The effects of bagging and debagging on external fruit quality, metabolites, and the expression of anthocyanin biosynthetic genes in 'Jonagold' apple (Malus domestica Borkh.)

- Bagging and debagging treatments did not affect the level of total non-structural carbohydrates or total organic acids.
- Bagging decreased the concentrations of total phenolics in the peel and flesh whereas de-bagging increased them.
- The expression of anthocyanin biosynthetic genes peaked at Hour 30 after bag removal.

'Jonagold' apple (Malus domestica Borkh.) was used to determine the effects of bagging and debagging on fruit external and internal qualities, as well as transcript levels for key genes involved in anthocyanin synthesis. Fruits from three treatments - control (never bagged), always bagged, or debagged - were harvested at 0 h, 6 h, 30 h, 3 d, 7 d, and 14 d after certain bags were removed. Peel and flesh tissues were sampled separately. Compared with the control, bagged fruit were yellowish, and had higher scores for lightness and hue angle, lower values for chroma, and the lowest concentration of anthocyanins. Fruit firmness was slightly increased by bagging treatment, whereas soluble solids content (SSC) was decreased. Compared with the control, bagged fruit had more starch, glucose, and galactose, but less of the other sugars and sugar alcohols. The concentrations of fructose and total non-structural carbohydrates were not obviously affected by bagging. Although the malic acid concentration was not influenced by bagging, other acids were either increased (quinic, shikimic, and citric) or decreased (ascorbic). The concentration of total organic acids did not show large difference among three treatments except that it tended to be lower at 14 DABR (days after bag removal). Phenolic compounds, especially flavonols in the peel, were severely inhibited, but their concentrations were increased by re-exposure to sunlight after bag removal. The debagging treatment led to up-regulation of the expression of MdMYB10 and seven structural genes involved in anthocyanin biosynthesis, and subsequent accumulation of anthocysnins. However, the peel of debagged fruit still have lower levels of anthocyanins and other phenolic compounds at harvest, although bag removal made up for the loss of total phenolics in the flesh.