Effects of continuous leaf wetness on photosynthesis: adverse aspects of rainfall

Above‐ground parts of Phaseolus vulgaris L. plants were treated with artificial misty rain (‘rain’) in a growth chamber to investigate the effects of leaf wetness on photosynthetic performance. The following results were obtained. (1) Stomata closed completely within 2 min of the onset of continuous ‘rain’ application and gradually opened to half the original aperture by 60 min. The rate of CO 2 exchange measured on such wet leaves changed in parallel with the changes in stomatal aperture and attained 60 to 70% of the control level by 1h. (2) The dependence of the rate of leaf photosynthesis, A, on the intercellular CO 2 concentration, c i [A(c i ) relationship], examined in thoroughly dried leaves which had been treated with ‘rain’ did not change until after 4 h of treatment. However, leaves treated for 6h showed discernible decreases in A at high c i (c i >500μmolmol −1 ). The photosynthetic rate of leaves treated with ‘rain’ for 24 h was reduced at all c i , and A at the ambient CO 2 concentration of 350μmolmol −1 was 60 to 70% of that of the control level. The rate of photosynthesis did not recover even after 3 d of treatment of the plants in a dry environment. These results clearly indicate that leaf wetness causes not only instantaneous suppression of photosynthesis but also chronic damage to the photosynthetic apparatus. Potential effects of leaf wetness on photosynthetic performance in nature are also discussed.