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 CO₂ (16 μM) and also at 0.55 mm DIC with elevated CO₂ (350 μM). For plants grown at air-equilibrium CO₂ , 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 CO₂ was 50% higher than the carbon saturated rates obtained at high DIC concentrations and air-equibrium CO₂ , where HCO₃ ^- is dominant. This difference is suggested to be caused by differences in uptake mechanisms for CO₂ and HCO₃ ^- Uptake of CO₂ is a diffusive process, whereas HCO_a ^- 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 CO₂ and HCO₃ ^- was quantitatively similar, except for the HCO₃ ^- uptake capacity which increased at high DIC and air-equilibrium CO₂ but declined at elevated CO₂ . 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.