer, 1988; Bewley and Black, 1994).
Many dormant seeds possess more than one dormancy mechanism (Tran and Cavanagh, 1984). Therefore, it must not be assumed that if a dormant seed contains a dormant embryo the seed coverings cannot contribute to the dormancy of that seed (Bradbeer, 1988).
1.4 Germination requirements
There are rneny factors that control germination of seeds, including light, temperature, moisture, soi1 nutrients and other soi1 chemicals. It is the combination of al1 of these factors acting together that determine when a seed will germinate (Fenner, 1985, 1991). Every seed has its own 'germination window", which can be changed with tirne. The germination window is defined as the range of conditions under which a seed will germinate (Karssen, 1982).
Seeds are more germinable if the range of germination conditions is wider.
Dormancy relieving factors (such as temperature) cause a widening of the window (range), while donnancy-inducing factors cause a narrowing of this range
(Vleeshouwers et al., 1995). Three conditions are neœssary for a seed to germinate. It has to be (a) viable, (b) nondomant and (c) provided with favorable environmental conditions. When a seed and its environment meet these requirements, germination will take place in five consecutive stages: (1) water uptake by the seed, (2) activation of hormones (such as gibberellins and cytokinins), enzymes and cellular respiration, (3) breakdown and movement of stored seed reserves to the embryo, (4) use of the mobilized reserves by the ernbryo as a source of energy for cellular activity and growth, (5) cell division and elongation in the embryo, which enable the embryo to protnide from the seed coat (Pearson and Ison, 1997).
Salisbury (1961 ) described germination patterns in nature. They range from quasi-simultaneous, where the seeds germinate as soon as they have sufficient moisture in the soil, to intermittent, where seeds from single samples germinate at irregular intervals over weeks, months or years. This latter pattern is very important for short-lived monocarpic weed species because new populations quickly replace those that have been eradicated.
1.5 Seed banks
Some seeds may germinate shortly after dispersal, while others may remain dormant in the soi1 for several rnonths to many years until suitable environmental conditions are available for seedling establishment. In general, there are two types of seed banks: (a) transient, and (b) persistent. ln the transient type, al1
viable seeds in the soi1 germinate or die within one year and there is no cany-
over until a new crop is deposited. Species with a transient seed bank during the
summer may germinate in the fall, while species with a transient seed bank
dunng the winter may germinate in the following spring. In the persistent type, at
least some seeds survive in the soi1 for more than one year and there is always
some carrysver until a new crop is deposited. There are some species with
persistent seed banks in which most seeds germinate quickly but some of them survive for at least one year. In contrast, other species with persistent s8ed banks have only a few seeds genninating soon after dispersal (Thompson and
Grime, 1979; Pearson and Ison, 1997).
1.6 Cypsela germination and seedling emergence in Scotch thistle
Scotch thistle, Onopordum acanthium, is a weed with an unusual pattern of seed population dynamics. Cypselas of this species exhibit strongly intermittent germination. Some germinate shortly after dispersal, while others remain dormant for several months to many years. In long-tenn seed bank studies it has proved to be one of the longest-lived weed species. For example, in the Duvel buried seed experiment, up to 46% of Scotch thistle cypselas germinated after 39 years and the percent germination increased with increasing duration of storage, unlike most other species (Toole and Brown, 1946). Cypselas must persist in the soi1 for several years since plants an appear after cultivation on areas where there has been no seeding for several years (Parsons, 1973).
Roberts and Chancellor (1979) declared that the level of innate dormancy in their sample of O. acanthium appeared to be somewhat greater than that in the other species they tested. They found that Scotch thistle not only differed in emergence pattern, but the cypselas appeared to have a greater capacity for persistence in cultivated soil.
Cavers et ai. (1995) reported that in Onopordum, cypselas varying greatly in dormancy were found in the same population and Meier (1995) found that cypselas from a single plant could differ greatly in seed domancy. Various authors have reported on germination responses in Scotch thistle, narning high temperatures, exposure to gibberellic acid, increased soi1 nitrate level, scarification, stratification, leaching, exposure to red light and other factors as stimulating germination (Scifres and McCarty, 1969; Young and Evans, 1972;
Perez-Garcia, 1993). These studies have been done in many parts of the world and many publications reported contradictory results. Even within London,
Ontario different scientists have reported great differences in percent germination from different seed samples in response to a common treatment (Threadgill,
1986; von Zuben, 1993; Meier, 1995). Despite these findings 1 did not find any previous reports of comparisons among populations of a species concerning responses to gibberellic acid, potassium nitrate or other chemicals.
Scifres and McCarty (1969) reported that cypselas of Scotch thistle contain a water-soluble germination inhibitor and are sensitive to differences in light quality. Young and Evans (1972) declared that this sensitivity to light quality is a symptom of the regulation of germination by phytochrome. They said that both the soluble inhibitor and the sensitivity to light quality apparently fundion in the embryo and not in the cypsela coat.
1.7 Thesis objectives
The objectives of this thesis are to examine germination behaviour in Scotch thistle, testing the following hypotheses: (a) cypselas matured under higher temperatures are more geminable than those matured under lower temperatures, (b) gibberellic acid and potassium nitrate have enhancing and sodium bicarbonate inhibiting effects on cypsela germination, (c) higher incubation temperatures have stimulatory effects on cypsela germination, and (d) different local populations respond differently to the same germination regime.
1.8 References
Alex, JmFw and Switzer, C.M. (1985) Ontario weeds. Ontario Ministry of Agriculture and Food, Toronto, ON, Publ. No. 505.208 pp.
Atwar, BmR (1980) Germination, donancy and morphology of the seeds of herbaceous omamental plants. Seed Science and Technology 8,523573.
Bewley, J.D. (1997) Seed germination and dormancy. Plant Ce11 9, 1055-1066.
Bewley, J.D. and Black, M. (1982) Physiology and biochemistry of seeds. 2. viability, dormancy, and environmental control. BerlinlHeidelberg , Springer- Verlag. 375 pp.
Bewley, J.D. and Black, M. (1994) Seeds. Physiology of development and germination. (2nd edition) New York, Plenum Press. 445 pp.
Bradbeer, J.W. (1988) Seed dormancy and germination. New York, Chapman and Hall. 146 pp.
Bryant, JwA. (1985) Seed physiology. London, Edward Arnold (Publishers) Ltd. 77 PP* Cavem, P.B., Groves, RH. and Kaye, P.€. (1995) Seed population dynamics of Onopordum over 1 year in southem New South Wales. Journal of ~pplied Ec010gy 32,425433.
Chancellor, R.J. (1982) Weed seed investigations. pp 9-29 in Thomson, J.R. (Ed.) Advances in research and technology of seeds. Part 7. Wageningen, Centre for Agricultural Publishing and Documentation.
Cdme, D. and Thdvenot, Cm(1982) Environmental control of embryo dormancy and germination. pp 271-298 in Khan, A.A. (Ed.) The Physiology and biochemistry of seed development, domancy and germination. Amsterdam, Elsevier Biomedical Press.
Crocker, W. (1916) Mechanics of domancy in seeds. American Journal of Botany 3,994 20. Crocker, W. (1948) Gmwth of Plants; fwenty years' tesearch at Boyce Thompson lnstitute. New York, Reinhold. 459 pp.
Fenner, M. (1985) Seed ecology. London, Chapman and Hall. 151 pp.
Fenner, M. (1991) The effects of the parent environment on seed germinability. .Seed Science Research 1.75-84.
Harper, J.L. (1959) The ecological significance of dormancy and its importance in- weed control. pp 415420. in Proceedïngs of the IP hternational Congress of Cmp Pmtection, Hamburg 1957, Vol. I.
Harper, J.L. (1977) Population biology of plants. London, Academic Press. 892 PP* - Hilhorst, HmW. M. (1995) A critical update on seed dormancy. 1. Primary dormancy. Seed Science Research 5,61073.
Huxley, AmJ., Griffiths, Mi and Levey, M. (1992) The new Royal Horücultural Society dictionary of gardening. Part 3 - L to Q. New York, The Royal Horticultural Society. pp 373-374.
Karssen, C.M. (1982) Seasonal patterns of dormancy in weed seeds. pp 243- 270 in Khan, A.A. (Ed.) The physiology and biochemistry of seed development, dormancy and germination. Amsterdam, Elsevier Biomedical Press.
Li, B. and Foley, M.E. (1997) Genetic and molecular control of seed dormancy. Trends in Plant Science 2, 384389.
Meier, L.R. (1995) Variation in seeds of Onopordum acanthium. MSc Thesis. Department of Plant Sciences, University of Western Ontario, London, Ontario, Canada. 113 pp.
Nikolaeva, M.G. (1977) Factors affecting the seed dormancy pattern. pp 51-76 in Khan, A.A. (Ed.) The physiology and biochemistry of seed dormancy and germination. Amsterdam, North-Holland Publishing Company.
Parsons, W.T. (1973) Noxious weeds of Victoria. Melbourne, lnkata Press. 300 PP* Pearson, C.F. and Ison, RL. (1 997) Agronomy of grassland systems. (2nd edition) Cambridge, Cambridge University Press. 222pp. Perez-Garcia, F. (1993) Effect of the origin of cypsela on germination of Onopordum acanthium L. (Asteraceae). Seed Science and Technology 21, 187-1 95.
Roberts, H.A and Chancellor, R.J. (1979) Periodicity of seedling emergence and achene survival in some species of Carduus, Chium and Onopodum. Journal of Applied Ecology 16,641-647.
Sal bbury, E. J. (1961 ) Weeds and aliens. London, Collins. 384 pp.
Scifres, C.J. and McCarty, M.K. (1969) Some factors affecting germination and seedling growth of Scotch thistle. Research Bulletin, Nebraska Agricultural Experiment Station 228,1029.
Silvertown, J. W. and Lovett Doust, J. (1993) introduction to plant population biology. (3rd edition) Oxford, Blackwell Scientific Publications. 210 pp.
Street, H.E. and Opik, Hm(1 984) The physiology of flowering plants: Their growth and development. (3rd edition) London, Edward Arnold. 279 pp.
Thompson, K. and Grime, J.P. (1979) Seasonal variation in the seed banks of herbaceous species in ten contrasting habitats. Journal of Ecology 67, 893- 921.
Threadgill, P.F. (1986) Variations in the biennial life history strategy among 15 nideral species in an abandoned gravel pit near London, Ontario. PhD Thesis, Department of Plant Sciences, University of Western Ontario, London, Ontario, Canada. 356 pp.
Toole, EH. and Brown, E. (1946) Final results of the Duvel buried seed experiment. Journal of Agncultural Researd, 72,201-21 0.
Tran, V.N. and Cavanagh, A.K. (1984) Structural aspects of dormancy. pp 1-44 in Murray, D.R. (Ed.) Seed physioogy. 2. Germination and =serve mobilization. Sydney, Academic Press.
Vleeshouwers, L. Mm, Boumeestet, H.J. and Karssen, C.M. (1995) Redefining seed dormancy: an attempt to integrate physiology and ecology. Journal of Er010gy 83,1031 -1 037. von Zuben, P. (1993) The effects of different outdoor storage conditions and positions on the germination, donancy, and viability of two populations of Onopordum acanthium. Honors Thesis. Department of Plant Sciences, University of Western Ontario, London, Ontario, Canada. 31 pp. Weed Science Society of America. (1984) Cmp losses due to weeds. Weed Science Society of America, 309 West Clark St. Champaign, IL 61820.
Young, J.A. and Evans, R.A. (1972) Gemination and persistence of achenes of Scotch aiistfe. Weed Science 2498-101. CHAPTER 2. DESCRIPTION OF ONOPORDUMACANTHWM L.
2.1 Geographical distribution
Scotch thistle, Onopordum acanthium L., is a member of the Asteraceae, a carduine thistle. The genus Onopordum has more than 50 species (Feinbnin-
Dothan, 1978) and of these, 0. acanthium is the only one found in six continents;
Asia (Danin, 1975; Rechinger, 1979), Europe (Tutin et al., 1W6), Africa (Harden,
1992), Australia (VVillis, 1972; Parsons and Cuthbertson, l992), North America
(Britton and Brown, 1970; Gleason and Cronquist, 1991) and South America
(Cabrera, 1971; Boelcke, 1986; Matthei and Marticorena, 1990).
Scotch thistle, a native of Europe, western and central Asia and Asia
Minor (Young and Evans, 1969) is naturalized in eastern North America (Bailey,
1949). From the genus Onopordum, only one species (0. acanthium) has been naturalized in Canada for at least 130 years (Hubbert, 1867). The first confimiad collection of this species in Canada was at Buctouche, New Brunswick in 1870 by Fowler (DAO, Ottawa). Later, it was collected Rom London, Ontario in 1878 by
Burgess (MTMG, Montreal), from Nova Scotia in 1880, from Woodstock, Ontario in 1885 by D. Millman (MTMG, Montreal), from Paisley, Ontario in 1886 by R.
Campbell (MTMG, MontrBal), from Nanaimo, British Columbia in 1887 by
Macoun (CAN, Ottawa), from Edmonton, Alberta in 1889 by White (CANI
Ottawa), from Outremont, Quebec in 1927 by Edmond Roy (DAO, Ottawa) and from Louis-Marie and Oka, Quebec in 1941 (QFA, Quebec) (Rousseau, 1968). It has also been found in Regina, Saskatchewan (Thomas, 1976) and near
Winnipeg, Manitoba (Scoggan, 1957). This species apparently spreads slowly in
Canada, and only in southern Ontario is it a noxious weed (Govemment of
Ontario, 1993) with more than a localized occurrence (Moore and Frankton,
1974). It occun in scattered locations throughout southem Ontario in disturbed areas, along fence lines, and around old buildings, usually in well-drained gravelly soils and grave1 pits. It is distinguished from al1 other thistles in Ontario by the very dense, white woolly covering on stems and leaves (Alex and Switzer,
1985). The Ontario distribution of Scotch thistle is centred on the London area. In this study, the characteristics of collection sites and the species associated with
O. acanthium in natural habitats are given in Appendices I and II, respectively.
2.2 Botanical description
The following description is based on plants growing at London, Ontario. In
Scotch thistle, the first year rosettes are very large, with leaves up to 65 cm long,
35 cm wide and 1.5 mm thick. The mature plant has a large and fleshy taproot.
The stem of the flowering plant is yellowish-green, erect, branched and winged.
A mature plant grows up to 3 metres in height. The leaves, with triangular lobes, are oblong in young plants and rectangular in older plants. The leaves and stems are usually covered with fine silvery white hairs, which give the plant a greyish appearance. The regular flower colour is purple, but white-flowered populations have also been reported (Danin, 1975). The capitula, 10-32 mm in diameter and
10-21 mm in height are flat, solitary or in terminal clusters of 2-7 with short spines on the bracts. Each capitulum produces from O (if al1 aborted) to 400 cypselas.
The cypselas are oblong, 4.07-5.98 mm in length, 1.73-3.25 mm in width, 0.86-
2.20 mm in depth and 4.02-18.68 mg in weight. They are deep brown to black and straight (from the centre of the capitulum) to curved (from the periphery of the capitulum). The pappus hairs are 4-10 mm long, yellowish at maturity, unequal in length and up to twice as long as the cypselas (M. Qaderi, personal observation).
2.3 Life cycle
O. acanthium is a monocarpic winter annual, biennial or short-lived perennial
(Hyde-Wyatt, 1968; Young and Evans, 1972). It is claimed that it reproduces entirely by cypselas (Alex and Switzer, 1985) but vegetative propagation (clonal growth) has been noted in at least ten plants (M. Qaderi, personal observation).
