Water-use efficiency – indications from carbon and oxygen stable isotope composition of crop plants

Abstract Background & Aims Enhancing water use efficiency (WUE) is critical in the regulation of water stress in plants. Since both WUE and 13 C discrimination depend on stomatal conductance ( gₛ ) and CO 2 assimilation rate ( A ), Δ 13 C values have been widely utilized as a proxy for WUE. However, to distinguish whether changes in WUE are attributable to variations in A or gₛ , δ 1 ⁸O values are increasingly incorporated into the analysis. In this review, we aim to analyze the advantages and limitations of this dual isotopic model for identifying the drivers of changes in plant WUE. Methods We conducted a literature review to examine the factors governing the correlation between WUE and isotopic values. Additionally, we collate existing studies to discuss where the dual isotope model offers valuable insights into changes in WUE and where it reaches its limits. Results While previous studies on trees revealed that the dual-isotope model effectively identified the relationship of WUE to climatic factors, WUE in crops is additionally controlled by plant group characteristics and fertilization effects. These determining factors can also be identified through the application of the dual isotope model, however, under conditions of low vapor pressure deficit, the applicability of the dual isotope model appears to be limited. Conclusion The dual isotope model holds significant potential for elucidating the mechanisms controlling WUE. This is particularly relevant for crops, as it facilitates the identification of cultivars and soil management practices that ensure high biomass production, even under conditions where gₛ is reduced due to water stress.