Oxygen Inhibition of Photosynthesis and Stimulation of Photorespiration in Soybean Leaf Cells

Abstract

The occurrence of photorespiration in soybean (Glycine max [L.] Merr.) leaf cells was demonstrated by the presence of an O(2)-dependent CO(2) compensation concentration, a nonlinear time course for photosynthetic (14)CO(2) uptake at low CO(2) and high O(2) concentrations, and an O(2) stimulation of glycine and serine synthesis which was reversed by high CO(2) concentration. The compensation concentration was a linear function of O(2) concentration and increased as temperature increased. At atmospheric CO(2) concentration, 21% O(2) inhibited photosynthesis at 25 C by 27%. Oxygen inhibition of photosynthesis was competitive with respect to CO(2) and increased with increasing temperature. The Km (CO(2)) of photosynthesis was also temperature-dependent, increasing from 12 mum CO(2) at 15 C to 38 mum at 35 C. In contrast, the Ki (O(2)) was similar at all temperatures. Oxygen inhibition of photosynthesis was independent of irradiance except at 10 mm bicarbonate and 100% O(2), where inhibition decreased with increasing irradiance up to the point of light saturation of photosynthesis. Concomitant with increasing O(2) inhibition of photosynthesis was an increased incorporation of carbon into glycine and serine, intermediates of the photorespiratory pathway, and a decreased incorporation into starch. The effects of CO(2) and O(2) concentration and temperature on soybean cell photosynthesis and photorespiration provide further evidence that these processes are regulated by the kinetic properties of ribulose-1,5-diphosphate carboxylase with respect to CO(2) and O(2).