LIMITATION OF NET CO2 ASSIMILATION RATE BY INTERNAL RESISTANCES TO CO2 TRANSFER IN THE LEAVES OF 2 TREE SPECIES (FAGUS-SYLVATICA L AND CASTANEA-SATIVA MILL)

Using a combination of gas-exchange and chlorophyll fluorescence measu rements, low apparent CO2/O-2 specificity factors (1300 mol mol(-1)) w ere estimated for the leaves of two deciduous tree species (Fagus sylv atica and Castanea sativa), These low values contrasted with those est imated for two herbaceous species and were ascribed to a drop in the C O2 mole fraction between the intercellular airspace (C-i) and the cata lytic site of Rubisco (C-c) due to internal resistances to CO2 transfe r, C-c was calculated assuming a specificity of Rubisco value of 2560 mol mol(-1). The drop between C-i and C-c was used to calculate the in ternal conductance for CO2 (g(i)), A good correlation between mean val ues of net CO2 assimilation rate (A) and g(i) was observed within a se t of data obtained using 13 woody plant species, including our own dat a, We report that the relative limitation of A, which can be ascribed to internal resistances to CO2 transfer, was 24-30%, High internal res istances to CO2 transfer may explain the low apparent maximal rates of carboxylation and electron transport of some woody plant species calc ulated from A/C-i curves.