(b) Hypogeal Germination . Factors affecting germination Seed germination is directly affected by external and internal factors: i. External factors a. Water : It activates the enzymes which digest the complex reserve foods of the According to Lysenko, development of an annual seed plant consists of two phases.
First phase is thermostage , which is vegetative phase requiring low temperature and suitable moisture. Next phase is photo stage which requires high temperature for synthesis of florigen (flowering hormone). ii. Hypothesis of hormonal involvement According to Purvis ( ), formation of a substance A from its precursor, is converted into B after chilling.
The substance B is unstable. At suitable temperature B is converted into stable compound D called Vernalin . Vernalin is converted to F (Florigen). Florigen induces flower formation.
At high temperature B is converted to C and devernalization occurs (Figure . ). A B D F Translocation of flower inducing substance C Chilling Precursor Vernalin Florigen High temperature Devernalization Figure . : Vernalization and Flowering .
Technique of Vernalization: The seeds are first soaked in water and allowed to germinate at o C to o C. Then seeds are transferred to low temperature ( o C to o C) from few days to days. Germinated seeds after this treatment are allowed to dry and then sown. The plants will show quick flowering when compared to untreated control plants.
. Devernalization Reversal of the effect of vernalization is called devernalization . . Practical applications .
Vernalization shortens the vegetative period and induces the plant to flower earlier. seed. If the water content of the seed goes below a critical level, seeds fail to germinate. b.
Temperature : Seeds fails to germinate at very low and high temperature. The optimum temperature is o C to o C for most tropic species. c. Oxygen : It is necessary for germination.
Since aerobic respiration is a physiological requirement for germination most will germinate well in air containing % oxygen. d. Light : There are many seeds which respond to light for germination and