Responses of roots to simulated weightlessness on the fast-rotating clinostat.

Sedimentable cell particles are distributed randomly along the horizontal axis of the fast-rotating clinostat. They neither sediment in the direction of gravity, nor in the direction of the centrifugal force, nor in the direction of the resultant force of both. The effect of this "weightlessness" and that of very small centrifugal forces on the perception of mass acceleration was examined using young primary roots of Lepidium sativum L. (Cruciferae) during their early development on the fast-rotating clinostat. The results of the experiments show: 1) there is no response of the roots in the direction of gravity, 2) at small centrifugal forces (< 2.2 x 10(-2) g) a curvature response occurs in the direction of the stimulus, 3) the threshold value for the perception of mass acceleration lies at 4.3 x 10(-3) g, and 4) below the threshold value the existence of an autonomous root curvature has been proved for the first time, which is not caused by mass acceleration.