Interaction of mineral nutrition with growth and productivity

1.1. Essentiality and the Fundamental Role of Mineral Elements

Before discussing limitations, we must understand why a plant depends on external mineral supply at all. Unlike animals, plants are autotrophic organisms capable of synthesizing all necessary organic compounds from carbon dioxide, water, and light. However, for this synthesis they require elements that they cannot produce on their own (Taiz et al., 2023). These elements are called essential—their absence makes it impossible for the plant to complete its life cycle.

Essential elements perform fundamental functions that can be grouped as follows (Marschner, 2012):

1. Constituents of organic compounds – nitrogen (proteins, nucleic acids, chlorophyll), sulfur (amino acids cysteine and methionine), phosphorus (nucleic acids, ATP, phospholipids).

2. Participation in energy metabolism – phosphorus in ATP, magnesium as a cofactor of ATPases, potassium for enzyme activation (Blevins, 1994).

3. Maintenance of structural integrity – calcium and boron in cell walls, silicon for mechanical strength.

4. Osmoregulation and transport – potassium is the main osmotic ion, determining turgor and stomatal movement (Marschner, 2012).

5. Participation in redox reactions – iron, copper, manganese in enzymes of electron transport chains (Blevins, 1994; Taiz et al., 2023).

It is precisely this fundamentality that creates the precondition for the law of the minimum in its physiological interpretation.

1.2. The Law of the Minimum: Not a Barrel but a Physiological System

In popular literature, the law of the minimum is often illustrated by “Liebig’s barrel”—if one stave is shorter, the water level is determined by that stave. This is a useful metaphor, but it is too mechanistic for understanding real plant physiology.

The physiological interpretation of the law of the minimum is fundamentally different:

Plant growth and productivity are determined by the essential element that is in the least availability relative to physiological demand, because that element is a necessary component of a critical function without which other processes cannot proceed fully.

Why is this important? Because a deficiency does not simply “reduce one thing”—it disrupts the operation of the entire system. Let us consider key examples.

1.3. Critical Concentrations and the Physiological Threshold

One of the most important physiological principles is: yield is determined not only by the presence of an element, but also by its concentration in a specific organ at a specific time.

Critical concentration is the minimum concentration of an element in the plant (usually in leaves at a certain development stage) required to achieve, for example, 90% of maximum productivity (Marschner, 2012; Gastal et al., 2015).

This concentration is not a constant. It depends on:

• Plant species – in C4 plants the critical nitrogen concentration is lower than in C3 (Connor et al., 2011)
• Age – in young plants the critical concentration is higher than in old ones (Marschner, 2012)
• Organ – in young leaves the critical concentration is different from that in stems
• Element interactions – high potassium requires more magnesium, and vice versa

For example, the critical phosphorus concentration for soybean during reproductive growth is higher than during vegetative growth, and it is at this time that phosphorus deficiency is particularly dangerous because it limits assimilate transport to developing seeds (Blevins, 1994).

1.4. The Systemic Nature of Limitation: From Deficiency to Symptoms

It is important to understand: deficiency is not simply a low concentration of an element. It is a functional disturbance that triggers a cascade of secondary changes.

Consider the chain for magnesium deficiency:

Mg²⁺ deficiency → chlorophyll decrease → photosynthesis drop → carbohydrate accumulation in leaves → feedback inhibition of photosynthesis → reactive oxygen species generation → oxidative stress → chlorosis and necrosis → premature senescence → yield reduction

This is not just “little magnesium → leaves turn yellow”. It is a systemic failure affecting photosynthesis, carbohydrate metabolism, antioxidant defence and, ultimately, productivity (Marschner, 2012).

A similar cascade can be traced for any element:

1.5. Why Does a Deficiency of One Element Limit the Entire Plant?

Now we can formulate the answer to the key question of the first part of the lecture.

One deficiency limits the whole plant for three reasons:

Reason 1. Fundamentality of functions. Every essential element participates in processes without which plant life itself is impossible. No phosphorus – no ATP. No nitrogen – no proteins. No potassium – no turgor and no transport. These elements are not interchangeable (except for partial replacement of K by Na in some species).

Reason 2. Integrativeness of regulation. The plant is a system with feedbacks. Deficiency of an element in one place triggers a regulatory response throughout the organism: the root/shoot ratio changes, carbon flows are redistributed, hormonal status is altered. Deficiency is not “treated locally”—it affects the entire system.

Reason 3. Nonlinearity. The relationship between element concentration and growth rate is threshold‑like. Up to a certain concentration, growth proceeds at maximum speed. Just below the threshold, growth slows sharply (Gastal et al., 2015; Connor et al., 2011). This means that even a small deficiency can cause a significant reduction in productivity if the concentration falls below the critical level.

1.6. Practical Implication: Diagnosis, Not Just Application

From an understanding of the systemic nature of limitation follows an important practical conclusion: deficiency must not simply be corrected by fertilizer application; it must be diagnosed at the level of physiological indicators.

The classical approach is to determine the element concentration in leaves at a given stage (Marschner, 2012). However, this is insufficient because:

• The same element may be present in different forms (active and inactive)
• Concentration depends on leaf age and water status
• Element interactions change the critical concentration

A more modern approach uses the Nitrogen Nutrition Index (NNI) and analogous indicators for other elements (Gastal et al., 2015). NNI is the ratio of the actual nitrogen concentration in the plant to the critical concentration for a given biomass. When NNI = 1, nutrition is optimal; when < 1, deficiency; when > 1, luxury consumption.

This allows not just to say “nitrogen is low”, but to determine the degree and timing of deficiency—exactly what is needed for precise management of nutrition throughout the growing season.

Conclusions to Part 1

1. Each essential element performs a unique fundamental function – without it the plant cannot exist normally.

2. Deficiency of any element causes not a local but a systemic disturbance affecting many processes (photosynthesis, transport, growth, development).

3. The relationship between element concentration and productivity is nonlinear – there is a critical threshold below which productivity drops sharply.

4. The law of the minimum in its physiological interpretation means that it is the element in deficit that determines the maximum possible productivity at any given moment, and its shortage cannot be compensated by an excess of other elements.

5. Management of mineral nutrition requires not only fertilizer application but also physiological diagnosis – assessment of critical concentrations and nutrition indices.

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2.1. Nonlinearity – The Main Property of Physiological Systems

In the first part we established that the plant is a complex integrated system where deficiency of one element limits overall productivity. From this, an erroneous conclusion is often drawn: “if there is too little, give more.” But physiology is nonlinear, and the relationship “fertilizer dose → yield” is never a straight line.

Key idea: between the concentration of an element in the medium and the growth rate there is not a linear but a curvilinear relationship with three clearly defined zones (Marschner, 2012; Connor et al., 2011):

1. Deficiency zone – growth is limited by lack of the element; every addition gives a yield increase.

2. Optimal supply zone – growth is maximal and does not change with further increases in concentration.

3. Toxicity zone – excess of the element begins to inhibit growth and development.

This is the classic response curve (dose‑effect), which in nutrition physiology is described not by a straight line but by an S‑shaped or sigmoid curve, or in the simplest case by a hyperbola (Marschner, 2012; Schopfer & Brennicke, 2016).

2.2. Deficiency Zone: Response Exists but Is Not Infinite

In the deficiency zone, the plant indeed responds to every increase in element availability – but with diminishing returns. This is because:

• Limitation by other resources – if you give much nitrogen but little phosphorus, the limit remains phosphorus (law of the minimum).
• Limitation of metabolic capacity – the rate of incorporation of the element into organic compounds has its enzymatic ceiling.
• Limitation by other factors – light, water, temperature can become limits even before the element ceases to be deficient.

