Shading in fruit changes the polyphenol accumulation of pellicle by regulating activity of key enzymes and expression of their gene related polyphenol anabolism of Juglans sigillata Dode

Polyphenols, abundant in the pellicle of walnuts, confer health benefits to the human body. The intricate relationship between light and phenolic compounds underscores the impact of shading, which can effectively diminish phenolic content and enhance the palatability of food. In this study, we delved into the connection between polyphenol accumulation, related enzyme activities, and gene expression in the pellicle of Juglans sigillata Dode under varying degrees of shading. The results show that different shade intensities can change polyphenols' content and affect the activity of key polyphenols anabolism enzymes and their gene expression. With the increase of the shading intensity, the total phenolics content (TPC) and total flavonoids content (TFC) were lower than the nomal light. In the mature stage, shading 60 % was lowest. 7 individual phenolics were identified in the pellicle, and the content of each varied significantly with the decrease of light intensity. Except for p-coumaric acid and ferulic acid, which were higher than the control at mature stage, all phenolics were lower than the control, but the overall trend was the same as that of the control. The activity of key enzymes in the anabolism of polyphenols was significantly affected after shading. In complete darkness phenylalanine ammonia lyase (PAL) was highest at the kernel-filling stage, shading 20 % of chalcone synthase (CHS) and shading 60 % of flavonol synthase (FLS) responded most strongly at the hard-core stage, while flavanone 3-hydroxylase (F3H) was most active at shading 60 % of kernel-filling stage, which was 2.03, 1.38, 4.67, and 2.08 times that of the control, respectively. The expression levels of key genes showed that the expression level of JsPAL was significantly increased at hard-core stage and mature stage, and the expression of JsPAL was 3.85 times higher than that of the control at the mature stage with 60 % shading. Correlation analysis showed that PAL enzyme activity was highly correlated with TPC, TFC and various individual phenolics with the increase of shading intensity. The content of p-coumaric acid was positively correlated with the activities of 4-coumarate coenzyme A ligase (4CL), F3H and FLS. The increase of shading intensity enhanced the correlation between total phenolics content, total flavonoids content, individual phenolics substances and JsPAL, JsC4H, JsCHS, JsCHI and JsFLS. Overall, PAL was a key enzyme affecting polyphenol accumulation under different shading levels, while at the genetic level, JsPAL, JsC4H, JsCHS, JsCHI and JsFLS were the key genes affecting them. This study will potentially provide more information for light management and polyphenol regulation in walnut cultivation.