Influx into the Apoplasm

Movement of low-molecular-weight solutes (e.g. ions, organic acids, amino acids, sugars) from the external solution into the cell walls of individual cells or roots (the free

Fig. 2.1 Cross section of two rhizodermal cells of a maize root. V, vacuole; C, cytoplasm; W, cell wall; E, external solution. (Courtesy of C. Hecht-Buchholz.)

space) is a nonmetabolic, passive process, driven by diffusion or mass flow (Fig. 2.1). Nevertheless, the cell walls can interact with solutes and thus may facilitate or restrict further movement to the uptake sites of the plasma membrane of individual cells or roots.

Primary cell walls consist of a network of cellulose, hemicellulose, (including pectins) and glycoproteins, the latter may represent between 5% and 10% of the dry weight of the cell walls (Cassab and Varner, 1988). This network contains pores, the so-called interfibrillar and intermicellar spaces, which differ in size. For root hair cells of radish a maximum diameter of 3.5-3.8 nm (35-38 A) has been calculated; maximum values for the pores in plant cell walls are in the range of 5.0 nm (Carpita et al., 1979). In comparison hydrated ions such as K+ and Ca2+ are small as shown below being only in the order of 10-20% of the cell wall pore size. The pores themselves would thus not be expected to offer any restriction to movement of ions in the free space.

Diameter (nm)

Rhizodermal cell wall (maize; Fig. 2.1) 500-3000

Cortical cell wall (maize) 100-200

Pores in cell wall <5.0

Sucrose 1.0 Hydrated ions

In contrast to mineral nutrients and low-molecular-weight organic solutes, high-molecular-weight solutes (e.g., metal chelates, fulvic acids, and toxins) or viruses and other pathogens are either severely restricted or prevented by the diameter of pores from entering the free space of root cells.

Fulvic Acid Diagram
  1. 2.2 Schematic diagram of the pore system of the apparent free space. DFS, Donnan free space; WFS, water free space.
  2. 2.2 Schematic diagram of the pore system of the apparent free space. DFS, Donnan free space; WFS, water free space.

In this network, a variable proportion of the pectins consist of polygalacturonic acid, originating mainly from the middle lamella. Accordingly both in roots and in the cell wall continuum of other plant tissue, the so-called apoplasm, the carboxylic groups (R • COO-) act as cation exchangers. In roots therefore cations from the external solution can accumulate in a nonmetabolic step in the free space, whereas anions are 'repelled' (Fig. 2.2).

Because of these negative charges the apoplasm does not provide a free space for the movement of charged solutes, and Hope and Stevens (1952) introduced the terms apparent free space (AFS). This comprises the water free space (WFS), which is freely accessible to ions and charged and uncharged molecules, and the Donnan free space (DFS), where cation exchange and anion repulsion take place (Fig. 2.2). Ion distribution within the DFS is characterized by the typical Donnan distribution which occurs in soils at the surfaces of negatively charged clay particles. Divalent cations such as Ca2+ are therefore preferentially bound to these cation-exchange sites. Plant species differ considerably in cation-exchange capacity (CEC), that is, in the number of cation-exchange sites (fixed anions; R • COO"), located in cell walls, as shown in Table 2.3.

As a rule, the CEC of dicotyledonous species is much higher than that of monocotyle-donous species. The effective CEC decreases as the external pH falls (Allan and Jarrell, 1989), and is usually much lower in intact roots than the values shown in Table 2.3.

Table 2.3

Cation Exchange Capacity of Root Dry Matter of Different Plant Species"

Table 2.3

Cation Exchange Capacity of Root Dry Matter of Different Plant Species"

Cation exchange capacity

Plant species

meq (100 g)_1 dry wt

Wheat

23

Maize

29

Bean

54

Tomato

62

"Based on Keller and Deuel (1957).

"Based on Keller and Deuel (1957).

Table 2.4

Uptake and Translocation of Zinc by Barley Plants0

Table 2.4

Uptake and Translocation of Zinc by Barley Plants0

Rate of uptake and translocation

(ßg Zn g"

dry wt per 24 h)

Zinc supplied asè

Roots

Shoots

ZnS04

4598

305

ZnEDTA

45

35

"Based on Barber and Lee (1974). ^Concentration of zinc in nutrient solution: 1 mg I'1.

