A group of organic compounds based on a heterocyclic ring containing three carbon atoms and three nitrogen atoms and including several commonly used agricultural herbicides, such as simazine and atrazine. The triazine herbicides work by inhibiting electron transport of photosynthesis. They are absorbed primarily through the plant root and bind to a particular site in photosystem II, thereby preventing binding of plastoquinone, a key electron acceptor in the electron transport chain. Certain crop plants are resistant to triazines—for example, maize roots contain an enzyme that renders them inactive—hence their usefulness as selective weedkillers. Attempts have been made to introduce the resistance gene into other crop species, to broaden the applicability of triazine herbicides. However, these efforts have been hampered because the target gene is part of the chloroplast genome, not the nuclear genome. Also, many weeds have now evolved triazine resistance.