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

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). Prasanna Mohanty is an INSA Honorary Scientist and Professor on Courtesy, DAVV, Indore.