Inorganic carbon acquisition in Riccia fluitans L

The response of photosynthetic rate to pH indicates that CO2 is the inorganic carbon (Ci) species preferentially used by the liverwort Riccia fluitans for photosynthesis. The absence of external carbonic anhydrase (CA) activity and insensitivity to the anion-exchanger inhibitor 4,4-diisothiocyanostilbene-2,2-disulphonate (DIDS) suggest that bicarbonate is not taken up. Cultivation with bicarbonate produces a decrease in the semi-saturation constant for Ci and an increase in soluble CA activity, but maximum photosynthetic rate decreases and no significant change in the Ci compensation point occurs. Plants cultivated at 1% CO2 show no significant differences in photosynthetic characteristics and CA activity from control plants. Electrophysiological measurements also suggest that CO2 is the form that crosses the plasmalemma. Application of 1% CO2 results in a transient hyperpolarization of the membrane potential (Em) and also a transient acidification of the cytoplasmic pH (pHc). Addition of 1 mM bicarbonate at pH 7.3 produces a similar but less marked response; at an external pH of 8.3 no acidification is observed. These results suggest that bicarbonate is not transported, because its effect mimics the response caused by CO2 which enters the cell inducing a fall in cell pH, and a hyperpolarization of Em probably due to stimulation of the proton pump.Key words: Riccia fluitans, inorganic carbon, membrane potential, cytoplasmic pH, photosynthesis.