The Role of Leaf Movements for Optimizing Photosynthesis in Relation to Environmental Variation

Abstract

There is an amazing array of leaf movements among plants, elicited by a wide variety of environmental signals. Leaf movements may occur over developmental time scales, involving growth processes, or over the scale of milliseconds involving rapid depolarization of membranes and ion fluxes. This chapter covers primarily leaf movements in response to light, although mechanical stimuli, temperature, and water are also discussed. The focus on phototropism, heliotropism, and thermonasty is due to the large effects these movements have on rates of photosynthesis. Heliotropism, whether it is due to differential growth on opposite sides of a plant stem, or turgor changes within a motor organ such as a pulvinus, has direct and large effects on light interception. Diaheliotropic, paraheliotropic, or a combination of the two movements may act to increase photosynthesis by increasing light interception, or optimize resource use efficiency by modulating incident light at intermediate levels. Paraheliotropic and wilting movements are major adaptive mechanisms in response to water deficits, high light, high temperatures, or a combination of these stresses. Thermonastic movements have been shown to play a major role in reducing damage to the photosynthetic machinery during low temperature periods, or conversely in high temperature periods. Despite the mounting number of case studies, a comprehensive understanding of the signal cascades and regulatory genetic pathways controlling rapid, reversible leaf movements has yet to be accomplished. An understanding of the phylogeographic distribution of rapid leaf movements is also lacking. The presence of rapid leaf movements in a number of commercially important crop species suggests that incorporation and modification of these properties may have potential to improve both productivity and stress resistance in cultivated species.