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

Abstract

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 ^2-₃ 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.