In the deficiency zone the plant is in a state of “starvation” for that element. Physiological signs: reduced element concentration in tissues, mobilisation from old organs to young ones, changes in root/shoot ratio (more on this in Part 3). Adding the element in this zone gives the maximum increase, but each subsequent addition gives a smaller increase because other factors begin to limit (Gastal et al., 2015).

Important: in the deficiency zone, the plant cannot “choose” not to use the excess – it absorbs the element to the extent it absorbs it, and if the element is in excess, it may store it as a reserve (luxury consumption). But that is already a transition to the next zone.

2.3. Optimal Supply Zone: Physiological Plateau

When the concentration of the element in the medium is sufficient to achieve maximum growth rate, further increase does not increase yield. This is the zone where the plant is no longer limited by that element.

What happens physiologically?

• All enzyme systems using this element are saturated.
• Storage forms of the element (vacuolar deposits, storage proteins, phytates) are already filled to some level.
• Further flow of the element goes into “luxury consumption” – accumulation in vacuoles, formation of inactive forms, deposition in cell walls or as insoluble salts.

For example, in plants evolutionarily adapted to phosphorus‑poor soils (many Proteaceae from Australia and South Africa), the regulation mechanism of phosphorus uptake is impaired – they cannot reduce uptake when phosphorus is abundant, and phosphorus accumulation leads to toxicity at concentrations that are “normal” for ordinary crops (Lambers & Oliveira, 2019). This shows that the boundaries of the zones depend on the species and its evolutionary history.

Physiological plateau – a state where the element ceases to be a factor limiting productivity. All other factors (light, water, temperature, other elements) come to the forefront. It is here that the zone of luxury consumption begins.

2.4. Luxury Consumption: Storing for the Future

The term “luxury consumption” means uptake of an element in amounts exceeding the immediate needs of growth (Blevins, 1994; Marschner, 2012). This is a physiological survival strategy.

Why does the plant accumulate excess?

• Reserve for possible deficiency – if soil supply temporarily ceases (drought, salinity, leaching), the stored element can be mobilised.
• Buffer for regulation – the vacuolar store helps maintain a stable concentration in the cytosol, where the main metabolism takes place.
• Osmotic function – many elements (K, Na, Cl, NO₃⁻) serve as osmotica, and their excess in vacuoles creates turgor.

Where is the excess stored?

• In vacuoles – as soluble salts (nitrate, chloride, potassium) or as organic acids (malate, citrate) for charge balance.
• In cell walls – calcium, boron, silicon.
• In special deposits – phytic acid (phytate) in seeds and roots binds potassium, magnesium, calcium, zinc, iron (Blevins, 1994; Marschner, 2012).
• As storage proteins – nitrogen in vegetative storage proteins (e.g., in alfalfa, soybean).

Luxury consumption is not always good. In the optimal supply zone it is harmless (the plant simply accumulates reserves). But if the concentration continues to rise, we enter the toxicity zone.

2.5. Toxicity: When Excess Becomes Poison

Excess of an element inhibits the plant by various mechanisms:

Direct ion toxicity: Some elements at high concentrations disrupt membrane transport, compete with other ions for binding sites, inhibit enzymes, and cause oxidative stress. For example:

• Boron – toxicity appears as necrosis of leaf margins, especially old leaves where boron accumulates because of the transpiration stream (Marschner, 2012). In sensitive crops (stone fruits, citrus), toxicity occurs already at 1–2 mg/kg in soil solution.
• Manganese – in acid soils and under waterlogging its mobility increases sharply, and even in tolerant species toxicity occurs, manifested as brown spots on leaves (Marschner, 2012).
• Sodium and chloride – the main cause of salinisation. Even in salt‑tolerant species, excess NaCl causes osmotic shock, disturbance of the K/Na balance, and ion toxicity (Marschner, 2012; Taiz et al., 2023).

Imbalance with other elements: Excess of one element can cause deficiency of another (antagonism). Classic examples:

• Excess potassium → magnesium and calcium deficiency – potassium competes with magnesium and calcium for binding sites on membranes and for uptake (Blevins, 1994; Marschner, 2012). In ruminants this can cause grass tetany – a disturbance of magnesium metabolism (Blevins, 1994).
• Excess phosphorus → zinc and iron deficiency – phosphorus forms sparingly soluble compounds with zinc and iron, reducing their availability (Marschner, 2012).
• Excess ammonium → potassium and calcium deficiency – ammonium inhibits cation uptake due to rhizosphere acidification and competition for transporters (Harper, 1994).

Disturbance of osmotic balance and water relations: High salt concentration in the soil solution lowers the water potential, making it harder for the plant to extract water (osmotic stress). Even if there is no direct ion toxicity, the plant may suffer from “physiological drought”.

Oxidative stress: Excess of many metals (Fe, Cu, Mn, Zn) triggers Fenton reactions, generating reactive oxygen species that damage membranes, proteins and DNA (Marschner, 2012). Symptoms – necrosis, chlorosis, accelerated senescence.

2.6. What Determines the Individual Toxic Dose?

Toxicity depends not only on absolute concentration, but also on:

• Species and cultivar specificity – for example, rice is tolerant to ammonium, while barley is sensitive (Harper, 1994). Species from poor soils (Proteaceae) are sensitive to phosphorus, whereas agricultural crops (maize, wheat) can tolerate high concentrations.
• Form of the element – for example, ammonium‑N is more toxic than nitrate‑N at high concentrations, especially if the soil has low buffering capacity (Harper, 1994).
• pH of the medium – in acid soils many metals (Al, Mn, Fe) become more soluble and toxic; in alkaline soils – boron and molybdenum (Taiz et al., 2023).
• Presence of other elements – for example, calcium reduces sodium toxicity, silicon reduces manganese and aluminium toxicity (Marschner, 2012).
• Developmental stage – seedlings are often more sensitive to salt excess than mature plants (Marschner, 2012).

2.7. Why Is “More Fertilizer” Not the Answer?

Now we can give a systemic answer to the question of the second part.

Reason 1. Law of diminishing returns. In the deficiency zone each unit of fertiliser gives a smaller and smaller increase until a plateau is reached where the increase stops completely. Further application is economically unprofitable and ecologically harmful.

Reason 2. Luxury consumption and immobilisation. Excess of the element is not used for growth but is deposited in storage forms (vacuoles, phytates, cell walls) or leached from the rhizosphere. The plant cannot “discard” the excess – it accumulates it, and this may create problems for product quality (e.g., excess nitrates in vegetables).

Reason 3. Antagonism with other elements. Excess of one element causes deficiency of another, which had been normal. For example, heavy potassium fertilisation can provoke magnesium deficiency, and excess phosphorus – zinc deficiency.

Reason 4. Environmental consequences. Unused nitrogen in the form of nitrates leaches into groundwater, causing eutrophication. Ammonia and nitrous oxide volatilise to the atmosphere, enhancing the greenhouse effect. Excess phosphorus is a limited resource; its irrational use depletes global phosphate reserves (Connor et al., 2011).

Reason 5. Deterioration of product quality. High doses of nitrogen fertilisers reduce sugar content, increase nitrate content, and impair storability of fruits and grain (Marschner, 2012). For example, in wheat, high nitrogen applied at late stages reduces grain protein content – this is a separate physiological pattern related to nitrogen remobilisation.

Reason 6. Reduction of stress tolerance. Plants with excess nitrogen have looser tissues, higher water content, and lower concentrations of protective compounds (phenolics, lignin). They are more susceptible to diseases, pests, lodging, drought, and frost (Marschner, 2012).

2.8. What Does “Optimal Dose” Mean from a Physiological Point of View?

Optimal dose is not the maximum possible, but the one that provides maximum economic and physiological efficiency.

