Sucrose and Glucose Uptake into Beta vulgaris Leaf Tissues : A Case for General (Apoplastic) Retrieval Systems

Concentration curves for sugar and amino acid uptake by Beta vulgaris L. leaf tissues contained both 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 versus linear uptake. Two millimolar 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 was found to inhibit the linear component of both 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 millimolar N-ethylmaleimide, 2 micrograms per milliliter cycloheximide, and high levels of mannitol acting as osmoticum. These results support the proposal that the linear component is due to a process more complex than simple, or exchange, diffusion. It would also appear that 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.