The UV-A (320-400 nm) component of sunlight is a significant damaging factor of plant photosynthesis, which targets the photosystem II complex. Here we performed a detailed characterization of UV-A-induced damage in photosystem II membrane particles using EPR spectroscopy and chlorophyll fluorescence measurements. UV-A irradiation results in the rapid inhibition of oxygen evolution accompanied by the loss of the multiline EPR signal from the S2 state of the water-oxidizing complex. Gradual decrease of EPR signals arising from the QA -Fe2+ acceptor complex, Tyr-D°, and the ferricyanide-induced oxidation of the non-heme Fe2+ to Fe3+ is also observed, but at a significantly slower rate than the inhibition of oxygen evolution and of the multiline signal. The amplitude of Signal IIfast, arising from Tyr-Z° in the absence of fast electron donation from the Mn cluster, was gradually increased during the course of UV-A treatment. However, the amount of functional Tyr-Z decreased to a similar extent as Tyr-D as shown by the loss of amplitude of Signal IIfast, that could be measured in the UV-A-treated particles after Tris washing. UV-A irradiation also affects the relaxation of flash-induced variable chlorophyll fluorescence. The amplitudes of the fast (600 μs) and slow (2 s) decaying components, assigned to reoxidation of QA- by QB and by recombination of (QAQB)- with donor side components, respectively, decrease in favor of the 15-20 ms component, which reflects PQ binding to the QB site. In the presence of DCMU, the fluorescence relaxation is dominated by a 1 s component due to recombination of QA- with the S2 state. After UV-A radiation, this is partially replaced by a much faster component (30-70 ms) arising from recombination of QA- with a stabilized intermediate PSII donor, most likely Tyr-Z°. It is concluded that the primary damage site of UV-A irradiation is the catalytic manganese cluster of the water-oxidizing complex, where electron transfer to Tyr-Z° and P680+ becomes inhibited. Modification and/or inactivation of the redox-active tyrosines and the QAFe2+ acceptor complex are subsequent events. This damaging mechanism is very similar to that induced by the shorter wavelength UV-B (280-320) radiation, but different from that induced by the longer wavelength photosynthetically active light (400-700 nm).
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