Review on carbon isotope composition (δ¹³C) and its relationship with water use efficiency at leaf level

Water deficiency is a major global constraint for plant productivity that is likely to be exacerbated by climate change. Hence, improving plant water use efficiency (WUE) has become a major goal in the near future. WUE reflects the coupling of carbon and water cycles in plant-soil-atmosphere continuum. Analyzing WUE can improve our understanding of the interaction between carbon and water cycles in terrestrial ecosystems. Stable carbon isotope analysis has become the most effective techniques in plant ecological research. As an indicator, foliar carbon isotope discrimination (δ¹³C) is often used to evaluate long-term WUE in C₃ plants. Study of δ¹³C and WUE can help to reveal and predict the response and adaptation of forest vegetation to global climate change. In this paper, the mechanism of characterization of δ¹³C and WUE, the factors influencing plant δ¹³C and WUE, including leaf traits, plant ecophysiology, climate factors, genetic control and genetic variation were summarized. The impacts of water stress and acid deposition on plant δ¹³C and WUE were also discussed. Plant stomatal conductance, specific leaf area, leaf nitrogen content, intercellular CO₂ concentration, and atmospheric CO₂ concentration were proposed to be the dominant factors influencing WUE variations due to their direct or indirect effects on plant net photosynthesis and transpiration rate. Generally, plants display higher WUE and lower δ¹³C when exposed to drought stress, and lower stomatal conductance and photosynthesis under long-term acid deposition. Nitrogen input will enhance plant productivity by improving water use efficiency. We suggest that the key role of quantitative trait loci, carbonic anhydrase, aquaporins, large and small subunits gene of Rubisco in the process of WUE genetic control must be highlighted when using stable isotope technique to study plant WUE. Finally, we must strengthen the study of multiple temporal and spatial scale variation and explore the application of combing analysis of δ¹³C and δ¹⁸O on the dual isotope conceptual model.