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Seed Biology Franklin T Chapter 1 genera Layout 1/30/08 12:38 PM Page 3 Chapter 1 1 Seed Biology Franklin T. Bonner Dr. Bonner is a scientist emeritus at the USDA Forest Service’s Southern Research Station, Mississippi State, Mississippi Contents Introduction 4 Insects 20 Flowering Plants 4 Pathogens 21 Reproductive Cycles 4 Birds 22 Flowering 4 Mammals 22 Initiation 5 Maturity and Dispersal 23 Phenology 5 Indices of maturity 23 Influencing factors 6 Physical 23 Temperature 6 Chemical 24 Light 6 Shedding and dispersal 24 Moisture 6 Dormancy 25 Nutrition 7 Types of Dormancy 25 Physiology 7 Seedcoat (or external) dormancy 25 Manipulation of flowering 7 Embryo (or internal) dormancy 25 Juvenile Phase 7 Morphological dormancy 26 Mature Phase 7 Combined dormancy 26 Structure and Development 8 Double dormacy 26 Pollination 10 Secondary dormancy 26 Pollen grain development 10 Overcoming Dormancy 26 Pollen dispersal 11 Seedcoat dormancy 26 Pollen viability and flower receptivity 11 Cold water soak 26 Pollination in angiosperms 11 Hot water soak 26 Pollination in gymnosperms 12 Hot wire 26 Fertilization 12 Acid treatment 27 Reproductive Abnormalities 12 Mechanical treatments 27 Polyembryony 12 Internal dormancy 27 Parthenocarpy 12 Stratification (chilling) 27 Agamospermy 13 Incubation and stratification 27 Fruit and Seed Development 13 Chemical treatment 27 Morphological Development 13 Combined treatments 27 Angiosperms 13 Variation in Dormacy 28 Gymnosperms 14 Germination 28 Physiological Development 16 Environmental Factors 28 Moisture content 16 Moisture 28 Stored food reserves 17 Temperature 29 Hormones 18 Light 29 Factors That Influence Seed Production 20 Aeration 30 Physiological factors 20 Biochemical Changes 30 Weather 20 Physical Development 31 Biotic factor 20 References 32 Chapter 1: Seed biology • 3 Chapter 1 genera Layout 1/30/08 12:38 PM Page 4 Introduction year; pollination occurs in the next spring, closely followed 1 Seeds are the principal means of regeneration of most by fertilization. The embryo grows rapidly, and seeds are woody plants. They serve as the delivery system for the mature by summer or early fall of the second year. This is transfer of genetic materials from one generation to the the cycle of most gymnosperms and angiosperms of North next. The part of a tree’s life cycle that involves seed forma- America. Detailed studies of individual species provide tion, maturation, dissemination, and germination is a good descriptions of the cycle in birch (Macdonald and complex—yet fascinating—chain of events, many of which Mothersill 1987), larch (Larix Mill.) (Owens and Molder are still poorly understood. Yet some knowledge of these 1979), spruce (Owens and others 1987), Douglas-fir events is necessary for successful collection and utilization (Pseudotsuga menziesii (Mirb.) Franco) (Owens and Smith of seeds to produce the means for artificial regeneration. 1964; Owens and others 1991a), thuja (Thuja L.) (Owens This chapter presents basic information on the biology of and Molder 1984a), and fir (Abies Mill.) (Owens and seeds and how this knowledge can be used in collecting, Molder 1977b). conditioning, storing, and sowing seeds. The second type of reproductive cycle is the 3-year cycle that is common to most species of pines (Owens and Flowering Plants Blake 1985). Buds form in late summer or early fall as before, followed by pollination the following spring. Pollen The seed-producing organisms of the plant kingdom tube and ovule development then stop in mid- or late- belong to the division Spermatophyta and are further classi- summer and resume the following spring. Fertilization fied into 2 sub-divisions—Gymnospermae (gymnosperms) occurs that spring, and the seeds mature in the fall. and Angiospermae (angiosperms). Gymnosperms are further Descriptive work on pines includes western white pine divided into orders. Only 2 are of interest here: Ginkgoales, (Pinus monticola Dougl. ex D. Don) (Owens and Molder which is represented by a single species, ginkgo (Ginkgo 1977a) and lodgepole pine (P. contorta Dougl. ex Loud.) biloba L.), and Coniferales (conifers), by far the most (Owens and Molder 1984b). Other gymnosperms in this important group of gymnosperms. The conifers contain 4 book that exhibit this type of reproductive cycle include families in North America: Pinaceae, Taxodiaceae, araucaria (Araucaria Juss.) and sciadopitys (Sciadopitys Cupressaceae, and Taxaceae. These include economically Sieb. & Zucc.). Among angiosperms of North America, the important genera such as pine (Pinus L.), spruce (Picea A. 3-year reproductive cycle occurs only in the black oak group Dietr.), sequoia (Sequoia Endl.), cypress (Cupressus L.), and (Erythrobalanus) of oak (Mogensen 1965). Like that in pine, yew (Taxus L.). fertilization in black oak does not occur until 13 months Angiosperms are divided into 2 classes—Monocotyle- after pollination. doneae and Dicotyledoneae. Monocotyledonous trees are The third type of reproductive cycle, found in members not very common in North America, but they are represent- of the Cupressaceae family, is