Plant Physiology 87:731-736 (1988)
© 1988 American Society of Plant Biologists
Metabolism and Enzymology
Sink Metabolism in Tomato Fruit 1
II. Phloem Unloading and Sugar Uptake
Sue Damon,
John Hewitt2,
Matt Nieder3 and
Alan B. Bennett
Mann Laboratory, Department of Vegetable Crops, University of California, Davis, California 95616
Analysis of [3H]-(fructosyl)-sucrose translocation in tomato (Lycopersicon esculentum Mill.) indicates that phloem unloading in the fruit occurs, at least in part, to the apoplast followed by extracellular hydrolysis. Apoplastic sucrose, glucose, and fructose concentrations were estimated as 1 to 7, 12 to 49, and 8 to 63 millimolar, respectively in the tomato fruit pericarp tissue. Hexose concentrations were at least four-fold greater than sucrose at all developmental stages. Short-term uptake of [14C]sucrose, -glucose, and -fructose in tomato pericarp disks showed first order kinetics over the physiologically relevant concentration range. The uptake rate of [14C]-(glucosyl)-1'-fluorosucrose was identical to the rate of [14C]sucrose uptake, suggesting sucrose may be taken up directly without prior extracellular hydrolysis. Short-term uptake of all three sugars was insensitive to 10 micromolar carbonyl cyanide m-chlorophenylhydrazone and to 10 micromolar p-chloromercuribenzene sulfonic acid. However, long-term accumulation of glucose was sensitive to carbonyl cyanide m-chlorophenylhydrazone. Together these results suggest that although sucrose is at least partially hydrolyzed in the apoplast, sucrose may enter the metabolic carbohydrate pool directly. In addition, sugar uptake across the plasma membrane does not appear to be energy dependent, suggesting that sugar accumulation in the tomato fruit is driven by subsequent intracellular metabolism and/or active uptake at the tonoplast.
2 Present address: Northrup King Co., P.O. Box 1827, Gilroy, CA 95021.
3 Present address: Escagen Corp., 830 Bransten Rd., San Carlos, CA 94070.
1 Research supported by gifts from Campbells, Beatrice/Hunt-Wesson, and Chesebrough/Ponds.
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