Constant ratio of C c to C i under various CO 2 concentrations and light intensities, and during progressive drought, in seedlings of Japanese white birch

Constant mesophyll conductance (gm), and two-resistance gm model (involved in resistances of cell wall and chloroplast), where gm reaches maximum under higher CO2 concentrations, cannot describe the phenomenon that gm decreases with increasing intercellular CO2 concentration (Ci) under relatively higher CO2 concentrations. Yin et al. (2020) proposed a gm model, according to which the ratio of chloroplastic CO2 concentration (Cc) to Ci is constant in the two-resistance gm model, which can describe the decreasing gm with increasing Ci. In the present study, we investigated the relationship between Cc and Ci in leaves of Japanese white birch by using simultaneous measurements of gas exchange and chlorophyll fluorescence under various CO2 concentrations, light intensities, and during progressive drought. Across the range of ambient CO2 from 50 to 1000 μmol mol-1, and light intensities of 50 to 2000 μmol m-2 s-1, measured under well irrigation, the ratio of Cc to Ci kept constant. During the progressive drought, overestimated Ci due to stomatal patchiness and/or cuticular transpiration was empirically corrected (threshold: stomatal conductance < 0.08 mol H2O m-2 s-1) from the A/Ci response measured under adequate irrigation. The ratio of Cc to Ci during progressive drought (predawn leaf potential reached ≈ - 2 MPa) also remained constant irrespective of soil drying rate in various pot sizes. The present study suggests the involvement of some physiologically regulative mechanisms to keep Cc:Ci ratio constant, which might act on gm in addition to the physical interaction of diffusive resistances in the cell components.