Average global surface temperatures are predicted to rise due to increasing atmospheric CO2 and other greenhouse gases. Attempts to predict plant response to CO2 must take into account possible temperature effects on phenology and reproductive sink capacity for carbohydrates. In this study, we investigated the effects of atmospheric CO2 partial pressure [35 Pa ambient CO2 (aCO2) vs. 70 Pa elevated CO2 (eCO2)] and temperature (26/15 vs. 35/21 C day/night) on short- and long-term net CO2 assimilation (An) and growth of red kidney bean (
Phaseolus vulgaris). During early vegetative development [14-31 days after planting (DAP)], An, and relative growth rate (RGR) at eCO2 were significantly greater at the supra-optimum (35/21 C) than at the optimum (26/15 C) temperature. At 24 DAP, the CO2 stimulation of An by eCO2 was 49% and 89% at optimum and supra-optimum temperature, respectively, and growth enhancement was 48% and 72% relative to plants grown at aCO2. This high temperature-induced growth enhancement was accompanied by an up-regulation of An of eCO2-grown plants. In contrast, during later reproductive stages (31-68 DAP) the eCO2 stimulation of An was significantly less at the supra-optimum than at optimum temperature. This was associated with reduced seed set, greater leaf carbohydrate accumulation, and down-regulation of An at the higher temperature. At final harvest (68 DAP), the eCO2 stimulation of total dry weight was 31% and 14% at optimum and supra-optimum temperature respectively, and eCO2 stimulation of seed dry weight was 39% and -18% at optimum and supra-optimum temperature, respectively. These data indicate substantial shifts in the response to eCO2 during different phenological stages, and suggest that impaired reproductive development at high temperature could reduce the potential for CO2 stimulation of photosynthesis and productivity in bean and possibly other heat-sensitive species.