Abscission is a physiological process whereby plants shed a part, such as leaf, flower or fruit, and retard their vegetative growth. It is a survival mechanism adopted by plants to live through adverse conditions and is promoted by a plant hormone called abscisin (also abscisic acid) produced by leaves and fruits. Extreme temperatures limit the metabolic activities such as respiration, of plants. Such a reduction consequently necessitates only a low level of photosynthetic activity.
The reduction in the requirement of energy can be to such a level which could be got from the photosynthesis activity of a few green cells, present in the terminal regions, after all the leaves fall.
Sometimes, in winter, ice crystals begin to form in the extra- cellular spaces and the viscosity of the cell protoplasm increases. To counter this, compatible osmotic such as betaine (an alkaloid) begins to build up. This process, osmo-regulation, helps plants to overcome the stress due to frost.