Tannins: Does Structure Determine Function? An Ecological Perspective

Abstract

Tannins are phenolic compounds of widespread occurrence in higher plants. They have complicated structures that have only recently been unraveled by natural products chemists. They are not known to have any physiological function. So what do they do? Examination of their structures and consideration of several possible roles, especially as defensive compounds against plants' enemies, allows much speculation. It is proposed that the enormous structural heterogeneity and natural division of tannins into two broad classes, the condensed and hydrolyzable, has important ecological consequences. It is argued that their structures suggest strong specificities for a variety of target molecules including proteins, digestive enzymes, and polysaccharides. These specificities insure mainline defense against plant enemies, but are nevertheless subject to the coevolutionary "arms race" (Feeny 1976). The idea that condensed tannins are reserved primarily for defense against microbes and pathogens, while the hydrolyzable tannins protect the plant against herbivores is discussed. What structure suggests about function, and how these ideas fit into the current scheme of antiherbivore chemistry, evolution of the arms race between plants and their parasites, and ecological theory is considered.