Effects of water stress on photochemical function and protein metabolism of photosystem II in wheat leaves

The effects of osmotic dehydration in wheat leaves ( Triticum aestivum L. cv. Longchun No. 10) on the photochemical function and protein metabolism of PSII were studied with isolated thylakoid and PSII membranes. The results indicated that PSII was rather resistant to water stress as mild water deficit in leaves did nut significantly affect its activity. However, extreme stress conditions such as 40% decrease in relative water content (RWC) or 1.8 MPa in water potential (Ψ) caused ca 50% reduction in O₂ evolution and ca 25% inhibition of DCIP (2.6-dichlorophenol indophenol) photoreduction of PSII. In addition, it was found that the inhibited DCIP photoreduction of PSII could not be reversed by DPC (2.2-diphenylcarbazide), a typical electron donor to PSII, suggesting that water stress did not affect electron donation to PSII. Urea-SDS-PAGE and western blot analysis showed that the steady slate levels of major PSII proteins, including the D1 and D2 proteins in the PSII reaction center, declined on a chlorophyll basis with increasing water stress, possibly as a result of increased degradation. In vitro translation experiments and quantitative analysis of chloroplast RNAs indicated that the potential synthesis of chloroplast proteins from their mRNAs was impaired by water stress. From the results it is concluded that the effects of water stress on PSII protein metabolism, especially on the reaction center proteins, may account for the damage to PSII photochemistry.