Oxygen Inhibition of Photosynthesis

Abstract

The response of whole leaf photosynthesis of wheat (Triticum aestivum L.) in relation to soluble CO2 available to the mesophyll cells, under low (1.5%) O2 at 25, 30 and 35.degree. C, followed Michaelis-Menten kinetics up to saturating CO2 but deviated at high CO2 levels where the experimental Vmax is considerably less than the calculated Vmax. The affinity of the leaves for CO2 during photosynthesis was similar from 25-35.degree. C with Km(CO2) values of approximately 3.5-5 .mu.M. In considering the effect of O2 on photosynthesis at 25, 30 and 35.degree. C where O2 and CO2 are expressed on a solubility basis: the effect of O2 on carboxylation efficiency was similar at the 3 temperatures; increasing temperature caused only a slight increase in kinetic constants Ki(O2) and Km(CO2), while the ratio of Ki(O2)/Km(CO2) was similar at the 3 temperatures; and the reciprocal plots of apparent rate of photosynthesis vs. (CO2 -.GAMMA. [compensation point]) at various O2 levels showed O2 to be a competitive inhibitor of photosynthesis. A model for separating O2 inhibition of photosynthesis into 2 components, direct competitive inhibition and inhibition due to photorespiration, was presented from simulated and experimental data of photosynthetic response curves to varying CO2 concentrations at low O2 vs. 21% O2. The photorespiratory part of O2 inhibition is considered as a major component at .GAMMA. and increases with increasing temperature and with increase in O2/CO2 solubility ratio. The competitive component of O2 inhibition is considered as a major component of O2 inhibition under atmospheric CO2 levels and is relatively independent of temperature at a given O2/CO2 ratio.