# Nutrient Transport in the Soil-Plant System

## Nutrient Transfer in Soil to Root Surface

In this section of the course we are concerned with the availability of nutrients at the root surface. At this stage we will confine our attention to nutrients that are distributed throughout the soil, and are not being directly influenced by processes or microbiological activity in the rhizosphere.

Nutrients come into contact with the root surface by root interception or by the processes of diffusion and mass flow.

Mass flow involves the movement of nutrients along with the water that is being absorbed by the root. The amount of nutrient per unit area being transported by mass flow is given by the product of the volume of water absorbed and the concentration of the nutrient in the soil solution.

Mass Flow (µg cm-2 s-1) =qvC1

where qv = inward flux of water (cm3 cm-2 s-1

C1 = concentration of nutrient in soil solution (µg cm-3

If the nutrient is absorbed by the root at a rate, In, that is greater than that at which the water can transport it, the concentration in soil solution at the root surface will decrease. On the other hand, if the water is absorbed at a rate that is  relatively greater than the nutrient is taken up, the nutrient will accumulate at the root surface, and in the root free space.

Example of calcium accumulation in soil around an old root channel (courtesy of  G.Callot)

## Diffusion

If the concentration of the nutrient at the root surface is different from that in the bulk soil solution the nutrient will move by diffusion from the zone of higher to lower concentration, i.e., either towards or away from the root (Figure 1).

Diffusion in water follows Fick's first law

where

J = flux (g s-1

C = concentration of labile nutrient in soil (g cm-3

x = distance (cm)

D0 = diffusion coefficient (cm2 s-1), and varies according to the size of the ion.

A = area available for diffusion (cm2

The flux per unit area = J/A

In soil, the effective diffusion coefficient (De) is less than that in free solution (D1) because of the complex 3-dimensional structure of pores, and the variation in water content, which affects the range of pore sizes that remain water-filled.

De is influenced by the impedance factor, f1, which takes into account the tortuous pathway the nutrient must follow through the soil pores, as well as the effects of increased viscosity of water at the soil surface, and negative adsorption effects on anions. The impedance factor increases with increasing soil moisture.qv is important for normalizing the units of Cl (mass per unit volume of liquid) and Cs (mass per unit volume of soil).

Note: Use of the term impedance factor is unfortunate. It is really a permeability value that is used i.e., the less the impedance to ion diffusion the greater the impedance factor. Increasing soil moisture decreases the tortuosity of the diffusion path and hence increases the impedance factor.

For ions such as NO3-

Because NO3- can be formed in soil, Cl- is normally used to determine the impedance factor.

De is also proportional to the inverse of the buffer power b

b = dCs/dC1

where Cs = concentration of labile ions in the soil and C1 = concentration in the soil solution).

The buffer power is given by the slope.

Note that the two concentrations are not based on the same volumes. The concentration in the soil is based on unit volume of soil, whereas that in solution is based on unit volume of solution. These two volumes are related by the volume of water per unit volume of soil i.e., the volumetric water content v

For nutrients that are not adsorbed by the soil - e.g. N03-

dCl/dCs is constant

The slope of the line will be qv

For nutrients that are adsorbed - e.g. K+, H2P04-

qvdC1/dCs may be as small as 10-3

For the Brookston clay sample, the buffer power for P, which was adsorbed, varied widely from about 30 to less than 4.

Note, however, that in some organic soils in Ontario there are few minerals to adsorb P, phosphate behaves like nitrate and the buffer power approaches qv

In soils the largest variation in De is due to b

If ions are being absorbed by the root more rapidly than the supply by mass flow, a depletion zone will develop around the root. The spread of this zone is the maximum distance from which ions are diffusing to the root.

The flux per unit area = J/A

and in soil -

The maximum distance can be given approximately by (2Det)1/2 , where t = time.

Q How is the distance from which roots are able to obtain nutrients affected?

• For N03-, De is at least 100 times that for H2P04- and so will move from about 10 times the distance in a given time.

Nitrate is therefore considered a relatively mobile nutrient while phosphorus is considered to be a relatively immobile nutrient.

The plant requirements for some nutrients - e.g. Ca2+, Mg2+ in calcareous soils -may be supplied entirely by mass flow. Others - e.g. H2P04- - are supplied almost entirely by diffusion.

