Involvement of Ethylene in the Induction of Cotton Defoliation by the Isoindoldione Cinidon-ethyl

The effect of the isoindoldione herbicide cinidon-ethyl on leaf abscission and the role of ethylene synthesis in the mechanism of action were studied in cotton (Gossypium hirsutum L. cv. Stoneville 825). Cinidonethyl is a new inhibitor of protoporphyrinogen IX oxidase in tetrapyrrole biosynthesis, which leads to the formation of I-aminocyclopropane-l-carboxylic acid (ACC)-derived ethylene as a side effect (K. Grossmann and H. Schiffer, Pestic. Sci. 55, 687 (1999)). Foliar application of cinidon-ethyl to young cotton plants at the fourth leaf stage resulted in a concentration-dependent increase in ethylene evolution and leaf abscission. Concomitantly, the fresh weight of abscised leaves was decreased. Within 2 days, cinidon-ethyl, strictly localized to blades of primary leaves of plants, induced maximum levels of ethylene, ACC, and conjugated ACC in the tissue. Ethane evolution as an indicator for membrane peroxidation remained unchanged within this period. Between the 2nd and 3rd days, a transient rise in ethylene and particularly ACC was observed in the petiole and abscission zone. Subsequently, leaf abscission was induced 4 days after treatment. Radiotracer studies showed that cinidon-ethyl is effective through contact action. After leaf blade application of [C]cinidon-ethyl, only trace amounts of C were translocated basipetally to the abscission zone. Therefore, it appears that ACC is transported as a signal from the leaf blade to the abscission zone, where metabolic conversion releases ethylene, which is implicated in leaf abscission. In accordance, cinidon-ethylinduced defoliation was gradually reduced by increasing concentrations of aminoethoxyvinyiglycine, an inhibitor of ethylene formation. The effect was completely restored by the addition of ACC.