STOMATAL RESPONSES TO LIGHT AND CARBON DIOXIDE

Stomatal pores in the leaf epidermis permit diffusion of CO(,2) into the photosynthetic mesophyll, but also unavoidably allow water loss. Control of gaseous diffusion is achieved by guard cell pairs which regulate pore apertures in response to factors affecting photosynthesis and plant water status.;Whole-cell patch clamping of guard cell protoplasts showed that blue light causes hyperpolarization of the plasmalemma following a 25-35 s delay. The response was observed in the absence of K('+), and when gradients for passive ion fluxes had been eliminated, and was blocked by the H('+)-ATPase inhibitor diethylstilbestrol. These results indicate that blue light stimulates proton extrusion by an ATPase which does not require K('+). The resulting electrical gradient drives K('+) influx, guard cell swelling, and stomatal opening.;In Commelina and Paphiopedilum, the blue light-stimulated conductance response was enhanced by background red light and inhibited by high vapor pressure deficits (VPD's). Low intercellular concentrations of CO(,2) also enhanced the response. Such interactions may fine tune conductance to an appropriate level, given prevailing environmental conditions.;Stomata responded to pulses of blue but not red light, hence the blue light response may also mediate conductance increases during sunflecks, possibly relieving CO(,2) limitation of carbon fixation. Under continuous illumination, both the blue and the red light response decrease stomatal limitation of assimilation.;An analysis was made of the energy costs and sources underlying stomatal opening. The cellular mechanisms of the stomatal responses to CO(,2) and light were then investigated experimentally. Paphiopedilum harrisianum, an orchid with achlorophyllous guard cells, had a weaker conductance response to CO(,2) than its close relative, Phragmipedium longifolium, with normal chlorophyllous guard cells, suggesting that chloroplasts are involved in the response. Paphiopedilum stomata responded to blue but not red illumination, supporting the hypotheses that guard cells have a non-chlorophyllous blue light photoreceptor, and that their red light response is chlorophyll-mediated.;Environmental "stresses" including light quality may also directly affect photosynthetic rates. These effects can be quantified as the depression in assimilation which remains after manipulation of VPD has been used to return conductance to pre-stress levels.