Light energy become effective when it is absorbed by a pigment. These pigments are called phytochromes. It occurs in two forms namely Pr and P_fr. The two forms are inter convertible. The P_r form absorbs red light of 660 nm and changed to p_fr. The p_fr form absorbs far red light of 730 nm and changed to P_r. The P_r form stimulates whereas P_fr form inhibit flowering. The P_r form stimulates whereas p_fr form inhibit flowering. P_fr form of phytochrome is the active form. The P_r form is not considered to be active.

In most fruits the rate of respiration will undergo a sharp rise and then fall near the end of ripening. Kidd and West termed this phenomenon "climacteric rise". The climacteric acts as a trigger that sets in progress those changes that rapidly transform the fruit from an unripe to a ripe condition. Finally, application of ethylene to unripe fruit will bring on a premature climactric and accelerate ripening

The rate of shoot elongation is greatest when the seedling is in the dark, and decreases upon exposure to light.

Long-day plants usually flower in the spring or early summer; they flower only if the light periods are longer than a critical length, which is usually 9-16 hours. For example, wheat plants flower only when light periods exceed fourteen hours.

The induction of flowering in response to the relative length of daily light and dark period is called photoperiodism.

Short-day plants flower only if light periods are shorter than some critical length. In these plants the dark period is critical and must be continuous. If this dark period is interrupted even with a brief exposure of light, the short day plant will not flower. Strawberries, Dahlias,Soyabeans and Chrysanthenums are short day plants.

Gibberellin overcomes vernalization requirement (low temperature) for flowering.