Physiology of nitrogen nutrition

Nitrogen as the Basis of the Photosynthetic Apparatus

Let us start with an obvious but important fact. In the dry mass of plant tissues, nitrogen accounts for about 1–5% (Marschner, 2012). That seems not so much—calcium or potassium can be present in similar or even greater amounts. So why does nitrogen end up in the spotlight?

The key lies in how this nitrogen is distributed within the cell. If we look at a leaf—the main organ of photosynthesis—we see an astonishing picture. Up to 50% of all nitrogen contained in the leaf is found in a single enzyme—ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known as Rubisco (Harper, 1994). And when we also consider that nitrogen is part of chlorophyll, light-harvesting complex proteins, and electron transport chain enzymes, it becomes clear that a significant portion of the plant’s total nitrogen is concentrated precisely in the photosynthetic apparatus.

Think about it: if a plant lacks nitrogen, it cannot build enough functional photosynthetic structures. And without them—neither energy nor biomass.

Nitrogen and the Intensity of Growth Processes

But the link between nitrogen and productivity is not limited to photosynthesis alone. Nitrogen also determines the intensity of growth—the rate at which the plant produces new organs. This is associated with two major groups of nitrogen‑containing compounds:

1. Proteins—enzymes, structural proteins, carrier proteins. Without nitrogen, no enzyme can be built, and therefore, metabolism as a whole is impossible.

2. Nucleic acids—DNA and RNA. Without nitrogen, new cells cannot be formed, because every cell division requires the synthesis of nucleic acids.

That is why, under nitrogen deficiency, we observe growth inhibition first and foremost. The plant becomes stunted, leaves become smaller, and the number of shoots and roots decreases (Tretyakov et al., 2000).

Here a fundamental physiological principle emerges: nitrogen is not just "nutrition" but a key regulator of growth activity. Nitrogen deficiency signals to the plant that it cannot increase biomass, so it switches to a survival strategy—slows growth, accelerates senescence of lower leaves, and mobilises nitrogen from them to young organs (Medvedev, 2012).

Nitrogen as a Link Between Photosynthesis and Growth

Let us note another important relationship. Leaf nitrogen content is closely linked to the rate of photosynthesis. Studies show a strong positive correlation between leaf nitrogen content and the rate of CO₂ assimilation (Sinclair and Horie, 1989). This is no accident: if we build the photosynthetic apparatus from proteins, the more nitrogen we can invest in it, the higher its performance.

However, here another important physiological principle comes into play. Nitrogen in leaves is distributed unevenly. Young, well‑illuminated leaves receive more nitrogen than old, shaded ones. This process is called reutilization—the plant’s ability to redistribute nitrogen from old, senescing organs to young, actively growing ones (Gastal and Lemaire, 2002).

Such a mechanism has profound meaning: the plant strives to use every atom of nitrogen as efficiently as possible, investing it where it will yield the greatest benefit for biomass accumulation. And since most biomass is created through photosynthesis, nitrogen is naturally directed to photosynthetically active tissues.

The Difference Between C₃ and C₄ Plants: Nitrogen Economy

Interestingly, the nitrogen requirement is not the same for different plant groups. C₄ plants, which possess an additional CO₂‑concentrating mechanism, have a markedly lower nitrogen requirement for photosynthesis. The leaf nitrogen content of C₄ plants is 1.5–2 times lower than that of C₃ plants, yet they can achieve comparable or even higher photosynthetic productivity (Marschner, 2012).

This is because C₄ plants contain less Rubisco in their leaves—since the CO₂‑concentrating system allows this enzyme to work more efficiently at lower contents. The paradox is that C₄ plants are often more productive while having a lower nitrogen requirement. This phenomenon is a striking demonstration that nitrogen efficiency is the key to productivity.

Why Does Nitrogen Most Often Limit Yield?

Now we can answer the main question: why is it nitrogen that most often determines productivity?

The answer lies in the combination of several factors:

1. Nitrogen is the basis of the photosynthetic apparatus. Without sufficient nitrogen, efficiently functioning chloroplasts and photosynthetic enzyme systems cannot be built.

