Testing a new method for extracting plant water for isotopic analysis

In ecohydrology, stable water isotopes (δ²H and δ¹⁸O) are valuable tools for investigating the water’s movement through the soil-plant-atmosphere continuum. Recent tracer-based studies using stable water isotopes showed that different methods for extracting water from plant tissues can return different isotopic compositions due to the presence of organic contaminants and because these methods extract different plant water domains. 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 vapour equilibration, mechanical squeezing and centrifugation. However, there remains a necessity to develop a cost and time efficient method to discriminately extract xylem water, which better represents the source waters used by plants for transpiration.

In this work, we evaluated the viability of Vacuum Extraction (VAC) - a method previously used in ecophysiology for chemical analysis - for the extraction of plant water for isotopic analysis. The specific objectives were to i) assess the likely influence of organic contaminants (glucose, fructose, sucrose, ethanol and methanol) in water samples extracted by VAC, ii) determine whether there is a significant difference in the isotopic signature of plant water extracted by VAC from lignified samples with and without bark, iii) compare the isotopic composition of plant water extracted by VAC and CVD.

The comparison tests were carried out in late March and early July 2024 on trees or shrubs of Cornus sanguinea, Carpinus orientalis, Prunus cerasifera, Photinia serratifolia, and Populus canadensis, located in a village close to Padua (Italy). In March, samples were taken from lignified twigs, and we prepared replicates with and without bark for extraction by VAC. In July, twig samples were collected for extraction by VAC and by CVD. Given the negligible presence of organic contaminants in VAC samples, we performed their isotopic analysis by laser spectroscopy. Conversely, CVD samples were analysed by isotope-ratio mass spectrometry.  

Our results showed no significant differences in the sugar levels of samples with and without bark, and no clear relation between the sugar content and the isotopic composition of plant water extracted by VAC. Additionally, when comparing CVD and VAC, the δ¹⁸O values were similar, but there were significant differences in the δ²H between the two methods, with VAC samples plotting significantly closer to the Local Meteoric Water Line compared to CVD samples. These first results indicate that VAC is a promising and effective method for the extraction of plant water for isotopic analysis. However, further tests should be performed for other species and under different environmental conditions.

 

Acknowledgements: This study was carried out within the Agritech National Research Center and received funding from the European Union Next-Generation EU (PIANO NAZIONALE DI RIPRESA E RESILIENZA (PNRR) – MISSIONE 4 COMPONENTE 2, INVESTIMENTO 1.4 – D.D. 1032 17/06/2022, CN00000022). This abstract reflects only the authors’ views and opinions, neither the European Union nor the European Commission can be considered responsible for them.