Dormancy is a period in an organism’s life cycle when growth, development, and physical activity are temporarily stopped. Dormancy is a period of reduced metabolic activity and therefore helps an organism to conserve energy. Dormancy can be observed in plants as well as in animals. In animals, mechanisms of dormancy occur through hibernation, aestivation, diapause, and brumation. Likewise in plants, dormancy can be seen in seed or a higher emerged plant.
Seed dormancy is defined as the condition in which a viable seed fails to dominate even under favorable conditions until they are given dormancy-breaking treatment. It is the temporary suspension of growth of the embryo which is accompanied by reduced metabolic activity. Seed dormancy can be innate (dormancy is present when the seed is still attached to the mother plant) or induced (dormancy occurs after the seed is detached from the mother plant).
CAUSES OF SEED DORMANCY:
- Underdeveloped embryo – Embryo is rudimentary and poorly developed at seed maturity. Example: palms, orchids.
- Immature embryo – Embryo is fully developed but not properly matured and needs low-temperature treatment (stratification) for germination. Example: apple, pear, peach, blackberry.
- Hard seed coat – The presence of a hard seed coat prevents the expansion and emergence of the embryo during germination. Example: Amaranthus, Capsella.
- Impermeable seed coat – The impermeable seed layer formed from polysaccharides, hemicelluloses, fats, wax, and protein prevents the uptake of water and gases necessary for seed germination. Example: a family of Leguminosae, Malvaceae, Chenopodiaceae.
- Chemical inhibitors- Chemicals present in the seed that suppress the germination process are known as inhibitors. They can be present in testa, endosperm, embryo, or embryo surrounding structures like juice or pulp of fruit (tomato) and glumes (oats). Examples of inhibitors are Abscisic acid (ABA), Coumarin, HCN, Ferulic acid, etc.
2. External Factors:-
- Chilling requirement of embryo- Temperate species like apple, pear, plum, cherry remain dormant in winter and require a chilling temperature of 3-10°C for germination.
- High-temperature requirement of embryo- Seeds of rice, Cyperus rotundus requires a temperature of 40-50°C under moist conditions for germination.
- Light requirement of embryo- Seeds which are sensitive to light are called photoblastic Seeds can be positively photoblastic (germinate only after the exposure of light) like tobacco, lettuce, or negatively photoblastic (germination is inhibited in presence of light) like Allium spp.
METHODS OF BREAKING DORMANCY:
1.Seed coat treatment/scarification:- The process of weakening hard and impermeable seed coat by breaking, rupturing, or scratching through mechanical or chemical methods is known as scarification.
- Mechanical scarification = Rubbing seed against an abrasive surface like rotating drums, sandpaper.
- Physical scarification = Soaking seed in normal water for 12-24 hours or soaking in hot water for 1-2 minutes.
- Chemical scarification = Seeds are soaked in conc. H2SO4 or HNO3 for some minutes to several hours. Example: Blackgram in conc.HSO4 for 3minutes, rice seed in 3% HN03 for 6-8 hours.
- Stratification = It is the process of treating seed in a moist medium at low temperature (5-10°C) for a sufficient period. Seeds are placed in alternate layers of moist sand or wet sphagnum at low temperature. Example: Apple, Pear, etc.
- High temperature treatment = Seeds are incubated at a temperature of 40-50°C with moisture less than 15%. Example: Rice seeds having moisture less than 15% are incubated at 40-50°C for 4-5 days.
- Alternate temperature treatment = Treatment of seed with alternate high and low temperature of temperature difference 10-12°C. When the dormant seeds are subjected to alternate temperature of 20°C for 16 hr and 30°C for 8 hr, the seed lot recorded 90% germination
3.Chemical treatment:- Chemicals are used for initiating dormancy like gibberellic acid(100ppm), kinetin(10-15ppm), ethrel, thiourea, KNO3, benzyl adenine etc. Example: 5-3% Thiourea is used for light and chilling temperature requiring seeds like lettuce, 2ppm Benzyl adenine for sunflower, 75ppm Ethrel for groundnut, etc.
4.Light treatment:- Exposure of positive photoblastic seed like lettuce, tobacco to red light of wavelength 670 nm activates germination.
5.Pressure treatment:- Application of high hydraulic pressure of 2000 atm at a temperature of 18 degrees Celsius for 5-20 minutes initiates germination by increasing permeability of testa to water and oxygen. Example: sweet clover, alfalfa.
SIGNIFICANCE OF SEED DORMANCY:-
- Protects the seed from damage arising due to unfavorable climatic conditions ensuring survivability of seed.
- Assists in the storage of grain which can be later used for human consumption processes.
- Helps in dispersal of seed over long distance.
- Dormant seed can remain alive in the soil for several years and hence become the source of new plants.
- Prevents seed from sprouting on panicles before harvesting.
- Dormancy induced by a hard seed coat can pass through the alimentary canal of animals and still remain unharmed.
- Creation of seed bank.
DORMANCY AS A SURVIVAL MECHANISM OF SEED:
Seed usually germinates under optimum conditions of moisture, air with favorable temperature, and light. Dormancy allows seed to postpone the germination process under environmental conditions and become appropriate for germination and seedling development. Dormancy allows the seeds to remain in suspended animation without any harm during drought, cold or high summer temperature, as well as dormant seeds, can live in the soil for several years. Dormancy allows the germination of seed only when the competition becomes less intense in the surrounding for resources among other species hence enhancing the seed survivability. Dormancy prevents the seed from deterioration during storage. Thus, seed dormancy extends the seed longevity i.e the lifespan of seed and enhances seed storability.
Dormancy is found to be a temporary state of reduced metabolic activity under harsh environmental circumstances. Dormancy occurring in seed either in innate or induced form can play a crucial role in survivability of seed. Dormancy permits the seed to withstand the environmental stress which otherwise would not be survived by ordinary seeds. In the havoc of the COVID-19 where seed shortage and late planting have been a major problem, the implication of seed dormancy can assist in maintaining longevity and storability of seed and hence ensuring productivity. Thus, dormancy is said to be an adaptive mechanism enhancing seed survivability and extending seed longevity and storability.