From a physiological point of view, the optimal dose is the concentration of the element in the medium at which:

1. The plant reaches maximum growth rate (the plateau of the response curve) – but without entering the luxury consumption zone.

2. The concentration of the element in tissues is within the adequate range – above the critical but below the toxic level (Marschner, 2012).

3. The nutrition index (e.g., NNI) is close to 1 – neither deficiency nor excess (Gastal et al., 2015).

4. Balance with other elements is maintained – ratios N:P, K:Mg, Ca:B, etc., are within the normal limits for the given crop and stage.

5. Losses to the environment are minimised – no excessive nitrate leaching, ammonia volatilisation, or nitrous oxide emissions.

Practical conclusion: The optimal dose is the result of an integrated assessment of the plant status, soil, climate, and economic conditions, not simply “more is better”. That is why modern agronomy is moving towards variable‑rate application of fertilisers based on physiological diagnostics (SPAD meters, NNI, remote sensing).

Conclusions to Part 2

1. The relationship “element dose → yield” has three zones: deficiency, optimal supply, and toxicity.

2. In the deficiency zone each addition gives a yield increase, but with diminishing returns – because of limitations by other resources and enzymatic capacity.

3. In the optimal supply zone a plateau is reached – further dose increase does not raise yield, only luxury consumption.

4. Luxury consumption is the storage of the element in vacuoles, phytates, and cell walls. It may be useful as a reserve, but does not increase current growth.

5. Toxicity occurs when a threshold is exceeded, and manifests as direct cell damage (necrosis, chlorosis), imbalance with other elements, osmotic stress, or oxidative stress.

6. Excess fertiliser not only fails to increase yield but also reduces product quality, plant resistance, and causes environmental damage.

7. The optimal dose is not the maximum, but the one that ensures maximum efficiency of element use while maintaining balance, quality, and environmental safety.

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3.1. The Plant Is Not a Passive Object but an Actively Managed System

In the previous parts we established that mineral nutrition acts non‑linearly: from deficiency through optimum to toxicity. But the most remarkable feature of plant physiology is the capacity for plastic adjustments. The plant does not simply “suffer” from deficiency or “enjoy” excess. It actively changes its morphology, architecture, physiology, and hormonal status to make the best use of available resources.

Key idea: a change in mineral nutrition is not only a change in growth rate, but also a change in the direction of development. The plant redistributes resources among organs, alters their shape and function, adjusts the root system to soil conditions, and the shoot system to light conditions. All this is controlled by a complex network of signalling systems in which mineral elements act simultaneously as resources and signals.

3.2. Root‑to‑Shoot Ratio: The First and Principal Response

The fastest and most noticeable adjustment to changes in nutrition is the change in the root‑to‑shoot mass ratio (R/S).

Under deficiency of mobile elements (nitrogen, phosphorus, potassium):

• The plant increases the proportion of roots in total biomass.
• Reason: roots are the main organ for mineral uptake; if minerals are scarce, more carbon must be “invested” in root development to obtain the deficient resource (Marschner, 2012; Connor et al., 2011).
• This is a classic example of functional equilibrium (Brouwer, 1963): the plant strives to balance resource availability – if the soil resource (water, minerals) limits, it invests in roots; if light limits, it invests in shoots.

Examples of quantitative changes:

• In beans, under phosphorus deficiency the root/shoot ratio can almost double (Whiteaker et al., 1976; cited in Marschner, 2012).
• In cereals under nitrogen starvation, the root fraction increases through finer and longer roots, not through increased diameter.

Why is this important? Increasing the root fraction means redistributing carbon from shoot to roots. This reduces photosynthetic surface (leaves) in the short term, but in the long term enhances the ability to acquire the deficient element. This is an example of a trade‑off between current growth and long‑term survival (Lambers & Oliveira, 2019).

The opposite situation – under excess of elements:

• The plant reduces the root fraction, investing more resources in the shoot, where they can be used for photosynthesis and reproduction.
• That is why on well‑fertilised soils plants appear more “green” and taller, but the root system becomes less branched.

Important: the change in R/S is not a passive consequence, but an active regulatory process governed by hormones, signalling peptides, and carbohydrate status.

3.3. Root System Architecture: Plasticity for Exploration

The root system changes not only its total mass but also its architecture – shape, branching, growth direction, root hair length. This allows the plant to “explore” the soil as efficiently as possible.

Nitrogen – local and systemic responses:

• When nitrate is locally available in the soil, roots proliferate specifically in that zone, producing more lateral roots (Drew, 1975; cited in Marschner, 2012).
• This phenomenon is mediated by auxin (as the key branching hormone) and nitrate signalling through the transporter NRT1.1, which can act as a nitrate sensor (Taiz et al., 2023; Gastal et al., 2015).
• If nitrate is uniformly distributed but at low concentration, roots elongate to “comb” a larger volume.

Phosphorus – foraging in the topsoil:

• Phosphorus is immobile in soil and is concentrated in the upper layer. Therefore, under deficiency, plants direct roots more horizontally (reduce the growth angle) and increase root density in the surface layer (topsoil foraging) (Lynch, 2007; Taiz et al., 2023).
• In many species (legumes, some grasses), under phosphorus deficiency root hair length and density increase, enlarging the effective absorbing surface at minimal carbon cost (Lambers & Oliveira, 2019).

Cluster roots (proteoid roots) – extreme adaptation:

• Some families (Proteaceae, Fabaceae, Cyperaceae, etc.) form specialised brush‑like clusters of root tips – cluster roots (Fig. 9.12 in Lambers & Oliveira, 2019).
• They exude large amounts of carboxylates (citric, malic acids) into the rhizosphere, which mobilise phosphorus bound to iron and aluminium, as well as organic phosphorus through phosphatase activation (Lambers & Oliveira, 2019).
• Cluster‑root formation is suppressed under adequate phosphorus supply – a striking example of an inducible structural adjustment.
• In many species, cluster roots develop only under severe phosphorus deficiency, and their appearance is controlled by a systemic signal (probably leaf phosphorus concentration), not by local concentration in the soil (Lambers & Oliveira, 2019).

Aerenchyma – adaptation to flooding:

• Under flooding (hypoxia), many plants form aerenchyma – air‑filled tissue in roots, stems, and sometimes leaves, which facilitates oxygen transport from shoot to roots (Marschner, 2012; Schopfer & Brennicke, 2016).
• This process is induced by ethylene, which accumulates in tissues under oxygen deficiency. Ethylene triggers programmed cell death of cortical parenchyma cells, creating air spaces.
• Aerenchyma allows the plant to survive in flooded soils, where other species die. This is an example of phenotypic plasticity at the tissue level.

3.4. Hormonal Regulation of Adjustments

All adjustments – from changes in R/S to cluster‑root formation – are mediated by the hormonal network. Mineral nutrition affects the synthesis, transport, and sensitivity to hormones.

Cytokinins – signal from roots to shoots:

• Synthesised in roots and transported to the shoot via the xylem.
• Under nitrogen deficiency, cytokinin synthesis decreases, leading to reduced shoot branching, slower leaf growth, and senescence of lower leaves (nitrogen re‑utilisation) (Marschner, 2012; Taiz et al., 2023).
• Under adequate nitrogen, cytokinins stimulate shoot growth, delay senescence, and maintain photosynthesis.

Auxin – regulator of root architecture:

• The main hormone stimulating lateral root formation.
• Under local nitrogen or phosphorus enrichment, auxin is transported to those zones, inducing branching.
• However, at high local nitrate concentrations, auxin synthesis may be activated, but then – through interaction with other signals – lateral root growth is suppressed (to avoid excessive branching) (Taiz et al., 2023).

