In constant conditions that follow daily temperature cycles, a circadian rhythm of cell division (20-h period) was apparent at the shoot apex of seedlings ofChenopodium rubrum. There was also rhythmicity in capacity to flower (21- to 24-h period) when the seedlings were exposed at various ages to a 12-h dark period. Neither oscillation was evident in seedlings raised under constant conditions. Rhythmical changes in flowering (20- to 24-h period) were also found upon treatment of seedlings for 6ensp;h at different times with solutions of glucose (0.4ensp;M), gibberellic acid (10minus;6ensp;M), or ethanol (0.1). This latter rhythm was apparent in constant conditions in the light long after export of floral stimulus from the cotyledons. It is argued that rhythmicity at the shoot apex, possibly in cell division, confers a 20- to 24-h periodicity on the flowering responses ofC.rubrum.By contrast, exposure of seedlings or adult plants to a dark period of varied duration revealed a rhythm in capacity to flower but with a considerably longer period (about 30ensp;h). This rhythm was present whether or not seedlings had been raised in fluctuating temperatures. The oscillation was also distinctive in that it originated in the leaf. Exposure of only one leaf on an adult plant to different durations of darkness induced the 30-h rhythm of flowering. Therefore, more than one rhythm, a leaf and an apex rhythm, may control flowering inC.rubrum.
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