Long distance transport of mineral elements

2.1. The Last Barrier: the Endodermis as “Customs”

Recall the anatomical structure of the root. Between the cortex and the central cylinder, where the xylem vessels are located, lies the inner layer of cortical cells—the endodermis. Endodermal cells have a unique feature: their radial and transverse walls are impregnated with an impermeable substance—suberin—forming the so‑called Casparian strips (Marschner, 2012).

The Casparian strips act as a hydraulic seal (Tret'yakov et al., 2000). They interrupt the apoplastic transport pathway—the route through cell walls and intercellular spaces. If such a barrier did not exist, water and dissolved ions could leak freely along cell walls directly into the xylem. In that case, the root could not regulate which substances and in what quantities enter the upward flow. The plant would be a “hostage” of the soil solution.

The presence of Casparian strips forces all ions moving toward the centre of the root to pass through the plasmalemma of endodermal cells and then travel via the symplast—the network of protoplasts connected by plasmodesmata (Marschner, 2012; Tret'yakov et al., 2000). This means that on the way into the xylem, each ion must cross yet another cell membrane and thus undergo control by the cell. The endodermis acts as the “last checkpoint” through which all substances destined for the shoot must pass. This switch from the apoplastic to the symplastic pathway is an obligatory condition for ions to enter the central cylinder (Marschner, 2012).

2.2. Xylem Loading: Active Secretion into the Vessels

After passing through the endodermis, ions move via the symplast (through pericycle cells) to the parenchyma cells adjacent to the xylem vessels. Here the final step before long‑distance transport occurs—xylem loading. Ions present in the symplast must be “unloaded” into the lumen of the dead xylem vessels.

For a long time, this process was thought to be passive: ions simply leak out of living cells into the vessels. However, research has shown that xylem loading is an active, regulated process. On the plasma membrane of the cells surrounding the xylem vessels (called xylem parenchyma cells), transport proteins are located that secrete ions into the vessel apoplast (Marschner, 2012; White, 2012).

This process can occur via several routes:

• Passive transport: Some ions, for example potassium (K⁺), can exit into the vessels through specialised potassium channels, such as the SKOR channels in Arabidopsis thaliana (White, 2012). These channels open at a certain membrane potential, and potassium moves down its electrochemical gradient. However, even this passive efflux is regulated, because channel activity is controlled by the plant (Marschner, 2012).

• Active transport: For ions that need to be concentrated in the xylem sap, or for anions (e.g., nitrate NO₃⁻) that are transported against an electrochemical gradient, active mechanisms are used. The key player here is the H⁺‑ATPase—a proton pump that, as in the plasmalemma of root hairs, pumps protons out of the cell into the vessel apoplast. This creates an electrochemical gradient that is used for secondary active transport (antiport or symport) of other ions (Marschner, 2012). For example, nitrate is loaded into the xylem through transport proteins of the NRT1 family (White, 2012; Tret'yakov et al., 2000).

• Vacuolar contribution: It is important to note that ions previously stored in cortical cell vacuoles can also enter the xylem sap. During root ageing or when soil element supply suddenly ceases, the plant can mobilise these stores and “release” them into the upward flow (Tret'yakov et al., 2000).

That xylem loading is a genetically controlled process is demonstrated by experiments with Arabidopsis mutants. For instance, the pho1 mutant has impaired phosphorus transport from root to shoot. The roots of this mutant take up phosphate from solution perfectly well but cannot load it into the xylem, so the plant suffers from phosphorus starvation (Marschner, 2012; White, 2012).

Thus, xylem loading is not just a passive leakage of ions but a crucial regulatory step. It is here that the amount of each element to be sent to the above‑ground part is determined. This allows the plant, on the one hand, to meet the demands of the shoot and, on the other, to avoid toxic ion concentrations in the leaves. In addition, there is an apoplastic “bypass” route around the endodermis. This is possible in the root cap zone, where the endodermis has not yet formed, or at the sites of lateral root emergence, where the integrity of the Casparian strips is disrupted. This pathway, called bypass flow, is especially important for the entry of calcium (Ca²⁺) into the xylem and, under salinity conditions, sodium (Na⁺) (Marschner, 2012).

We see that entry of elements into the xylem is a complex, energy‑consuming, and tightly regulated process that ensures selectivity and controllability of the entire mineral nutrition system. In the next chapter, we will consider in what exact form elements travel along this transport route.