Abscisic acid (ABA) and ethylene – stress hormones:

• Under water deficit or salinity, ABA activates stomatal closure and slows growth, conserving resources.
• Ethylene, as mentioned, induces aerenchyma under flooding and can stimulate root hair formation under phosphorus deficiency (Marschner, 2012).
• Ethylene also participates in inhibition of lateral root growth at high ammonium concentrations, which is a protective response.

Gibberellins – growth stimulators:

• Under nitrogen deficiency, gibberellin content decreases, restricting stem elongation and promoting carbon accumulation in roots.

Important: hormones do not act alone. They form complex interaction networks where one hormone can affect the synthesis or sensitivity of another. For example, cytokinins and auxin often act antagonistically on root branching, while ethylene can modulate auxin transport.

3.5. Systemic Regulation: How Shoot Controls Roots and Vice Versa

Adjustments do not occur only locally. The plant is a single whole, and information about element deficiency is transmitted from root to shoot and back.

Signals from root to shoot:

• Cytokinins (as above) – report nitrogen availability in the root zone.
• Peptide signals (CEP – C‑terminally Encoded Peptides) – synthesised in roots under nitrogen deficiency, transported via xylem to leaves, where they activate receptors that then, through other signals (glutaredoxins), return to roots and stimulate nitrate uptake in nitrate‑rich patches (Taiz et al., 2023) – this is an example of systemic coordination.
• Sugars (sucrose) – transported from leaves via phloem to roots, providing energy for uptake and assimilation processes. Under nitrogen deficiency, sugar concentration in roots may increase (as shoot growth is slowed), and this serves as an additional signal for up‑regulation of nitrate transporters.

Signals from shoot to roots:

• Auxin is synthesised in young leaves and transported basipetally to roots, regulating branching.
• miRNAs (e.g., miR399) – participate in systemic regulation of phosphorus homeostasis. Under phosphorus deficiency, miR399 is transported via phloem to roots, where it suppresses the expression of a ubiquitin ligase, leading to activation of phosphate transporters (Taiz et al., 2023; Lambers & Oliveira, 2019).

Example of systemic control – cluster roots:

• Their formation is suppressed when leaf phosphorus concentration is high, even if the local root zone has low phosphorus (Lambers & Oliveira, 2019).
• This means that the deficiency signal originates from the shoot, not the root. It is probably related to leaf phosphate concentration or the sugar‑to‑phosphorus ratio.

3.6. Developmental Strategies: Fast and Slow, Frugal and Profligate

Different species have different evolutionarily developed strategies for responding to nutrient shortage.

Fast‑growing species (ruderals, agricultural crops):

• Under element deficiency they increase R/S, but moderately – they tend to reduce overall growth rather than invest too much in roots.
• They rely on high growth rate to outcompete neighbours for light, even if the soil is poor (Connor et al., 2011; Lambers & Oliveira, 2019).
• Their plasticity is limited: they do not form specialised structures (cluster roots) or form them weakly. They rely more on fertilisers than on their own adaptations.

Slow‑growing species from poor soils (e.g., Proteaceae, many heath shrubs):

• Under element deficiency they greatly increase R/S and form highly effective structures (cluster roots, mycorrhizas).
• They have low growth rates, even under high nutrition – because their metabolism is geared towards resource economy rather than rapid growth (Lambers & Oliveira, 2019).
• They have high leaf longevity and efficient re‑utilisation of elements before abscission (high use efficiency – more in Part 4).
• They may have low capacity to down‑regulate uptake under excess (e.g., Hakea prostrata accumulates phosphorus to toxic concentrations if supplied, because its regulatory mechanisms did not evolve for high availability) (Lambers & Oliveira, 2019).

Intermediate strategies:

• Many species form mycorrhizas (arbuscular or ectomycorrhizal), which increase the efficiency of phosphorus and other element uptake without drastic root restructuring (Taiz et al., 2023).
• Mycorrhiza is not a morphological change of the plant itself, but a symbiotic adaptation that extends its capabilities.

3.7. Root Adaptations to Iron and Zinc Deficiency: Strategy I and II

This is one of the most striking examples of physiological adjustments. Iron is an immobile element in soil, especially under alkaline conditions. Plants have evolved two fundamentally different mechanisms for its mobilisation (Marschner, 2012; Lambers & Oliveira, 2019):

Strategy I (in all plants except grasses):

• Under iron deficiency, roots release protons (acidifying the rhizosphere), reduce Fe³⁺ to Fe²⁺ via a plasma‑membrane ferredoxin reductase, and release phenolic compounds (chelators) to solubilise iron.
• These processes are localised in the root elongation zone and require energy (ATP).
• In response to deficiency, transcription of genes encoding H⁺‑ATPase, reductase, and Fe²⁺ transporters is activated.
• Importantly, this is an inducible system, switched on only when iron is lacking.

Strategy II (in grasses, including wheat, barley, maize, rice):

• Under iron deficiency, roots release phytosiderophores – low‑molecular‑weight chelators (e.g., mugineic acid) that bind Fe³⁺ with high affinity.
• Then the Fe³⁺‑phytosiderophore complex is taken up via a specific transporter (YSY proteins) without reduction at the root surface (Lambers & Oliveira, 2019; Taiz et al., 2023).
• Phytosiderophores are synthesised from nicotianamine and require several enzymes; their synthesis increases strongly under iron deficiency.
• Grasses also have zinc‑phytosiderophores, as zinc can also be mobilised by the same chelators.

These strategies are a clear example of how the plant restructures root biochemistry and morphology in response to deficiency of a specific element. Strategy I requires rhizosphere acidification, which may be inefficient in highly buffered (alkaline) soils, where Strategy II works better.

3.8. Deficiency and Restructuring at the Cell and Organ Level

Restructuring occurs not only at the whole‑plant level, but also at the level of individual cells and organs.

• Change in specific leaf area – under nitrogen deficiency, leaves become thicker (more mesophyll per unit area) and have lower specific area, which reduces photosynthesis per unit mass but saves nitrogen (Gastal et al., 2015).
• Change in chlorophyll and protein content – under nitrogen deficiency, Rubisco content decreases, but other proteins may accumulate if they do not contain nitrogen (e.g., carbohydrates).
• Elongation of internodes and change in leaf orientation – under phosphorus deficiency, soybean exhibits paraheliotropism (turning leaves edge‑on to the sun) to avoid photoinhibition, because phosphorus deficiency impairs assimilate transport and light energy utilisation (Blevins, 1994).
• Accelerated senescence of lower leaves – under deficiency of nitrogen, phosphorus, potassium, or magnesium, lower leaves yellow and die, releasing elements for young organs. This is a programmed restructuring governed by cytokinins and ethylene (Gastal et al., 2015).

3.9. Temporal Dynamics: Early and Late Responses

Restructuring has different time scales:

• Fast (minutes to hours) – changes in membrane transport, transporter activation, rhizosphere pH changes, rapid hormonal signals (ABA, ethylene).
• Medium‑term (hours to days) – changes in transporter gene expression, onset of phytosiderophore synthesis, metabolic reorganisation (e.g., phosphatase activation).
• Long‑term (days to weeks) – morphological changes: increase in R/S, formation of cluster roots, alteration of shoot architecture, acceleration or deceleration of senescence.

Important: the plant does not activate all responses at once. It assesses the situation and initiates the most effective strategy depending on the duration and intensity of the deficiency. For example, under short‑term nitrogen deficiency, transporters and re‑utilisation are activated; under long‑term deficiency, the entire root system is restructured (Gastal et al., 2015).

Conclusions to Part 3

1. The plant actively restructures its development in response to mineral nutrition – this is not passive suffering but purposeful adaptation.

