Effect of cultivation factors on flavonoid content in yellow onion (Allium cepa L.)

Onions are grown all over the world. In northern Europe, the most commonly grown is yellow onion (Allium cepa L.). It is rich in flavonoids, especially quercetin glucosides. These compounds have been proposed to have beneficial health effects which may be mediated by their antioxidative properties. The uptake in the human body of quercetin glucosides from onion has been reported to be relatively high in comparison to other quercetin sources like apple and tea. Many studies of onion flavonoids have been performed on onions from "the local market," resulting in limited knowledge on the effects of various cultivation factors on flavonoid content in onions. From a grower's point of view, it is of interest to know which cultivation and handling techniques affect onion quality. The focus of our work has been on the role of field factors such as fertiliser level and time of harvest on flavonoid biosynthesis in onion. Flavonoids have been analysed at lifting, after field curing, and during the storage period. Yearly variation in onion flavonoid content was considerable, probably due to variations in global radiation (amount of sunlight measured as MJ/m(2)) at the end of the growing period. Cultivar differences were inconsistent throughout the years and this suggests that they reacted differently to changes in weather conditions. Additional amounts of nitrogen fertiliser available in soil seemed not to affect onion flavonoid levels, which means that it may be possible to grow onions with limited nitrogen leakage without reduced yield or antioxidant concentration. Time of lifting from the soil may be important because most of the flavonoid synthesis during the onion growth period seemed to occur during the last weeks before lifting. Field curing after lifting, leaving the onions for about ten days in windrows on the field, significantly increased the flavonoid content without any effect on onion dry weight in the edible part. High concentrations of quercetin glucosides were maintained throughout the storage period in cold storage at constant temperature and constant relative humidity.