Apoplastic mesophyll signals induce rapid stomatal responses to CO 2 in C ommelina communis

Summary Previous studies have suggested that the mesophyll contributes to stomatal CO 2 responses. The effects of changes in CO 2 concentration (100 or 700 ppm) on stomatal responses in red or white light were examined microscopically in a leaf segment, an epidermal strip and an epidermal strip placed on a mesophyll segment of C ommelina communis , all mounted on a buffer‐containing gel. In both red and white light, stomata of the leaf segment opened/closed rapidly at low/high CO 2 . In red light, epidermal strip stomata barely responded to CO 2 . In white light, they opened at low CO 2 , but hardly closed at high CO 2 . Stomata of the epidermal strip placed on the mesophyll responded in the same manner as those on the leaf segment. Insertion of a doughnut‐shaped cellophane spacer (but not polyethylene spacer) between the epidermal strip and the mesophyll hardly altered these responses. Stomata in leaf segments treated with 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea ( DCMU ), a photosynthesis inhibitor, did not open in red light, but opened/closed at low/high CO 2 in white light. These results indicate that the apoplast transfer of ‘mesophyll signals’ and the stomatal opening at low CO 2 are dependent on photosynthesis, whereas the stomatal closure at high CO 2 is independent of photosynthesis.