Temperature Dependence of Photosynthesis in Agropyron smithii Rydb. : I. FACTORS AFFECTING NET CO(2) 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 degrees C. Below 40 degrees C, temperature inhibition was primarily due to O(2) inhibition of photosynthesis, which reached a maximum of 65% at 45 degrees 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 degrees C. At leaf temperatures above 40 degrees C, decreases in photosynthetic capacity were partially dependent on temperature-induced irreversible reductions in the quantum yield for CO(2) uptake.