Seeds

Seeds

The seed is the dispersal stage of the life cycle of angiosperms and gymnosperms . It contains the embryo, the next generation of plant in miniature. Many seeds are dry when shed from their parent plant and are thus adapted to withstand harsh environments until conditions suitable for germination are achieved.

The evolution of plants to produce seeds is poorly understood because the fossil evidence is incomplete. The advantage of reproducing through seeds is apparent, however: The embryo is encased in a protective coat and is provided with a source of nutrients until, as a young seedling following germination, it becomes established as an independent photosynthetic (autotrophic ) entity.

Seeds account for 70 percent of food consumed by humans, and are also the major feeds for domestic animals. Their importance cannot be overstated. World seed production is dominated by the cereals, and even the production of wheat, maize, or rice alone by far exceeds that of all the other crops. As a concentrated source of carbohydrate, cereals provide for the human diet, livestock feed, and industrial raw materials. They are also an important source of protein, oil, vitamins, and fiber. Grain legumes , particularly soybeans and groundnuts (peanuts) are an important source of proteins and vegetable oils, which are used in margarine and cooking fats, and have applications in paints, varnishes, and plastics, as well as the manufacture of soaps and detergents. An understanding of seeds is therefore an essential prelude to human attempts to improve their quality and yield, whether it be by conventional breeding techniques or the novel approach of genetic engineering.

Seed Structure

A seed is a combination of maternal tissues, embryo tissues, and (in angiosperms) endosperm tissue. Seeds of different species are variable in size and internal structure at the time they are shed from their parent plant. They may be barely visible to the naked eye (for example, orchids), weigh a few micrograms to milligrams (for example, poppy, tobacco, and many annual weeds), weigh up to several hundred milligrams to grams (for example, soybean, maize, pea, and bean) or even several kilograms (coconut and Lodoicea maldivica ).

Non-Maternal Tissues

The seed develops from the ovule after the egg cell within has been fertilized by a male gamete from a germinated pollen grain. The resulting diploid zygote cell then undergoes extensive mitotic divisions to form the embryo. In angiosperms, the process of double-fertilization occurs when a second male gamete from the pollen tube fuses with two female nuclei in the ovule, yielding a triploid nucleus containing one set of paternal genes and two maternal sets. This also undergoes extensive mitotic divisions to produce the endosperm, usually a storage tissue that may (cereals, castor bean) or may not (peas, beans) persist in the mature seed, or it may be reduced to a thin layer of cells (lettuce, tomato, soybean).

Maternal Tissues

The seed coat (testa) develops from the outer layers of the ovule, the integuments, and is a diploid maternal tissue. In many angiosperm species the ovary wall surrounding the ovule also divides and develops at the same time as the seed to form an enclosing fruit. While many species form a fleshy fruit, in others the fruit tissues (pericarp) develop as only a few layers of cells, become dry, and adhere to the outside of an equally thin or thinner seed coat. Strictly, by botanical definition, such dispersal structures are fruits (for example, cereal grains, lettuce, sunflower, oak). Nonetheless, for convenience, and because they contain the embryo, they are usually called seeds.

In some angiosperm seeds, following the completion of fertilization, another part of the ovule, the nucellar tissue, may divide mitotically and grow to produce a nutritive perisperm (sugar beet, coffee). In gymnosperm (conifer) seeds, the tissue surrounding the mature embryo is the megagametophyte, a haploid maternal tissue into which the fertilized zygote grows during seed development; it is still substantially present in the mature seed as a source of stored reserves.

The Embryo

The embryo is made up of an axis bearing one or more cotyledons. The axial region contains a hypocotyl to which the cotyledons are attached, a radicle that will become the primary root following germination, and the plumule, the shoot axis bearing the first true leaves. These parts are usually easy to discern in dicot angiosperm seeds and in those of the polycotyledonous (many cotyledons) gymnosperms. But in seeds of monocot plants, particularly the cereal grains, the single cotyledon is much reduced and modified to form the scutellum, an absorptive structure that lies against the endosperm and absorbs material from it. The basal sheath of the cotyledon is elongated to form a coleoptile , which covers the first leaves.

The shapes of the embryos and their position within the seed are variable between species. In those dicot species that have a substantial endosperm (endospermic seeds) the embryo occupies proportionately less of the seed than when the endosperm is rudimentary or absent (compare castor bean with lettuce or runner bean). In contrast, the cotyledons of nonendospermic seeds are much bulkier and are the storage tissues, and in peas and beans account for over 90 percent of the mass of the seed.

Several variations on this general theme occur. In the Brazil nut the cotyledons are much reduced and the bulk of the seed is occupied by a storage hypocotyl. Because the Brazil nut is primarily a single hypocotyl, it does not split in two like most other nuts made from two enlarged cotyledons. Cotyledons are absent from the seeds of many parasitic species. In orchids, seeds are shed when the embryos are extremely small and contain only a few cells, and completion of development occurs afterward.

Non-Embryonic Storage Tissues

In most species, the maternally derived perisperm fails to develop and is quickly absorbed by the developing embryo. Where it does persist, it is a major storage tissue, sometimes in conjunction with an endosperm, or the cotyledons (for example, sugar beet).

