Photosynthetic performance at low temperature ofBouteloua gracilisLag., a high‐altitude C₄grass from the Rocky Mountains, USA

The mechanisms controlling the photosynthetic performance of C 4 plants at low temperature were investigated using ecotypes of Bouteloua gracilis Lag. from high (3000 m) and low (1500 m) elevation sites in the Rocky Mountains of Colorado. Plants were grown in controlled‐environment cabinets at a photon flux density of 700 μ mol m −2 s −1 and day/night temperatures of 26/16 °C or 14/7 °C. The thermal response of the net CO 2 assimilation rate ( A ) was evaluated using leaf gas‐exchange analysis and activity assays of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPCase) and pyruvate,orthophosphate dikinase (PPDK). In both ecotypes, a reduction in measurement temperature caused the CO 2 ‐saturated rate of photosynthesis to decline to a greater degree than the initial slope of A versus the intercellular CO 2 response, thereby reducing the photosynthetic CO 2 saturation point. As a consequence, A in normal air was CO 2 ‐saturated at sub‐optimal temperatures. Ecotypic variation was low when grown at 26/16 °C, with the major difference between the ecotypes being that the low‐elevation plants had higher A; however, the ecotypes responded differently when grown at cool temperature. At temperatures below the thermal optimum, A in high‐elevation plants grown at 14/7 °C was enhanced relative to plants grown at 26/16 °C, while A in low‐elevation plants grown at 14/7 °C was reduced compared to 26/16 °C‐grown plants. Photoinhibition at low growth temperature was minor in both ecotypes as indicated by small reductions in dark‐adapted F v / F m . In both ecotypes, the activity of Rubisco was equivalent to A below 17 °C but well in excess of A above 25 °C. Activities of PEPCase and PPDK responded to temperature in a similar proportion relative to Rubisco, and showed no evidence for dissociation that would cause them to become principal limitations at low temperature. Because of the similar temperature response of Rubisco and A , we propose that Rubisco is a major limitation on C 4 photosynthesis in B. gracilis below 17 °C. Based on these results and for theoretical reasons associated with how C 4 plants use Rubisco, we further suggest that Rubisco capacity may be a widespread limitation upon C 4 photosynthesis at low temperature.