Phytotoxic substances may result from human activity, as with herbicides, or they may be produced by plants, by microorganisms, or by naturally occurring chemical reactions.
Most examples pertaining to this definition of phytotoxin are members of various classes of specialised or secondary metabolites, including alkaloids, terpenes, and especially phenolics, though not all such compounds are toxic or serve defensive purposes.
[4] They are medically important by interfering with components of the nervous system affecting membrane transport, protein synthesis, and enzyme activities.
Terpenes are made of water-insoluble lipids, and synthesized from acetyl-CoA or basic intermediates of glycolysis[5] They often end in -ol (menthol) and comprise the majority of plant essential oils.
Lignins, also a group of phenolics, are the most common compounds on Earth, and help conduct water in plant stems and fill spaces in the cell.