Q How do root hairs affect uptake?

• Root hairs will greatly increase the absorption of immobile ions such as P which have a small diffusion coefficient. The effect on absorption of mobile ions such as N03- that have a much greater diffusion coefficient will be much less.
A much higher root length density is required to deplete the soil of immobile than mobile nutrients.

Mobile nutrients are absorbed from essentially the total soil volume occupied by roots. This is termed the root system sorption zone. A plant can effectively absorb all of a mobile nutrient from this volume. Immobile nutrients, however, are absorbed from only a small volume of soil adjacent to the root. This is termed the root surface sorption zone. Because roots occupy only a small proportion of the total soil (usually less than 1%) only a small proportion of the immobile nutrient in the soil is accessible to plants in one season.

Root interception

This is calculated in terms of the volume of soil displaced by the root system.

For example, root interception of phosphate-P in the topsoil can be estimated from:

Depth of topsoil = 20 cm.

For 1 m2 of soil surface

Volume = 2 x 10 x10 cubic decimeters

= 200 L
and for 1 ha

V = 200 x 104 L = 2 x 106
If Cl = 50 mg L-1 soil, then total mass of P = 50 x 10-6 x 2 x 106 kg = 100 kg.

If roots occupy 1% of the soil volume, interception will be 100 x 0.01 kg = 1 kg.

### Modelling transport of nutrients to roots.

Much of the modelling approach was developed by P H Nye and co-workers in Oxford, UK, and by S A Barber in Purdue, Indiana.

The key equation to describe the inflow of nutrients to the root surface IN is given by:

Interception generally is considered to be included in the diffusion term.

Such a model is essentially mechanistic.

## Ion Uptake Into The Root

In this section we are concerned with the uptake of nutrients at the root surface.

Some definitions:

Absorption (or uptake) refers to the amount of solute which is removed from the external medium.

Translocation refers to the quantity of absorbed solute which is transported from the root along the stele to the stem.

Accumulation refers to that part of absorbed solute which is not translocated, but is retained within the root.

Ions are selectively accumulated in cells and in xylem sap, with the degree of accumulation varying with ionic species, plant species and external concentration.

### Transport of ions from the root surface to the site of transfer into the symplasm

Transport occurs in the cell walls by diffusion or mass flow. The primary cell wall consists of a network of cellulose microfibrils and hemicellulose, with pectins and glycoproteins acting as cross linkages.

Cell walls exhibit a cation-exchange capacity (CEC).

Some values of cation capacity for plant root walls (afterMarschner).

 Plant Cation exchange capacity  m eq (100 g)-1 dw Wheat 23 Corn 29 Faba bean 54 Tomato 62

Values for CEC are greater for dicots than monocots due to more pectic polysaccharides in the wall of dicots. CEC also varies with the pH of the bathing solution because of the variable charge on the walls, mainly due to carboxylic groups.

The maximum pore diameter within the cell wall is in the range 5 to 10 nm, about 10 to 20 times the diameter of hydrated cations. For example the diameter of a hydrated Ca++ ion is 0.82 nm.

Cations will accumulate in the cell wall because of the negatively charged exchange sites, where they are adsorbed or form a diffuse layer close to the walls of the pores. Although this exchange- adsorption is not an essential step in the ion absorption process it may affect it by either increasing or decreasing the availability of ions for uptake into the symplasm.

Radio-opaque salts can diffuse across the root as far as the Casparian band of the endodermal cells (see for example Robards and Robb, Planta, 120, 1-12, 1974). An appreciable volume of the root exterior to the endodermis is freely accessible to ions - the "apparent free space". Ions move into this "free space" by diffusion or by mass flow with water. Anions are excluded from pores narrower than 0.5 nm diameter, which suggests that their uptake into the symplasm occurs at the root epidermis or outer layer of the cortex.

### Ion Accumulation in Cells

Ions are transferred from the free space across the plasmalemma into the cytoplasm of the cortical or endodermal cells usually against a concentration gradient (Marschner pp 6-12).

The plasma membrane (plasmalemma) and the tonoplast are the main sites for ion absorption and transport in roots.

