Juice sac granulation is a common internal physiological disorder of citrus fruit. In the present study, we compared the physiochemical characteristics and transcriptome profiles of juice sacs in different granulation levels from Huyou fruit (Citrus changshanensis). The accumulation of cell wall components, including the water-soluble pectin, protopectin, cellulose, and lignin, were significantly correlated with the granulation process, resulting in the firmness increase of the juice sac. The in situ labeling of the cell wall components indicated the early accumulation of cellulose and high-methylesterified pectin in the outer layer cells, as well as the late accumulation of lignin in the inner layer cells of the juice sac. Several phytohormones, including auxins, abscisic acids, cytokinins, jasmonic acid, salicylic acid, and/or their metabolites, were positively correlated to the granulation level, indicating an active and complex phytohormones metabolism in the granulation process. Combining the trend analysis by the Mfuzz method and the module-trait correlation analysis by the Weighted Gene Co-expression Network Analysis method, a total of 2940 differentially expressed genes (DEGs) were found to be positively correlated with the granulation level. Gene Ontology (GO) enrichment indicated that the selected DEGs were mainly involved in the cell wall organization and biogenesis, cell wall macromolecule metabolic process, carbohydrate metabolic process, and polysaccharide metabolic process. Among these selected genes, those encoding β-1,4-xylosyltransferase IRX9, cellulose synthase, xyloglucan: xyloglucosyl transferase, xyloglucan galactosyltransferase MUR3, α-1,4-galacturonosyltransferase, expansin, polygalacturonase, pectinesterase, β-glucosidase, β-galactosidase, endo-1,3(4)-β-glucanase, endoglucanase and pectate lyase that required for the biosynthesis or structural modification of cell wall were identified. In addition, NAC, MYB, bHLH, and MADS were the top abundant transcription factors (TFs) families positively correlated with the granulation level, while the LOB was the top abundant TFs family negatively correlated with the granulation level.

Fruit bagging is a commonly used cultivation measure to protect citrus fruit from insects and adverse environments. Present study aimed to comprehensively investigate the effects of bagging on physiochemical characteristics of grapefruit. The grape fruit were bagged at about 110-120 days after anthesis with one-layer kraft paper bag with black coating inside (SL), double-layer kraft paper bag with one black paper as inner layer (DL), and three-layer kraft paper bag with two black papers as inner layers (TL) respectively. UPLC-HRMS technique was used to identify a total of 19 flavonoids, 2 phenylpropanoids, 9 coumarins, and 5 limonoids. By using HPLC method, 50 carotenoids were identified. GCMS method was used to identify 3 soluble sugars, 3 organic acids, and 11 amino acids. In the quantitated components in the peel (albedo and flavedo), the chlorophylls and the carotenoids components (such as luteoxanthin, violaxanthin, 9-cis-violaxanthin, xanthophyll, zeaxanthin and β-carotene) were significantly down-regulated by bagging, causing the surface color of bagged fruit turn yellow earlier but paler than unbagged control, particularly in the three-layer kraft bag treatment. Unlike the peel, the color and the carotenoids contents of the juice sacs were less affected. The physiochemical compounds other than pigments, including soluble sugars, organic acids, amino acids, flavonoids, coumarins, and limonoids were minimally affected by bagging treatments. Our results indicated that bagging at about 110-120 days after anthesis exerted influence mainly on peel color, while less on sugars, acids, amino acids, flavonoids, limonoids and coumarins of grapefruit.