Residual nutational activity of the sunflower hypocotyl in simulated weightlessness

Does gravity drive circumnutation? One model ascribes hypocotylcircumnutation to a continuing series of geotropic responseseach of which overshoots thereby maintaining a sustained oscillation.However, some features of the observed movements are not easilyreconciled with the model. The critical feature of this modelis the requirement that circumnutation must exhibit an absolutedependence on a g-force. Experiments with Helianthns annum onhorizontal clinostats demonstrated an 80% reduction in the amplitudeof hypocotyl circumnutation (compared with upright plants atone g) although the oscillations continued at simulated "zerog". It is not certain that the clinostat environment adequatelysimulates the weightless environment of space but, if it does,we may expect a space experiment to demonstrate that hypocotylnutation in Helianthus annuus is not fully dependent on gravity. (Received October 13, 1978; )