GRAVITROPIC RESPONSES OF THE AVENA-COLEOPTILE IN-SPACE AND ON CLINOSTATS .3. HYPOGRAVITY EFFECTS ON COLEOPTILE CURVATURES

The precise description of the gravitropic curvature response of plant s is often difficult due to the presence of gravity when the response is recorded. Therefore, unambiguous results on the gravitropic reactio ns in differents segments of coleoptiles, the participation of auxin t ransport in the curvature development and the reactions to hypogravity stimulations can only be achieved in weightlessness. To answer these questions, curvatures of Avena coleoptiles at different distances from the apex were studied after transverse, hypogravity stimulations. The experiments were conducted in Spacelab on the Shuttle in earth orbit. Test plants were cultivated on 1.0 g-centrifuges in Spacelab. The sti mulation accelerations, were between 0.1 and 1.0 g, and duration varie d between 2 and 130 min. All plant responses were recorded in weightle ssness by a video cassette recorder for analysis. The gravitropic curv ature started almost simultaneously in apical segments, and occurred l ater in the basal ones. After maximum response, the curvature in the t op segments showed a clear autotropic reaction, that was not seen in t he basal segments. Initial wrong-way curvatures were recorded in the b asal part of the coleoptiles at several g-levels. The further down alo ng the coleoptile, the later the occurrence of the maximum gravitropic curvature of each segment. For example, after a stimulus of 25 g min the maxima appeared to travel down the coleoptile with a speed of abou t 50 mm h(-1) in the upper and 20 mm h(-1) in the lower part of the co leoptiles. It is concluded that the basipetal auxin transport can cont ribute only marginally to the gravitropic curvature pattern due to its much lower transport rate. Local reactions control the curvature patt ern in each segment (even if the basipetal auxin transport must add to the reactions). Extrapolations from stimulus response data curves all owed some determinations of threshold stimulation times for different segments along the coleoptiles. Data for the individual segments along the coleoptiles were not at variance with the reciprocity law.