PHOTOINHIBITION OF PHOTOSYNTHESIS DURING RAIN TREATMENT - IDENTIFICATION OF THE INTERSYSTEM ELECTRON-TRANSFER CHAIN AS THE SITE OF INHIBITION

Continuous wetness of leaves in the light causes a reduction in the ca rbon exchange rate (CER) in Phaseolus vulgaris L. [Ishibashi and Teras hima (1995) Plant Cell Environ. 18: 431]. In this study, we investigat ed the initial cause of photoinhibition upon application of water, des ignated rain treatment, and me found a large decrease in the rate of e lectron transport through the whole chain from water to methyl viologe n via PSII and PSI. In spite of the decrease in the rate of electron t ransport, there was no decrease in the activity of either PSI or PSII when these activities were measured separately. The intactness of PSI was also confirmed by the absence of any change in the photooxidizable amount of P-700, the reaction centre of PSI, and the intactness of PS II was confirmed by measurements of Chi fluorescence. The results sugg est that the inhibition by the rain treatment, which occurs at the sit e between PSI and PSII, might be a novel type of photoinhibition, unli ke the conventional types of photoinhibition that involve PSI and PSII .