#### Active and Passive Absorption (Marschner pp 19-26)

Ion uptake can result in concentrations in the plant that are greater than those in the external medium. Furthermore there is considerable selectivity between ions.

Changes in the ion concentration in the external medium and in sap expressed from the roots of maize and faba bean (after Marschner ).

 Concentration in external medium (mM) Concentration in root-press sap  (mM) Initial After 4 days Ion Corn Faba bean Corn Faba bean Potassium 2.00 0.14 0.67 160 84 Calcium 1.00 0.94 0.59 3 10 Sodium 0.32 0.51 0.58 0.6 6 Phosphate 0.25 0.06 0.09 6 12 Nitrate 2.00 0.13 0.07 38 35 Sulphate 0.67 0.61 0.81 14 6

The plasma membrane (plasmalemma) and the tonoplast are the main sites for ion absorption and transport in roots.

#### Movement of solutes by diffusion

The steady state flux is given by:

For water movement the chemical potential given by the Gibbs free energy could be used to determine flow:

where Nw is the molar fraction of water

In dilute solutions the chemical potential of an ion is given by:

µ again represents the reference state

R = gas constant (8.3 J K-1 mol-1

T = absolute temperature (K)

Cs = concentration in solution (In concentrated solutions activity rather than concentration should be used)

If the ion is in an electrical field, the effect of that electrical potential on the ion also has to be considered and the chemical potential must be replaced by the electrochemical potential:

where Z = valence of ion (e.g. +1 for K+, -2 for S042-

F = Faraday constant - charge carried by one mole of ions (96,400 JV-1 mole-1

y= electrical potential of the system

Ions move from a region of high to a region of lower electrochemical potential. Thus the direction of movement is determined by the difference in electrochemical potential.

Electrochemical potential gradients exist across the plasmalemma and determine the amount of energy required to move an ion across the membrane.

If for a given ion, the electrochemical potential inside the cell (in cytoplasm) is the same as that in the solution outside, there will be no spontaneous movement across the membrane and the system is in a passive flux equilibrium.

µi = µo
RT ln Ci + ZFyi = RT ln Co + ZFyo

EN is referred to as the Nernst potential.

The Nernst potential is the electrical potential difference necessary to maintain a given concentration difference. If the electrical potential in the cytoplasm is negative compared to the outside, a cation will be at a higher concentration inside than outside at a passive equilibrium.

At a temperature of 20oC (293K) the equation reduces to:

and if EN is expressed in mV

Thus if the electrical potential inside the cell is -58 mV relative to outside, a monovalent cation would be 10 times as concentrated inside at passive equilibrium. In contrast, the concentration of a monovalent anion in the cell would be one tenth of that outside.

An electrical potential usually exists across a cell membrane with the cytoplasm being more negative than the external solution. This electrical potential is maintained by ion transport systems or "pumps" in the cell membrane which use metabolic energy to transport ions against a concentration gradient.

The dominant system is a proton pump which transports protons (H+) out of the cell. This pump uses energy from ATP to effect the transfer of the proton. ATP phosphohydrolase (generally referred to as ATPase) is located in the cell membrane and hydrolyses ATP to form ADP and release the energy in the pyrophosphate bond. A cation (eg. K+) may move into the cell in exchange for the H+, resulting in no net change in electrical charge in the cell. If a cation is not exchanged for H+ there will be a residual negative charge in the cell. Because the electrical potential of the cytoplasm is usually negative, cations may accumulate in the cell without direct expenditure of energy.

Work must be done to move anions in against the negative electrical potential. Transport across membranes can be defined as active only after an assessment of the combined driving forces (chemical and electrical potential differences) which may act upon the ion. If the ion's distribution cannot be accounted for by the passive driving forces, active transport is invoked.

Passive movement of ions may be facilitated by carriers and channels, taking advantage of the electrochemical potential gradient established by an ATPase. Specific ions may move through ion channels 3-5 orders of magnitude faster than carriers. Channel opening responds to the voltage gradient across the membrane and to external stimuli such as light or hormones. However, the evidence suggests that they are closed most of the time.

