Increases in both atmospheric CO2 concentration ([CO2]) and temperature on the Earth's surface is likely to alter the photosynthetic responses of C4 plants. An experiment was conducted in sunlit, controlled environment chambers known as Soil-Plant-Atmosphere-Research (SPAR) units to determine if elevated [CO2] and temperature interactively affect photosynthetic parameters of the C4 species, big bluestem. Ten treatments were comprised of two CO2 levels of 360 (ambient) and 720 (elevated) mL L-1 and five day/night temperature of 20/12, 25/17, 30/22, 35/27 and 40/32ºC. Treatments were imposed from 10 days after emergence (DAE) through 125 DAE. Plants grown in elevated [CO2] had significantly higher leaf net photosynthetic rates (PN) and internal [CO2] (Ci), but had lower conductance (gs) and transpiration rate (Tr) compared to that at ambient [CO2]. At a given [CO2], increase in temperature increased PN and Tr. Plants grown at elevated [CO2] did not differ for dark respiration (Rd), maximum quantum yield (F) and curvature (θ); had lower light compensation point (LCP); and higher maximum photosynthesis (Pmax) and light saturation estimate (LSE) than plants grown in ambient [CO2]. The maximum rate of Rubisco activity (Vcmax), or light-saturated rate of electron transport (Jmax) or triose phosphate utilization (TPU) were higher at elevated than at ambient [CO2]. Increase in temperature increased Pmax, LCP, LSE, Vcmax, Jmax and TPU , irrespective of the [CO2].
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