2. Nitrogen is a growth regulator. The rate of cell division and expansion directly depends on nitrogen supply through the synthesis of proteins and nucleic acids.

3. Nitrogen is a mobile element. The plant's ability to redistribute nitrogen among organs means that under deficiency, all functions suffer, but growth and yield formation are affected first.

4. Nitrogen is the most deficient element in soils. Unlike other elements, the content of available nitrogen in soil is often low, and its supply strongly depends on many factors (temperature, moisture, microbial activity).

5. Nitrogen is the basis of the energy balance. All carbon assimilation processes require the participation of nitrogen‑containing enzymes.

This is precisely why nitrogen starvation manifests earlier and more severely than deficiency of other elements. And that is why nitrogen fertilisers produce the fastest and most noticeable effect on productivity.

Transition to the Next Question

So, we have understood why nitrogen is so important. But here is the paradox: the atmosphere contains about 78% nitrogen, yet the plant cannot use it directly. Life requires fixed nitrogen—that which is already incorporated into chemical compounds.

A natural question arises: in what form does the plant obtain nitrogen from the soil? The answer is two main forms: nitrate (NO₃⁻) and ammonium (NH₄⁺). And the choice between them is not just a matter of taste. It is a matter of physiological consequences that determine how much energy the plant will spend and how effectively it can use this precious element.

It is precisely this—the physiological consequences of using different nitrogen forms, the energy costs of their assimilation, and how nitrogen becomes not just food but also a signal—that will be the subject of the next part of our lecture.

Key Concepts and Conclusions

Two Forms, One Goal

So, we have established that nitrogen is a key element determining plant productivity. But in the soil it is not present as organic compounds that the plant could use directly. The main available forms of inorganic nitrogen are the nitrate ion (NO₃⁻) and the ammonium ion (NH₄⁺).

At first glance, the difference is only in the chemical form in which nitrogen enters the root. But for plant physiology, this is a fundamental distinction, determining:

• the energy balance of the whole plant;
• the distribution of assimilation processes between root and leaves;
• the acid‑base status of both cells and the rhizosphere;
• the possibility or impossibility of storing nitrogen in tissues.

The choice between nitrate and ammonium is not a matter of plant "taste" but a matter of physiological strategy that evolved over time and depends on environmental conditions, plant species, and even developmental stage (Harper, 1994; Marschner, 2012).

Nitrate: Why It Cannot Be Used "As Is"

Let us start with nitrate. This is the most common form of nitrogen in aerobic soils, especially in agricultural regions. Nitrate is highly mobile in the soil solution, easily reaches the roots with water flow, and is actively taken up by specialised transport systems—proteins of the NRT1 and NRT2 families (Taiz et al., 2023; Morot‑Gaudry et al., 2012).

But here a fundamental problem arises: nitrate is an oxidised form of nitrogen. Nitrogen in nitrate is in the +5 oxidation state. In organic molecules (amino acids, proteins, nucleic acids), nitrogen must be in a reduced form—with an oxidation state of –3.

That is, nitrate is like ore that must first be smelted before it can be used. The plant cannot incorporate nitrate into an amino acid—it must first be reduced to ammonium.

This is why the utilisation of nitrate requires two‑step reduction:

1. Nitrate (NO₃⁻) → nitrite (NO₂⁻) — catalysed by nitrate reductase in the cytosol;

2. Nitrite (NO₂⁻) → ammonium (NH₄⁺) — catalysed by nitrite reductase in plastids (Taiz et al., 2023; Medvedev, 2012).

These reactions require:

• reducing equivalents (electrons)—NADPH or reduced ferredoxin;
• energy—in terms of one reduced nitrogen atom, the equivalent of 12 ATP molecules is consumed (Taiz et al., 2023).

Why spend so much energy if ammonium—an already reduced form—is available?

Ammonium: Cheap but Dangerous

Ammonium is indeed energetically more advantageous. It is already reduced, and there is no need to spend energy converting it from the oxidised form. Ammonium assimilation directly into amino acids proceeds via glutamine synthetase (GS) and glutamate synthase (GOGAT), and this process requires significantly less energy than nitrate reduction (Taiz et al., 2023; Schopfer & Brennicke, 2016).

