Distribution and Structural Variations within Plants

The tannin-containing vacuole may become very large and has been observed to occupy over 90% of the volume of Picea glauca cells (18) and also of tannin-containing cells in Pinus strobus needles (75). Tannins do not appear to be present in chloroplast cells and, in the case of Douglas-fir callus cell suspension cultures, some sort of metabolic balance appears to be struck between the chloroplast and tannin-producing cells. Too rapid production of the latter leads to death of the culture (H. A. Stafford, personal communication). Such specialized tannin cells have also been observed in Vitis vinifera phloem tissue and Lotus pedunculatus leaf and root tissues. Such a pattern may be the norm in the plant kingdom. 

Condensed tannins, when present in a woody plant, may not always exhibit a constant structure throughout the plant. There are a number of examples of this phenomenon known Acacia species commonly elaborate both 5-oxy and -deoxy-flavanoid tannins (for example, profisetinidins and procyanidins) in the wood (34) and bark (140), and exclusively 5-oxy-flavanoids and associated flavanoids (for example, quercetin glycosides) in the leaves (20, 138). Other examples include many Ribes species that commonly contain prodelphinidins. These are predominantly of the gallocatechin-4 (6) type in the leaves, but with the contrasting epigallocatechin-4 (3) stereochemistry in the fruits (37). Further, the tannins of Pinus radiata display contrasting structures between the bark in which 2,3-trans stereochemistry, catechin-4 (5), procyanidin polymers predominate, and tannins in the phloem, needles, male cones, and winter buds, in which a variety of mixed procyanidin and prodelphinidin polymers exist with predominantly 2,3-c/5 stereochemistry (Thble 7.7.5). 

The above observations tend to suggest that more than one metabolic pool for the production of proanthocyanidins may exist in a plant and these in some cases are localized in different tissues. There is some evidence for this in Stafford s observations on procyanidin biosynthesis in Douglas-fir callus tissue (130, 131). Here quite high concentrations of dimers and trimers based on catechin-4 (5) units are found, but the associated polymer is based almost entirely on epi-catechin-4 (2) units, and evidence was obtained that the latter are formed much more rapidly than the catechin-4 oligomers (131). Thus the main biosynthetic process appears to be directed towards the synthesis of epicatechin-4 units that are diverted to polymers, and it seems likely that the two types of unit may be derived from distinct metabolic pools. 

These observations may explain why, in surveys by Foo and Porter, of a wide range of plants reported to be rich sources of the dimers catechin-(4a- 8)-catechin and catechin-(4a- 8)-epicatechin, it was observed almost without exception that the polymers were based predominantly on epicatechin-4 units (37). 

The biosynthesis of flavonoids from the malonate/shikimate level to dihydroflav-onols is now well established (Fig. 7.7.5). Many of the enzyme systems responsible for the transformations have been isolated and the reactions demonstrated in cell-free media (28). 

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