Red light activates a chloroplast-dependent ion uptake mechanism for stomatal opening under reduced CO2 concentrations in Vicia spp.

Under red light in ambient CO2 guard cells of faba bean (Vicia faba) fix CO2 and accumulate sucrose, causing stomata to open. We examined whether at [CO2] low enough to limit guard cell photosynthesis stomata would open when illuminated with red (R) or far-red (FR) light. After illumination with R or FR in buffered KCl solutions, net stomatal opening was c. 3 mum (R and FR) in air containing 210-225 mul l(-1) CO2 and was 5 mum (R) or 6.5 mum (FR) in air containing 40-50 mul l(-1) CO2. Opening was fully inhibited by 3-(3,4-dichlorophenyl)-1,1 dimethyl urea, the calmodulin antagonist W-7, the ser/thr kinase inhibitor ML-9, and sodium orthovanadate, but not by dithiothreitol, which inhibits formation of zeaxanthin, the blue light photoreceptor of guard cells. Stomatal opening was accompanied by K+ uptake and starch loss. Similar results were obtained when leaves were exposed to conditions designed to lower intercellular leaf [CO2]. These data suggest that the guard cell chloroplasts transduce reduced [CO2], activating stomatal opening through an ion uptake mechanism that depends on chloroplastic photosynthetic electron transport and that shares downstream components of the blue light signal transduction cascade.