Long-term effects of light quality on leaf hydraulic conductance (K_leaf) and stomatal conductance (g_s) were studied in cucumber, and their joint impact on leaf photosynthesis in response to osmotic-induced water stress was assessed. Plants were grown under low intensity monochromatic red (R, 640 nm), blue (B, 420 nm) or combined red and blue (R:B, 70:30) light. K_leaf and g_s were much lower in leaves that developed without blue light. Differences in g_s were caused by differences in stomatal aperture and stomatal density, of which the latter was largely due to differences in epidermal cell size and hardly due to stomatal development. Net photosynthesis (A_N) was lowest in R-, intermediate in B-, and highest in RB- grown leaves. The low A_N in R-grown leaves correlated with a low leaf internal CO₂ concentration and reduced PSII operating efficiency. In response to osmotic stress, all leaves showed similar degrees of stomatal closure, but the reduction in A_N was larger in R- than in B- and RB-grown leaves. This was probably due to damage of the photosynthetic apparatus, which only occurred in R-grown leaves. The present study shows the co-ordination of K_leaf and g_s across different light qualities, while the presence of blue in the light spectrum seems to drive both K_leaf and g_s towards high, sun-type leaf values, as was previously reported for maximal photosynthetic capacity and leaf morphology. The present results suggest the involvement of blue light receptors in the usually harmonized development of leaf characteristics related to water relations and photosynthesis under different light environments.

ISHS VII International Symposium on Light in Horticultural Systems RED AND BLUE LIGHT EFFECTS DURING GROWTH ON HYDRAULIC AND STOMATAL CONDUCTANCE IN LEAVES OF YOUNG CUCUMBER PLANTS