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

A "quasi-diffusion resistance", r_q' 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 r_q to Rabinowitch's classical rectangular hyperbolic model of photosynthesis rate as a function of CO₂-concentration at the carboxylating surface is discussed. Examination of K_m^nPP for phos- Phopyruvate carboxylase (the predominant carboxylase in maize) suggests, as a reasonable hypothesis for light-saturated maize leaves, that r_q may be essentially independent of ambient CO₂-concentration up to at least 300 μI/I. A corollary of the hypothesis is that an increase of diffusion resistance, rather than of r_q, 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. [ABSTRACT FROM AUTHOR]