Leaf expansion - an integrating plant behaviour

Leaves expand to intercept light for photosynthesis, to take up carbon diox ide, and to transpire water for cooling and circulation. The extent to whic h they expand is determined partly by genetic constraints, and partly by en vironmental conditions signalling the plant to expand more or less leaf sur face area. Leaves have evolved sophisticated sensory mechanisms for detecti ng these cues and responding,vith their own growth and function as,well as influencing a variety of whole-plant behaviours. Leaf expansion itself is a n integrating behaviour that ultimately determines canopy development and f unction, allocation of materials determining relative shoot : root volume, and the onset of reproduction. To understand leaf development, and in parti cular, how leaf expansion is regulated, we must know at the molecular level which biochemical processes accomplish cell growth. Physiological experime ntation focusing on ion fluxes across the plasmamembrane is providing new m olecular information on how light stimulates cell expansion in some dicotyl edonous species. Genetic analyses in Arabidopsis, corn, and other species a re rapidly generating a list of mutations and enzyme activities associated with leaf development and expansion, Combination of these approaches, using informed physiological interpretations of phenotypic variation will allow us in the future to identify genes encoding both the processes causing cell expansion, and the regulators of these events.