ACTIVITIES OF CARBOXYLATING ENZYMES IN THE CAM SPECIES OPUNTIA-FICUS-INDICA GROWN UNDER CURRENT AND ELEVATED CO2 CONCENTRATIONS

Responses of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase) to an elevated atmosphe ric CO2 concentration were determined along with net CO2 uptake rates for the Crassulacean acid metabolism species Opuntia ficus-indica grow ing in open-top chambers. During the spring 13 months after planting, total daily net CO2 uptake of basal and first-order daughter cladodes was 28% higher at 720 than at 360 mu l CO2 1(-1). The enhancement, cau sed mainly by higher CO2 assimilation during the early part of the nig ht, was also observed during late summer (5 months after planting) and the following winter. The activities of Rubisco and PEPCase measured in vitro were both lower at the elevated CO2 concentration, particular ly under the more favorable growth conditions in the spring and late s ummer. Enzyme activity in second-order daughter cladodes increased wit h cladode age, becoming maximal at 6 to 10 days. The effect of elevate d CO2 on Rubisco and PEPCase activity declined with decreasing irradia nce, especially for Rubisco. Throughout the 13-month observation perio d, O. ficus-indica thus showed increased CO2 uptake when the atmospher ic CO2 concentration was doubled despite lower activities of both carb oxylating enzymes.