A comparative study of plant water extraction methods for isotopic analyses: Scholander-type pressure chamber vs. cryogenic vacuum distillation

Abstract

Abstract. Recent tracer-based studies using stable isotopes of hydrogen and oxygen showed that different methods for extracting water from plant tissues can return different isotopic composition due to the presence of organic compounds and the extraction of different plant water pools. One of the most used methods to extract plant water is the cryogenic vacuum distillation (CVD), which tends to extract total plant water. Conversely, the Scholander-type pressure chamber (SPC), which is commonly used by tree physiologists to measure shoot water potential and determine plant water stress, has been rarely applied to extract plant water for isotopic analyses. In this work, we analyzed the variability in the isotopic composition of plant water extracted by SPC and CVD, also considering the potential variability in the isotopic signature of the plant tissues (i.e., leaves, twig without bark, twig with bark, twig close to the trunk of the tree, and wood core) and plant species (i.e., alder, apple, chestnut and beech). The extraction of plant water by SPC is simple, can be carried out in situ, and it does not require specific laboratory work as in case of CVD. However, the main limitation of SPC is the very small water volume that can be extracted from the lignified shoots during conditions of water deficit, compared to CVD. Our results indicated that plant water extracted by SPC and CVD were significantly different. The difference in the isotopic composition obtained by the two extraction methods was smaller in the beech samples compared to alder, apple and chestnut samples. The isotopic signature of alder, apple and chestnut plant water extracted by SPC was more enriched in δ2H and δ18O, respectively, than the samples obtained by CVD. We conclude that plant water extraction by SPC is not an alternative for CVD, as SPC likely extracts only water within the xylem (dead cells), whereas CVD tends to retrieve all water stored in the sampled tissue, from both living and dead cells. However, studies aiming to quantify the relative contribution of the water sources to transpiration should rely more on the isotopic composition of xylem water transpiring during the sampling day (which is theoretically sampled by SPC), than the isotopic composition of total plant water (sampled by CVD), which also contains a fraction of water that could be stored in plant tissues for long time.

On the Spatio-Temporal Under-Representation of Isotopic Data in Ecohydrological Studies

“…(2) Artificial effects (ART in Figure 1) mainly include soil and xylem water field sampling and extraction methods as well as instrumental measurement uncertainty in laboratory analysis, as follows: a) Different methods for soil water sampling and extraction, such as tension lysimeters and cryogenic vacuum distillation returning different isotopic composition from the same sample (Geris et al, 2015;Orlowski et al, 2016Orlowski et al, , 2018Orlowski et al, , 2019. b) Different methods for plant water sampling and extraction such as cryogenic vacuum distillation, Scholander-type pressure chamber, and other destructive methods possibly producing artifacts and returning different isotope values (Thoma et al, 2018;Fischer et al, 2019;Zuecco et al, 2020), preferentially affecting 2 H rather than 18 O values (Chen et al, 2020). Moreover, these destructive methods might not be able to sample the accessible (or mobile) water as in-situ methods do (e.g., .…”

Ressi experimental catchment: Ecohydrological research in the Italian pre‐Alps

“…Plant water is extracted using cryogenic vacuum distillation (Koeniger et al, 2011). Additional plant water samples were extracted by the Scholander‐type pressure chamber in summer 2013 and 2017 (Penna et al, 2013; Zuecco et al, 2020).…”

“…Data collected in the catchment were used to examine the seasonality of the contributions of different water sources to streamflow, the soil moisture and precipitation threshold for event runoff generation (Figure 3; Penna et al, 2015), to test a hysteresis index to study how the relation between streamflow and soil moisture changes seasonally (Zuecco et al, 2016), and to investigate which water sources were most likely used for transpiration (Penna et al, 2013). Other studies used the precipitation and soil moisture datasets collected in Ressi to validate hydrological models (Brocca et al, 2015), to compare different types of throughfall collectors (Zuecco et al, 2014), and to examine the isotopic composition of plant water extracted by cryogenic vacuum distillation and the Scholander‐type pressure chamber (Zuecco et al, 2020).…”

“…In the legend, 'groundwater' indicates the location of the piezometers for the monitoring of the depth to the water table and the shallow groundwater sampling F I G U R E 2 Pictures of the Ressi experimental catchment: upper part of the catchment during summer (a), near the outlet after a snowfall event in winter (b), weather station and rainfall collector (c), V-notch weir and the stream gauge at the outlet (d), and shallow groundwater piezometer (e) water sources were most likely used for transpiration (Penna et al, 2013). Other studies used the precipitation and soil moisture datasets collected in Ressi to validate hydrological models (Brocca et al, 2015), to compare different types of throughfall collectors (Zuecco et al, 2014), and to examine the isotopic composition of plant water extracted by cryogenic vacuum distillation and the Scholandertype pressure chamber (Zuecco et al, 2020).…”

Reply on RC1

“…The cryogenic extraction biases estimated by Chen et al (2020) vary from -5.2 to -10.9 ‰ δ 2 H among different species, potentially complicating any cross-species comparisons that rely on xylem δ 2 H differences of similar magnitudes. Clearly more work is needed to determine how cryogenic extraction biases vary among plant species and among individuals within species, as well as how they vary with the extraction procedures that are used (Barbeta et al, 2020b;Fischer et al, 2019;Millar et al, 2018;Zuecco et al, 2020).…”