Changes in quercetin glucoside concentrations of onion bulbs by scales, during storage, and in sprouting leaves exposed to UV

• Total quercetin (TQ) concentrations were greatest in the dry skins on a fresh weight basis.
• TQ contents peaked in the first fresh scale on an area basis.
• The QG content in the bulbs stored in a CA did not change significantly during storage.
• Q4′G and Q3,4′G contents increased when leaves were exposed to UV light.

Onion is an excellent source of quercetin (Q), which is known to have various health benefits. In this study, we investigated the distribution of quercetin glucoside (QG) compounds among dry skins, semi-dry skins, and fresh onion scales; changes in the concentrations of QG compounds in onion bulbs stored at various temperatures for five months; and changes in QG concentrations in the etiolated leaves irradiated by ultraviolet (UV) or visible light. Among scales of a bulb, total quercetin (TQ) concentrations were greatest in the outer-most scales (dry skins) and were gradually reduced toward the inner fresh scales on a fresh weight basis. However, on an area basis, TQ content peaked in the first fresh scale and was lower in the dry skins or the inner fresh scales. The QG content in the bulbs stored in a controlled atmosphere (CA) did not change significantly, while the concentrations in bulbs stored at 30 °C increased by about 50% and then returned to the initial levels. We observed increases of quercetin-3,4′-diglucosde (Q3,4′G), accompanied by a reduction of quercetin-4′-glucoside (Q4′G), at 30 and 24 °C storage, suggesting a relationship between QG concentration and storage temperature or metabolic changes. However, it was not clear if there was a net synthesis of the TQ compounds at the 30 and 24 °C storage. In the sprouting leaves under the different lights, Q4′G and Q3,4′G concentrations increased the most when exposed to UV light and, to a lesser extent, when exposed to visible light; however, even the samples in the dark condition showed a slight increase in QG compounds. We did not observe the synthesis of QG in the white onion leaves. Our results confirm that the synthesis of QG compounds is enhanced by UV light and, to a lesser extent, by visible light.