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

Constant mesophyll conductance (g), and two-resistance g model (involved in resistances of cell wall and chloroplast), where g reaches maximum under higher CO concentrations, cannot describe the phenomenon that g decreases with increasing intercellular CO concentration (C) under relatively higher CO concentrations. Yin et al. (2020) proposed a g model, according to which the ratio of chloroplastic CO concentration (C) to C is constant in the two-resistance g model, which can describe the decreasing g with increasing C. In the present study, we investigated the relationship between C and C in leaves of Japanese white birch by using simultaneous measurements of gas exchange and chlorophyll fluorescence under various CO concentrations, light intensities, and during progressive drought. Across the range of ambient CO from 50 to 1000 μmol mol, and light intensities of 50 to 2000 μmol m s, measured under well irrigation, the ratio of C to C kept constant. During the progressive drought, overestimated C due to stomatal patchiness and/or cuticular transpiration was empirically corrected (threshold: stomatal conductance < 0.08 mol HO m s) from the A/C response measured under adequate irrigation. The ratio of C to C 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 C:C ratio constant, which might act on g in addition to the physical interaction of diffusive resistances in the cell components.