Growth, CO2 uptake, and responses of the carboxylating enzymes to inorganic carbon in two highly productive CAM species at current and doubled CO2 concentrations

In Agave salmiana Otto ex Salm. var. salmiana grown for 4½ months in open-top chambers, 55% more leaves unfolded and 52% more fresh mass was produced at 730 than at 370μmol CO₂ mol^?1. A doubling of the CO₂ concentration also stimulated growth in another highly productive CAM species, Opuntia ficus-indica (L.) Miller, leading to earlier initial ion and 37% more daughter cladodes. Substantial net CO₂ uptake occurred earlier in the afternoon and lasted longer through the night for A. salmiana at 730 than at 370μmol CO₂ mol^?1, resulting in 59% more total daily net CO₂ uptake. The Michaelis constant (HCO₃^?) for PEPCasc was 15% lower for A. salmiana and 44% lower for O. ficus-indica when the CO₂ concentration was doubled; the percentage of Rubisco in the activated state in vivo was on average 64% higher at the doubled CO₂ concentration. Thus the substantial increases in net CO₂ uptake and biomass production that occurred in these two CAM species when the ambient CO₂ concentration was doubled resulted mainly from higher inorganic carbon levels for their carboxylating enzymes, a greater substrate affinity for PEPCase, and a greater percentage of Rubisco in the activated state.