Effect of Ethylene on Indole-3-Acetic Acid Transport, Metabolism, and Level in Leaf Tissues of Woody Plants during Abscission

Abstract

The ability of exogenously supplied ethylene to promote abscission of intact leaves varies greatly among different species (1). This is particularly true for woody plant species where some abscise one or two days after the beginning of treatment with ethylene while others respond to the hormone after much longer periods. Although it is well accepted that auxin-ethylene interactions control abscission processes (1,3), there are no conclusive data to explain why various species differ greatly in their response to ethylene. Beyer & Morgan (5) proposed a model for the role of ethylene in the regulation of abscission of intact leaves. The proposed regulatory system first involves a modification of the hormonal balance in the abscission zone achieved as ethylene reduces indole-3-acetic acid (IAA) transport capacity of the petioles. After the reduction of auxin levels, ethylene exerts direct action in the abscission zone such as stimulating synthesis of cell-wall degrading enzymes (1,3) and secretion of these enzymes into the cell-wall (2). Also, ethylene has been demonstrated to reduce endogenous IAA level (4,6,9) and there is some evidence that ethylene may reduce auxin level in the abscission zone either by stimulating destruction (6,7,10,11,15), or by inhibiting synthesis (6). Beyer (4) demonstrated that the leaf blade is the initial target tissue of exogenously supplied ethylene, where some essential function of the hormone must first be performed before abscission can occur.