Flower thinning effect of ammonium thiosulfate in Red Fuji apple

Abstract: 【Objective】The appropriate concentration and spraying period of a flower thinning agent: ammonium thiosulfate (ATS) for Tianhong 2 apple were explored, and the effects of ATS on fruit quality were studied so as to provide reference for the application of ATS in apple.【Methods】The experiment was conducted in Tianping lake base of Shandong Institute of Pomology from April 2019 to November 2020. The reagent used is analytical pure ATS (98% ), which was purchased from the official website of Aladdin reagent. The tested trees used were 9 a and 10 a Tianhong 2 /SH38/M. robusta. On April 8, 2019, ATS concentration screening test was carried out at full bloom stage, with four concentration treatments of 0.5%, 1.0%, 1.5% and 2.0%, and water spray was used as the control. In 2020, the spraying period screening test was carried out with six treatments including spraying at stage 1 (25% central flower opening), at stage 2 (75% central flower opening), at stage 3 (25% petal falling off), at stage1and stage 2 (stage 1+2), at stage 1 and stage 3 (stage 1+3), and at stage 2 and stage 3 (stage 2+3), with no treatment as the control. A knapsack electric sprayer was used to spray the agent until the flowers were moist and dripping slightly. The fruit setting rate and fruit vacancy rate were investigated after physiological fruit drop. Meanwhile, taking the treatment at stage 2 as the representative treatment, the central fruit were collected to determine the diameter size and mineral nutrient content of young fruit. Semi micro distillation method, molybdenum antimony anti absorption spectrophotometry, and flamephotometer method were used for the determination of total nitrogen, total phosphorus and total potassium, respectively. At the fruit ripening stage, samples from selected treatments were collected for fruit quality determination. Single fruit was weighed with an electronic platform scale; the vertical and horizontal diameters of fruit were measured with a digital vernier caliper; the peel hardness of fruit with a GY-1 fruit hardness tester; the content of soluble solids with a digital sugar meter; the peel color determined using a CR-410 color difference meter; and titratable acid content was determined with acid-base neutralization titration.【Results】0.5%-2.0% ATS treatments decreased inflorescence fruit setting rate and flower fruit setting rate, and the treatments with 1.5% and 2.0% ATS displayed significant effect. There was no significant difference between the two low concentration treatments and the control, and between the treatments of 1.5% and 2.0%. The ratios of single fruit and double fruit inflorescence in 2.0% treatment were the highest, which were 35.93% and 19.25%, respectively. However, the treatment burned the young leave. Therefore, the concentration of 1.5% was more appropriate. Compared with the control, fruit set decreased after spraying once and twice. Among them, stage 1+3 had the lowest inflorescence setting rate and flower fruit setting rate, which were 87.70% and 33.56%, respectively. The percentages of single fruit and double fruit were the highest in the treatment of stage 1+3, which were 38.33% and 33.33%, respectively. The single fruit rate was significantly higher than in the other treatment periods and in the control. It is considered that two-spray treatments at the 25% and 75% central flower opening were most suitable. The treatment of ATS significantly increased the vertical and horizontal diameters and total nitrogen content in young fruit, but had no significant effect on total phosphorus and total potassium. After ATS treatment, the quality indexes such as single fruit weight, red color a*, soluble solid content and vitamin C content were increased. The single fruit weight increased by 21.47 g or 11.7% compared with the control. However, it had no significant effect on fruit shape index, fruit hardness and titratable acid. The brightness value of L was slightly lower than that of the control. 【Conclusion】For ATS application to Fuji apple fruit thinning, the suitable concentration is 1.5%, and the suitable spraying period is the initial and full bloom periods. The treatment can significantly reduce the fruit set, increase the proportion of single fruit inflorescence. It can significantly increase fruit weight, red color, soluble solid content and vitamin C content, but has no significant effect on fruit hardness and titratable acid content. ATS can be used as an effective flower thinning agent in production.