2. The first and main restructuring is the change in root‑to‑shoot ratio: under deficiency of mobile elements, the root fraction increases (functional equilibrium).

3. The root system is plastic: branching, root hair length, growth angle, and specialised structures such as cluster roots (for phosphorus) and aerenchyma (for oxygen) are modified.

4. All restructuring is governed by the hormonal network: cytokinins, auxin, ethylene, ABA, and gibberellins act in a coordinated manner.

5. There are systemic signals between shoot and roots – peptides (CEP), miRNAs, sugars – that coordinate the response of the whole plant to local changes in nutrition.

6. Different species have different strategies: fast‑growing species invest less in roots; slow‑growing species from poor soils form effective adaptations but have low growth rates.

7. There are specialised element‑mobilisation strategies (e.g., Strategy I and II for iron) that demonstrate biochemical plasticity.

8. Restructuring occurs on different time scales – from minutes to weeks – allowing the plant to flexibly respond to changing conditions.

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4.1. Why Efficiency Is Not Just “Yield per Fertiliser”

In the previous parts we established that the plant is a nonlinear system that plastically restructures itself in response to mineral nutrition. Now we come to the central practical question: how to measure and improve how efficiently the plant uses the available elements?

When we say “fertiliser use efficiency”, we often think of a simple formula: “yield divided by applied fertiliser”. But the physiological reality is much more complex. The plant obtains elements not only from fertilisers but also from soil reserves, atmospheric deposition, symbiotic fixation (in legumes), and re‑utilisation (remobilisation) from senescing organs. Therefore, agronomic efficiency and physiological efficiency are different things.

Key idea: nutrient use efficiency (NUE) is an integrative characteristic reflecting how fully the plant is able to extract an element from available sources, use it to produce biomass, and, in crop plants, channel it into the economically valuable part of the yield.

A systematic analysis of NUE was first proposed by Moll et al. (1982) for maize, and their concept remains basic to this day (Gastal et al., 2015; Connor et al., 2011).

4.2. Definition and Components of NUE

NUE (Nitrogen Use Efficiency) – the most studied indicator, but analogous approaches exist for phosphorus (PUE), potassium (KUE), and other elements.

In general:

NUE = Yield / Available Nitrogen

or, if we speak of applied fertiliser:

NUE = (Yield) / (N fertiliser + N soil + N fixation + N precipitation)

But this is a black box. To understand the physiology, NUE is broken down into two main components (Moll et al., 1982; Gastal et al., 2015):

1. N‑uptake efficiency (NupE)

This is the plant’s ability to extract nitrogen from available sources (soil, fertiliser, fixation). It depends on:

• Root system architecture – root length, density, penetration depth, number of root hairs (Gastal et al., 2015; Marschner, 2012).
• Transporter activity – high‑affinity (HATS) and low‑affinity (LATS) nitrate and ammonium uptake systems (Harper, 1994; Gastal et al., 2015).
• Competition with microorganisms – for mineral nitrogen in the rhizosphere (Connor et al., 2011).
• Synchronisation – matching the peaks of plant demand and soil nitrogen availability.

2. N‑utilization efficiency (NutE)

This is the plant’s ability to produce biomass (or economically valuable yield) per unit of absorbed nitrogen. It depends on:

• Photosynthetic efficiency per unit of nitrogen (PNUE) – higher in C4 plants than in C3 (Connor et al., 2011; Gastal et al., 2015).
• Partitioning of nitrogen among organs – more nitrogen in leaves → higher photosynthesis, but more in stems → higher structural support.
• Re‑utilisation efficiency – ability to mobilise nitrogen from senescing organs to young ones or to seeds (Gastal et al., 2015; Marschner, 2012).
• Harvest index (HI) and Nitrogen harvest index (NHI) – the fraction of biomass and nitrogen that is transferred to the economically valuable part (grain, tubers, fruits).

Thus:

(for grain crops, where HI is the fraction of grain in total biomass) (Gastal et al., 2015).

4.3. Physiological Basis of NUE: The Critical Dilution Curve

To understand how NutE works, we must return to the concept of critical nitrogen concentration (%Nc), which we briefly mentioned in Part 1.

The critical dilution curve describes how the nitrogen concentration in the plant should decrease as its biomass increases, provided the plant experiences neither deficiency nor excess of nitrogen (Gastal et al., 2015; Lemaire & Gastal, 1997).

Empirically it has the form of a power function:

where:

• %Nc – critical nitrogen concentration (in % of dry matter),
• W – accumulated biomass (t/ha),
• a and b – species‑specific coefficients (e.g., for wheat a = 5.3, b = 0.44; for maize a = 3.4, b = 0.37) (Gastal et al., 2015).

What does this mean physiologically?

As the plant grows, the proportion of metabolically active tissues (leaves, meristems) decreases, while the proportion of structural tissues (stems, cell walls) increases. Since structural tissues contain less nitrogen, the overall nitrogen concentration in the plant naturally falls even under optimal nutrition. This is nitrogen dilution due to changes in plant architecture, not to deficiency.

If the actual nitrogen concentration in the plant (%N_actual) is above the critical (%Nc), this indicates luxury consumption (more nitrogen than needed for maximum growth). If below – deficiency.

On this basis, the Nitrogen Nutrition Index (NNI) is defined (Gastal et al., 2015; Lemaire & Gastal, 1997):

• NNI = 1 – optimal nutrition,
• NNI < 1 – deficiency (the lower, the stronger),
• NNI > 1 – luxury consumption.

Why is NNI important for understanding NUE?

• NNI allows comparison of different varieties and conditions regardless of absolute biomass.
• NNI shows how efficiently the plant uses available nitrogen to maintain metabolically active tissues.
• Varieties with high NUE under low nitrogen often have a higher NNI at the same biomass – they better maintain the critical nitrogen concentration (Gastal et al., 2015).
• NNI is a bridge between agronomy (element concentration) and physiology (plant status).

4.4. Why High Yield ≠ High NUE?

This is one of the most important and counter‑intuitive conclusions of modern plant physiology.

Historical context: Breeding in the 20th century (especially the “Green Revolution”) was aimed at increasing yield under high mineral nutrition. Varieties were selected under abundant nitrogen, phosphorus, potassium, and irrigation. They did produce high yields – but they were not efficient in nutrient use (Gastal et al., 2015; Connor et al., 2011).

Why did this happen?

• Breeders selected plants with high Harvest Index – a larger proportion of grain in total biomass. This increased yield but reduced vegetative mass (stems, leaves), which serve as a nitrogen reservoir for grain.
• Modern varieties often have lower root mass (invest less in roots) because under high fertilisation this is unnecessary. But under deficiency they are poorer at extracting nitrogen from the soil (Gastal et al., 2015).
• Many modern varieties have shorter post‑flowering photosynthetic duration (early senescence), limiting nitrogen influx to grain and reducing NUE under low nitrogen (Gastal et al., 2015; Sadras & Calderini, 2015).

Example: In wheat over the last 50 years, yield has increased significantly, while NUE has hardly changed or even declined at high nitrogen rates (Gastal et al., 2015). This means we feed plants more, but they use this nitrogen inefficiently – most of it goes to the environment.

Important distinction:

• NUE at high nitrogen – the ability of a variety to give high yield when nitrogen is abundant. This was the target of classical breeding.
• NUE at low nitrogen – the ability of a variety to maintain yield when nitrogen is scarce. This is what breeding for sustainable agriculture is now targeting.

These two abilities do not always correlate. A variety yielding 10 t/ha at 200 kg N/ha may yield only 2 t/ha at 50 kg N/ha, while another variety yielding 8 t/ha at 200 kg N/ha may yield 4 t/ha at 50 kg N/ha. The second variety has higher NUE at low nitrogen – precisely the type needed for environmentally sustainable agriculture (Gastal et al., 2015; Borrelli et al., 2015).

