Studies on fruit puffing and storability characteristics of Miyagawa satsuma orange and its puffing-resistant bud mutant

Abstract:【Objective】Puffiness is a common physiological disorder occuring in the ripening and post- harvest period in mandarin citrus fruits, which greatly affects the storage and transportation perfor-mance of citrus fruits. Normally oranges and pummelos are less prone to puffing disorder, while manda-rin oranges are more prone to it. Miyagawa (Citrus unshiu Marc.) is the main satsuma orang cultivar in China, especially in Zhejiang province. In recent years, delayed cultivation techniques have improved the fruit quality of Miyagawa, but it greatly aggravated the occurrence of fruit puffiness. A bud mutant of Miyagawa was found to be resistant to puffiness in previous study. This study aimed to explore the difference of fruit quality, puffing characteristics and storage performance between Miyagawa and its bud mutant, and provide references for the selection of puffing-resistant citrus varieties.【Methods】The main fruit quality traits of Miyagawa and its mutant grown in the same orchard including single fruit  weight, fruit shape index, color, hardness, soluble solid content, titratable acid content, vitamin C (Vc) content were measured in 2014, 2015, 2017, 2019, 2020 respectively. Also the puffiness index were in-vestigated in each year. The characteristics of the peel of Miyagawa and its bud mutant were compared,such as the thickness and the density of oil gland. Using paraffin sections, the anatomical structures offruit peel of Miyagawa and its mutant were observed with an optical microscope. Also, resin ultrathin sections of the albedo were made to observe the ultrastructure of albedo with a transmission electron mi- croscope. The mature fruits of Miyagawa and its mutant in 2020 were selected and stored under (20±1) ℃ with a constant relative humidity of 75% for 100 d. During the storage period the fruit shape,thepuffiness index, peel color, fruit weight loss rate, hardness, soluble solid, titratable acid, and Vc con-tent were detected and analyzed every 20 days.【Results】There was no fruit neck in fruits of Miyagawa and its mutant. The fruit base of Miyagawa was slightly convex with a cone-shaped, while the bud mu-tant was relatively flat-shaped without protrusions. The mutant fruit core was more full and smaller than that of Miyagawa. According to fruit quality test, the fruit of the mutant was 48.79 mm long, 63.08 in di- ameter, 0.78 in shape index, weighing about 102.90 g. The fruit was orange-yellow with h0 68.19 and CCI index 6.51. The fruit hardness was 0.96 kg·cm-2, the soluble solid content was 13.88%, and the Vccontent was 29.87 mg·100 g-1, which were all significantly higher than those of Miyagawa. The fruit ti-tratable acid content was 0.62%, and the fruit puffiness index was significantly lower than that of Miya-gawa. The peel thickness of the bud mutant fruit was 1.65 mm, which was extremely significantly lowerthan that of Miyagawa. There was no significant difference in thickness of the peel and the density of the oil gland between Miyagawa and its mutant. Cell structure observation results showed that the albe-do cells of the bud mutant were relatively neatly arranged, and the cell structure was relatively com- plete. Some subcellular organelles could be observed obviously, including chloroplasts, mitochondria,while the albedo cells wall of Miyagawa separated from cytoplasm, and the subcellular organelles were degraded, which tended to age. During storage at room temperature, the CCI of both Miyagawa and its mutant fruits increased, the weight loss rate continued to rise, the hardness gradually went lower, and the soluble solid content increased. The titratable acid content decreased rapidly in the first 20 days,then slowly declined, and the Vc content gradually went down. The hardness, soluble solids and titrat-able acid content of the mutant fruits were higher than those of Miyagawa in the same period. The fruit quality and storability of the mutant were better than those of Miyagawa, especially the puffing-resis-tant performance.【Conclusion】The fruits of the bud mutant not only maintain the excellent fruit quali-ty of Miyagawa, but also have better puffing-resistance and storage performance. It has the potential to become a new puffing-resistant satsuma mandarin orange variety in the future, and also can be a perfect material for the study of puffiness mechanism.