Constant ratio of Ccto Ciunder various CO2concentrations and light intensities, and during progressive drought, in seedlings of Japanese white birch

Constant mesophyll conductance (g(m)), and two-resistanceg(m)model (involved in resistances of cell wall and chloroplast), whereg(m)reaches maximum under higher CO(2)concentrations, cannot describe the phenomenon thatg(m)decreases with increasing intercellular CO(2)concentration (C-i) under relatively higher CO(2)concentrations. Yin et al. (2020) proposed ag(m)model, according to which the ratio of chloroplastic CO(2)concentration (C-c) toC(i)is constant in the two-resistanceg(m)model, which can describe the decreasingg(m)with increasingC(i). In the present study, we investigated the relationship betweenC(c)andC(i)in leaves of Japanese white birch by using simultaneous measurements of gas exchange and chlorophyll fluorescence under various CO(2)concentrations, light intensities, and during progressive drought. Across the range of ambient CO(2)from 50 to 1000 mu mol mol(-1), and light intensities of 50 to 2000 mu mol m(-2) s(-1), measured under well irrigation, the ratio ofC(c)toC(i)kept constant. During the progressive drought, overestimatedC(i)due to stomatal patchiness and/or cuticular transpiration was empirically corrected (threshold: stomatal conductance < 0.08 mol H2O m(-2) s(-1)) from theA/C(i)response measured under adequate irrigation. The ratio ofC(c)toC(i)during progressive drought (predawn leaf potential reached approximate to - 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 keepC(c):C(i)ratio constant, which might act ong(m)in addition to the physical interaction of diffusive resistances in the cell components.

Automated summary

The study found that the ratio of C(c) to C(i) in leaves of Japanese white birch remains constant under different CO(2) concentrations and light conditions. During progressive drought, the overestimated C(i) was corrected by stomatal patchiness and/or cuticular transpiration, leading to a constant ratio of C(c) to C(i).