4.5. Physiological Mechanisms of High NUE

What makes a variety efficient under low nitrogen? Several key mechanisms (Gastal et al., 2015; Marschner, 2012; Taiz et al., 2023):

1. Efficient root architecture:

• Greater root length per unit biomass, finer roots, more root hairs – this increases the soil volume available for nitrogen extraction.
• Ability of roots to penetrate deep soil layers where nitrate may remain, not leached from the topsoil.
• Inducible expression of high‑affinity nitrate transporters (NRT2) and ammonium transporters (AMT1) under deficiency (Gastal et al., 2015; Taiz et al., 2023).

2. Efficient nitrogen re‑utilisation (remobilisation):

• Ability to mobilise nitrogen from senescing leaves and stems to developing organs (especially grain) – a key process for NUE in cereals (Gastal et al., 2015; Marschner, 2012).
• In wheat, up to 80‑90% of grain nitrogen may come from remobilised reserves rather than current uptake (Harper, 1994; Gastal et al., 2015).
• Varieties with delayed senescence (stay‑green) can maintain photosynthesis and nitrogen uptake longer after flowering, enhancing NUE (Gastal et al., 2015; Sadras & Calderini, 2015).

3. High photosynthetic nitrogen use efficiency (PNUE):

• In C4 plants (maize, sorghum, sugarcane) PNUE is higher than in C3 (wheat, rice, soybean), because they have less Rubisco per unit nitrogen (Connor et al., 2011; Gastal et al., 2015).
• In C3 plants, PNUE can be increased by optimising nitrogen distribution in the canopy – more nitrogen in upper, well‑lit leaves, less in shaded ones (Gastal et al., 2015).
• This requires regulation of the vertical nitrogen profile in the canopy, which is a physiologically controlled process.

4. High Nitrogen Harvest Index (NHI):

• NHI = (nitrogen in grain) / (total nitrogen in plant).
• In highly efficient varieties, NHI can reach 0.7‑0.8 (Gastal et al., 2015; Marschner, 2012).
• NHI is determined by efficiency of phloem loading with amino acids and efficiency of unloading in the grain.

5. Reduction of “non‑productive” nitrogen losses:

• Minimisation of ammonia leakage through stomata (especially during senescence), reduction of denitrification in the rhizosphere, and decrease in nitrate leaching (Connor et al., 2011).

4.6. NUE and Grain Quality: An Inevitable Trade‑off?

One of the most challenging problems: high NUE at low nitrogen often means low grain protein content.

This is due to a simple physiological fact:

If we want high NUE (much grain per unit N), we must either increase grain mass with the same N, or reduce N with the same grain mass. But protein is N in grain. Therefore, yield increase at low N often comes at the expense of protein content.

This is a classic trade‑off between quantity and quality (Gastal et al., 2015; Sadras & Calderini, 2015). It is seen in wheat (protein content falls as yield rises), in rice, soybean, and potato.

Can this trade‑off be broken?

• Theoretically – yes, if we can increase uptake efficiency (NupE) without reducing the proportion of nitrogen going to grain.
• In practice – this is difficult because the mechanisms of nitrogen accumulation in grain and carbohydrate accumulation are often regulated by different genes and may have different hormonal sensitivities (Gastal et al., 2015).
• Some wheat varieties show higher NHI without reducing protein – but these are rare exceptions, not the rule.

Example: In winter wheat, over the past 30 years breeding has increased yield by about 1% per year, but grain protein content has either remained unchanged or declined (Gastal et al., 2015). This means that the amount of protein per hectare increased, but the protein concentration fell – which is important for baking quality.

4.7. How to Improve NUE: Breeding and Agronomic Approaches

Breeding approaches (Borrelli et al., 2015; Gastal et al., 2015):

• Selection under low nitrogen – this allows identification of genotypes with high NUE under deficiency, which can then be used in crosses with high‑yielding lines.
• Genomics and marker‑assisted selection (MAS) – identification of QTL (quantitative trait loci) associated with nitrogen uptake and utilisation efficiency. Such QTL have already been identified in maize, wheat, and rice (Gastal et al., 2015; Borrelli et al., 2015).
• Genetic engineering – for example, transformation with genes for high‑affinity nitrate transporters (NRT2), assimilation enzymes (GS, GOGAT), or senescence regulators (stay‑green).
• Use of wild relatives – wild species often carry alleles that increase NUE and were lost during domestication (Borrelli et al., 2015).

Agronomic approaches (Gastal et al., 2015; Connor et al., 2011):

• NNI diagnosis – allows determination of when and how much nitrogen to apply to maintain NNI = 1.
• Split application – dividing the total dose into several top‑dressings at key developmental stages (tillering, stem elongation, heading) to synchronise nitrogen supply with demand.
• Use of nitrification inhibitors – slows conversion of ammonium to nitrate, reducing leaching losses (Connor et al., 2011).
• Use of green manures and organic fertilisers – improves soil structure and gradual nitrogen release, but requires accurate calculation.
• Optimisation of planting density – at high density, competition for nitrogen increases, and NUE may decline if the dose is not adjusted.

Important: improvement of NUE cannot be achieved solely by breeding or solely by agronomy. It is an integrative task requiring collaboration among physiologists, breeders, agronomists, and ecologists (Gastal et al., 2015; Borrelli et al., 2015).

4.8. NUE in a Global Perspective: Challenges and Opportunities

Facts (Connor et al., 2011; Gastal et al., 2015):

• Global NUE for cereal crops is about 40‑50% – that is, half of the applied nitrogen does not end up in the yield.
• In developed countries (Europe, USA), NUE is often lower due to excessive fertiliser doses; in developing countries it is higher, but because of low yields.
• Nitrogen losses as nitrates (_{20% of applied), ammonia (}10‑15%), and nitrous oxide (~1‑2%) cause enormous environmental damage.

What can be done?

• Increase NUE to 60‑70% – this is a realistic goal for the coming decades (Gastal et al., 2015).
• Reduce fertiliser doses without yield loss through precision agriculture (remote sensing, NNI diagnosis, variable‑rate application).
• Increase the proportion of legumes in crop rotations – biological nitrogen fixation does not require fossil fuels and does not emit N₂O (Connor et al., 2011; Denison, 2015).
• Develop varieties with high NUE at low nitrogen – a key direction of modern breeding (Borrelli et al., 2015; Gastal et al., 2015).

Conclusions to Part 4

1. NUE is an integrative indicator reflecting how fully the plant uses available nitrogen to produce biomass and economically valuable products.

2. NUE is decomposed into uptake efficiency (NupE) and utilization efficiency (NutE), and for cereals also into Nitrogen Harvest Index (NHI).

3. The critical dilution curve and the Nitrogen Nutrition Index (NNI) allow objective assessment of NUE and comparison of varieties and conditions.

4. High yield ≠ high NUE – classical breeding selected varieties for abundant nutrition, and they are often inefficient under deficiency.

5. Improving NUE requires multiple physiological mechanisms: root architecture, efficient re‑utilisation, high PNUE, and high NHI.

6. There is a trade‑off between yield and protein content – increasing NUE often reduces grain quality, but this trade‑off can be alleviated by breeding.

7. Improving NUE is possible through breeding (under low nitrogen, using markers and genomic technologies) and through agronomy (NNI diagnosis, split application, green manures).

