A simple glasshouse experiment to test the isotopic fractionation in olive trees

Plant transpiration is a main component of the global water cycle and plays a key role in regulating ecohydrological process. Stable isotopes of oxygen and hydrogen are often used for the identification and quantification of plant water sources in ecohydrology. However, the isotopic tracing technique assumes that the isotopic signal in the water taken up by the plants remains unaltered during uptake at the soil-roots interface and transport to the distal twigs, i.e., isotopic fractionation does not occur during the water uptake and along the transport pathway. Nevertheless, recent studies showed that isotopic fractionation can occur under different environmental conditions. In this study, we performed a simple experiment with two olive (Olea europaea) trees utilizing labelled water to test isotopic fractionation of plant water during uptake and transport within the plants under controlled conditions. In addition, we performed the cryogenic vacuum distillation in two different laboratories to examine any possible effects of the extraction system on the isotopic composition of plant water extracts. We set up the olive trees in pots inside a glasshouse and measured sap flow rates with Granier thermal dissipation probe, and shallow soil moisture by using a portable soil moisture probe. Air temperature, global solar radiation, and relative humidity were measured by a weather station installed inside the glasshouse nearby the olive trees. We irrigated the two plants with water of known isotopic composition and sampled the twigs, wood cores, roots, and soils at different depths (0-5, 5-15, and 15-25 cm). We extracted plant and soil waters by means of cryogenic vacuum distillation performed in two different laboratories. Our results showed that the plant water samples reflected the isotopic signature of labelled water and mobile soil water, suggesting no isotopic fractionation during water transport. No significant differences were detected for twigs and wood cores extracted from distinct sections of the tree. However, only significant differences were obtained between plant tissue water (twigs, cores) and cryogenically-extracted deep soil water (i.e., >15 cm depths). Furthermore, we found no significant effects of the two cryogenic extraction systems on the isotopic composition of water extracts. Our results indicate that isotopic fractionation might not occur during root water uptake and transport processes in olive trees, at least under the specified experimental conditions, validating the conventional isotope-tracing approach. Further work both in the field and under controlled conditions, and on different plant species, is needed to check for this consistency, as well as testing other plant water extraction methods. Keywords: olive tree; stable isotope analysis; plant water; cryogenic vacuum distillation; fractionation; labelled water.