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Removal by a Trace of Sodium of the Period Lengthening of the Potassium Uptake Rhythm Due to Lithium in Lemna gibba G3 ¹

National Institute for Basic Biology, Myodaiji, Okazaki 444 Japan

The effect of Li⁺ on the period of the K⁺ uptake rhythm in the flow medium culture of the duckweed (Lemna gibba G3) was investigated under various ionic conditions. In the presence of Li⁺ at 0.2 millimolar or higher concentrations, the period was longer than the normal level of 25.4 hours by 2 hours. Li⁺ also lowered the amplitude of the rhythm. Although Na⁺ itself did not change any parameter of the rhythm, simultaneous application of Na⁺ at a very low concentration (20 micromolar) almost completely removed the effects of 0.5 millimolar Li⁺ on both the period and the amplitude. However, divalent cations (Ca²⁺ and Mg²⁺) or Rb⁺ did not remove the Li⁺ action on the period. The effect of Li⁺ and its removal by Na⁺ corresponded to intracellular Li⁺ and Na⁺ levels. The period was prolonged when the duckweed contained more Li⁺ than 5 micromoles/gram fresh weight. But the Li⁺ effect was cancelled when the in vivo Na⁺ level was greater than one-fifth that of Li⁺, even if the Li⁺ level exceeded over 5 micromoles/gram fresh weight.