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

Abstract

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.

Severe simulated herbivory constrains Sclerocarya birrea saplings regardless of resource availability

“…Our results corroborated the findings of other studies of savanna woody plant responses to browsing that have shown that compensatory or over‐compensatory shoot production is a common response to various intensities of browsing (Scogings & Gowda 2020). However, because reduced stem diameter can be indicative of C exhaustion (Hester et al .…”

“…The instrument has been successfully used to determine chemical responses of woody plants to browsing (Pellisier 2013). It has been successfully calibrated for indirectly measuring condensed tannin concentrations (acid-butanol assay, Sorghum tannin standard) in some savanna woody species including S. birrea (Scogings et al 2014) and has been successfully used to determine S. birrea chemical responses to nutrient fertilisation (Scogings 2018).…”

Ethnobotany, Phytochemistry, Ethnopharmacology, and Toxicity of Euclea divinorum Hern (Ebenaceae): A Review

“…The roots, leaves, and other parts of E. divinorum showed high tannin contents (36,37), which constitutes an inherent defense mechanism against herbivory of species in the Euclea genus (10). They are therefore widely used in Tanzania, Zimbabwe, Kenya, Botswana, and Yemen as purple ink, black dye for mats, wool, for tanning leather, basket ware, and other textiles, especially when a mordant of iron-rich mud is used after dyeing (11,(38)(39)(40)(41)(42).…”

Evolutionary response to coexistence with close relatives: increased resistance against specialist herbivores without cost for climatic‐stress resistance

“…These measures are non-destructive and much more rapid than quantifying the within-leaf concentrations, but nevertheless strongly correlated to within-leaf concentrations (Goulas et al 2004). Moreover, anthocyanins and flavonols reflect well overall polyphenol concentrations under comparable conditions (Scogings 2014). We measured three leaves per tree, between leaf veins on both leaf faces.…”