Abstract

The activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) from the floating angiosperm Spirodela polyrhiza (L.) Schleid. (giant duckweed) grown at a photon irradiance of 200 or 400 μmol photons m-2 s-1 was consistently low, in the range of 56-62. Similarly low values were observed with four other emergent aquatic species growing under full sun irradiance. Transference of Spirodela plants for short (minutes) or long (days) periods to the higher or lower irradiance increased or decreased, respectively, the activation by only about 15. Activation was not greatly altered by exposure of the plants to full sun irradiance of >2000 μmol photons m-2 s-1 or CO2 concentrations in air of 0 and 1170 μmol mol-1, but darkness caused a slow decline to 20 activation. Transient oscillations were observed following a change in irradiance or CO2 concentration indicating that Rubisco was responsive to environmental perturbations. The low Rubisco activation was not due to the tight binding of inhibitors such as carboxyarabinitol-1-phosphate. It is concluded that a substantial proportion of the Rubisco protein in these naturally-occurring species may not be used for CO2-fixation at any given moment.