During germination the cells of the embryo resume metabolic activity. The stored fats, proteins and starch are digested. The insoluble food is made soluble and complex food is made simple. Assimilation of this food by the growing embryo induces growth and the seedling assumes its shape.
As the food is translocated, the cells of the embryo become active and grow in size and divide rapidly to form the seedling.
Based on the behaviour of cotyledons, germination is of two types
i) Epigeal Germination
ii) Hypogeal Germination
In this type of germination, the cotyledons come above the surface of the soil into the air due to the rapid growth and elongation of the hypocotyl. The cotyledons turn green and make food. They finally dry up and fall off once the seedling becomes an independent plant.
Examples: Bean, gourd, castor and cotton.
In this type, the cotyledons remain in the soil and the epicotyl elongates pushing the plumule upwards. The cotyledons do not turn green and slowly dry and fall off.
Examples: Maize and coconut
Plants vary greatly in response to light with respect to seed germination. Seeds which respond to light for their germination are named as photoblastic
. Non-photoblastic seeds germinate irrespective of the presence or absence of light.
For the light sensitive seeds, the red region of the visible spectrum, is most effective for germination. The far red region reverses the effect of red light and makes the seed dormant.
The red light and far-red sensitivity of the seeds is due to the presence of a photo receptor pigment called phytochrome
Functioning of Phytochrome System
The pigment phytochrome that absorbs light occurs in two inter convertible forms Pr
is metabolically inactive and absorbs red light (660nm). On absorption of light it gets converted into a metabolically active Pfr
promotes germination. Pfr
converts back to Pr
after absorbing far red light (730nm). Treatment with red light (R) stimulates seed germination, whereas far red light (FR) inhibits seed germination. The quality of light to which seeds are exposed last, determine seed germination. This can be shown as
R ……. germination
R + FR ….. no germination
R + FR + R ….. germination
R + FR + R + FR….. no germination
Light requirement for seed germination may be replaced by hormones such as gibberellins or cytokinins.