Collection and composition of xylem sap and root structure in two halophytic species

Abstract

Leptochloa fusca (L.) Kunth and Atriplex hortensis (L.) were grown on quartz sand or in liquid culture in the presence of varied concentrations of NaC1. Xylem sap was collected as (a) root pressure exudate, in L. fusca even at 100 mM NaC1, (b) by applying pressure to excised roots of L. fusca and (c) from leaves of the whole plant growing in quartz sand by pressurizing the root system. The latter procedure failed in L. fusca due to the passage of air and soil solution into the leaves. This was caused by an extensive aerenchyma in root cortex. In Atriplex hortensis remarkably high pressures were required to induce a flow of sap. The mineral cation and anion and the amino acid composition of the xylem sap obtained by the different methods was measured and is examined in view of using it for determining the flows of minerals in the whole plant and in relation to the effects of salinity. The spacious aerenchyma in roots of L. fusca has been found to persist also after prolonged exposure to dry air.

Importance of Cycling and Recycling of Mineral Nutrients within Plants for Growth and Development

“…Substantial cycling of Na is a typical feature not only in salt sensitive species (Lessani and Marschner. 1978) but also in salt tolerant species (Gouia et al, 1994;Jeschke et al. 1995) and fulfills a dual purpose, as a driving force in the phloem and as a means of counteracting the accumulation of Na in source leaves by xylem import.…”

“…Nutrient fluxes in the xylem driven by root pressure can be measured after decapitation. To estimate nutrient fluxes under conditions where transpiration occurs, either negative pressure (suction) may be applied to the cut stem or positive pressure on the medium surrounding the root system, however, there are associated complications with these methods (Salim and Pitman, 1984;Else et al, 1995;Jeschke et al, 1995). For phloem sap, direct measurements are not possible and data on fluxes have to be obtained from indirect measurements, e.g.…”

Root Methods

“…To pressurize root systems (method 1) the culture vessels includ ing the roots are placed in Scholander-bomb-like vessels in which the pressure can be raised with compressed air (see Fig. Aiso roots (including the pot) of intact plants can be placed in pressure chambers allowing collection of xylem sap exuding from the cut leaf tips Jeschke et al 1996). By varying the hydrostatic pressure, the sap flow rates can be adjusted to those measured in intact plants.…”

Partitioning and flows of ions and nutrients in salt‐treated plants ofLeptochloa fuscaL. Kunth

“…The collection of xylem sap from L. fusca has been described in detail by Jeschke, Klagges & Bhatti (1995). For the present modelling, root pressure exudates were collected either from excised single roots or from the shoot stumps after shoot excision at the first and second harvests.…”

“…Moreover, for modelling of fiows in a whole plant ideally the ion: Ca ratios in xylem sap collected at the base of each organ would be needed. Collection of in situ xylem sap from individual leaves is possible by applying pressure (Munns & Passioura, 1984;Passioura, 1988), but his method failed in L. fusca because pressure forced air and soil solution through the large air spaces transversing roots, sheaths and leaves in this species (Jeschke et al, 1995). Therefore, only ion:Ca ratios in root pressure exudates from the shoot base were available and the modelling required the additional assumption that these refiected those in xylem sap higher up the stem.…”