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

Abstract

Summary The seasonal pattern in the rate of sucrose accumulation in the tracheal sap of isolated shoots of willow was studied at 21° and at 2°C. Maximum accumulation rates are obtained during deep winter, reaching 3 to 8 mg ml-1 d-1, while much smaller rates are found during the mobilization period in spring. The appropriate rates of sucrose efflux from the ray parenchyma into the vessels are computed to be 30 to 80 pmol cm-2 min-1 at its maximum. Efflux at 2° and at 21°C were different by ⅓ only and were inhibited similarly in the presence of the sulfhydrylreactive compound PCMB. From this result, passive and facilitated diffusion at sulfhydrylsensitive carriers are indicated to be involved when sucrose permeates into the tracheal sap in winter. The sucrose accumulation of the tracheal sap itself is suggested to be influenced mainly by four parameters: the abundant starch-sucrose conversion in the symplast in the cold, the following passive and facilitated efflux of the sucrose into the apoplast, the absence of efficient reabsorption of the sugars and the missing resynthesis of starch at low temperatures.