Effect of foliar application of Naphthalene acetic acid and Carbaryl and leaf nutrition of urea and magnesium sulfate on alternate bearing reduction of Kinnow mandarin in Dezful region

Introduction: Kinnow mandarin is a high yield cultivar that has special importance due to its interesting flavor and high tolerance againts high temperature conditons (Nawaz et al., 2008). Unfortunately, it has alternate bearing habit that affecteing its economic efficiency (Safaei-Nejad et al., 2015) . Foliar application of fruit thining compounds and nutrientes in on-year could be efective to adjust alternate bearing in fruits (Rabe, 1994; Safaei-Nejad et al., 2015; Sebastian et al., 2019). This research was carried out in order to the evaluation of the possibility of alternative bearing adjustment of Kinnow mandarin using foliar application of thinning compounds and nutrients in Dezful region of Iran.
Material and methods: To cope with alternative bearing problem of Kinnow mandarin trees in Dezful region of Iran, two separate experiments were conducted during 2018. In the first experiment, in order to increase the yield of Kinnow mandarin trees in off-crop year, the trees were treated with foliar application of urea (0, 0.3 %, and 0.5%) and magnesium sulfate (0, 0.1%, and 0.3%) fertilizers and the mixture of them in the early of January, February and March in on-crop year. In the second experiment, trees were treated by spray application of Naphthalene acetic (NAA) (zero, 200, 300, and 400 mg/l) and Carbaryl (zero, 700, 1200, and 1700 mg/l) at 40 days after full bloom stage during on-year. At the end, yield, fruit characteristics including fruit weight, fruit length and diameter and their ratio, total soluble solids, vitamin C, total acidity and TSS/TA ratio were measured. Analysis of variance was performed by ANOVA procedures (SAS 9.1 for Windows) and significant differences were calculated according to Duncan’s multiple range tests (P < 0.05). Results and discussion: according to the results, yield and fruit weight were significantly affected by the interaction of type of fertilizer × time of foliar application, while fruit length and diameter were not significantly affected by them. Also, fruit length and diameter were affected significantly by fertilizer type. Foliar application of urea (0.3%) + Magnesium sulphate (0.1%) in the early of February at on-crop year significantly increased the yield of Kinnow mandarin trees in off-year. It highlights the role of time of foliar application and ratio of nutrients on adjustment of alternative bearing of Kinnow mandarin in a special region. Results shows, total acidity of fruits was significantly affected by time of foliar application while other chemical properties of fruits were not significantly affected by time of foliar application, type of fertilizers and their interaction. The results indicated, the effects of foliar application of NAA and Carbaryl at 40 days after full bloom stage in on-year on yield and fruit length were significant while fruit weight and diameter and the ratio of fruit length to diameter were no significantly affected by these compounds. The highest and lowest yield of kinnow mandarin trees was related to the treatments of control and foliar application of NAA (400 mg/l), respectively. Regarding to the yield of Kinow mandarin trees, there were no statically significant difference between foliar application of Carbaryl and control treatment. Conclusion: Overall, the highest yield of trees in off-crop year was related to the treatment of foliar application of urea (0.3%) + Magnesium sulphate (0.1%) in the early of February (on-crop year) that had significant difference with control trees and it was able to adjust alternative bearing to a satisfactory level. Based on the results, Carbaryl compound is not able to adjust alternative bearing of Kinnow mandarin trees at 40 days after full bloom stage in Dezful region. Among different concentration of NAA, NAA spraying with concentration of 200 mg/l adjust alternative bearing more appropriately and had significant difference with control trees, but the concentration of 300 and 400 mg/l NAA reduced the level of yield with a great intensity, which is not recommended.