But ammonium has serious drawbacks.

First, ammonium is toxic to plants when it accumulates in tissues. Why? Ammonium is a weak base. When it accumulates in the cell, it disrupts:

• transmembrane proton gradients—the basis for the operation of photosynthetic and respiratory electron transport chains, ATP synthesis, and secondary active transport of ions and metabolites (Taiz et al., 2023; Lambers & Oliveira, 2019).
• acid‑base balance of the cytoplasm and organelles. Ammonium can cross membranes and alter pH, thereby disrupting enzyme function.

Therefore, when taking up ammonium, the plant must assimilate it immediately—incorporate it into organic compounds to avoid toxic effects. This requires a constant supply of carbon skeletons (α‑ketoglutarate and other keto acids) and energy for GS and GOGAT activity.

Ammonium nutrition is like a quick loan: it comes cheap but requires immediate repayment and does not forgive mistakes.

Second, ammonium cannot be stored in vacuoles in large amounts, unlike nitrate, which can be accumulated in high concentrations without harming the cell. This means that under ammonium nutrition, the plant has no "strategic reserve" of nitrogen and must assimilate it as it arrives.

Effect of Nitrogen Form on Rhizosphere pH: A Key Physiological Consequence

This aspect is often underestimated, but it has enormous importance for mineral nutrition as a whole. Recall that to absorb an ion, the cell must maintain electroneutrality. When the root absorbs the ammonium cation (NH₄⁺), it is forced to release a hydrogen ion (H⁺) into the rhizosphere. As a result, the rhizosphere pH decreases—the environment becomes more acidic (Morot‑Gaudry et al., 2012; Medvedev, 2012).

Conversely, absorption of the nitrate anion (NO₃⁻) is accompanied by the release of hydroxyl ions (OH⁻) or bicarbonates, which raises the rhizosphere pH—the environment becomes more alkaline (Marschner, 2012).

This simple pattern has far‑reaching consequences:

• Acidification of the rhizosphere (ammonium nutrition) can mobilise unavailable forms of phosphorus and micronutrients (iron, manganese, zinc), but can also lead to toxic effects of aluminium and manganese on acidic soils.
• Alkalisation of the rhizosphere (nitrate nutrition) favours nitrate and potassium uptake, but can cause iron deficiency on carbonate soils (so‑called "lime‑induced chlorosis").
• Mixed nitrate‑ammonium nutrition is often physiologically optimal because it allows the plant to regulate rhizosphere pH more effectively and also save energy by distributing the load between roots and shoots (Harper, 1994; Schopfer & Brennicke, 2016).

This is a fine example of how plant physiology "manages" soil chemistry around the root, creating more favourable conditions for its own nutrition.

Where to Reduce Nitrate—in the Root or in the Leaf? Energy Economy

The fate of nitrate that enters the root can be twofold:

1. It may be reduced directly in the root to ammonium, and then transported as amino acids and amides via the xylem to the shoot.

2. It may be transported in the xylem sap to the leaves and reduced there (Harper, 1994; Morot‑Gaudry et al., 2012; Connor et al., 2011).

What difference does it make? But the physiological difference is enormous and is related to the source of reducing equivalents and energy.

• In the root, reducing equivalents (NADPH) are supplied mainly by the pentose phosphate pathway of glucose oxidation—i.e., through respiration. This is expensive: reducing 1 mole of nitrate to ammonium in the root consumes about 60 g of glucose equivalent (Tretyakov et al., 2000; Medvedev, 2012).

• In the leaf, reducing equivalents can come directly from photosynthetic electron transport in the form of reduced ferredoxin. This is much cheaper—only about 15 g of glucose for the same work, and the energy comes not from reserves but from light (Taiz et al., 2023).

Therefore, many plants "prefer" to reduce nitrate in the leaves when light is sufficient, saving the carbon resources of the root. However, this ratio varies greatly among species:

• In lupine, pea, and most woody species, most nitrate is reduced in the roots (Harper, 1994).
• In sugar beet, cotton, and sunflower, reduction occurs predominantly in the leaves.
• In cereals (wheat, maize), both locations occur, depending on conditions and age (Tretyakov et al., 2000).

