Differential Oxygen Response of Photosynthesis in Soybean and Panicum milioides

The carbon dioxide compensation concentration of Panicum milioides was less than that of soybean over the range of 15 to 35 C. In soybean (Glycine max [L.] Merr. cv. Wayne), the compensation concentration was directly proportional to O(2) concentration. In P. milioides, the compensation concentration was near zero up to 10% O(2) and then increased linearly with higher O(2), although the slope of the response was less than that in soybean. Leaf extracts of P. milioides contained 3-fold higher phosphoenolpyruvate carboxylase activity than soybean leaf extracts. Oxygen inhibition of photosynthesis and carboxy-lation efficiency was less in P. milioides than that observed in soybean. The affinity of P. millioides ribulose-1,5-di-P carboxylase for CO(2) appeared to be slightly greater than that of soybean. The affinity of both enzymes for O(2) was similar. The reduced response of the compensation concentration and photosynthesis to O(2) in P. milioides may be explained by photosynthetic phosphoenolpyruvate carboxylase fixation and by an apparent increased affinity of ribulose-1,5-di-P carboxylase for CO(2).