GRAVITROPIC RESPONSES OF THE AVENA-COLEOPTILE IN-SPACE AND ON CLINOSTATS .4. THE CLINOSTAT AS A SUBSTITUTE FOR SPACE EXPERIMENTS

Gravitropic responses of dark grown oat coleoptiles were measured in w eightlessness and under clinorotation on earth. The tests in micrograv ity were conducted in Spacelab during the IML-1 mission and those on c linostats were conducted in laboratories on earth. The same apparatus was used for both kinds of tests. In both cases autotropism and gravit ropic responsiveness were determined. This allowed a quantitative comp arison between the plants' responses after receiving the same tropisti c stimulations either in weightlessness or on clinostats. Autotropism was observed with oat coleoptiles responding in weightlessness but it did not occur on clinostats. Gravitropic responsiveness was measured a s the ratio between the incremental bending response (degrees curvatur e) and the corresponding incremental g-dose (stimulus intensity times duration for which it was applied). Plants were tested at either of tw o stages of coleoptile development (i.e. different coleoptile lengths) . From a total of six different kinds of critical comparisons that cou ld be made from our rests that provided data for clinorotated vs weigh tless plants, three showed no significant difference between responses in simulated vs authentic weightlessness. Three other comparisons sho wed highly significant differences. Therefore, the validity of clinoro tation as a general substitute for space flight was not supported by t hese results.