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

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.