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Butyrivibrio sp. C3 degrades rutin anaerobically to yield phloroglucinol, carbon dioxide, 3,4-dihydroxy-benzaldehyde, and 3,4-dihydroxyphenylacetic acid. The sugars are first released by intracellular glycosidases and then the heterocyclic ring of the aglycone undergoes hydrolytic cleavage. The glycosides of quercetin added to the medium are readily metabolized, but the aglycone is not.

The effect on the conversion of carbon14 to quercetin by a buckwheat plant caused by variation in the dose rate of a carbon 14-labelled precursor in μmoles over a 100-fold range has been studied. The results expressed as dilution of carbon14 varied over a 200-fold range for the same precursor with most of the variation being found at low dose rates. When expressed as the percentage of administered carbon14 converted to quercetin the results for acetate, D-glucose, and L-phenyllactic acid were mostly constant over this range but L-phenylalanine had a maximum percentage conversion or an optimal dose rate. An explanation of the results in metabolic terms has been attempted.It is recommended that the "percentage of C14 converted" be used to express results in plant biosynthetic work.

Various C14-labelled compounds were administered to Fagopyrum tataricum. After allowing 24 hours for metabolism the quercetin was isolated and degraded, by a modified alkaline fusion, into phloroglucinol (ring A) and protocatechuic acid (ring B plus carbon 2). Uniformly labelled sucrose or C14O2 (assimilated photosynthetically) formed quercetin equally labelled in the A and B rings, but sodium acetate-1-C14 (or acetate-2-C14) gave quercetin labelled almost entirely in ring A. Isotope competition experiments indicated meso-inositol and phloroglucinol were not precursors of ring A. The biosynthesis of quercetin from C14O2 or acetate required the combined action of stem and leaves. Light was not required for synthesis of quercetin from acetate.

Cuttings of Fagopyrum tataricum were allowed to metabolize various labelled compounds for 24 hours in the light. Quercetin was then isolated and degraded by alkaline hydrolysis of its pentamethyl ether into veratric acid (ring B plus carbon 2) and 2′-hydroxy-2,4′,6′-trimethoxyacetophenone (ring A plus carbons 3 and 4). Eleven amino acids, including tyrosine, were poor precursors of quercetin as were also protocatechuic acid and p-hydroxybenzoic acid. Shikimic acid, phenylalanine, p-hydroxycinnamic acid, and cinnamic acid were very good precursors followed by (in decreasing order of effectiveness) caffeic acid (fair), sinapic acid, m-methoxycinnamic acid, and ferulic acid (very poor). Quercetin formed from β- or ring-labelled cinnamic acid gave labelled veratric acid on degradation, and that from α- or carboxyl-labelled cinnamic acid gave labelled 2′-hydroxy-2,4′,6′-trimethoxyacetophenone. Two-ninths of the activity of quercetin formed from uniformly labelled phenylalanine was found in the acetophenone derivative and seven-ninths in the veratric acid. These results show that C6.C3 compounds are used with the carbon skeleton unchanged for the synthesis of ring B and carbons 2, 3, and 4 of quercetin and that the ring hydroxylation pattern of the compound used determines its effectiveness. None of the compounds were good precursors of ring A.