Pathways of Uptake and Accumulation of Sugars in Tomato Fruit

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Abstract
The route of sucrose unloading from the conducting tissue, the regulation of sucrose hydrolysis and the uptake and subsequent metabolism of sugars were investigated in the rapidly growing tomato fruit. During the first two weeks of fruit enlargement, the vacuole accounted for more than 85% of the protoplast volume and the intercellular space accounted for 20% of the fruit placental tissue. The plasmodesmatal frequency was highest between phloem parenchyma cells and lowest between phloem sieve cells and phloem parenchyma. The total invertase activity was about 8 μmol glucose g−1 d. wt min−1 during the rapid growing period and increased six-fold at ripening. The wall-bound invertase accounted for less than 11% of the total activity. Invertase activity increased with increasing sucrose concentrations (up to 50 mM) in the incubation medium, but decreased at higher concentrations. Sucrose synthase activity could only be detected when fruit was older than 19 d. The uptake and metabolism of sugars by fruit cells were investigated by incubation of fruit slices with 14C-sugars for 3 h. The uptake of sucrose increased with the sucrose concentration up to 200 mM. The rate of glucose uptake and its conversion to the ethanol-insoluble fraction were higher than those of sucrose. The uptake of sucrose did not compete with that of glucose or vice versa, provided the osmotic potential of the incubation solution was maintained constant. The uptake of sucrose was not inhibited by metabolic inhibitors such as PCMBS, CCCP, sodium azide or vanadate. The ATPase activity in the fruit tissue was low. These findings did not identify conclusively the mode of sucrose unloading. However, the uptake of sugars by fruit cells is non-specific and does not appear to require a membrane carrier or plasmalemma ATPase to provide energy for sucrose uptake.