Effects of alternate bearing and 2,4-D application on fruit growth pattern, abscission enzymes activity, ACC content of calyx and carbohydrates partitioning of fruits in Satsuma mandarin (Citrus unshiu Marc.) cv. Miyagawa

Considerable fluctuations in annual yield (alternate bearing) is one of the noticeable characteristics of some citrus cultivars. Application of chemical thinners at high yield (On) years is an efficient way for ameliorating this problem. The aim of this work was to evaluate the effects of crop load and 2,4-D foliar application at full bloom on Satsuma mandarin (Citrus unshiu Marc.) cv. Miyagawa fruits at On year under rainfed farming. The 24-year-old ̒Miyagawa̕ mandarin trees grafted on Poncirus (Poncirus trifoliata Raf.) rootstock grown at North Iran were examined for ethylene precursor (ACC) content of calyx, activity of cellulase and polygalacturonase in abscission zone of fruitlets, and also the growth pattern and carbohydrates partitioning in fruits. Results showed that 2,4-D was effective in intensifying the activity of abscission enzymes and ACC synthesis during fruitlet abscission and increased fruitlets drop. In addition, 2,4-D significantly increased fruit growth rate from 73 days after anthesis (DAA), resulting to significantly heavier fruits than those of On (check) trees. The other consequence of 2,4-D treatment was the increase in the fruits' sucrose, fructose, and glucose levels, resulting in fruit quality improvement. As a result, 2,4-D spray of On ̒Miyagawa̕ trees during full bloom period can be a suitable solution to control alternate bearing and to enhance fruit quality.