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3.1. Ionic Composition: The Basis, but Not Everything

Undoubtedly, the main components of xylem sap are macronutrients in ionic form: cations of potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), as well as anions of nitrate (NO₃⁻), phosphate (H₂PO₄⁻), sulphate (SO₄²⁻), and chloride (Cl⁻). These ions constitute the major part of the osmotic pressure of xylem sap and meet the shoot’s requirements for macronutrients.

For example, analysis of xylem sap from tobacco (Nicotiana glauca) showed that potassium concentration is about 200–250 mg/L, and phosphorus about 68 mg/L (Boote et al., 1994; Hocking, 1980). The element ratios in the xylem can differ greatly from those in the external environment, which is a consequence of the selective work of the root. As a rule, potassium and nitrate predominate in the xylem, while phosphate and sulphate concentrations are much lower.

However, ions do not travel alone in the xylem. Their transport form strongly depends on interactions with organic acids, amino acids, and specialised chelating agents.

3.2. Organic Acids as the Main “Companions” of Cations

One of the key features of xylem sap is the presence of organic acids, primarily citrate, malate, and succinate (Marschner, 2012; White, 2012). Their role cannot be overstated.

First, organic acids perform a compensatory function. Since the root often absorbs more cations than anions (especially under ammonium nutrition or nitrogen fixation), to maintain electroneutrality in the xylem, the plant synthesises and releases organic anions (e.g., malate) into the vessels. They neutralise the positive charge of potassium and calcium (Marschner, 2012; Tret'yakov et al., 2000). Therefore, a certain proportion of organic acids is always present in xylem sap, serving as a “buffer” for cations.

Second, and even more important physiologically, organic acids are natural chelators (complexing agents) for many cations. This is especially relevant for polyvalent cations, which in free form can be poorly mobile and even toxic. In particular:

• Iron (Fe³⁺) in xylem sap is transported predominantly as a complex with citrate (Marschner, 2012; White, 2012). This protects iron from precipitation and ensures its delivery to leaves. It is known that Arabidopsis mutants with impaired citrate loading into the xylem (e.g., frd3) suffer from iron‑deficiency chlorosis despite sufficient iron content in the roots (White, 2012).
• Magnesium (Mg²⁺) and zinc (Zn²⁺) can also move as complexes with organic acids.
• Calcium (Ca²⁺), although a divalent cation, is largely transported in free ionic form, but part of it is also bound to organic anions.

3.3. Nitrogen: Nitrates and Organic Forms

Nitrogen is the only element that can be transported in the xylem in both inorganic and organic forms. In plants receiving nitrate nutrition, a significant portion of nitrogen moves precisely as NO₃⁻ (Hopkins & Hüner, 2009). However, depending on the species and conditions, part of the nitrate is reduced to ammonium in the roots and incorporated into amino acids. These organic compounds—amino acids and amides (glutamine, asparagine)—also enter the xylem and are transported to the shoots (Marschner, 2012; Lambers & Oliveira, 2019).

The proportion of organic nitrogen in xylem sap varies greatly. In plants that reduce nitrate predominantly in the roots (e.g., pea and lupine), organic nitrogen (glutamine, asparagine) predominates. In plants that reduce nitrate in the leaves (e.g., sugar beet or cotton), nitrate prevails in the xylem (Tret'yakov et al., 2000). In legumes that actively fix atmospheric nitrogen, xylem sap is rich in amides and ureides (allantoin, allantoic acid), which are the main transport forms of fixed nitrogen (Hopkins & Hüner, 2009).

3.4. Metal Chelates: Protection and Transport

For micronutrients such as iron, zinc, copper, and manganese, transport in free ionic form through the xylem would be extremely inefficient because of their tendency to hydrolyse and precipitate. Therefore, evolution has developed special mechanisms for their transfer as chelates—stable complexes with organic molecules.

• Nicotianamine is a universal chelator found in the xylem sap of many plants. It binds Fe²⁺, Zn²⁺, Cu²⁺, and Mn²⁺ ions, ensuring their solubility and transport. Nicotianamine is also a precursor of phytosiderophores in grasses (Marschner, 2012; White, 2012).
• Citrate and malate, as already mentioned, perform the same function for Fe³⁺ and other cations.
• Histidine—an amino acid that in some species (e.g., nickel hyperaccumulators) participates in the transport of nickel and zinc as histidine complexes (Marschner, 2012).
• Phytosiderophores are a special class of chelators excreted by grass roots (strategy II for iron uptake), which can also be loaded into the xylem as Fe‑phytosiderophore complexes (Marschner, 2012).

