Is a low internal conductance to CO2 diffusion a consequence of succulence in plants with crassulacean acid metabolism?

Leaf internal conductance to CO2 (g(i)) from substomatal cavity to the carboxylation sites of Rubisco was measured in the leaf succulent CAM species, Kalanchoe daigremontiana Hamet et Perr. Measurements were made during Rubisco-mediated atmospheric C-3 carboxylation in phase IV photosynthesis. Using simultaneous gas exchange and chlorophyll fluorescence techniques, internal conductance was calculated to be 0.05 mol m(-2) s(-1) bar(-1), when measured at both saturating and limiting light. This is one of the lowest recorded values for g(i) as compared to a range of C-3 species with comparable Rubisco content and indicates a large diffusion limitation to atmospheric CO2 fixation through the C-3 pathway in K. daigremontiana. In ambient air, CO2 partial pressure at the carboxylation sites of Rubisco was 109 mu bar. Internal diffusion is limited by a thick leaf consisting of densely packed, succulent mesophyll with a small, portion of airspace. We speculate that a low internal conductance to CO2 diffusion results from the compromise between a succulent mesophyll required for C-4 acid storage and access for CO2 diffusion to both PEPC in the cytoplasm and Rubisco in the chloroplasts, Restricted diffusion of CO2 within the leaf makes CO2 assimilation less efficient during the transient phases of crassulacean acid metabolism.