In Ontario, it flowers from late June to Odober and ovule ferülization occurs by
self or cross-pollination that can be performed by wind and insects (Appendix III).
Depending on size, a single Scotch thistle plant can produce from about 100 to
as many as 50,000 cypselas per plant (M. Qaderi, unpublished data).
After maturation, cypselas are released from the parent plant. Some are
retained in the capitulum for weeks or months. The only significant method of
dispersal is by cypselas, each of which is equipped with a stout pappus.
Compared to other thistles, Scotch thistle has a poorîy developed pappus that is
not readily wind-borne. However, whole plants can be carried considerable
distances by fall or winter gales. There is also some local spread of roots by cultivation equipment, as parts of the root system can be established (clonal growth) in areas where suitable germination conditions are available (Parsons,
1973; Hyde-Wyatt and Morris, 1980).
- If germination requirements are adequate, some cypselas germinate as soon as they reach the soi1 in late summer or early fall. The resulting seedlings form sizable (> 50 cm in diameter) rosettes before the onset of winter. The next year, in late summer or early fall, they flower, set cypselas and then die. If cypselas do not germinate right after dispersal, they are incorporated into seed banks and pass the winter in a dormant state. The next year they geninate in spring or early summer, stay at the rosette stage until the summer of the following season, then flower, set cypselas, and die. These two kinds of plants are winter annuals and biennials, respectively. If flowefing plants are damaged by ineffective cutting, cultivation or herbicides, they may mature some cypselas but then become shortlived perennials by producing regrowth that will bolt in the following year, set cypselas and Vien die (Parsons, 1973; M. Qaderi, personal observation). During this study period, Vie first two types of life cycle were noted in the Scotch thistle populations in London, Ontario. Also, two plants that flowered in two successive years have been observed by P. Cavers (personal communication). 2.4 Economic importance
2.4.1 Detrimen tal
Scotch thistle is one of Ontario's 23 noxious weeds (Government of Ontario,
1993). It has also been recognized as a noxious weed around the world
(Parsons, 1973; Briese, 1988; Keil and Turner, 1993). 0. acanthium with its intermittent germination and its prickly stem and leaves at maturity, causes problems for both agricultural products and poultry fams (Auld et al., 1979;
Wheatley and Collett, 1981; Alex and Switzer, 1985). Where Scotch thistle is present in dense stands, it eliminates clovers and desirable grasses (Hyde-
Wyatt, 1968).
Hooper et al. (1970) stated that Scotch thistle's infestations in northern
California (USA) caused annual losses to ranchen of approximately $25.20Ba in wet meadows, $16.60/ha in wheatgrass stands, and $8.40/ha in downy brome rangelands.
In Australia, O. acanthium and 0. illyncum are considered to be the worst and rnost costly weeds of the genus Onopordum. These two thistles fom
problem infestations in 57% of the counties in New South Wales. The infestations are centred mainly in the southern and central tablelands of New South Wales
(Briese, 1988). The area infested by these thistles reaches ca. 1.i million ha in
southeastem Australia (Briese et al., 1990). The median annual cost of control
including labour was as high as $50 per ha (Btiese, 1996). The annual costs of
these thistles were estimated to be $1 5-20 million in 1987. They are considered
to be gradually spreading and difficult and expensive to control by herbicides (D.T. Briese, personal communication), particularly because they are resistant to cheap and mild hormonal herbicides (e.g. 2,4-D = 2,4-dichlorophenoxyacetic acid and MCPA = 2-methyl-4chlorophenoxyacetic acid). If famen want to eradicate Scotch thistle from their lands they need to use more potent and expensive herbicides that can also destroy legumes in the pastures (Davidson, 1990).
Scotch thistle can be an impassable obstacle to livestock on rangelands and pastures; it also causes both wool flaw and injury to animals (Auld et al., 1979).
These infestations almost totally exclude livestock from grazing and access to water (Hooper et al., IWO; Sindel, 1991 ).
2.4.2 Beneficial
Leaf juice of Scotch thistle has been used for a long time to treat cancers, ulcers, rickets and nervous complaints. A root decoction can reduce mucus discharges
(Bremness, 1989). After the outer bracts are removed, the flower heads can be boiled or steamed and served with butter. The young stems are eaten raw with oils and vinegar or steamed and eaten hot, after blanching and peeling. The cypselas formerly were used to produce oil for cooking and lamps and the white hairs of leaves and stems were collected as pillow stuffing (Steyermark, 1963;
Bremness, 1989). The whole plant forms a striking decorative feature in gardens
(Haughton, 1978). 2.5 Methods of control
Different control methods such as mechanical (e.g. cutting) (Auld, 1988), pasture cornpetition (e.g. cropping) (Michael, 1968), chernical (e.g. herbicides)
(Matthews, 1975), grazing management (e.g. goats) (Popay and Field 1996) and biological (e.g. insects) (Delfosse, 1990) have been applied to eradicate O. acanthium from infested areas. Some of these methods are not very effective and just temporarily remove a thistle population from the site, while some other methods are costly or detrimental to aops (Michael, 1968; Young and Evans,
1969; Wheatley and Collett, 1981). The effects of these control methods on the suppression of 0. acanthium have been studied in many parts of the world including Australia. The control techniques that are currently applied mate many practical problems (Minehan, 1996).
2.5. i Mechanical control
A study has shown that in pastures, previously given weed control treatments, cultivation and cropping was a successful control method. After first flowering, mowing and slashing seemed to be useful, but they were not very effective because of variation in cypsela maturity. However, repeated rnowing throughout the entire growing season was successful (Wheatley and Collett, 1981). It is worth mentioning that this kind of control is very labour-intensive. 2.5.2 Ch emical control
Most herbicides give temporary control of thistles. Young and Evans (1969) reported that application of the expensive and extremely phytotoxic herbicide picloram (4-amino-3,5,6-tflchloropicolinic acid) was the only chemical control method that consistently suppressed O. acanthium in northern California, USA.
In Tasmania, for broad-acre (overall) spraying, 2,4-D, and for spot treatment, amitrole (3-arnino-1,2,4-triazole), were rewrnmended (Hyde-Wyatt, 1968). A study in New Zealand showed that seedlings of O. acanthium are susceptible to the emulsifiable esters of 2,4-D at 1.12 kglha and as young plants to amitrole
(2.24 kglha), dicamba (1.12 kgha) and picloram (0.28 kglha to 0.56 kglha)
(Matthews, 1975).
Arnitrole, at 150 ml in 181 L water and dicarnba, at 180 ml in the same volume of water gave a slow kill of Scotch thistle, whereas diquat, at 60 ml or 90 ml in the same volume of water gave a rapid kill. However, the first two caused unrecoverable damage to adjacent pasture plants, while after application of diquat, pasture plants tecovered quickly and even occupied the spaces where there were thistles before (Hyde-Wyatt, 1968).
At the rosette stage, amitrole, dicamba and diquat have been shown to give effective chemical control of O. acanthium (Hyde-Wyatt, 1968). To control small rosettes, application of dicamba at 0.7 Uha of the 200 g/L amine salt has been recornmended (Wheatley and Collett, 1981). 21
Michael (1968) showed that the combined effects of amitrole and competition from five perennial grasses decreased the yield of O. acanthium for the first year of application, but in two or three years these effects disappeared.
2.5.3 Gmhgmanagement
The ability of thistles to invade pastures can be changed by grazing management
(Sindel, 1991) primarily by changing the competitiveness of the desirable pasture species (Sindel, 1996). Stocking pastures is an essential step in thistle control.
Sheep, goats and horses, but not cattle, have a signifiant effect on thistles in the early stages of infestation when they eat young thistle plants (Wheatley and
Collett, 1981). In a study J. Leigh (in Davidson, 1990) showed that goats, which have a reputation for eating everything, ignored the leaves of 0. acanthium, but they ate al1 the capitula (flowering heads) and thus completely prevented seed dispersal fiom mature plants.
2.5.4 Other biological control
One technique tested for control of O. acanthium and 0. illyticum in Australia was the use of biocontrol agents (Delfosse, 1990). The first biocontrol agent against Onopordum spp. in Australia was released in 1987. Several potential agents such as the seed weevil, Lannus latus, or the stem-boring weevil, iixus cardui have been released and confimed as established in the field in 1992 and
1993, respectively. In France, studies are also king done on the biology and
impacts of two more potential agents, the crown weevil, Tnchosirocalus hom'dus, and the crown fly, Botanophila spinosa (D.T. Briese, personal communication).
Surveys in Greece have shown that the , weevil L. latus, found only on
Onopordum thistles, is one of the best candidates for biological control
(Davidson, 1990). Scientists are currently evaluating the effectiveness of these control agents on O. acanthium and other Onopordum species (Pettit et al., 1996).
Alex, J.F. and Switzer, C.M. (1985) Ontario weeds. Ontario Ministry of Agriculture and Food, Toronto, ON. Publ.No. 505.208 pp.
Auld, B.A. (1988) Dynamics of pasture invasion by three weeds, Avena fatua L., Carduos tenuiflorus Curt. and Onopordum acanthium L. Australian Journal
of Agncultural Research 39,589-596. +
Auld, BA., Menz, R.M. and Medd, RmW. (1979) Bioeconomic model of weeds in pasture. Agro-Ecosystem 5,69-84.
Bailey, L.H. (1949) Manual of cunivated plants. New York, The Macmillan Company. 1029 pp.
Boelcke, 0. (1986) Plantas Vasculares De La Argentina. nativas y exdlicas. Buenos Aires, Editorial Hernisferio Sur S.A. p 269.
Bremness, Lm(1989) The complete book of herbs. Montreal, The Reader's Digest Association (Canada) Ltd. 265 pp.
Briese, D.T. (1988) Weed status of twelve thistle species in New South Wales. Plant Protection Quarîetly 3, 135-1 41 .
Briese, D.T. (1996) Landholder attitudes to Onopordum thistles and their control: A preliminary view. Plant Protection Quartedy 11,281 -284.
Briese, D.T., Lane, De, Hyde-Wyatt, B.H., Crocker, J. and Diver, R.G. (1990) Distribution of thistles of the genus Onopordum in Australia. Plant Protection Quartedy 5,23027. Britton, N.L. and Brown, A. (1970) An iltustrated nota of the hotthem United States and Canada. Vol. III. (2nd edition) New York, Dover Publications, Inc. pp 555556.
Cabrera, A.L. (1971) Compositae. pp 283 8 285 and Figure 291 in Correa, M.N. (Ed.) Flora Patagonica. Part VII. Buenos Aires, Coleccion Cientifica del INTA, Institut0 Nacional de Tecnologia Agropecuaria.
Danin, A. (1975) Onopodum. pp 356-369 in Davis, P.H. (Ed.) Flora of Turkey and the East Aegean Islands. Vol. 5. Edinburgh, University Press.
Davidson, S. (1990) Goats help eliminate thistles. Rural Research 147, 16-1 9.
Delfosse, ES. (1990) Biological wntrol of weeds and the dried fruits industry. Plant Protection Quarterly S,91-97.
Feinbrun-Dothan, N. (1978) Ericaceae to Compositae. pp 382-387 in Zohary, M. (Ed.) Flora Palaestina. Part Three. Text. Jenisalem, Jenisalem Academic Press.
Gleason, H.A. and Cronquist, A (1991) Manual of vascular plants of northeasfern United States and aaacent Canada. (2nd edition) New York, New York Botanical Garden. p 614.
Government of Ontario. (1 993) Weed Control Act 1990 and Ontado regulation 1096. Queen's Printer, Toronto, Ontario.
Harden, G.H. (1 992) Flora of New South Wales. Vol. 3. Kensington, NSW, New South Wales University Press. pp 324-325.
Haughton, C.S. (1978) Green immigrants: the plants that transformed Ametica. New York, Harcourt Brace Jovanovich, Inc. 450 pp.
Hooper, J.F., Young, J.A. and Evans, RmA (1970) Economic evaluation of Scotch thistle suppression. Weed Science 18,583-586.
Hubbert, J. (1867) Catalogue of the flowering plants and ferns indigenous to, or naturalized in Canada. Montreal , Dawson Brothem.
Hyde-Wyatt, B.H. (1968) Cotton thistle. Tasmania Journal of Agriculture 39, 43- 46.
Hyde-Wyatt, B.H. and Morris, D.I. (1980) The noxiuos and secondary weeds of Tasmania, Tasmania, De partment of Agriculture. p 25. Keil, D.J. and Turner, C.E. (1 993) Onopordum. p 320 in Hickman, J.C.(Ed.) The Jepson manual: Higher plants of California. Berkeley and Los Angeles, University of California Press.
Matthei, 0. and Marticorena, Cm(1990) Weeds of the farnily Asteraceae new for the flora of Chile. Gayana, Botanica 47,5763.
Matthews, L.J. (1975) Weed control by chemical methods. New Zealand, A.R. Shearer, Government Printer. p 222.
Michael, P.W. (1968) Control of the biennial thistle, Onopordum, by amitrole and five perennial grasses. Australian Journal of Expefimental Agriculture and Animal Husbandry 8,331 -339.
Minehan, D. (1996) Practical problems with existing thistle control: where is more research needed? Plant Protection Quarterly 11,279-280.
Moore, R.J. and Frankton, C. (1974) The thistles of Canada. Canada Department of Agriculture, Ottawa, Ontario. Monograph No. 10, 111 pp.
Parsons, W.T. (1973) Noxious weeds of Victda. Melbourne, lnkata Press. 300 PP* Parso.ns, W.T. and Cuthbertson, E.G. (1992) Noxious weeds of Australia. Melbourne, lnkata Press. 692 pp.
Pettit, W.J., Briese, 0.T. and Walker, A. (1996) Aspects of thistle population dynamics with reference to Onopordum. Plant Protection Quarterly 11, 232- 235.
Popay, 1. and Field, R (1996) Grazing animals as weed control agents. Weed Technology 1O, 21 7-231.
Rechinger, K.H. (1979) Flora lranica. Compositae Ill-Cynareae. Graz, Akademische Dnick-u. Verlagsanstalt pp 156-163.
Rousseau, C. (1968) Histoire, habitat et distribution de 220 plantes intruites au Quebec. Le Naturaliste Canadien 195,494 7 1.
Scoggan, H.J. (1957) Flora of Manitoba. National Museum of Canada, Ottawa, Bulletin No. 140,619 pp.
Sindel, B.M. (1991) A review of the ecology and control of thistles in Australia. Weed Research 31,189-201. Sindel, B.M. (1996) Overview of thistle management in Australia. Plant Protection Quarterly 1 1,285-289.
Steyennark, J.A. (1963) Flora of Missouri. Arnes, The Iowa State University Press. 1725 pp.
Thomas, AeGw(1976) Weed survey of cultivated land in Saskatchewan. Agriculture Canada, Research Station, Regina, Saskatchewan. p 17.
Tutin, T.G., Heywood, V.H., Burges, N.A., Moore, D.M., Valentine, D.H., Walters, S. M. and Webb, D.A. (1976) Flora Europaea. London, Cambridge University Press. pp 244-248.
Wheatley, W.M. and Collett, I.J. (1981) Winning the thistle war. Agncultural Gazette of New South Wales 92.25-28.
Willis, J.H. (1 972) A handbook to plants in Victoria. Carlton, Victoria, Melbourne University Press. pp 762-763.
Young, J.A. and Evans, RA(1969) Control and ecological studies of Scotch thistle. Weed Science 17,6M3.
