Influence of a Selenium Biofortification on Antioxidant Properties and Phenolic Compounds of Apples (Malus domestica)

Abstract

Biofortified apples seem to be a suitable produce. In this study, different selenium forms and application levels were applied to the two apple varieties 'Golden Delicious' and 'Jonagold', grown in the years 2017 and 2018 in order to increase the selenium uptake within a typical Western diet. It was shown that the biofortification, which was performed as a foliar application implemented in usual calcium fertilization, led to significantly increased selenium contents in the fruits. Furthermore, biofortification affected the total phenolic content (TPC), the polyphenol oxidase activity (PPO), as well as the antioxidant activity (AOA), the latter measured with the two well-known assays Trolox Equivalent Antioxidant Capacity Assay (TEAC) and Oxygen Radical Absorbance Capacity Assays (ORAC). The varying selenium forms and application levels showed a differing influence on the parameters mentioned before. Higher fertilizer levels resulted in higher selenium accumulation. It was found that PPO activity fluctuates less in biofortified apples. With regard to TPC, selenate led to higher amounts when compared to the untreated controls and selenite resulted in lower TPC. AOA analysis showed no clear tendencies as a result of the selenium biofortification. In the case of 'Jonagold', a higher AOA was generally measured when being biofortified, whereas, in the case of 'Golden Delicious', only one form of application led to higher AOA. Additionally, differences in the amount of major phenolic compounds, measured with High Performance Liquid Chromatography Mass Spectrometry (HPLC-DAD-ESI-MSⁿ), were observed, depending on the conditions of the biofortification and the variety.

Keywords: TEAC; Total Phenolic Content; agronomic biofortification; antioxidant activity; apple; phenolic compounds; phenoloxidase; selenium.

Figure 1

Polyphenol oxidase activity in units/100 g fresh weight (f.w.) for the apple samples of the variety ‘Golden Delicious’. Foliar Se application per hectare and meter canopy height: 0.15 kg as selenite (A), or selenate (B), 0.075 kg as selenate (C) (n = 4). Different letters are significantly different (p < 0.05).

Figure 2

Results of polyphenol oxidase activity in units/100 g fresh weight for the apple samples of the variety ‘Jonagold’. Foliar Se application per hectare and meter canopy height: 0.15 kg as selenite (A), or selenate (B), 0.075 kg as selenate (C) (n = 4). Different letters are significantly different (p < 0.05).

Figure 3

Total phenolic content (TPC) in mg GAE/100 g d.w. for the apple samples, depending on harvest year, apple variety, and form of selenium. Foliar Se application per hectare and meter canopy height: 0.15 kg as selenite (A), or selenate (B), 0.075 kg as selenate (C) (n = 4). Different letters are significantly different (p < 0.05).

Figure 4

Results of the antioxidant activity (Trolox Equivalent Antioxidant Capacity Assay (TEAC) assay) in mmol TE/100 g d.w. for the apple samples. Foliar Se application per hectare and meter canopy height: 0.15 kg as selenite (A), or selenate (B), 0.075 kg as selenate (C) (n = 4). Different letters are significantly different (p < 0.05).

Figure 5

Results of the antioxidant activity (Oxygen Radical Absorbance Capacity Assays (ORAC) assay) in mmol TE/100 g d.w. for the apple samples. Foliar Se application per hectare and meter canopy height: 0.15 kg as selenite (A), or selenate (B), 0.075 kg as selenate (C) (n = 4). Different letters are significantly different (p < 0.05).

Figure 6

High Performance Liquid Chromatography Mass Spectrometry (HPLC-MS)ⁿ-chromatogram at 280 nm of apple cultivar ‘Jonagold’, biofortified with 0.075 kg Se/ha in the form of selenate in 2018. Peak numbers: 1, procyanidin dimer; 2, chlorogenic acid; 3, epicatechin; 4, procyanidin trimer; 5, phloretin-2-xylosyl-glucoside; 6, phloretin-2-glucoside.