Differences in instantaneous water use efficiency derived from post-carboxylation fractionation respond to the interaction of CO₂ concentrations and water stress in semi-arid areas

In the context of global warming attributable to the increasing levels of CO2, severe drought can be anticipated in areas with chronic water shortages (semi-arid areas), which necessitates research on the interaction between elevated atmospheric concentrations of CO2 and drought on plant photosynthetic discrimination. As δ13C of water-soluble compounds in leaves was depleted from extracellular CO2 to primary assimilates, no explanation has been offered for 13C fractionation before leaf-exported transportation of photosynthate. Either its variation according to the CO2 concentration and/or water stress gradients, or their interaction have not yet been identified. Therefore, saplings of species typical to a semi-arid area of Northern China that have similar growth status – Platycladus orientalis and Quercus variabilis – were selected and cultivated in growth chambers with orthogonal treatments (four CO2 concentrations [CO2] × five soil volumetric water contents (SWC)). The δ13C of water-soluble compounds extracted from leaves of potted saplings was measured to determine the instantaneous water use efficiency (WUEcp) after cultivation. Instantaneous water use efficiency derived from gas exchange (WUEge) was integrated to estimate differences in δ13C signal variation before leaf-exported translocation of primary assimilates. The WUEge of the two saplings both decreased with increased soil moisture, and increased with elevated [CO2] at 35 %–80 % of Field Capacity (FC) by strengthening photosynthetic capacity and reducing transpiration. Differences in instantaneous water use efficiency (iWUE) according to distinct environmental changes differed between the species. The WUEge of P. orientalis was significantly greater than that of Q. variabilis, while the opposite results were obtained in a comparison of the WUEcp of the two species. The differences between WUEge and WUEcp were clearly species-specific, as demonstrated in the interaction of [CO2] and SWC. Rising [CO2] coupled with moistened soil generated increasing disparities between WUEge and WUEcp in P. orientalis with an amplitude of 0.0328 ‰–0.0472 ‰. Further, the differences between WUEge and WUEcp of Q. variabilis increased as CO2 concentration increased and water stress alleviated (0.0384 ‰–0.0466 ‰). The 13C fractionation in post-photosynthesis was linearly dependent on gs, and was attributed to environmental variation. Thus, cautious descriptions of the magnitude and environmental dependence of apparent post-carboxylation fractionation are worth our attention in photosynthetic fractionation.