Fructo‐oligosaccharide content and fructosyltransferase activity during growth of onion bulbs

SUMMARY The accumulation of fructo‐oligosaccharides and the activities of fructosyltransferase (sucrose: sucrose 1F‐fructosyltransferase (SST), 1F‐fructosyltransferase (1F‐FT) and 6G‐fructosyltransferase (6G‐FT)) in the bulbs of three onion cultivars were investigated from June to September 1993. The total fructo‐oligosaccharide content increased from June until August, then decreased in September, except in one cultivar. The levels of neokestose and its related tetrasaccharides (lF,6G‐di‐β‐d‐fructofuranosyl sucrose and 6G(1‐β‐d‐fructofuranosyl)2 sucrose) were higher than those of 1‐kestose and nystose throughout growth. The activities of 6G‐FT, 1F‐FT and SST were high in June and July, then decreased; SST activity was very low in September. The activity ratios of 6G‐FT to 1F‐FT varied between 1.86 and 2.65 over the growth period. Two trisaccharides, three tetrasaccharides and four pentasaccharides were identified, together with a mixture of hexa‐ and heptasaccharides, all of which were synthesized in vitro from 0.1 M sucrose by an enzyme preparation of onion bulbs harvested in August. Octa‐ and nonasaccharides other than the saccharides formed from sucrose were also synthesized from O.1 m 1‐kestose or 0.1 m neokestose. All the saccharides produced from sucrose, 1‐kestose or neokestose by the crude enzyme prepared from onion bulbs were identical to the saccharides occurring naturally in onion bulbs.