As noted, seeds are categorized as endospermic or nonendospermic in relation to the presence or absence of a well-formed endosperm within the mature seed. The relatively massive endosperm is the major source of stored seed reserves in species such as the cereals, castor bean, date palm, and endospermic legumes (carob, fenugreek). In the cereal grains and seeds of some endospermic legumes (for example, fenugreek) the storage cells of the endosperm are nonliving at maturity, and the cytoplasmic contents have been replaced entirely by the stored reserves (starch and protein in cereals; hemi-cellulose cell walls in fenugreek). But on the outside of the endosperm there remains a living tissue of one to a few cell layers in thickness, the aleurone layer, whose role is to synthesize and secrete enzymes to mobilize those reserves following germination.

The Seed Coat (Testa)

The anatomy of the seed coat is highly variable, and differences among species have been used for taxonomic purposes. The coat is of considerable importance to the seed because it is a protective barrier between the embryo and the outside environment (in some species the fruit coat may augment or be a substitute for this role). Protection by the seed coat is aided by the presence of an inner and outer cuticle , impregnated with fats and waxes, and lignified cell walls. Phenolics or crystals (of calcium oxalate, for example) may be deposited in the coat to discourage predation by insects. Mucilage-containing cells may be present that burst on contact with water, retaining and absorbing moisture as a supply to the germinating embryo. Rarely, hairs or wings develop on the seed coat to aid dispersal (willow, lily); more frequently the dispersal structures are a modification of the surrounding fruit coat.

Quiescence and Dormancy

The completion of seed development and the acquisition of the mature structure is marked in many species by a loss of water, so that the mature seed can be dispersed in the dry state. The water content of a dry seed is usually 5 to 15 percent, versus 70 percent or more for the plant as a whole. When dry, a seed can withstand extremes of temperature that would rapidly result in death in the hydrated state. Not surprisingly, then, dry seeds are more or less in a state of suspended animation, with little or no metabolic activity. As such, they are said to be quiescent. When introduced to water again, under favorable conditions such seeds will rapidly resume metabolism and complete germination.

The phenomenon of seed quiescence is very different from that of dormancy. The latter is when seeds in a hydrated state fail to complete germination even when conditions are favorable; that is, temperature, water and oxygen supply are not limiting. Dormant seeds are metabolically active, in fact as active as their nondormant counterparts, but there exists within the seed a block (or blocks) that must be removed before germination can be completed. To be released from dormancy, a seed must experience a particular stimulus, or undergo certain metabolic changes. The cause of dormancy is not clearly understood, but at least one factor is the growth regulator hormone abscisic acid (ABA), which is imported from the parent plant into the seed during its development.

Many seeds lose dormancy (while remaining quiescent) while still in the mature dry state, in a process called after-ripening, which may extend over several weeks to many years. Dormancy of hydrated seeds in the soil may be broken by one or more environmental cues, whose effectiveness depends on the species. These cues include: 1) light, usually for a short duration, with sunlight being the most effective; 2) low temperature, around 1 to 5°C for several to many weeks; 3) fluctuating temperatures, usually day-night

CONDITIONS THAT TERMINATE SEED DORMANCY
Species	Common Name	Light	Chilling	Alternating Temperatures	After-ripening
Acer pseudoplatanus	Great maple		+		+
Avena fatua	Wild oat		+		+
Betula pubescens	Birch	+	+		+
Hordeum species	Barley		+		+
Lactuca sativa	Lettuce	+	+		+
Nicotiana tabacum	Tobacco	+		+
Pinus sylvestris	Scot's pine	+	+
Prunus domestica	Plum		+		+
Triticum aestivum	Wheat		+		+

fluctuations of 5 to 10°C; and 4) chemicals, of which nitrate is the most important in the soil.

Dormancy is a mechanism to ensure the optimum distribution of seed germination in time and space. For example, seeds that require weeks of cold temperatures to break their dormancy cannot complete germination immediately after being shed from their parent plant in early fall, but will do so only following the cold winter months. This ensures that they are not in the delicate seedling stage at the onset of winter, which would be detrimental to their survival. Dormancy of light-requiring seeds will be removed only when seeds are at the soil surface, a mechanism that prevents germination at too great a depth. This is crucial for small seeds whose stored reserves are insufficient to support growth through the soil to carry the seedling leaves into the light to begin photosynthesis. Seeds on the forest floor receive light that is poor in the red wavelengths, since this is absorbed by the leaves of the overarching canopy. Thus, seeds in this environment must wait for the appearance of gaps in the canopy (tree fall or logging) before their dormancy can be broken, and they can then emerge in situations where there is reduced competition for resources from established plants. Phytochrome is the light-perception system in dormant seeds and is activated by wavelengths rich in red.

While the significance of dormancy can best be understood in an ecological context, it is important in agriculture too. Prolonged dormancy in crop species is undesirable since germination could be spread out over several years, resulting in unpredictable and low annual yields. On the other hand, lack of at least a temporary dormancy can be harmful also because, for example, mature seeds of barley or wheat could germinate on the ear if wetted by rain before harvest, resulting in crop spoilage.

see also Embryogenesis; Flowers; Fruits; Fruits, Seedless; Germination; Germination and Growth; Grains; Phytochrome; Pollination; Reproduction, Fertilization and; Reproduction, Sexual; Seed Dispersal; Seed Preservation.

J. Derek Bewley

Bibliography

Bewley, J. Derek, and Michael Black. Seeds: Physiology of Development and Germination, 2nd ed. New York: Plenum, 1994.

Bradbeer, J. W. Seed Dormancy and Germination. Glasgow and London: Blackie, 1988.

Chrispeels, Maarten J., and David E. Sadava. Plants, Genes and Agriculture. Boston and London: Jones and Bartlett, 1994.