Differential Oxygen Response of Photosynthesis in Soybean and Panicum milioides

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Abstract
The CO2 compensation concentration of P. milioides was less than that of soybean over the range of 15-35.degree. C. In soybean (Glycine max [L.] Merr. cv. ''Wayne''), the compensation concentration was directly proportional to O2 concentration. In P. milioides, the compensation concentration was near zero up to 10% O2 and then increased linearly with higher O2, 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. O2 inhibition of photosynthesis and carboxylation efficiency was less in P. millioides than that observed in soybean. The affinity of P. millioides ribulose-1,5-diphosphate carboxylase for CO2 appeared to be slightly greater than that of soybean. The affinity of both enzymes for O2 was similar. The reduced response of the compensation concentration and photosynthesis to O2 in P. millioides may be explained by photosynthetic phosphoenolpyruvate carboxylase fixation and by an apparent increased affinity of ribulose-1,5-diphosphate carboxylase for CO2.