Phase Control of the Potassium Uptake Rhythm by the Light Signals in Lemna gibba G3

Abstract

Summary The action of light in controlling the phase of the potassium uptake rhythm in the flow medium culture of Lemna gibba G3 was investigated. The flow medium used contained 1% sucrose. Light (80, 500 or 4200 lux) and dark cycles reset the rhythm with a transient of about 3 d. Daily light pulses (4200 lux for 15 min) also reset the rhythm with the peak coming earlier with a transient for 3 d. The phase of the reset rhythm was mainly determined by the light-on signal with a slight modification due to the duration and intensity of the light regime. The potassium uptake rhythm has the basic properties of circadian rhythm as below. i) Among two skeleton light regimes constituted by two pulses per day (each at 4200 lux for 15 min), the shorter one simulated the complete light regime. ii) Brief light pulse inserted in the prolonged darkness induced a typical phase response curve. Red light acted on the rhythm similarly to white light and blue light casued a halfway resetting of the rhythm when they were given as light and dark cycles or as pulses at 24 h intervals. A brief far-red light promptly diminished almost all the uptake activity. White light reversed this far-red effects. Additional far-red given to the duckweeds under continuous white light also diminished the amplitude of the rhythm without lowering an arrhythmic uptake. These results suggests the phytochrome reaction is involved in the control of the amplitude. However, the far-red interruption of continuous white light for 6 h failed to change the phase of the rhythm, indicating that the phase of the rhythm was not fully reset by P fr (far-red absorbing form of phytochrome) level.