Rethinking isotope-based plant water uptake tracing in viticulture: alternative sampling approaches

Up to date, isotopic techniques in ecohydrology have been rarely applied in viticulture, despite their potential to identify plant water uptake sources and resolve their temporal dynamics, largely because traditional approaches for sampling grapevine xylem sap are destructive and often impractical in productive agroecosystems such as commercial vineyards, particularly when a high number of replicates is required. Moreover, recent evidence indicates that cryogenic vacuum distillation (CVD), commonly used to extract xylem water, can induce a methodological bias, yielding water that is artificially depleted in δ²H relative to the original xylem water.

These limitations have stimulated interest in alternative, non-destructive approaches. Recent studies have shown that transpired water can be collected by enclosing grapevine branches in plastic bags and sampling condensed water. Nevertheless, it remains unclear whether the isotopic composition of transpired water can be reliably used to infer plant water sources when the true isotopic signature of xylem water is unknown.

Here, we tested whether transpired water condensation can be used to retrieve the isotopic composition of plant water sources. We conducted a controlled experiment on potted grapevine plants irrigated with water of known isotopic composition, under contrasting water availability and different atmospheric conditions.

Multiple plant water sampling techniques were applied to detect isotopic changes along the soil–plant–atmosphere hydraulic continuum and to evaluate the validity of using transpired water to infer plant water uptake sources. In particular, we employed a vacuum pump–based sap extraction method designed to retrieve flowing xylem sap water and expected to closely reflect source water isotopic composition. Xylem bulk water, leaf bulk water, and bulk soil water were extracted using CVD.

The isotopic composition of vacuum-extracted sap water and of CVD-extracted waters were compared with transpired water and the original source of water (irrigation). Vacuum-extracted sap water closely reflected the isotopic composition of source water. Interestingly, transpired water collected in plastic bags also showed potential to be used as a proxy to infer the source water; however, its interpretation is less straightforward, requiring many replicates and explicit consideration of atmospheric conditions.

Overall, our results provide a methodological framework for evaluating non-destructive approaches to trace plant water sources and contribute to a better understanding of isotopic fractionation processes along the soil–plant–atmosphere continuum, with implications extending beyond viticulture to ecohydrological studies in managed and natural ecosystems.