Carbon based secondary metabolites in African savanna woody species in relation to anti-herbivore defense
Chemical defense in African savanna woody species have been inferred from foliar condensed tannin and total phenolic concentrations. However these measures are controversial since condensed tannin chemical heterogeneity compromises its simple quantification and total phenolics, which is a measure of general foliar phenolics, includes many different low molecular weight phenolics. Some of these low molecular weight phenolics have been shown to deter herbivory individually or in combination with others. The growth differentiation balance hypothesis predicts a trade-off between costs of chemical defenses or carbon based secondary metabolites relative to the demand for photosynthate by growth. However, this hypothesis was developed in northern boreal and temperate forests and the extent to which it applies to African savanna woody species remains unclear. In this study we: 1) Compared the use of Quebrachowith Sorghumtannin as standards for condensed tannin quantification, relative to absolute condensed tannin concentration in selected species; 2) Determined low molecular weight phenolic profiles of 6 species with different life history, morphological and functional traits; and 3) Investigated whether the growth differentiation balance hypothesis explains allocation to carbon based secondary metabolites in Combretum apiculatumalong a 6-level N gradient; and following 100% simulated herbivory along the N gradient.Condensed tannins were extracted from 5 tree species and purified. Condensed tannin concentrations were determined using the acid-butanol assay and expressed as Quebracho, Sorghumand absolute tannin concentrations. Chemical composition of different tannin polymers were determined by thiolysis using HPLC and HPLC-MS. Photosynthesis, growth variables andphenotypic phytochemical responses to the different N and N x herbivory treatments were measured in C. apiculatum. Low 7molecular weight phenolics were determined and identified using HPLC and HPLC-MS respectively.Results from this study suggest: that thecontinued use of Quebrachotannin as condensed tannin standard in the acid-butanol assay is unjustified; that generalizations based on life history, morphological and functional traits may be misleading since foliar low molecular weight phenolics are not necessarily related to these traits; and that the growth differentiation balance hypothesis does not explain allocation to carbon based secondary metabolites in C. apiculatum.
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