KINETIC RESOLUTION OF DIFFERENT RECOVERY PHASES OF PHOTOINHIBITED PHOTOSYSTEM-II IN COLD-ACCLIMATED AND NON-ACCLIMATED SPINACH LEAVES

Leaf discs from spinach were exposed to a photon flux density of 1 250 mumol m-2 s-1 at 5-degrees-C for 2 or 3 h in ambient air. Photoinhibition of photosystem II (PS II) was measured by means of chlorophyll fluorescence. Recovery of photosystem II was followed at 6-degrees-C and 20-degrees-C in low light or darkness for periods up to 12 h. The experimental setup allowed kinetic resolution of different phases of recovery. The experiments revealed a temperature dependent dark recovery phase and two distinct light- and temperature dependent phases: (1) A relatively fast, light dependent recovery phase occurred in parallel with partial recovery of basic fluorescence at 6-degrees-C and 20-degrees-C. A population of PS II centers with very slow fluorescence induction kinetics, which had accumulated during photoinhibition treatment, disappeared during this phase. This fast recovery phase is proposed to represent reactivation of photoinhibited PS II, without dissassembly or incorporation of new D1-protein. (2) A relatively slow light-dependent recovery phase took place at 20-degrees-C, but not at 6-degrees-C. In the presence of the chloroplast translation inhibitor streptomycin, part of the 2nd phase was inhibited. This phase is proposed to involve assembly of new Photosystem II centers, which is partly dependent on de novo synthesis of D1-reaction center protein, but presumably is also using a preexisting pool of D1-protein. Cold acclimation of the leaves resulted in a decreased sensitivity for photoinhibition of photosystem II. Recovery of photoinhibited photosystem II at 6-degrees-C of the cold-acclimated leaves was faster than in non-acclimated leaves, but this effect can be ascribed to diminished photoinhibitory damage.