Light quality shapes mesophyll conductance in cucumber leaves in association with coordinated anatomical changes.

Light quality is a pivotal environmental cue shaping leaf anatomy and photosynthetic performance, but how anatomical changes are associated with internal CO2 diffusion remains incompletely understood. Mesophyll conductance, a key determinant of photosynthetic rate, is strongly impacted by anatomical traits, yet the specific structural correlates of mesophyll conductance under varying light spectra remain incompletely understood. We investigated how varying red (R) and blue (B) light ratios affected leaf anatomical traits and how these traits were associated with changes in mesophyll conductance and photosynthetic rate in cucumber. The results demonstrated that plants grown under monochromatic red light exhibited impaired photosynthetic performance, together with reductions in leaf thickness, leaf mass per area, stomatal density, and chloroplast exposure to the intercellular airspace. These anatomical characteristics were associated with lower mesophyll conductance, consistent with greater resistance to CO2 diffusion across both gas- and liquid- phases. In contrast, blue-enriched spectra (25B, 75B, 100B) were associated with greater chloroplast exposure to the intercellular airspace and larger intercellular airspace, which coincided with higher mesophyll conductance and photosynthetic rate. The 75B treatment showed the highest photosynthetic performance, coinciding with a combination of favorable mesophyll and chloroplast anatomical characteristics. Analysis of modeled diffusion resistance indicated that cell wall and chloroplast stroma resistances accounted for major components of the modeled resistance to CO2 diffusion and varied with corresponding anatomical features. Quantitative limitation analysis confirmed that mesophyll limitation was the predominant constraint under all light regimes. Our findings demonstrate that light quality associated anatomical variation is closely linked to mesophyll conductance and internal CO2 diffusion, providing physiological insight into how spectral composition influences photosynthetic performance in controlled-environment agriculture.