Is a Low Internal Conductance to CO₂ Diffusion a Consequence of Succulence in Plants with Crassulacean Acid Metabolism?

Abstract

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 C₃ 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 C₃ species with comparable Rubisco content and indicates a large diffusion limitation to atmospheric CO₂ fixation through the C₃ pathway in K. daigremontiana. In ambient air, CO₂ partial pressure at the carboxylation sites of Rubisco was 109 µbar. 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 C₄ 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.