Foliar water uptake as a source of hydrogen and oxygen in plant biomass

Abstract

Introductory biology lessons around the world typically teach that plants absorb water with their roots, but, unfortunately, absorption of water through leaves and subsequent transport and use of this water for biomass formation remains a field limited mostly to specialists. Recent studies have identified foliar water uptake as a significant net water source for terrestrial plants. The growing interest in the development of a new model that includes both foliar water uptake (in liquid form) and root water uptake to explain hydrogen and oxygen isotope ratios in leaf water and tree rings demands a method for distinguishing between these two water sources. I therefore devised a labelling method that utilizes two different water sources, one enriched in deuterium (HDO + D2O; δD = 7.0 × 104‰, δ18O = 4.1‰) and one enriched in oxygen-18 (H218O; δD = −85‰, δ18O = 1.1 × 104‰), to simultaneously label both foliar-absorbed and root-absorbed water and quantify their relative contributions to plant biomass. Using this new method, I here present evidence that, in the case of well-watered Cryptomeria japonica, hydrogen and oxygen incorporated into new leaf cellulose in the rainy season derives mostly from foliar-absorbed water (69% from foliar-absorbed water and 31% from root-absorbed water), while that of new root cellulose derives mostly from root-absorbed water (20% from foliar-absorbed water and 80% from root-absorbed water), and new branch xylem is somewhere in between (55% from foliar-absorbed water and 45% from root-absorbed water). The dual labelling method first implemented in this study enables separate and simultaneous labelling of foliar-absorbed and root-absorbed water, and offers a new tool to study the uptake, transport, and assimilation processes of these waters in terrestrial plants.

Updating the dual C and O isotope—Gas‐exchange model: A concept to understand plant responses to the environment and its implications for tree rings

“…Depending on g s , the ambient water vapour mole fraction (e a ) and its isotopic value (δ 18 O V , which is not isotopically in equilibrium with the source water) will alter the δ 18 O LW . Particularly, high ambient humidity conditions and open stomata (mostly low E) facilitate a high vapour exchange rate between substomatal cavities and ambient air (Kagawa, 2022;Lehmann et al, 2018Lehmann et al, , 2020, impacting δ 18 O LW considerably and thus δ 18 O organic . The influx of 18 O-depleted water vapour (δ 18 O V < δ 18 O LW ) from the ambient air into the leaf intercellular spaces and its mixing with leaf water diminish the δ 18 O LW enrichment (Farquhar et al, 2021;Lehmann et al, 2020); similar to what we observe with source water replenishing the transpirative water loss (E), g s modulates both processes (replenishment and air to substomatal cavity exchange rate), resulting in a negative g s -δ 18 O LW relationship.…”

Detecting long‐term changes in stomatal conductance: challenges and opportunities of tree‐ring δ18O proxy

“…precipitation and water vapour, which could easily be achieved using existing monitoring networks world‐wide, e.g. Fluxnet, NEON, ICP Forests), and a complete characterization of the δ 18 O P pathway from soil through tree xylem (Barbeta et al ., 2022 ) and from water vapour to leaf water (bidirectional diffusion – Lehmann et al ., 2019 ; Kagawa, 2022 ) to tree rings under field conditions (both at intra‐ and inter‐annual resolution). Recent progress in deriving precipitation isotope time series using machine learning or isotope‐enabled general circulation models can help to reduce uncertainties related to source water δ 18 O input (Nelson et al ., 2021 ).…”

Updating the dual C and O isotope – gas exchange model: A concept to understand plant responses to the environment and its implications for tree rings

“…Another process, which significantly impacts δ 18 O LW is iii) theback diffusion of ambient water vapor into the substomatal cavities (bidirectional fluxes, Seibt et al, 2006, Farquhar et al, 2021, depending on g s , the ambient water vapor mole fraction (e a ) and its isotopic value (δ 18 Ο ατμ-απ +ε ε ). Particularly at high ambient humidity conditions and high g s (but low E), facilitate a high bidirectional diffusion rate of water molecules (Lehmann et al, 2018;Kagawa, 2022), impacting δ 18 O LW considerably and thus δ 18 O organic . Similar to the Péclet effect, the influx of 18 O depleted water vapor into the leaf intercellular spaces diminishes the δ 18 O LW enrichment, with an increasing water vapor influx resulting in a negative relationship between g s and δ 18 O LW (Farquhar et al, 2021).…”