Young, J.A. and Evans, R.A. (1972) Germination and persistence of achenes of Scotch thistle. Weed Science 20.98-1 01 . CHAPTER 3. INTERPOPULATION VARIANCE IN GERMfNATlON RESPONSES OF CYPSELAS OF SCOTCH THISTLE, ONOPORDUM ACANTHlUM L., MATURED UNDER GREENHOUSE AND FIELD CONDITIONS~*~
3.1 Introduction
The conditions under which seeds mature on the mother plant can determine subsequent germination responses and the fate of the next generation. Both physiological and environmental factors experienced during maturation will affect the geminability of seeds (Gutterman, 1992). Of these, temperature has a particularly important role (Wulff, 1995). Seeds of different species respond differently to maturation temperature (Koller, 1962). This topic needs more detailed study (Gutterman, 1980181 , 1992).
Higher maturation temperatures inmase seed geminability in many species, such as Rosa sp. (Von Abrams and Hand, 1956), Anagallis arvensis
(Grant-Lipp and Ballard, 1963), Avena fatua (Sexsmith, 1969; Peters, 1982),
Beta vulgaris (Heide et al., 1W6), Chenopodium bonus-henncus (Dorne, l98l),
Plantago lanceolata (Alexander and Wulff, 1985) and Lactuca &va (Drew and Brocklehurst, 1990). In some cases, higher temperatures have an opposite effect on germinability of seeds. For example, in Syringa spp. (Junttila, 1973a,b),
Stylosanthes hamata (Argel and Humphreys, 1983), Glycine max (Keigley and
Mullen, 1986), Panicum dichotomiforum (Govinthasamy, 1994) and Zhlaspi
"' A version of this chapter will be submiüeâ for publicaiion in îhe Canadan Jomal of Botany. awense (Hume, 1994) higher maturation temperatures led to stronger domancy.
Seeds maturing in different years or even seeds from the same plant maturing at different times cm exhibit differences in dormancy and germination responses (Karssen, 1982). Such differences in seed genninability may be related to the conditions under which seeds mature on the mother plant (Baskin and Baskin, 1973). For example, Chadoeuf-Hannel and Barralis lnra (1983) showed that in Amaranthus retrMexus seeds from plants that developed later in the season (July) were less dormant than those from plants that appeared earlier
(April).
We worked with Scotch thistle, Onopordum acanthnrm L., a noxious weed that grows in a range of different climates, from cool Scandanavia (Boissier,
1875) and Western and Eastern Siberia (Czerepanov, 1995) to warm areas such as North Africa (Harden, 1992), Italy, Greece, Spain, Portugal (Tutin et al., 1976),
Australia (Jessop and Toelken, 1986), California (Keil and Turner, 1993) and
Texas (Correll and Johnston, 1970).
Scotch thistle is usually a monocarpic biennial, but under certain
conditions, it can be an annual or short-lived perennial, reproducing almost
entirely by cypselas (Hyde-Wyatt, 1968; Alex and Switzer. 1985). It occurs most
commonly in waste lands, pastures, fence lines, gravelly riverbanks and well-
drained sandy or gravelly soils (Moore and FranMon, 1974). The density and
vigour of this species Vary from year to year, probably because of climatic
conditions, but this is not well understood (Michael, 1968).
O. acanthium has an unusual pattern of population dynamics. Some cypselas may germinate shortly after dispersal (in fall), while others may remain donnant for at least 40 years (Toole and Brown, 1946). Roberts and Chancellor
(1979) found that the level of innate dormancy in Scotch thistle appeared to be somewhat greater than in other species with respect to both ernergence pattern and. capacity for persistence in cultivated soils. In other species, the degree of innate dormancy or the ' germination window' of seeds can be altered by the conditions under which they mature (Baskin and Baskin, 1975; Fenner, 1991;
Hilhorst and Karssen, 1992; Gutteman, 1994). A 'germination window" is the range of conditions under which a seed will gerrninate (Karssen, 1982).
Even though the environmental factors involved in the onset of innate dormancy or the subsequent germination of seeds have been studied in many species, few reports are available where more than one population per species was used for this purpose. In 0. acanthium, Meier (1995), dealing indirectly with this phenomenon without designing a particular experiment, has found that cypselas from two populations colleded early in the season (August) were more germinable than those collected ffom the same populations later in the season
(October). This was strongly correlated with maturation temperature.
The purpose of this study was to compare the germination responses of cypselas of different collections of each of several local populations of O. acanthium matured under both greenhouse and field conditions in London,
Ontario. The following hypotheses were tested: (1) a higher maturation temperature has a widening effect on the 'germination window"; (2) cypselas collected early in the season are more germinable than those collected later in the season; (3) different local populations respond differently to the same maturation temperature.
3.2 Materials and Methods
3.2. f Cypsela collection
3.2. f . 1 1996 (greenhouse and field) Bulk collections of cypselas of Scotch thistle were taken fiom at least 15 plants
on August 5, August 20, September 4 and Septernber 19, 1996 in each of Wo populations (ESW = Environmental Sciences Western, FCA = Fanshawe
Conservation Area) grown in both the greenhouse and the field in or near
London, Ontario. A third bulk colledion (GP = Gibbons Park) was taken on
August 31, September 4, September 19 and October 4, 1996. For further details
of the populations see Appendix 1. The parent plants from the FCA population were naturally occurring in the field while those of ESW and GP had been
transplanted from the original sites to the ESW Field Station since they would
have been destroyed in their original habitats. Rosettes of al1 three populations
were transplanted to pots in the greenhouse in May 1996. Mean daily
temperature and rainfall data for London Airport for the entire collection period
are shown in Appendix IV.
3.2.1.2 1997 (greenhouse and field)
Bulk collections of cypselas of Scotch oiistle were taken from at least 15 plants
on August 9, August 24, September 6,and September 21, 1997 in each of three
populations in or near London, Ontario (ESW, Q = Quarry, Clarke Sideroad, Ola = Olalondo Rd. - Appendix 1). The parent plants from the Q and Ola populations were naturally occurring while those of the ESW population had been grown from cypselas colleded in September 1995, incubated on April 21, 1996 and transplanted to the ESW Field Station at the rosette stage on May 28; 1996.
Mean daily temperature and rainfall data for the entire collection period are shown in Appendix IV. Shortly after collection, the cypselas were deaned manually and used for germination tests within three days. Aborted cypselas were discarded.
3.2.2 Germination tests
For every collection tirne, from each population, 10 lots of 100 cypselas each
(five lots from each of the two conditions, greenhouse and field) were counted.
Each lot was placed in a 9 cm diameter glass Petri dish on one layer of Anchor blue germination filter paper initially moistened with 10 ml of distilled water. More water was added each day as needed. The cypselas were set to genninate in an incu.bato at 25OC for 14h light, and 1O°C for IOh dark Light was provided by four cool white fluorescent tubes (mean photosynthetically active radiation or PAR of
73.3 pmol photons m-2 s*1 at the level of the Petri dishes, n=lO) situated ca. 25 cm above the surfaces of the Petri dishes. Geminated cypselas (radicles 2 mm or longer) were comted and removed from Petri dishes every day. The experiments were terminated after a five day period with no germination (after 45 days in both years). Firrn ungerminated cypselas at the end of experiments in
1996 were subjected to a viability test by using a 1% (w/v) solution of tetrazolium chloride (2'3,s-triphenyl tetrazolium chloride, C19Ht NICI, Sigma)(method of
Delouche et al., 1962). In 1997 viability of ungeminated cypselas was tested by cutting them 1 mm from the cotyledonary end and putting them to geminate.
3.2.3 Statisfical analyses
Final germination percentages for the 1996 and 1997 data were analyzed by means of a balanced ANOVA (McKenzie et al., 1995), after arc-sine square mot transformation to nonnalize the variance (Zar, 1984). Then a Tukey's test was used to detemine differences between treatments (SAS lnstitute Inc., 1982;
Sokal and Rohlf, 1995).
The coefficient of germination rate (CGR) was calculated for each replicate by dividing the total percent germination (N) by the number of geminated cypselas on the particular day on which a munt was made (ni) multiplied by the number of deys from the start of the expen'rnent (di) and
summing for al1 days on which germination occurred:
CGR = N I Enidi
All values of CGR are between O and 1 (Am et al., '1993). Then, a balanced
ANOVA and a Tukey's test were applied (SAS lnstitute Inc., 1982; McKenzie el
al., 1995; Sokal and Rohlf, 1995).
3.3 Results
3.3.1 Cypsela matutafion (rrreenhouse and tleM, 1996)
For al1 populations and collection times, cypselas matured in the greenhouse had higher germination percentages than those matured in the field. Populations (P <
0.001), conditions (P < 0.001), collections (P = 0.001), the two-way interactions between population x condition (P c 0.001), population x collection (P < 0.001) and condition x collection (P = 0.037) and the three-way (P < 0.001) interacüon had significant effects on germination percentages.
Except for the GP population at collection four, al1 populations at al1 collection times had significantly higher total percent germination for cypselas matured in the greenhouse than those matured in the field. For every collection, more than threequarters of the cypselas collected from the greenhouse gerrninated. In contrast, total percent germination for fieldcollected cypselas ranged from 0.2 to 18% for ESW, 4.8 to 59.8% for FCA and 47.4 to 87.6% for
GP, depending on the collection date (Table 3.1 ).
Within populations, there was no significant difference in germination percentage among collection times for the cypselas matured in the greenhouse, but there were differences for the cypselas matured in the field (Table 3.1). The general trend for the field collections was that successively higher total percent germination values were recorded ffom progressively later collections. The first collection from GP with the highest total percent germination was a glaring exception to this trend (Table 3.1 ).
For cypselas matured in the field, large differences among populations were obtained for al1 collections, especially between GP and ESW, which fiowered and matured cypselas in the same location. Much smaller but significant differences among populations (except for the last collection) were found in the Table 3.1 Total percentages (mean I SE) of freshly-harvested cypselas of Onopordum acanthium, from three populations grown under greenhouse and field conditions in London, ontario'.
Condition Collectionz Population
(Mean Maturation ESW FCA GP Temperature) Greenhouse 1 76.8018,178a3 93.2ûkl.32Aa 95.0011.30Aab (21.PC) 2 78.40f3.42Ba 84.40f1.81 Ba 99.40H ,60Aa
Field (16. @C)
ESW = Environmental Sciences Western, FCA = Fanshawe Conservation Area and GP = Gibbons Park.
Cypselas were collected from Aug. 05 to 0d. 04,1996 and incubated at 25OC for 14h light, and 1O°C for 1Oh dark.
Meam followed by different upper-case letlem within mws or by different lower-case letters within columns are significantiy different (P c 0.05) according to Tukey's Honestly Significant Difference multiple range test (Tukey's HSD). greenhouse. In collections from FCA and ESW up to 116 and 114 of cypselas rernained dormant in the germination test (Table 3.1).
In every collection.from the greenhouse and field, ungerminated cypselas for al1 populations had more than 98% viability.
Germination rates, in general, were less variable for cypselas matured in the greenhouse than those matured in the field (Table 3.2). Populations (P <
0.001), conditions (P = 0.044), collections (P c 0.001), the two-way interactions between population x condition (P < 0.001), population x collection (P < 0.001), condition x collection (P = 0.049) and the three-way (P < 0.001) interaction had significant effects on germination rates.
Within populations, cypselas matured in the field showed much greater variability in rates than those matured in the greenhouse. For al1 collection times, both the fastest (0.165 in GP) and the slowest (0.01 1 in ESW) germination ocairred for cypselas matured in the field. The germination rate for field-colleded cypselas ranged from 0.01 1 to 0.126 for ESW, from 0.098 to 0.1 39 for FCA and from 0.069 to 0.165 for GP. Cypselas matured in the greenhouse showed an
intermediate germination rate that ranged from 0.077 to 0.091 for ESW, from
0.08 to 0.107 for FCA and from 0.118 to 0.159 for GP (Table 3.2). Arnong populations, except for the second collection in the field, there were significant differences in germination rates for different collection times. For the greenhouse cypselas, germination rates were always faster for GP than ESW and for two collections than QI but for the field cypselas, GP genninated faster for two
collections than ESW (Table 3.2). Table 3.2 Coefficient of germination rates (mean I SE) of fresh~~haniested cypselas of Onopordum acanthium from three populations grown under greenhouse and field conditions and incubated at 25OC for 14h light, and 10°C
1Oh dark.
Condition Collection Population
(Mean Maturation ESW FCA GP Temperature) Greenhouse 1 0.091 iO.0068bc' 0.1 07kO.006ABabc 0.1 25iô.003Abc (21.7OC) 2 0.077Io.OOZBcd 0.1 07iû.OOSABabc 0.1 32I0.002Aab
Field (9 6. @C)
' Means followed by different upper-case letters within rows or by different lower-caw letteo, within columns are significantly different (P < 0.05) according to Tukey's Honestly Signifiant Difference rnuttiple cange test (Tukey's HSD), 3.3.2 Cypsela maturation (greenhouse and Md, 1997) For al1 populations and collection times, cypselas matured in the greenhouse had higher total germination percentages than those matured in the field (Table 3.3).
There was great variability in these results since populations (P < 0.001), conditions (P c 0.001), collections (P < 0.001) and heir two-way (P < 0.001) and three-way (P < 0.001) interactions had significant effects on germination percentages. The same general trend - higher total percent germination for the greenhouse than the field - was obtained as in the first year (1996) but the last collection from the greenhouse had low total percent germination. The lower values for both the greenhouse and the field than under the same treatrnents for
1996 are probably due to the lower maturation temperature in 1997 than in 1996
(see Appendix IV). Differences among populations were greater for cypselas matured in the field than those matured in the greenhouse. For the field cypselas,
Q always germinated to higher percentages than ESW and for three collections higher than Ola. Further, two collections of Ola germinated to higher percentages than ESW (Table 3.3).
In every collection from the greenhouse and field, ungertninated cypselas had more than 98% viability.
Germination rates, in general, were significantly faster for cypselas matured in the. field than for thos8 matured in the greenhouse (Table 3.4), although there were no differences for the ESW population and only two from Ola and one from Q field germinated faster. Populations (P < 0.001), conditions (P < Table 3.3 Total germination percentages (mean * SE) of freshly-harvested cypselas of Onopordum acanthium, from three populations grown under greenhouse and field conditions in London, ontario'.
Condition CollectionZ Population
(Mean Maturation ESW a Ola Temperature) Greenhouse 1 36.60524483 86.00I3.70Aa 96.001i .40Aa (2 1.3%) 2 80.00k3.61ABa 74,6011.97Bb 92.4011.36Aa
Field (15.TC)
' ESW = Environmental Sciences Western, Q = Quarry and Ola = Olalondo Rd. Cypselas were collecteci from Aug. 09 to Sept. 21,1987 and incubated at 25OC for 14h light, and 1O0C forl0h da&. ' Means followed by different upper-casa letten within rows or by different lower-case lettero within columns are significantly different (P < 0.05) according to Tukey's Honestly Significant Difference multiple range test (Tukey's HSD). Table 3.4 Coefficient of germination rates (mean I SE) of freshlyhanrested cypselas of Onopordum acanthium from three populations grown under greenhouse and field conditions and incubated at 25OC for 14h light, and 10 OC for ?Ohdark.
