|Germination and Seedling Development|
Wild seeds display many adaptations that predispose germination within specific habitats and seasons. By contrast, seeds of crops and other cultured plants usually lack controls that prevent germination.
The control system was lost because some seeds in the population lacked controls and were chosen when they germinated in the care of a culturist. For that reason, most cultivated plants that start from seeds show little or no germination control. Most of the information on germination control, therefore, covers wild species of plants.
Seeds and Dormancy
Seeds are the exclusive means of regeneration for the annual flowering plants. In other plants, seeds are an alternative strategy to regeneration by buds, bulbs, rhizomes, stolons, or tubers. In those plants, the primary roles of the seed are to disperse the population and to reinvigorate the genetic diversity of the germ line.
Seed dormancy occurs in most plants, and when a seed is dormant it will not germinate, even if it is in the right environmental conditions. The dormant state may begin with maturation of the seed embryo, or it may develop in climate extremes after the seed falls from its parent.
Dormancy prevents immediate germination when the mature seed is in an inappropriate environment, and it is a programmed phase in the life cycle. Dormancy’s function ends, and a germination window opens, at a time when the emerging seedling will have the optimum chance for survival.
After-ripening removes the dormancy and allows the seed to respond to germination stimuli. Seeds of summer annuals after-ripen when exposed to winter and early spring temperatures, a treatment called stratification. Exposure to cold temperatures can also promote dormancy in some seeds.
Not all nongerminating seeds are innately dormant. There are also nondormant and conditionally dormant seeds. Neither type may germinate when the seeds mature, simply because the parent prevents contact with the soil and absorption of water or because the temperature range is below that necessary for germination.
|Seeds and Dormancy|
Wild seeds may experience a deepening of dormancy as a result of exposure to the temperatures of the dormant season. Nondormant seeds may simultaneously experience biochemical reactions that deepen dormancy and cause them to after-ripen.
Dormancy is caused by one or more conditions of the seed. Physiological dormancy of the embryo is the most common. It may be caused by the presence of an inhibitor molecule, an inadequate level of a growth hormone, or some other internal factor. Examples of the latter include blockages in membrane function or in synthesis of an enzyme or its nucleic acid messenger.
Other causes of dormancy are a hard or impervious seed coat, an underdeveloped embryo, or some combination of those factors. Some hard-coated seeds require physical scraping, such as tumbling down a swift-flowing stream and being scraped on the stream bed. Others may require exposure to a forest fire or passage through the gut of an animal to weaken the seed coat.
The seeds from the previous year’s crop of summer annuals wait to germinate in spring. Seeds of many species will germinate when soil temperatures reach a threshold constant. Others require daily fluctuations of temperature, waiting until the daily fluctuation becomes sufficiently large.
Seeds of most species require a light stimulus to germinate. The light is absorbed by the pigment phytochrome, which is positioned in the cotyledons of the embryo. Phytochrome acts as a shade detector.
White light and, especially, red light, convert the phytochrome molecule to an active form. The rearrangement of the molecule causes it to attach a different part of itself to a new location on a cell membrane within the cotyledons of the seed.
Transformed phytochrome allows seeds to germinate. By contrast, far-red light, which is absorbed by the transformed phytochrome molecule, transforms the molecule back to its original shape—that is, it deactivates it.
When sunlight is transmitted through green leaves to the forest floor, much of the visible light with wavelengths shorter than 700 nanometers is absorbed or scattered. This shade light is rich in far red, and it tends to deactivate phytochrome.
Not all seeds germinate at the beginning of the following growing season. Light-demanding seeds that have become buried or have fallen into the shade will be stressed by the absence of an activating light signal, while the embryos experience an environment that is otherwise growth-promoting. Stressed seeds may enter a secondary dormancy and will need to undergo a second interval of after-ripening before again becoming nondormant.
Seeds emerging from secondary dormancy may require a smaller light stimulus. Following primary dormancy, one or more complete light cycles may be necessary. By contrast, seeds may be fully activated by only a brief pulse of light when they emerge from secondary dormancy.
Many seeds become dry on the parent plant as a part of maturation. Drying is believed to end the seed-building phase and start the pregermination phase. Most domestic seeds require no after-ripening but will germinate if allowed to take up water. Water uptake is a first step in germination.
Biochemical events that begin with water uptake in domestic seeds include metabolism along three separate pathways and an increased use of oxygen. Excessive moisture, even at temperatures too low for germination, may lower seed viability. Thus, seeds are best preserved under cold,dry conditions.
With proper storage, a reasonable percentage of seeds may live for many years. Record long-lived species include seeds of Canna (arrowroot, six hundred years), Albizia (mimosa and silk trees), and Cassia (about two hundred years).
Seeds of Verbascum (mullein) have survived at 40 percent viability for one hundred years. Weed seeds in soil banks may need to wait for hundreds of years before the forest is cleared by catastrophe and the environment once again is favorable for such pioneer species.
By contrast, recalcitrant seeds require their embryos to be kept moist, or viability is quickly lost; examples include trees with large seeds, such as walnut, oak, hazel, and chestnut. These seeds usually live less than one year.
Seedling development begins with the close of germination. Cells in the embryonic root (the radicle) begin to divide and grow. In some seeds, or in unusual environmental conditions, other parts of the embryo emerge before the radicle. Development of the seedling is marked by the growth and elongation of the embryo stem. Seeds are classified according to which part of the stem grows more rapidly.
In the epigeal type, the hypocotyl, which is the basal part of the stem between the radicle and the embryonic leaves (the cotyledons), grows, thereby thrusting the cotyledons above the soil. In the hypogeal type, the epicotyl or upper part of the stem elongates, and the cotyledons remain underground.
Exposed to light, phytochrome in the cotyledons calls for an end to subterranean elongation, called etiolation, and the beginning of plantlike growth. Among its functions, phytochrome triggers the synthesis of chlorophyll; photosynthesis soon turns the cotyledons into sugar factories.
At the same time, the epicotyl region of the embryo above the cotyledons is extending the plumule to form the first true, or foliage, leaves. In hypogeal seeds the first leaves emerge from the plumule.
Food reserves that are stored in the endosperm, cotyledons, and embryo will nourish the early growth of the plant until it can synthesize the necessary machinery for making its own food. Foods are stored in seeds as starch and other complex carbohydrates, fats, and proteins.Cereal grains contain large amounts (65-80 percent) of carbohydrates, which are stored in the endosperm.
Seeds of legumes are famous for their high protein contents,which reach 37 percent in soybeans. The peanut, a legume, stands out by having both high protein (30 percent) and high fat content (50 percent). Legumes store food reserves in the embryonic leaves (cotyledons).
Food is transferred to the growing sites, primarily as sucrose and amino acids. Starches and other carbohydrates and fats are first converted to simple sugars and then to sucrose for transport.
Synthesis of active phytochrome, second messengers, and the plant hormone gibberellin are involved in determining the rate of mobilization and transport. They promote synthesis of enzymes, such as those that break down food reserves into simple sugars.
The embryo selects one part of the root-stemaxis for rapid growth, changing the relationship of the other parts to the external environment. The cotyledons are also versatile: They may act as first leaves or may remain attached to another part of the embryo, such as the endosperm.
There, they act as absorptive organs to transport mobilized food reserves to the growing parts of the seedling. In onions, a single cotyledon performs both functions. The exposed part carries out photosynthesis, while the buried part absorbs foods from the endosperm.