Analysis of Bundle Sheath Conductance and C4 Photosynthesis using a PEP-Carboxylase Inhibitor

Abstract

High [CO₂] in bundle sheath cells (BSC) allows high rates of CO₂ assimilation (A) and minimises photorespiration in C₄ leaves. The [CO₂] in BSC is likely to be low when PEPcase is inhibited by 3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate (DCDP), but Rubisco is still functional. The degree of C₄ photosynthesis can be assessed by decreased A and the conductance of BSC walls by the slope of A versus [CO₂] during application of DCDP. Inhibition of A by 4.0 mM DCDP was 87-100% in C₄ species and was overcome by increasing [CO₂] to 2.0-2.5 kPa. In C₃ -C₄ species in Moricandia, Panicum and Neurachne, A and its inhibition by O₂ were unchanged by DCDP, but in C₃ -C₄ and C₄ -like species and interspecific hybrids of Flaveria, A was reduced and its inhibition by O₂ was increased. Conductance of BSC walls was 1.13, 1.96, and 2.35 mmol m^-2 s^-1 in the C₄ species, Sorghum bicolor (L.) Moench, Panicum miliaceum L., and Panicum maximum Jacq., respectively. There was no effect of O₂ on A in DCDP-treated C₄ leaves, apparently because the low conductance of BSC walls masks the influence of O₂ on CO₂ exchange in these cells.