### Kinetics of Ion Absorption (Marschner pp 26-30)

Kinetics of ion absorption can be described by the same Michaelis - Menten relations used to describe the kinetics of enzyme reactions:
V = Vmax Cs
Km + Cs

V = velocity of reaction

Vmax = maximum velocity

Km = constant - concentration when V = ½ Vmax

Cs = concentration of ion in solution

Since there is some efflux of ions from cells, the net intake can be given by:

In = Imax (Cl - Cmin
Km + (Cl - Cmin )

I = intake (equivalent to V)

Cl = Conc. of ion in solution at root surface

Cmin = Concentration at which efflux equals influx

Imax and Km vary with species, age of plant, age of root and nutrient demand of the shoot. Mechanisms for the regulation of nutrient uptake are uncertain, but could involve increased rates of efflux or feedback control as tissue concentrations rise (see Siddiqi et al. Plant Physiol, 93,1426-32,1990), and there may also be transcriptional control (i.e. rates of synthesis of carrier molecules).

### Effect of Other Ions on Absorption (Marschner pp 38-42)

Nutrient absorption is also influenced by the presence of other ions. Addition of an ion of one charge - e.g. K+ - may decrease the absorption of other similarly charged ions - e.g. Mg2+ - and increase the absorption of oppositely charged ions. Other more specific interactions also occur. One example is the antogonism between N03- and Cl. Ammonium ions can also inhibit the uptake of nitrate ions. The evidence suggests that the uptake of NH4+ increases the potential in the cytosol and thereby supresses the proton- N03- symport action.
Effects of nitrate concentration in the rooting medium and chloride uptake (after Glass and Siddiqi, J.Exp.Bot, 36,556-566, 1985 ).
 Concentration in nutrient solution (m mol L1) Cl- NO3- Cl- in roots  (µmol g1 fresh weight) Cl- in shoots  (µmol g1 fresh weight) 1 0 52 93 1 0.2 26 73 1 1 13 54 1 5 9 46

## Uptake Along and Movement Across the Root (Marschner pp 63-78)

Ion uptake along a root tends to decline with distance from the tip but basal root zones have a considerable capacity for ion uptake. The decline in uptake with distance from the tip is much greater for Ca or Mg than for other ions.

Ions may move to the stele in the free space (apoplastic pathway) or in the symplast (symplasmic pathway) but must be actively transferred across the plasmalemma of the endodermis if they have not been absorbed by cortical cells. Cell to cell movement in the symplast is through the plasmdesmata, which are more frequent on tangential walls of the endodermis and xylem parenchyma than on cells of the outer cortex.

Plasmadesmata numbers per unit area on tangential walls of root cells in barley (Robards et al.,1973).

 Location Frequency of plasmadesmata  (µm-2) Cortex 0.28 Cortex / Endodermis 0.37 Endodermis / Pericycle 0.75

In the stele, the ions must be transferred from living cells in the stele to non-living xylem vessels. This may occur as leakage but evidence suggests that an active process requiring metabolic energy is involved. The movement of calcium poses a significant problem for the idea that a general apoplastic barrier exists near the root periphery (the exception being where the endodermis is breeched by emerging laterals). Symplastic flow of Ca2+ is thought to be restricted because its presence in the cytoplasm would tend to remove phosphate by precipitation. It has been suggested that in the endodermal cells there may be a preponderance of Ca2+ channels in the plasmalemma on the outer tangential wall, and Ca2+-translocating ATPase units in the membrane.on the inner wall (Clarkson, Plant, Cell Environment, 7, 449-456).

### Long-Distance Transport of Nutrients

#### Uptake and transport of cations within the root

The figure below shows the difference between cations in their longitudinal movement within the root.

Accumulation and movement of cations within the root of maize. (After Marschner)

Whereas K+ moved and was accumulated both acropetally and basipetally, Ca+ showed no acropetal movement and Na+ only a small amount. Sodium showed some movement but was strongly accumulated in the basal direction along the root so that almost none reached the shoot. Calcium was not accumulated in the root to any significant extent, but did move to the shoot. Potassium showed most movement to the shoot.

### Xylem Transport (Marschner 81-91, 70-73)

Transport from the roots to the shoot occurs in the non-living cells of the xylem and is driven by: (1) the gradient in hydrostatic pressure (root pressure) resulting from the release of nutrients into the xylem; and (2) the gradient in water potential resulting from water loss primarily at leaf surfaces (transpiration).

