Using willow stem segments it has been possible to show that the presence of gibberellic acid in the xylem can influence the lateral mobility of radioactive indolylacetic acid and its principal metabolite, indoleacetylaspartic acid. The extent of the lateral movement of indolylacetic acid depends on the concentration of endogenous gibberellin in the xylem, and movement may be increased by exogenous gibberellin applications to the xylem. The mechanism controlling the lateral movement of 2, 4-dichlorophenoxyacetic acid and 2, 4, 5-trichlorophenoxyacetic acid appeared to be somewhat different and there was no positive movement towards gibberellin.
These results are discussed in relation to previous experimentation on the interaction of plant growth regulators during transport. It is suggested that gibberellins may influence cell membranes and this may account for their effects on the transport of indolylacetic acid.