Seed dormancy

seed dormancy

1. SEED DORMANCY

2. CONTENT Introduction Classification Mechanism of Dormancy Factors causing Dormancy Methods to Overcome Dormancy Biological Importance of Seed Dormancy

3. Endosperm Aleurone layer Cotyledon Epicotyl Radical Testa

4. Introduction During the developmental cycle of the plant, at some phase certain structures like buds, tubers, seeds, go through a period of temporary suspension of growth activity, such a state is called dormancy. In plant physiology , dormancy is a period of arrested plant growth . It is an survival strategy exhibited by many plants under unfavorable conditions. To overcome unfavorable conditions , lower plants produces Endospores,Zygospores,Auxospores,.. etc

5. Seed dormancy: Seed dormancy is defined as a state in which seeds are unable to germinate even under environmental conditions normally favorable for germination.

6. HORMONAL MECHANISM : Studies with ABA and GA biosynthesis and signaling pathway demonstrated that these two hormones have important and antagonistic roles in germination and dormancy respectively. ABA induces and maintains seed dormancy during embryo maturation. The key to ABA metabolism are some specific genes encoding key enzymes are of NCED family ( NCED6 and NCED9 ). ( Nine-cis-epoxy carotenoid dioxygenase.) Embryonic identity protein (FUS3), which positively regulates ABA synthesis.

7.   Dormancy factor   DOG1  transcription levels are enhanced by low temperatures during seed maturation.

8. CLASSIFICATION OF DORMANCY

9. Primary Dormancy : Due to internal causes even if all external environmental conditions are duly present. Ex:Hormonal factor (ABA-GA imbalance), Genetic factors Secondary Dormancy : When dormancy caused due to external factors like absence of sunlight, CO 2 , very high or low temperature. Ex: Nicotiana attenuata only germinate if there is presence of smoke.

10. Classification of seed dormancy Exogenous dormancy Physical dormancy Mechanical dormancy Chemical dormancy Endogenous dormancy Physiological dormancy Morphological dormancy Combined dormancy

11. Physical dormancy : Dormancy that is caused by an impermeable seed coat. It is the result of impermeable layer that develops during maturation and drying of the seed or fruit. It prevents the seed from taking up water or gases. As a result, the seed is prevented from germinating until dormancy is broken. Generally, physical dormancy is the result of one or more palisade layers in the fruit or seed coat.

12. Mechanical dormancy: Mechanical dormancy occurs when seed coats or other coverings are too hard to allow the embryo to expand during germination. Chemical dormancy- Presence of inhibitors within the seed (seed coat) can cause dormancy. E.g.: Cotton seed pericarp contains Abscisic acid Coriander seed pericarp contains Coumarin Paddy hull contain Abscisic acid

13. Physiological dormancy Physiological dormancy prevents embryo growth and seed germination until chemical changes occur. Photo dormancy : light sensitivity affects germination of some seeds. These photoblastic seeds need a period of darkness or light to germinate. Thermo dormancy : seed sensitivity to heat or cold. Some seeds germinate only at high temperature. like  A maranth .

14. Morphological dormancy In morphological dormancy, the embryo is underdeveloped or undifferentiated. These seeds have both morphological and physiological dormancy. Combined dormancy

15. Mechanism of Dormancy Hormonal control theory. Temperature and gaseous exchange restriction theory. Metabolic control theory.

16. Hormonal control theory Dormancy causing hormones are ABA and ethylene. Dormancy causing hormones and dormancy provoking hormones (GA & Cytokinin) ratio determine the fate of dormancy. Chilling treatment, light exposure increases Gibberellin level in seeds & causes germination.

17. Temperature and gaseous exchange theory Restricted oxygen uptake ( due to hard seed coat) and high temperature induces dormancy in seeds. Oxygen deficiency leads to inhibition of Cytochrome oxidase activity which leads to dormancy.

