Xylem Transport

3.2.1 Composition of the Xylem Sap

The composition and concentration of mineral elements and organic solutes in the xylem sap depend on factors such as plant species, mineral element supply to the roots, assimilation of mineral nutrients in the roots and nutrient recycling. Composition and particularly concentration of solutes are also strongly influenced by the degree of dilution by water (Section 2.9) and are therefore dependent on the transpiration rate and the time of day. Composition and concentration of xylem sap also change typically during the ontogenesis of plants (Tàble 3.2). In soybean during the reproductive stage xylem sap volume flow declines and the concentrations of some mineral nutrients in the sap decrease and of others increase. This decline in mineral nutrient concentrations can be reverted by depodding the plant, reflecting an easing in sink competition for photosynthates between pods and roots, thus, leading to higher uptake and xylem loading of mineral nutrients (Nooden and Mauk, 1987).

In perennial species in temperate climates composition of the xylem sap changes typically during the season not only in organic solutes (e.g. remobilized in spring) but also in nitrate concentrations and pH (Glavac et al., 1991). Polyvalent heavy metal cations in the xylem sap exist mainly in organic form complexed with organic acids, amino acids and peptides (White et al., 1981a,b). Both number and distribution of the complexes vary with plant age in annual species (Cataldo et al., 1988).

The form and proportion of the various nitrogen fractions in the xylem sap depend on the form of nitrogen supply (NOf; NH4"; N2 fixation), the predominant site of nitrate reduction (roots or shoots) and the proportion of recycled nitrogen (Section 3.4.4). Except at very high external NH4" supply the concentration of NH4 in the xylem is very low (Van Beusichem et al., 1988), in maize in the range of 1 mM, irrespective of whether nitrogen is supplied as NH4 or NOJ (Engels and Marschner, 1993). The concentration of organic acids in the xylem sap depends primarily on the cation-anion uptake ratio by the roots (Table 2.39) and the form of nitrogen supply (Arnozis and Findenegg, 1986). In the xylem sap of annual species high concentrations of sugars may also occur, for example, in maize in which up to 5 mM may be present (Canny and McCully, 1989), and in soybean sugars may account for about 15% of the total organic carbon in the sap (Cataldo et al., 1988). Xylem sap may also contain enzymes like peroxidases (Biles and Abeles, 1991) which probably derive from maturing xylem elements (Section 2.8).

Phytohormones are a normal constituent of xylem sap, particularly cytokinins which are mainly synthesized in the roots (Section 5.6). Recently the concentration of abscisic acid (ABA) in the xylem sap has attracted wide interest as a possible nonhydraulic chemical root signal to the shoot of the root water status and also on the strength of the soil (Passioura and Gardner, 1990). As the soil dries out stomatal conductance decreases prior to decrease in leaf turgor, and inverse relationships have been shown to occur between stomatal conductance and xylem sap ABA concentrations (Zhang and Davies, 1989, 1990). Under field conditions too, for example, in maize, stomatal conductance has been found to be closely related to the ABA concentration of the xylem sap but not the current leaf water status nor ABA concentrations in the bulk leaf (Tardieu et al., 1992). There is a substantial body of evidence that high concentrations of ABA, or of 'inhibitors' other than ABA (Munns, 1992), in the xylem sap are also causally involved in a decrease in cell extension and division and, thus, also in leaf elongation in response to drying or compacting soil (Randall and Sinclair, 1988; Gowing et al., 1990; Blum et al., 1991). As soil dries out both the ionic composition and pH of the xylem sap increases (Gollan et al., 1992), and this may also alter the partitioning of ABA in the leaf cells and lead to preferential transport of ABA to the guard cells (Hartung et al., 1988; Hartung and Slovik, 1991; Section 5.6).

The increase in ABA concentrations in the xylem sap of nitrogen deficient plants and its consequences for plant water relations and leaf growth are discussed in Chapter 6. The root-derived hormonal signals in the xylem sap also distinctly affect long-distance transport of mineral nutrients, for example, via the volume flow rate in the xylem, the rate of xylem-phloem transfer (Section 3.3.4) and the mineral nutrient distribution within the shoot (Section 3.2.4).

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