Tannins — Their Place in Plant Metabolism

Terrestrial vascular plants synthesize, in addition to structural polymers like cellulose and lignin, a rather bewildering array of metabolic products, such as lignans, phenolic acids, tannins, alkaloids, terpenoids etc. Excluding the structural polymers, the functions of many of these compounds (i.e., so-called secondary metabolites) are not well understood. This chapter presents an overview of the metabolism of phenylpropanoids, particularly hydrolyzable and condensed tannins, and then reexamines these pathways in the context of their relationships to general plant metabolism. Calculations show that the cost of diversion of biochemical energy of living plants into tannins is high compared to energy requirements for the structural cell wall polysaccharides. Hence, in many high-tannin-content plants, tannin synthesis contributes significantly to captured photosynthetic energy usage. Phenylpropanoid (hence, tannin) synthesis is also particularly important to nitrogen recycling.