The isotopic composition of phloem water and its relations to xylem water at daily and sub-daily resolutions
Bibliographic record
Abstract
Isotopic tracing is de rigueur in ecohydrology and for quantifying tracing water sources that contribute to xylem water. But, tree transpiration is not a one dimensional process from roots to leaves. Three dimensional storages actively participate in water transport within the stem complicating in unknown ways, the otherwise straightforward tracing from source to xylem. Phloem is the largest elastic storage and works as a hydraulic capacitor, and as such is of great importance to tree water transport and functioning. Water stored in phloem moves into xylem vessels buffering changes in xylem water potential and sustaining tree hydraulic integrity. Although phloem water is of great importance to transpiration, we lack understanding about the relationship between xylem and phloem water isotopic composition. Assessing the isotopic composition of phloem is a needed next step to fully comprehend patterns of tree water use and improve understanding about isotopic offset between xylem and source water. Here we show daily and sub-daily dual-isotope measurements of phloem water in relation to xylem and leaf water in Salix viminalis along with high-resolution measurements of plant water status and transpiration rates in a large lysimeter. We found that phloem was more depleted in heavier isotopes than xylem and leaves. On average δ2H phloem water was 2.05 ‰ and δ18O phloem water was 0.66 ‰ more negative than xylem water. The largest difference observed between phloem and xylem isotopic composition occurred at night during a period of tree water deficit. Although, there was variability in the observed difference between xylem and phloem throughout the experiment, xylem and phloem isotopic composition were highly correlated (δ2H r = 0.89; δ18O r = 0.75). Our sub daily measurements showed that xylem and phloem differences decreased during predawn and morning compared to previous evening and midday measurements. We observed that the δ2H difference between phloem and xylem increased with the increase in daily use of phloem water storages, while δ18O difference between phloem and xylem increased with transpiration rate. Our results show that xylem and phloem isotope composition are in sync and that observed differences can be related to changes in plant water status and possible fractionation associated with transport within phloem-xylem. Further studies are necessary to understand how phloem affects source water interpretations across different tree species and larger trees, where phloem contribution to daily transpiration may be larger.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".