Assessing the Effects of Alloxydim Phototransformation Products by QSAR Models and a Phytotoxicity Study

Abstract

Once applied, an herbicide first makes contact with leaves and soil. It is known that photolysis can be one of the most important processes of dissipation of herbicides in the field. However, degradation does not guarantee detoxification and can give rise to byproducts that could be more toxic and/or persistent than the active substance. In this work, the photodegradation of alloxydim herbicide in soil and leaf cuticle surrogates was studied and a detailed study on the phytotoxicity of the main byproduct on sugar beet, tomato, and rotational crops was performed. Quantitative structure⁻activity relationship (QSAR) models were used to obtain a first approximation of the possible ecotoxicological and environmental implications of the alloxydim and its degradation product. The results show that alloxydim is rapidly degraded on carnauba and sandy loam soil surfaces, two difficult matrices to analyze and not previously studied with alloxydim. Two transformation products that formed in both matrices were identified: alloxydim Z-isomer and imine derivative (mixture of two tautomers). The phytotoxicity of alloxydim and the major byproduct shows that tomato possesses high sensitivity to the imine byproduct, while wheat crops are inhibited by the parent compound. This paper demonstrates the need to further investigate the behavior of herbicide degradation products on target and nontarget species to determine the adequate use of herbicidal products to maximize productivity in the context of sustainable agriculture.

Keywords: degradation; herbicide; phytotoxicity; quantitative structure–activity relationship; transformation products.

Figure 1

Kinetic evolution of the photodegradation products (open symbols and dotted lines) formed during the irradiation of alloxydim (filled symbols and solid line) on (a) carnauba wax films and (b) sandy loam soil.

Figure 2

Representative HPLC-DAD chromatogram of alloxydim photodegraded on carnauba wax films and sandy loam soil under simulated sunlight (example on carnauba at irradiation time of 1 h).