Three Methods of Estimating Mesophyll Conductance Agree Regarding its CO₂ Sensitivity in the Rubisco-Limited C_i Range

Whether the mesophyll conductance to CO₂ movement (g_m) within leaves of C₃ plants changes with CO₂ concentration remains a matter of debate, particularly at low CO₂ concentrations. We tested for changes in g_m over the range of sub-stomatal CO₂ 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.