The Anisotropy of the Mesophyll and CO₂Capture Sites inVicia fabaL. Leaves at Low Light Intensities

Experiments are reported on the spatial distributions of isotopic carbon within the mesophyll of detached leaves of the C3 plant Vicia faba L. fed 14CO2 at different light intensities. Each leaf was isolated in a cuvette and ten artificial stomata provided spatial continuity between the ambient atmosphere (0.03–0.05% v/v CO2) and the mesophyll from the abaxial leaf side. Paradermal leaf layers exhibited spatial profiles of radioactivity which varied with the intensity of incident light in 2 min exposures. At low light, when biochemical kinetics should limit CO2 uptake, sections through palisade cells contained most radioactivity. As the light intensity was increased to approximately 20% of full sunlight, peak radioactivity was observed in the spongy cells near the geometric mid-plane of the mesophyll. The results indicate that diffusion of carbon dioxide within the mesophyll regulated the relative photosynthetic activity of the palisade and spongy cells at incident photosynthetically active light intensities as little as 110 μE m−2 s−1 when CO2 entered only through the lower leaf surface.