Covering the trees of Kinokuni tangerine with plastic film during fruit ripening improves sweetness and alters the metabolism of cell wall components

The persistent consumption of citrus fruit mainly depends on its sweet–sour taste as well as pulp texture. To date, the simple facility by covering plastic film just over trees is commonly used in different citrus species for harvest time extension and quality improvement. In this study, the effect of covering trees of Citrus reticulata cv. Kinokuni with plastic films was investigated. Results indicated that simply covering plastic film over trees significantly decreased the soil water content, the light intensity, and then the photosynthetic rate, however, moderately increased the internal average temperature of 0–3 °C. Moreover, it significantly facilitated soluble sugar accumulation, increased water soluble pectin content, and decreased protopectin content. Investigating transcript profile showed that only 351 gene transcripts were significantly influenced by covering plastic film. Interestingly, most differently expressed genes (DEGs) were involved in carbohydrate and cell wall metabolisms except some DEGs were related to signal transduction and environmental adaption. Taken together, covering plastic film over trees comprehensively modified the internal environmental condition and improved the fruit sweetness and mastication trait by promoting soluble sugar accumulation, increasing water soluble pectin content, and decreasing protopectin content through the alteration of transcript level.