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 Main subject - july/aug 2005
Seed Dormancy
Dra. Denise Cunha F. S. Dias
dcdias@ufv.br
Once mature, a seed disperses from the mother-plant, becoming an autonomous organism because it has an embryo that, under suitable environment conditions, will develop originating a seedling. However, as it does not always happen, one may question: why some species seeds do not germinate, even when sown under suitable moisture and temperature conditions?
The answer may seem to be simple: it is because they have entered an advanced deterioration process, culminating in embryo's death or, otherwise, they are dormant. In the first instance, seeds absorb water but do not complete the essential metabolic activities for embryonal axis growth, i. e., they do not originate a full seedling with root and shoot portions. Dormant seeds are those ones that, even though being alive and under environment conditions that usually favor gemination process, do not germinate due to an internal restriction preventing them from embryo development. Germination will only occur when such restriction is overpassed, which in nature can take some days, months or even years, depending on the species.
Dormancy is an ordinary phenomenon, mainly in some vegetables and grasses, some fruit and forest and ornamental plants that do not germinate right after harvest due to internal mechanisms, either physical or physiological in nature, that block germination. Those are genetical mechanisms occurring along the species life cycle, during seed maturation, so that just after the dispersion, the seed will not be able to germinate yet. Such dormancy, that installs during seed maturation stage, is called primary. However, in some species, germination impediment gets established after seed dispersion, induced by given stress conditions or germination unfavorable environment, characterizing another kind of dormancy, called secondary. Non-dormant lettuce seeds can get into secondary dormancy when allowed to germinate under high temperatures.
Thus, seeds dormancy is an important life cycle stadium of plants, featured by the transitory absence of germinating capacity, allowing plant species to survive to adversities, mainly those ones that make difficult or even impossible the plant vegetative growth. Therefore, it is a crucial phenomenon for many plant species in several ecosystems to perpetuating and surviving. It is also due to dormancy that the seeds of many species do not germinate inside the fruit when it is still attached to the plant because, after physiological maturity, in case of favorable environment conditions to germination - for example, increasing moisture due to rain excess - , seeds with no embryo growth impediment will be allowed to germinate when still attached to the mother-plant. Practically, it is common to note the germination of beans and peanuts seeds inside the pod under such conditions, as well as cotton seeds in the seed vessel and wheat seeds in the ear. It is remarkable that most plants currently cultivated are represented by varieties, cultivars, and hybrids genetically improved through selection processes that have eliminated dormancy, because the goals of modern agriculture are both seed germination fastness and uniformity and seedling emergency in the field. This is the case for soybean, beans, sunflower, maize, and other seeds whose survival depends on man.
Nevertheless, there are many species whose survival is guaranteed by dormancy. Dormancy duration varies significantly among plant species, either in days, months or even several years. For one single specie such period can vary as a function either of the genotype, the environment where the seed was produced and other factors. Besides that, seeds from the same plant have distinct dormancy intensities so that germination can take place along the time, at regular intervals, while dormancy is overpassed, so increasing the probability of individuals survival. Thus, the prevention to seed germination established by dormancy constitutes in a beneficial strategy because it distributes germination across time and allows the species to "escape" from adverse conditions to seedlinggrowth. Therefore, it is a phenomenon that contributes to seed germination only during a given time or season that favors seedling establishment, assuring its survival. Anyway, there is a certainty: no matter the species, dormancy becomes less intense with time until it is totally overpassed. So, even when viable and under suitable environmental germination conditions, (temperature, water and oxygen), the seed will not germinate until the avoiding mechanism ceases its action.
This way, under an developmental view, dormancy is an adaptive feature that guarantees species survival in different ecosystems. Consequently, it is one of the factors contributing to weeds establishment, making its control and eradication difficult, causing losses to farmers. Despite the systematic control of such plants, weeding and herbicides, thousands of them emerge during crops cultivation, since these species seeds survive on the soil for several years due to dormancy. An example that illustrates such situation is the difficulty in eradicating red rice from rice crops in different regions.
Red rice (Oryza sativa L.)
Species that have evolved in humid areas have developed water-absorption preventing mechanisms, to avoid germination from occurring right after seed dispersion, so assuring its survival. In a semi-arid climate, the scant rain level is the factor limiting species survival; seeds that are typical from these regions often have water-soluble, germination-inhibiting substances that will be leached only after pouring rains, so that germination will only occur when water availability on the soil is enough to establish the seedling. In very dense forests, light absence under the canopy is the factor that prevents seed germination in several species which either only germinate in the presence of light or need light to overpass dormancy.
Strelitzia reginae
Dormancy can also be an obstacle to agriculture since it generates a non uniform seedling emergence in the field, several times making necessary the use of suitable treatment to overpassing dormancy before sowing, what is not always either practical or viable. In vegetables seeds just harvested that are used in green fertilization, such as Stizolobium aterrimum, broad beans, pig beans, dalichas, dormancy can cause two kinds of problems:
1) increase of the amount of seeds for sowing because dormancy post-harvest is marked, so that a great portion of the seed lot will not germinate;
2) many dormant seeds keep viable on the soil and will germinate during the development of future crops cultivated in that area, so becoming invader plants.
Stizolobium aterrimum
It is both ecological and economically important to know dormancy mechanisms as well as their duration among the different species because it helps if it is necessary or not to make use of specific treatments to act on the seed metabolism, releasing the embryo for development or making it able to germination.
page 2 ->> Seed Dormancy
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