An Endogenous 24-hour Rhythm in the Growth Rate of theAvenaColeoptile

Abstract

The rates of growth of coleoptiles of intact Avena seedlings were studied by means of time-lapse photography, using infra-red radiation. When the seedlings are germinated in red light and subsequently transferred to darkness, a growth rhythm is established in which the first peak in the growth-rate curve occurs about 16–17 hours after the transfer, and the second peak 24 hours later. When the transfer is made sufficiently early, three peaks may occur before growth ceases. The occurrence of the peaks and the emergence of the primary leaf are independent of one another. Alteration of the point in the life-history at which the seedlings are transferred from light to darkness changes the times of occurrence of the peaks, but does not affect the period of the rhythm. The incidence of the rhythm shows no correlation with time of day; therefore the rhythm is not due to diurnal changes in external conditions. Interruption of the dark period by several hours' exposure to red light causes the suppression of a previously induced rhythm and the establishment of a new one which commences at the time the seedlings are restored to darkness. When they are grown under continuous red light no rhythm is induced. Within the range 16° to 28° C., temperature has little or no effect on the period of the rhythm. When seedlings of Triticum are grown under the same conditions as those which induce a rhythm in Avena, no rhythmical variation in the growth rate can be detected.