Although is has long been recognized that water deficit in plants can result in a redcution in protein synthesis, the detailed mechanisms behind this have nor been thoroughly elucidated. In the present study, experiments were conducted to investigate the effects of water stress on protein synthesis at both transcrip-tional and psot-transcriptional levels in spirng wheat leaves (Triticum aestivum L. cv. Longchun No.10). The results reveales that stressing wheat seedlings in -0.5MPa PEG solutions for 0.24, 48 and 72h resulted in a progressive decrease in leaf protein synthesis as reflected by the incorporation fo 3 H-Leucine into chloroplast protein. Accompanying the decrease of protein synthesis, the synthetic rate of leaf RNA, the steady state levels of cellular mPNAS and rNAS, ad well as the translatbility of poly (A)+- RNAs the steady state levels of cellular mPNAs and rNAs, as well as the translatbility of poly (A)+- RNAs decreased significantly, indicating that water stress nto only affected gent transcription and RNA accumula-tion in wheat leaves but imparied the messenger-like properties of mPNAs. Furthermore, as the loss of rRNA species mighr result in disruption of the ribosomal subunits and that of mRNAs might reduce the proportion of ribosomes organized as polyriosomes it is conceivable that water stress also affected the translational level of leaf gene expresion. Based on these results, the slowdown of protein synthesis in water-stressed wheat leavs can be attributed to both transcriptional and post-transcriptional contol mechanisms.

The effctts of osmotic dehydration in wheat leaves (Triticum aestivum L.cv. Long-chun No.10) on the photochemical function and protein metabolism of PSII were studied with isolated thylakoid and PSIImembranes. The results indicated tha PSIIwas rather resistant to water stress as mild water deficit in leaves did not sigificantly affect its activity. However, extreme stress conditions such as 40% decrease in relative water content (RWC) or 1.8Mpa in water potential (Ψw) caused ca 50% reduction in O2 evolution and ca 25% inhibtion of DCIP (2.6-dichloophenol indophenol) photore-duction of PSII.In addition. it was found that the inhibited DCIP photreduction of PSII, suggesting that water stress did not affect electron donation to PSII. Urea-SDS-PAGE and westen blot analysis showed that the steady state levls of major PSII proteins, including the DI and D2 proteins in the PSII reaction center, declined on a chlorophyll basis with increasing water stress, possibly as a result of increased degra-chlorophyll bisis with increasing water stress, possibyl as a result of increased degra-duction. In vitro translation experiments and quantitative analysis of chlorplast RNAs indicated that the potntial 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 reation center protenins, may account for the damage to PSII photochemistry.

The electrophoretic migratio rates of several proteins of photosystem 2 particles from spinach were much higher in gels containing 1m M Ca2+ than in gels containing 1m M ethylenglyocl-bis (β-aminoethyl ether) N,N,N1,N1,-tetraactic acid (EGTA). Incubation of gels with terbium (Tb3+) and the corresponding Tb3+-fluorescence were used to identify the Ca2+-binding proteins on the basis of selective occupation of Ca2+-binding sites with Tb3+. The 47, 43 and particularly 33 kDa polypeptides were most probably involed in Ca2+-binding.