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The supply of energy for stomatal opening was investigated with epidermal peels of Commelina communis L. and Vicia faba L., under white, blue and red irradiation or in darkness. Fluencerate response curves of stomatal opening under blue and red light were consistent with the operation of two photosystems, one dependent on photosynthetic active radiation (PAR) and the other on blue light, in the guard cells. The PAR-dependent system was 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-sensitive and KCN-resistant and showed a relatively high threshold irradiance for its activation; its activity was most prominent at moderate to high irradiances. The blue-light-dependent photosystem was KCN-sensitive, was active at low irradiances, and interacted with the PAR-dependent photosystem at high blue irradiances. Stomatal opening in darkness, caused by CO2-free air, fusicoccin or high KCl concentrations, was KCN-sensitive and DCMU-resistant. These data indicate that stomatal opening in darkness depends on oxidative phosphorylation for the supply of high-energy equivalents driving proton extrusion. Light-dependent stomatal opening appears to require photophosphorylation from guard-cell chloroplasts and the activation of the blue-light photosystem which could rely either on oxidative phosphorylation or a specific, membrane-bound electron-transport carrier.

Blue light-stimulated stomatal opening in detached epidermis of Vicia faba is reversed by green light. A 30 s green light pulse eliminated the transient opening stimulated by an immediately preceding blue light pulse. Opening was restored by a subsequent blue light pulse. An initial green light pulse did not alter the response to a subsequent blue light pulse. Reversal also occurred under continuous illumination, with or without a saturating red light background. The magnitude of the green light reversal depended on fluence rate, with full reversal observed at a green light fluence rate twice that of the blue light. Continuous green light given alone stimulated a slight stomatal opening, and had no effect on red light-stimulated opening. An action spectrum for the green light effect showed a maximum at 540 nm and minor peaks at 490 and 580 nm. This spectrum is similar to the action spectrum for blue light-stimulated stomatal opening, red-shifted by about 90 nm. The carotenoid zeaxanthin has been implicated as a photoreceptor for the stomatal blue light response. Blue/green reversibility might be explained by a pair of interconvertible zeaxanthin isomers, one absorbing in the blue and the other in the green, with the green absorbing form being the physiologically active one.

Osmoregulation in guard cells of intact, attached Vicia faba leaves grown under growth chamber and greenhouse conditions was studied over a daily light cycle of stomatal movements. Under both growth conditions guard cells had two distinct osmoregulatory phases. In the first (morning) phase, opening was correlated with K+ uptake and, to a lesser extent, sucrose accumulation. In the second (afternoon) phase, in which apertures were maximal, K+ content declined and sucrose became the dominant osmoticum. Reopening of the stomata after a C0,-induced closure was accompanied by accumulation of either K+ or sucrose, depending on the time of day, indicating that a single environmental signal may use multiple osmoregulatory pathways. Malate accumulation, correlated with K+ uptake, was detected under growth chamber but not greenhouse conditions, whereas CI-was the main K+ counterion in the greenhouse. These results indicate that guard-cell osmoregulation in the intact leaf depends on at least two different osmoregulatory pathways, K+ transport and sucrose metabolism. Furthermore, the relative importance of the K+ counterions malate and CI-appears to be environment-dependent.

Changes in neutra1 sugar and organic acid content of guard cells were quantitated by high-performance liquid chromatography during stomatal opening in different light qualities. Sonicated Vicia faba epidermal peels were irradiated with 10 pmol m-' s-' of blue light, a fluence rate insufficient for the activation of guard cell photosynthesis, or 125 pmol m-'s-' of red light, in the presence of 1 mM KCI, 0

