Effects of Increased Gravity Force on Nutations of Sunflower Hypocotyls

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Abstract
A centrifuge was used to provide sustained acceleration to study the hypocotyl nutation of 6 day old Helianthus annuus L. over a range of g-forces, up to 20 times normal g. At the upper end of this g-range, nutation was impeded and at times was erratic evidently because the weight of the cotyledons exceeded the supportive abilities of the hypocotyls. Over the range 1-9 g, the period of nutation was independent of the resultant force vector. Over the same g-range, the amplitude of nutation was nearly independent of the chronic g-force. If nutation in sunflower seedlings is an oscillation caused by a succession of geotropic responses which continue to overshoot the equilibrium position (plumb line), its amplitude might be expected to be more sensitive to changes in magnitude of the sustained g-force. In order to preserve the geotropic model of nutation, i.e., that it is a sustained oscillation driven by geotropic reactions, it is necessary to assume that geotropic response must increase with increasing g most rapidly in the region of the g-parameter below the terrestrial value of 1 g.

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