Contrasting photosynthetic, stomatal and mesophyll mechanisms drive common reductions in leaf water-use efficiency under blue light.

Forests experience complex light environments, yet the detailed roles of photosynthetic, stomatal, mesophyll and biochemical responses to dynamic blue light remain unclear in tree species. We measured the blue-light responses of leaf gas exchange, online isotope discrimination, photorespiration and chlorophyll fluorescence in grey alder and holm oak, and investigated the underlying biochemical and physiological mechanisms. With increasing blue light, differing photosynthetic and stomatal responses consistently led to a decrease in water-use efficiency (WUE) in the two species. For grey alder, the WUE decline was primarily due to reduced photosynthesis rate (An); for holm oak, although An also decreased, blue-light-stimulated stomatal opening played a major role. Although the An reduction was linked to blue-light-induced photoprotective processes in grey alder, An was coordinated with mesophyll conductance (gm) in both species. The maximum carboxylation rate of Rubisco and gm imposed considerable photosynthetic limitations, especially at high blue-light levels. However, the component of gm responding to blue light and coordinating with An differed: the chloroplast membrane in grey alder and the cell wall and plasma membrane in holm oak. Our findings highlight species-specific physiological strategies of blue-light response and underscore the importance of considering spectral composition when assessing tree carbon-water trade-offs in forests.