Inorganic carbon acquisition in Riccia fluitans L.

The response of photosynthetic rate to pH indicates that CO2 is the inorganic carbon (C-i) 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 C-i and an increase in soluble CA activity, but maximum photosynthetic rate decreases and no significant change in the C-i 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 (E-m) and also a transient acidification of the cytoplasmic pH (pH(c)). 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 E-m probably due to stimulation of the proton pump.