Carbon isotope discrimination in C3‐C4 intermediates

Abstract

ABSTRACTCarbon isotope discrimination in C3–C4 intermediates is determined by fractionations during diffusion and the biochemical fractionations occurring during CO2 fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO2 fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO2 fixation occurring in the bundle sheath, the extent of bundle‐sheath leakiness and the contribution which C4‐cycle activity makes to the CO2 pool there. In most instances, carbon isotope discrimination in C3–C4 intermediates is C3‐like because only a small fraction of the total carbon fixed is fixed in the bundle sheath. In particular, this must be the case for Flaveria intermediates which initially fix substantial amounts of CO2 into C4‐acids. In C3–C4 intermediates that refix photorespiratory CO2 alone, it is possible for carbon isotope discrimination to be greater than in C3‐species, particularly at low CO2 pressures or at high leaf temperatures. Short‐term measurements of carbon isotope discrimination and gas exchange of leaves can be used to study the photosynthetic pathways of C3‐C4 intermediates and their hybrids as has recently been done for C3 and C4 species.