On the thresholds of gravitational force perception by plants.
Changes in growth and development of plants in "weightless" environments is a focus of several biosatellite projects. Implicit in experimental designs is the consideration that the force vectors from vibration and vehicular spin are near or below the thresholds of perception. Thresholds of 10(-3) x g have been assigned based upon results from centrifugal stimulation of organs essentially fixed with respect to earth gravity. A value of 2 x 10(-5) x g was derived from vibrational (discontinuous) acceleration of a clinostat wire on which seedlings were mounted. We have developed apparatus that will enable the imposition upon seedlings of a continuous centrifugal force of constant sign concomitant with the nullification of the directional component of earth gravity. Recent experiments evaluating the responses of Avena roots and shoots in such apparatus indicate a threshold on the order of 10(-3)-10(-2) x g. These values are supported by results of experiments where organs were grown on clinostats whose rotational axes were at various tilt angles with respect to earth gravity; this technique enables minute increments of longitudinal gravitational stimulation as a vector function of the tilt angle, concomitant with compensation for earth gravity. We will discuss the possibility that assigned thresholds of micro-g magnitude are derived as a consequence of undefined forces that are artifacts of apparatus function.
Gordon
S A
SA
Division of Biological and Medical Research, Argonne National Laboratory, Argonne, Illinois, USA.
Shen-Miller
J
J
D000054:Acceleration; D018554:Avena / Q000254:growth & development*; D002498:Centrifugation; D006112:Gravitation; D018522:Gravitropism / Q000502:physiology*; D018517:Plant Roots / Q000254:growth & development*; D018520:Plant Shoots / Q000254:growth & development*; D012399:Rotation; D014732:Vibration