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

Forests experience complex light environments, but 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 (Alnus incana) and holm oak (Quercus ilex), and investigated the underlying biochemical and physiological mechanisms. With increasing blue light, differing photosynthetic and stomatal responses consistently led to a decrease in intrinsic water-use efficiency (iWUE) in the two species. For alder, the decline in iWUE was primarily due to a reduced photosynthesis rate (An); for oak, although An also decreased, blue-light-stimulated stomatal opening played a major role. Although the reduction in An was linked to blue-light-induced photoprotective processes in alder, it 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 that responded to blue light and coordinated with An was the chloroplast membrane in alder whereas it was the cell wall and plasma membrane in oak. Our findings highlight species-specific physiological strategies in the response to blue light and underscore the importance of considering spectral composition when assessing carbon-water trade-offs in forest trees.