Plant transpiration is the largest contributor to continental water flux (Jasechko, 2018;Jasechko et al., 2013). Understanding the sources of transpiration water is important for predicting the effects of global change on water security and ecosystem services. Stable isotope tracing is one of the most useful tools for the aforementioned purpose (Evaristo et al., 2015;Sprenger et al., 2016). A central assumption in the use of stable isotopes to investigate plant water sources is that, excluding leaf-evaporative effects, the stable isotopes 2 H and 18 O act as conservative tracers. Therefore, the isotopic composition of xylem water effectively reflects that of source water (Ehleringer & Dawson, 1992). This requires that there is no occurrence of significant isotopic fractionation during root water uptake and the subsequent water transport in plant organs (Ehleringer & Dawson, 1992;Washburn & Smith, 1934;Zimmermann et al., 1968); and sampling and isotopic analysis of plant water do not introduce errors (Brunel et al., 1995).However, recent studies have shown that isotopic signatures of the extracted xylem water do not always match those of the water source(s), thus creating isotopic offsets between stem water and the water source. Isotopic offsets, usually depleted in 2 H, of stem water relative to the water source, have now been reported in ecologically diverse plant species (de la Casa et al., 2021). Lin and Sternberg (1993) and Ellsworth and Williams (2007) first reported that fractionation, associated with transmembrane water transport at the root-soil interface, results in the depletion of stem water for drought-tolerant and salinity-tolerant plants. Zhao et al. (2016) did not find evidence for deuterium fractionation during water uptake; they attributed the observed soil-stem water isotopic offsets as a putative discrimination during water transport and redistribution within the plant. Other alternative explanations for the isotopic offsets of stem water have also been hypothesized, such as soil pore-scale isotopic heterogeneity (Lin & Horita, 2016;Oerter & Bowen, 2019;Oerter et al., 2014), vapor transport and condensation on the root tips (Vargas et al., 2017), and a mixture of sap and tissue water (Barbeta et al., 2022). However, a recent study by Chen et al. (2020) proved that little isotopic fractionation occurs during water uptake and transport among diverse habitats/species, and the offsets in δ 2 H are artifacts of cryogenic vacuum extraction (CVE).
