Evolution of Polyphenolic Compounds

Lignin plays an enormously important role in determining the utility of cellulose as described above, by adding strength to it as well as by protecting it from pathogens. Lignin, however, also plays a number of different roles in plants and has done so through time. It is thought that the aromatic amino acids, which are the precursors to lignin, as well as tannins and flavonoids, evolved in aquatic algae as protectants against high UV-B radiation (Rozema et al, 1997; Lowry et al., 1980). Subsequently, derivative phenolic-acid products provided protection against microbial predation in terrestrial nonvasuclar plants and finally with the evolution of land plants, polyphenolics such as tannins and lignins provided further protection from herbivores and microorganisms, respectively, as well as mechanical strength as noted above. Also derived from the same phenolic pathway was the large class of compounds known as flavonoids, which have multiple roles, including plant-insect interactions (Rozema et al., 1997).

The consequences of the evolution of lignin to the operation of the carbon cycle have been considerable. Lignin is one of the most refractory organic compounds produced by plants. It resists decay partly because it is insoluble and it has a very high C/N ratio. Lignin and lignin-degradation products inhibit the breakdown of complex carbohydrates. Complexes resulting from lignin breakdown can persist for thousands of years in aerobic soils (Robinson, 1990).

Robinson (1990) traces the evolutionary history of lignin. There was no lignin in plants in the Ordovician. Ligninlike compounds were found in some of the first vascular plants in the Silurian. She estimates that the content of lignin in these small plants was comparable to that of herbs today (10-15%). Subsequently, lignification rose to 40%) in the Late Devonian, dropped to 30-35%) at the end of the Mesozoic, and has subsequently declined to an average of about 20%).

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