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

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