Transformation kinetics of 6-methoxybenzoxazolin-2-one in soil
Wheat (Triticum aestivum L.) and other cereals produce allelochemicals as natural defense compounds against weeds, fungi, insects and soil-borne diseases. The main benzoxazinoid allelochemical of wheat is 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), bound as β-glucoside and released upon plant injury. When leached from wheat to soil, DIMBOA is microbially transformed to 6-methoxy-benzoxazolin-2-one (MBOA). Exploiting benzoxazinoids and their degradation products as substitutes for synthetic pesticides depends on knowledge of transformation pathways and kinetics. In an MBOA degradation experiment at a concentration of 2400 nmol g–1 soil, the previously identified transformation products 2-amino-7-methoxy-phenoxazin-3-one (AMPO) and 2-acetylamino-7-methoxy-phenoxazin-3-one (AAMPO) were quantified. Three different kinetic models were applied to MBOA transformation kinetics; single first-order (SFO), first-order multi-compartment, and double first-order in parallel. SFO proved to be adequate and was subsequently applied to the transformations of MBOA, AMPO and AAMPO. Degradation endpoints, expressed as degradation time (DT), were calculated for MBOA, AMPO and AAMPO to test whether the maximum values for synthetic pesticides set by the European Commission and the Danish Environmental Protection Agency were exceeded. DT50 values for MBOA and AMPO were 5.4 d and 321.5 d, respectively, and DT90 values were 18.1 d and 1068 d, respectively. The DT50 value for AMPO exceeded the maximum value. The persistence, concentrations and toxicity of metabolites such as AMPO should be considered when breeding cereal crops with increased levels of benzoxazinoids.