Seasonal Variation and Fate in Senescent Tissues

A number of useful observations have been made on the variation of extractable tannin concentrations in leaves and fruits. These show that in a species with an annual cycle of growth, tannin levels are generally low in newly formed leaves and concentrations peak in late summer/early autumn. The decrease in the level of extractable tannins is often attributed to a continuous process of increasing condensation. A number of explanations are possible  

It is still not known whether tannins are actively metabolized by the plant or are metabolic endproducts. Current evidence favors the view that flavonoids in general are turned over by the plant (6) and, if this is the case for tannins, then the decrease in concentration may merely be due to a lowering of their rate of production. Haslam has suggested that proanthocyanidih production may be limited by the reductive capacity of a plant (56). Thus, late in the season proantho-cyanidin production, a net reductive process, is diverted to anthocyanidin production, which accounts in part for the autumnal reddening of leaves (56). There is some support for this as willow stems are a rich source of procyanidins in summer, but in winter only cyanidin and catechin may be extracted (L. J. Porter, unpublished observations). 

Senescence in leaf tissue leads to a general desiccation of the organ and consequent breakdown of the cells. Possibly proanthocyanidins are further polymerized by the native oxidase enzymes of the leaf, and hence rendered insoluble. 

Hathway has shown that oak bark contains large concentrations of oxidase enzymes and demonstrated their effect on the flavan-3-ols which occur in oak bark (59). Ahn and Gstirner have reported the presence of oxidatively coupled flavan-3-ol dimers in oak bark, in addition to the normal proanthocyanidin dimers (2). Such products are consistent with the presence of oxidase/peroxidase enzymes. Such secondary processes would explain the very high dispersitivities observed for Firms sylvestris tannins, compared with values for tannins isolated from living plant tissue (R. Marutzky and L. J. Porter, unpublished observations). It would also account for the extremely high MW tannin-like material isolated by Yazaki and Hillis from Pinus radiata bark methanol extracts by ultrafiltration (145). 

More recently, Roux and co-workers (149) have isolated a complex array of oligomeric polyphenols based on (2jR,3S)-3,3, 4, 7,8-pentahydroxyflavan from the heartwood of mesquite (Prosopis glandulosa). These include some oligomers linked by normal proanthocyanidin interflavanoid linkages (i.e. 4- 6), but predominantly the form of linkage has arisen from oxidative coupling to form biphenyl and m-terphenyl interflavanoid bonds. These are just the sort of bonds expected to be formed in secondary processes in any proanthocyanidin polymer, and it is still an open question as to the extent of these processes in outer bark and heartwood. 

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