Condition Collection Population
(Mean Maturatfon ESW Q Ola Temperature) Greenhouse 1 0.076kû.004~a' 0.1 15Hl.004Abc 0.1 1510.003Aab (21 3%) 2 0.11 110.003Aa 0.143f0.002Aabc 0.094k.004~b
Field 1 0.09210,Ol OAa 0.086fl.007Ac 0.1 I9&0,009Aab (15. TC) 2 0. 124I0.033Aa 0,180~.007Aab 0.182=K).OIOAa
' Means followed by different uppercaw letters within rowr or by different lower-case lettm within columns are significanUy different (P < 0.05) according to Tu key's Honestly Significant Difference multiple range test (Tukey's HSD). 0.001), collections (P c 0.001), the two-way interactions between population x condition (P = 0.028), population x collection (P = 0.024) and condition x collection (P < 0.001 ) had significant effects on germination rates. The three-way interaction was not significant (P = 0.1 52).
Unlike the first year (1996) the germination rates were faster for cypselas matured in the greenhouse than for those matured in the field, with the exception of the first collection of Q (Table 3.4). Neither within nor between population significant differences were obtained for different collections of cypselas matured in the greenhouse. For cypselas rnatured in the field, the fourth collection of Q germinated significantly faster than the first collection of the same population and the fourth collection of ESW (Table 3.4).
3.4 Discussion
3.4.1 The role of temperature during maturation
Overall, cypselas matured under wamer temperatures (greenhouse) were more germinable than those matured under cooler temperatures (field) (Tables
3.1,3.3). Temperature is one environmental factor that aflects the physiological and sometimes morphological status of seeds during maturation (Baskin and
Baskin, 1975). Even small differences in the temperature environment of the mother plant may affect the geminability of seeds. In general, higher temperatures are positively related with higher geminability (Guttenan, 1992).
Cypselas matured under wamer conditions were lighter in colour than those matured under cooler conditions. Bewley and Black (1982) reviewed the literature on seed colour and reported that dark coloured seeds of n'ce and wheat had more dormancy than light' coloured seeds. This suggests that thos8 chernicals that are responsible for colour and probably have inhibitory effects on cypsela germination, are synthesized less under wamer conditions. Such differences can be due to changes in the hormone (e.g. abscisic acid) levels andlor sensitivity of tissues to these hormones (Karssen, 1995). However, Black
(1991 ) and Bewley (1997) claimed that at present little or nothing is known about the mechanism of dormancy induction in seeds. Hilhorst (1998) supported their claim by stating ' it is clear that we are still far from understanding the principles and regulation of dormancy".
Koller (1962) showed a direct quantitative relationship between maturation temperature and germinability of lettuce seeds. In Aegilops ovata (A. geniculata),
Datta et al. (1972) found that al1 orders of caryopses matured at 28122OC were lighter and germinated to higher percentages after 24h of imbibition than those matured at 1511O°C. In Chenopodium bonus-henricus, Dorne (1981 ) reported that the mean of the average daily temperature in the last 30 days of maturation had a high positive relationship with seed germination. Von Abrams and Hand
(1956) also found this relationship in Rosa sp. In Avena fafua, Sawhney et al.
(1985) showed that the duration of dormancy in mature seeds was increased by low temperatures during seed development and diminished by high temperatures. Peters (1982) also reported that seeds of three types of wild oat matured at a constant 15OC versus constant 20°C, had 97 and 63% domancy, respectively. His fînding suggests that seeds produced in wam summers were less donnant than those produced in cool ones.
The effect of higher temperatures during maturation is likely related to a decrease in the synthesis of inhibitors or an increase in promoters, but it is not understood clearly (Fenner, 1991). Weisner and Grabe (1972) found that low temperatures during seed maturation increased the degree of dormancy in dormant cultivars of both perennial and annual ryegrass (Lolium spp.). Jouret
(1974 in Maguire, 1976) showed that in cool weather impatiens pamiflora produced seeds with greater dormancy than seeds produced in a warmer climate.
In their study on a clone of Anagallis amensis, using three different maturation temperature regimes and keeping al1 other environmental conditions constant, Grant-Lipp and Ballard (1963) found that the degree of dormancy was strongly correlated with temperature dunng maturation. Seeds from the highest temperatures (30125OC) had no dormancy, while those at the lowest temperatures (20115OC) had strong dormancy.
Our findings do net correspond with the results of Karssen (1970) who found that the germination of chenopodium album seeds was lower after maturation at 22/22'C than at W12OC. Also, Keigley and Mullen (1986) found that after flowering in soybeans (Glycine max), the more accumulated days at higher temperatures (32128OC) the lower was percent germination, in cornparison with seed maturation under temperatures of 27/Z°C. This pattern was found in other species such as Sytinga spp. (Junttila, 1973a,b), Stylosanthes hamata
(Argel and Humphreys, l983), Panicum dichotomiflorum (Govinthasamy, 1994) and Thlaspi arvense (Hume, 1994).
There exists a correlation (? = 0.35), although a weak one, between 1 maturation temperature and cypsela gerrninability for al1 populations used in
1996 and 1997.' A cornparison of the ho years' data in the ESW population shows that cypselas that matured in 1996 had higher percent germination than those that matured in 1997 (Fig. 3.1 A-D). In London, Ontario, the mean daily average temperature for the entire duration of experiments was 16.0°C for 1996 and 15.7'C for 1997 and fluctuations from day to day were greater in 1997
(Appendix IV).
3.4.2 Seasonal responses - early versus late In general, cypselas that matured early in the season were less dormant than those that matured late. However, this response was not consistent among populations. It has been shown that preconditioning not only causes differences between populations but also within populations in different years or seasons
(Baskin and Baskin, 1975).
Meier (1995) showed that cypselas of O. acanthium from October collections had much lower percent germination than those from an August one.
Our findings, with a few exceptions; agree with Meier's results. To some extent differences in germination patterns were related to the collection times, but it was temperature and other environmental factors that detenined the seed germination performance. In London, Ontario, early collections usually have a relationship with higher ripening temperatures, but this relationship can be Figure 3.1 Cumulative percent germination of cypselas of Onopordum acanthium from the (A) first, (B) second, (C) third, and (D) fourth collections of the ESW population matured under greenhouse and field conditions in 1996 (e, a, respectively) and 1997 (A,V,respectively), and incubated at 25OC for 14h light, and 1O0C for IOh dark. Final germination percentages not followed by the same letter are significantly different (P < 0.05) according to Tukey's Honestly Significant Difference multiple range test (Tukey's HSD). changed by other environmental factors; that is why higher germination percentages were obtained from the later collections than the earlier ones in some of the populations. For example, in the ESW population grown in the greenhouse, cypselas from the second collection in 1997 germinated to higher percentages than the ones from the other collections of that year (Table 3.3).
Hilhorst and Karssen (1992) stated that the depth of innate dormancy is dependent on temperature during growth and development of the parent plant; higher temperatures correspond to less dormant seeds.
Thompson (1937 in Baskin and Baskin, 1973) found that lettuce seeds that matured early in the season had significantly lower germination percentages than those that matured later. Differences in the germination patterns were probably due to variation in temperature and other environmental factors at different collection times.
In general, our findings (Table 3.3) correspond with results obtained for other members of the Asteraceae, by Stergios (1976) for Hieracium aurantiacum and by Venable and Levin (1985) for Hetemtheca lafifollia, and also by Meier
(1995) for the same species, 0. acanthium. They found that as the growing season progressed, the germination of cypselas decreased.
von Zuben (1993) reported that cypselas of Scotch thistle collected from near Gibbons Park, London later in the season (Oct. 13, 1992) were more germinable than those collected earlier (Sept. 26 and Od. 7, 1992). In our field study in 1996 (Table 3-11, cypselas from the fourth and last colledion of the GP population (06.4) had higher percent germination than those in the second and third collections, which is sirnilar ta the trend obtained by von Zuben (1993). In these cases, some other environmental factors that we did not monitor probably were more involved than maturation temperatures in affecting germination.
3.4.3 Diflerences among populations
Overall, cypselas from al1 populations matured under warmer conditions gerrninated to higher percentages than those matured under woler conditions
(Tables 3.1,3.3). However, there were also significant differences between populations in response to difïerent maturation ternperatures.
McWilliams et al. (1968) showed that in Amaranthus retroflexus populations from a northem part of the range had significantly higher percent germination at 20°C than those from a southem part of the range.
Differences in total germination percentages and rates of germination of different populations (Tables 3.2,3.4) can be explained by referring to environmental variations during cypsela maturation. Factors such as temperature, light, moisture and nutrients cause variations among populations
(Fenner; 1991). Variability in germination patterns among populations was probably caused by both genetic and environmental factors. When the two populations (ESW and Q) grown under the same conditions (ESW, Field Station) were compared, cypselas from all four collecüons of Q matured at the Field
Station had significantly lower total percent germination (1 1.1 I2.7, mean of four collections) than those of Q (23.5 I 1.9, mean of four collections) matured in their
natural habitat. Nevertheless, the former cypselas still had significantly higher total percent germination than those from the ESW (2.6 i 0.7, mean of four collections) that ripened in the same site (M. Qaderi, unpublished data).
3.4.4 The genmination window
In general, cypselas of al1 populations that matured under higher temperatures had wider germination windows (se8 section 1.4) than those that matured under lower temperatures (Table 3.1,3.3). Changing the environment in which seeds mature or in which seeds germinate can change the germination window. Seeds are more germinable if the range of germination conditions is wider. Changes to the germination window can be acquired during maturation on the mother plant
(Vleeshouwers, 1997) or after shedding through after-ripening or storage
(Stokes, 1965).
These findings are useful for elucidating the population dynamics of O. acanthîum. For example, if the temperatures during maturation (usually between
August and September) are high, cypselas will germinate mainly in the fall shortly after dispersal and there will be an increase in the size of the Scotch thistle population for the next year. If the maturation temperature is low, .then there will be less germination in the fall and consequently few bolting rosettes will appear in the following spn'ng. If we know the maturation temperature that determines the germination window of a particular species, we can roughly predict the size and persistence of the resulting seed bank and possibly the density of plants in a subsequent population. 3.5 References
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4.1 Introduction
Seed dormancy is regulated by the balance of stimulators and inhibitors (Amen,
1968; Wareing and Saunders, 1971; Varner and Ho, 1976; Bewley and Black,
1982) with overall controll by abscisic acid (ABA) (Bewley, 1997). This is related to both pre- and postdispersal chernical environments (Fenner, 1985; Fenner,
1991). In many species, seed dormancy can be broken by the exogenous application of chemicals such as gibberellic acid (Forsyth and Brown, 1982;
Perez-Garcia and Pita, 1989; Campbell et ai., 1991; Lecat et al., 1992; Bell et al.,
1995; Plummer and Bell, 1995; Evans et al., 1996; Afolayan et al., 1997) or potassium nitrate (Popay and Roberts, 1970a; Hendricks and Taylonon, 1972;
Hilhorst and Kansen, 1989; Ellis et al., 1990; Karssen and Hilhorst, 1992;
Carmona and Murdoch, 1995). It cm also be imposed by the change in pH
caused by chemicals such as sodium bicarbonate, which, in tum, are naturally
dependent on the presence of sodium and dissolved carbon dioxide in soi1 (Kidd,
1914a, 1914b; Thompson, 1957; Lindsay, 1979; Donahue et al., 1983; Tan,
4 994; White, 1997). Even though the effects of dormancy - breaking chemicals have been
'O' A version of this chapter will be submitîed for publication in Seed Science Research. studied extensively for many species (Kolk, 1962; Hogue, 1976; Medd and
Lovett, 1978; Hilton, 1984; Xia et al., 1992; Adkins and Adkins, 1994; Derkx and
Karssen, 1994; Laura et al., 1994), little is known about interpopulation variance in response to these chemicals. Perez-Garcia et al. (1995) in their study on seed germination patterns of four populations each of two weedy mustards, Diplotaxis erucoides (L.) OC. and D. virgata (Cav.) DC. have shown that at maturity seeds of D. virgata were dormant but not those of D. erucoides. The application of gibberellic acid to seeds of D. virgata stimulated germination, especially in the populations with stronger dormancy. In Onopordum nervosum, Perez-Garcia and
Duran (1990) showed that gibberellic acid clearly stimulated cypsela (seed) germination in two populations and significant dnerences at the 1% level were obtained between the different GA3 concentrations used. In their study, an increase in GA3 concentration resulted in higher germination percentages, and this increase was slightly greater in one population than the other. Meier (1995) found significant differences in germination between two populations of O. acanthium in response to gibberellic acid (GA3) at a concentration of 0.1% but not at a concentration of 0.01%. We could not find any study that has been done on the simultaneous affects of G&, KNQ andor NaHC03 on germination responses of different populations of the same species.
We chose to work with Scotch thistle, Onopordum acanthium L., a species with a broad spectrum of dormancy. 0. acanthium is monocarpic and reproduces almost entirely by cypselas (Hyde-Wyatt, 1968; Aiex and Switzer, 1985; M.
Qaderi, personal observation). Sorne cypselas will geminate in the fall shortly after dispersal from the mother plant, some in the following spfing and some remain as a dormant component of the persistent seed bank, gerrninating at intervals over several years (M. Qaderi, unpublished data).
Several authors have reported that donancy can be broken in cypselas of Scotch thistle by applications of gibberellic acid (Scifres and McCarty, 1969;
Young and Evans, 1972; Groves and Kaye, 1989; Perez-Garcia, 1993) or potassium nitrate (Young and Evans, 1972; Groves and Kaye, 1989) but no study has been done on the effects of sodium bicarbonate on this or any other species.
The purpose of this study was to determine the effects of gibberellic acid, potassium nitrate and sodium bicarbonate on the germination patterns of cypselas of four local populations of O. acanthium collected from London,
Ontario. The following hypotheses were tested: (1) germination responses are positive to GA3 and KN03 and negative to NaHC03, (2) the higher the concentrations of G& or KN03 the faster and more complete will be the germination, whereas the higher the concentrations of NaHC03 the slower and less complete will be the germination, and (3) at different concentrations of these chemicals, different local populations will have a variety of responses and the patterns of response will Vary with year of collection and duration of storage between collection and testing.
4.2 Materials and Methods
4.2. i Cypsela collectfon
Bulk collecüons of cypselas of Scotch thistle, Onopordum acanfhium L., were 56 taken from 100 randomly selected plants on September 4, 1996 and from 50 randomly seleded plants on September 6, 1997 in each of four populations in or near London, Ontario (ESW, Q, Ola, GP - Appendix 1). The parent plants from the Q and Ola populations were naturally occumng while those from the ESW and GP populations had been transplanted from the original sites (at the rosette stage in the spting of 1996) to ESW since they would have been destroyed in their original habitats. The cypselas were cleaned manually and stored in the laboratory at room temperature (20-25OC) until they were tested. Cypselas from the 1996 collection were stored for 67 days before testing while those from the
1997 collection were used fresh, after 5 days storage. Aborted cypselas were discarded.
4.2.2 Germination tests
Three (1996) and four (1997- includes 0.01 25%) concentrations (see Table 4.1 ) of GA3, KNOj and NaHC03 were used. For each of GA3, KNO, and NaHCO3,20 lots of 1O0 cypselas each from each population in 1996 and 25 lots in 1997 were each placed in a 9 cm diameter glass Petri dish on one layer of Anchor blue germination filter paper initially moistened with 10 ml of one of the prepared solutions or distilled water (control). The cypselas were set to geminate in incubators at 25OC for 14h light, and 1O0C for 1Oh dark. The dishes containing gibberellic acid were topped up with the same concentrations of G& and for sodium bicarbonate with the same concentrations of NaHC03 'but for KNOj distilled .water was added as needed. Light was provided by four cool white Table 4.1 The pH of solutions used as germination media for cypselas of
Onopordum acanthium L. collected in London, Ontario in 1996197.
