Efflux and Reabsorption of Sugars in the Xylem II. Seasonal Changes in Sucrose Uptake in Salix

Abstract

The seasonal pattern in sucrose content, in the rate of sucrose accumulation in the tracheal sap, and in the rate of sucrose uptake from the trachéal sap in isolated shoots of Salix × smithiana has been studied at 21° and at 2°C. Considerable uptake of 20 to 40 mg ml-1 d-1 appeared in particular during three periods, during the rapid growth of catkins, during the outgrowth of vegetative buds, and especially during the stage of starch deposition in the ray cells in summer. The appropriate rates of sucrose uptake into the paratracheal ray parenchyma are computed to be 400 to 800 pmol cm-2min-1 at its minimum. The highest rates of sucrose efflux, in contrast, reached only about ⅕ of the highest uptake rates. Because a clear-cut increase in hexose content in the tracheal sap always was following the infusion of the sucrose solution, and because of the relative higher accumulation of hexoses at 2° as compared to 21°C, sucrose reabsorption is suggested to be preceeded by its extraplasmatic hydrolysis and to proceed by a hexose uptake system. Unsufficient reabsorption is observed at the stage of highest sucrose efflux and thus is considered to be one essential prerequisite of the sugar accumulation in the xylem sap.