Effect of enhanced UV-B radiation on cotton growth and photosynthesis

It has been shown that the thinning of ozone layer continuously enhances ambient ultraviolet-B (UV-B) radiation. Enhanced UV-B radiation influences the growth, development and metabolism of crops, of which photosystem is the initial and most important target. In this study, UV-B radiation was increased by 20% and 40% by using ultraviolet lamp during the whole growth period of cotton under field condition, and its effect on cotton morphology, dry matter accumulation, photosynthetic pigment content and seed cotton yield were analyzed. The influencing mechanism of enhanced UV-B radiation on photosynthesis was also investigated by determining gas exchange parameters and chlorophyll fluorescence parameters in functional leaves. The results showed that the growth of cotton stems, leaves and dry matter accumulation were significantly inhibited by enhanced UV-B radiation. The inhibition effects of enhanced UV-B radiation on cotton were more obvious at seedling stage than that at later growth stages. Seed cotton yield also remarkably decreased with increasing UV-B radiation. The contents of chlorophyll a (Chl_a) and chlorophyll b (Chl_b) increased under the treatment of 20% above ambient UV-B radiation and there was no change in Chl_a/Chl_b. When UV-B radiation increased to 40% above ambient UV-B radiation, Chl_a, Chl_b and Chl_a/Chl_b significantly decreased. With increasing UV-B radiation, net photosynthetic rate (P_n) of functional leaves on cotton main stem significantly decreased. Although there were no change in stomatal conductance (G_s) and transpiration rate (T_r) under 40% increase in UV-B radiation, while intercellular CO₂ concentration (C_i) increased, which indicated that the decline in photosynthesis was mainly caused by non-stomatal limitation factors. The results of chlorophyll fluorescence parameters analysis showed that with increasing UV-B radiation, maximum quantum efficiency (F_v/F_m), operating efficiency (Φ_PSⅡ), linear electron transport rate (ETR) and photochemical quenching (qP) of PSII remarkably decreased, but non-photochemical quenching (NPQ) increased. All the chlorophyll fluorescence parameters were significantly correlated with P_n changes. Slowly relaxing NPQ (NPQ_S) and its proportion in NPQ significantly increased under enhanced UV-B radiation, which indicated that the photochemical efficiency of PSⅡ decreased as its reaction center was damaged by elevated UV-B radiation. The results demonstrated that photosynthetic leaf area, chlorophyll content and photosynthetic rate of cotton dropped under enhanced UV-B radiation during the cotton growth period. This inhibited cotton growth, material accumulation and seed cotton yield. Decrease in P_n due to enhanced UV-B radiation was closely related with the destruction of PSⅡ reaction center.