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

Han, Qingmin and Iio, Atsuhiro and Naramoto, Masaaki and Kakubari, Yoshitaka (2010) Response of Internal Conductance to Soil Drought in Sun and Shade Leaves of Adult Fagus crenata. ACTA SILVATICA ET LIGNARIA HUNGARICA: AN INTERNATIONAL JOURNAL IN FOREST, WOOD AND ENVIRONMENTAL SCIENCES, 6 (1). pp. 123-133. ISSN 1786-691X

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Abstract

The internal conductance to CO2 (gi) is an important constraint of photosynthesis, and its acclimation to environmental factors varies widely within plant functional groups, genera and even species. We measured photosynthesis and chlorophyll fluorescence response curves to intercellular CO2 concentrations simultaneously in attached sun and shade leaves of 90-year-old Fagus creneta Blume 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 intercellular space (Ci) to the site of carboxylation inside the chloroplast stroma (Cc). The results suggest that gi is as important as stomatal conductance for photosynthetic efficiency in F. crenata. If this large decrease from Ci to Cc was not accounted for, the maximum rate of carboxylation (Vcmax) in sun and shade leaves was underestimated ca 36% and 24%, respectively. When soil water supply was sufficient, leaf water potential dropped to a daily minimum in early morning, facilitating CO2 transfer and thus photosynthesis. When soil water potential at 25 cm depth fell to –0.015 MPa, gi and stomatal conductance decreased by 20–40% in comparison with their respective values under sufficient soil water supply. In contrast, Vcmax decreased by 7% in sun leaves, but there was no change in this parameter in shade leaves. Ignoring the effect of gi on Cc under stressed conditions would lead to up to 22% underestimates of Vcmax, and consequently overestimates of biochemical limitations. These results suggest that CO2 diffusional limitations have more significant effects than biochemical limitations on the rate of photosynthesis in F. crenata during soil drought.

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