CO2 diffusion resistance of bundle sheath cells in Amaranthus edulis, as revealed from the study of a PEP carboxylase deficient mutant

Abstract

A mutant of the NAD-ME type C4 plant Amaranthus edulis, that lacks PEP carboxylase in the mesophyll cells was studied. Analysis of CO2 response curves of the mutant in the absence of a functional C4 cycle, provided a direct means of determining the liquid phase diffusive resistance of atmospheric CO2 to the sites of RuBP carboxylation inside the bundle sheath (BS) chloroplasts (rbs) within intact plants. The magnitude of rbs has been difficult to determine and previous estimates using different methods have ranged from 15 -1400 m2 s mol-1. The values of rbs in A. edulis were 70-180 m2 s mol-1 and they increased as the leaf matured. This is about 70-fold higher than the liquid phase resistance or diffusion of CO2 to Rubisco in the mesophyll cells of C3 plants. The values of rbs in A. edulis are sufficient for C4 photosynthesis to elevate the CO2 concentration inside the BS cells and thus reduce photorespiration. High reassimilation of photorespired CO2 was demonstrated in both mutant and wild type plants at limiting CO2 concentrations, which can be explained by high rbs. Based on O2 sensitivity analysis of wild type plants, the calculated CO2 concentration in BS cells under maximum rates of CO2 fixation in young leaves (lower rbs) and in expanded leaves (higher rbs) was about the same (1000-1200 m bar), which is sufficient to minimise photorespiration. This indicates that overcycling of the pathway and the rate of leakage of CO2 from the BS cells was higher in the younger leaves.