Abstract
Phenolic acids and condensed tannins have been implicated in plant resistance to insects and diseases. The objective of this study was to determine phenolic acid and condensed tannin concentrations in six cotton ( Gossypium hirsutum L.) genotypes that display varying degrees of spider mite [ Tetranychus urticae (Koch)] resistance. Since phenolic acids are precursors for condensed tannins in the shikimate pathway, the levels of these compounds should be positively correlated. Leaf blades from whole cotton plants grown in a controlled environment were analyzed using HPLC techniques at two growth stages (pinhead-square and first-bloom) for concentrations of 13 tentatively identified phenolic acids, while condensed tannin concentrations were determined spectrophotometrically from extracts of the topmost unfolded leaf and the fourth mainstem leaf from the terminal. Caffeic, p-coumaric and salicylic acids occurred at high concentrations relative to the other phenolic acids. Gallic, gentisic, protocatechuic, benzoic and ferulic acids occurred at intermediate concentrations; p-hydroxybenzoic, vanillic, syringic, sinapic and cinnamic acids occurred at low concentrations. Genotypes that exhibited higher levels of condensed tannins also had significantly higher levels of total phenolic acids, syringic acid and p-coumaric acid concentrations. Additionally caffeic, p-hydroxybenzoic, syringic, p-coumaric and benzoic acids and total phenolic acids were highly correlated with condensed tannin concentrations. Phenolic acid concentrations decreased, while condensed tannin concentrations increased, from pinhead-square to first-bloom growth stages in all genotypes. While genotypes displaying some degree of resistance to the spider mite may have been selected for elevated condensed tannin levels, these genotypes also may have been inadvertently selected for high phenolic acid levels. Therefore, the observed resistance in these genotypes may be attributed to high levels of not only condensed tannins, but also phenolic acids.
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