No plant can survive for very long in the dark. One low-tech way to kill an unwanted plant, therefore, is to deprive it of
light. Gardeners who want to convert a lawn into a garden, for example, might kill the grass by covering it with layers of newspaper or cardboard for several weeks. The light reactions of photosynthesis cannot occur in the dark; the plants die.
Another way to kill plants is to apply chemicals called herbicides. Some of these plant poisons also stop the light reactions. For example, a weed killer called DCMU (also called diuron) blocks electron flow in photosystem II. Paraquat, noted for its use in destroying marijuana plants, diverts electrons from photosystem I. Either way, blocking electron flow prevents the production of ATP and NADPH. Without these critical products, photosynthesis cannot continue.