Submitted to: Journal of Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/27/2003
Publication Date: 4/1/2004
Citation: Zhao, D., Reddy, R., Kakani, V.G., Mohammed, A.R., Read, J.J., Wei, G. 2004. Leaf and canopy photosynthetic characteristics of cotton (Gossypium hirsutum L.) under elevated Co2 concentration and UV-b radiation. Journal of Plant Physiology. 161:581-590. Interpretive Summary: Combustion of fossil fuels (gas and oil) and clearing of forests have led to increased levels of harmful 'greenhouse' gases, especially carbon dioxide (CO2) in the atmosphere and destruction of the ozone layer in the upper atmosphere. Depletion of stratospheric ozone increases levels of ultraviolet-B radiation (UV-B) reaching the earth's surface. Increasing the atmospheric concentration CO2 is expected to stimulate plant growth and photosynthesis, but increasing the UV-B radiation may be detrimental to plants. Because CO2 and UV-B are expected to increase together in future climate-change scenarios, this study determined the combined and interactive effects of these environmental factors on growth and net photosynthesis (Pnet) in cotton. As expected, increased atmospheric CO2 concentration led to greater leaf and canopy Pnet. Although a moderate increase in the level of UV-B did not affect Pnet, the high UV-B treatment led to 40% less leaf photosynthesis, 50% smaller leaf area, and significant reduction of some key enzymes. Decreased Pnet, which is a measure of growth efficiency, in cotton by UV-B was due to the combined effects of decreased photosynthesis and smaller leaves and this decrease was not offset by the positive effects of increased CO2. Results show the potential effect of unabated, concurrent increases of CO2 and other greenhouse gases, when accompanied by increased UV-B radiation, is a net reduction in production of this important fiber crop.
Technical Abstract: Increases in both atmospheric CO2 and ultraviolet-B (UV-B) radiation are features of current climate change patterns. Studies were conducted in sunlit, controlled-environment chambers to determine the interactive effects of elevated (CO2) and UV-B radiation on photosynthetic C assimilation in cotton (Gossypium hirsutum L.). Two CO2 levels of 360 (ambient) and 720 (elevated) uL L-1, and three UV-B levels of 0 (control), 8 and 16 kJ m-2 d-1 were imposed for a 66-d period, between crop emergence and flowering stages. Plants grown at elevated CO2 had greater leaf and canopy net photosynthetic rates (Pnet), lower dark respiration, and lower light compensation point than plants grown at ambient CO2. Doubling the CO2 concentration, however, did not affect values for leaf CO2 compensation point, maximum rate of RuBisco activity, or light-saturated rate of electron transport. The highest UV-B dosage of 16 kJ m-2 d-1 led to 40% lower leaf Pnet, 50% smaller leaf area, 72% lower RuBisco activity, and 64% lower electron transport, as compared to control plants. Values for Pnet did not differ between controls and plants receiving 8 kJ m-2 d-1 UV-B radiation. Results indicate decreased canopy photosynthesis in cotton under 16 kJ m-2 d-1 UV-B is due to the combined effects of decreased leaf area and leaf Pnet. The loss of RuBisco activity and electron transport are two major factors in UV-B inhibition of leaf Pnet.