Abstract
The response of whole leaf photosynthesis of wheat (Triticum aestivum L.) in relation to soluble CO(2) available to the mesophyll cells, under low (1.5%) O(2) at 25, 30, and 35 C, followed Michaelis-Menten kinetics up to saturating CO(2) but deviated at high CO(2) levels where the experimental V(max) is considerably less than the calculated V(max). The affinity of the leaves for CO(2) during photosynthesis was similar from 25 to 35 C with Km (CO(2)) values of approximately 3.5 to 5 muM.In considering the effect of O(2) on photosynthesis at 25, 30, and 35 C where O(2) and CO(2) are expressed on a solubility basis: (a) the effect of O(2) on carboxylation efficiency was similar at the three temperature; (b) increasing temperature caused only a slight increase in kinetic constants Ki(O(2)) and Km(CO(2)), while the ratio of Ki(O(2))/Km(CO(2)) was similar at the three temperatures; and (c) the reciprocal plots of apparent rate of photosynthesis versus (CO(2) - Gamma) at various O(2) levels showed O(2) to be a competitive inhibitor of photosynthesis.A model for separating O(2) inhibition of photosynthesis into two components, direct competitive inhibition and inhibition due to photorespiration, was presented from both simulated and experimental data of photosynthetic response curves to varying CO(2) concentrations at low O(2)versus 21% O(2). The photorespiratory part of O(2) inhibition is considered as a major component at Gamma and increases with increasing temperature and with increase in O(2)/CO(2) solubility ratio. The competitive component of O(2) inhibition is considered as a major component of O(2) inhibition under atmospheric CO(2) levels and is relatively independent of temperature at a given O(2)/CO(2) ratio.