Such diversity of transport forms is not accidental. It allows the plant to flexibly regulate the supply and distribution of elements, prevent their toxic effects, and ensure their availability at the sites of utilisation.

3.5. Factors Affecting Xylem Sap Composition

The composition of xylem sap is not constant. It changes dynamically under the influence of many factors:

• Element supply: When a particular element is deficient, its concentration in the xylem decreases; when in excess, it increases, which can lead to toxic effects.
• Transpiration rate: The higher the intensity of water evaporation, the faster the xylem flow. However, the concentration of elements in it may change: under low transpiration (at night, in high humidity), the xylem sap is more concentrated, while under strong transpiration it is diluted (Marschner, 2012).
• Time of day and season: In legumes, for example, ureide transport increases during the light part of the day, while in many plants the proportion of organic nitrogen rises at night, which is related to nitrate reductase activity (Tret'yakov et al., 2000).

In summary, xylem sap is not just water with salts. It is a dynamic transport system containing ions, organic acids, amino acids, and chelates in complex proportions, which ensures not only the delivery of macro‑ and micronutrients but also the maintenance of plant ion homeostasis. Understanding this diversity of transport forms is key to understanding how the plant manages its mineral nutrition.

In the next chapter, we will consider why xylem alone is insufficient for the full distribution of elements and what role phloem plays in this.

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4.1. Phloem Transport: Fundamental Differences from Xylem

Unlike the xylem, the phloem is a system of living cells (sieve tubes) that transport a concentrated solution of organic substances, primarily sucrose, from sites of synthesis (photosynthetic source leaves) to sites of consumption or storage (actively growing organs, fruits, seeds, storage roots, and tubers). This flow is called downward, but in fact it can be directed downward (to roots), upward (to young leaves, flowers, and fruits), or even horizontally (to lateral shoots). The direction is determined by the sucrose concentration gradient: substances always move from source to sink (Boote et al., 1994; Hopkins & Hüner, 2009).

However, the phloem is not only for sucrose transport. Along with organic substances, mineral elements also move through the sieve tubes, and in considerable quantities (Boote et al., 1994; Marschner, 2012). Analysis of phloem sap (which is much more difficult to collect than xylem sap) shows that the concentration of many elements is higher than in the xylem. For example, in tobacco phloem sap, potassium concentration reaches 3673 mg/L, which is 18 times higher than in the xylem; phosphorus 6.4 times; magnesium 3.1 times (Boote et al., 1994). This means that the phloem is a major pathway for element redistribution within the plant.

4.2. What Moves in the Phloem and What Are the Features of This Transport?

Phloem sap has a high pH (7.8–8.0), which is fundamentally different from the acidic reaction of xylem (pH about 5.6). This creates specific conditions for complexation and element transport. The main groups of substances in the phloem flow are:

• Macronutrients: Potassium (K⁺) is the dominant cation of the phloem, serving as an osmotic agent that maintains the pressure gradient for mass flow of assimilates (Boote et al., 1994; Marschner, 2012). Phosphorus (P) is mainly in the form of phosphates and organic phosphate esters. Nitrogen is in the form of amino acids and amides (glutamine, asparagine). Magnesium is well mobilised and transported in the phloem.
• Micronutrients: Significant amounts of zinc, iron, copper, and molybdenum are also found in the phloem (Marschner, 2012). The form of their transport is particularly interesting. Unlike the xylem, where micronutrients are often bound to organic acids, in the phloem they appear to be transported as complexes with nicotianamine, as well as in association with specialised carrier proteins (e.g., for iron—the ITP protein) (White, 2012).
• Limitations: Two elements are virtually absent from the phloem flow (or present in negligible amounts)—calcium (Ca²⁺) and, to a lesser extent, boron (B) (Boote et al., 1994; Marschner, 2012). The concentration factor of calcium in the phloem relative to the xylem is only 0.44 (Boote et al., 1994). This means that calcium is practically not redistributed via the phloem. This fact is key to understanding its deficiency symptoms, to which we will turn in the next chapter.