Flow to the shoot is induced by secretion of solutes into the xylem vessels, so lowering the water potential and consequently there is an inflow of water, the permeability of membranes being much greater for water than for ions. Nutrient flow in the xylem is therefore only in the upward direction.

Protons are pumped into the apoplast of the xylem by the xylem parenchyma cells.

Anions may move in co-transport with the protons, or move along the electrochemical potential gradient.

Reabsorption of protons might result in providing the driving force for cations to move into the apoplast (antiport)

The preferential loading of nitrate compared with glutamine may be important in partitioning different forms of N between roots and shoots

Processes in the xylem

Interactions between positively-charged ions (inorganic and organic) and negatively-charged groups on the cell walls of the xylem (exchange adsorption) can reduce the speed of movement. The magnitude of these interactions depends on the ion and its concentration, and the presence of competing ions.

The role of exchange adsorption is quite variable in intact plants. The following table is an example of the main metal-organic ligand complexes formed in xylem fluid.

Chemical speciation (% of total metal) of metals in tomato xylem exudate as calculated by the computer programs (CHELATE AND GEOCHEM).

 Tomato Species CHELATE GOECHEM Ca2+ 83.8 81.1 Ca-citrate 8.3 10.8 Ca-malate 7.0 7.8 Mg2+ 88.7 88.9 Mg-citrate 6.2 -- Mg-malate 4.1 8.5 Mg-P04 -- 1.3 Mn2 72.2 -- Mn-citrate 10.6 100.0 Mn-malate 15.7 -- Mn-malonate 1.3 --

Nutrients are also absorbed (resorption) into or released from the living cells that surround the route from roots to leaves. The concentration and composition of the xylem fluid changes, therefore, along the route.

Occurrence of Mo toxicity occurs inversely proportional to absorption of the element in the xylem

Both organic molecules and mineral nutrients exchange between the xylem vessels and the parenchyma cells.

Some xylem parenchyma cells have a transfer cell appearance.

Xylem parenchyma is important in ion selectivity - ie what moves to the shoot

1. There is preferential accumulation of some mineral nutrients in the root eg Ca, Cd, and occurrence of Mo toxicity occurs inversely proportional to absorption of the element in the xylem
2. pH stasis is important in the selectivity. For different anions the balance exported from the root differs considerably - eg more NO3 than SO42-, the pH balance is maintained by organic acid production and other mineral anions
3. Temperature can also affect the phenomenon - eg the ratio of K : Ca increases with root temperature
4. When nutrient supply from the roots declines, the parenchyma cells can release stored nutrients into the vessels.
Transpiration rate is a key factor influencing the uptake and distribution of nutrients.

Effect of transpiration rate on uptake and transport of nutrients in maize.

 Relative humidity  (%) Nutrient content  (mg g1 dry weight) K Mg Ca Fe >95 47.0 1.4 1.6 0.04 60 49.9 1.5 1.5 0.06

Effect of transpiration rate on uptake and transport of nutrients in peppers.

 Nutrient content of fruits  (mg g1 dry weight) Transpiration K Mg Ca 35 91 3.0 2.8 100 88 2.4 1.5

Effect of transpiration rate on uptake and transport of nutrients in sugar beet

 Relative  Transpiration Nutrient (mole plant1 in 4h) K Na Uptake Translocation Uptake Translocation 15 10.3 6.5 12.0 3.4 100 11.0 7.0 19.1 8.1

Amino acid transport is also affected by transpiration rate.

Transfer to sinks

Where mineral nutrients end up in the shoot depends on the relative water use by the various components. Even within a leaf the evaporation is variable, which can lead to localized toxicity symptoms (eg B)

Transfer out of the xylem considered to involve proton pumps. Co-transport of NH4+ and ureides involves a proton pump

Some minerals get precipitated in the apoplasm - avoidance of excess concentration in the apoplasm

Phloem Transport (Marschner 92-98)

Long-distance translocation in the phloem with its living cells, occurs in the upward and downward directions. The direction is determined by the nutritional requirements of the various plant organs. Phloem translocation is independent of transpiration and supplies the major proportion of nutrient requirements for actively growing areas, such as young leaves, fruits and seeds, organs that lose little, if any, water. This is also the route for the movement of sugars formed as a result of carbon dioxide (C02) assimilation in the leaves.

