Photosynthesis can be enhanced by lateral CO2 diffusion inside leaves over distances of several millimeters

This study examines the extent to which lateral gas diffusion can influence intercellular CO2 concentrations (c(i)) and thus photosynthesis in leaf areas with closed stomata. Leaves were partly greased to close stomata artificially, and effects of laterally diffusing CO2 into the greased areas were studied by gas-exchange measurement and chlorophyll fluorescence imaging. Effective quantum yields (Delta F/F-m') across the greased areas were analysed with an image-processing tool and transposed into c(i) profiles, and lateral CO2 diffusion coefficients (D-C'(lat)), directly proportional to lateral conductivities (g(lat)(*)), were estimated using a one-dimensional (1D) diffusion model. Effective CO2 diffusion distances in Vicia faba (homobaric), Commelina vulgaris (homobaric) and Phaseolus vulgaris (heterobaric) leaves clearly differed, and were dependent on D-C'(lat), light intensity, [CO2], and [O-2]: largest distances were approx. 7.0 mm for homobaric leaves (with high D-C'(lat)) and approx. 1.9 mm for heterobaric leaves (low D-C'(lat)). Modeled lateral CO2 fluxes indicate large support of photosynthesis over submillimeter distances for leaves with low D-C'(lat), whereas in leaves with large D-C'(lat), photosynthesis can be stimulated over distances of several millimeters. For the plant species investigated, the surplus CO2 assimilation rates of the greased leaf areas (Agr) differed clearly, depending on lateral conductivities of the respective leaves.