Bitter problems in ecological feeding experiments: commercial tannin preparations and common methods for tannin quantifications

• In the field of plant-herbivore interactions research methods manipulating plant secondary metabolites are becoming more and more popular. In most cases food offered to herbivores is manipulated with commercially available compounds, or more specifically, compound mixtures. Among the most commonly used is tannic acid, the almost sole commercial source of hydrolysable tannins. However, recent studies have shown that different tannic acid preparations are not comparable in their tannin structures. While tannic acids are meant to contain only gallotannins (GTs), some commercial preparations compose mainly of more simple galloglucoses (that have e.g. much lower protein precipitation capacity than GTs) or even of gallic acid (the hydrolysis product of GTs). Another commonly used group of tannins used in feeding trials is condensed tannins (CTs), usually in the form of quebracho tannin. One of the problems in using quebracho is that it contains different CT structures than e.g. leaves of many deciduous trees. Additionally, when analysed with the common acid-butanol assay for total CTs, quebracho tannins give even 30-fold lower absorbance than the CTs of those deciduous trees. This dilemma is highlighted by the fact that quebracho tannin is commonly used as a standard when analysing CT concentrations in plant tissues. In addition to above problems, different herbivore species are known to react differently to varying tannin structures. Therefore, ecologically the most serious concern lies in that our commercial tannin compounds might have biological activities highly different from the tannins of our study plants. Below we discuss these problems, as well as some means to deal with them.

Nyckelord

hydrolysable tannins

condensed tannins

plant-herbivory interaction

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