Response of Internal Conductance to Soil Drought in Sun and Shade Leaves of Adult Fagus crenata

The internal conductance to CO2 (gi) is an important constraint of photosynthesis, and itsacclimation to environmental factors varies widely within plant functional groups, genera and evenspecies. We measured photosynthesis and chlorophyll fluorescence response curves to intercellularCO2 concentrations simultaneously in attached sun and shade leaves of 90-year-old Fagus crenetaBlume trees to determine (1) how gi varies within the crown and (2) whether soil drought affects gi.Internal conductance to CO2 was found to be 0.058 and 0.185 mol m-2 s-1 in lower and upper crowns,respectively, resulting in a decrease of about 70 μmol mol-1 in CO2 concentration from the intercellularspace (Ci) to the site of carboxylation inside the chloroplast stroma (Cc). The results suggest that gi isas important as stomatal conductance for photosynthetic efficiency in F. crenata. If this large decreasefrom Ci to Cc was not accounted for, the maximum rate of carboxylation (Vcmax) in sun and shadeleaves was underestimated ca 36% and 24%, respectively. When soil water supply was sufficient, leafwater potential dropped to a daily minimum in early morning, facilitating CO2 transfer and thusphotosynthesis. When soil water potential at 25 cm depth fell to –0.015 MPa, gi and stomatalconductance decreased by 20–40% in comparison with their respective values under sufficient soilwater supply. In contrast, Vcmax decreased by 7% in sun leaves, but there was no change in thisparameter in shade leaves. Ignoring the effect of gi on Cc under stressed conditions would lead to up to22% underestimates of Vcmax, and consequently overestimates of biochemical limitations. These resultssuggest that CO2 diffusional limitations have more significant effects than biochemical limitations onthe rate of photosynthesis in F. crenata during soil drought.