Changes of phenolics contents, antioxidant activities, and enzyme activities in pellicles of Juglans sigillata Dode during fruits development

Phenolic compounds are important components that affect the taste and health-care function of walnuts. And phenolic compounds in walnut (Juglans sigillata Dode. cv. 'Qianhe 7') pellicles at three developmental stages were explored to identify their role. The results showed that the total phenolic content, total flavonoid content, ferric reducing antioxidant power, 2, 2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) and 1, 1-diphenyl-2-picrylhydrazyl free radical scavenging activities in pellicles increased gradually during fruits development, and reached the maximum value at maturation period. Syringic acid, gallic acid, catechin, and rutin contents were highly significantly positively correlated with reducing power or highly significantly negatively correlated with IC50 of free radical scavenging activities. Gallic acid and rutin contents were highly significantly positively correlated with phenylalanine ammonia-lyase (PAL) activity. Total phenolic content, syringic acid, catechins, and rutin contents were highly significantly negatively correlated with cinnamate 4-hydroxylase (C4H) and 4-coumarate CoA ligase (4CL) activities. Ferulic acid content was highly significantly positively correlated with polyphenol oxidase (PPO) activity. Overall, PAL, C4H, 4CL and PPO were key enzymes in the metabolism of polyphenols, and they affected the antioxidant capacity of the pellicles by regulating the content of polyphenol during walnut fruit development.