Plant secondary metabolites are a diverse group of molecules that are
involved in the adaptation of plants to their environment but are not part of the
primary biochemical pathways of cell growth and reproduction. In general, the
terms plant secondary compounds, phytochemicals, antinutritional factors,
and plant xenobiotics have been used in the literature to refer to this group of
compounds. There are well over 24,000 structures, including many compounds
that have antinutritional and toxic effects on mammals. This number does
not include the oligomeric polyphenolic compounds (proanthocyanidins and
hydrolyzable tannins) that are just now being more accurately described
and will increase the number by several thousand. Some major plant secondary
metabolites or phytochemicals that occur in plants include protease inhibitors,
lectins, alkaloids, nonprotein amino acids, cyanogenic glycosides, saponins,
and tannins. These compounds are involved in defense against herbivores and
pathogens, regulation of symbiosis, control of seed germination, and chemical
inhibition of competing plant species (allelopathy), and therefore are an integral
part of the interactions of species in plant and animal communities and the
adaptation of plants to their environment.