Characterization of Photosynthetic14Carbon Assimilation byPotamogeton lucensL.

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Abstract
Photosynthetic assimilation of exogenous 14CO2 and H14CO3− by the aquatic angiosperm Potamogeton lucens L. is reported. Equivalent maximum rates of assimilation (1.5 μmol s−1 m−2) were obtained in the presence of saturating levels of 14CO2 (1.0 mol m−3, pH 5.3) or H14CO3− (1.5 mol m−3, pH, 9.2). Under subsaturating 14CO2 levels, both gaseous diffusion and H14CO3− transport were shown to operate simultaneously, such that maximal photosynthetic rates were established. An induction lag of approximately 3 min was observed when exogenous 14CO2 was assimilated. A longer lag of approximately 12 min was required, however, before linear assimilation rates were established when H14CO3 acted as the carbon source. The light-activated H14CO3− transport system was found to be quite labile. A brief (5 min) dark treatment returned the system to the inactive state. Bicarbonate transport was shown to be competitively inhibited by CO32−ions. The possibility is discussed that this form of inhibition may be common to many HCO3− assimilators. Preliminary polar cation transport studies (from lower to upper leaf surface) indicated an almost exact one to one relationship between the rates of Na+ influx and efflux and H14CO3− assimilation. The possible relationship(s) between these transport processes and the requirement for electrical neutrality is briefly discussed.