Photoinhibition and light-dependent turnover of the D1 reaction-centre polypeptide of photosystem II are enhanced by mineral-stress conditions

The function of photosystem II (PSII) and the turnover of its D1 reaction-center protein were studied in spinach (Spinacia oleracea L.) plants set under mineral stress. The mineral deficiencies were induced either by supplying the plants with an acidic nutrient solution or by strongly reducing the supply of magnesium alone or together with sulfur. After exposure for 8-10 weeks to the different media, the plants were characterized by a loss of chlorophyll and an increase in starch content, indicating a disturbance in the allocation of assimilates. Depending on the severity of the mineral deficiencies the plants lost their ability to adapt even to moderate irradiances of 400 micromole photons.m-2.s-1 and became photoinhibited, as indicated by the decrease in Fv/Fm (the ratio of yield of variable fluorescence to yield of maximal fluorescence when all reaction centers are closed). The loss of PSII function was induced by changes on the acceptor side of PSII. Fast fluorescence decay showed a loss of PSII centers with bound QB, the secondary quinone acceptor of PSII, and a fast reoxidation kinetic of QA-, the primary quinone acceptor of PSII, in the photoinactivated plants. No appreciable change could be observed in the amount of PSII centers with unbound QB and in QB-nonreducing PSII centers. Immunological studies showed that the contents of the D1 and D2 proteins of the PSII reaction center and of the 33-kDa protein of the water-splitting complex were diminished in the photoinhibited plants and the occurrence of a new polypetide of 14 kDa that reacted with an antibody against the C-terminus of the D1 protein. As shown by pulse-labelling experiments with [14C]leucine both degradation and synthesis of the D1 protein were enhanced in the mineral-deficient plants when compared to non-deficient plants. A stimulation of D1-protein turnover was also observed in pH 3-grown plants, which were not inhibited at growth-light conditions. Obviously, stimulation of D1-protein turnover prevented photoinhibition in these plants. However, in the Mg- and Mg/S-deficient plants even a further stimulation of D1-protein turnover could not counteract the increased rate of photoinactivation.