Effects of Safeners on Herbicide Metabolism

With few exceptions, herbicides are subject to metabolic transformations both in weed and crop species, after they have penetrated the plant tissue and are under way to their target site. As a rule, the herbicide metabolites are more polar than the herbicidal parent compound, and they exhibit reduced phytotoxicity or are completely non-phytotoxic. While often the first step of herbicide metabolism entails a partial or total detoxification of the parent compound, there are other cases where the herbiddaDy active form is generated in the first metabolic reaction (e.g., the hydrolysis of the inactive fenoxaprop-P-ethyl to the herbicidally active free add fenoxaprop-P) followed by detoxification of the molecule in the subsequent metabolic step. 

These chemically diverse safeners all need to be applied to the crop (maize, sorghum) by seed dressing to obtain the selective safener effect. The oldest and best 

NA had also a stimulatory effect on the oxidative metabolism of the herbicide bentazone. Microsomal preparations of etiolated shoots from maize, which had received a seed treatment with NA, showed activity of a bentazone hydroxylase, which was not detectable in extracts from controls without safener pre-treatment [35]. Also, the improved tolerance of maize to the imidazolinone AC263222 after NA seed treatment could be related to enhanced AC 263222 hydroxylation by stimulation of a cytochrome P450 monooxygenase [31]. 

Seed treatment of wheat with fluxofenim increased GST activity nine-fold, when assayed with the herbicide dimethenamid as a substrate. This increase correlated well with accelerated herbicide metabohsm in wheat shoots, which was observed as a response to fluxofenim treatment [38]. 

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