This distribution is a result of evolution, linked to the ecological strategy of the species. For example, plants growing under light‑limited conditions cannot rely on photosynthetic reduction in the leaves and must use the root system.

Why Mixed Nutrition Is Often Better

In practice, many crops give higher yields under mixed nitrate‑ammonium nutrition. Reasons:

1. Energy saving: part of the nitrate is reduced in leaves (using photosynthesis), part in roots, which reduces the respiratory load.

2. pH regulation: ammonium acidifies the rhizosphere, nitrate alkalinises it; their combined presence allows pH to be maintained in the optimal range for the uptake of most elements.

3. Reduced risk of ammonium toxicity: the presence of nitrate lowers the proportion of ammonium nitrogen, thus reducing its uptake rate, allowing the plant to assimilate it in time.

4. Signalling role of nitrate (discussed in the next section): nitrate induces the expression of genes for assimilation enzymes, preparing the plant for efficient use of nitrogen in any form.

In natural conditions, nutrition with only one form is rare—both nitrate and ammonium are usually present in different proportions. Plants are evolutionarily adapted to use both forms, and their physiological systems can switch depending on availability (Sadras & Calderini, 2015).

Transition to the Next Question

So, we have analysed why the choice between nitrate and ammonium is not just a matter of availability but a strategic decision affecting energy, pH, and plant safety. We have seen that nitrate is an "expensive but safe and transportable" form, while ammonium is a "cheap but dangerous and requiring immediate use" form.

But nitrogen physiology is not limited to transport and assimilation. We have already mentioned that nitrate is not just nutrition but also a signal. It is precisely this—the role of nitrate as a regulator of gene expression, a coordinator of carbon and nitrogen metabolism, and an integrator of environmental signals—that will be the subject of the next part of the lecture.

Key Concepts and Conclusions (Section 2)

From Chemical Fact to Physiological Problem

In the previous part, we established that nitrate is an oxidised form of nitrogen (oxidation state +5), whereas the construction of organic molecules requires reduced nitrogen (oxidation state −3). This chemical discrepancy gives rise to a fundamental physiological problem: nitrate cannot be incorporated into an amino acid or protein—it must first be reduced.

But if it were only a matter of a chemical reaction, the issue would be solved at the biochemical level. The physiological aspect is much deeper. It concerns:

• Where reduction occurs—in the root or in the leaf?
• Where does the energy and reducing equivalents come from?
• How does the plant distribute energy costs among different organs?
• Why have different species developed different nitrate reduction strategies?

The answers to these questions determine not only the efficiency of nitrogen nutrition but also the competitiveness of the species, its adaptation to environmental conditions, and ultimately productivity (Harper, 1994; Taiz et al., 2023).

Two Steps of Reduction: Not Just Biochemistry but a Physiological Compromise

The nitrate reduction process comprises two steps, and it is important to understand their physiological meaning:

Root vs Leaf: Energy Arithmetic

We now come to the central question: where is it more advantageous for the plant to reduce nitrate—in the root or in the leaf?

Reduction of nitrate to ammonium requires 8 electrons per molecule. But the cost of obtaining them differs greatly depending on the site:

Why Don’t Plants Shift All Reduction to Leaves?

If leaf reduction is so advantageous, why do many plants still reduce nitrate in roots? Why not transport all nitrate to the leaves?

The reasons are purely physiological:

Regulation of Nitrate Reduction Distribution: Physiological Plasticity

It is important to understand that the distribution between root and leaf reduction is not a rigid species characteristic but a dynamic process regulated by many factors:

Connection with Photosynthesis: Why Nitrate Reduction Is Not Just "Consumption" but Coordination

Now we see that nitrate reduction is not an isolated process but part of a unified carbon‑nitrogen metabolic system. Here are the key points of interaction:

Why Nitrate Reduction Is a Physiological Choice, Not a Necessity

In light of the above, it becomes clear that the decision to reduce nitrate in root or leaf is not just a "place" where the enzyme works. It is:

• An energy‑saving strategy. With sufficient light, leaf reduction is more advantageous; under light deficiency, root reduction.
• Adaptation to environmental conditions. Forest and shade species—root strategy; light‑loving species—leaf strategy.
• Balance regulation. Mixed reduction allows optimal distribution of energy costs between roots and shoots.

