Flavonoid Stability and Biotransformation in Agricultural Soils: Effects of Hydroxylation, Methoxylation, and Glycosylation

Abstract

Stricter pesticide regulations are increasing the demand for environmentally acceptable alternatives with flavonoids seen as promising candidates for use as biopesticides. However, the current limited understanding of the environmental fate of flavonoids in soils restricts their assessment as active pesticide ingredients. To address this knowledge gap, we conducted laboratory incubation experiments with LC-MS-based quantification to determine the half-lives of 18 structurally related flavonoids in three agricultural soils. Hydroxylated flavonoids were rapidly transformed (t_1/2: 3-12 h), while methoxylated derivatives exhibited substantially longer half-lives, which increased with the number of methoxy groups (t_1/2: 5-460 h). Glycosylated flavonoids were primarily transformed into their aglycones (t_1/2: 0.5-5 h). Incubation experiments with autoclaved soil indicated that biotic processes primarily catalyzed the observed transformations. All trends were consistent across different soil types and pH values. This study provides a comprehensive overview of flavonoid stability in agricultural soils, enhancing our understanding of their potential as alternative pesticides.

Keywords: biopesticide; soil half-life; structure−stability relationship; sustainable agriculture.

1

Structures and names of the investigated flavonoids. IUPAC nomenclature is added to naringenin. Conjugated hydroxyl groups that can be oxidized to quinoids are shown in bold. Methoxy groups are marked in orange, and glycosyl groups are marked in purple. Note: flavonoids grouped according to Schnarr et al., who categorized flavonoids by their core structure. Methoxylated and glycosylated flavonoids are derivatives of these core structures and are named accordingly.

2

(A) Concentration of core flavonoids during their incubation in Lufa 5M soil. Concentrations below the LOQ are marked with an “X”. The gray horizontal dashed lines indicate the spiked concentration. The dark gray solid lines indicate the fitted first-order kinetics (including data points >5% of the initial concentration). (B) Soil half-lives of core flavonoids derived from fitted first-order kinetic. Error bars represent the standard error of the fit.

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(A) Concentration of methoxylated flavonoids during their incubation in Lufa 5M soil. The gray horizontal dashed lines indicate the spiked concentration. The orange solid lines indicate the fitted first-order kinetics (including data points >5% of the initial concentration). For Tri-Me-Apigenin and Tetra-Me-Luteolin, the fitted first-order kinetics were adjusted to account for the observed lag phase, which lasted 3 days. (B) Soil half-lives of methoxylated flavonoids and their respective core flavonoid derived from fitted first-order kinetic. Error bars represent the standard error of the fit. Half-lives marked with an asterisk (*) could not be determined due to rapid removal of the flavonoid (i.e., recovery <5%). Half-lives marked with two asterisks (**) are greater than 450 h.

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(A) Concentration of glycosylated flavonoids during their incubation in Lufa 5M soil with their transformation product. Concentrations below the LOQ are marked with an “X”. The gray horizontal dashed lines indicate the spiked concentration. The purple solid lines indicate the fitted first-order kinetics (including data points >5% of the initial concentration). (B) Soil half-lives of glycosylated flavonoids derived from fitted first-order kinetics. Error bars represent the standard error of the fit.