The Threshold Stimulus for Geotropism

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Abstract
The minimum effective geotropic stimulation time is very short for coleoptiles, hypocotyls and roots. New data from the literature are used to emphasize that the minimum time is of the order of 10–20 s. The existence of such a short minimum time should be carefully considered in the design of geotropic experiments in Spacelab, which are now at the planning stage. Even brief acceleration periods at 1 g or less during a flight might cause unintentional geotropic effects.The short minimum stimulation time also has relevance for the statolith theory: since the distance which an amyloplast can sediment during this time is very small, no long sedimentation distance is likely to be required for georeception. Moreover, the magnitude of thermal noise must be considered in evaluating proposals for the georeceptive mechanism.