Leaf and canopy photosynthetic characteristics of cotton (Gossypium hirsutum) under elevated CO₂ concentration and UV-B radiation

Increases in both atmospheric CO₂ concentration ([CO ₂]) and ultraviolet-B (UV-B) radiation on the Earth's surface are features of current climate change patterns. An experiment was conducted in sunlit, controlled environment chambers known as Soil-Plant-Atmosphere-Research (SPAR) units to determine interactive effects of elevated [CO₂] and UV-B radiation on leaf and canopy photosynthetic characteristics of cotton. Six treatments were comprised of two CO₂ levels of 360 (ambient) and 720 (elevated) μL L^-1 and three levels of 0 (control), 8, and 16 kJm^-2d^-1 biologically effective UV-B radiation. Treatments were imposed for 66 days from crop emergence through three weeks after the first flower stage. Plants grown in elevated [CO₂] had significantly greater leaf area, higher leaf and canopy net photosynthetic rates (P _N), lower dark respiration rate (Rd), and lower light compensation point (LCP) than plants grown in ambient [CO₂]. There was no difference in CO₂ compensation point (Γ), maximum rate of Rubisco activity (V_cmax), or light-saturated rate of electron transport (J_max) between ambient and elevated CO₂ treatments. When plants were grown in 8 kJm^-2d^-1 UV-B radiation, most of the measured photosynthetic parameters did not differ from control plants. High UV-B (16 kJ) radiation, however, caused 47-50% smaller leaf area, 38-44% lower leaf P_N, 72-74% lower V_cmax, and 61-66% lower J_max compared to the control. There were no interactive effects of [CO₂] and UV-B radiation on most of the photosynthetic parameters measured. From the results, it is concluded that decreased canopy photosynthesis due to enhanced UV-B radiation in cotton is associated with both smaller leaf area and lower leaf P_N, and loss of Rubisco activity and electron transport are two major factors in UV-B inhibition of leaf P _N.