Effects of plant growth regulators on fruit quality and peel thickness of Jinfeng kiwifruit

【Objective】Kiwifruit is a berry with pericarp that can be divided into exocarp, mesocarp, and endocarp. The exocarp, known as the peel, protects the flesh from the adverse external environment and plays an important role in maintaining the hardness of the fruit and moisture in the fruit. 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 such strong vigor, large single fruit weight, uniform fruit shape, delicate flesh, and high sugar and dry matter contents. This cultivar is a medium-to-late cultivar. It has the characteristics of strong adaptability, good stress resistance, high quality, and high yield. However, 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 and thus limits the production and sales of the cultivar. In this study, plant growth regulators and mineral elements were used to treat the young fruit of Jinfeng, to screen out the effective treatment measures suitable for thickening the peel and improving the fruit quality of Jinfeng kiwifruit and provide technical support for its application.【Methods】Three plant growth regulators, 6-BA (6-benzylaminopurine, at25 mg·L-1 , 50 mg·L-1 or 75 mg·L-1 ), GA3 (gibberellic acid, at 50 mg·L-1 or 100 mg·L-1 ), and NAA (naphthylacetic acid, at 25 mg·L-1 or 50 mg·L-1 ) were selected. At the same time, different concentrations and combinations of CaCl2 and Zn (NO3)2 were also applied, and the treatments were divided into two types: fruit soaking and foliar spraying. For fruit soaking treatments, the fruit was completely soaked in the treatment solutions for at least three seconds 15 days after full bloom to ensure that the entire fruit surface was wetted. Three trees with basically the same vigor were selected for each treatment, and at least 50 fruit were randomly treated for each tree. For foliar spraying treatment, a sprayer was used to spray the leaves of the test trees at 15, 25, and 35 days after full bloom until both sides of the leaves were dripping. After the fruit reached the commercial maturity (soluble solids content ≥ 8%), at least 30 fruit of the same size and free of diseases and pests were picked for each treatment. The fruit were brought back to the laboratory immediately after harvest, and the fruit appearance indexes (transverse diameter, longitudinal diameter, single fruit weight, and fruit shape index) were measured. After the fruit placed at room temperature reached the edible state (soluble solids content ≥ 17.5%), the internal quality indexes of the fruit were determined. The soluble solid 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 at the equatorial part of the fruit was fixed with FAA (alcohol formalin acetate mixed fixative solution), stained with the saffron solid green, and then observed and photographed with an upright white light photographing microscope. The number of epidermal cell layers, epidermal thickness, length and width of the peel cells were measured with the 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 analyses of difference significance and correlation. Origin 2018 was used for graph drawing. 【Results】All the treatments increased the length and single fruit weight of Jinfeng kiwifruit. The average single fruit weight in the foliar treatment of 1 g ·L- 1 Zn (NO3)2 + 25 mg ·L- 1 NAA 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, and 25 mg·L-1 and 50 mg·L-1 6-BA fruit soaking treatments 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 in 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.