Improving photosynthetic characteristics and antioxidant enzyme activity of capsule wall and subtending leaves increases cotton biomass under limited irrigation system

The photosynthetic performance of the canopy boll-leaf system (BLS) reflects the material and energy exchange abilities between plant and external environment. A two-year field experiment determined the response to irrigation regimes [600 (W1), 480 (W2), and 360 (W3) m3 ha-1] of cotton BLS physio-biochemical traits. Decreasing irrigation, photochemical quenching coefficient, the electron transfer rate of PSII, chlorophyll, and stomata width of the BLS decreased; nonphotochemical quenching (NPQ), the activity of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), hydrogen peroxide (H2O2), and malondialdehyde (MDA) contents increased at 7-21 day interval after anthesis (TAA). Among them, W2 increased SOD and POD by 3.5-42.1% and 1.4-57.8%, respectively, compared to W1 treatment. NPQ and carotenoid contents of capsule wall and CAT of subtending leaves increased. Principal component analysis showed that NPQ, MDA, H2O2, POD, and CAT were positively correlated with the seed biomass. Therefore, cotton could protect photosynthetic apparatus by maintaining lower membrane lipid peroxidation and higher heat dissipation capacity of capsule wall and subtending leaves to ensure higher biomass accumulation under limited irrigation.