Three Methods of Estimating Mesophyll Conductance Agree Regarding its CO(2) Sensitivity in the Rubisco-Limited C(i) Range

Whether the mesophyll conductance to CO(2) movement (g(m)) within leaves of C(3) plants changes with CO(2) concentration remains a matter of debate, particularly at low CO(2) concentrations. We tested for changes in g(m) over the range of sub-stomatal CO(2) concentrations (C(i)) for which Rubisco activity limited photosynthesis (A) in three plant species grown under the same conditions. Mesophyll conductance was estimated by three independent methods: the oxygen sensitivity of photosynthesis, variable J fluorescence combined with gas exchange, and the curvature of the Rubisco-limited A vs. C(i) curve. The latter assay used a new method of rapidly obtaining data points at approximately every 3 μmol mol(−1) for Rubisco-limited A vs. C(i) curves, allowing separate estimates of curvature over limited C(i) ranges. In two species, soybean and sunflower, no change in g(m) with C(i) was detected using any of the three methods of estimating g(m). In common bean measured under the same conditions as the other species, all three methods indicated large decreases in g(m) with increasing C(i). Therefore, change in g(m) with C(i) in the Rubsico-limited region of A vs. C(i) curves depended on the species, but not on the method of estimating g(m).