Answer:
Dormant seeds remain
under non-germination conditions only for a specific period of timethat 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 towater,
(e.g., Chenopodium, Trigonella, Meliotus) or oxygen (e.g., Brassica alba,
Pyrusmalus-Apple, Sinapisarvensis) or chemicals, (e.g., Xanthium)
(b) In many
plants, tough (hard) seed coats are the cause of dormancy as they
providemechanical 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 seedgermination. These inhibitors may be present inside the fruit, (e.g.,
the fruit juice oftomato contains ferulic acid), in the embryo (e.g., Xanthium),
endosperm, (e.g., Iris) orseed-coat, (e.g., Cucurbita)
(d) Shedded
seeds like those of wheat, barley, oat etc., need an interval for ripening
andgaining the ability to germinate. During the interval, the seeds produce
necessarygrowth 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 irrigation water.
(v) Maturation of
embryo.
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