Temperature Dependence of Photosynthesis in @Agropyron smithii Rydb.
Plant Physiology, (1982)Abstract
As part of an extensive analysis of the factors regulating photosynthesis in Agropyron smiih Rydb., a C3 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 CO2 uptake were fully reversible up to 40C. Below 40'C, temperature inhibition was primarily due to 02 inhibition of photosynthesis, which reached a maximum of 65\% at 45C. 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 CO2 increased over the entire expermental temperature range, having a QiLo of 1.2 to 1A. Increases in the apparent Michaelis constant (KR) for RuBP carboxylase were observed in both in vitro and in vivo assays. The Qlo values for the maximum velocity (V,,) of CO2 fixatio by RuBP carboxylase in vivo was lower (1.3-1.6) than those calculated from in vito 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 400C. At leaf temperatures above 40C, decreases in photosynthetic capacity were partially dependent on temperature-induced irreversible reductions in the quantum yield for CO2 uptake.