Seed dormancy and germination
Introduction
The seed is an important stage in the higher plant life cycle with respect to its survival as a species. It is the dispersal unit of the plant, which is able to survive the period between seed maturation and the establishment of the next generation as a seedling after it has germinated. For this survival, the seed, mainly in a dry state, is well equipped to sustain extended periods of unfavorable conditions. To optimize germination over time, the seed enters a dormant state. Dormancy prevents pre-harvest germination as well. Numerous studies have been performed to better understand how germination is controlled by various environmental factors and applied chemicals. However, still very little is known about the process by which the embryo emerges from the seed to complete germination and how embryo emergence is blocked in dormant seeds.
Arabidopsis possesses seed dormancy, as is the case for many other plant species. This property is controlled by environmental factors such as light, temperature and time of seed dry storage as well as by genetic factors. The use of genetics and molecular genetics in Arabidopsis is starting to shed light on some aspects of the mechanism of dormancy and germination by the identification of mutants and genes that control these processes. This review provides an overview of current knowledge of seed dormancy and germination in Arabidopsis based mainly on the contribution that molecular genetics made to the study of this process including a table with genes that are related to germination/dormancy. Several recent reviews describes more details of the molecular mechanism that were derived from the mainly molecular genetic studies combined with physiological experiments.
Seed dormancy
Seed dormancy has been defined as the incapacity of a viable seed to germinate under favorable conditions Dormancy in Arabidopsis should be described as physiologically non-deep, meaning that embryos released from surrounding structures grow normally, and that dormancy is lost through moist chilling (stratification) or after-ripening However, in addition to the testa and endosperm layer surrounding the embryo, the growth potential of the embryo is also important to overcome the constraint of these structures and thereby affects the dormancy state of a seed
Since dormancy is regulated at different developmental phases, in interaction with environmental factors, it is difficult to detect when the genetic and physiological differences are established. This difficulty arises because all dormancy assays are based on seed germination, which is the result of the balance between the degree of dormancy and the capacity of the embryo to overcome dormancy. Mechanistically one can distinguish factors that influence dormancy and germination on the basis of their effect on germination, being either inhibiting or promoting. Mutants that germinate better or faster can represent genes that promote dormancy or those that repress germination. A further distinction can be made by defining the timing and site of action of these factors (during maturation or during imbibition of the seeds, in theembryo, the endosperm or in the testa). The interaction between these factors and the large effect of the environment, both during seed development and during imbibition, make seed dormancy a very complex trait.
Importance
The phenomenon of seed dormancy is best understood at three levels. At the population level, seed dormancy enables the formation of a soil seed bank from which plants can emerge at different times of year or in response to ecosystem disturbances. At the single-plant level, individual mothers have evolved mechanisms for maintaining control of progeny seed-germination behaviour and generating heterogeneity in progeny seed properties. This enables mothers to hedge their bets by producing seeds with different propensities to germinate or that are likely to germinate at different times and places. Finally, at the level of the individual progeny seed, mechanisms exist to maintain and break dormancy, often in response to environmental stimuli that limit germination to specific annual time windows, or enable seeds to wait for gaps in the canopy to appear.
1.Perennation:
favourable conditions.
Seed Dormancy: Reasons and Importance
Reason of seed dormancy
1.IMMATURITY OF EMBRYO
Embryo is immature at the time of seed shedding. The seed will remain dormant till the embryo becomes mature, e.g., Anemone nemorosa, Ranunculus ficaria.
2. After ripening
The seeds require a period of dry storage for developing the ability to germinate, e.g., Wheat, Oat, Barley.
3.IMPERMEABLE SEED COAT
The seed coat is impermeable to water and gases, e.g., Apple, Chenopodium.
4.HARD SEED COAT
The seed coat is mechanically resistant and does not allow the embryo to grow, e.g., Amaranthus, Lepidium.
5.GERMINATION INHIBITORS
They occur in the seed coats and cotyledons of the embryos. The important germination inhibitors causing seed dormancy are abscisic acid, phenolic acid, ferulic acid, coumarin, short fatty acids and cyanogenic chemicals, e.g., Apple, Peach, Ash, Cucurbita, Iris, Xanthium.
NATURAL BREAKDOWN OF SEED DORMANCY
In nature seed dormancy is broken automatically due to:
(i) Development of growth hormones to counter growth inhibitors,
(ii) Leaching of germination inhibitors,
(iii) Maturation and after-ripening of embryo,
(iv) Weakening of impermeable and tough seed coats by microbial action, abrasion, passage through digestive tract of animals, etc.
Artificial Breaking of Seed Dormancy:
1.Scarification:
Hard, impermeable seed coat is weakened or ruptured by filing, chipping, hot water and chemicals.
2. Stratification:
Seeds are moistened and exposed to oxygen for variable period at low or high temperature.
3. Counteracting Inhibitors:
Inhibitors are destroyed by dipping seeds in KNO3, thiourea, ethylene chlorohydrin and gibberellin.
4.SHAKING AND PRESSURE
Vigorous shaking and hydraulic pressure are used to weaken seed coats.
Importance of Seed Dormancy:
1. Perennation:
Seed dormancy allows seeds to pass through drought, cold and other un-favourable conditions.
2. DISPERSAL
It is essential for dispersal of seeds.
3. Germination under Favourable Conditions:
Seeds germinate only when sufficient water is available to leach out inhibitors and soften the seed coats.
4. Storage:
It is because of dormancy that human beings are able to store grains, pulses and other edibles for making them available throughout the year and transport to the areas of deficiency.
Seed germination
By definition, germination incorporates those events that commence with the uptake of water by the quiescent dry seed and terminates with the elongation of the embryonic axis. Water uptake by a seed is triphasic; phase I rapid initial uptake; phase II plateau phase and in phase III further increase of water uptake, however, only when germination occurs. The first signs of germination are the resumption of essential processes, including transcription, translation and DNA repair followed by cell-elongation and eventually at the time of radicle protrusion, resumption of cell division. Physically germination is a two-stage process, where testa rupture is followed by endosperm rupture. Following rupture of the micropylar endosperm by the emerging radicle, germination is complete.
Germination assays in Arabidopsis are often performed in light on seeds freshly harvested or stored for a limited time. Other parameters are the germination rate after different periods of cold treatment) and germination in darkness). In addition to testing mature seeds, germination of immature seeds, either excised from the silique or within fruits detached from the plant, can be used to investigate genetic variation during the early stages of seed development.
Writer: Shushil Pandey (Student, Bsc Ag 4th Sem, IAAS, PAKLIHAWA)