Preharvest Application of Phenylalanine Induces Red Color in Mango and Apple Fruit's Skin

Abstract

Anthocyanins are secondary metabolites responsible for the red coloration of mango and apple. The red color of the peel is essential for the fruit's marketability. Anthocyanins and flavonols are synthesized via the flavonoid pathway initiated from phenylalanine (Phe). Anthocyanins and flavonols have antioxidant, antifungal, and health-promoting properties. To determine if the external treatment of apple and mango trees with Phe can induce the red color of the fruit peel, the orchards were sprayed 1 to 4 weeks before the harvest of mango (cv. Kent, Shelly, and Tommy Atkins) and apple fruit (cv. Cripps pink, Gala and Starking Delicious). Preharvest Phe treatment increased the red coloring intensity and red surface area of both mango and apple fruit that was exposed to sunlight at the orchard. The best application of Phe was 2-4 weeks preharvest at a concentration of 0.12%, while a higher concentration did not have an additive effect. A combination of Phe and the positive control of prohydrojasmon (PDJ) or several applications of Phe did not have a significant added value on the increase in red color. Phe treatment increased total flavonoid, anthocyanin contents, and antioxidant activity in treated fruit compared to control fruits. High Performance Liquid Chromatography analysis of the peel of Phe treated 'Cripps pink' apples showed an increase in total flavonols and anthocyanins with no effect on the compound composition. HPLC analysis of 'Kent' mango fruit peel showed that Phe treatment had almost no effect on total flavonols content while significantly increasing the level of anthocyanins was observed. Thus preharvest application of Phe combined with sunlight exposure offers an eco-friendly, alternative treatment to improve one of the most essential quality traits-fruit color.

Keywords: anthocyanin; apples; flavonoids; fruit quality; mango; phenylalanine; preharvest; prohydrojasmon; red color.

Figure 1

Red color evaluation of ‘Kent’ mango fruit peel. ‘Kent’ mango orchards were sprayed with 0.12% phenylalanine (Phe) or 0.2% prohydrojasmon (PDJ), one or two weeks preharvest. The fruit was evaluated at harvest (T0), after cold storage (CS, 3 weeks at 12 °C), and after shelf life (SL, 7 days at 22 °C). (A) Red color intensity (index 0–5). (B) Red surface area (% of fruit coverage). (C) Color (Hue value of the reddest point). (D) Green-red color range (a* value). (E) Representative pictures of ‘Kent’ mango boxes after shelf-life storage. Mean values and standard errors are presented. Statistical analysis was conducted for each time point separately (small or capital letters). Different letters represent a significant difference (p ≤ 0.05).

Figure 2

Red color evaluation of apple (cv. Cripps pink) peel. Apple orchard was sprayed preharvest with 0.12% phenylalanine (Phe) and/or 0.2% prohydrojasmon (PDJ). The fruit was evaluated at 3-time points: at harvest, after cold storage (CS, 3 weeks at 2 °C), and after shelf life (SL, 7 days at 20 °C). (A) Red color intensity (index 0–5). (B) Red surface area (% of fruit coverage). (C) Color (hue value at the reddest point). (D) Green-red color range (a* value at the reddest point). (E,F). Representative pictures of Cripps pink apple box after shelf-life storage. Mean values and standard errors are presented. Statistical analysis was conducted for each time point separately (small or capital letters). Different letters represent a significant difference (p ≤ 0.05).

Figure 3

Effect of preharvest treatment with 0.12% Phe or 0.2% PDJ on antioxidant, phenolic, and flavonoid content in mango (cv. Kent) and apple fruit (cv. Cripps pink) at harvest, after cold storage (CS), and shelf-life (SL). Antioxidant activity, total phenolic, and total flavonoid content were evaluated from mango peels (cv. Kent) (A–C), apple (cv. Cripps pink) (D–F) peels. Mean values and standard errors are presented. Statistical analysis was conducted for each time point separately (small or capital letters). Different letters represent significant differences (p ≤ 0.05).

Figure 4

Effect of preharvest treatment of Phe and PDJ on chlorophyll, anthocyanin, and flavonoids fluorescence. Mango (cv. Kent, A–C) and apple (cv. Cripps pink, D–F) orchards were treated 1–3 weeks (1W, 2W, 3W) preharvest with 0.12% Phe or 0.2% PDJ and chlorophyll, anthocyanin, and flavonoids fluorescence at the reddest point of the fruit peel was analyzed at 3-time points: after harvest, after cold storage (CS), and after shelf-life (SL). (A,D) Chlorophyll fluorescence (SFR_R). (B,E) Flavonoids fluorescence (FLAV). (C,F) Anthocyanin fluorescence (ANTH_RG). Mean values and standard errors are presented. Statistical analysis was conducted for each time point separately (small and capital letters). Different letters represent significant differences (p ≤ 0.05).

Figure 5

Quantification of metabolites using HPLC in mango and apple fruit peel. Mango (cv. Kent) and apple (cv. Cripps pink) were sprayed with 0.12% phenylalanine (Phe) or 0.2% prohydrojasmon (PDJ) one or two weeks preharvest. Apple (A–C) and mango (D,E) peels were analyzed after 3 weeks of cold storage at 2 °C and 12 °C, respectively. (A,D) Anthocyanin content in apple and mango, respectively. (B,E) Flavonol content in apple and mango, respectively. (C) Dihydrochalcones in apple. All the values expressed as Peak area/gFW (×10⁶). Mean values and standard errors are presented. Different letters represent significant differences for total content (p < 0.05).