18. Metabolic control theory Dormancy in seed is controlled by metabolic energy produced in seeds. During dormancy Respiratory system blocked ATP synthesis blocked increase in AMP and ADP leads to increase in the activity of Glycolytic pathway. Production of growth inhibitors at lower concentration of oxygen.

19. Factors causing dormancy of seeds Hard seed coat Dormancy due to temperature Immature Embryo Light Sensitive Seed Dormancy due to growth inhibitory chemicals Excessive Salts

20. Dormancy due to hard seed coat – Testa of some seeds are hard and multilayered (Leguminaceae, Solonaceae). Hard seed coat prevents germination due to following reasons: Hard seed coat prevents the entry of water into the seed. Hard seed coat obstruct exchange of gases, especially oxygen. Oxygen is necessary for respiration. Hard seed coat causes mechanical resistance. Thus radicle does not come out.

21. Dormancy due to temperature Some seeds require chilling treatment for germination. Examples are apple, walnut, and Pinus. Such seeds need chilling temperature (1 -5 ºC) for few weeks. Immature Embryo Embryo is not matured at the time of harvesting for maturity of such embryo, resting period is required. Example: Caltha palustris.

22. Light Sensitive Seed There two type of seeds for light sensitivity: Positive photoblastic seeds: Seeds of some species like lettuce remain dormant in the dark, they germinate when they are exposed to light. Negative photoblastic seeds: Seeds of some species like Phlox, Nemophila and Silene remain dormant when exposed to light. They germinate only in the dark.

23. Dormancy due to growth inhibitory chemicals There are several naturally occurring compounds which inhibit germination of seeds, like Coumarin, Cyanides, Ammonia, Dinitrophenols, Organic acid, Aldehydes, Alkaloids, Fluorides. Example: Tomato fruit pulp – Caffeic acid and Ferulic acid which induce dormancy.

24. METHODS TO OVERCOME SEED DORMANCY Mechanical scarification Impaction Cold stratification Removal of inhibitors Treatment with growth hormones Specific light requirement Pressure

25. Scarification : Any treatment i.e. physical or chemical that weakens the seed coat. Rubbing or Puncturing : By rubbing them against rough surface or puncturing the seed coat with pointed needle. E.g. Coriander, Castor. Chemical treatment includes treating seeds with strong acids and organic solvents. E.g.Sulphuric acid.

26. 2. Impaction : it is process of shaking of seeds to clear blockage of testa pores. E.g. Seeds of Crotolaria egyptica (legume)

27. 3. Stratification Treatment of seed at low temperature under moist condition. During this anatomical and biochemical changes takes place in the seed. Growth promoting hormones increases and Growth retarding hormones decreases.

28. 4 . Removal of inhibitors : Growth inhibiting chemicals removed by repeated washing with water. 5. Treatment with growth hormones : Gibberellins substitute the cold treatment requirement in many seeds. Cytokinin promotes germination of many seeds. 6. Pressure : Dormancy can be broken by exposing seeds to high atmospheric pressure. E.g. Sweet Clover seeds subjected to hydraulic pressure of 2000 atm favors the increase in germination at 18 ºC.

29. 7. Specific light requirement : Dormancy of Photoblastic seeds can be overcome by light exposure. Red light promotes germination in lettuce seeds.

30. Biological Importance of Seed Dormancy 1. Dormancy allows the seeds to remain in suspended animation without any harm during drought, cold or high summer temperature. 2. The dormant seeds can remain alive in the soil for several years. 3. It helps the seed to get dispersed over long distances through unfavorable environment or inhospitable area. 4. Dormancy induced by the inhibitors present in the seed coats is highly useful to desert plants. The seeds germinate only after a good rainfall which dissolves away the inhibitors. 5. It follows storage of seeds for later use by man.

31. REFERENCES Modern plant physiology - R K Sinha

32. THANK YOU NAVINKUMAR Jr. M.Sc.(agri.) PG18AGR10057