Role of Indole-3-Acetic Acid in Modification of Geotropic Responses in Clinostat Rotated Avena Seedlings

Oat seedlings were grown in a sand medium on clinostats with horizontal axes of rotation to nullify the directional component of the gravity-force vector. Coleoptile segments from such seedlings showed an enhanced absorption of apically applied exogenous auxin (indole-3-acetic acid), compared to segments from vertically rotated or stationary controls. Absorption of basally applied auxin and auxin transport were unaffected by the gravity treatments. Horizontal rotation did not materially change the amount of auxin produced and transported from excised coleoptile tips; however, plants so rotated showed an enhanced curvature response to unilaterally applied auxin. Collectively, these experiments indicate that enhanced plant responses to horizontal clinostat rotation, where rates of rotation are sufficient to nullify the directional component of the gravity-force vector, are caused primarily by increases in metabolism and not by a modification of auxin availability. These data do not support recently advanced hypotheses that the polarity of auxin transport is based on gravitational sedimentation of cell inclusions.