Leaf anatomical traits shape lettuce physiological response to vapor pressure deficit and light intensity.

Main conclusion: Anatomical plasticity in stomatal, vascular, and mesophyll traits enables lettuce to partially buffer high evaporative and irradiance stress, advancing understanding of crop acclimation under a changing environment. Phenotypic plasticity in leaf anatomical and physiological traits is fundamental for plant acclimation to variable environmental conditions. While the individual effects of light intensity and vapor pressure deficit (VPD) on plant performance are relatively well understood, their interactive influence on leaf structure and function remains underexplored. Here, we investigated the response of Lactuca sativa L. var. capitata ('Salanova') to combinations of two VPD levels (0.78 and 1.4 kPa) and three daily light integrals (DLIs; 8.6, 12.9, and 15.5 mol m⁻² d⁻¹) in a vertical farming system. Contrary to our initial hypothesis that high irradiance combined with elevated VPD would impair mesophyll development and photosynthetic performance, plants under high VPD and high DLI exhibited pronounced anatomical plasticity. These included a 40% increase in stomatal density, a 24% increase in minor vein density, enhanced palisade mesophyll thickening, and elevated chloroplast surface exposure to intercellular airspaces (Sc), enabling partial maintenance of CO₂ diffusion despite reductions in mesophyll gas phase conductance (g_ias). Multivariate analyses revealed a strong coordination among anatomical and physiological traits under high VPD, with vascular and stomatal traits emerging as critical nodes. Although plants under low VPD consistently achieved higher biomass, photosynthesis, and water use efficiency, those under high VPD and high light conditions activated structural and biochemical compensations (e.g., increased Vcmax and Jmax), mitigating the detrimental effects of environmental stress. Our findings emphasize the essential role of leaf anatomical plasticity in facilitating plant acclimation to combined high evaporative demand and irradiance, offering novel insights for optimizing crop performance in controlled environment agriculture.