Abaxial and Adaxial Stomatal Behaviour and Responses to Fusicoccin on Isolated Epidermis Ofcommelina Communisl.

S ummary A comparative study was made of the behaviour of abaxial and adaxial stomata on isolated epidermis of Commelina communis incubated under controlled conditions. The degree of opening and of K + accumulation, and the responsiveness to light, carbon dioxide, abscisic acid and KC1 concentration were substantially higher in abaxial than in adaxial stomata; thus their differential behaviour is mainly a result of inherent, rather than of environmental, differences between the two epidermes. The isolation of guard cells, by killing other epidermal cells, failed to annul the disparity, suggesting that any difference in subsidary cell resistance between the two epidermes is not responsible for this phenomenon. Fusicoccin was able to overcome the smaller opening of adaxial stomata, indicating that there is no fundamental difference in the potential opening ability between abaxial and adaxial stomata. Even in a medium free of KC1, fusicoccin stimulated exceptionally high K + accumulation in adaxial guard cells, demonstrating that endogenous K + available in the adaxial epidermis is sufficient for wide opening. Accordingly, an inherent metabolic condition of adaxial stomatal cells appears to prevent them opening as wide as abaxial stomata. The effectiveness of fusicoccin to produce exceptionally wide adaxial apertures suggests that, when necessary, the proton pump can operate as efficiently in adaxial as in abaxial guard cells.