Plant Physiol, October 2002, Vol. 130, pp. 964-976

Bundle Sheath Diffusive Resistance to CO₂ and Effectiveness of C₄ Photosynthesis and Refixation of Photorespired CO₂ in a C₄ Cycle Mutant and Wild-Type Amaranthus edulis¹

School of Biological Sciences, Washington State University, Pullman, Washington 99164-4236 (O.K., V.R.F., G.E.E.); and Department of Biological Sciences, Lancaster University, Lancaster LA1 4YQ, United Kingdom (P.J.L.)

A mutant of the NAD-malic enzyme-type C₄ plant, Amaranthus edulis, which lacks phosphoenolpyruvate carboxylase (PEPC) in the mesophyll cells was studied. Analysis of CO₂ response curves of photosynthesis of the mutant, which has normal Kranz anatomy but lacks a functional C₄ cycle, provided a direct means of determining the liquid phase-diffusive resistance of atmospheric CO₂ to sites of ribulose 1,5-bisphosphate carboxylation inside bundle sheath (BS) chloroplasts (r_bs) within intact plants. Comparisons were made with excised shoots of wild-type plants fed 3,3-dichloro-2-(dihydroxyphosphinoyl-methyl)-propenoate, an inhibitor of PEPC. Values of r_bs in A. edulis were 70 to 180 m² s¹ mol¹, increasing as the leaf matured. This is about 70-fold higher than the liquid phase resistance for diffusion of CO₂ to Rubisco in mesophyll cells of C₃ plants. The values of r_bs in A. edulis are sufficient for C₄ photosynthesis to elevate CO₂ in BS cells and to minimize photorespiration. The calculated CO₂ concentration in BS cells, which is dependent on input of r_bs, was about 2,000 µbar under maximum rates of CO₂ fixation, which is about six times the ambient level of CO₂. High re-assimilation of photorespired CO₂ was demonstrated in both mutant and wild-type plants at limiting CO₂ concentrations, which can be explained by high r_bs. Increasing O₂ from near zero up to ambient levels under low CO₂, resulted in an increase in the gross rate of O₂ evolution measured by chlorophyll fluorescence analysis in the PEPC mutant; this increase was simulated from a Rubisco kinetic model, which indicates effective refixation of photorespired CO₂ in BS cells.