A Global Meta-Analysis of Water Use Efficiency Proxies Reveals That UV Radiation Decreases Transpiration Without Improving WUE

Abstract

Plant water use efficiency (WUE) links physiological processes to ecosystem-scale carbon and water cycles, making it a crucial parameter for climate change adaptation modelling. Climate and stratospheric ozone dynamics expose plants to varying intensity of ultraviolet-B radiation (UV-B), which affects stomatal function and transpiration. This meta-analysis evaluates UV-B effects on WUE using gas exchange and isotopic proxies. While UV-B radiation reduces stomatal conductance and transpiration, it also suppresses photosynthesis, particularly under non-saturating light. As a result, WUE remains unchanged or declines in UV-B exposed plants, depending on the measurement method. Instantaneous gas exchange-based WUE proxies indicate a decrease, whereas isotope-based proxies, integrating long-term fluxes, show no significant UV-B effect. The suppression of photosynthesis due to UV-B occurs only when UV-B lamps are used to increase the UV-B dose; when UV-B is excluded under field conditions there is no significant effect on WUE. Only some field studies report improved WUE under ambient UV-B, suggesting potential adaptive benefits. Overall, the findings challenge the assumption that UV-B-induced decreases in transpiration enhance WUE. Instead, they highlight a complex interplay between UV radiation, photosynthesis, and stomatal regulation, emphasizing the need to reconsider UV-B's role in plant water relations under future climate conditions.

Keywords: photosynthesis; stable carbon isotopes; stomata; ultraviolet radiation.

Figure 1

The overview of UV effects on CO₂ assimilation rate (A), stomatal conductance (Gs), transpiration rate (E), and water use efficiency calculated on the basis of Gs (sWUE; A/Gs), E (eWUE; A/E), and carbon stable isotope ratio (isWUE). Left panel: Meta‐analysis significance of UV‐induced effects: SDM, standard difference in means; 95% CI, 95% confidence interval. The numbers indicate the number of case studies included in the meta‐analysis. Right panel: Quantitative changes induced by UV radiation: vertical bars = medians, boxes = inter‐quartile range (IQR), whiskers = 5th and 95th percentiles, points = outliers identified.

Figure 2

A detailed analysis of UV effects on water use efficiency based on stomatal conductance (sWUE), transpiration rate (eWUE), and abundance of stable carbon isotopes (isWUE). Symbols represent standard difference in means (SDMs), with error bars indicating 95% confidence intervals (CIs). Data are categorized by plant functional groups (Woody vs. Non‐woody), growth environment (C – growth chamber, G – greenhouse, F – field conditions), applied biologically effective UV dose in kJ m^–2 day^–1 (Amb – ambient UV intensity; supplementary doses: Sup_low – below 5 kJ m^–2 day^–1; Sup_med – 5–10 kJ m^–2 day^–1; Sup_high – above 10 kJ m^–2 day^–1, and UV treatment duration (Dur_short ≤ 30 days; Dur_med 30–90 days; Dur_long > 90 days). Asterisks denote statistical significance of UV effects within each group: n.s. – p > 0.05, * – 0.05 ≥ p > 0.01; ** – p ≤ 0.01; *** – p ≤ 0.001. Numbers in brackets indicate the count of case studies analysed per group. In addition, numerical p‐values indicate statistically significant differences within tested categories. For detailed analysis of UV effects on CO₂ assimilation rate (A), stomatal conductance (Gs), and transpiration rate (E), see Figure 4. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

Meta‐analysis of UV‐induced effects on CO₂ assimilation rate (A), stomatal conductance (Gs), transpiration rate (E), stomata‐based water use efficiency (sWUE; A/Gs), and transpiration‐based water use efficiency (eWUE; A/E). Symbols represent standard differences in means (SDMs) for non‐saturating light conditions (light grey triangles; non) and saturating light conditions (dark grey triangles; sat), with error bars showing 95% confidence intervals (CIs). Asterisks indicate the statistical significance of UV effects within each group: n.s. – p > 0.05, * – 0.05 ≥ p > 0.01; ** – p ≤ 0.01; *** – p ≤ 0.001. Numbers in brackets show the number of case studies analysed per group. p‐values on the left indicate statistically significant differences between SDM values under saturating ( > 1000 µmol m^–2 s^–1) and non‐saturating intensities of photosynthetically active radiation (PAR).

Figure 4

Analysis of instantaneous UV effects on CO₂ assimilation rate (A; upper panel), stomatal conductance (Gs; middle panel), and transpiration rate (E; bottom panel) determined via gas‐exchange techniques. Symbols represent the standard difference in means (SDMs), with error bars showing 95% confidence intervals (CIs). Red circles indicate SDM values for all studies regardless of light intensity, while light grey and dark grey triangles represent SDM values for studies conducted under non‐saturating and saturating light intensities, respectively. All parameters are further categorized by plant functional groups (Woody vs. Non‐woody), growth environment (C – growth chamber, G – greenhouse, F – field conditions), applied biologically effective UV dose in kJ m^–2 day^–1 (Amb – ambient UV intensity (exclusion type of UV experiments), supplementary doses (Sup_low – below 5 kJ m^–2 day^–1; Sup_med – 5–10 kJ m^–2 day^–1; Sup_high – above 10 kJ m^–2 day^–1), and UV treatment duration (Dur_short ≤ 30 days, Dur_med 30–90 days, Dur_long > 90 days). Asterisks denote the statistical significance of UV effects within each group: n.s. – p > 0.05, * – 0.05 ≥ p > 0.01; ** – p ≤ 0.01; *** – p ≤ 0.001. Numbers in brackets indicate the number of case studies analysed per group. Additionally, p‐values highlight statistically significant UV effects within tested categories for all light conditions (red), non‐saturating light conditions (light grey), and saturating light intensities (dark grey). [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5

Relationship between UV‐induced effects on CO₂ assimilation rate (A) and stomatal conductance (Gs; upper panel) or transpiration rate (E; lower panel). White circles indicate case studies where UV effects on water use efficiency (WUE) ranged between −10% (decrease) and +10% (increase), reflecting proportional changes in A and Gs (sWUE) or A and E (eWUE). Red (blue) circles represent case studies showing a UV‐induced decrease (increase) in WUE due to disproportional changes in A/Gs or A/E. Arrows depict increase (↑) or decrease (↓) in each parameter within all segments of the graph, with double arrows indicating stronger UV effects. The total number of case studies per category is indicated by n‐values. [Color figure can be viewed at wileyonlinelibrary.com]