Evaluating the response of δ¹³C in Haloxylon ammodendron, a dominant C ₄ species in Asian desert ecosystems, to water and nitrogen addition as well as the availability of its δ¹³C as an indicator of water use efficiency

Variations in precipitation and atmospheric N deposition affect water and N availability in desert and thus may have significant effects on desert ecosystems. Haloxylon ammodendron is a dominant plant in Asian desert, and addressing its physiological acclimatization to the changes in precipitation and N deposition can provide insight into how desert plants adapt to extreme environments by physiological adjustment. Carbon isotope ratio (inline-formulaδ¹³C) in plants has been suggested as a sensitive long-term indicator of physiological acclimatization. Therefore, this study evaluated the effect of precipitation change and increasing atmospheric N deposition on inline-formulaδ¹³C of H. ammodendron. Furthermore, H. ammodendron is a Cinline-formula₄ plant; whether its inline-formulaδ¹³C can indicate water use efficiency (WUE) has not been addressed. In the present study, we designed a field experiment with a completely randomized factorial combination of N and water and measured inline-formulaδ¹³C and gas exchange of H. ammodendron. Then we calculated the degree of bundle-sheath leakiness (inline-formulaφ) and WUE of the assimilating branches of H. ammodendron. inline-formulaδ¹³C and inline-formulaφ remained stable under N and water supply, while N addition, water addition and their interaction affected gas exchange and WUE in H. ammodendron. In addition, inline-formulaδ¹³C had no correlation with WUE. These results were associated with the irrelevance between inline-formulaδ¹³C and the ratio of intercellular to ambient COinline-formula₂ concentration (inline-formulac_i inline-formula $M17inlinescrollmathml/$ 8pt14ptsvg-formulamathimga3a809672b156f3719eee3cbaf593ee5 bg-18-2859-2021-ie00001.svg8pt14ptbg-18-2859-2021-ie00001.png  inline-formulac_a), which might be caused by a special value (0.37) of the degree of bundle-sheath leakiness (inline-formulaφ) or a lower activity of carbonic anhydrase (CA) of H. ammodendron. In conclusion, inline-formulaδ¹³C of H. ammodendron is not sensitive to global change in precipitation and atmospheric N deposition and cannot be used for indicating its WUE.