And most importantly: the ability of plants to switch between strategies is a key adaptation mechanism that allows them to maintain high productivity under different conditions.

Transition to the Next Question

We have examined why nitrate cannot be used directly and how the plant solves the problem of its reduction by distributing energy costs between root and leaf. We have seen that nitrate reduction is not just a biochemical process but a physiological compromise depending on light, carbohydrates, temperature, and species.

But the physiology of nitrate does not end there. It turns out that nitrate performs not only a nutritional but also a signalling function. It not only "feeds" the plant but also informs it about the state of the external environment, triggering cascades of regulatory reactions that remodel metabolism, growth, and development.

It is precisely about this—nitrate as a signalling molecule, how the plant "senses" nitrogen and uses this information to optimise its life—that we will discuss in the next part of the lecture.

Key Concepts and Conclusions (Section 3)

From Nutrition to Information: A New Paradigm

In the previous sections, we considered nitrogen as a nutrient element—a building material for the photosynthetic apparatus and an energy resource requiring reduction costs. But modern plant physiology has added a fundamentally new dimension to this picture: nitrogen (and especially nitrate) is also a signalling molecule.

This idea changes the very concept of mineral nutrition. The plant does not merely "ingest food"—it perceives information about the presence of nitrate in the environment and remodels its physiology accordingly. This allows the plant to:

• prepare the enzymatic machinery for nitrogen assimilation before it starts arriving in large amounts;
• coordinate nitrogen uptake with carbon metabolism to avoid imbalance;
• regulate root architecture, directing growth towards nitrate‑rich zones;
• integrate information about nitrogen status at the whole‑organism level.

Nitrate is not just "food"—it is a "letter" from the environment: "There is nitrogen here, get ready for active growth!"

Primary Nitrate Response: How the Plant "Senses" Nitrate

One of the most striking manifestations of nitrate’s signalling function is the so‑called Primary Nitrate Response. This phenomenon was discovered when studying gene expression: within 30 minutes after adding nitrate to roots grown without nitrogen, transcription of more than 500 genes sharply increases (Taiz et al., 2023; Morot‑Gaudry et al., 2012).

Among these genes are those encoding:

• nitrate reductase and nitrite reductase—enzymes of nitrate reduction;
• nitrate transporters (NRT1 and NRT2)—to enhance uptake;
• enzymes of ammonium assimilation (glutamine synthetase, glutamate synthase);
• factors regulating root growth.

This response is extremely fast for a plant cell—it does not require de novo protein synthesis but is based on the activation of pre‑existing transcription factors. The speed of the response indicates that this is a specialised signalling mechanism, not merely a consequence of substrate appearance.

The Transceptor CHL1/NRT1.1: How Nitrate Is "Recognised"

The key molecule in nitrate perception is CHL1/NRT1.1—a protein that is simultaneously a transporter and a receptor (a so‑called transceptor—transporter + receptor) (Taiz et al., 2023; Morot‑Gaudry et al., 2012).

What makes CHL1/NRT1.1 unique?

From Perception to Response: The Transcription Factor Cascade

After CHL1/NRT1.1 "recognises" nitrate, the signal is transmitted to the nucleus through a cascade of protein kinases and activation of transcription factors. The key player here is NLP7 (NIN‑LIKE PROTEIN 7)—a transcription factor that binds to the promoters of genes for nitrate reductase, nitrite reductase, and nitrate transporters (Taiz et al., 2023).

