Conversion of Labeled Substrates to Sugars, Cell Wall Polysaccharides, and Tartaric Acid in Grape Berries

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Abstract
[U-14C]Sucrose, myo-[U-14C]inositol, [6-14C]- and [U-14C]glucuronate, UDP-[U-14C]glucuronate, [U-14C]gluconate and L-[1-14C]ascorbic acid were fed into grape berries, Vitis labrusca L. cv. Delaware, at intervals throughout the ripening process and incorporation of 14C into several metabolites was studied. [U-14C]Sucrose was the most effective precursor of cellulose in young grape berries and of glucose and fructose in mature berries. UDP-[U-14C]glucuronate was the best precursor of pectic substance, followed by [14C]glucuronate and myo-[U-14C]inositol. L-[1-14C]Ascorbic acid was the most effective precursor of tartaric acid. In young berries, [U-14C]sucrose and [U-14C]gluconate also produced labeled tartaric acid, the latter a somewhat better precursor in the 3 wk following flowering. The remaining test compounds were only poor sources of 14C for tartaric acid although all 3, glucuronate, UDP-glucuronate and myo-inositol, were utilized by the grape berry for pectin biosynthesis. Apparently tartaric acid is synthesized by a C-1 oxidation mechanism of hexose in young grape berries.