Different effects of light irradiation on the photosynthetic electron transport chain during apple tree leaf dehydration

Effects of light irradiation on the photosynthetic electron transport chain between P-680 and P-700 in apple tree leaves was probed with chlorophyll a fluorescence transient and 820 nm transmission measurements during dehydration under different light intensities. The results showed that light accelerated the leaf water-loss rate during dehydration. Leaf dehydration lowered the maximum quantum yield of PSII and the far-red light induced maximal transmission change at 820 nm, but increased the relative variable fluorescence intensity at J-step, especially under increasing irradiation conditions. During leaf dehydration, irradiation lowered the relative variable fluorescence intensity at l-step. At the beginning of leaf dehydration, moderate light accelerated the leaf water-loss rate and then lowered the maximal light-trapping efficiency of P-680. Upon further dehydration under moderate light or dehydration under high light, light accelerated the water-loss rate and also directly decreased the maximal light-trapping efficiency of P-680. The more significant decrease in the exchange capacity of plastoquinones at the Q(B) site was mainly attributed to the faster water-loss rate under moderate light than in the dark. Under high light, irradiation also directly lowered the capacity. The reoxidation of PQH(2) in the dehydrated leaves was enhanced by the light irradiation. The rapidly decreased contents of P-700 + plastocyanin were mainly attributed to the faster water-loss rate under light conditions in contrast with that in the dark. The different effects of light irradiations on the photosynthetic electron transport chain might be involved in the acclimation of apple tree leaves to dehydration. (C) 2012 Elsevier Masson SAS. All rights reserved.