Responses of photorespiration and thermal dissipation in PSII to soil water in cotton bracts

Aims Photorespiration and non-photochemical quenching of chlorophyll fluorescence in photosystem II (PSII) were studied in bracts and leaves of cotton (Gossypium hirsutum) plants, in order to investigate the photoprotective mechanisms and drought tolerance in cotton bracts under field conditions of water-saving drip irrigations.
Methods Gas exchange and chlorophyll fluorescence parameters of PSII were analyzed in bracts and leaves of cotton plants after anthesis. The study was conducted with two treatments comprising normal drip irrigation (5 228.5 m³·hm^–2) and water-saving drip irrigation (3 874.1 m³·hm^–2) under field conditions.
Important findings The actual photochemical efficiency of PSII (Φ_PSII) decreased in both bracts and leaves of cotton plants after anthesis under water-saving drip irrigation, but the magnitude of decrease was less in bracts than in leaves. Results showed the bracts experienced less severe photoinhibition than leaves. The rate of net photosynthesis (P_n), Φ_PSII, net photorespiration (P_r), photochemical quenching (q_P), and non-photochemical quenching (NPQ) decreased in the leaves of cotton plants under water-saving drip irrigation, but no significant difference was observed in the bracts. With decreasing water supply, the P_r/P_n in bracts was much higher than that in leaves and water deficit had no significant effect on the P_r/P_n in bracts. The results of chlorophyll fluorescence parameters showed that the quantum yield of regulated energy dissipation (Y(NPQ)) was higher in bracts than in leaves under high irradiance and temperature conditions, and that the thermal dissipation in bracts was not susceptible to water deficit, thus protecting the photosynthetic apparatus against photodamage. Overall, both photorespiration and energy dissipation in bracts were found to alleviate photoinhibition and played important roles in protecting PSII in cotton plants.