M04_DAVI4493_08_SE_C04.qxd 8/6/10 11:56 PM Page 109 part two Seed Propagation CHAPTER 4 Seed Development CHAPTER 5 Principles and Practices of Seed Selection CHAPTER 6 Techniques of Seed Production and Handling CHAPTER 7 Principles of Propagation from Seeds CHAPTER 8 Techniques of Propagation by Seed During much of human existence, special kinds of crops, referred to as landraces, were maintained by farmers who kept a portion of each year’s seed to produce the crops for the following year. These landraces received local names and represented some of our most important agricultural crops coming from Asia (rice, millet, soybean, many vegetables), southwest Asia (wheat, barley, oats, rye), Africa (rice, sorghum, watermelon), and the Americas (corn, squash, beans, pepper, potato, sunflower, cotton, tobacco). Modern agriculture (agronomy, horticulture, and forestry) relies on seeds and seedlings to produce most of the world’s food and fiber resources. Great advances have been made in the past century that permit seed companies to provide high quality seeds with superior genetics. Public and private plant breeders use the principles and practices of genetic research to breed new seedling cultivars that have superior growth characteris- tics, crop yields, pest resistance, and nutrition. Seed companies maintain germplasm for parental seed stocks and are responsible for production, storage, and distribution of seeds to producers. Millions of kilograms (pounds) of seeds are produced each year for use by propagators worldwide. This section deals with all aspects of the seed industry including genetic selection, seed production, and germination. M04_DAVI4493_08_SE_C04.qxd 8/6/10 11:56 PM Page 110 4 Seed Development learning objectives INTRODUCTION • Trace the origin of seeds. Four hundred million years ago, plants moved out of the oceans to col- • Follow the relationship onize land. Two major adaptations made this possible. The first was the between flower parts and evolution of the root. The root not only anchored the plant in soil but seed parts. also allowed the plant to obtain water and minerals no longer brought • Explain the general parts to the plant by ocean water. A second adaptation that increased a plant’s of a seed. success on land was the development of a vascular system. This allowed • Describe the stages of seed materials obtained by the root system to be efficiently transported to the development. leafy photosynthetic parts of the plant. However, the price of these • Explain unusual types of seed adaptations to land habitation was relative immobility. The first vascu- formation. lar plants (e.g., ferns) used spores to spread the result of sexual repro- • Observe how plant hormones duction. However, plants that used spores to reproduce required a wet are important to seed environment to allow male sperm to swim to fertilize the female egg. development. The development of the seed habit (dispersal of seeds rather than spores • Describe ripening and dissem- for reproduction) permitted plants to move away from perpetually wet ination of fruits and seeds. environments and colonize areas with drier climates. This initiated the proliferation of the marvelous diversity found in seeds and their accom- panying fruit structures. Seed-producing plants (especially angiosperms) became incredibly successful, and it is estimated that there are currently over 250,000 species of flowering plants, easily the most diverse group found in the plant kingdom. Propagation by seeds is the major method by which plants repro- duce in nature, and one of the most efficient and widely used propaga- tion methods for cultivated crops. Plants produced from seeds are referred to as seedlings. Sowing seeds is the physical beginning of seedling propagation. The seed itself, however, is the end product of a process of growth and development within the parent plant, which is described in this chapter. REPRODUCTIVE LIFE CYCLES OF VASCULAR PLANTS Plant life cycles are characterized by alternate sporophytic and gametophytic generations. The sporophyte is usually plant-like in appearance with a diploid genetic composi- tion. The sporophyte produces specialized reproductive structures that facilitate gamete production through meiosis. This initiates the gametophytic generation. Male and female gametes have a haploid genetic composition, and fusion of these gametes (fertilization) results in a fertilization The reproductive zygote (embryo) that sexual union of a male restarts the sporophytic generation. and female gamete. M04_DAVI4493_08_SE_C04.qxd 8/6/10 11:56 PM Page 111 seed development chapter four 111 Vascular plants are separated into those that dissemi- (usually wet conditions). The life cycle of a fern is nate the next generation by spores or