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

Increases in both atmospheric CO2 concentration ([CO2]) 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) muL L-1 and three levels of 0 (control), 8, and 16 kJ M-2 d(-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 (R-d), and lower light compensation point (LCP) than plants grown in ambient [CO2]. There was no difference in CO2 compensation point (Gamma), 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 kJ m(-2) d(-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.