Contents lists available at ScienceDirect Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti 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 Babak Adouli a, ⁎ , Zabiollah Zamani a , Mohammad Reza Fattahi-Mohgadam a , Behrouz Golein b , Keramatollah Rezaei c a Department of Horticulture Science, University of Tehran, Karaj, Iran b Citrus and Subtropical Fruits Research Center, Ramsar, Iran c Department of Food Science and Technology, University of Tehran, Karaj, Iran ARTICLE INFO Keywords: Chemical thinners Cellulase Polygalacturonase Crop load Rainfed farming ABSTRACT 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 ̒ Miyagawa  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 im- provement. As a result, 2,4-D spray of On ̒ Miyagawa  trees during full bloom period can be a suitable solution to control alternate bearing and to enhance fruit quality. 1. Introduction Alternate bearing is characteristic of some citrus cultivars, including Satsuma mandarin (Citrus unshiu Marc.) cv. Miyagawa in north of Iran. The main problem associated with alternate bearer cultivars is the ir- regular cropping, i.e. a large number of small fruits in one season fol- lowed by few and large fruits the next season (Izak, 2012). Because the price of fruit fluctuates according to yield and fruit size, this phenom- enon is undesirable for growers. Therefore, the flowering and fruit load control in citrus is indispensable to the industry (Koshita and Takahara, 2004). Satsuma mandarin as a parthenocarpic and alternate bearer cultivar can produce a high number of flowers in some years resulting to high yield of small sized fruits leading to reducing the return bloom the next year. The mechanisms by which the developing crop influences the return bloom and yield in the following year is not fully understood (Monerri et al., 2011) but two hypotheses (hormonal and nutritional) have been suggested. The hormonal hypothesis proposes that devel- oping fruits produce inhibitors that directly or indirectly reduce flow- ering in the spring following the On season. The nutritional hypothesis holds that fruits may influence flowering by reducing plant reserves, mainly carbohydrates which accumulate in the root system and leaves. The rate of this reserves depletion has been related to flower and fruit number (Shalom et al., 2012; Moreira et al., 2013; Martinez-Alcantara et al., 2015; Rebolledo et al., 2015). Flower and fruit abscission is a metabolic process that occurs in response to developmental, environmental and hormonal cues. The principal hormonal regulators of abscission are ethylene and auxin, which accelerate and inhibit the process, respectively (Macnish et al., 2005; Iglesias et al., 2006; Rebolledo et al., 2012). In citrus, two https://doi.org/10.1016/j.scienta.2018.04.042 Received 8 April 2018; Received in revised form 18 April 2018; Accepted 19 April 2018 ⁎ Corresponding author. E-mail addresses: b.adouli@areeo.ac.ir (B. Adouli), zzamani@ut.ac.ir (Z. Zamani), fattahi@ut.ac.ir (M.R. Fattahi-Mohgadam), bgoleincitrus@yahoo.com (B. Golein), krezaee@ut.ac.ir (K. Rezaei). Scientia Horticulturae 238 (2018) 58–65 0304-4238/ © 2018 Elsevier B.V. All rights reserved. T