1) Cells undergo changes in structure.
2) Vacuole of the cell acts as lysosome and secretes hydrolytic enzymes.
3) The starch content is decreased in the cells.
4) Photosynthesis is reduced due to loss of chlorophyll accompanied by synthesis and accumulation of anthocyanin pigments, therefore the leaf becomes red.

1l) The knowledge of photoperiodism plays an important role in hybridisation experiments.
2) Photoperiodism is an excellent example of physiological pre-conditioning that is using an external factor to induce physiological changes in the plant.

Phytochrome is a bluish biliprotein pigment responsible for perception of light in photo physiological process. It was named by Butler et a1 and exists in two inter convertible forms - P_r (red . r light absorbing pigment)' and (P_fr - far red light absorbing pigment).

Agricultural role:
1. Formation of seedless fruits'without fertilization is induced by gibberellins Example: Seedless tomato, apple and cucumber.
2. It promotes the formation of male flowers in cucurbitaceae. It helps in crop improvement.
3. Uniform bolting and increased uniform seed production
4. Improves number and size of fruits in grapes
5. Promotes elongation of inter-node in sugarcane without decreasing sugar content.
6. Promotion of flowering in long day plants even under short day conditions.

lnternal Factors:
1. Genes are intracellular factors for growth.
2. Phytohormones are intracellular factors for growth. Example: auxin, gibberellin, cytokinin, etc.
3. C/N Ratio: The ratio of carbohydrates and nitrogenous compounds regulate the specific pattern of growth in plants