Solutions Concentration (%, wlv) pH
Gibberellic acid (Gk) 0.0125 3.80
Potassium nitrate (KNOI)
Sodium bicarbonate (NaHCQ) 0.0125
0.025 0.05
O*1
Distilled water (control) - 5.70 fluorescent tubes (PAR of 39.07 pmol photons m-2 s-1 at the belof the Petri dishes, n=30) situated ca. 25 cm above the surfaces of the Petri dishes.
Geminated cypselas (radicle 2 mm or longer) were counted and removed from
Petri dishes every day. The experiments were terminated after a five day period with no germination (after 45 and 65 days in 1996 and 1997, respectively). Firm ungerrninated cypselas at the end of the experiments in 1996 were subjected to a viability test by using a i% (wlv) solution of tetrazolium chloride (2,3,5-triphenyl tetrazolium chloride, CloH15N4CI, Sigma) (method of Delouche et al., 1962). In
1997 viability of ungerminated cypselas was tested by cutting them 1 mm frorn the cotyledonary end and putting them to germinate.
Germination was also tested in solutions with a pH of 3.1 (using H2S04) and a pH of 8.1 (using NaOH) to se0 if enhancing or inhibiting effects of the solutions used were due to the actual chemicals or to the change in pH. Also, a
0.1% solution of Na2C03 (pH of 11.1) was used to see whether NaHCO3 or
Na2C03,both of which can occur naturally in the soil, has more inhibitory effectf.
Cypselas from the ESW population colleded on Sept 6, 1997 were used for these treatments,
4.23 Statisfica/ ananelyses
Final germination percentages for each of the G&, KN03 and NaHCQ experiments were analyzed by rneans of a balanced ANOVA for each year's data
separately (McKenzie et al., 1995). Then, a Tukey's test was used to detemine
differences between treatments (SAS Institut0 Inc., 1982; Sokal and Rohlf, 1995). Before analysis, an arc-sine square root transformation was used to
normalize the variance (Zar, 1984).
The coefficient of germination rate (CGR) was calculated for each replicate by dividing the total percent germination (N) by the number of geminated seeds on the particular day on which count was made (ni) multiplied by the number of days from the start of the experiment (di) and summing for al1 days on which germination occurred:
CGR =-N1 Xnid1
All values of CGR are between O and 1 (Alm et al., 1993). Then, a balanœd
ANOVA and a Tukey's test were applied (SAS Institute Inc., 1982; Sokal and
Rohlf, 1995).
4.3 Results
4.3.1 The efktsof gibberellic acid
Most fresh (1997) and stored (1996) cypselas germinated after G& application, regardless of the concentration used (Table 4.2). Populations (P e 0.001), concentrations (P c 0.001) and their interactions (P < 0.001) had significant effects on germination percentages. For all but one populationlyear combination
(ESW-1997) there was no difference in total percent germination among the different concentrations of G& and in all G& treatments 85-100% of the cypselas germinated. In contrast, cypselas in control treatments had between 4 and 27% germination (1997) and between 42 and 89% germination (1996).
Despite the similar response to al1 concentrations of G& by al1 four 60
Table 4.2 Total germination percentages (mean f SE) of cypselas of Onopordum acanthium, cdlected from four populations in or near London, Ontario in Sept.
1996 and Sept. 1997'.
Year Concentration Population
of G& (%)' ESW Q Ola GP
1996 O(control) 41,8k4.3cb3 64.2i2.08b 53.617.6BCb 89.0I2.3Ab
0.025 96.2i1.2Aa 91.M.OAa 97.611 .OAa 98.0M.6Aab
0.05 96.4e.lABa 90.411.9Ba 96.2I1.6ABa 99.0IO.SAa
0.1 94.0k1.9Aa 90.2i1.7Aa 97.4i1 .Ma 97.4H.8Aab
1 ESW = Environmental Sciences Western, Q = Quarry, Ola = Olalondo Rd. and GP = Gibbons Park. Cypselas wen subjeded to various concentrations of gibberellic acid (w)and incubated et 25OC for 14h light, and 1O°C for 1Oh da* frorn Dec. 22, 1996 to Feb. 05,-1997 (stored) and from Sept. 11 to Oct, 15, 1997 (fresh).
Means followed by different uppercase letters within mws or by different lawer-uise letters within columns for each year are significantly different (P < 0.05) according to Tukey's Honestly Significant Difference multiple range test (Tu key's HS D). populations there were difierences among populations in total percént germination. For example, the GP population had 97.4-100% germination in al1 treatments whereas only 89-93% of the cypselas from the Q population germinated. Viability tests at the end of each experiment revealed that more than
97% of ungeminated cypselas were viable. Thus, there were significant differences among populations in the proportion of viable cypselas that did not respond to G& treatment. The population with the least germination in the control treatment, ESW 1997, was the only population with significant differences in total percent germination among the G& treatments. The 0.0125% G& treatment increased percent germination from 4.4 to 85%; the 0.025% treatment increased it again to 95.2% and the two highest concentrations of G& significantly increased it further to 99.6 and 98.4% total germination.
Significant effects on germination rates were obtained between populations (P c 0.001.), concentrations (P c 0.001) and their interactions (P =
0.004, 1996; P = 0.002, 1997). For the stored cypselas (1996), germination was significantly faster in al1 but one G& treatment (0.025% GA3 - OP) than in the control for the same population (Table 4.3). For the fresh cypselas (1997), germination rates in al1 G& treatments for the ESW population and the 0.025%
~Ctreatmentfor the Ola population were slower then in the control treatment. In ail other cases, there was no difference between the control and any G& treatment nor between any GA3 treatments (concentration) for the same population (Table 4.3).
In general, Q cypselas geminated more rapidly and ESW cypselas more
slowly than those from other populations (Table 4.3). Only for fresh control
cypselas (1 997) was this difference not significant.
4.3.2 The effecfs of potassium nitrate
In general, application of KN03 increased percent germination of cypselas of O.
acanthium (Table 4.4). Populations (P < O.W1), concentrations (P c 0.001) and
their interactions (P c 0.001) had significant effects on germination percentages.
Unlike the pattem of response to GA3, there was an increase in percent germination in al1 populations (except GP in 1996) with an increase in the concentration of KN03 in solution. In comparison to the control, the lowest
concentration, 0.01 25%, did not increase percent germination, nor did it affect the rate of germination.
The other concentrations of KNO3 had a variety of effects on the different
populations in the two years of the experiment. The ESW population, with the
lowest percent germination in the control, had a significant increase in total
percent germination from the 0.025% to the 0.05% treatment (1997), and from
the 0.05% to the 0.1% treatment in both years (Table 4.4). In the 1997 test, the other three populations had large inaeases in percent germination beîween the
0.025 and 0.05% treatments and no difference between the 0.05 and 0.1% treatments. Only the Ola population showed this pattern in 1996 (Table 4.4). The
only population that did not show inqeased total percent germination in response to KN03 was GP in 1996, but that collection had 89% germination of control
cypselas (Table 4.4). Table 4.4 Total germination percentages (mean ISE) of cypselas of Onopordum acanthium, collected from four populations in or near London, Ontario in Sept.
1996 and Sept. 1997'.
Year Concentration Population
of KNO, (%12 ESW Q Ola GP
i996 O(contro1) 41 ,8i4.382 64.M.OBb 53.617.686 89.0a.3Aa
0.025 53.4I2.6Bbc 81.013.5Aab 57.4f7.7Bb 92.21.2Aa
O. OS 66.6S.OBb 82.6e.2ABab 91.4e. IAa 93.0k1.6Aa
0.1 88.N.2Aa 86.M.4Aa 86.013.2Aa 90.22.2Aa
' ESW = Environmental Sciences Western, Q = Quarry, Ola = Olalondo Rd. and OP = Gibbons Park. * Cypselas were subjeded to various concentrations of potassium nitrate (KNOd and incubated at 25OC for 14h light, and 10°C for 10h dark fmm Dec. 22, 1996 to Feb. 05, 1997 (dored) and from Sept. 11 to Od. 15,1997 (fresh).
~eansfollowed by different upper-ease letten within mws or by different lowercase lettecs within columns for each year are significantly different (P < 0.05) according to Tukey's Honestly Sig nificant Difference multiple range test ('ïukey's HSD). Table 4.5 Coefficient of germination rates (mean I SE) of cypselas of Onopordum acanthium from four populations subjected to various concentrations of potassium nitrate (KNOa) and inwbated at 25OC for 14h light, and 10°C for 1Oh
Year Concentration Population of KNOt (%) ESW Q Ola GP 1996 O(contto1) O. 10610.01 OB^' 0,17110.018Aa O, 104îO,013Ba 0.4 74k0,OOSAa 0.025 O.O75~,004Ba 0.158H.01 OAab 0.091M.013Ba 0.143fl.OOBAab 0.05 O, 1OW.011 Ba 0.142B.011ABab O. 108a. W9Ba 0.1GûkO.OO7Aab 0.1 0.097I0.003Aa 0.1 1710.007Ab 0.10110.WSAa 0.1 1610.006Ab
f997 O(control) O. 169îO,OlSABa 0,l97SOOSAa O. 165~.009ABa O. 132M.019Ba 0.01 25 0.162st0.016Aa 0.139M. 03 3ABab 0.117I0.024ABab 0,087f0.011Bab 0.025 0,099f0.015Ab 0,147I0.005Aab 0.104M.015Ab 0.097I0.016Aab 0.05 0.090f0.003ABb 0.134&0.004Ab 0.073fO.Oû6Bb O.û66fl,003Bb 0.1 0,074iû.WSAb 0.116f0.004Ab 0.072H.003A b 0.083I0.W5Ab
' Means idlowed by dlfferent upper-case letterxi within rows or by different Iower-case lenefs within cdumns for each year are dgniflcantly dWTemnt (f < 0.05) accordhg to Tukey's Honestly Significant Difference muitipie range test (Tukey's HSD). Figure 4.1 Cumulative percent germination of cypselas of Onopordum acanthium from the ESW (Environmental Sciences Western), Q (Quarry), Ola (Olalondo
Rd.) and GP (Gibbons Park) populations cdlected in or near London, Ontario in
1997. For al1 populations the KN03 treatments with 0.05% (A), 0.025% (1)and
0% (control (@)) concentrations that were incubated at 25OC for 14h Iight, and
1OoC for 1Oh dark are shown. Final germination percentages not followed by the same letter are significantly different (P < 0.05) according to Tukey's Honestly
Significant Difference multiple range test (Tukey's HSD).
Significant effects on germination rates were obtained behnreen
populations (P < 0.001), concentrations (P < 0.001) and their interactions (P =
0.01 4, 1996; P = 0.046, 1997). There was a trend towards slower germination with the application of KN03, especially at the highest concentration (Table 4.5).
Only the Ola and ESW populations in 1996 failed to show significantly slower germination in the 0.1 % treatment as compared to the control (Table 4.5).
The complexity of responses by the.diflerent populations to different KNQ concentrations is illustrated in Figure 4.1, which portrays the cumulative
germination patterns for the 1997 collections of the four populations given the
control, 0.025% and 0.05% KN03 treatments. Only the 0.05% KN03 treatment
stimulated increased germination of ESW cypselas, but it caused 90%
germination compared to less Vian 10% in the 0.025% KN03 and control
treatments. In contrast, the Q and Ola populations had significant differences
among all treatrnents but the range in total percent germination was much less
than for the ESW population. The GP population resembled the ESW population
in its response to 0.05% KNOa but was similar to the Ola and Q populations for
the other two treatments. Initial germination occurred soonest for the Q
population but the relative rate of germination among treatments varied greatly
among populations (Figure 4.1). In al1 treatments for al1 populations there was
more than 98% viability.
4.3.3 The effecfs of sodium bicarbonate
In general, sodium bicarbonate decreased total germination percentages in both Table 4.6 Total germination percentages (mean ISE) of cypselas of Onopordum acanthium, collected from four populations in or near London, Ontario in Sept.
1996 and Sept. 1997'.
Year Concentration Population
of NaHC03 (%)' ESW Q Ola GP
0.1 37,611.1 BCa 47.2I1 .OBb 33.2I1.2Cbc 62.M.9Ab
1997 O(contro1) 4.4I0.8Ca 26.6I0.7Aab 17.6k1.8Ba 21.2I3.2ABa
0.01 25 3.4iO.8Ca 32.2kl.OAa 14.Oiû.6Bab 17.2k1.78ab
0.025 4.4a.fCa 25.411.1 Aab 11 .&1.2Bbc 11.6a.88bc
' ESW = Environmental Sciences Western. Q = Quarry, Ola = Olalondo Rd. and OP = Gibbons Park. * Cypselas were subjected to various concentrations of wdium bicarbonate (NaHC03) and incubated at 25OC for 14h light, and 10°C for 10h dark fmm Dec. 22, 1996 to Feb. 05, 1997 (stored) and from Sept. 11 to Oct. 15, 1997 (fresh).
Means followed by different upper-case letten within rom or by different lowe~caseletters within columns for each year are significantly different (P c 0.05) according to Tukey's Honestly Signifiant Difference multiple range test (Tukey's HSD). the 1996 and 1997 tests (Table 4.6). Populations (P * 0.001 ), concentrations (P
< 0.001) and their interactions (P c 0.001, 1996; P = 0.002, 1997) had significant effects on germination percentages. However, there was no change Rom the control in the ESW population in either year, nor in the Q population in 1997.
Also, the 0.0125% NaHCQ treatment (1997) did not change total percent germination compared to the control. Most other NaHCOs treatments reduced total percent germination but in al1 1996 treatments at least one third of the cypselas geminated.
In al1 1996 treatments, including the control, the GP population had the highest total percent germination and the ESW population the lowest. In the 1997 treatments the Q population had the highest percent germination and the ESW population the lowest. There was no obvious relationship beîween total percent germination and the effects of sodium bicarbonate.
There was no effect of sodium bicarbonate on the rate of germination of fresh (1 997) cypselas (Table 4.7). However, three populations, excluding GP, had faster germination of stored (1996) cypselas under al1 three sodium bicarbonatœeconcentrations than in the control (Table 4.7).
In 1996, significant effects on germination rates were obtained between populations (P c 0.001), concentrations (P c 0.001) and their interactions (P c
0.001 ). In 1997, significant effects on germination rates were found only between populations (P ( 0.001) but not between concentrations (P = 0.266) nor in the interactions between populations and concentrations (P = 0.518).
There was only one difference between populations in the rate of Table 4.7 Coefficient of germination rate (mean i SE) of cypselas of Onopordum acanthium from four populations subjected to various concentrations of sodium bicarbonate (NaHC03) and incubated at 25OC for 14h Iight, and 10°C for
Year Concentration Population of NaHCO, (%) ESW Q 01a GP 1936 O(contr01) O. 10610.01 O& O, 171I0.018Ac 0.104+0.013Bb 0.174kû.OOSAa. 0.025 O, 175M.W5Ba 0.264iû.013Aa O. 158dXl.01 OBa 0.16310.01 1Ba 0.05 0.173~.003ABa 0,21810, W3Ab 0.167~.004Ba O, 190~.003ABa 0.1 0,171M.004Ba 0.232HI.OO6Aab 0.1 71B.OOSBa O. 186I0.012Ba
~97~(controi) 0.16W.OlSAa 0,197M.008Aa 0.1653O.009Aa 0.132I0.019Aa 0.01 25 0.1 9W.019Aa 0.180M.01 OAa 0.18W.006Aa 0,135I0,Ol 3Aa 0.025 0. 164kO.016Aa 0.176M.014Aa 0.160H.014Aa 0.134I0.014Aa 0.05 0.2~.005Aa 0,175kû.012Aa 0.184I0,OllAa 0.148~.011Aa 0.1 O. 181 M.020ABa 0.196M. 007Aa O, 149î-û. OOGABa 0. 127fO.Oû68a
' Means followed by different upper-case letters wiîhin rows or by dmerent lower-case letters wlthin columns for each year are signiRcantly differenl (P < 0.05) accordhg to Tukey's Honestly Signifmnt Difierence muiüple range test Vukey's HSD). germination of fresh cypselas; under the 0.1 % NaHC03 treatment Q was faster than GP. In contrast, stored (1996) cypselas of the Q population germinated faster in al1 concentrations of NaHC03 and in the control than cypselas of Ola and al1 but the 0.05% NaHC03treatment of ESW.
