Effects of water stress on photosystem II photochemistry and its thermostability in wheat plants

Modulated chlorophyll fluorescence, rapid fluorescence induction kinetics and the polyphasic fluorescence transients (OJIP) were used to evaluate PSII photochemistry in wheat plants exposed to water stress and/or heat stress (25-45°C). Water stress showed no effects on the maximal quantum yield of PSII photochemistry (F(v)/F(m), the rapid fluorescence induction kinetics, and the polyphasic fluorescence transients in dark-adapted leaves, indicating that water stress had no effects on the primary photochemistry of PSII. However, in light-adapted leaves, water stress reduced the efficiency of excitation energycapture by open PSII reaction centres (F'(v)IF'(m)) and the quantum yield of PSII electron transport (φ(PSII)), increased the non-photochemical quenching (q(N)) and showed no effects on the photochemical quenching (q(p)). This suggests that water stress modified the PSII photochemistry in the light-adapted leaves and such modifications may be a mechanism to down-regulate the photosynthetic electron transport to match a decreased CO₂ assimilation. In addition, water stress also modified the responses of PSII to heat stress. When temperature was above 35 °C, thermostability of PSII was strongly enhanced in water-stressed leaves, which was reflected in a less decrease in F(v)/F(m), q(p), F'(v)/F'(m), and φ(PSII) in water-stressed leaves than in well-watered leaves. There were no significant variations in the above fluorescence parameters between moderately and severely water-stressed plants, indicating that the moderate water stress treatment caused the same effects on thermostability of PSII as the severe treatment. It was found that increased thermostability of PSII may be associated with an improvement of resistance of the O₂-evolving complex and the reaction centres in water-stressed plants to high temperature.