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Articles

Oxygen Inhibition of Photosynthesis

II. Kinetic Characteristics as Affected by Temperature ¹

^a Department of Horticulture, University of Wisconsin, Madison, Wisconsin 53706

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 C, followed Michaelis-Menten kinetics up to saturating CO₂ but deviated at high CO₂ levels where the experimental V_max is considerably less than the calculated V_max. The affinity of the leaves for CO₂ during photosynthesis was similar from 25 to 35 C with Km (CO₂) values of approximately 3.5 to 5 µM.

In considering the effect of O₂ on photosynthesis at 25, 30, and 35 C where O₂ and CO₂ are expressed on a solubility basis: (a) the effect of O₂ on carboxylation efficiency was similar at the three temperature; (b) 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 three temperatures; and (c) the reciprocal plots of apparent rate of photosynthesis versus (CO₂ - ) at various O₂ levels showed O₂ to be a competitive inhibitor of photosynthesis.

A model for separating O₂ 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 CO₂ concentrations at low O₂versus 21% O₂. The photorespiratory part of O₂ inhibition is considered as a major component at 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.