Foliar application of ascorbic and citric acids enhanced 'Red Spur' apple fruit quality, bioactive compounds and antioxidant activity

This study aimed to assess the effect of the foliar application of ascorbic acid (AA) and citric acid (CA) on total antioxidant activity (TAA), total phenolics, total flavonoids, total anthocyanin content, antioxidant enzymes, phenylalanine ammonialyase (PAL), and polyphenol oxidase (PPO) activities in apple 'Red Spur'. The experiment was conducted on 12-years-old trees 'Red Spur' grafted on MM106 rootstock. The trees were sprayed with AA (0, 200 and 400 mg L-1) and/or CA (0, 200 and 400 mg L-1) at three different times during summer. Foliar application with AA and CA significantly (p < 0.01) enhanced all measured quality attributes and decreased the activity of PPO. Fruit from trees treated with AA at 400 mg L-1 and CA at 200 mg L-1 showed the highest TAA and catalase (CAT) enzyme activity. Total phenolics increased in fruits when trees were sprayed with AA and CA. Contrasting, AA treatment, CA had no significant effect on guaiacol peroxidase (G-POD). A significant decrease in PPO activity was detected in fruits when treated with both AA and CA. Both treatments significantly decreased the activity of PAL at 400 mg L-1. Considering the results, foliar application of AA and CA, either alone or in combination improved the quality and nutraceutical properties of 'Red Spur' apple.