Graduate Thesis Or Dissertation

 

Sugar transport in strawberry fruit : uptake competition and developmental changes in invertase levels Public Deposited

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  • Mechanisms by which plant organs accumulate photoassimilate are important in determining final sink size and may involve invertase activity. Because strawberry fruit are strong sinks for sugars, changes in invertase levels during development and mechanisms of sugar uptake within the fruit were examined. Fruit from greenhouse-grown, day-neutral strawberries, (Fragaria X ananassa Duch. cv Brighton), were used for all studies. Fruit invertase levels were analyzed from 0 to 2 5 days post anthesis (DPA). Soluble and insoluble acid invertases were found, both with optimal activity at pH 4.6. Soluble invertase activity was highest at anthesis and decreased during development. Insoluble invertase activity was low at anthesis, but increased by 5 DPA where it remained for the following 20 days. Insoluble invertase was solubilized in 1 M NaCl and is presumably ionically bound to the cell wall. At 15 DPA, histochemical staining showed invertase activity (mostly insoluble) throughout the tissue, but denser around vascular bundles; cortex tissue stained predominantly in the cell walls. Location of invertase near vascular bundles suggests a function in phloem unloading. Sugar uptake kinetics of cortex protoplasts from 16-18 DPA fruit were similar to those found previously in tissue disks. Glucose uptake was higher than sucrose or fructose and exhibited simple saturation kinetics. Sucrose and fructose had similar uptake rates with biphasic curves; saturable kinetics was observed at low sugar levels (below 10 mM) and first-order (linear) kinetics at higher levels. Apoplastic sucrose, glucose and fructose concentrations were estimated to be 80, 80, and 120 mM, respectively. Higher fructose levels may result from sucrose hydrolysis by cell wall invertase followed by more rapid uptake of glucose. The high apoplastic sugar concentrations indicate that the linear component of sucrose and fructose uptake is important in sugar uptake in vivo. No competition was seen among the three sugars, suggesting separate uptake pathways. 1'-Fluorosucrose, a sucrose analog, was not hydrolyzed by cell wall invertase and showed uptake kinetics similar to sucrose in cortex tissue. This, coupled with a lack of competition in sucrose uptake and demonstration of sucrose uptake by protoplasts, suggests that hydrolysis of sucrose by cell wall invertase is not a prerequisite for its uptake in strawberry fruit.
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