Variation of Fructan and Its Metabolizing Enzymes in Onions with Different Storage Characteristics

Abstract

Onions (Allium cepa L.) accumulate fructans, which are fructose polymers, in their bulbs as reserve carbohydrates and as a source of energy for sprouting. Onions with high fructan content and slow fructan degradation by hydrolysis are considered suitable for long-term storage. We have previously found that ‘Pole Star’ accumulates fewer fructans than ‘Kita-momiji 2000’ in their bulbs. In this study, we attempted to clarify the differences in storage characteristics, fructan content, and fructan metabolizing enzyme activities between the two cultivars during storage. Sprouting was not observed in the bulbs of ‘Kita-momiji 2000’ stored at 15 °C for up to 20 weeks, whereas it was observed in ‘Pole Star’ from around 14 weeks. The fructan content during storage showed a gradual decrease in the inner and outer scales of the bulbs in ‘Kita-momiji 2000’, whereas a rapid decrease was observed in ‘Pole Star’. In the basal plate, the fructan contents in ‘Kita-momiji 2000’ were higher than those in ‘Pole Star’ after 16 weeks of storage. Fructan metabolizing enzyme activities were low and constant in ‘Kita-momiji 2000’, whereas their activities increased in ‘Pole Star’ during storage. The low fructan content of ‘Pole Star’ was thought to be due to the high activity of fructan metabolizing enzymes, but the variation of fructan content was difficult to clearly explain using the balance of fructan hydrolase and synthase activities alone.