In the first figure, both Na+ and K+ moved acropetally in the phloem of the root, and K+ is mobile in both phloem and xylem.

Composition of xylem and phloem exudate

 Component Xylem (mg mL1) Phloem (mg mL1) Dry matter 1.2 184 Sucrose - 162 Amino compound 283 10808 NH4+ 9.7 45.3 K 204 3673 P 68 435 Ca 189 83 Mg 34 104 Mn 0.2 0.9 Na 46 116 Cl 64 486 S 43 139 Fe 0.6 9.4 Zn 1.5 15.9 Cu 0.1 1.2

Plant growth regulators are present in both xylem and phloem sap.
Xylem/Phloem Exchange (Marschner 98-99)

During translocation, nutrients are transferred between the xylem and the phloem by extensive exchange processes. In the stem and leaf veins xylem and phloem are separated by only a few cell layers, and direct exchange of unmetabolized nutrients between these two streams is mediated by specialized cells known as transfer cells, which posses extensive wall ingrowth that enhances the cells' surface area. Indirect transfer occurs when nutrients are transformed in the leaf and immediately exported in the phloem, or stored and exported at a later stage of development. The latter process is often associated with leaf senescence during the growth of reproductive structures (flowers and fruits) and involves the mobilization of reserve materials such as proteins and carbohydrates. It is less associated with new vegetative growth unless the plant has experienced a period of insufficient nutrient supply.

The relative supply of nutrients in the xylem and phloem changes with development, with the xylem contributing progressively more as the surface area for transpirational water loss increases. In organs such as legume fruits, rutabaga roots and potato tubers, which transpire relatively little during growth, mineral nutrients must be provided almost exclusively by the phloem stream.

Cycling of mineral nutrients between shoots and roots

20% of K in xylem may have recycled from the shoot.

For amino-N it may be 60%

Mobile and Immobile Nutrients (Marschner 100-105)

Different nutrients have different mobilities in phloem fluids and considerable variability in nutrient mobility exists between different plant species and between plants of different nutrient status. With mobile nutrients such as nitrogen, the growth of developing organs may continue using nutrients mobilized from mature or old organs even when there is no external supply of these mobile nutrients. Localized deficiencies do not develop until the total amount of nutrient in the plant becomes insufficient. Thus, the concentration of nutrients in the whole plant or in old leaves is a good indicator of nutrient status and an early indicator of deficiency. The concentration of a mobile nutrient in young leaves is a poor guide to nutrient status since it remains high even when the plant is deficient. Other nutrients such as calcium, manganese and boron are accumulated in transpiring tissues and are rarely mobilized. In general, plant organs can only develop properly if they receive a continuous supply of immobile nutrients from the external medium in the xylem stream. Deficiency symptoms are first evident on young organs. Clearly, in devising diagnostic procedures and in making recommendations fro treatment of nutrient disorders, the contrasting behaviour of mobile and immobile nutrients must be considered.

Mobility of mineral nutrients in the phloem

Minerals show major differences in mobility
Mobility of mineral nutrients in the phloem

 High mobility Intermediate Low mobility K Fe Ca Mg Zn Mn P Cu S B amino-N Mo Cl (Na)

Relative importance of xylem and phloem for supply of highly mobile and low mobility ions.

 High mobility (eg K, P) Low mobility (eg Ca) Xylem Phloem Xylem Phloem Growth ++ + ++ t production ++ - ++ t senescence + -- -

Aspects of transport of specific ions

Phosphorus

Radial transfer may be as ATP or sugar phosphates. Mineral P is then released into the xylem. Glucose-6-phosphate and glucose-6-phosphatase are both present in high concentration in xylem parenchyma cells. And inhibition of the enzyme greatly reduces the xylem loading of P but not uptake into roots.

Potassium

Some requirement for K in the shoot may be met by the release from loaded late metaxylem vessels in corn

K is the counter-ion for NO3- transport in the xylem and may be transported back to the root in the phloem as companion cation to malate. The malate being formed to balance the charge as nitrate is reduced in the leaf.

Sodium

Within the xylem of the stem, Na is removed from vessels into parenchyma cells and then transferred back to the roots.