INORGANIC CARBON ACQUISITION IN RICCIA-FLUITANS L

The response of photosynthetic rate to pH indicates that CO2 is the in organic carbon (C-i) species preferentially used by the liverwort Ricc ia fluitans for photosynthesis. The absence of external carbonic anhyd rase (CA) activity and insensitivity to the anion-exchanger inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulphonate (DIDS) suggest that bica rbonate is not taken up. Cultivation with bicarbonate produces a decre ase in the semi-saturation constant for C-i and an increase in soluble CA activity, but maximum photosynthetic rate decreases and no signifi cant change in the C-i compensation point occurs. Plants cultivated at 1% CO2 show no significant differences in photosynthetic characterist ics and CA activity from control plants. Electrophysiological measurem ents also suggest that CO2 is the form that crosses the plasmalemma. A pplication of 1% CO2 results in a transient hyperpolarization of the m embrane potential (E-m) and also a transient acidification of the cyto plasmic 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 acidific ation is observed. These results suggest that bicarbonate is not trans ported, because its effect mimics the response caused by CO2 which ent ers the cell inducing a fall in cell pH, and a hyperpolarization of E- m probably due to stimulation of the proton pump.