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Labeling of nonstructural carbohydrates in winter wheat stems




Hogan, Mary Ellen, author

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Fructans are polymers of fructose, which contain a terminal or subterminal glucose moiety. The biosynthetic pathway for the levan-type of fructan, found in wheat, has not been elucidated. These compounds are a significant component of the nonstructural carbohydrate fraction in winter wheat stems. They accumulate when photosynthetic production exceeds their utilization. Based on a study of the translocation of 14C-sucrose in winter wheat, Robinson (44) suggested that the fructose moiety of translocated sucrose might be used preferentially for biosynthesis of fructans. The present study tested this hypothesis. Six winter wheat plants were grown to anthesis, then labeled with 14C-glucosyl, 3H-fructosyl-sucrose for three hours. The plants were then dissected and extracted in boiling 80% ethanol for 48 hours. They were subsequently extracted with water (20°C) for 48 hours. Water extracts were hydrolyzed with 2 M trifluoroacetic acid. The hydrolyzed samples were chromatographed using one-way, descending paper chromatography. Chromatograms were cut into one cm strips and eluted with water (20°C). These eluates were analyzed using liquid scintillation spectroscopy. The same technique was used to analyze aliquots of both the original ethanol and water extracts directly. Results indicated that a greater amount of the 3H from the fructose moiety of the translocated sucrose appeared in the water extracts, than in the ethanol extracts. Ratios of 3H:14C in the water extracts were greater than those in the supplied sucrose, indicating that the fructose moiety of translocated sucrose was used preferentially in the biosynthesis of water soluble fructans. This suggests that hexoses derived from translocated sucrose were not as readily interconverted in wheat stems as in other plants. The differing 3H:14C ratios of labeled fructans along the stem may indicate the degree of tissue maturity. Older stem sections contained greater amounts of labeled fructans.


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