Seasonal Variation of Sucrose Content in the Xylem Sap of Salix

Abstract

The sucrose content of the vessel water extracted from young shoots of willow has been studied over a three years period. The seasonal variation is described. Maximum sucrose content appears in late winter and reaches as much as 3 to 5 % (w/v). Content of hexoses is usually less than 1/10 of sucrose content. During blossoming, sucrose decreases sharply in the catkin-bearing shoots indicating an uptake in these parts. Experiments with isolated shoots showed that during late winter sucrose can enter the vessels at a rate of up to 10 nmol ml-1 min-1 (=0.5 % increase in sucrose content per day). The flux of sucrose that is necessary for this increase in sucrose content of the vessel sap has been calculated to be 50, 100, or 200 pmol cm-2 min-1, depending on whether all bordering ray cells, or the contact cells, or their large pit fields, respectively, had been taken as permeation area. Sodium fluoride, p-chloromercuribenzoate, and prolonged storage at 21 °C markedly affected the sucrose efflux into the vessels. The transpiration rates observed at the time of blossoming of catkins and the high sucrose content of the vessels suggest a considerable transport of carbohydrates via the xylem pathway at this short period.