those who do so depicted in Figure 4–1. Spores are produced in sporan- with seeds. gia within a sorus produced on the underside or edge of the fern frond (sporophyte). The spore (1n) germi- Seedless Vascular Plants nates and produces a small leafy structure called a pro- Seedless vascular plants reproduce from spores, and thallus. On the mature prothallus, male (antheridia) include horsetails (Equisetum), wiskferns (Psilotum), and female (archegonia) are formed. The male lycopods (Lycopodium), Selaginella, and ferns. The antheridium releases the motile sperm (1n) that swims spore is a protective structure that is tolerant of envi- into the archegonium uniting with a single egg cell ronmental conditions, germinating when conditions (1n). Following fertilization, the zygote develops into a are conducive for the gametophytic generation new fern. (a) (b) (c) (f ) (d) (e) Figure 4–1 A representative fern life cycle includes alternate sporophytic and gametophytic generations. (a) A mature fern sporophyte produces fronds that typically produce (b) sori (spore producing structures) on the underside of the leaf-like frond. (c) Within the sori are sporangia that contain the spores that initiate the gametophytic gerneration. (d) When the spore germinates it produces a leaf-like gametophyte called the prothallus. On the prothallus, several female archegonia and many male antheridia are formed. (e) Fertilization occurs when the male sperm unites with the female egg within the archegonium. (f) The resultant young sporophyte becomes the long-lived fern. Adapted from Linda R. Berg. 1997. Introductory Botany. Saunders College Publishing. M04_DAVI4493_08_SE_C04.qxd 8/6/10 11:56 PM Page 112 112 part two seed propagation Seed Plants located between the scales of the female cones. The seed habit developed during the Devonian period Haploid male and female gametes fuse to form a about 350 to 385 million years ago in an extinct group of diploid zygote that devel- plants called the progymnosperms (47). ops into the embryo endosperm The major Progymnosperms are only known from the fossil record within the seed. Storage storage tissue in seeds. (Fig. 4–2) and produced seedlike structures enclosed in tissue (endosperm) in a It is derived from the female tissue called cupules. They are considered the pro- gymnosperm seed is from haploid female genitors to our current-day gymnosperms and the haploid female game- gametophyte in gym- angiosperms. The seed habit is characterized by several tophyte. nosperms, while in anatomical features that differentiate them from spore- Angiosperms are angiosperms it is the producing plants: true flowering plants. result of gamete fusion The term angiosperm that forms a triploid 1. Rather than producing a single spore type (homo- means “enclosed seeds” (3n) storage tissue. spory), seed plants produce a separate female and refers to the female megaspore and male microspore (heterospory). ovary tissue (carpels) that forms the fruit surrounding 2. The female gametophyte is retained on the mother angiosperm seeds. Angiosperms are the dominant plant plant (sporophyte) and is enclosed within a protec- type on Earth with approximately 250,000 species, tive maternal seed coat. compared with only about 8,000 living species of gym- 3. The ovule has an opening designed to receive nosperms. One reason for angiosperm success and pollen that does not depend on water for male diversity is the mutualistic co-evolution of animals gamete transfer. (especially insects) as pollinators and seed dispersers. Seed plants are separated into gymnosperms and A representative angiosperm life cycle is presented in angiosperms. Gymnosperms include the cycads, Figure 4–4 (page 114). A key development in the ginkgo, gnetophytes (Ephedra, Gnetum) and the angiospermic life cycle is the presentation of the female conifers (like pine, fir, and hemlock). The term gym- megagametophyte as a multi-celled (usually 8) embryo nosperm means “naked seeds” and refers to the absence sac within the ovule. Male gametes from the pollen of ovary tissue covering the seeds, which is a character- enter the ovule. One gamete fuses with the egg cell to istic of angiosperms (flowering plants). Pine is repre- form a zygote, and the second fuses with two polar sentative of a gymnosperm life cycle (Fig. 4–3). nuclei to form the endosperm. This double fertilization Conifers produce separate male and female reproduc- is a characteristic of angiosperms and leads to a triploid tive cones (strobili) on the same plant. Male cones pro- endosperm rather than the haploid endosperm seen in duce winged pollen that is dispersed by wind. Egg cells gymnosperms. Based on seedling morphology, are produced within the female megagametophyte angiosperms can be separated into dicotyledonous