KINETICS OF SUPPRESSION OF CIRCUMNUTATION BY CLINOSTATTING FAVORS MODIFIED INTERNAL OSCILLATOR MODEL

According to a well-conceived gravity-driven model, oscillatory growth movements called circumnutations are produced by a continuing set of gravitational responses with overshoot. In simplest form that model predicts that if the gravity stimulus could be removed completely and abruptly, growth oscillations would damp out within a time less than the period of oscillation. That condition was simulated by imposing on horizontally clinostatted sunflower shoots a unit g centripetal force in the direction that coincided with the plants' morphological axis. Circumnutations proceeded in quantitatively normal manner. By stopping the centrifuge, the plants were subjected abruptly only to clinostat simulated weightlessness. Circumnutations decreased but did not disappear. The half-time for decline to a final low level of activity was about 4-fold the period of oscillation at 1 g—about 8-fold the period at simulated zero g. That slow damping was not consistent with the prediction based on the gravity driven model of circumnutation. The model needs to be revised.