o C). This process is called Vernalization. The term Vernalization was first used by T. D.
Lysenko ( ). . Mechanism of Vernalization: Two main theories to explain the mechanism of vernalization are: i. Hypothesis of phasic development ii.
Hypothesis of hormonal involvement i. Hypothesis of phasic development ( ) named this pigment and it exists in two interconvertible forms: (i) red light absorbing pigment which is designated as P r and (ii) far red light absorbing pigment which is designated as P fr . The P r form absorbs red light in 660nm and changes to P fr. The P fr form absorbs far red light in 730nm and changes to P r .
The P r form is biologically inactive and it is stable whereas P fr form is biologically active and it is very unstable. In short day plants, P r promotes flowering and P fr inhibits the flowering whereas in long day plants flowering is promoted by P fr and inhibited by P r form. P fr is always associated with hydrophobic area of membrane systems while P r is found in diffused state in the cytoplasm. The interconversion of the two forms of phytochrome is mainly involved in flower induction and also additionally plays .
It increases the cold resistance of the plants. . It increases the resistance of plants to fungal disease. .
Plant breeding can be accelerated. . Seed Germination and Dormancy I. Seed Germination The activation and growth of embryo from seed into seedling during favourable conditions is called seed germination .
. Types of germination There are two methods of seed germination. Epigeal and hypogeal. i.
Epigeal germination During epigeal germination cotyledons are pushed out of the soil. This happens due to the elongation of the hypocotyl. Example: Castor and Bean. ii.
Hypogeal germination During hypogeal germination cotyledons remain below the soil due to rapid elongation of epicotyls (Figure . ). Example: Maize, Pea. Foliage leaves Hypocotyl Radicle (b) Pea Epicotyl Cotyledon Hypocotyl Epicotyl Hypocotyl Cotyledon Foliage leaves Hypocotyl Radicle (a) Bean Figure .
: (a) Epigeal Germination