Aspen knots, a rich source of flavonoids

Hydrophilic extractives in heartwood, sapwood, and knots of three aspen species were analyzed by gas chromatography, gas chromatography-mass spectrometry, and high-performance size-exclusion chromatography. The amounts of flavonoids were considerably higher in the knots relative to the stem wood. Flavonoids identified were dihydrokaempferol, naringenin, kaempferol, catechin, and taxifolin. In addition, glycosides of dihydrokaempferol, naringenin, and kaempferol were identified by mass spectrometry and shown to be glucosides by enzymatic hydrolysis by β-D-glucosidase. Dihydrokaempferol and its glucoside dominated in all knot samples although there were variations in the amounts and composition of the extractives. The total amounts of flavonoids varied between 11 and 43 mg/g in Populus tremula, 12 and 62 mg/g in Populus tremuloides and 47 and 82 mg/g in Populus grandidentata. The aspen knots were found to be a rich source of bioactive flavonoids.