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Gravity Functions of Circumnutation by Hypocotyls of Helianthus annuus in Simulated Hypogravity ^1,2

Plant Centrifuge Laboratory, University City Science Center, University of Pennsylvania, Philadelphia, Pennsylvania, Biology Department, University of Pennsylvania, Philadelphia, Pennsylvania

For more than a decade research on the botanical mechanism responsible for circumnutation has centered on whether or not these nearly ubiquitous oscillations can be attributed to a hunting process whereby the plant organ continuously responds to the gravity force and, by overshooting each stimulus, initiates a sustained oscillation or, driven by a not yet defined autogenic mechanism, performs oscillatory activities that require no external reinforcement to maintain the observed rhythms of differential growth.

We explore here the effects of altered gravity force on parameters of circumnutation. Following our earlier publication on circumnutation in hypergravity we report here an exploration of circumnutation in hypogravity.

Parameters of circumnutation are recorded as functions of the axially imposed gravity force. The same method was used (two-axes clinostat rotation) to produce sustained gravity forces referred to as hypergravity (1 < g), hypogravity (0 [unk] g < 1), and negative gravity (–1 < g < 0). In these three regions of the g-parameter nutational frequency and nutational amplitude were influenced in different ways.

The results of our tests describe the gravity dependence of circumnutation over the full range of real or simulated gravity levels that are available in an earth laboratory. Our results demonstrated that nutational parameters are indeed gravity-dependent but are not inconsistent with the postulate that circumnutation can proceed in the absence of a significant gravity force.

² Correspondence concerning this paper should be addressed to A. H. Brown, Biology Department, G-5, University of Pennsylvania, Philadelphia, Pa. 19104.

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