Putative effects of pH in intra-chloroplast compartments on photoprotection of functional photosystem II complexes by photoinactivated neighbours and on recovery from photoinactivation in Capsicum annuum leaves

Leaf segments from Capsicum annuum L. plants grown at 100 (low light) or 500 (high light) μmol photons m^-2 s^-1 were illuminated in the presence of nigericin, dithiothreitol (DTT), or high [CO₂] (1% in air), with or without lincomycin, an inhibitor of chloroplast-encoded protein synthesis. At various times, the remaining fraction (f) of functional PSII complexes was measured by a dark-adapted chlorophyll fluorescence parameter (1/F_o - 1/F_m; where F_o and F_m are the fluorescence yields corresponding to open and closed PSII traps, respectively), which was calibrated by the oxygen yield per saturating single-turnover flash. The results were interpreted according to a simple kinetic model incorporating the hypothesis that photoinactivated PSII complexes photoprotect functional neighbours (Lee et al. 2001, Planta 105, 377-384), yielding the rate coefficients of photoinactivation and repair, and a parameter, a, which phenomenologically describes the effectiveness of photoprotection by photoinactivated PSII complexes. The presence of the uncoupler nigericin during illumination greatly decreased a by an order of magnitude, suggesting that a sufficiently acidic thylakoid lumen may be required for the photoprotective mechanism to operate. Both nigericin and high [CO₂] decreased the rate coefficient of repair several fold, suggesting that the stromal pH was non-optimal for protein synthesis in the presence of nigericin or high [CO₂]. The xanthophyll cycle, inhibited by DTT, seemed to have a minimal effect on the rate coefficients of photoinactivation and repair, and on the parameter a. The results underline the importance of optimal pH in both the stroma and lumen for photoprotection, and recovery from photoinactivation of PSII.