Summary The responses of abaxial and adaxial stomata to indole‐3‐acetic acid (IAA) were examined on isolated epidermis of Commelina communis. IAA was remarkably effective in producing wide adaxial apertures accompanied by a massive increase in the stomatal K+ content, and, therefore, eliminating the normal disparity in both the opening and K+ accumulation between abaxial and adaxial stomatal cells; it also reversed the suppressive effect of abscisic acid. IAA may affect the transport of K+ either directly or indirectly through its control of the proton pump. If the IAA‐induced adaxial opening and K+ accumulation is a result of a stimulation of proton extrusion, then, under normal conditions, the proton pump should operate at a lower capacity in adaxial than in abaxial guard cells. Alternatively, the lack of stomatal sensitivity to IAA in the presence of Na+ ions suggests a possibility that it might have a direct effect on the K+ transport. In either case, as reported here, an external supply of IAA would enhance K+ accumulation in adaxial guard cells. The inherent disparity in opening and accompanied K+ accumulation between abaxial and adaxial stomatal cells is postulated to be a result of differences in endogenous auxin level between the two epidermes and/or of differential adaxial and abaxial sensitivity to auxin. Thus, the results indicate a hormonal basis for the differential adaxial and abaxial opening.

Summary 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.

SUMMARYA comparative study was made of diurnal movements, and responses to a range of light intensities, of abaxial and adaxial stomata on normal and inverted leaves of Stachytarpheta indica by microscopic examination of epidermal strips fixed in absolute ethanol. Additional observations were made on Coreopsis grandifiora, Crotalaria retusa and Tridax procumbens. The natural diurnal movements in the field and light-induced opening in the laboratory and accompanied changes in concentrations of starch and potassium in the guard cells were all substantially more pronounced in the shaded abaxial stomata than in the exposed adaxial stomata on normal leaves. The effect of leaf inversion was to accentuate these differences considerably. Carbon dioxide-free air was not effective in overcoming the differences in opening ability of abaxial and adaxial stomata. The differential behaviour of abaxial and adaxial stomata is related, to a great extent, to differential changes in concentrations of starch and potassium in abaxial and adaxial guard cells. The greater photosensitivity of abaxial stomata is a widespread phenomenon occurring in both monocotyledons and dicotyledons irrespective of differences in stomatal density; the adaptive significance of this phenomenon is brieffy mentioned.