Diffusion and Quasi‐diffusion Resistances in Relation to the Carboxylation Kinetics of Maize Leaves

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Abstract
A “quasi‐diffusion resistance”, rq, is defined to accommodate the role which the thermochemical and photochemical phenomena of photosynthesis play in the control of CO2 fixation to the terminology and approach of the diffusion resistance analogue for the CO2 exchange of leaves. The relationship of rq to Rabinowitch's classical rectangular hyperbolic model of photosynthesis rate as a function of CO2 concen‐tration at the carboxylating surface is discussed. Examination of Kmapp for phos‐phopyruvate carboxylase (the predominant carboxylase in maize) suggests, as a reasonable hypothesis for light‐saturated maize leaves, that rq may be essentially independent of ambient CO2‐concentration up to at least 300 μ1/l. A corollary of the hypothesis is that an increase of diffusion resistance, rather than of rq, may account for the observed curvature of the response curves of light‐saturated maize leaf photosynthesis to ambient CO2‐concentration. An experiment carried out on fieldgrown maize plants, using a well controlled leaf chamber as a nitrous oxide diffusion porometer, gave evidence which strongly supported the hypothesis.