Rhythm in potassium uptake by a duckweed, Lemna gibba G3

The potassium uptake activity of the “flow-medium culture” of a long-day duckweed, Lemna gibba G3, followed a circadian rhythm which persisted for more than 5 days under continuous light. The period of the rhythm was about 25 hr under 3000 lux at 26°C and was slightly over-compensated against temperature, Q10 being a little less than 1.0. The amplitude of the rhythm was dependent on light intensity, and there was no potassium uptake in the dark. Magnesium uptake was affected by the potassium movement and showed circadian rhythmicity with a small amplitude under conditions where the potassium uptake was already saturated. Calcium uptake did not show any obvious rhythm. In Contrast to L. gibba, a short-day duckweed L. perpusilla 6746 displayed circadian rhythm of potassium uptake only in the dark and not in the light. This rhythm did not persist beyond the second cycle.