THE EFFECT OF THE EXTERNAL MEDIUM ON THE GRAVITROPIC CURVATURE OF RICE (ORYZA-SATIVA, POACEAE) ROOTS

The roots of rice seedlings. growing in artificial pond water, exhibit robust gravitropic curvature when placed perpendicular to the vector of gravity. Te determine whether the statolith theory (in which intrac ellular sedimenting particles are responsible for gravity sensing) or the gravitational pressure theory (in which the entire protoplast acts as the gravity sensor) best accounts for gravity sensing in rice root s, we changed the physical properties of the external medium with impe rmeant solutes and examined the effect on gravitropism. As the density of the external medium is increased. the rate of gravitropic curvatur e decreases. The decrease in the rate of gravicurvature cannot be attr ibuted to an inhibition of growth, since rice roots grown in 100 Osm/m (2) (0.248 MPa) solutions of different densities all support the same root growth rate but inhibit gravicurvature increasingly with increasi ng density. By contrast, the sedimentation rate of amyloplasts in the columella cells is unaffected by the external density. These results a re consistent with the gravitational pressure theory of gravity sensin g, but cannot be explained by the statolith theory.