"Based on Barber and Lee (1974). ^Concentration of zinc in nutrient solution: 1 mg I'1.

Because of spatial limitations (Casparian band and exodermis; Section 2.5.1) only part of the exchange sites of the AFS are directly accessible to cations from the external solution. Nevertheless, the differences shown are typical of those that exist between plant species.

Exchange adsorption in the AFS of the apoplasm is not an essential step for ion uptake or transport through the plasma membrane into the cytoplasm. Nevertheless, the preferential binding of di- and polyvalent cations increases the concentration of these cations in the apoplasm of the roots and thus in the vicinty of the active uptake sites at the plasma membrane. As a result of this indirect effect, a positive correlation can be observed between the CEC and the ratio of Ca2+ to K+ contents in different plant species (Crooke and Knight, 1962; Haynes, 1980). Effective competition between H+ or mono- and polyvalent aluminium species or both with magnesium for binding sites in the apoplasm of roots is obviously a main factor responsible for the depression in magnesium uptake and appearance of magnesium deficiency in annual (Rengel, 1990; Tan et al., 1991) and forest tree species (Marschner, 1991b) grown on acid mineral soils (Section 16.3).

The importance of cation binding in the AFS for uptake and subsequent shoot transport is also indicated by experiments with the same plant species but with different binding forms of a divalent cation such as zinc (Table 2.4). When zinc is supplied in the form of an inorganic salt (i.e., as free Zn2+), the zinc content not only of the roots but also of the shoots is several times higher than when zinc is supplied as a chelate (ZnEDTA), that is, without substantial binding of the solute in the AFS. In addition, restricted permeation of the chelated zinc within the pores of the AFS may be a contributing factor. Using these differences in uptake rate between metal cations like Zn2+ (and also Cu2+ and Mn2+) and their complexes with synthetic chelators in so-called chelator-buffered solutions calculations can be made of the concentrations of free metal cations in the external solution required for optimal plant growth (Bell et al., 1991; Laurie et al., 1991). According to these calculations, extremely low external concentrations at the plasma membrane of root cortical cells appear to be adequate to meet plant demand for these micronutrient cations (Section 2.5.4).

With heavy-metal cations in particular, binding in the apoplasm can be quite specific. Copper, for example, may be bound in a nonionic form (coordinative binding) to nitrogen-containing groups of either glycoproteins or proteins of ectoenzymes, such as phosphatases or peroxidases, in the cell wall (Harrison et al., 1979; Van Cutsem and

Fig. 2.3 Time course of influx (I) and efflux (E) of 45Ca and 42K in isolated barley roots. After 30 min (arrow) some of the roots were transferred to solutions with nonlabelled Ca2+ and K+. The proportion of the exchangeable fraction in the apparent free space is calculated by extrapolation to zero time (x).

Fig. 2.3 Time course of influx (I) and efflux (E) of 45Ca and 42K in isolated barley roots. After 30 min (arrow) some of the roots were transferred to solutions with nonlabelled Ca2+ and K+. The proportion of the exchangeable fraction in the apparent free space is calculated by extrapolation to zero time (x).

Gillet, 1982). This cation binding in the apoplasm can contribute significantly to the total cation content of roots, as shown by studies of the uptake of polyvalent cations such as copper, zinc, and iron. This is also demonstrated by the data in Table 2.4. When supplied in non-chelated form high contents of polyvalent cations in the roots compared to the shoots therefore not necessarily reflect immobilization in the cytoplasm or vacuoles but may result from preferential binding in the apoplasm of the root cortex.

The root apoplasm may also serve as transient storage pool for heavy metals such as iron and zinc which can be mobilized, for example by specific root exudates such as phytosiderophores, and translocated subsequently into the shoots (Zhang et al., 1991b, c). The size of this storage pool for iron probably plays a role for genotypical differences in sensitivity to iron deficiency in soybean (Longnecker and Welch, 1990). On the other hand, excessive uptake of calcium may be restricted by precipitation as calcium oxalate in the cell walls of the cortex (Fink, 1992b).

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