Thresholds for Georesponse to Acceleration in Gravity-Compensated Avena Seedlings

The magnitude of acceleration required to induce growth responses in Avena seedlings grown in the absence of tropic response to earth gravity has been investigated. For this purpose, a clinostat was developed that imposes accelerations from about 10(-9) g to 3 g upon the seedling; simultaneously, it nullifies, or compensates for, response to the directional component of the gravitational-force vector by rotating the seedling on a horizontal axis. When accelerations less than 10(-3) g are applied in either the acropetal or the basipetal direction, the growth in length and weight of the various organs is not materially different from that of compensated seedlings to which no longitudinal force is applied. At accelerations between 10(-3) and 10(-2) g, differences in growth become highly significant. When the centrifugal forces are transverse to the seedling during compensation, the threshold acceleration range for geoperception, as manifest by shoot reorientation, is again between 10(-3) and 10(-2) g. Geotropic reorientation of the root becomes apparent after exposures between 10(-4) and 10(-3) g.