Research NoteCalibration of a hand-held instrument for measuring condensed tannin concentration based on UV- and red-excited fluorescence

Condensed tannins are ecologically important compounds in plants for several reasons, e.g. as herbivore deterrents or protecting photosynthetic tissues against ultraviolet radiation. The hand-held Dualex Series 4 instrument was originally developed for indirect determination of flavonol concentrations in crops. Flavonols are low-molecular-weight phenolics from which condensed tannins are synthesised. Therefore, it was expected that the Dualex’s ‘flavonol index’ (range 0.00–3.00) could be used to predict condensed tannin concentrations, providing a rapid, cost-effective, non-destructive method to sample more species and more plants per treatment than destructive methods used for wet-chemistry tannin assays. Thirteen broad-leaved woody species were sampled at the Nkuhlu and Letaba exclosures in Kruger National Park during December 2011. Four plants were sampled per species by collecting 100 leaves per plant, measuring the flavonol index of each leaf and pooling them in classes of 0.10 flavonol units. Condensed tannin concentrations were determined according to the acid-butanol assay with Sorghum tannin as standard. Regression models (condensed tannin = a*flavonol index – b) were significant for only four species: Acacia nigrescens (Senegalia nigrescens), Euclea divinorum, Spirostachys africana and Combretum zeyheri. Other than high variation in chemistry among plants, poor calibration results were likely caused by high chemical variation within leaves.Keywords: Dualex, flavonol, herbivore, Multiplex, secondary metabolite African Journal of Range & Forage Science 2014, 31(1): 55–5