Among the many properties that tannins play in the physiology of plants and trees that synthesise them, is their ability to protect the plant from biotic and abiotic stress. The presence of tannins in the plant tissues is important because these compounds are efficient repellents against predators and parasites.
Tannins are powerful antibiotics that can defend the plant tissues from putrefaction of fungal origin due to their inhibitory activity against hydrolytic enzymes such as cellulase, pectinase, xylanase. These are used by pathogens to penetrate plant cells and their action on the cellular membranes of pathogens to inhibit oxidative phosphorylation. This is possible because of the antioxidant activity tannins possess, due to their capacity to act as oxidisable substrates and free radical scavengers to protect cellular constituents by oxidation.
The toxicity of tannins in relation to many fungal pathogens is well documented. In vitro tests have shown no significant differences between the toxicity of hydrolysable tannins compared to condensed tannins.
Tannins, in particular ellagitannins, are endogenous inhibitors of the growth of numerous species of pests. They act as an antibiotic substance or as an anti-nutritional deterrent against insects and aphids. This function is correlated with the toxicity that tannins may have because of their capacity to complex the proteins and decrease the digestibility of the food. This causes negative effects on the endogenous metabolism of insects. Another aspect of its deterrent power is the effect tannins have on taste. They cause the flavour to be astringent while hardening the tissues of the plant.
An example of this is the ellagic acid strongly inhibits the growth of the tobacco budworm Heliothis virescens. Similarly, ellagitannins have proven to be very effective to combat the infestations of different insect species, such as Schizaphis graminum and Myzus persicae, while the condensed tannins are efficient against the Gossypium hirsutum.