The functions of tannins
There are minor tannin constituents in fungi, algae and mosses. The tannins are present in both flowering and seed producing plants, in particular in many Dicotyledons. We can find these substances in significant amounts, so much so that these species constitute the source of material for the production of many commercial vegetable extracts including chestnut, quebracho, Tara and many others.
The deposit of tannins can occur in any type of plant tissue. Different parts of the plant contain different amounts of tannin, for example:
- In the roots: tannin is mainly found in the hypodermis, under the epidermal layer where it acts as a protection against pathogens
- In the trunk: tannin is present in the areas of active growth, such as the secondary phloem, the xylem and in the layer between the bark and the epidermis, where it regulates the growth of the tissues
- Inside the seeds: tannin is located in the layer between the external tegument and the aleuronic layer contributing to the maintenance of dormancy
- In the fruits and leaves: tannin provides an astringent flavour which reduces the appetite of herbivorous animals and insects and thus represents a natural line of defence
Other functions are related to the important role tannins play in the physiology and development of the seeds and in the activation of the nodulation gens which favour the fixation of nitrogen in the plants and in the attraction of pollinating insects (Debeaujon et al., 2000; Sagan et al., 1995).