available primarily as a solution in various strengths. It is classified as "restricted use", which means that it can be used by licensed applicators only. The estimated use of paraquat in US agriculture is mapped by the US Geological Survey and shows a doubling from 2013 to 2018, the latest date for which figures are available, and now reaching annually. There is an ongoing international campaign for a global ban, but the cheap and popular paraquat continues to be unrestricted in most developing countries. A small group of countries, including India and Guatemala and supported by manufacturers, have blocked the listing of paraquat as a hazardous chemical for the purposes of the Rotterdam Convention. Reactivity and mode of action Paraquat is an oxidant that interferes with electron transfer, a process that is common to all life. Addition of one electron gives the radical cation: [paraquat]2+ + e− [paraquat]•+ The radical cation is also susceptible to further reduction to the neutral [paraquat]0: [paraquat]•+ + e− [paraquat]0 As an herbicide, paraquat acts by inhibiting photosynthesis. In light-exposed plants, it accepts electrons from photosystem I (more specifically ferredoxin, which is presented with electrons from PS I) and transfers them to molecular oxygen. In this manner, destructive reactive oxygen species (ROS) are produced. In forming these reactive oxygen species, the oxidized form of paraquat is regenerated, and is again available to shunt electrons from photosystem I to restart the cycle. This induces necrosis, and unlike with some mechanisms of necrosis, does not produce double-stranded breaks. Paraquat is often used in science to catalyze the formation of ROS, more specifically, the superoxide free radical. Paraquat will undergo redox cycling in vivo, being reduced by an electron donor such as NADPH, before being oxidized by an electron receptor such as dioxygen to produce superoxide, a major ROS. Weed resistance management Problems with herbicide resistant weeds may be addressed by applying herbicides with different modes of action, along with cultural methods such as crop rotation, in integrated weed management (IWM) systems. Paraquat, with its distinctive mode of action, is one of few chemical options that can be used to prevent and mitigate problems with weeds that have become resistant to the very widely used non-selective herbicide glyphosate. One example is the "double knock" system used in Australia. Before planting a crop, weeds are sprayed with glyphosate first, then followed seven to ten days later by a paraquat