The Transport Behaviour of the Synthetic Auxin 2,4-Dichlorophenoxyacetic Acid in Decapitated Seedlings of Sunflower (Helianthus annuus L.)

Abstract

The transport behaviour of the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D), applied in aqueous solution to the cut stem surface of sunflower seedlings decapitated in the epicotyl, was studied using steam-ringing to differentiate between apoplastic and symplastic movement. Initially the 2,4-D moved in apoplastic tissue and was distributed rapidly throughout the plant, apparently as the result of a non-auxin-specific transport process. When an amount of 2,4-D sufficient to maintain apical dominance in the decapitated seedling was applied, the initially distributed material was subsequently redistributed acropetally in the stem apoplast and accumulated in the stump apex. When a lower level of 2,4-D, insufficient to maintain apical dominance, was applied, the initially distributed material was redistributed basipetally in the stem symplast, probably via the auxin polar transport system, and accumulated in the root. It is suggested that the 2,4-D loading capacity of the polar transport system is an important factor determining both the transport behaviour of the 2,4-D and its ability to maintain apical dominance in this system.