Testing a new method for extracting plant water for isotopic analysis

In ecohydrology, stable water isotopes are an invaluable tool for investigating water’s movement through the soil-plant-atmosphere continuum. Accurate tracing of water relies on the ability to extract plant water while maintaining its isotopic signature (δ2H and δ18O). While Cryogenic Vacuum Distillation (CVD) is widely recognized as a standard method of plant water extraction for isotopic analysis, its indiscriminate water extraction has proven problematic. Various other techniques have been developed and tested for plant water extraction such as Direct Vapor Equilibrium (DVE) and Centrifugation (CE). However, there remains a necessity to develop a cost and time efficient method to discriminately extract xylem water, which better represents recently up-taken source water. This master’s thesis evaluated the viability of Vacuum Extraction (VAC), a method previously used in ecophysiology for chemical analysis, for xylem water extraction for isotopic analysis (δ2H and δ18O). The thesis focused on assessing the likely influence of phloem’s presence on the quantity of organic contaminants (glucose, fructose, sucrose, ethanol and methanol) in water samples extracted by VAC and their influence on isotopic results, determining whether there is a significant difference in the isotopic signature of plant water extracted by VAC from lignified samples with and without bark. Furthermore, this thesis focused on comparing the isotopic composition of plant water extracted by VAC and CVD. Findings revealed no significant difference in the sugar levels between samples with and without bark and no clear relation between the sugar content and isotopic signature. Additionally, when comparing CVD and VAC, the δ18O values were found to be comparable, however, there were significant differences in the δ2H between the two methods with VAC samples plotting significantly closer to the Local Meteoric Water Line (LMWL) compared to CVD samples. These results guide the preparatory procedure for VAC extraction, indicate that the extract is mainly composed of xylem water and demonstrate the high potential associated with this new technique, highlighting the need for further research focused on its implementation.