Stomatal responses to carbon dioxide of isolated epidermis from a C3 plant, the Argenteum mutant of Pisum sativum L., and a crassulacean-acid-metabolism plant Kalanchoë daigremontiana Hamet et Perr.

The response of stomata in isolated epidermis to the concentration of CO2 in the gaseous phase was examined in a C3 species, the Argenteum mutant of Pisum sativum, and a crassulacean-acid-metabolism (CAM) species, Kalanchoë daigremontiana. Epidermis from leaves of both species was incubated on buffer solutions in the presence of air containing various volume fractions of CO2 (0 to 10000·10(-6)). In both species and in the light and in darkness, the effect of CO2 was to inhibit stomatal opening, the maximum inhibition of opening occurring in the range 0 to 360·10(-6). The inhibition of opening per unit change in concentration was greatest between volume fractions of 0 and 240·10(-6). There was little further closure above the volume fraction of 360·10(-6), i.e. approximately ambient concentration of CO2. Thus, although leaves of CAM species may experience much higher internal concentrations of CO2 in the light than those of C3 plants, this does not affect the sensitivity of their stomata to CO2 concentration or the range over which they respond. Stomatal responses to CO2 were similar in both the light and the dark, indicating that effects of CO2 on stomata occur via mechanisms which are independent of light. The responses of stomata to CO2 in the gaseous phase took place without the treatments changing the pH of the buffered solutions. Thus it is unlikely that CO2 elicited stomatal movement by changing either the pH or the HCO 3 (-) /CO 3 (2-) equilibria. It is suggested that the concentration of dissolved unhydrated CO2 may be the effector of stomatal movement and that its activity is related to its reactivity with amines.