Changes in composition of the xylem sap as well as in ion fluxes in populus tremula X alba L. xylem in dependence on exogenous factors

Abstract

This investigation shows diurnal variations in the xylem-sap composition of poplar. All major macronutrients reached a maximum concentration in the first half of the light period and decreased to the middle of the night. The relative abundance of the nutrients did not change during the day. The sap flow which responded very fast to the environmental changes (2.2 fold increase within 10-20 min of illumination) reached a maximum value in the second half of the light period. Transpiration (and photosynthesis) was constant throughout the light phase. The calculated translocation rates displayed a maximum in the first half of the light period and, therefore, did not fit the time course of sap flow. During the night, translocation rates were 63-69% lower than the maximum. The regulation of nutrient translocation is discussed taking the active xylem loading into account. The axial distribution located the nitrate assimilation in younger and storage of nitrate (and other macro nutrients) in older leaves. However, the sap flow was greater in younger shoot sections compared to older sections. We assume that the greater demand for nitrate in the younger shoot section was satisfied via an increased volume flow rather an increased nitrate concentration. Salt treatment mainly affected young leaves increasing their Na+ and Cl- content on the expense of Mg2+ and NO3- content respectively. A threshold (pH 3.25) was observed concerning the effect of low pH on in vivo sap flow.