Photoisomerization and degradation dynamics of (E/Z)-pyribencarb under UV irradiation: identification of antifungal activity, and implication of ZnO and quercetin as UV absorbers in a greenhouse

Abstract

Pyribencarb has two diastereomeric isomers, and these would be isomerized under sunlight photochemically; however, the antifungal activity of each diastereomer was not revealed, and the isomerization and degradation under sunlight were not characterized in terms of dynamics in detail. (E)-and (Z)-pyribencarbs were evaluated for their antifungal activities against Botrytis cinerea, Fusarium oxysporum, and Monilinia fructicola. The (E)-isomer showed a potent antifungal activity (MICs 0.025-0.05 mg kg(-1)), but not the (Z)-isomer (MICs > 0.25 mg kg(-1)). For the characterization of the degradation and diastereomeric isomerization, each of the isomers was treated with UVA, UVB, and UVC. Under the UVC irradiation (5.0 W m(-2)), pyribencarbs were degraded photochemically, and the kdegs were 0.0068-0.0089 min(-1) in aprotic solvent and 0.0151-0.0180 min(-1) in protic solvent. The photoinduced isomerization of pyribencarbs was found under all UVs, and the isomerization rate constants (kis) were 0.0529-0.0593 min(-1) under UVB (6 W m-2), and 0.4953-0.5466 min(-1) (5.0 W m-2) under UVC; the equilibrium ratios of (E/Z)-pyribencarb were 1:4 under UVB and 2:3 under UVC, thus finally the (Z)-isomer was preferentially accumulated under UVs. The organic UV absorber, quercetin, effectively inhibited the isomerization of (E)-pyribencarb; kis were decreased from 0.0524 to 0.0103 min(-1) at 200 mg L-1 of quercetin under UVB, and from 1.2040 to 0.1706 day(-1) under sunlight. On the other hand, ZnO catalyzed the degradation of pyribencarbs (kdeg 0.0072 min(-1), 0.1% ZnO) but inhibited the isomerization (k(i) 0.0058 min(-1)) under UVB. In addition, the 50% residue dissipation time of pyribencarb in eggplant in the greenhouse was 3.8 days, and the photochemical degradation and the crop dilution effects were suggested to be the main factors for the dissipation in the fruit. Furthermore, the equilibrium ratio of the (E/Z)-isomers was identified as 3:2 in the greenhouse. In conclusion, the photoinduced isomerization to (Z)-pyribencarb could reduce the antifungal efficacy, and quercetin was expected to be an isomerization inhibitor, whereas ZnO would be suggested as a residue reduction agent.