Graviresponse of cress roots under varying gravitational forces.

During the spacelab mission IML-2 threshold values concerning gravity controlled growth processes have been estimated in order to test the reciprocity law (dose = stimulus x time = constant) for the first time under exact physiological conditions. Cress seedlings have been cultivated from dry seeds under conditions of microgravity and on a 1 x g-centrifuge in the ESA-BIORACK. With the help of NIZEMI--the slow rotating centrifuge microscope--these seedlings have been stimulated by different doses ranging from 12 to 60 x g x s. Two different values of acceleration--0.1 x g and 1 x g--have been used. Graviresponses of the roots have been documented by video recording for 60 min under conditions of microgravity. The response of roots to accelerations of 0.1 x g was remarkably less than to 1 x g in spite of the same doses being applied to the seedlings. Roots cultivated under conditions of microgravity showed a higher sensitivity than those grown on the 1 x g-centrifuge. Displacement of statoliths in gravity perceiving cells was mainly less than 1 micron under the different stimulation procedures. These results together with results from former space flights do not confirm the validity of the reciprocity law. They indicate that transformation of the gravistimulus has to occur in close vicinity to the statoliths, probably mediated by the ground cytoplasm and the cytoskeleton suspended therein.