PARENT SESSION
Oral Session #68: Nutrient Cycling. Presiding: D. Johnson.
Thursday, August 9, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.
Tannins: the complexity of chemistry and structure.
Kraus, Tamara1, Yu, Zengshou1, Preston, Caroline2, Zasoski, Robert1, Dahlgren, Randy1, 1 2
ABSTRACT- The importance of tannins in ecosystem function and nutrient cycling has long been recognized. Tannins are estimated to be the fourth most abundant biochemical following cellulose, hemicellulose and lignin. To determine the role of tannins in ecological processes, it is important to accurately quantify their abundance and understand how structural properties affect reactivity. Our previous work along the Ecological Staircase on the northern California Coast showed that extractable polyphenols and condensed tannins in conifer foliage increase as soils become more acidic and infertile. In this study, purified tannins from the foliage of nine species growing on the Ecological Staircase were examined for chemical reactivity, protein precipitation capacity (PPC) and structural characteristics (13C NMR). Foliar tannin concentrations ranged between 10 and 25% for the nine species. Reactivity of the purified tannins varied among species 7-fold for the butanol assay, 3-fold for the vanillin assay and 1.5-fold for the Folin-Ciocalteau assay. There was a 5-fold difference in PPC. Variation in chemical reactivity and PPC can be partially explained by differences in structural characteristics of the tannins determined by 13C NMR. In particular, the content of condensed versus hydrolyzable tannin, as well as whether the condensed tannins are composed of procyanidin or prodelphinidin units, influences reactivity. Our results show that structural characteristics of tannins play an important role in regulating their reactivity in ecological processes.
KEY WORDS: tannin, polyphenol, 13C NMR, protein precipitation