Diurnal rhythm of uridine incorporation into RNA regulated by two light-perceiving systems in a long-day duckweed, Lemna gibba G3

A long-day duckweed, Lemna gibba G3, was found to be controlledby two lightperceiving systems; a system perceiving a prolonged,high-intensity white light and the phytochrome system, withrespect to the incorporation of radioactive uridine into RNA.When the duckweed was exposed to short or long days, the uridineincorporating activity into RNA changed diurnally reaching itshighest level at 18 hr and its lowest one at 6 hr after thebeginning of a light period. The level of maximum activity rosein proportion to an increase in the length of the light periodup to 12 hr or in light intensity up to 3000 ergs/cm²sec. Thefar-red light termination of the light period resulted in adecrease in uridine incorporation, the extent of which was constantirrespective of the length of the light period. The uridine incorporating activity changed diurnally when theduckweed was exposed to continuous light. The period lengthof the rhythm was circadian and was constant over a temperaturerange of 16° to 30°C. (Received September 1, 1975; )