The role of osmotic potential in spring sap flow of mature sugar maple trees (Acer saccharum Marsh)

Abstract

Field measurements of xylem sap osmotic and pressure potential were made on sugar maple trecs (Acer saccharum Marsh) during the winter and spring of 3 years to determine whether the hydrostatic pressure was osmotically generated. Sap osmotic potential was low enough to account for the hydrostatic pressure but the dynamics of its diurnal behaviour indicated that osmotic potential was not directly responsible for hydrostatic pressure. The diurnal course of hydrostatic pressure showed definite peaks but osmotic potential often did not. The magnitude of the diurnal changes in hydrostatic pressure was approximately 0·15 MPa whereas the changes in osmotic potential were only 0·05 MPa. Because the sap osmoticum is primarily sucrose, and starch is stored in the xylem throughout the tree, the temperature dependence of the sucrose-starch interconversion system was investigated. More active amylase was formed in maple twigs after incubation at 0°C and 4°C than at -3, 6 or 15°C. The rate of starch hydrolysis by maple amylase increased with temperature, reaching a maximum at approximately 45°C. There was some starch hydrolysis at -3°C. The starch hydrolysis system thus indicated no critical role for temperature fluctuations about 0°C. Starch was found to be densely stored in the rays of the trunk and twigs and around the central pith in the twigs. © 1985 Oxford University Press.