Short‐term effects of CO2 and O2 on citrate metabolism in illuminated leaves

Although there is now a considerable literature on the inhibition of leaf respiration (CO2 evolution) by light, little is known about the effect of other environmental conditions on day respiratory metabolism. In particular, CO2 and O2 mole fractions are assumed to cause changes in the tricarboxylic acid pathway (TCAP) but the amplitude and even the direction of such changes are still a matter of debate. Here, we took advantage of isotopic techniques, new simple equations and instant freeze sampling to follow respiratory metabolism in illuminated cocklebur leaves (Xanthium strumarium L.) under different CO2/O2 conditions. Gas exchange coupled to online isotopic analysis showed that CO2 evolved by leaves in the light came from ‘old’ carbon skeletons and there was a slight decrease in 13C natural abundance when [CO2] increased. This suggested the involvement of enzymatic steps fractionating more strongly against 13C and thus increasingly limiting for the metabolic respiratory flux as [CO2] increased. Isotopic labelling with 13C2-2,4-citrate lead to 13C-enriched Glu and 2-oxoglutarate (2OG), clearly demonstrating poor metabolism of citrate by the TCAP. There was a clear relationship between the ribulose-1,5-bisphosphate oxygenation-to-carboxylation ratio (vo/vc) and the 13C commitment to 2OG, demonstrating that 2OG and Glu synthesis via the TCAP is positively influenced by photorespiration.