Treatment of photosystem II particles with Nacl-washings, low-PH washings or detergent OG solubilizings inhibited oxygen evolution and the inhibition was reversed by addition of exogenous ca2+ Dynamic analysis with Ca2+ reconstitution revealed two Ca2+ binding sites with different affinities in the NaC1-washed PS II partides lr in the low-PH-treated ones. oxygen-evolving PS II core complex also contained the high and low affinity Ca2+ binding sites ca2+ enhanced the intensity of fluores cence emission of PS II core complex These results suggest that cacium play two roles in PS II, the low affinity Ca2+ is associated with energy transfer while the high affinity Ca2+ is concerned with water splitting reaction.

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.

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.

Peptide ranging from residue 229 to 240 of the Dl protein of Photosystem (PS) 11 was synthesized and Were used as candidates of calcium Fiuorescence and FTIR spectroscopy were used totest the conformation after lanthanide additions. FIuorescenee spectroscopy showed that the synthetic peptide provide binding site and that glutamic acids are lnvolved in lanthanide. binding Resolution enhancement tech combined with band curve fitting procedures to quamitate the FT1R special information from the am the relative areas of these component bands indicate that lanthanide anldeinduced induced a substantial decrease in of unordered structure and turns. while a corresponding increase in the amount of a helix and open loop was also observed This indicate that a relatively compact stfllcture of the synthetic peptide ls formed if lanthanides are applied The results may reflect on the physiological and binchemical funcfion of caleium in PSII including preventing Dl fromtrypsin digestion.

Photosystem II oxygen evolution capacity the steady-state level of photosyst II (PSII) reaction center polypeptide DI and its transcript and template levels in Zea mays L (Xinyu No.4) under waler stress and rewatering were studied. The results indicated that PSII and whole chain electron transport capacities decreased slightly under moderate water stress and appreciably under severe water stress and could not recover to control level upon rewatering the resuts of western and northem blots showed that the content of PSII reaction center polypeptide DI changed as a similar pattern to PSII and whole-chain electron transport capacitiesz. Dot blot analysis for DNA showed that there was no obvious response of the template level of Dl to water stress or rewatering. From the results, it was concluded that PSII was the major site affected by water stress, where the functional, loss of PaII could be attributed to the reducion-center polypeptide D1, which may be caused by the decrease in its transcript Ievel. Rewatefing codid only ameliorate slightly under moderate water stress but coutd not recover to control Level under severe water stress.