Differential response of chloride binding sites to elevated temperature: a comparative study in spinach thylakoids and PSII-enriched membranes

摘要

A study of heat effects was performed in thylakoids and photosystem II (PSII)-enriched membranes isolated from spinach in relation to Cl?-induced activation of PSII catalyzed oxygen evolution and the retention of Cl? in the PSII complex. For this, Cl?-sufficient membranes and low-Cl? membranes were used. The presence of Cl? in the reaction medium did accelerate oxygen evolution, which remained unaffected by heat treatment up to 40°C in PSII membranes and up to 42.5°C in thylakoids. Heat resistance of Cl?-induced activation of oxygen evolution was found to be independent of the presence of ‘bound Cl?’ in the preparations. However, the functional stability of the PSII complex during heat treatment showed a marked dependence on the presence of bound Cl? in PSII. Electron paramagnetic resonance study of manganese (Mn) release per reaction center/Y D + showed that there was little loss of Mn2+ up to 42°C in our preparations, although the PSII activity was significantly lowered. These observations together with data from steady state chlorophyll a fluorescence imply that the site of action of Cl? causing direct activation of oxygen evolution was different from the site of primary heat damage. A differential response of chloride binding sites to heat stress was observed. The high-affinity (tightly bound, slow exchanging) site of chloride is affected earlier (～37°C) while low-affinity (loosely bound, fast exchanging) site gets affected at higher temperatures (42.5°C in thylakoids and 40°C in the case of PSII-enriched membranes).