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 plants is often difficult due to the presence of gravity when the response is recorded. Therefore, unambiguous results on the gravitropic reactions in differents segments of coleoptiles, the participation of auxin transport 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 stimulation accelerations, were between 0.1 and 1.0 g, and duration varied between 2 and 130 min. All plant responses were recorded in weightlessness by a video cassette recorder for analysis. The gravitropic curvature started almost simultaneously in apical segments, and occurred later in the basal ones. After maximum response, the curvature in the top segments showed a clear autotropic reaction, that was not seen in the basal segments. Initial wrong-way curvatures were recorded in the basal part of the coleoptiles at several g-levels. The further down along 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 about 50 mm h(-1) in the upper and 20 mm h(-1) in the lower part of the coleoptiles. It is concluded that the basipetal auxin transport can contribute only marginally to the gravitropic curvature pattern due to its much lower transport rate. Local reactions control the curvature pattern in each segment (even if the basipetal auxin transport must add to the reactions). Extrapolations from stimulus response data curves allowed 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.