Table 4.8 shows that there was no significant difference in percent germination among cypselas that germinated at pH=3.1 (27%), at pH=8.1
(26.8%) and those of the control (28.8% in distilled water, pH-5.7) but there was a significant difference between cypselas that germinated in the 0.1% (wlv) solution of Na2C03(1 2.2%, pH=llA) and those of the other treatments.
In al1 treatments, ungenninated cypselas from al1 populations had more than 99% viability.
4.4 Discussion
Cavers et al. (1995) found that some cypselas of Onoponlum sp. that were extracted from the seed bank in Australia germinated immediately in a greenhouse when supplied with adequate moisture and suitable temperatures.
Some geminated after several weeks in the greenhouse but only after the soi1 had been stirred. Some others required stimulation with 0.01 % gibberellic acid and sorne did not gerrninate at al1 but were apparently still dormant and viable at the end of the experiment.
Our results suggest that cypselas of O. acanthium in London, Ontario also
have a range of germination responses, similar to those of Onopordum sp. examined by Cavers et al. (1995) ih Australia. Our results also indicate that the Table '4.8 The effects solutions of different pH on total percent germination
(mean ISE) of cypselas of OnoporPrum acanthium L. collected from the ESW population in London, Ontario on Sept. 6, 1997 and incubated at 25OC for 14h light, and 1O0C for 1Oh dark.
Solution PH Percent germination
Distilled water (control) 5.7
H2So4 3.1
NaOH 8.1
Na2COs 11.1
' Means followed by different lowe~caseletten within columns are signifantly different (P < 0.05) according to Tukey's Honestly Signifiant Difference multiple range test (Tukey's HSD). seed banks in the soi1 will be different for different populations of Scotch üu'stle, even if the populations mature in the same habitats.
4.4.1 GibbereIIic ecid and cypsela germination
Gibberellic acid is a very successful chernical in breaking donancy and promoting germination in seeds (Bewley and Black, 1982). For germination, seeds need gibberellic acid to offset the action of abscisic acid (ABA), a domancy regulator (Hilhorst and Karssen, 1992; Bewley, 1997) that is not resynthesized in mature seeds (Karssen, 1995). Gibberellic acids themselves do not regulate dormancy but their sensitivity is dependent upon light andlor temperature (Derkx and Kansen, 1994).
G&, even at a very low concentration compared to those used by many other scientists (e.g. Perez-Garcia, 1993; Meier, 1995), caused virtually al1 viable cypselas to geninate (Table 4.2). Thus, our findings suggest that for further study a range of concentrations between O and 0.0125% should be used in an attempt to find a linear relationship between the GA3 concentrations and total
percent germination. In general, stored (1996) cypselas that were treated with
G& germinated faster than those from the control, but fresh (1997) cypselas did
not show this pattern (Table 4.3). Faster germination by stored cypselas is
probably due to quick synthesis of G& from precursors, or its rapid transport from one part of the cypselas to another (Mayer and Poljakoff-Mayber, 1989).
In contrast to our results where there were few differences in response to
different concentrations of G&, Chen (1997) showed that 0.05% and 0.1% G& treatments produced significantly higher percent germiriation than the control but a concentration of 0.01% GA3 had no effed on Pitcher's thistle, Cirsium pitcheri.
Also, ferez-Garcia (1993) with Onopordum acanthium and Perez-Garcia and
~uran(1990) with O. nervosum have shown that the 'addition of 2mM G& enhanced the germination of cypselas of O. acanthiom and percent germination increased as G& concentration increased in 0. neNosum. Young and Evans
(1972) showed that addition of GA3 at a concentration they described as '0.14 mmole" to the substrate enhanced cypsela germination in 0. acanthium. Meier
(1995) showed that cypselas of Scotch thistle had increased total percent germination in a 0.1% concentration of G& but not at 0.01 %, which does not correspond to our results. These differences can be explained through differences in dates of collection. We used cypselas that were collected in early
September while Meier collected her samples in early and late October. In another experiment, we found that cypselas collected later in the growing season had stronger dorrnancy. It is probable that Meier's cypselas had stronger donancy and hence did not respond to G& treatment. In her study both populations (ESW and Q) gave less than 65 percent germination with the application of 0.1 % GA3, while in our study with the same concentration al1 populations had more than 90 percent germination.
Our findings contradict the results obtained by Scifres and McCarty
(1969). None of the concentrations of GAI (0, 8, 15, 31, 63, 125 or 250 ppm) that they tested stimulated germination of nonstratified cypselas but stratified ones were sensitive to GA3 treatment. In our study, GA3 always had enhancing effects on nonstratified cypselas, either stored or fresh. The differences may have resulted from the environmental conditions under which the cypselas matured (Nebraska, USA versus London, Canada) or from genetic differences.
In our tests it was only treatments containing G&, not al1 of thos8 at lower pH, that promoted germination in cypselas of Scotch thistle (Tables 4.1,4.8).
Derkx and Karssen (1994) found that seeds of Arabidopsis thaliana were dependent on exogenous GA'S for germination. Their mutants were not able to synthesire GAs and did not germinate unless treated with 0.1% GA The resulto of Scifres and McCarty (1969) support our findings regarding pH. In their experiment with pH treatments ranging from 3 to 8, germination percentages affer 10 days were between 70 and 80 percent. Neither ow experiments nor those of Scifres and McCarty (1969) had buffered solutions in pH studies. In a preliminary experiment Scifres and McCarty (1969) found that a salt effect from their buffers inhibited germination of 0. acanthium.
Populations responded differentiy to GAJ treatments. Obviously many cypselas in populations with strong dormancy (ESW and Ola) were triggered to geninate by GA3 (Table 4.2). Comparatively, there were fewer differences among populations in total percent germination with stored (1996) cypselas than with fresh (1997) ones (Table 4.2). It seems that storage, even at room temperatures (20-25OC), can reduœ differences among populations in their response to.G&. Our findings agree with Meier's (1995) results. In both studies, more cypselas in the Scotch thistle population with stronger dormancy responded to 0.1% G& than in the one with weaker dormancy. There were more differences among populations in the rate of germination than in total percent germination (Table 4.3). If we compare our results for total percent germination to those of other scientists, the differences among different concentrations 'of G& that they obtained can be explained through the shorter pefiod of time that they ran their experirnents. We ran our experiment over a longer period, differences among populations became less and less over tirne, due to the slower germination pattern of some populations.
4.4.2 Potassium nitrate and cypsela germination
Nitrates, especially potassium nitrate and related compounds, can replace the light requirement for germination in many positively photoblastic seeds (Evenati,
1956; Toole et al., 1956; Roberts, 1973; Rorison, 1973; Bewley and Black, 1994).
In field experiments, the density of O. acanthium was increased by the addition of nitrogenous fertilizers (Michael, 1970).
In Kolk's (1962) experiment, application of 0.2% KNOj reduced the domancy of seeds of Rumex crispus, Matricana inodora, Sinapis arvensis,
Chenopodium album and Thlaspi arvense, but did not increase the rate of germination of any of these species. Popay and Roberts (1970a) found that concentrations of 10" and IO-* M KNO~increased percent germination in
Capsella bursa-pastoris seeds compared to a concentration of 10~M KNOJ which did not differ from a distilled water control. In a greenhouse experiment in pots, using O, 100,200,350 or 500 mM KN03, Adkins and Adkins (1994) showed that an increase in the KN03 concentration from 100 to 200 mM increased 78 seedling emergence of Avena fatua. However, as the concentration was increased to 500mM effects on emergence were less pronounced because of reduced seed viability.
Young and Evans (1972) found that in O. acanthium addition of '1 .O rnmole" KNO~to the substrate, which was accepted as an optimum concentration, promoted germination while a lower concentration, '0.1 mmole', was not effective and a higher one, '10 mmole", depressed germination.
We found the same pattern of germination for 0. acanthium as found in other species by Kolk (1962), Popay and Roberts (1970a), Adkins and Adkins
(1994) and in O. acanthium by Young and Evans (1972). Our findings showed that in general th8 higher the concentration of KN03 the higher was the percent germination, but the rate of gemination was slower for KN03treatments than the control. Although the highest concentration (0.1% KN03 did not deaease percent germination, only one of our populations (ESW) had higher percent germination than in the 0.05% KNOJ treatment.
As show for the caryopses of Avena fatua, KN03 directly affects the respiratory system (Adkins et al., 1984). In some cases, nitrate can provide a gap-detection mechanism (Pons 1989), or ad as a cofactor to phytochrome action and the combined effects of the two can lead to gibberellin synthesis
(Mayer and Poljakoff-Mayber, 1989). It has also been postulated that KN03 might affect various kinds of seeds differently Ri connection with the breaking of domancy. Either the living parts of the seed may be influenced or the permeability of seed coverings to gaseow exchange may be increased by an interaction between KN03 and one or more compounds which block gaseous passage through the coverings (Kolk, 1962).
PH values in Petri dishes at the end of the experiment changed by less than 0.2. We concluded that there was no effect of pH changes in the results of our KN03experiments.
4.4.3 Sodium bicarbonate and cypsela germination
Alkali soils are those with enough sodium salts, especially sodium carbonate
(Na&03) and sodium bicarbonate (NaHC03) to raise the pH above 8.5. Sodium ions can cause dispersion of clays and make soils puddle and more slowly permeable (Thompson, 1957). The direct effect of NaHC03 on seed germination was likely a result of the reduction in seed respiration caused by CO2reducing Oz uptake (Corbineau and Côme, 1995).
For stored (1996) cypselas, except for GP, those in the NaHC03 treatments geninated more rapidly than in the control, but there was no significant difference in germination rate for fresh (1997) cypselas (Table 4.7).
This cmbe explained by the strong germination potential of stored cypselas. In this case, for some cypselas the inhibitory effeds of NaHCO3 were overmme and the cypselas eventually germinated.
NaHC03 caused inter- and intrapopulation differences. In general, there was a deaease in total percent germination as the concentration of sodium bicarbonate increased (Table 4.6). However, a significant decrease was only recorded for the populations with more gerrninability (GP, Q and Ola) but not for ESW, a population with strong donnancy. This result was found for bath fresh
(1997) and stored (1996) cypselas. It is logical that cypselas with strong domancy (e.g. ESW) would be afiected less than cypselas with weak domancy, since germinable cypselas would be inhibited but not ungerminable ones.
4.4.4 Ecological implications
In a number of species, seeds in the soi1 undergo an annual cycle of dormancy and non-donancy. The occurrence-of these cycles shows that there are always physiological changes in the state of seeds from dormant to nondomant and vice-versa (Fenner, 1985). Changes in seed dormancy can be related to the seed's microenvironment where gases (O2 and CO2), moisture, temperature and nitrate ions may have the ability to promote or inhibit the synthesis of gibberellins from precursors (Mayer and Poljakoff-Mayber, 1989).
An important factor for germination of seeds in soi1 is nitrate ion concentration (Steinbauer and Grigsby, 1957). When the soi1 temperature is high and the moisture percentage is low the level of nitrate rises. However, best germination occurs when the soi1 has a moderate level of nitrate ions, high percent moisture and higher day and night temperatures (Popay and Roberts,
1970b). The nitrate concentration of the soi1 increases in spring (Russell, 1961).
The levels of Oz and CO2 in the soi1 are also important for germination. In general, the greater the soi1 depth, the higher is the CO2 level (Popay and
Roberts, 1970a). CO2 in the soi1 can be dissolved and form different wrnpounds such as Na2C03and NaHC03, which in tum rnay enhance or inhibit germination, depending on the species. Sodium bicarbonate, which was used in this study, is found naturally in soils with alkaline pH levels. The higher the level of NaHC03 in the soi1 the higher was the pH and the lower was the germination percentage for
O. acanthium. As show in Appendix I and Table 4.1, al1 collection sites had pH values sirnilar to those used for the NaHC03 treatments. The possession of a variety of germination rates is of great advantage for populations of O. acanthium. Cypselas that gemiinate fast can establish a sizable seedling population in a few days under favorable conditions. Such seedlings often cm survive subsequent environmental problems whereas later-emerging seedlings at a younger phase of development will die. However, if under unexpectedly harsh conditions al1 emerged seedlings die, there are still some cypselas emerging later that can replace Vie dead ones, fiIl the gaps and perpetuate the populations. This is one reason for the success of Scotch thistle in many parts of the world.
4.4.5 Variation in response fo chemicals emong populations The most important point arising from mis work is that the vast majority of studies of G& or KNOâ have been dom with single populations of a species and conclusions based on one population cannot cbaracterize an entire species.
Here, we show that even populations wlleded from very close proximity to each other can Vary dramatically in th& responçe*tothese chemicals; in the case of
KN03 they Vary more than different species did in some earlier studies (e.g. Kolk,
1962).
The interpopulation variance coùld anse because of environmental or genetic factors or both. Cypselas ficim two populations studied (GP and ESW-
1997) came from plants transplanted to the Field Station in 1996. These plants grew there for more than one year before producing cypselas. Differences in germination patterns in these two populations must have resulted from genetic differences andlof differential sensitivity to environmental conditions between the populations.
We believe that this is the first study in which as many as four populations of a species were each subjected to several different concentrations of both G& and KNO3. It is also the first one that we know of to investigate the role of sodium bicarbonate in the germination processes of a seed population.
4.5 Refetences
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Hilhorst, H.W.M. and Karssen, C.M. (1989) The role of light and nitrate in seed germination. pp 191-205 in Taylorson, R.B. (Ed.) Recent advances in the development and germination of seeds. New York, Plenum Press.
Hilhorst, H.W.M. and Kansen, C.M. (1992) Seed domancy and germination: the role of abscisic acid and gibberellins and the importance of hormone mutants. Plant Growth Regulation 11,225-238.
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Karssen, C.M. (1995) Hormonal-regulation of seed development, domancy, and germination studied by genetic control. pp 333-350 in Kigel, J. and Galili, G. (Eds) Seed development and germination. New York, Maml Dekker, Inc. Karssen, CI. and Hilhorst, H.W.M. (1992) Effect of chernical environment on seed germination. pp 327-348 in Fenner, M. (Ed.) Seeds: the ecdogy of regeneration in plant mmunities. Wallingford, U K, CAB NTERNATION AL.
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Popay, A.I. and Roberts, EH. (1970b) Ecology of Capsella bursa-pastoris (L.) Medik and Senecio vulgans L. in relation to germination behaviour. Journal of E~ology58,123-1 39.
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5.1 Introduction
Many researchers who study the germination patterns of weed species use only
one population of each species for their investigations; hence they have Me
information about the variation that occurs between populations within a species
(Christal et al., 1998).
There have been several recent studies that report variation in
germination responses of different populations within a species (Andersson and
Milberg, 1998). Often, these differences are closely correlated with the habitats
in which the populations occur. For example, Beckstead et al. (1996) found that
populations of Bromus tectorum from five distinct geographic areas in Western
North America had large between-population differences in germination,
particularly for recently harvested seeds.