8. Globally, increasing NUE is an environmental and economic imperative for sustainable agriculture.

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5.1. Quality Is Not Only Taste, but Physiology

In the previous parts we focused on quantity – yield mass, growth rate, nutrient use efficiency. But for agriculture, the chemical composition of the product is equally important: protein, starch, sugar, oil, vitamin, nitrate, and structural fibre content. These parameters determine:

• Nutritional value for humans and animals,
• Technological properties (baking, brewing, oil‑processing),
• Storability during storage and transport,
• Environmental safety (nitrate and heavy metal content).

Key idea: crop quality is a direct consequence of how the plant distributes carbon, nitrogen, and mineral elements among different types of compounds. Distribution, in turn, is governed by physiological regulatory mechanisms that depend on mineral nutrition. That is, quality is not an “agronomic additive” but an integral outcome of the functioning of the whole physiological system.

5.2. Nutrition Determines Where Carbon Goes

Carbon enters the plant through photosynthesis in the form of sugars (mainly sucrose) and is then distributed among three main “flows”:

1. Structural carbohydrates – cellulose, hemicellulose, lignin (cell walls, stems, roots).

2. Storage carbohydrates – starch (in seeds, tubers, roots), sucrose (in fruits, root crops), fructans (in many cereals and composites).

3. Energy metabolism and synthesis of other substances – respiration, protein, lipid, and secondary metabolite synthesis.

Mineral nutrition switches carbon flows among these directions.

Nitrogen – the main regulator of carbon distribution:

• Under high nitrogen supply, the plant actively synthesises proteins (including Rubisco and other enzymes) and nucleic acids. Carbon is directed to leaf and stem growth. Less storage carbohydrate accumulates.
• Under nitrogen deficiency, protein synthesis is inhibited, and surplus sugars not used for protein synthesis are directed to starch, sucrose, and lignin. This leads to accumulation of carbohydrates in leaves, stems, and sometimes fruits (Marschner, 2012; Gastal et al., 2015).

Example – sugar beet: under high nitrogen, the sucrose proportion in root tubers decreases, while the proportion of “ballast” nitrogen‑containing compounds (amino acids, amides) increases. This reduces sugar extraction efficiency during processing (Marschner, 2012).

Phosphorus – regulator of carbon transport:

• Phosphorus is part of ATP and participates in phloem loading with sucrose. Under phosphorus deficiency, assimilate export from leaves to storage organs is impaired.
• In soybean, phosphorus deficiency leads to starch accumulation in leaves, reduced sucrose export to seeds, and smaller seeds (Blevins, 1994).
• In potato, phosphorus deficiency reduces tuber starch content because sugar supply from leaves is impaired.

Potassium – regulator of phloem loading:

• Potassium is required for H⁺‑ATPase activity, which creates the proton gradient for sucrose co‑transport into phloem cells. Under potassium deficiency, phloem loading with sucrose decreases, and carbon becomes “stuck” in leaves (Marschner, 2012; Blevins, 1994).
• In potato, potassium deficiency reduces tuber starch content; in tomato, it reduces fruit sugar content.
• Potassium also increases the sugar fraction in fruits by improving transport, whereas under low potassium fruits become more watery with low dry matter (Marschner, 2012).

Boron – regulator of sucrose synthesis and transport:

• Boron participates in sucrose synthesis in leaves and in its transport in the phloem (Blevins, 1994; Lambers & Oliveira, 2019).
• Under boron deficiency, sugar transport to roots and fruits is impaired, leading to carbohydrate accumulation in leaves and reduced sugar content in the harvested product (e.g., in beet, sunflower, and fruit trees).

5.3. Nutrition Determines Protein Composition and Nitrogen Metabolism

The quality of many crops is determined by protein content and composition:

• In wheat – the content of gluten proteins (gliadins and glutenins), which determine baking properties.
• In soybean and other legumes – the content of storage proteins (globulins) and essential amino acids (methionine, cysteine, lysine).
• In potato – the content of soluble proteins and amino acids.

Nitrogen nutrition – a key factor in protein composition:

• Under high nitrogen, synthesis of storage proteins is enhanced, but their amino acid composition may change: the proportion of nitrogen‑rich amino acids (glutamine, asparagine, arginine) increases, while the content of sulfur‑containing amino acids (methionine, cysteine) may decrease (Marschner, 2012; Blevins, 1994).
• In wheat, high nitrogen doses increase total protein content but may reduce baking quality if the gliadin/glutenin ratio is disturbed (Gastal et al., 2015).
• In legumes, high nitrogen suppresses symbiotic fixation, and the amino acid ratio in seeds may shift towards higher asparagine and glutamine (Denison, 2015).

Sulfur – an essential element for protein quality:

• Sulfur is part of methionine and cysteine – amino acids that are often limiting in plant protein.
• Under sulfur deficiency, the content of methionine and cysteine in storage proteins decreases, reducing the nutritional value of grain (Blevins, 1994; Marschner, 2012).
• In wheat, sulfur is important for disulfide bond formation, which determines gluten elasticity. Sulfur deficiency impairs baking properties even if total nitrogen is sufficient (Marschner, 2012).

Nitrogen and nitrates – a quality issue in vegetables:

• Under excess nitrogen in soil and insufficient light (e.g., in greenhouses in winter), nitrates accumulate in leafy vegetables (spinach, lettuce, cabbage) (Harper, 1994; Marschner, 2012).
• Nitrates themselves are not dangerous, but in the intestine they can be reduced to nitrites, which bind haemoglobin (methaemoglobinaemia). Therefore, nitrate content is an important quality parameter.
• Nitrate accumulation can be reduced by optimising nitrogen doses, using the ammonium form (which is more rapidly incorporated into amino acids), and improving light conditions (Harper, 1994).

5.4. Macronutrients and Oil Quality in Oilseed Crops

In sunflower, rapeseed, soybean, olive, and palm, quality is determined by lipid content and fatty acid composition.

Nitrogen and phosphorus:

• Lipid synthesis requires significant amounts of energy (ATP) and reducing equivalents (NADPH). Both depend on phosphorus (ATP) and nitrogen (enzymes, including Rubisco, which supplies carbon).
• Under phosphorus deficiency, lipid synthesis may be inhibited, reducing seed oil content (Marschner, 2012).
• Excess nitrogen, on the other hand, stimulates protein synthesis at the expense of lipids, reducing the oil fraction. In rapeseed, high nitrogen reduces oil content by 2‑5% (Gastal et al., 2015).

Potassium and magnesium:

• Potassium is important for transport of sugars from leaves to seeds – the substrate for lipid synthesis.
• Magnesium is a cofactor of many lipid‑metabolism enzymes, including acetyl‑CoA carboxylase, a key enzyme in fatty acid synthesis.
• In sunflower, potassium fertilisers increase seed oil content, especially on poor soils (Marschner, 2012).

Boron:

• Boron is important for pollination and seed set in oilseed crops. Under boron deficiency, seed number declines, and oil content may fall due to incomplete filling (Blevins, 1994).

5.5. Micronutrients – “Quality Additives” for Fine Tuning

Macronutrients determine the main carbon and nitrogen flows, but micronutrients often act as “quality regulators” (Blevins, 1994; Marschner, 2012).

5.6. Antagonism between Quantity and Quality

In Part 4 we already mentioned the trade‑off between yield and protein content. But this is a particular case of a broader phenomenon: any strong shift in nutrient balance towards one element can impair quality.

Excess nitrogen reduces the quality of many crops:

• In cereals – reduced gluten content and poorer baking properties (if excess nitrogen occurs in early stages rather than during grain filling) (Gastal et al., 2015).
• In sugar beet – reduced sucrose content and accumulation of “harmful nitrogen” (amino acids, amides) that impair processing technology (Marschner, 2012).
• In vegetables – nitrate accumulation.
• In fruit crops – reduced sugar content, increased wateriness, poorer storability (Marschner, 2012).

