Release of Ions into the Xylem

After radial transport in the symplasm into the stele, most of the ions and organic solutes (amino acids, organic acids) are released into the xylem. This release into fully differentiated non-living xylem vessels therefore represents a retransfer from the symplasm into the apoplasm. Crafts and Broyer (1938) postulated uphill transport in the symplasm across the cortex to the endodermal cells and, in the stele, a 'leakage' into the xylem. The distinctly lower oxygen tension in the stele than in the cortex of intact roots (Fiscus and Kramer, 1970) seemed to support this view of an oxygen deficiency-induced leakage. Also electrophysiological studies apparently indicate ion movement from the symplasm into the xylem along the electrochemical gradient (Bowling, 1981).

In contrast, Pitman (1972a) put forward a two-pump model for ion transport from the external solution into the xylem, one located at the plasma membrane of root cortical cells and the other at the symplasm-xylem interface (apoplasm) in the stele (Fig. 2.35).

Root Apoplasm

Fig. 2.35 Model for symplasmic (1) and apoplasmic (2) pathways of radial transport of ions across the root into the xylem. Key: -Q>, active transport; <—, resorption. (Modified from

Fig. 2.35 Model for symplasmic (1) and apoplasmic (2) pathways of radial transport of ions across the root into the xylem. Key: -Q>, active transport; <—, resorption. (Modified from

In this model the xylem parenchyma cells play a key role in ion secretion. High concentrations of ions such as K+ in these cells, together with transfer cell-like structures (Kramer et al., 1977) support this model. These cells seem to be involved also in reabsorption of ions from the xylem sap along the pathway to the shoot (Section 2.9).

The release of ions and organic solutes ('xylem loading') is not well understood. The key role of a respiratory-dependent proton pump at the plasma membrane of the parenchyma cells is now well established. Protons are pumped into the xylem (DeBoer et al., 1983; Mizuno et al., 1985) and acidify the xylem sap which has pH values between about 5.2 and 6.0 depending, for example, on plant species and source of nitrogen supply (Arnozis and Findenegg, 1986). Similarly to the pump at the plasma membrane of cortical cells (Fig. 2.9) the proton pump at the plasma membrane of xylem parenchyma cells transfers protons into the apoplasm of the xylem vessels and may thereby act indirectly by reabsorption as a driving force for the secretion of cations (antiport). Anions may be secreted either by cotransport with the protons or along the electrical potential gradient formed by the proton pump (e.g. transport into the vacuole, Fig. 2.9). As the oxygen tension is usually lower in the stele than in the cortex, the xylem loading pump is more quickly inhibited at decreasing oxygen tensions in the root environment (DeBoer et al., 1983).

Recently this concept of xylem loading as an energized process has been questioned by Wegner and Raschke (1994). By using isolated parenchyma cells from barley roots and measuring plasma membrane-potential related fluxes of cations and anions these authors suggest that similar to guard cells at closing (Section 8.7.6.2) also release of ions into the xylem sap occurs through ion channels in a process which is thermodynamically passive.

Irrespective of the different views on the mechanism there is general agreement that xylem loading is separately regulated from the ion uptake in cortical cells. This separate regulation step offers the plant the possibility of control selectivity and rate of longdistance transport to the shoot, for example as a feedback regulation depending on shoot demand [Fig. 2.23 (6)]. For example, preferential xylem loading of nitrate

Table 2.35

Root Uptake and Translocation to the Shoot of Phosphate and Sulfate in two Genotypes of Arabidopsis thaliana"

Table 2.35

Root Uptake and Translocation to the Shoot of Phosphate and Sulfate in two Genotypes of Arabidopsis thaliana"

Phosphate6

Sulfate

Translocation

Translocation

Root uptake

to shoot

Root uptake

to shoot

Genotype

(nmol g^h"1)

(%)

(nmol g"1 h"1)

(%)

Wildtype

1593

35

291

25

Mutant

1559

0.9

367

12

"Poirier et al. (1991). Reprinted by permission of the American Society of Plant

Physiologists.

"Poirier et al. (1991). Reprinted by permission of the American Society of Plant

Physiologists.

compared with the amino acid glutamine may play an important role in partitioning of different forms of nitrogen between roots and shoots (Schobert and Komor, 1990).

Selective inhibitors of protein synthesis strongly impair xylem loading of mineral nutrients such as potassium without affecting their accumulation in the roots (Láuchli et al., 1978; Morgan et al., 1985). An example of separate genetic control of the xylem loading step is also shown in Table 2.35. Compared with the wildtype the mutant of Arabidopsis requires very high external phosphate supply for normal growth. At low phosphate supply the mutant becomes severely phosphorus deficient because of impaired shoot transport of phosphorus whereas the uptake in the roots is not different from the wildtype (Table 2.35). In contrast to phosphate sulfate transport to the shoot is similar in the mutant and the wildtype. This defect in the mutant is caused by a single recessive gene locus which obviously regulates the xylem loading of phosphorus (Poirier et al., 1991). From results obtained by Sasaki et al. (1987) one may speculate on regulation of the enzyme glucose-6-phosphatase. In maize the activity of this enzyme and the concentration of its substrate glucose-6-phosphate are particularly high in xylem parenchyma cells. Inhibition of this enzyme by glucosamine severely depresses the xylem loading of phosphorus but not accumulation in the roots. Evidence for a particular fine regulation of phosphorus loading into the xylem is also the inability of maize plants to meet the phosphorus demand of the shoot at low root zone temperatures (Engels and Marschner, 1992a; Section 2.5.2).

The discovery of the abundance of living LMX vessels (see above) comprising more than half the total root length in maize up to 50 days old (Wenzel et al., 1989) renewed the view of leakage as a mechanism of ion release into the xylem (McCully etal., 1987). The concentrations of K+, for example, in the vacuoles of living LMX vessels are up to 400 ium. These high levels of K+, together with the other solutes in the vacuoles, are released into the transpiration stream at maturation of the LMX vessels. According to McCully and Canny (1988) this leakage from maturing xylem vessels can account for about 10% of the shoot demand of growing maize plants. Thus, at least part of the solutes in the xylem sap (including proteins) may derive not from active xylem loading but leakage from maturing xylem vessels.

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