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Thiago Vieira da Costa, João Alexio Scarpare Filho and Matthew W. Fidelibus

In two experiments, various combinations of ethephon, with or without 1-aminocyclopropane carboxylic acid (ACC), were applied to the fruiting zone of ‘Selma Pete’ raisin grapes (Vitis vinifera) to determine whether any could serve as a defoliant, and if so, whether defoliation improved subsequent vine drying of the grapes. In the first experiment, the fruiting zone was treated on 8 Aug. 2013 with a control (water) and one of four plant growth regulator (PGR) treatments: 1000 ppm ethephon, 1000 ppm ethephon plus 1000 ppm ACC, 2000 ppm ethephon, and 2000 ppm ethephon plus 1000 ppm ACC. In the first experiment, treatment with any of the PGRs hastened leaf senescence, but leaf greenness, measured with a SPAD meter, declined most rapidly in leaves from vines treated with 2000 ppm ethephon or 2000 ppm ethephon plus 1000 ppm ACC, and defoliation was best in vines treated with 2000 ppm ethephon plus 1000 ppm ACC. None of the treatments in the first study affected berry composition, hastened berry drying, or ultimately affected raisin moisture or quality. In a second experiment, initiated 18 days later, a factorial design was employed to determine whether three chemical treatments, a control (water spray), 2000 ppm ethephon, and 2000 ppm ethephon plus 1000 ppm of ACC, might interact with fruiting zone orientation (east or west facing) to affect leaf senescence or berry drying. The second study confirmed that 2000 ppm ethephon and 2000 ppm ethephon plus 1000 ppm ACC induced rapid leaf senescence. Defoliation proceeded more rapidly in the second study and by 13 days after treatment, vines treated with 2000 ppm ethephon plus 1000 ppm ACC had less than one leaf layer remaining in the fruiting zone compared with more than 2.5 leaf layers in untreated vines. Treatments again had no effect on berry fresh weight or composition, but grapes on west-facing vines treated with 2000 ppm ethephon plus 1000 ppm ACC dried significantly better than grapes on vines subjected to other treatments, possibly because the higher temperatures of west-facing vines coupled with better defoliation of the 2000 ppm ethephon plus 1000 ppm ACC treatment was sufficient to improve grape drying compared with vines subjected to other trellis orientation and chemical treatment combinations. Therefore, we conclude that treatment with ethylene-promoting PGRs can defoliate the fruiting zone of ‘Selma Pete’ grapes with divided canopies, and such defoliation treatments may enhance berry drying when drying is initiated later than normal.