SPECIFICITY FACTOR OF RIBULOSE-BISPHOSPHATE CARBOXYLASE OXYGENASE OF QUERCUS-ROBUR/

The specificity factor (S) of Rubisco (EC 4.1.1.39) controls the relat ive rates of carboxylation vs. oxygenation of RuBP. Values of S may va ry among species, and in vivo measurements sometimes resulted in low v alues of apparent specificity in tree species. We therefore measured S of Rubisco either purified to homogeneity or semipurified from oak le aves (Quercus robur L.). Activities were determined in an oxygen elect rode using the total consumption of RuBP. A value of 103.8 was obtaine d at 25 degrees C and pH 8.2. Use of semi-purified and completely puri fied extracts yielded similar results. Identical values of S were foun d in Rubisco extracted from leaves grown at 12 or 100% incident irradi ance in a nursery. The values of S obtained in oaks ranged within the published values from other C-3 plants. It was concluded that trees do not differ in the properties of their Rubisco, and that reported low values measured in vivo have probably to be ascribed to low diffusivit y of CO2 from intercellular spaces to chloroplasts.