4.3. Why Does the Plant Need Phloem Transport: Main Functions

Thus, the phloem solves several fundamentally important tasks in the distribution of mineral elements:

1. Recirculation (cycling) of elements. Elements that entered leaves via the xylem can be loaded into the phloem and sent back to the roots. This allows the plant to reallocate resources according to the needs of different organs. For example, potassium and phosphorus can circulate between roots and shoots several times a day, acting as osmotically active substances and signalling molecules (Marschner, 2012; Tret'yakov et al., 2000).

2. Reutilisation (reuse) of elements. This is a key mechanism that allows the plant to “recycle” elements from senescing organs. For instance, from yellowing lower leaves where proteins and nucleic acids are broken down, amino acids and phosphate esters are loaded into the phloem and transported to young leaves or developing fruits (Hopkins & Hüner, 2009). Thanks to this, grain in cereals can be filled even after nitrogen uptake from the soil has virtually ceased (Tret'yakov et al., 2000).

3. Delivery to non‑transpiring organs. Fruits, seeds, rhizomes, and tubers have very low transpiration, so they receive almost no elements via the xylem. Their main source of mineral nutrition is the phloem, which brings sugars and accompanying ions. For example, this is why gypsum is used for top‑dressing developing peanut fruits (which are in the soil and do not transpire water)—calcium must reach the fruits through the apoplast from the soil, since it is not transported via the phloem (Boote et al., 1994).

4. Signalling function. The phloem is not only a transport channel but also a pathway for rapid spread of signalling molecules. Potassium concentration in the phloem can regulate the rate of sucrose export, and changes in phloem sap composition inform the roots about the shoot’s requirements for specific elements (Marschner, 2012).

Thus, the xylem and phloem form a single, interconnected transport network. The xylem is the “artery” delivering water and minerals from the root to the leaves, while the phloem is the “vein” that not only removes photosynthetic products but also ensures fine, targeted redistribution of elements, returning them from some organs to others and delivering them where they are most needed. Without this second system, the plant could not survive in a changing environment nor successfully form seeds and fruits.

In the next, concluding chapter, we will see how these fundamental features of transport explain what we observe in the field—deficiency symptoms on different leaves.

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5.1. Mobility of Elements: A Fundamental Difference

In the previous chapter, we established that the phloem enables redistribution (reutilisation) of elements from senescing organs to young ones. However, the ability of elements to undergo such reutilisation is not uniform. Based on analyses of phloem sap composition and experiments with labelled atoms, all mineral elements can be divided into three groups according to their physiological mobility (Boote et al., 1994; Marschner, 2012; Tret'yakov et al., 2000):

This distinction is not accidental. It is determined by the chemical form in which the element is present in tissues and how easily it can be loaded into the phloem.

Why are potassium, magnesium, and nitrogen so mobile?

• Potassium (K⁺) is present in cells predominantly in free ionic form. It does not form part of strong organic structures and is readily loaded into the phloem (Boote et al., 1994; Marschner, 2012).
• Magnesium (Mg²⁺) is an important component of chlorophyll, but when the leaf senesces, chlorophyll breaks down, and the released magnesium can be mobilised and redistributed (Boote et al., 1994).
• Nitrogen (N) in organic form (amino acids, amides) is a major component of phloem sap. Proteins and nucleic acids in senescing leaves are actively hydrolysed, and the resulting amino acids are transported to young organs (Hopkins & Hüner, 2009).

Why are calcium and manganese virtually immobile?

• Calcium (Ca²⁺) is a unique element. Most calcium in the plant is found in cell walls as insoluble salts of pectic acids (calcium pectate) and also as calcium oxalate crystals in vacuoles (Boote et al., 1994; Marschner, 2012). These forms are structural and practically insoluble. Moreover, even if calcium is in soluble form, its loading into the phloem is extremely difficult, and the calcium concentration in phloem sap is always very low (Boote et al., 1994). Therefore, calcium is unequivocally an immobile element. This is precisely why gypsum is used for fertilising peanut fruits that are in the soil: calcium must reach them directly from the soil via the apoplast, because the phloem cannot deliver it (Boote et al., 1994).
• Manganese (Mn²⁺) also has very low phloem mobility, although the mechanisms are not fully understood. It is often bound in tissues as insoluble oxides or tightly associated with proteins.

The medium mobility of iron, zinc, boron, and copper is explained by the fact that they can be loaded into the phloem, but only as specific chelates (e.g., with nicotianamine) and provided they are in excess in the cell. However, the rate of this loading and redistribution is considerably lower than for potassium or magnesium (Marschner, 2012; White, 2012).