Differences between populations can be created, at least in part, by non-
- genetic factors, especially the matemal environment during seed maturation
(Andersson and Milberg, 1998; Fenner, 1991). Other authors have stressed the
impact of genetic differences among populations. Christal et al. (1 998) compared
5.1 A version of this chapter has been submitted for publication in the Canadian Journal of Botany, April21,1998. the germination of 18 British populations of Chenopodium album under Wo contrasting germination regimes. All of their populations, from widely separated sites in Britain, were grown in a common garden. Seeds for their experiment were taken frorn the common garden plants. Despite this procedure designed to remove effects of the matemal environment, there were many differences in germination behaviour among populations.
Relatively few scientists have compared germination for seeds from different local populations of the same species. Perez-Garcia et al. (1995) obtained great differences in percent germination among three populations of
Diplotaxis virgata that were set to geminate under five contrasting germination regimes. AH of these populations had been collected from the province of
Madrid, Spain. McWilliams et al. (1968) also obtained large differences in some germination regimes frorn seeds of Ameranthus retroflexus collected from different sites within the same city. Local populations should experience very similar climates for seed production but their sites could differ in sail fertility, drainage, shading and other environmental factors. There could also be genetic difierences among populations.
We chose to work with Scotch thistle, Onopordum acanthium L., a constituent of the flora of southem Ontario for more than 100 yean (herbarium records- DAO, CAN). Despite its long existence as a constituent of the local flora, 0. acanthium has never been abundant, being restricted to riverbanks, abandoned gravel pits and waste places. O. acanthium is a biennial or short-lived monocarpic perennial. Flowering plants produce ripe cypselas from late July or early August until late October; with large numbers of cypselas being shed in late August and early September.
Fresh cypselas can germinate and emerge from mid summer (early August) until late autumn (November) of the same growing season (Meier, 1995; P. Cavers, unpublished data). Obviously, these cypselas will have been exposed to a wide range of temperature regimes from the earliest to the latest dates of emergence.
In a current wntinuing experiment (M. Qaderi and P. Cavers, unpublished data), up to 10% of cypselas wllected as soon as they were ripe and then immediately buried 3 cm deep in either sand or silt loam soil, germinated and emerged as seedlings in the same growing season. Some cypselas left on the soi1 surface after dispersa1 also germinated and emerged but the totals were fewer than 1%. Seedling emergence occurred over a prolonged period; from
August until late October. There were differences between cypselas from two local populations, one from a grave1 pit and one that originated on waste ground within the flood plain of the Tharnes River.
The purpose of this study was to record germination responses of recently ripened cypselas of O. acanthium under a range of temperature and light regimes such as might be experienced either on or just under the soi1 surface between mid-summer and late autumn in London, Ontario. A second aim was to determine the amount of variation in response of cypselas of four local
populations to common light and temperature regimes. 5.2 Materials and Methods
5.2.1 Cypsela collection
In September 1996 bulk collections of ripe cypselas (= seeds) of Scotch thistle,
O. acanthium L., were taken from 100 plants in each of four populations (ESW,
Q, Ola, GP) in or near London, Ontario (Appendix 1). The parent plants from populations Q and Ola were naturally occurring while those of ESW and GP had been transplanted to the Environmental Sciences Western Field Station (ESW) of the University of Western Ontario at the rosette stage (in the spring of 1996) since they would have besn destroyed in their original habitats. Shortly after collection, the cypselas were cleaned manually and stored dry in the laboratory
(20-25°C) until they were used (October 24, 1996). Aborted cypselas were discarded.
5.22 Germination tests
For each population there were seven light and temperature treatments as follows: 35OC light 14h, 20°C dark 10h; 25OC light 14h, 10°C dark 1Oh; lO0C light
14h, SOC dark 1Oh; 35'C 14h, 20°C 1Oh, continuous dark; 25OC 14h, 10°C 1Oh, continuous dark; 1O0C 14h, 5'C IOh, continuous da*; constant 20°C, continuous dark. For illumination the light source consisted of four cool white fluorescent tubes (mean PAR of 86.3, 89.7 and 51.3 pmol photons m 2 s' t at the level of the
Petri dishes for 35/20, 25/10 and 1015°C, respectively, n=lO). A separate experiment showed that there was no difference in germination response for cypselas exposed to PAR values from 35 to 105 pmol photons m-2 s-1 at the level of the Petri dishes. There were five replicates per treatment for a total of 35 Petri dishes per population.
In each replicate Petri dish (9 cm diameter) 100 cypselas were placed on one layer of Anchor blue germination filter paper and moistened with 10 ml of distilled water. Water was added each day as needed. Germinated cypselas were counted and removed every day for light treatments, but the kntinuous dark treatments, in sealed metal canisters, were not opened until the end of the experiment. Protrusion of the radicle 2 mm or more was accepted as germination. The experiment was terminated after five days with no germination.
Ungerminated cypselas were subjected to a viability test using a 1 % (wlv) solution of tetrazolium chloride (method of Delouche et al., 1962).
5.2.3 Sta tistkal analyses
Final germination percentages were analyzed by means of a balanced ANOVA
(McKenzie et al., 1995), after an arc-sine square root transformation to nomalite the variance (Zar, 1984). Then, a Tukey's test was used to determine differences between treatments (SAS lnstitute Inc., 1982; Sokal and Rohlf,
1995).
For results from al1 lightldark treatments, the coefficient of germination rate (CGR) was calculated for each replicate by dividing the total percent germination (N) by the number of geminated cypselas on the particular day on which count was made (ni) muitiplied by the number of days from the start of the experiment (di) and summing for al1 days on which germination occurred: CGR = N 1 Cnidi
All values of CGR are between O and 1 (Alm et al., 1993). Then, a balanced
ANOVA and a Tukey's test were applied (SAS lnstitute Inc., 1982; McKenzie et al., 1995; Sokal and Rohlf, 1995).
5.3 Results
5.3. f Temperature eflects
The temperature at which cypselas were incubated strongly affected the germination pattern. Populations (P < 0.001), temperatures (P c 0.001), the two- way interactions between population x temperature (P c 0.001), population x light (P = 0.001), temperature x light (P < 0.001) and the three-way (P < 0.001) interaction had significant effects on germination percentages.
The largest proportion of cypselas geminated in the 35/20°C treatment
(Table 5.1). There was significantly less germination in total at 25/10°C, but at least 35% of cypselas geninated from every population. In contrast, there was less than 8% germination from any population at a constant 20°C, and less than
1.5% germination from any population at 1015*C (Table 5.1).
The rate of germination for al1 populations was significantly faster at
25/10°C than at 35/20°C or the 1015°C (P c O.OOl)(Table 5.2). There were differences in rate of germination among populations at 25/10°C but not at
35/20°C or 1015'C (Table 5.2). Table 5.1 Total germination percentages (mean f SE) of cypselas of
Onopordum acanthium, collected frorn four populations in London, Ontario in
Septernber 1996'.
~empenture~Light Populations Treat. ESW Q. Ola GP
' ESW = Envimnmental Sciences Western, Q = Quarry and Ola = Olalondo Rd. ' Cypselas were incubated at seven temperature and light regirnes (35120, 25/10, 10e; 14h light at the higher temperature, 1Oh dark at the lower temperature; the same three altemating temperature regimes in constant daik; and 20°C, constant da&) hmOd. 24,1996 to Feb. 12, 1997.
Means followed by different upper-case leners within nm, or by different lowet-case letters within columns are significantly different (P < 0.05) according to Tukey's Honestly Signifiant Difference multiple range test vukey's HSD). 'Altemating light and daric (14MlOh) Continuous darkness Table 5.2 Coefficient of Germination Rate (CGR) + SE for cypselas of four populations of Onoporduin acanthium germinated under three ternperature regimes, each with 14h light at the higher temperature and 10h dark at the lower temperature.
Temperature Population
ESW Q Ola GP
' Means followed by different upper-case letten wnhin mws or by-different lower-case lettea within columns are significantly different (P < 0.05) accoding to Tukey's Honestly Signiflcant Difference multiple range test (Tukey's HSD). 5.3.2 Light - temperature intemctions The overall effect of light on total percent germination was significant (P <
0.001). At 35/20°C, there was a significant difference for the GP population only
(Table 5.1), where germination in the dark was significantly higher than in light.
In contrast, at 25110°C at least three times as many cypselas (in total) germinated in light than in dark (Table 5.1). In the Ola population 13 times more cypselas germinated in light than in dark.
5.3.3 DiHemnt populations
Cypselas from GP gave the highest and those from ESW the lowest percent germination (Table 5.1 ). Arnong populations and between lightldark treatments, cypselas from GP germinated to the highest percentage (95.4 k 0.68) at
35120°C, and ESW to the lowest percentage (O) a! 10/5°C (Table 5.1). At 1015°C, there were no significant differences in germination percentage in light or dark, either within or between populations.
Under alternating light and darkness, al1 populations except Ola (Fig.
5.1A-O) had the highest percent germination at 25/10°C, while Ola had higher percent germination at 35I2O0C. Under both temperature regimes, on day 61 cypselas from Ola had almost equal percent germination (67.2%) at 25/10°C and at 35120°C (66.1%). After this date, there was a gradua1 increase in percent germination at 35120°C, but no increase at 25/10°C (Fig. 5.1 C). At 35120°C, three populations germinated to higher percentage in darkness than in light (Table
5.1 ). Figure 5.1 Cumulative percent germination of cypselas of Onopordum acanthium from (A) the ESW (Environmental Sciences Western) population, (6) the Q (Quarry) population, (C) the Ola (Olalondo Rd.) population, and (D) the
GP (Gibbons Park) population, incubated under 35/20 (O) and 25/10°C (@); 14h light, 10h dark. Final germination percentages not followed by the same letter are significantly different (P < 0.05) according to Tukey's Honestly Significant
Difference multiple range test (Tukey's HSD).
At constant 20°C, total germination percentages were very low for al1 populations. The GP, Q and ESW populations germinated to significantly higher percentage than did the Ola (P 0.05) (Table 5.1).
Cypselas from al1 populations had higher rates of germination at 25/10°C than at 35/20°C (Table 5.2). At 35/20°C there was no significant difference in rate among populations but at 25/10°C Q and GP germinated significantly faster
(P < 0.001) than ESW and Ola. Over the first five days, the germination rate was fastest for the Q population at 25/10°C, and slowest for the ESW population at
35/20°C (Fig.B. 1A-B).
In every light and dark treatment, ungerminated cypselas from al1 populations had more than 99% viability.
5.4 Discussion
5.4.1 The Impact of different femperafure regimes
Seeds from a large number of species need altemating temperatures to germinate (Bradbeer, 1988). For many species the highest percent germination occurs in light at fluctuating temperatures (Bewley and Black, 1982), and this is true for 0.acanfhium at 25110°C (Table 5.1).
The minimal (QI)germination recorded at 10/5*C indicates that such lower temperatures are not suitable for germination of most cypselas of O. acanthium, even though other germination requirements such as light and moisture are provided (Table 5.1). Nevertheless, the few cypselas that germinate at low temperatures could contribute to a natural population, since some seedlings of O. acanthium emerging in late October or early November in
London, Ontario survive the winter (P. Caven, unpublished data).
At 35/20°C the interior of the Petri dishes becarne warmer than the air in the incubator outside the Petfi dishes (37.4OC inside; 35.3OC outside). This result did not occur inside the dishes that were kept in the dark. The higher percent germination in darkness at these temperatures indicates that the slightly lower daytime temperatures in the dark treatment were more favorable for germination.
Also, during the experiment, water often evaporated and condensed on the lower surface of Petri dish lids in this treatment. Thus, when less water was available
(or water supplies fluctuated), fewer cypselas were able to germinate. This result
suggests that during the summer when the temperature on the soi1 surface rises,
soi1 moisture decreases, and wnsequently very few or sometimes no cypselas
germinate. Results from an outdoor seed bank study agree with this (M. Qaderi,
unpublished data). Similar results at high temperatures were obtained by Groves
and Kaye (1989) using fresh cypselas of Onopordum aff. illyncum. Their findings
correspond closely with the results obtained at 2511O0C from the Q, Ola and
ESW populations (Table 5.1).
The rate of germination was significantly faster at 2511 O°C than at 35/20°C
(P < 0.001) (Table 5.2). This result may help to explain patterns of seedling
emergence in our clirnate. During spring, when the diumal temperatures are
around 25I1O0C and other germination requirements are met, flushes of
seedlings arise within a-few days. However, during mid-summer when the soi1
surface temperature goes up to 40°C or above, the number of seedlings appearing is less and emergence is slow. In summer, favorable conditions do not last as long as the period that we provided, thus germination of cypselas is usually interrupted by lack of moisture or some other factor. Also, it appears that the 25/10°C regime is closer to the optimum for this species, based on the definition of optimum as the temperature regime under which germination is most rapid and most wmplete (Wilsie, 1962).
5.4.2 The role of light
A light requirement is an obvious adaptation to stimulate germination and enable seeds to germinate in open habitats. Germination of seeds of many species that do not have a strong light requirement for germination can be promoted by fluctuating temperatures in continuous darkness (Fenner, 1985).
Cornparison of overall mean germination percentages under altemating temperatures with light (36.5) versus the same temperature regime and continuous darkness (26.5) indicates that light is necessary for the germination of some cypselas of 0. acanthium. In fad, under favorable conditions of temperature and moisture, percent germination is higher in spring if cypselas of this species are exposed to full sun (M. Qaderi, unpublished data). Of 112 herbaceous species investigated by Thompson and Grime (1983), 46 had germinated to higher percentages in alternating temperature treatments under light conditions than in constant darkness with the same temperature regimes.
At 20°C in the dark, few cypselas of O. acanthium geminated (Table
5.1). Although by comparison to the 25110 and 35/20°C treatments, these numben are very low, they are higher than in the 1015°C treatment (Table 5.1). It seems that even though constant temperature and continuous darkness do not provide suitable conditions for germination of most cypselas of this species, a substantial number will germinate under temperatures found deep in the soil, where cypselas that geninate will die before they reach the surface. Scifres and
McCarty (1 969) found that under constant temperatures, nonstratified cypselas of Scotch thistle had higher percent germination at 25 and 28OC than at higher or lower constant temperatures. They obtained ca. 5% germination at 15'C, which is similar to the results we obtained at 20°C (Table 5.1).
5.4.3 Four populations from one local area
Some cypselas collected from al1 four local populations genninated. Germination at 25110°C was always the most rapid. In contrast, germination patterns at
35/20°C were slow and prolonged, continuing throughout the 111 day germination period. Also, there was very little germination from any population in the constant 20°C or 1015°C treatments.
Nevertheless, there were also several striking and significant differences in germination response among the populations. The GP population had the highest percent germination in five of the seven regimes. In two treatments, 90% or more of the cypselas germinated, suggesting that under certain conditions only a srnall persistent seed bank would forni. In light at 35/20°C, the Ola population had a much higher germination percentage than any other population. The Q population began to geminate most rapidly after the start of incubation. The highest percentage of cypselas remaining dormant at 35/20°C was in the ESW population. Sinœ cypselas of the four populations matured within a few km of each other and plants of the ESW and GP populations both ripened in field plots at ESW, there is a strong indication that the observed differences in germination behaviour were genetic, at least in part.
