The kinetic analysis of fructan biosynthesis in excised leaves of Lolium temulentum L.

Excised leaves of Lolium temulentum L. were fed 1CO. at 99% isotopic abundance in order to study the kinetics of incorporation of photosynthetically fixed carbon into different fructans and to determine the pathways of synthesis between sucrose and high‐molecular‐weight fructan. Following 6 h exposure to CO2 of detached and illuminated leaves, analysis of the accumulation of I‐keatose, 6–kestose and neokestose, and of the pattern of labelling observed in these isomers, suggested that they were each formed directly from sucrose at different rates. Similarly, the data suggested thai the tetrasaccharide isomers were formed directly from their trisaccharide precursors. Changes in the concentration of 13C both in the total water‐soluble component of the leaves and the fructan pool after 20h exposure to 13Co2 suggested that there was a continual turnover of frunctosyl residues. The formation of trisaccharides From sucrose and subsequent frunctosyl transfer to oligo‐ and polysaccharides of progressively higher molecular weight was consistent with a direct precursor product relationship. The stable concenttation of sucrose observed in leaves between 14 h and 48 h after excision indicated that fructan metabolism in L. temulentum may serve to regulate the sucrose content of the tissue. Copyright © 1993, Wiley Blackwell. All rights reserved