CARBON-DIOXIDE ENRICHMENT AND TEMPERATURE EFFECTS ON COTTON CANOPY PHOTOSYNTHESIS, TRANSPIRATION, AND WATER-USE EFFICIENCY

The objectives of this study were to evaluate effects of ambient and double ambient [CO2] at a range of growing temperatures on photosynthesis, respiration, transpiration, water-use efficiency and dry matter accumulation of cotton plants (Gossypium hirsutum L., cv. DPL 50). In Experiment I, plants were grown outdoors until first bloom, then transferred into naturally lit growth chambers and grown for 22 days at 30/ 18 degrees C with five CO2 concentrations varying from 350 to 900 mu l l(-1). In Experiment II, air temperatures were maintained at 20/12, 25/17, 30/22, and 35/27 degrees C day/night during a 70-day experimental period with [CO2] of 350 and 700 mu l l(-1) at each temperature, Photosynthesis increased with [CO2] from 350 to 700 mu l l(-1) and with temperature. Plants grown at 35/27 degrees C produced fewer bells due to abscission compared with plants grown at optimum temperatures (30/20 degrees C). At higher [CO2], water-use efficiency increased at all temperatures due mainly to increased canopy photosynthesis but also to more limited extent to reduced canopy transpiration. Increased photosynthesis at higher [CO2] resulted in greater dry matter accumulation at all temperatures except at 20/12 degrees C. Respiration increased as dry matter and temperature increased. Plants grown at higher [CO2] had less respiration per unit dry matter but more per unit area. These results indicate that future increases in [CO2] are likely to benefit cotton production by increasing carbon assimilation under temperatures favorable for cotton growth. Reduced fruit weights at higher temperatures indicate potential negative effects on production if air temperatures increase as projected in a high-CO2 world.