New HPPD-Inhibitors – A Proven Mode of Action as a New Hope to Solve Current Weed Problems
HPPD herbicides are one of the two types of 'bleaching' herbicides being inhibitors of hydroxyphenylpyruvate dioxygenase. Today it is known that inhibition of the HPPD enzyme stops the catabolic degradation of tyrosine to plastoquinones (important for photosynthesis and carotenoid biosynthesis) and tocopherol (vitamin E). HPPD inhibitors stop the catabolic degradation of tyrosine to plastoquinones (important for photosynthesis), restrict the weed's ability to produce vitamin E, which protects biological membranes against oxidative stress and the photosynthetic apparatus against photo-inactivation and prevent carotenoid biosynthesis. This results in the destruction of chlorophyll and the weed turns white. Although the mode of action of HPPD-inhibitors is a very powerful one, the first rice HPPD herbicides were not very active in the field requiring high application rates of several kilograms per hectare for effective weed control. The main reason for such very high rates was because the active ingredient is released, for example by hydrolysis, over a long period of time. This is an important factor especially for the Japanese rice market, where season-long weed control with a herbicide is highly desired. This would not be achievable with the more polar and more water-soluble HPPD-inhibitor itself. As more became known about these inhibitors new chemistries were invented that increased the biological effects and broadened the range of crops on which the herbicides could be used. The discovery of pyrazolate, pyrazoxyfen, benzofenap, sulcotrione, isoxaflutole, benzobicyclon, mesotrione, topramezone, tembotrione and pyrasulfotole and the improved biological efficacy are described.
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Document Type: Research Article
Publication date: 01 February 2009
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