Phenylurea, triazine and diazine represent economically very important compounds since they are used in chemical, pharmaceutical and agricultural industries. Although new herbicides are now available, they still represent the basic products for weed control. All the compounds of these classes constitute about 40 % of all herbicides used at present in agriculture, amounting to thousands of tons all over the world. These compounds are absorbed through the roots and then translocated via the xylem to the leaves. Some of these herbicides are directly absorbed by the leaves. They act by inhibiting photosynthesis at the level of the photosystem 11-dependent electron transfer and further block production of ATP and NADPH. In soils, these compounds are quite persistent and they are adsorbed on soil colloids and on organic substances in proportion to the cation exchange capacity of these soil constituents.
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