1-aminocyclopropane-1-carboxylic acid (ACC) is assumed to cause flower abscission via the ethylene pathway when used as a chemical thinner in peaches. The objectives of this study were to investigate the uptake of ACC via flower parts, and determine if ethylene is the main cause of flower abscission by ACC. In a series of field trials in Oregon, we determined that ACC is a non-mobile compound when applied as a foliar spray to peach trees. ACC does not translocate between branches and movement of externally applied ACC is very limited even between flower parts. When the ACC solution was applied via paintbrush to various parts of the flowers, it became evident that for sufficient flower thinning activity, the presence/uptake of ACC is needed through the pedicel and/or Abscission Zone tissues of the pedicel (AZ1-2). Application of ACC to the petals only, resulted in petal drop only but not in flower abscission. When evaluating the mode of action of ACC, we established a close relationship between ethylene production of the flowers and flower abscission. However, when ethylene production of the flowers was reduced with the addition of ACC oxidase inhibitors (2-picolinic acid, pyrazinecarboxylic acid) in the ACC spray solution, the flower abscission rate remained the same as in the ACC application alone. These results indicate that ethylene might not be the only factor in flower abscission caused by ACC.