5.2. How Does Mobility Determine the Pattern of Deficiency Symptoms?

Now we can give a clear physiological explanation for what is visually observed in the field.

1. Deficiency of mobile elements (N, P, K, Mg):

• Why do symptoms appear on old leaves? Because under deficiency conditions, the plant “sacrifices” old leaves that have already fulfilled their function to supply young, actively growing tissues that determine future productivity (Tret'yakov et al., 2000). The process proceeds as follows: when the element is lacking in the soil, its concentration in the xylem drops. In response, the plant activates mechanisms of organic compound degradation in old leaves (e.g., chlorophyll breakdown to release magnesium, protein hydrolysis to obtain nitrogen). The released elements are loaded into the phloem and transported to the growing points.
• Result: old leaves turn yellow (chlorosis), redden (excess anthocyanins), or die (necrosis), while young leaves remain green for some time. A typical example is yellowing of lower leaves under nitrogen deficiency, or marginal scorch of old leaves under potassium deficiency (Tret'yakov et al., 2000).

2. Deficiency of immobile elements (Ca, Fe, Mn, B):

• Why do symptoms appear on young leaves? If an element cannot be redistributed from old tissues, then young, growing organs are entirely dependent on its current supply via the xylem. As soon as root uptake decreases, an immediate deficiency arises at the growing points (Boote et al., 1994; Tret'yakov et al., 2000).
• Result: symptoms first appear on the youngest, actively growing parts of the plant—on upper leaves, growing points, and fruits. Classic examples:
  • Iron (Fe) deficiency: chlorosis (yellowing) of young leaves, while old ones remain green (Tret'yakov et al., 2000). This is because iron is a component of enzymes required for chlorophyll synthesis, and it cannot be redistributed from old leaves.
  • Calcium (Ca) deficiency: dieback of apical buds, young leaves, and disorders such as blossom‑end rot in tomatoes or bitter pit in apples. These organs have low transpiration and receive almost no calcium via the xylem, and because of calcium immobility in the phloem, they cannot obtain it from other parts of the plant (Boote et al., 1994; Marschner, 2012).
  • Boron (B) deficiency: death of growing points, deformation of young leaves, and impaired pollination.

5.3. Exceptions Confirming the Rules

It is important to understand that element mobility is not an absolute characteristic. In some cases, immobile elements can be redistributed.

Reproductive stage: During seed and fruit maturation, senescence processes intensify throughout the plant. During this period, enhanced breakdown of organic substances in vegetative organs occurs, and even elements such as iron and zinc can be significantly mobilised and transported into seeds (Marschner, 2012; Tret'yakov et al., 2000). This is why zinc and copper deficiencies in cereals often manifest not immediately, but specifically during grain filling.

Severity of deficiency: Under very severe, chronic deficiency of an immobile element, the plant may start to break down old tissues to release it, but the efficiency of this process is very low, and symptoms still remain on young leaves.

5.4. Summary: A Knowledge System for Diagnostics

Thus, visual diagnosis of nutrient deficiencies is not merely memorising pictures (chlorosis on young leaves—iron, on old leaves—nitrogen). It is a deep physiological regularity based on:

1. Understanding the transport systems (xylem and phloem).

2. Knowing the mobility of each element (its ability to be reutilised).

3. Understanding the element’s function (if it is part of chlorophyll—deficiency causes chlorosis; if part of cell walls—necrosis and deformation).

Now, when a student sees yellowing upper leaves on maize, they do not simply say “it is iron deficiency”—they see a whole chain: iron is immobile → it is not redistributed from old leaves → young tissues depend on current supply → therefore, the problem lies in poor iron uptake from the soil or in disrupted xylem transport. And if the lower leaves yellow, it indicates that nitrogen, potassium, or magnesium is being actively withdrawn from old tissues to supply the young ones. This is how physiology becomes a tool of practical agronomy.

Conclusion of the Lecture:

We have travelled the full path: from the question “what next?” after ion uptake to understanding why deficiency diagnostics is an applied task of transport physiology. The lecture has shown that the mineral element distribution system in plants is a two‑component system (xylem + phloem) that ensures not only the delivery of resources from the soil but also their flexible redistribution within the plant. Knowledge of these mechanisms is the basis for skilful management of crop nutrition, timely diagnostics, and ultimately for achieving high yields.