The isotopic composition of phloem water and its relations to xylem water at daily and sub-daily resolutions

Isotopic tracing is de rigueur in ecohydrology and for quantifying tracing water sources that contribute to xylem water. But, tree transpiration is not a one dimensional process from roots to leaves. Three dimensional storages actively participate in water transport within the stem complicating in unknown ways, the otherwise straightforward tracing from source to xylem. Phloem is the largest elastic storage and works as a hydraulic capacitor, and as such is of great importance to tree water transport and functioning. Water stored in phloem moves into xylem vessels buffering changes in xylem water potential and sustaining tree hydraulic integrity. Although phloem water is of great importance to transpiration, we lack understanding about the relationship between xylem and phloem water isotopic composition. Assessing the isotopic composition of phloem is a needed next step to fully comprehend patterns of tree water use and improve understanding about isotopic offset between xylem and source water. Here we show daily and sub-daily dual-isotope measurements of phloem water in relation to xylem and leaf water in Salix viminalis along with high-resolution measurements of plant water status and transpiration rates in a large lysimeter. We found that phloem was more depleted in heavier isotopes than xylem and leaves. On average δ²H phloem water was 2.05 ‰ and δ¹⁸O phloem water was 0.66 ‰ more negative than xylem water. The largest difference observed between phloem and xylem isotopic composition occurred at night during a period of tree water deficit. Although, there was variability in the observed difference between xylem and phloem throughout the experiment, xylem and phloem isotopic composition were highly correlated (δ²H r = 0.89; δ¹⁸O r = 0.75). Our sub daily measurements showed that xylem and phloem differences decreased during predawn and morning compared to previous evening and midday measurements. We observed that the δ²H difference between phloem and xylem increased with the increase in daily use of phloem water storages, while δ¹⁸O difference between phloem and xylem increased with transpiration rate. Our results show that xylem and phloem isotope composition are in sync and that observed differences can be related to changes in plant water status and possible fractionation associated with transport within phloem-xylem. Further studies are necessary to understand how phloem affects source water interpretations across different tree species and larger trees, where phloem contribution to daily transpiration may be larger.