42. Seeds of some plants germinate immediately after shedding from the plants while in other plants they require a period of rest before germination. The later phenomena is called as dormancy. Give the reasons for seed dormancy and some methods to break it.


Dormant seeds remain under non-germination conditions only for a specific period of time that may vary from days to years. This specific period is called dormancy period.
Causes of Seed Dormancy
(a) In many plants the cause of dormancy is due to the impermeability of seed coat to water, (e.g., Chenopodium, Trigonelia, Meliotus) or oxygen (e.g., Brassica alba, Pyrus malus-Apple, Sinapis arvensis) or chemicals, (e.g., Xanthium)

(b) In many plants, tough (hard) seed coats are the cause of dormancy as they provide mechanical resistance to embryo growth, e.g., Capsella, Lepidium.
(c) Some seeds produce certain chemical substances, such as abscisic acid (ABA,most common), phenolic acids, coumarin, short chain fatty acid, etc. which inhibit the seed germination. These inhibitors may be present inside the fruit, (e.g., the fruit juice of tomato contains ferulic acid), in the embryo (e.g., Xanthium), endosperm, (e.g., Iris) or seed-coat, (e.g., Cucurbita)
(d) Shedded seeds like those of wheat, barley, oat etc., need an interval for ripening and gaining the ability to germinate. During the interval, the seeds produce necessary growth hormones.
Methods of breaking dormancy are as follows
(i) Inactivation of growth inhibitors by heat or cold treatment.
(ii) Mechanical abrasions weaken the tough and impermeable seed coat.
(iii) Microorganisms present in the soil weaken and decompose hard seed coat.
(iv) Washing away of inhibitors by rain or imigation water.
(v) Maturation of embryo.