A plant pigment, phytochrome, controls response to light. The inactive form, Pr, absorbs red light to become Pfr, the active form. It is reconverted to Pr by absorbing far-red light. The ratio between these two forms provides information about daylight because sunlight has more red than far-red light. Phytochrome also controls early seedling growth, provides information about shading, and helps direct shoot phototropism and seed germination.
Senescence is an active and passive cessation of growth.
Growth becomes dormant during cold or dry times and resumes when environmental conditions are more favorable. Internal biological clocks control daily responses, or circadian rhythms. Environmental changes can alter, or entrain, these clocks. The McGraw-Hill Companies. Any use is subject to the Terms of Use and Privacy Policy .