Excess potassium can induce magnesium deficiency, which reduces chlorophyll, photosynthesis, and consequently sugar accumulation (Blevins, 1994; Marschner, 2012).

Excess phosphorus can induce zinc and iron deficiency, reducing auxin and chlorophyll synthesis, impairing fruit and grain quality (Marschner, 2012).

Optimal quality is achieved not at the maximum dose of each element, but with balanced nutrition, where all elements are in the optimal supply zone (NNI ≈ 1, similarly for P, K, Mg, and micronutrients) (Gastal et al., 2015; Connor et al., 2011).

5.7. Effect of Nutrition on Secondary Metabolites and Flavour

Quality is not only proteins, fats, and carbohydrates, but also secondary metabolites: aroma compounds, phenolics, alkaloids, glucosinolates, anthocyanins. They affect taste, aroma, colour, antioxidant activity, and disease resistance.

Nitrogen and secondary metabolites:

• High nitrogen often reduces the synthesis of phenolic compounds (flavonoids, phenolic acids) in fruits and leaves, because carbon and energy are switched to protein synthesis rather than defence substances (Marschner, 2012).
• This may reduce the antioxidant value of vegetables, berries, and fruits.
• In many crops (grape, tomato, tea), excess nitrogen reduces aroma and flavour intensity (Marschner, 2012).

Sulfur and glucosinolates (flavour of brassicas):

• Glucosinolates are sulfur‑containing compounds responsible for the pungent flavour of cabbage, radish, horseradish, and mustard.
• Under sulfur deficiency, their synthesis decreases, and the flavour becomes less pronounced (Marschner, 2012).
• Under excess sulfur, the opposite occurs – flavour intensifies, which may be a plus (spicy vegetables) or a minus (fodder crops).

Potassium and anthocyanins:

• Potassium influences anthocyanin (red, blue, purple pigments) synthesis in fruits and leaves. In grape, potassium fertilisation enhances berry colour, important for winemaking (Marschner, 2012).

5.8. Nutrition and Product Storability

Crop quality includes not only consumer properties at harvest, but also the ability to be preserved during storage and transport.

Calcium – the main element for storability:

• Calcium strengthens cell walls, reduces membrane permeability, and slows fruit senescence.
• In apples – calcium deficiency causes bitter pit, which greatly reduces appearance and storability (Marschner, 2012).
• In tomatoes – calcium deficiency causes blossom‑end rot, rendering fruits unmarketable.
• In potato – calcium deficiency impairs skin quality and reduces resistance to late blight (Marschner, 2012).

Potassium and storability of fruits and tubers:

• Potassium increases turgor and resistance of cell walls to damage, reduces wilting.
• In apples, high potassium improves storage, reducing subcutaneous spots (Marschner, 2012).
• In potato, potassium improves skin quality, reduces flesh browning during slicing and cooking.

Boron – storability of root crops and fruits:

• Boron participates in cell‑wall formation, reducing cracking during storage (beet, carrot).
• Boron deficiency causes necrosis of internal tissues in root crops, leading to rapid spoilage during storage (Blevins, 1994).

5.9. Managing Quality through Nutrition: Practical Examples

Wheat: to obtain high protein content and good baking quality:

• Ensure adequate nitrogen nutrition, but not excessive in early stages (to avoid excessive vegetative growth).
• Apply the main nitrogen dose at heading‑grain filling stages, when nitrogen loading of grain occurs (Gastal et al., 2015).
• Provide sulfur (for gluten disulfide bonds) and micronutrients (Zn, Cu, Mn) for full protein synthesis (Marschner, 2012).

Grape (for wine):

• Moderate nitrogen deficiency and adequate potassium favour accumulation of sugars and aroma compounds, rather than “green” notes (Marschner, 2012).
• Potassium enhances colour in red varieties and increases lodging resistance.
• Boron is important for cluster formation and pollination.

Tomato (for fresh market and processing):

• Optimal K/Mg ratio determines firmness, colour, and sugar content.
• Calcium prevents blossom‑end rot.
• Moderate nitrogen increases sugars; excess impairs flavour and increases wateriness.

Potato:

• Potassium determines starch content, flesh colour, skin quality, and resistance to browning.
• Excess nitrogen reduces starch and increases reducing sugars, worsening chip colour during frying (Marschner, 2012).
• Magnesium and phosphorus are important for proper tuber filling.

5.10. Quality as an Integrative Characteristic of Physiology

Now we can summarise and give a physiological definition of crop quality.

Crop quality is the distribution of carbon, nitrogen, sulfur, phosphorus, and other elements among different classes of organic compounds (proteins, carbohydrates, lipids, secondary metabolites, structural components) in the economically valuable organs, determined by the balance of mineral nutrition, hormonal status, and the genetic developmental programme.

This definition emphasises that:

• Quality is not an additive sum of individual properties, but a result of flow regulation.
• By changing mineral nutrition, we simultaneously affect yield and quality, and these effects may be in opposite directions.
• The optimum for quantity does not coincide with the optimum for quality – and the task of physiology is to find a balanced regime that maximises the integral value of the harvest.

Conclusions to Part 5

1. Product quality is a direct consequence of physiological processes, namely the distribution of carbon, nitrogen, and other elements among different classes of organic compounds.

2. Nitrogen is the main regulator of the protein/carbohydrate ratio: at high nitrogen, protein increases but sugars and starch decrease; under deficiency, the opposite occurs.

3. Potassium, phosphorus, boron, and magnesium determine sugar transport and accumulation in storage organs, and also affect lipid synthesis (oil content).

4. Sulfur is essential for the synthesis of methionine and cysteine – amino acids that determine protein quality and, through disulfide bonds, the baking properties of wheat.

5. Micronutrients (Zn, Mn, Cu, Fe, Mo, B, Ni) act as “quality regulators”, affecting protein synthesis, nitrate assimilation, enzyme formation, disease resistance, and storability.

6. There is an antagonism between quantity and quality: excess nitrogen often reduces sugar, aroma, and protective substances, and impairs storability and technological properties.

7. Optimal quality is achieved not at maximum fertiliser doses, but with balanced nutrition, where macro‑ and micronutrients are in the optimal supply zone.

8. Managing quality through nutrition requires diagnosis of plant status (NNI, micronutrient content) and variable‑rate fertiliser application according to developmental stage and target quality (e.g., for cereals, emphasis on nitrogen during grain filling).

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Final Summary of the Entire Lecture

We have travelled from the simple question “why does one element limit the whole plant?” to a comprehensive understanding that mineral nutrition is not just an “additive” but a fundamental regulator of the entire physiological system.

1. The law of the minimum is not a mechanical “barrel” but a systemic limitation, because each essential element performs a critical function without which growth, photosynthesis, transport, and reproduction stop.

2. The dose‑response relationship is nonlinear: there are zones of deficiency, optimum, and toxicity. Luxury consumption is storage, not growth. Excess does not increase yield, but degrades quality and causes environmental harm.

3. The plant actively restructures its development: it changes the root/shoot ratio, root architecture, hormonal status, forms specialised structures (cluster roots, aerenchyma), and uses systemic signals to coordinate responses.

4. Nutrient use efficiency (NUE) is an integrative indicator that depends on uptake, utilisation, and re‑utilisation efficiency. High yield does not guarantee high NUE, especially under low nitrogen.

5. Crop quality is determined by the distribution of carbon, nitrogen, and other elements among proteins, carbohydrates, lipids, and secondary metabolites. Optimal quality requires balanced nutrition, not extreme doses.

The main practical conclusion: managing mineral nutrition is not just “applying fertilisers”. It is physiological management of the plant, requiring diagnosis of plant status, knowledge of critical concentrations and nutrition indices, consideration of developmental stages, element interactions, and target product quality.