The mechanism of stomatal closing in response to O_3 was indirectly investigated by using H_2O_2 which is the intermediate product of O_3 metabolites. Stomata in epidermal strips close in response to H_2O_2. The effect of H_2O_2 on stomatal closing was dependent on the concentration of H_2O_2. 10 ppm H_2O_2 showed a clear effect on stomatal closing and 1000 ppm H_2O_2 induced complete stomatal closing after the treatment of 3 hours. Stomatal closing by H_2O_2 in intact leaf was also observed by measuring the diffusion resistance with porometer. It was found that the stomatal closing by H_2O_2 was not mediated by Ca^2+, and that was a different result observed in stomatal closing by water stress. Reversely, Ca^2+, showed a great inhibition on stomatal closing. The leakage of K^+ in epidermal strips was doubled in response to H_2O_2 when it was campared to the control. 10 ppm H_2O_2 decreased photosynthetic activity. Fv/Fm representing the activity of Photosystem Ⅱ was reduced about 4 % in 10 ppm H_20_2 and 8 % in 100 ppm H_2O_2 in the treatment of 1.5 hour. However, stomatal closing by 10 ppm H_2O_2 was reduced about 56 %. Accordingly, it can be suggetsted that stomatal closing by H_2O_2 is related with the decrease of photosynthetic activity, but it was chiefly induced by the change of the membrane permeability.

ABA and SA showed different effect on stomatal closing on same condition. The addition of 1 mM salicylic acid to fully opened stomata resulted in a significant reductionn of 22 % in stomatal aperture. However, 1 mM ABA reduced 73 % of stomatal aperture. The light absorption spectra of the salicylic acid solution showed that SA was degraded within 1 hour. Therefore, SA solution was resupplied to the detached epidermis every 30 min. during incubation and it was found that even at 10 μM SA induced stomatal closing significantly. Its effect was also greatly pH dependent. The reduction of stomatal aperture caused by 1 mM SA was most effective at lower pH (pH 7.2, 5 %; pH 6.2, 40 %; pH 5.2, 78 %). Therefore, if SA was properly treated to the epidermal strips in the medium, the effects of SA on stomatal closing were similar with those of ABA.