Sun-Ben Ku and Gerald E. Edwards a Department of Horticulture, University of Wisconsin, Madison, Wisconsin 53706 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 K m (CO 2 ) values of approximately 3.5 to 5 μM. 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 K i (O 2 ) and K m (CO 2 ), while the ratio of K i (O 2 )/K m (CO 2 ) was similar at the three temperatures; and ( c ) the reciprocal plots of apparent rate of photosynthesis versus (CO 2 - Γ) 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 Γ 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.
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