Inhibition of photosystem I and photosystem II in chloroplasts by UV radiation

The effects of UV radiation on the low temperature fluorescence and primary photochemistry of PSII and PSI of spinach chloroplasts were studied. Fluorescence induction curves at −196°C were measured at 695 nm for PSII fluorescence and at 730 nm for PSI fluorescence to determine both the initial Fo and final FM levels. The primary photochemistry of PSII was measured as the rate of photoreduction of C-550 at − 196°C, that of PSI as the rate of photooxidation of P700 at −196°C. The results were analyzed in terms of a model for the photosynthetic apparatus which accounts for the yields of fluorescence and primary photochemistry. According to this analysis UV radiation increases nonradiative decay processes at the reaction center chlorophyll of PSII. However, the effect of UV radiation is not uniform throughout the sample during irradiation so that account must be taken of the fraction of PSII reaction centers which have been irradiated at any given time. UV radiation also inactivates P700 and causes a slight increase in nonradiative decay in the antenna chlorophyll of PSI. All fluorescence of variable yield, FV = FM−Fo, at 730 nm is due to energy transfer from PSII to PSI so that the sensitivity of Fv to UV radiation is the same at 730 and 695 nm.