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Photoinhibition of photosynthesis is a process by which excessive light radiation, which is absorbed by the leaves, results in the inactivation and/or impairment of the chlorophyll-containing reaction centers of the chloroplasts, thus inhibiting photosynthesis. The susceptibility of leaves to photoinhibition is known to be enhanced by additional stress factors, which coincide with the light. In this work, we have studied photoinhibition of photosynthesis in control and low-night temperature (LNT) leaves of grapevine ( Vitis vinifera L. cv. Riesling) under controlled conditions (irradiation of detached leaves to about 1900 μmol m −2 s −1 ). The degree of photoinhibition 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 PSII, F v / F m declined, F 0 increased significantly in high light (HL) irradiated LNT leaves than in control leaves. In isolated thylakoids, the rate of whole chain and PSII activity markedly decreased in HL irradiated more in leaves of LNT than in leaves of control. A smaller inhibition of PSI activity was also observed in both leaves. The artificial exogenous electron donors DPC, NH 2 OH and Mn 2+ failed to restore the HL induced loss of PSII activity in control leaves, while DPC and NH 2 OH were restored in LNT leaves. It is concluded that HL in LNT leaves inactivates both acceptor and donor side of PSII whereas it does at the acceptor side only in control leaves. Quantification of the PSII reaction center protein D1 and 33 kDa protein water splitting complex following HL exposure of leaves showed pronounced differences between control and LNT. The marked loss of PSII activity in HL irradiated LNT leaves were due to the marked loss of D1 and 33 kDa proteins. The high degree of photoinhibition observed in LNT leaves probably represents a dynamic regulatory process protecting the photosynthetic apparatus from severe damage by excess light. |
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