Fructosyl transfer from sucrose and oligosaccharides during fructan synthesis in excised leaves of Lolium temulentum L.

summary Fructan biosynthesis begins with the transfer of a fructosyl moiety from one sucrose molecule to another to yield a trisaccharide. Trisaccharides may also arise by the reversible transfer of a fructosyl moiety from higher oligomers to sucrose but in this case there is no net fructan synthesis. Short‐term and long‐term exposure of detached illuminated leaf blades of Lolium temulentum (L.I to 14 CO 2 was used to examine the mechanism of transfer of fructosyl residues to sucrose. Two trisaccharides, 1‐kestose and neokestose, were found to be radioactive when leaves excised and illuminated for 15 h ‐were exposed to N CO 2 for 30 min. The label increased in neokestose during the chase period, while that in 1‐kestose increased for the first 2 h of the chase period then declined for the remaining 4h. With a longer exposure to 14 CO 2 during the first 6 h of the induction period, three trisaccharides, neokestose, 1‐kestose and 6‐kestose were radiolabelled. The label turned over in neokestose and 1‐kestose, but continued to accumulate in 6‐kestose during a subsequent 18 h chase period. The specific activities of glucose and fructose of the sucrosyl portion and the terminal fructosyl moiety of the various trisaccharides were compared. In the rapid pulse‐chase experiment the specific activity of the 1 terminal fructosyl moiety was consistently less than that of the sucrosyl moiety. During the chase period, the specific activity of the terminal and internal fructose moieties became similar. These results indicate that in addition to trisaccharide formed by transfer of fructosyl units from sucrose, substantial amounts of both neokestose and 1‐kestose are made by transfer of fructosyl units from higher oligomers onto sucrose in reactions probably localized in the vacuole.