Abstract:【Objective】Kiwifruit belongs to the category of berry, and its pericarp can generally be divided into exocarp, mesocarp, and endocarp. The exocarp, also known as the peel, separates the flesh from the external environment and plays an important role in maintaining the hardness of the fruit, maintaining the water balance, and resisting the damage of the external adverse environment. ‘Jinfeng’ kiwifruit (also known as ‘Fenghuang No.1’) is a new yellow-fleshed kiwifruit cultivar selected from the seedlings of Actinidia chinensis. It has the comprehensive advantages of strong growth potential, large single fruit weight, uniform fruit shape, delicate flesh, high sugar and dry matter content. This cultivar is a medium-to-late maturing yellow-fleshed kiwifruit in Fengxin County, Yichun City, Jiangxi Province. It has the characteristics of strong adaptability, good stress resistance, high quality, and high yield in local cultivation. However, it was found that the peel of this cultivar is extremely thin and prone to bruises, abrasions, scratches, etc., which is very unfavorable for storage and long-distance transportation, which has a great impact on production and sales and has affected the promotion and application of this cultivar to a certain extent. In this study, plant growth regulators and mineral elements were used to treat the young fruits of ‘Jinfeng’, to screen out effective treatment measures suitable for thickening the peel and improving the fruit quality of ‘Jinfeng’ kiwifruit and provide technical support for its popularization and application. 【Methods】Three plant growth regulators, 6-BA (6-benzylaminopurine), GA3 (gibberellin), and NAA (naphthylacetic acid) were selected with concentrations set as follows: 6-BA was 25 mg·L-1、50 mg·L-1 and 75 mg·L-1, respectively, GA3 was 50 mg·L-1 and 100 mg·L-1, NAA was 25 mg·L-1 and 50 mg·L-1. At the same time, different concentrations and combinations of CaCl2 and Zn(NO3)2 were also applied, and the treatment was divided into two types fruit soaking and foliar spraying. For fruit soaking treatment, the fruit was completely soaked in the treatment solution for at least three seconds 15 days after full bloom to ensure that the entire fruit surface was wet. Three trees with basically the same growth potential were selected for each treatment, and at least 50 fruits were randomly treated for each tree. For foliar spraying treatment, the agricultural sprayer was used to spray the leaves of the test trees at 15, 25, and 35 days after full bloom, respectively, until both sides of the leaves were dripping. After the fruits reached the commercial picking period (soluble solids ≥ 8%), at least 30 fruits of the same size and free of diseases and pests were picked for each treatment and control. The fruits were transported back to the laboratory immediately after harvest, and the fruit appearance quality indexes (transverse diameter, longitudinal diameter, single fruit weight, fruit shape index) were detected. After the fruit was placed at room temperature to reach the edible state (soluble solids ≥ 17.5%), the internal quality indexes of the fruit were determined. The soluble solids content was determined by a handheld digital sugar meter. The soluble sugar content was determined by anthrone colorimetry, and the total content of titratable acid was determined by the NaOH neutralization titration method. The content of ascorbic acid was determined by molybdenum blue colorimetry. The dry matter content of the fruit was determined by the drying method. The peel of the equatorial part of the fruit was fixed with FAA (alcohol formalin acetate mixed fixative solution), stained with the saffron solid green staining method, and then observed and photographed with an upright white light photographing microscope. The number of epidermal layers, epidermal thickness, long diameter, and short diameter of peel cells were measured by Image-Pro Plus 6.0 software. Finally, the activities of enzymes related to cell wall metabolism, including phenylalanine ammonialyase (PAL), peroxidase (POD), polyphenol oxidase (PPO), cellulase, and pectinase, were measured. SPSS 22.0 was used for data difference and correlation analysis. Origin 2018 was used for graph drawing.【Results】All treatments promoted the growth of longitudinal diameter and single fruit weight of ‘Jinfeng’ kiwifruit. The average single fruit weight of 1 g·L-1 Zn(NO3)2 + 25 mg·L-1 NAA foliar spraying treatment was the highest. Saffron solid green staining showed that the main component of the peel of ‘Jinfeng’ is lignin. Paraffin section observation showed that different plant growth regulator treatments significantly increased the thickness of ‘Jinfeng’ kiwifruit peel, among which 25 mg·L-1 NAA foliar spray treatment, 25 mg·L-1 and 50 mg·L-1 6-BA fruit soaking treatment had the most significant effect on the thickening of ‘Jinfeng’ kiwifruit peel. The combination of 100 mg·L-1 GA3 + 5 g·L-1 CaCl2 treatment had the best fruit quality, but the effect on improving peel thickness was not obvious. The storage time of ‘Jinfeng’ kiwifruit treated with 25 mg·L-1 NAA was significantly prolonged with the increase of peel thickness.【Conclusion】Based on the effects of each treatment on fruit quality and peel thickness of ‘Jinfeng’ kiwifruit, as well as the simplicity of operation technique, 25 mg·L-1 NAA foliar spray is the best treatment.
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