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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 HCOa - 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.