The temperature dependence of the stimulation of photosynthesis by elevated carbon dioxide in wheat and barley

The temperature dependencies of the solubility of carbon dioxide and oxygen in water and the temperature dependency of the kinetic characteristics of the ribulose-1,5 bisphosphate carboxylase/oxygenase (Rubisco) enzyme result in the short-term stimulation of photosynthesis with a doubling of carbon dioxide from 350 to 700 μmol mol−1 usually decreasing from about 90% at 30°C to about 25% at 10°C at high photon flux. In field-grown wheat and barley, the expected values at 30°C were observed, but also values as high as 60% at 10°C. The much larger than expected stimulation at cool temperatures in these species also occurred in plants grown at 15°C, but not at 23°C in controlled environment chambers. Gas exchange analysis indicated that an unusually high diffusive limitation was not an explanation for the large response. Assessment of the apparent in vivo specificity of Rubisco by determining the carbon dioxide concentration at which carboxylation equalled carbon dioxide release from oxygenation, indicated that growth at low temperatures altered the apparent enzyme specificity in these species compared to these species grown at the warmer temperature. Inserting the observed specificities into a biochemical model of photosynthesis indicated that altered Rubisco specificity was consistent with the observed rates of assimilation. Whether altered apparent Rubisco specificity is caused by altered stoichiometry of photo respiration or an actual change in enzyme specificity, the results indicate that the temperature dependence of the stimulation of photosynthesis by elevated carbon dioxide may vary greatly with species and with prior exposure to low temperatures.