Decreases in photosynthetic rate, stomatal conductance ( g s), and mesophyll conductance ( g m) are often observed under elevated CO 2 conditions. However, which anatomical and/or physiological factors contribute to the decrease in g m is not fully understood. Arabidopsis thaliana wild-type and carbon-metabolism mutants ( gwd1, pgm1, and cfbp1) with different accumulation patterns of non-structural carbohydrates were grown at ambient (400 ppm) and elevated (800 ppm) CO 2. Anatomical and physiological traits of leaves were measured to investigate factors causing the changes in g m and in the mesophyll resistance (expressed as the reciprocal of mesophyll conductance per unit chloroplast surface area facing to intercellular space, S c/ g m). When grown at elevated CO 2, all the lines showed increases in cell wall mass, cell wall thickness, and starch content, but not in leaf thickness. g m measured at 800 ppm CO 2 was significantly lower than at 400 ppm CO 2 in all the lines. Changes in S c/ g m were associated with thicker cell walls rather than with excess starch content. The results indicate that the changes in g m and S c/ g m that occur in response to elevated CO 2 are independent of non-structural carbohydrates, and the cell wall represents a greater limitation factor for g m than starch.
Changes in mesophyll conductance of Arabidopsis wild-type and carbohydrate-metabolism mutants grown at elevated CO 2 are explained by changes in cell wall mass and thickness rather than by non-structural carbohydrates.