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

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Abstract
Summary • Under red light in ambient CO 2 guard cells of faba bean ( Vicia faba ) fix CO 2 and accumulate sucrose, causing stomata to open. We examined whether at (CO 2 ) 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 µm (R and FR) in air containing 210-225 µl l -1 CO 2 and was 5 µm (R) or 6.5 µm (FR) in air containing 40-50 µl l -1 CO 2 . 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 inter- cellular leaf (CO 2 ). • These data suggest that the guard cell chloroplasts transduce reduced (CO 2 ), 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.