Temperature Dependence of Photosynthesis in Agropyron smithii Rydb. : I. FACTORS AFFECTING NET CO2 UPTAKE IN INTACT LEAVES AND CONTRIBUTION FROM RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE MEASURED IN VIVO AND IN VITRO

As part of an extensive analysis of the factors regulating photosynthesis in Agropyron smithii Rydb., a C 3 grass, we have examined the response of leaf gas exchange and ribulose-1,5-bisphosphate (RuBP) carboxylase activity to temperature. Emphasis was placed on elucidating the specific processes which regulate the temperature response pattern. The inhibitory effects of above-optimal temperatures on net CO 2 uptake were fully reversible up to 40°C. Below 40°C, temperature inhibition was primarily due to O 2 inhibition of photosynthesis, which reached a maximum of 65% at 45°C. The response of stomatal conductance to temperature did not appear to have a significant role in determining the overall temperature response of photosynthesis. The intracellular conductance to CO 2 increased over the entire experimental temperature range, having a Q 10 of 1.2 to 1.4. Increases in the apparent Michaelis constant ( K c ) for RuBP carboxylase were observed in both in vitro and in vivo assays. The Q 10 values for the maximum velocity ( V max ) of CO 2 fixation by RuBP carboxylase in vivo was lower (1.3-1.6) than those calculated from in vitro assays (1.8-2.2). The results suggest that temperature-dependent changes in enzyme capacity may have a role in above-optimum temperature limitations below 40°C. At leaf temperatures above 40°C, decreases in photosynthetic capacity were partially dependent on temperature-induced irreversible reductions in the quantum yield for CO 2 uptake.