Blue light reduces leaf-level water-use efficiency via contrasting physiological mechanisms in two tree species

Blue light-dependent photosynthesis and stomatal opening have been intensively studied in herbaceous crops but less so in tree species, where forests face more complex light environments compared with crops in agricultural fields. The light spectral environment in forests is influenced by factors such as the multi-layered canopy structure, dynamic light availability and shading through canopy gaps, and the occurrence of sun flecks. These factors result in dynamic variability in blue-to-red light ratio perceived by trees. Therefore, we conducted leaf gas exchange measurements, combined with online isotope discrimination, photorespiration and chlorophyll fluorescence on two contrasting tree species grey alder (Alnus incana) and holm oak (Quercus ilex) across a full gradient of blue light fraction, with the remaining fraction supplied as red light to maintain a constant total light intensity. Photosynthetic and stomatal responses to increasing blue light differed markedly between the two species but led to a consistently decreasing water-use efficiency (WUE). For grey alder, the decrease in WUE was primarily due to blue light-induced photosynthesis reduction, which is associated with light stress on the photosynthetic apparatus as detected by chlorophyll fluorescence; whereas for holm oak, blue light-stimulated stomatal opening played the major role in reducing WUE. Although isotope-based estimates of mesophyll conductance were lower at higher blue light levels, especially in grey alder, the changes in mesophyll conductance did not result in a CO₂ shortage at the site of Rubisco under higher compared with lower blue light levels. However, across species, the component responding to blue light differed: the chloroplast membrane in grey alder and the cell wall and plasma membrane in holm oak. We suggest that, in tree species, blue light decreases WUE through distinct coordination between photosynthesis and stomata and species-specific blue light sensitivities of underlying mechanisms influencing the CO₂ diffusion pathway.