The effect of prohexadione-calcium on the phenolic content in developing fruits and leaves of apple trees

We studied the effect of prohexadione-calcium (Pro-Ca) on the content of hydroxycinnamic acids (chlorogenic acid, p-coumaric acid, ferulic acid and caffeic acid), flavonols (rutin, quercetin-3-O-rhamnoside), flavan 3-ols (catechin, epicatechin) and dihydrochalcones (phloridzin, phloretin) in the leaves and fruits of the scab resistant apple cultivar ‘Florina’ and the scab susceptible cultivar ‘Jonagold’. Apple leaves were sampled 7 times at 2- day intervals after each Pro-Ca treatment. Fruits were harvested after the second Pro-Ca treatment six times during the growing season until their technological maturity. The phenolic compounds were detected using a HPLC system with a diode array detector at 280 and 350 nm. The antioxidant potential of apple pulp and peel, respectively, was measured with the DPPH (2,2-diphenyl-1-picrylhydrazyl) method and the total phenolic content in apple leaves and fruits was determined spectrophotometrically using the Folin-Ciocalteu phenol reagent method. Phloridzin was identified as the major phenolic compound in apple leaves in both cultivars, followed by rutin, quercetin-3-O-rhamnoside, epicatechin, chlorogenic acid, catechin, caffeic and p-coumaric acid. The treatment with Pro-Ca had similar effects on scab-susceptible and resistant apple cultivars. We showed that Pro-Ca caused a significant decrease in the synthesis of catechin, epicatechin, rutin, quercitrin, phloretin and phloridzin in the treated leaves and fruits. In contrast, the content of hydroxycinnamic acids increased significantly after treatment with Pro-Ca. In the treated leaves, the content of total phenolic compounds increased by approximately 23% compared to untreated leaves. In contrast, Pro-Ca caused a significantly lower content of total phenolics in the fruits which is also linked with a lower antioxidant activity.