Photoinhibition of Photosynthesis in Sun and Shade Grown Leaves of Grapevine (Vitis vinifera L.)

The degree of photoinhibition of sun and shade grown leaves of grapevine was determined by means of the ratio of variable to maximum chlorophyll (Chl) fluorescence (F_v/F_m) and electron transport measurements. The potential efficiency of photosystem 2 (PS2), F_v/F_m, markedly declined under high irradiance (HI) in shade leaves with less than 10 % of F₀ level. In contrast, F_v/F_m ratio declined with about 20 % increase of F₀ level in sun leaves. In isolated thylakoids, the rate of whole chain and PS2 activity in HI shade and sun leaves was decreased by about 60 and 40 %, respectively. A smaller inhibition of photosystem 1 (PS1) activity was also observed in both leaf types. In the subsequent dark incubation, fast recovery was observed in both leaf types that reached maximum PS2 efficiencies similar to non-photoinhibited control leaves. The artificial exogenous electron donors DPC, NH₂OH, and Mn²⁺ failed to restore the HI-induced loss of PS2 activity in sun leaves, while DPC and NH₂OH were significantly restored in shade leaves. Hence HI in shade leaves inactivates on the donor side of PS2 whereas it does at the acceptor side in sun leaves, respectively. Quantification of the PS2 reaction centre protein D1 and the 33 kDa protein of water splitting complex following HI-treatment of leaves showed pronounced differences between shade and sun leaves. The marked loss of PS2 activity in HI leaves was due to the marked loss of D1 protein of the PS2 reaction centre protein and the 33 kDa protein of the water splitting complex in sun and shade leaves, respectively.