Q. What is paraquat’s mode of action in the plant?
A. Paraquat acts in the presence of light to desiccate the green parts of all plants with which it comes into contact. After application, penetration through the leaf surface occurs almost immediately. This absorption is increased by high light intensity, humidity, and by the specific adjuvant system usually built into the formulation, which ensures good spray retention and wetting of target foliage. Tank mix adjuvants are required if not built into the formulation. The site of action for paraquat is in the chloroplast. Chloroplasts contain the photosynthetic systems of green plants, which absorb the light energy used to produce sugars. Paraquat is known to act on the photosynthetic membrane system called photosystem I, which produces free electrons to drive photosynthesis. The free electrons from photosystem I react with the paraquat ion to give the free radical form. Oxygen rapidly reconverts this free radical and in that process produces super oxides. Chemically highly reactive, the super oxides attack unsaturated membrane fatty acids, rapidly opening up and disintegrating the cell membranes and tissues. The paraquat ion/free radical process then recycles producing further quantities of super oxide until the supply of free electrons ceases. Visible wilting of treated plants is apparent within hours under warm, bright conditions, but may take longer under dull, cool conditions. This is quickly followed by the appearance of brown, desiccated or chlorotic tissue. Light, oxygen and chlorophyll are therefore all required for the rapid and characteristic herbicidal effects of paraquat. It is the rupturing of the cell membranes allowing water to escape from the plant material that leads to the rapid desiccation of the foliage.