Responses of stomata to environment have been intensively studied, but little is known of genetic effects on stomatal conductance or their consequences. In Pima cotton (Gossypium barbadense L.), a crop that is bred for irrigated production in very hot environments, stomatal conductance varies genetically over a wide range and has increased with each release of new higher-yielding cultivars. A cross between heat-adapted (high-yiekling) and unadapted genotypes produced F2 progeny cosegregating for stomatal conductance and leaf temperature. Within segregating populations in the field, conductance was negatively correlated with foliar temperature because of evaporative cooling. Plants were selected from the F2 generation specifically and solely for differing stomatal conductance. Among F3 and F4 populations derived from these selections, conductance and leaf cooling were sgnificantly correlated with fruiting prolificacy during the hottest period of the year and with yield. Conductance was not acted with other factors that might have affected yield potential (singleleaf photosynthetic rate, leafwater potential). As breeders have increased the yield of this crop, genetic variability for conductance has allowed inadvertent selection for "heat avoidance" (evaporative cooling) in a hot environment.Stomata represent a unique adaptation of terrestrial plants that couples leaf gas exchange to water availability. As a result, stomatal function has been a focus of interest for plant physiologists at least since the 17th century. The evolution, development, bioenergetics, biophysics, biochemistry, and environmental responses of stomata have since then been well-documented (1, 2).There is substantial variation in the stomatal responses of higher plant species to atmospheric humidity, soil water availability, and other environmental factors (3-7 (12).The physiological benefits of very high conductance are unexplained. One hypothesis is that, when ambient daytime temperatures greatly exceed the optimum plant temperature, increasing stomatal conductance promotes evaporative cooling of leaves and thereby reduces thermal stress. For both upland cotton and Pima cotton, the optimum daytime temperature is <300C (13-15), which is well below commonly occurring air temperatures in most cotton-growing areas. Many studies show substantial cooling of upland cotton foliage (16-18), and a role of stomata in promoting a "heat avoidance" type of heat resistance through evaporative cooling has been proposed (12,19,20). Tests of this hypothesis have been equivocal, though, primarily because the lack of identified genetic variability for stomatal conductance in upland cotton precludes comparison to appropriate control plants. We took advantage of genetic variability for conductance in Pima cotton to create populations segregating for conductance level. Here we describe the association between conductance and leaf temperature in these populations, and we report the relationship of these traits to heat resistance (defined as yield potential ...

Stomatal opening in response to light has a component that matches the absorption spectrum of chlorophyll; however, the intervening sensory transduction steps are not well understood. To study this process, we illuminated Vicia faba guard cell protoplasts with red light and simultaneously recorded current flow across the plasma membrane, utilizing the patch clamp technique in the whole cell configuration. We report evidence that under voltage clamp conditions, red light (1 mmol of photonsm2s 1) stimulated an outward current. This response required ATP (2.5 mM) and orthophosphate (1 mM) at the cytoplasmic side of the membrane. Both red-light-stimulated currents and currents activated in the dark by the proton pump agonist fusicoccin (10 !LM) were abolished by the protonophore carbonylcyanide m-chlorophenylhydrazone at 10 ImM, indicating that these responses were carried by protons. Pump currents were inhibited by orthovanadate applied to the cytoplasmic side of the membrane (50% inhibition at 3.5 FM), implicating a H+ -ATPase. Elimination of the current by the photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea, in the presence of saturating concentrations of ATP, pointed to a requirement for photosynthetically active chloroplasts. We conclude that red light stimulates an electrogenic proton pump at the plasmalemma of Vicia guard cells and that chloroplasts modulate this response.The importance of proton transport across the plasma membrane of fungi and green plants has long been recognized, and it is well established that active proton extrusion is a major transport process in these cells (1, 2). Proton transport is mediated by a proton-translocating ATPase located in the membrane, and the proton gradient established by the H+-ATPase provides the driving force for secondary transport processes (3). The electrogenic properties of the proton pump have been more thoroughly characterized in algae (4) and fungi (5). Cells from higher plants also rely on a H+-ATPase to drive secondary transport processes (2, 3), although many properties of that pump remain speculative.Guard cells are a valuable system for the study of ion transport in higher plants because ion transport is central to guard cell regulation of stomatal aperture. Guard cells transduce many environmental signals, such as light, hormones, intercellular CO2 concentrations, and relative humidity, into changes in turgor pressure which are dependent on ion transport. The degree of turgor in guard cells controls the dimension of the stomatal pore, which in turn controls gas exchange in leaves (6).Light is an important environmental signal for stomatal opening (7). Stomatal opening in response to light has two distinct components, one matching the absorption spectrum of chlorophyll, with peaks in the blue and the red, and the other restricted to the blue region of the spectrum. Consequently, stomatal responses to red light depend on sensory transduction events that follow chlorophyll excitation, whereas responses to blue light represent the ...