Phloem water isotopically different to xylem water: Potential causes and implications for ecohydrological tracing

Abstract

The stable isotopes of hydrogen and oxygen in xylem water are often used to investigate tree water sources. But this traditional approach does not acknowledge the contribution of water stored in the phloem to transpiration and how this may affect xylem water and source water interpretations. Additionally, there is a prevailing assumption that there is no isotope fractionation during tree water transport. Here, we systematically sampled xylem and phloem water at daily and subdaily resolutions in a large lysimeter planted with Salix viminalis. Stem diurnal change in phloem water storage and transpiration rates were also measured. Our results show that phloem water is significantly less enriched in heavy isotopes than xylem water. At subdaily resolution, we observed a larger isotopic difference between xylem and phloem during phloem water refilling and under periods of tree water deficit. These findings contrast with the expectation of heavy‐isotope enriched water in phloem due to downward transport of enriched leaf water isotopic signatures. Because of previous evidence of aquaporin mediated phloem and xylem water transport and higher osmotic permeability of lighter hydrogen isotopologues across aquaporins, we propose that radial water transport across the xylem–phloem boundary may drive the relative depletion of heavy isotopes in phloem and their relative enrichment in xylem.

Does back-flow of leaf water introduce a discrepancy in plant water source tracing through stable isotopes?

“…The discrepancy between the isotopic signatures of xylem water and below ground and stream water observed by Brooks et al (2010) was reconfirmed in our meta-analysis for groundwater (p < 0.01; Fig. 1) and could have numerous explanations, e.g., fractionation by roots during uptake (Barbeta et al, 2019;Ellsworth and Williams, 2007;Lin and Sternberg, 1993;Zhao et al, 2016), which could be enhanced by mycorrhizae (Poca et al, 2019), xylem-phloem water exchange (Cernusak et al, 2005;Nehemy et al, 2022) and absorption and evaporation through bark (Ellsworth and Sternberg, 2015;Gimeno et al, 2022), including an alternative water source, as we hypothesise. While it is important to note that 3 out of the 25 datasets used in our meta-analysis indicate enrichment in groundwater rather than xylem water, the overall combined dataset indicates the opposite, with an average xylem isotopic δ 18 O enrichment of 2.21‰ compared to groundwater (Fig.…”

“…composition occurs during this uptake (Dawson and Ehleringer, 1991;Zimmermann et al, 1967); and (ii) there is no fractionation during sap transport through the xylem until it reaches the leaves and is transpired into the atmosphere (Dawson and Ehleringer, 1993). However, over recent decades, an increasing amount of stable isotope research has shown that: (i) fractionation during root water uptake can occur, mainly, but not exclusively (Vargas et al, 2017;Poca et al, 2019), in xerophytic and halophytic species (Lin and Sternberg, 1993;Ellsworth and Williams, 2007;Zhao et al, 2016;Barbeta et al, 2019); and (ii) a range of processes can affect the isotopic composition during sap transport in the xylem, such as xylemphloem water exchange (Cernusak et al, 2005;Nehemy et al, 2022) and absorption and evaporation through bark (Ellsworth and Sternberg, 2015;Gimeno et al, 2022).…”

Flushing or mixing? Stable water isotopes reveal differences in arctic forest and peatland soil water seasonality

“…Isotopic composition of stem water depends on many factors, such as response to drought (Bertrand et al, 2014; Csank et al, 2016; Voltas et al, 2015), daily (Filella & Peñuelas, 2003) or sub‐daily fluctuations (Martín‐Gómez et al, 2016), permafrost conditions (Young‐Robertson et al, 2017) evaporative enrichment due to restricted sap flow (del Castillo et al, 2016), foliar uptake of water (Berry et al, 2014; Eller et al, 2013), fluctuations along the stem height (de Deurwaerder et al, 2020; Vega‐Grau et al, 2021), tree internal water storage (Knighton et al, 2020), water transport across the xylem‐phloem boundary (Nehemy, Benettin, et al, 2022), magnitudes of winter snow fall (Jespersen et al, 2018), proximity to stream channels (Alstad et al, 1999) or evaporation and diffusion of stem water (Tetzlaff et al, 2021). Recent studies in field of ecohydrology indicate that isotopic composition of bulk stem water differs from the sap water in xylem conduits (Barbeta et al, 2022), and water age distribution differences from tree base to tree crown further complicate root water uptake investigation (Seeger & Weiler, 2021).…”

The natural abundance of stable water isotopes method may overestimate deep-layer soil water use by trees

“…Although we did not collect soil water isotope samples in the isotopic labeling experiments, this may have little effect on determining the soil layer depths from which trees derive their water source due to the high δD values in the injected solution. It should be noted that isotopic spatial heterogeneity of xylem water induced by sampling position and time (Nehemy et al, 2022) and soil water induced by uneven distribution of throughfall and preferential flow (Xiang et al, 2019;Yang and Fu, 2017) could lead to an isotopic offset. Recently, isotopic offsets between plants and their potential water sources have also been found in various ecosystems, which may hinder the unambiguous identification of water sources and influence the accurate assessment of DLSW utilization (Barbeta et al, 2022;de La Casa et al, 2022;Zhao et al, 2016).…”