Responses of Soybean Canopy Photosynthesis and Transpiration to Whole‐Day Temperature Changes in Different CO2 Environments1

Abstract

AbstractResponse of plants to increased temperatures, as well as increased CO2, is one of the concerns about the global consequences of continued burning of fossil fuel. In this study, soybeans [Glycine max (L.) Merr. cv. Bragg] were grown in controlled‐environment chambers located outdoors and acclimated to two environmental CO2 levels, 330 and 800 µmol mol−1. Canopies were exposed to different day‐long dry bulb temperature treatments that were maintained for a few days. Comparisons of CER‐light response curves and daytime total CO2 exchange from both low and high CO2 treatments indicated that canopy CER was not significantly affected by dry bulb temperature in the range 28°C to 35°C (nighttime temperature was 23°C). Canopy water use increased significantly in both CO2 treatments as dry bulb temperature was increased and the dewpoint temperature was kept at 21°C. Calculated total canopy resistances were constant throughout the temperature treatments in each of the CO2 treatments. This indicated that between dry bulb treatments, changes in canopy water use resulted strictly from temperature‐driven changes in the vapor pressure gradients within the canopy. Characteristic ratios of internal to external canopy CO2 concentrations were calculated and found to be similar among all temperature treatments and in both CO2 treatments. Across all temperature treatments, cumulative daytime transpiration was nearly equal between the two CO2 treatments, however, water‐use efficiency (WUE) decreased in both CO2 treatments as dry bulb temperature increased. Since CER response in both CO2 treatments was unaffected by air temperature across the range of 28 to 35°C, the decrease in WUE with increasing temperature was due solely to increasing transpiration.