Transport and Degradation of Auxin in Relation to Geotropism in Roots of <i>Phaseolus vulgaris</i>

Abstract

Abstract The movement of auxin in Phaseolus vulgaris roots has been examined after injection of IAA −3 H into the basal root/hypocotyl region of intact, dark‐grown seedlings. Only a portion of the applied IAA −3 H was transported unchanged to the root tip. The major part of the chromatographed, labelled compounds translocated to the roots was indole‐3‐acetylaspartic acid (IAAsp) and an unidentified compound running near the front in isopropanol, ammonia, water. The velocity of the auxin transport (7.2 mm per hour) was calculated from scintillation countings of methanol extracts from serial sections of the root. An accumulation of radioactive compounds in the extreme root tip, was observed 5 h after the injection of IAA. The influence of exogenous IAA on the geotropical behaviour of the bean seedling roots was examined. Pretreated roots were stimulated for 5 min in the horizontal position and then rotated parallel to the horizontal axis of the klinostat for 60 or 90 min. The resulting geotropic curvature of IAA‐injected and control roots showed significantly different patterns of development. When the stimulation was started 5 h after application of the auxin, the geotropic curvature became larger in roots of the injected plants than in the controls. If, however, the translocation period was extended to 20 h the geotropic curvature was significantly smaller in the roots of the injected plants. The auxin injection did not significally affect the rate of root elongation. The change in geotropical behaviour of the roots is interpreted as a result of the influence of the conversion products of the applied IAA on the geotropical responsiveness.