Chapter 19. Photomorphogenesis and photoperiodism in plants

It has long been observed that light affects the way plants grow and develop. Photomorphogenesis encompasses all responses to light that affect plant form. Phototropic responses involve the reorientation of plant organs with respect to an asymmetry in the incident light Photoperiodic responses are those in which various aspects of development are modified in response to changes in the daily light/dark cycle, and involve a circadian timing mechanism. This chapter deals with the discovery and nature of the photoreceptors involved in these phenomena, their physiological roles as determined in the laboratory, and their possible significance in the natural environment. Although lower plants also show clear photomorphogenic responses, they have in general been less intensively studied, and we will restrict this discussion to higher plants.