After reading this article you will learn about:- 1. Meaning of Seed Dormancy 2. Ecological Significance of Seed Dormancy 3. Types 4. Causes 5. Effects.
Meaning of Seed Dormancy:
Dormancy is a delaying mechanism which prevents germination under conditions which might prove to be unsuitable for establishment. As long as the seed remains viable the possibility exists that it may eventually find itself more favourably placed.
Ecological Significance of Seed Dormancy:
The ability to remain dormant for a long period is associated with seeds of species from unpredictable environments such as bare ground and swamps. Even in more stable environments dormancy may occur if regeneration is associated with haphazard local disturbance.
Another unpredictable feature of the environment in certain regions is the climate. This is especially true in the case of arid climates whose rainfall tends to be very variable. As a consequence, a high level of seed dormancy is a characteristic feature of many plants of arid zones.
The absence of dormancy would at first sight appear to be a disadvantage to any species as it reduces the opportunities for dispersal even in predictable environments. But in arid zone plants there is definite dormancy.
The breaking of dormancy does not in itself constitute germination, but is a necessary prerequisite of it. Thus a seed may need to experience some environmental conditions which act as a trigger for germination, but which would be quite unsuitable for germination as such.
Types of Seed Dormancy:
Harper (1977) recognizes three types of seed dormancy depending on the how each of them arises: viz., innate, enforced and induced. Although these categories are not completely tidy, they probably represent the neatest classification for ecological purposes.
The seed dormancy is of following types:
i. Innate dormancy:
A seed which in innately dormant is one which is incapable of germination when freshly dispersed even if conditions suitable for seedling growth are supplied. This inability to germinate may be due in certain species to the embryo being immature at the time of dispersal.
Innate dormancy may be imposed chemically by the presence of inhibitory compounds either in the seed coat or in the embryo. Often these can be simply leached out (as in the case of the desert annuals).
In other cases the seed must first experience some special environmental conditions such as chilling, fluctuating temperature of specific photoperiods, which appear to initiate the biochemical processes which break dormancy. Initial dormancy often seems to wear off with time regardless of environmental conditions.
The presence of innate dormancy can usually be interpreted as an adaptation either for staggering germination (as in the case of the hard coated seeds), or for delaying germination until the appearance of most favourable season. Many species have a chilling requirement which imposes winter dormancy and delays germination until spring.
ii. Enforced dormancy:
It occurs when the seed is simply being deprived of its requirements for germination, for example, by the absence of sufficient moisture, oxygen, light or a suitable temperature. No special physiological mechanism is involved here, and the seeds might more properly be considered merely quiescent. Seeds lying deep in the soil are probably prevented from germination by a lack of oxygen.
Those on the surface are exposed to extremes temperature and irradiance. Those in the shade of other plants may suffer enforced dormancy because of the inhibitory effects of the quality of leaf-filtered light. In all of these cases enforced dormancy prevents germination in unfavourable circumstances. Seeds which fall on stony ground are more likely to experience enforced dormancy than to follow the fate of their biblical counterparts.
iii. Induced dormancy:
It is caused by the unfavourable conditions. In many species newly dispersed seeds have no innate dormancy, but if they fail to meet suitable conditions for germination, they acquire an induced (or secondary) dormancy.
Polygonum persicaria exhibits all three types of dormancy in the field during its first year after dispersal. The seeds, which are shed in autumn, have an innate dormancy which imposes an after-ripening requirement which prevents germination immediately prior to winter.
An enforced dormancy is imposed by low temperatures during winter itself. If the seeds fail to germinate in spring (through exposure to hot or dry conditions) an induced dormancy is acquired which can only be broken by a second period of chilling. Thus each dormancy type prevents germination under a different set of unfavourable conditions.
Causes of Seed Dormancy:
The major causes of seed dormancy are following:
i. Hard seed coat:
The hard seed coat is impermeable to water and oxygen.
ii. Immature embryo:
In some Ranunculus species the embryos are incompletely developed when seeds are shed. In such cases embryos mature during the dormant period.
iii. Presence of inhibitors:
Inhibiting chemicals in the fruit or seed coats or within die embryo or endosperm, may delay germination.
Light sensitive seeds do not germinate in absence of light; whereas light hard seeds do not germinate on the exposure to light.
Generally the low temperatures promote and high temperatures inhibit the germination. In the Indian desert many weed seeds must pass through very high temperature (60°-70C) of sand in day and at the same time low temperatures (5°-10°C) of night before they are stimulated for germination.
Effects on Seed Dormancy:
i. Influence of parental environment on seed dormancy:
The germination characteristics of a seed are laid down during the course of its development, and it is not surprising to find that the environmental conditions experienced by the parent plant during seed maturation can strongly influence the degree and type of dormancy in the seed.
Harrington and Thompson (1952) found that germ inability was inversely proportional the mean temperature of the parents’ growing conditions during the 30 days before seeds harvest.
The day length experienced by the parent plant (especially during the last few days of seed maturation) also affects dormancy in certain species. This has been extensively studied in desert annuals by Gutterman (1982). In le species studied there is a clear relationship between day length and germ inability.
ii. Influence of seed position in plant on seed dormancy:
The germination behaviour of seeds from the same parent is often correlated with their position in the inflorescence of the parent plant. In some case the plant may produce two or more distinct types of seeds which different from each either in size, shape and colour, as well as in their requirement for germination.
This phenomenon-Polymorphism, is part: particularly well developed in certain families such as the Asteraceae, the Chenopodiaceae md the Poaceae.
Seed polymorphism is most common in arid zone plants. In these plants occurrence of differences in seed shape, colour, size and weight have been reported. The variations in colour, size and weight of seeds are controlled by the genetic constitution.
The seed polymorphism is very common in Crotalaria medicaginea, Indigofera tinctoria, Euphorbia caducifolia, Tephrosia purpurea, Cenchrus spp. etc.
It is possible that many species whose seeds are not visibly polymorphic may have seeds which are at least physiologically polymorphic n their germination requirements.
The physiological mechanism involved in both morphological and physiological polymorphism would have the effect of broadening the range of conditions which the population can exploit for regeneration.
Differences in germinating behaviour between seeds from an individual parent plant may be due to variations in the microenvironments experienced by seeds in different parts of an inflorescence.
The seeds once germinated have lost all resistance against drought. This is true not only for desert seeds but also for seeds from plants from temperate regions. Thus, in arid zone seeds prevented from germinating by being buried too deep may retain their viability for scores of years.
An arid zone seed which has inhibitors may not germinate. But as linseeds become older the inhibitor activity decreases; at the same time the mechanical strength of the seed coat decreases.