Sucrose and Glucose Uptake into Beta vulgaris Leaf Tissues

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Abstract
Concentration curves for sugar and amino acid uptake by B. vulgaris L. leaf tissues contained a saturable and a linear component. Similarly shaped curves were obtained for influx of sucrose, glucose and 3-O-methyl glucose by leaf discs, whole petiole slices, petiole segments containing pith tissue only and petiole segments containing vascular bundles, although the tissues took up the various sugars via different proportions of saturable vs. linear uptake. Two mM p-chloromercuribenzenesulfonic acid selectively inhibited the saturable component of sucrose uptake, but had almost no effect on the linear component. Uptake of glucose and 3-O-methyl glucose remained unaffected by p-chloromercuribenzenesulfonic acid treatment. Anoxia inhibited the linear component of sucrose and 3-O-methyl glucose influx, while the saturable component remained unaffected. The linear component of sucrose uptake was also competitively inhibited by maltose, as well as being selectively promoted by certain exposures to 5 mM N-ethylmaleimide, 2 .mu.M/ml cycloheximide and high levels of mannitol acting as osmoticum. Apparently, the linear component is due to a process more complex than simple, or exchange, diffusion. Evidently, the linear transport component utilizes a separate energy source than does the saturable component of sucrose influx. Evidence for phloem loading from the apoplast was re-examined with respect to the present findings. Saturable sucrose uptake by minor vein tissues may represent retrieval of solute from the free space, which could explain the apoplastic loading phenomenon.