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

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.