Auxin Factor in Branch Epinasty

Abstract

A clone of Coleus blumei was grown in pots that were attached to horizontal clinostats. Rotation at rates from 1 rpm to 1 rph produced epinastic curvatures of branch tips. Angular movement was measured from periodic shadowgraphs. Curvatures were produced in leafless branches by applying 1% indoleacetic acid (IAA) in lanolin near the disbudded tips. Auxin supply to the lower side was as effective as that absorbed at the tip, indicating lateral transport toward the upper side of the branch. The blocking effect of 2% tri-iodobenzoic acid was used to confirm this path of transport by an unknown mechanism. Radioactive IAA-2-C14 was also supplied to leafless branches as terminal caps of auxin paste. Radioactivity in extracts of the upper and lower halves of the bisected curvatures revealed lateral transport of IAA toward the upper side. The excess of radiocarbon on the convex (upper) side of the curvature was in the ratio of 95. Bioassays of chloroform extracts from similarly halved curvatures in leafy branches confirmed the excess of IAA in the upper half by the same ratio. The imbalance in auxin transport is considered to be the physiological basis of branch epinasty.