Growth and photosynthetic acclimation by Ranunculus aquatilis L. in response to inorganic carbon availability.

Relative growth rates of Ranunculus aquatilis L. were measured in the laboratory at dissolved inorganic carbon (DIC) concentrations between 0.2 and 5.2 mM at air-equilibrium CO (16 μM) and also at 0.55 mm DIC with elevated CO (350 μM). For plants grown at air-equilibrium CO , growth was limited by inorganic carbon below 1.6 mM DIC and the apparent half saturation constant was 0.5 mM. The growth rate at elevated CO was 50% higher than the carbon saturated rates obtained at high DIC concentrations and air-equibrium CO , where HCO is dominant. This difference is suggested to be caused by differences in uptake mechanisms for CO and HCO Uptake of CO is a diffusive process, whereas HCO use is an active process which involves uptake/transport systems in the cell membranes. The plants acclimated to the DIC regime for growth by reductions in carboxylation efficiency and bicarbonate affinity, but enhanced photosynthetic capacity at elevated DIC. Within the range of concentrations used, the acclimation to CO and HCO was quantitatively similar, except for the HCO uptake capacity which increased at high DIC and air-equilibrium CO but declined at elevated CO . Dark respiration was unaffected by inorganic carbon per se, but increased with growth rate. Maintenance respiration was constant among treatments. It is concluded that inorganic carbon, apart from being the primary substrate for photosynthesis, has secondary growth regulatory effects which affect the photosynthetic apparatus of the plants.