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

•  Under red light in ambient CO guard cells of faba bean (Vicia faba) fix CO and accumulate sucrose, causing stomata to open. We examined whether at [CO ] 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.