The physiological inability of a seed to germinate even under favourable condition is called seed dormancy. Many kinds of seeds can germinate immediately after maturation. The seeds in some species of citrus may germinate in the fruit. But in some cases seed passes a period of rest or inactivity before germination. Such seeds do not germinate even in favourable external conditions. Something blocks the processes germination. It is seed dormancy .
CAUSES OF SEED DORMANCY
I here are following causes of seed dormancy.:
- Hard Seed Coat
Seeds of many species possess hard seed coat. Most of these seeds belong to the family. Leguminosae and Malvaceae. Such seeds remain dormant. Hard seed coat prevents germination due to following reasons:
(a) Hard seed coat prevents the entry of water into the seed.
(b) Hard coat obstruct exchange of gases, especially oxygen. Oxygen is necessary for respiration.
(c) Hard seed coat causes mechanical resistance. Thus radicle does not come out.
- Immature Embryo
The embryo is fully developer inside the seed in most of the plants before shedding from the parent plant. But development of the embryo is incomplete at dispersal in few cases like Fraxinus and Anemone. Such seeds remain dormant and fail to germinate. Such seeds imbibe water and the development of the embryo is completed in a few weeks. Then the embryo is fully developed Now the seed can germinate under favourable conditions.
- Light Sensitive Seeds
There two type of seeds for light sensitivity:
(i) Positive photoblastic seeds: Seeds of most of the species germinate equally well both in the dark and in light. Therefore light is not necessary for germination. However, seeds of some species like lettuce remain dormant in the dark. –They do not germinate till these are exposed to light.These are called positive photoblastic seeds. Therefore dark inhibits germination in these seeds.
(ii) Negative photoblastic seeds: Seeds of some species like phlox. Nemophila and Silene remain dormant when exposed to light. They germinate only in the dark. These are called negative photoblastic seeds.
- Chilling Requirement
Seeds of many temperate trees remain dormant and do not germinate after harvest. Its examples are apple. walnut and pinus. Such seeds need chilling temperature (1 -5o) for a few weeks.This requirement is met in nature. These seeds lie buried in the soil during winter. Therefore they get chilling temperature. But some seed become dormant in nature. They need artificial chilling.
- Dormancy due to growth inhibitory chemicals
There are several naturally occurring compounds which inhibit germination of seeds. Seeds of many species contain several phenolic compounds. These phenolic compounds are ferulic acid, parasorbic acid and coumarin. These compounds are present in the embryo or in the seed coats. They inhibit the germination. Sometimes, an inhibitory hormone abscisie acid prevents germination.