Tannin assays in ecological studies Precipitation of ribulose-1,5-bisphosphate carboxylase/oxygenase by tannic acid, quebracho, and oak foliage extracts

Abstract

Tannic acid and quebracho precipitate many times their weight of the abundant leaf protein, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPC). The use of this protein in protein precipitation assays for tannin content is described. Extracts of mature foliage of pin, bur, and black oak precipitate 2.01, 0.69, and 0.09 mg RuBPC/ mg (dry wt) of leaf powder extracted, respectively, at pH 6.1. From these measurements it can be calculated that all three of these oak species have sufficient tannins to precipitate all of the RuBPC present in their foliage. At mildly alkaline pHs, however (pH ⩾ 7.5), RuBPC is not precipitated by tannins. Since RuBPCis the most abundant protein present in photosynthetic tissues, often constituting as much as 50% of the soluble proteins and 25% of the total proteins in leaf tissue, the interactions of this protein with tannins are highly relevant to an evaluation of the role of tannins as antiherbivore, digestibility-reducing substances. Our measurements provide no basis for arguing that differences in tannin levels in different species reflect differences in the digestibility of leaf proteins or that tannins have any effect whatsoever upon the digestibility of leaf protein under conditions which normally prevail in most insects' guts. These findings emphasize the need to test more of the assumptions underlying contemporary interpretations of the importance of tannins in plant herbivore interactions.