Evidence for a Role for NAD(P)H Dehydrogenase in Concentration of CO₂ in the Bundle Sheath Cell of Zea mays

Prior studies with Nicotiana and Arabidopsis described failed assembly of the chloroplastic NDH [NAD(P)H dehydrogenase] supercomplex by serial mutation of several subunit genes. We examined the properties of Zea mays leaves containing Mu and Ds insertions into nuclear gene exons encoding the critical O- and N-subunits of NDH, respectively. In vivo reduction of plastoquinone in the dark was sharply diminished in maize homozygous mutant compared to normal leaves but not to the extreme degree observed for the corresponding lesions in Arabidopsis. The net carbon assimilation rate (A) at high irradiance and saturating CO₂ levels was reduced by one-half due to NDH mutation in maize although no genotypic effect was evident at very low CO₂ levels. Simultaneous assessment of chlorophyll fluorescence and A in maize at low (2% by volume) and high (21%) O₂ levels indicated the presence of a small, yet detectable, O₂-dependent component of total linear photosynthetic electron transport in 21% O₂. This O₂-dependent component decreased with increasing CO₂ level indicative of photorespiration. Photorespiration was generally elevated in maize mutant compared to normal leaves. Quantification of the proportion of total electron transport supporting photorespiration enabled estimation of the bundle sheath cell CO₂ concentration (Cb ) using a simple kinetic model of ribulose bisphosphate carboxylase/oxygenase function. The A versus Cb relationships overlapped for normal and mutant lines consistent with occurrence of strictly CO₂-limited photosynthesis in the mutant bundle sheath cell. The results are discussed in terms of a previously reported CO₂ concentration model [Laisk A, Edwards GE (2000) Photosynth Res 66: 199–224].