Abstract
The relationship between snow tolerance and fructan accumulation before winter snow cover was investigated in fall-planted onions grown in a snowfall zone. In cultivation tests where the date of planting was changed, planting date was found to affect both growth before snow cover and fructan accumulation. Fructan content was very low and sugar content was low in early-planted onions. In contrast, with a conventional planting date, the degree of fructan polymerization was 9 and total sugars, including monosaccharides and disaccharides, were high. Overwintering ability was also high with the conventional planting date; specifically, although growth was limited, fructan accumulation and total sugar content were high. Thus, sugar and fructan accumulation before snow cover play an important role in providing energy for metabolism under snow, while snow tolerance improves with increased fructan content. Moreover, the survival rate was high with the conventional planting date, suggesting that fructan also provides energy for recovery after the snow cover has melted. When grown at 15 ℃, a decrease in sugar accumulation was observed compared with growth at 5 ℃. Then, after 9 weeks of growth at 0.5 ℃ in the dark, the onion plants initially grown at 15 ℃ died. In contrast, all plants initially grown at 5 ℃ survived, suggesting that the sugar content is related to survival under low-temperature dark conditions. A change from sugar to fructan was observed in dark conditions at 0.5 ℃; however, fructan accumulation did not occur at 5 ℃, suggesting that fructan was synthesized at 0.5 ℃ despite the dark conditions.