When plants from two populations (ESW and Q) were grown under the same condition (ESW, Field Station) differences in germination pattern between these populations were due to both genetic and environmental factors. Cypselas from Q population that were produced at the Field Station had lower percent germination in test at 25/10°C than cypselas of Q that ripened in their natural habitat. Nevertheless, the former cypselas still had higher percent germination than. those from the ESW population that ripened in the same site (M. Qaderi, unpublished data). Genetic differences in germination arnong populations of a single species have seldom been documented (Christal et a1.,1998). However, the same authors have clearly shown that the differences they obtained in germination among populations were genetic in origin.
Baskin and Baskin (1973) summarized many studies of donancy and germination responses for different populations of a species collected from wide altitudinal and geographic ranges. In many of these -studies, there were significant differences in germination andor domancy-breaking requifements. In this and another paper (Baskin and Baskin, 1975) they concluded that the preconditioning environment during seed maturation could modify any natural genetic differences among populations in germination and domancy behaviour. Collections from the same. population of Amnaria patula var. robusta in three successive years at the same site yielded significant differences in germination tests (Baskin and Baskin, 1975), which the authors ascribed to differences in the precipitation patterns prior to seed maturity.
Most populations of O. acanthium from London, Ontario occur in or near gravel pits or on gravelly soi1 along the Thames River. Two of our populations,
Ola and Q came from gravel pits, while the ESW population originated in a field beside the Thames River in London. The GP population also came from a gravel soi1 beside the Thames, but its substrate had largely been brought to the site from an unknown source area by a company that was attempting to stabilize the river bank. There is no evidence from our results of a consistent difference in germination behaviour between populations from gravel pits and those from river banks.
Differences in germination behaviour among local populations could allow a species to exploit a variety of local habitats and could also be of value in areas where the local climate varies markedly from year to year. One year the environment would be favorable for one population and another year for another.
Variation amongst local populations gives a species a greater range of responses and the opportunity to survive under changeable conditions.
5.5 References
Alm, D.M., Stoller, E.W. and Wax, LM. (1993) An index model for predicting seed germination and ernergence rates. Weed Technoogy 7,560-569. Andersson, Lmand Milberg, P. (1998) Variation in seed domancy arnong mother plants, populations and years of seed collection. Seed Science Research 8,29-38.
Baskin, J.M. and Baskin, C.C. (1973) Plant population diffefences in dorrnancy and germination characteristics of seeds: Heredity or environment? Amencan Midland Naturalist 90,493498.
Baskin, JoM. and Baskin, C.C. (1975) Year-to-year variation in the germination of freshly harvested seeds of Arenana patula var. robusta from the same site. Journal of the Tennessee Academy of Science 50,106-1 08.
Beckstead, J., Meyer, S.E. and Allen, P.S. (1996) Bromus tectorum seed germination: between-population and between-year variation. Canadian Journal of Botany 74,875882.
Bewley, J.D. and Black, M. (1982) Physiology and biochemist~of seeds. 2. viability, dormancy, and environmental control. BerlinlHeidel berg , Spr inger- Verlag. 375 pp.
Bradbeer, J.W. (1988) Seed dormancy and germination. New York, Chapman and Hall. 145 pp.
Christal, Ao, Davies, D.H.K. and van Gardingen, P.R (1998) The germination ecology of Chenopodium album populations. Aspects of Applied Biology 51,127-1 34.
Delouche, J.C., Still, T.W., Raspet, M. and Lienhard, Mo (1962) The tetrazolium test for seed viability. Technical Bulletin, Mississ@pi Agficultural Expetifnent Station 5l,l-64.
Fenner, M. (1985) Seed ecology. London, Chapman and Hall, 1SI pp.
Fenner, M. (1991) The effects of the parent environment on seed germinability. Seed Science Research 1,7584.
Groves, RoHm and Kaye, PoEm (1989) Germination and phenology of seven introduced thistle species in southem Australia. Australian Journal of Botany 37,351-359.
McKenzie, Je, Schaefer, RL. and Farber, E. (1995) The student edition of Minitab for Windows. (2nd edition) Reading, Massachusetts, Addison- Wesley. McWilliams, E.L., Landen, RaQ. and Mahlstede, J.P. (1968) Variation in seed weight and germination in populations of Amaranthus retroflexus L. Ecdogy 49,290-296.
Meier, L.R. (1995) Variation in seeds of Onopordum acanthium. MSc Thesis. Department of Plant Sciences, University of Western Ontario, London, Ontario, Canada. 113 pp.
Perez-Garcia, Fa, Idondo, J.M. and Martinez-Laborde, J.B. (1995) Germination behaviour in seeds of Diplotaxis erucoides and D. virgsta.
Weed Research 35,495502. '
SAS Institut0 Inc. (1 982) SAS user's guide: statistics. North Carolina, Cary. 584 PP*
Scifres, C.J. and McCarty, M.K. (1 969) Some factors affecting germination and seedling growth of Scotch thistle. Research Bulletin, Nebraska Agricultural Ekpenment Station 228, 1 -29.
Sokal, RmR and Rohlf, F.J. (1995) Biometry. The pdnciples and practice of statistics in biology research. (3rd edition) New York, W.H. Freeman and Company. 887 pp.
Thompson, K. and Grime, J.P. (1983) A comparative study of germination responses to diumally-fluctuating temperatures. Journal of Applied Emlogy 20, 141-156.
Wilsie, C.P. (1962) C~opadaptation and distribution. San Francisco, W.H. Freeman and Company. 448 pp.
Zar, J. Hm(1 984) Biostatisticai anaîysis. (2nd edit ion) hglewood Cliffs, New Jersey, Prentice-Hall Inc. 718 pp. CHAPTER 6. GENERAL DISCUSSION
variation in germination pattern not only occurs between species, but also within species (Fenner, A985), within populations or even within plants (Caven and
Steel, 1984). This vanability allows seeds to germinate intemittently over days, months or years, which is an advantage br most weed species because it prevents them from being eradicated by long-lasting control mechanisms
(Harper, 1959). Variability in germination pattern or change in the 'germination windoM is caused by both pre- and postdispersal environmental factors
(Gutterman, 1980181; Fenner, 1991). Mile seeds are on the mother plant, they experience environmental factors such as temperature, light, drought and moisture and nutrients that affect their subsequent germination responses
(Fenner, 1991). After seed dispersal, both the chernical environment (0.g. concentrations of water and oxygen andlor noxious inhibitors) and the physical environment (e.g. temperature and light) detemine when and where a seed will genninate (Bewley and Black, 1994). Changing the environment under which seeds mature or in which seeds geminate can change the germination window.
8.1 The higher the maturation temperature the wider the germination window
A small change in the environment affects the range of conditions under which a seed may germinate (the germination window). Previous studies have shown that in Onopordum acanthium, germination percentages gradually deaease with collections made later in the growing season (Meier, 1995). In my experiment, plants grown under two conditions, greenhouse and field, showed that change in the germination patterns was mainly related to the temperature under which cypselas matured. In general, cypselas from al1 five populations that matured under warmer temperatures (e.g. greenhouse) had higher percent germination than those that matured under cooler temperatures (e.g. field). There is no doubt that in London, Ontario air temperature, in general, decreases from early to late in the ripening season. However, if the air temperature happens to go up late in the season (e.g. late Septernber or early October), the cypselas that mature under this temperature obviously will have more geminability than t hose t hat mature early in the season but under cooler conditions.
6.2 Changes in the germination window by chemicals
A seeds chemical environment has an important role in detenining the range of the germination window (Fenner, 1985). This environment can be intemal (e.g. gibberellic aa'd) or extemal (e.g. potassium nitrate and sodium bicarbonate). The effects of exogenous application of these and other chemicals have been seen on Onopordum species including O. acanthium (Scifres and McCarty, 1969;
Young and Evans, 1972; Perez-Garcia and Duran, 1990; Campbell et al., 1991).
Work with Scotch thistle in this study showed that G& and MO3,in general, had
a widening effect on the germination window, while NaHC03had almost no effed
or a narrowing effect on some populations. It is parüculerly important that al1 the
populations used in these experiments responded differently to these chemicals. 1O9
The information obtained frorn these experiments shows that populations with strong dormancy such as ESW were more sensitive to G& and KN03 than those with lesser domancy such as GP. It seems clear that in do~antpopulations, parent plants do not synthesize enough gibberellic acid or reserve nitrogenous compounds during maturation for later use in triggering the production of enzymes that are responsible for resource mobilization and embryo elongation, respective1y (Black, 1972).
6.3 A wider germination window at higher germination temperatures
The germination window can be changed, not only by maturation temperatures and the chernical environment, but also by the physical environment (e.g. temperature and light) under which a seed geminates. Fluctuations in temperature and light conditions are important in widening or narrowing the germination window (Kansen, 1982). Results from my experiment reveal that
light has a determining role in the germination pattern at 25/10°C or lower, but
not at 35î20°C. One possibility is that in this species the effect of light can be
replaced by higher fluctuating temperatures (35/#C) from which higher percent
germination was obtained under dark conditions. A second possibility is that
cypselas in the light at 35/20°C absorb light energy and become warmer than
cypselas in the same temperature regime in the dark The higher temperature in
the light is less suitable for germination. Less germination at 20% than at
2511O0C (daylnight average 18.75 OC) indicates that temperature fluctuations are
important for the germination of cypselas of 0. acanthium. Higher percent germination under light at 25/10°C and in the dark at 35/20°C suggests that in nature in London, Ontario the optimum soi1 temperature for germination of cypselas of Scotch thistle that are exposed to light ocairs in spring and fall, but there is also some germination in summer. Very little or no germination at 1015°C suggests that germination hardly ever occurs in late fall, winter or early spring when the soi1 temperature is typically around 10°C or less. In this expefiment the response of different populations to different temperatur.e and light regimes suggests that every single population has its own germination pattern, which is different from others, and this individuality has both genetic and environmental aspects.
6.4 Concluding remarks
Intermittent germination is an important feature in the success of Scotch thistle.
The results obtained from this research show that intermittency in this species decreases if cypselas mature under wamer temperatures. The positive response of cypselas to gibberellic acid and potassium nitrate and the neutral or negative response to sodium bicarbonate indicate that germination is dependent on a) the amount of gibberellic acid synthesized in cypselas during maturation, b) the level of nitrogen reserves in cypselas through the mother plant or augmented by uptake from the soi1 in the seed bank and c) the concentration of hydrogen ions and carbon dioxide in the soil. In general, germination regimes of 25i100C
lightldark and 35/20°C in the dark were equally effective in promoting
germination. Further study on the exact roles of light and temperature in germination is required. Finally, the different responses of different populations to these environmental factors help us to realize that a variety of wntrol methods may be needed for different populations of this species even if they originate within the same local area.
6.5 References
Bewley, J.D. and Black, M. (1994) Seeds. Physiology of development and germination. (2nd edition) New York, Plenum Press. 445 pp.
Black, M. (1972) Controf processes in gennination and dormancy. London, Oxford University Press. 16 pp.
Campbell, M.H., Nielsen, W.J. and Nicol, H.I. (1991) Some factors affecting the germination of achenes of Onopordum illyricum L. Plant Protedion Quartedy 10, 70-72.
Caven, P.B. and Steel, M.G. (1984) Pattern of change in seed weight over tirne on individual plants. Amencan Naturalist 124,324-335.
Fenner, M. (1985) Seed ecology. London, Chapman and Hall. 1SI pp.
Fenner, M. (1991 ) The effects of the parent environment on seed gerrninability. Seed Science Research 1,7584.
Gutterman, Y. (1980181) Influences on seed geminability: phenotypic matemal effects during seed maturation. lsrael Journal of Botany 29, 105-1 17.
Harper, J.L. (1 959) The ecological significance of domancy and its importance in weed control. pp 415420 in Pmedings of the IVh International Congress of Crop Pmtection, Hamburg 1957, Vol, I.
Karssen, C.M. (1982) Seasonal patterns of dormancy in weed seeds. pp 243- 270 in Khan, A.A. (Ed.) The physiology and biochemistry of seed developmenl, domancy and germination. Amsterdam, Elsevier Biomedical Press.
Meier, L.R. (1995) Variation in seeds of Onopordum acanthium. MSc Thesis. Department of Plant Sciences, University of Western Ontario, London, Ontario, Canada. 113 pp. Parez-Garcia, F. and Duran, J.M. (1990) The effect of gibberellic acid on germination of Onopordum nervosum Boiss. seeds. Seed Science and Technology 18,83-88.
Scifres, C.J. and McCarty, MA. (1969) Some factors affecting germination and seedling growth of Scotch thistle. Research Bulletin, Nebraska Agn%ultural Ekperiment Station 228, 1-29.
Young, J.A. and Evans, RA (1972) Germination and persistence of achenes of . Scotch thistle. Weed Science 20,984 01.
Appendix II (continued) Echium vulgare L. Blueweed Etigemn annuus (L.) Pers. Annual fleabane Erigeron philadelphh~sL, Philadelphia fleabane Etydimurn cheiranthoMes L. Wormseed mustard Festuca longifo/ia Thui Il. Hard fescue Festuca spp. Feswe Fmxinus sp. Ash Galium mollugo L. Smooth bedstraw Hypencum perforatum L. St. John's-wort Lactuca scano/a L. Prickly lettuce Lepidium campestre (L.) R. Br. Field peppergrass LitMa vulganb Mill. Yellow toadfîax Lolium prenne L. Perennial ryegrass Medicago lupulina L. Black medick Melilotus alba Desr. White sweet-clover Melilotus sp. Sweet-clover Nepeta catatia L. Catnip Oenothera biennis L. Yellow evening-prirnose Phleum pratense L. Timothy Picea sp. Spruce Plantago lanceolata L. Narrow-leaved plantain Plantago major L. Broad-leaved plantain Poa compressa L. Canada bluegrass Poa pratensis L. Kentucky bluegrass Pdygonum aviculam L. Prostrate knotweed Potentitla maL. Sulfur cinquefoil Pnrnus vi~iniana1. Red choke cherry Rumex c?ispusL. Curled dock 54 Silene vulgaris (Moench) Garcûe (=S. cucubalus Wibel) Bladder campion Appendix II (continued) 55 Solidago canadensis L. Canada goldenrod + 56 Solidago spp. Goldenrod + 57 Taraxacum oîYWna/e Weber Dandelion + 58 Ti-ifolium hybndum L. Alsike clover 59 TrifoIium pratense L. Red clover + 60 Tiifolium sp. Clover 6 1 Tiifolium stoloniferum Muhl. Buffalo clover + 62 U/mus sp. Elm + 63 Verbascum thapsus L. Common mullein + Appendix III. lnsects collected from sites of two Scotch thistle, Onopordum acanthium L., populations (UWO campus, outside the greenhouse and ESW= Environmental Sciences Western) in or near London, Ontario from June 28 to September 2, 1996.
Pollinators' Collection sites Scientific name Common name UWO campus ESW (transplants)
Andrena sp. Andrenid Bee - +2 Apis mellifera L. Honey Bee - + Augochloropsis sp. Sweat Bee + - Bombus bimaculatus Cr. Foraging Bumble Bee - + Bombus impatiens Cr. Bumble Bee Bombus vagans Smith Bumble Bee CeMsspp. Digger Wasp Ensta/is tenax (L.) Drone Fly
' obsewed colleding pollen andior nectar from flowen. + = colleded. - = not colleded. Total 528.60 1 I Total 306.30
Jul. AU^. Sept. Octi Jul. AU^. Sept. Octm 1996 1997
Appendix IV. Mean daily air temperature and total rainfall in London, Ontario for four months in each of 1996 and 1997. (data taken from Environment Canada, Ontario Region, London Airport) IMAGE EVALUATION TEST TARGET (QA-3)