AMMONIUM-STIMULATED POTASSIUM RELEASE FROM LEMNA-MINOR-L GROWN ON A MEDIUM CONTAINING NITRATE AS N-SOURCE

Net K-efflux from duckweed (Lemna minor L.) was initiated by the addition of NH4Cl to nutrient medium with NO3 as the only nitrogen source. Other NH4-salts (sulfate or nitrate) were also effective, while LiCl, NaCl, RbCl or CsCl caused no major K-release under the same conditions. Sodium, Rb and Cs ions lowered the net K-uptake observed in controls with no effector added, while in presence of Li ions the absorption of K was stimulated. These results provide putative evidence that K-release into the medium is specific for NH4 addition and not a general effect of monovalent cations. The level and the time course for K-efflux depended on the NH4 concentration added (1-30 mM), but the pH in the medium (3.7, 6.8 or 7.9) caused only minor effects. These findings suggest that the K-release was initiated by NH4 ions rather than by NH3 passing through the plasmalemma. Magnesium ions were released in a similar manner after NH4 additions, but only to about 10% of the rate observed for K. No clear NH4-stimulated net efflux was observed for Na and Ca ions. Potassium-release was not detectable after the addition of high NH4 concentrations to duckweed grown on NH4-medium. The release of K was apparently associated with the transition from NO3 to NH4 utilization and might be linked directly or indirectly to the high initial absorption rate for NH4.