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 o xygen in water and the temperature dependency of the kinetic character istics 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 mu mol mol(-1) usually dec reasing from about 90% at 30 degrees C to about 25% at 10 degrees C at high photon flux. In field-grown wheat and barley, the expected value s at 30 degrees C were observed, but also values as high as 60% at 10 degrees C. The much larger than expected stimulation at cool temperatu res in these species also occurred in plants grown at 15 degrees C, bu t not at 23 degrees C in controlled environment chambers. Gas exchange analysis indicated that an unusually high diffusive limitation was no t an explanation for the large response. Assessment of the apparent in vivo specificity of Rubisco by determining the carbon dioxide concent ration at which carboxylation equalled carbon dioxide release from oxy genation, indicated that growth at row temperatures altered the appare nt 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 s pecificity was consistent with the observed rates of assimilation. Whe ther altered apparent Rubisco specificity is caused by altered stoichi ometry of photorespiration or an actual change in enzyme specificity, the results indicate that the temperature dependence of the stimulatio n of photosynthesis by elevated carbon dioxide may vary greatly with s pecies and with prior exposure to low temperatures.