Recovery of photoinactivated photosystem II in leaves: retardation due to restricted mobility of photosystem II in the thylakoid membrane

Abstract

The functionality of photosystem II (PS II) following high-light pre-treatment of leaf segments at a chilling temperature was monitored as Fv/Fm, the ratio of variable to maximum chlorophyll fluorescence in the dark-adapted state and a measure of the optimal photochemical efficiency in PS II. Recovery of PS II functionality in low light (LL) and at a favourable temperature was retarded by (1) water stress and (2) growth in LL, in both spinach and Alocasia macrorrhiza L. In spinach leaf segments, water stress per se affected neither Fv/Fm nor the ability of the adenosine triphosphate (ATP) synthase to be activated by far-red light for ATP synthesis, but it induced chloroplast shrinkage as observed in frozen and fractured samples by scanning electron microscopy. A common feature of water stress and growth of plants in LL is the enhanced anchoring of PS II complexes, either across the shrunken lumen in water-stress conditions or across the partition gap in larger grana due to growth in LL. We suggest that such enhanced anchoring restricts the mobility of PS II complexes in the thylakoid membrane system, and hence hinders the lateral migration of photoinactivated PS II reaction centres to the stroma-located ribosomes for repair.