somewhat similar to the sec- ed in this book by the families Palmae (genera Roystonea ond type. The primary difference is that fertilization occurs O.F. Cook, Sabal Adans., and Washingtonia H. Wendl.) and within a few weeks of pollination during the second year, Liliaceae (genus Yucca L.). Dicotyledonous species number with embryo development going into a dormant phase in over 30 families in North America and are by far the largest late summer or early fall (Owens and Blake 1985). This class of woody plants. This class includes such common type of cycle has been described for Alaska-cedar genera as maple (Acer L.), acacia (Acacia Mill.), birch (Chamaecyparis nootkatensis (D. Don) Spach) (Owens and (Betula L.), ash (Fraxinus L.), holly (Ilex L.), oak (Quercus Molder 1984a) and probably occurs in some juniper L.), and blueberry (Vaccinium L.). (Juniperus L.) species (Johnsen and Alexander 1974). Reproductive Cycles Flowering The reproductive cycles of flowering plants begin with Botanically speaking, angiosperms produce true flowers, initiation of reproductive buds and end with maturation of but gymnosperms do not. Gymnosperm reproductive struc- the seeds. There are 3 types of reproductive cycles that have tures are actually strobili, but for this discussion, they will been recognized in trees of the temperate zone (Owens and be considered flowers in the broad sense. All trees propagat- Blake 1985). ed from seeds pass through a period of juvenility before The 2-year cycle is the most common type. Repro- they acquire the capability of flowering and producing seeds ductive buds form late in the growing season of the first of their own. The length of this juvenile period is extremely 4 • Woody Plant Seed Manual Chapter 1 genera Layout 1/30/08 12:38 PM Page 5 varied among species, ranging from as little as 3 years for Initiation 1 Pinus gieggii Engelm. (Lopez-Upton and Donahue 1995), to In numerous woody plants, flower initiation and devel- 40 years for sugar pine (P. lambertiana Dougl.) (Krugman opment is a lengthy process extending over several months. and Jenkinson 1974). The majority of tree species in the During this period, environmental factors and the internal temperate zone, however, begin flowering at the age of 10 to physiological condition of the trees interact to produce the 15 and produce significant seedcrops by the age of 25 to 30 flower crops. The effects of some environmental factors (Owens and others 1991b). Woody shrubs generally flower have been observed through the years, and these relation- and fruit at earlier ages. Extensive data on seed-bearing ages ships have been used to influence flowering and seed pro- are presented for all species in part 2 of this book. duction in some species (see below). The internal factors Among species with unisexual flowers (flowers of one involved are still poorly understood, as are their interactions sex only, either staminate or pistillate), flowers of one sex with the environment. may be produced long before flowers of the other sex. For Phenology. Flower buds on most trees and shrubs of example, Scots pine (P. sylvestris L.) may produce female the temperate regions are initiated late in the growing sea- strobili at age 5 to 7, but no male strobili until age 10 to 15 son of the year preceding flowering (table 1). In species (Matthews 1970). Many other pines are the same. The with unisexual flowers, male flowers may start earlier and extent of this phenomenon in angiosperms is not known, but differentiate more rapidly as well. Flowers may bloom from it does occur in some species, for example, yellow birch late winter to fall, depending on the species and the loca- (Betula alleghaniensis Britton) (Erdmann 1990). tion. In temperate trees, flowering is primarily seasonal, that The length of the juvenile period can be affected by is, production only occurs in certain times of the year. Most many factors other than age. Physical size of the plant species bloom in the spring, but there are numerous excep- seems to be important in some cases (Hackett 1985; tions to this rule. Witch-hazel (Hamamelis virginiana L.) Schmidtling 1969). Genetic differences are often obvious in flowers from September to mid-November; California-laurel even-aged plantations where spacing and tree size are equal, (Umbellularia californica (Hook & Arn.) Nutt.) from and there is experimental evidence to confirm the genetic December to May; September elm (Ulmus serotina Sarg.) in effect in a few species (Sedgley and Griffin 1989). September; and deodar cedar (Cedrus deodara (Roxb.) Furthermore, tree improvement programs have demonstrated Loud.) in September to October. The times reported in this that selections for early flowering within species have the book for flowering are typically expressed as a range of sev- potential to produce clones with precocious flowering traits eral months to allow for the latitudinal and elevational dif- (Krugman and others 1974). ferences throughout the range of a given species. Local vari- ations in weather may also affect the time of flowering from year to year on the same tree.
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