Elevated CO2-induced changes in mesophyll conductance and anatomical traits in wild type and carbohydrate metabolism mutants of Arabidopsis thaliana.

Decreases in photosynthetic rate, stomatal conductance (g(s)), and mesophyll conductance (g(m)) are often observed under elevated CO2 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) CO2. 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 CO2, 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 CO2 was significantly lower than at 400 ppm CO2 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 CO2 are independent of non-structural carbohydrates, and the cell wall represents a greater limitation factor for g(m) than starch.