Photosynthetic carbon fixation by plant leaves uses atmospheric CO2 as a substrate. In terrestrial plants, the carbon dioxide entering the leaf shares its diffusion pathway with the water lost by evaporation at the leaf surfaces. To avoid desiccation, plants regulate their gas exchange, minimizing water loss with minimal curtailment of CO2 uptake. Gas exchange in leaves is controlled by a pair of guard cells surrounding the stomatal pores in the leaf epidermis. Guard cells function as turgor valves: when the plant has an abundant water supply and the environmental conditions favor high photosynthetic rates, guard cells are turgid and the stomatal pores are wide open. At night, or under stress, stomata close and water evaporation is reduced. Guard cells are continuously sensing the leaf environment and the perceived environmental signals are transduced into appropriate turgor levels. We study sensory transduction in guard cells at different levels of organization. Stomatal responses in the whole leaf are analyzed with gas exchange techniques. An attached, intact leaf is placed in a gas exchange cuvette and the rates of photosynthesis and stomatal conductances are measured. Using gas exchange analysis we recently found that new cotton varieties bred for heat resistance and higher yields, have higher stomatal conductances than older varieties. Using a number of these varieties chosen along a gradient of increasing yields and increasing conductance levels, we are studying the inheritance of stomatal regulation and the mechanisms determining the stomatal responses to temperature. At the cellular level, we study sensory transduction in isolated guard cells and their organelles. The protoplasts, obtained by enzymatic treatment of epidermal strips, respond to many of the signals perceived by the guard cells in the intact leaf. Using guard cells protoplasts and the electrophysiological technique of patch clamping, we have characterized a light activated, electrogenic ion pump at the guard cell plasma membrane. Several lines of evidence, including the measurements illustrated in the accompanying figure, indicate that the pump is a proton pumping ATPase, that generates an electrochemical gradient used by the guard cells to drive the active ion transport required for turgor build-up and stomatal opening. We are currently studying different aspects of the sensory transduction of the light signal during stomatal movements, including the properties of the guard cell chloroplast, the identity and localization of the blue light photoreceptor in guard cells and the relationship between ion transport and carbon metabolism. Characterization of the properties of the guard cells in isolation and in the intact leaf enriches our understanding of plant biology and will hopefully help agronomist and breeders design plants that require less water for their optimal productivity.