Studies on the lateral transport of auxin in horizontal organs

Abstract

The lateral transport of 2,4-D-C in horizontally orientated segments of Helianthus annuus hypocotyls has been studied. This auxin was asymmetrically supplied to one semi-circular cut end of the segments and the radioactivity transported to upper and lower receivers at the other cut end was determined. Since the total transport to receivers was independent of the position of the donors, and the lateral transport downwards to lower receivers through segments having upper sources was more than the lateral movement upwards to upper receivers through segments having lower sources, an actual polarized lateral transport from the upper to the lower side of the horizontal segments was indicated. The basipetally moving auxin made major contribution in giving rise to the lateral auxin asymmetry. C moving in the acropetal direction was less sensitive to the lateral effect of gravity. The polarized lateral transport did not rise in proportion to the auxin concentration supplied. At external auxin concentrations of 0.2 and 1.0 mg/l, 1411 per cent of the total C transported appeared to be moving laterallydownwards from the upper to the lower part of the horizontal segment. When the source concentration was 5.0 mg/l the polarized lateral transport was 9 per cent of the total transport. The lateral transport continued to occur over the 11-hour period tested. The lateral transport responded to temperature variations and increased with the rise in temperature to 35° C. It is susceptible to inhibition by NPA and 2-Chloroetliylphosphonic acid. In addition there are several similarities between the characteristics of the lateral and the longitudinal transport. Therefore, it is suggested that the lateral transport may be an active transport. The lateral and the longitudinal transport showed different sensitivities to the transport inhibitors NPA and 2-Chloroethylphosphonic acid. The lateral as well as the longitudinal transport of C represented the movement of 2,4-D-C.