This is how it works:

1. CHL1/NRT1.1 perceives nitrate at the plasma membrane.

2. A protein kinase cascade is activated.

3. NLP7 moves to the nucleus.

4. NLP7 binds to regulatory regions of nitrogen metabolism genes.

5. Transcription begins—within 30 minutes of the signal.

Importantly: NLP7 itself does not "know" about nitrate—it is activated via a signalling cascade that depends on the state of CHL1/NRT1.1. That is, the entire system is built on nitrate being perceived at the membrane and the response being launched in the nucleus.

Why Is the Nitrate Signal Not Just "Turn on Enzymes"?

The primary nitrate response is not simply the switching on of assimilation enzymes. It is a comprehensive remodelling of entire metabolism, including:

Signalling Role of Nitrate Compared to Other Forms of Nitrogen

It is important to note that not only nitrate but also ammonium can serve as a signal, but the nature of the signal differs.

• Nitrate signals the presence of "safe" and "storable" nitrogen. It induces long‑term adaptations: root growth, transporter expression, metabolic remodelling.

• Ammonium signals the presence of "fast" but potentially dangerous nitrogen. It can repress the nitrate response (when ammonium is abundant, nitrate is not needed) and stimulate immediate assimilation to avoid toxicity (Marschner, 2012; Lambers & Oliveira, 2019).

These differences are important because they allow the plant to integrate information about different nitrogen forms and choose the optimal strategy.

Nitrate Signal and Productivity: Practical Significance

Understanding the signalling function of nitrate has direct practical implications:

From Signal to System: Integration with Other Regulators

The nitrate signal is only one element of a complex system of information integration in the plant. It interacts with:

• Light signals—via phytochrome and cryptochrome, which also influence the expression of nitrate reduction genes.
• Hormonal signals—auxins, cytokinins, abscisic acid, ethylene, which also modulate the nitrogen response.
• Carbohydrate signals—sucrose, glucose, which inform about the carbon nutrition status.

The result is a signalling network in which nitrate is one of the nodes, but not the only one. This makes the system robust and flexible: even if one signal changes, others can compensate.

Transition to the Next Question

So, we have analysed that nitrate is not just a nutrient but a signalling molecule that informs the plant about nitrogen availability, triggers cascades of regulatory reactions, and coordinates carbon and nitrogen metabolism. We have seen how the transceptor CHL1/NRT1.1 works, how the signal is transmitted via NLP7 to the nucleus, how it affects root growth and the physiology of the whole plant.

But we have not yet fully answered one important question: why can ammonium, despite being so energetically favourable, be dangerous? In the next part, we will examine the physiological mechanisms of ammonium toxicity—a problem closely related to acid‑base balance, respiration, assimilate transport, and even photosynthesis.

Key Concepts and Conclusions (Section 4)

The Paradox of Ammonium Nutrition

In the previous sections, we established that ammonium (NH₄⁺) is an energetically favourable form of nitrogen: it is already reduced, and its assimilation requires significantly less energy than nitrate reduction. It would seem that the plant should prefer this form. But in nature, ammonium is rarely the main nitrogen source for most crop plants, and many species suffer from its excess.

Why does ammonium, being "cheap" in energy terms, become dangerous?

The answer lies in the fact that ammonium is not just an ion but a powerful physiological irritant that, when accumulated, disrupts several key life‑support systems of the cell. Ammonium toxicity is not the action of the ion itself but a disruption of physiological balance: respiration, carbon metabolism, acid‑base status, assimilate transport, and membrane transport (Taiz et al., 2023; Marschner, 2012; Britto & Kronzucker, 2002).

Three Pillars of Ammonium Toxicity

The toxic effect of ammonium is exerted through three interconnected mechanisms:

Disruption of Ionic Balance: Ammonium vs Potassium

Ammonium and potassium are monovalent cations with similar ionic radii and hydration energies. Therefore, transport systems that take up K⁺ can also capture NH₄⁺, and vice versa (Marschner, 2012; White, 2012). At high ammonium concentrations in the rhizosphere:

• Potassium uptake is suppressed—due to direct competition for common transporters (e.g., AKT1 channels and HAK5 transporters in Arabidopsis).
• Secondary potassium deficiency develops, even if potassium is present in the soil. This manifests as chlorosis, weakened turgor, and reduced drought and disease resistance.
• Uptake of Ca²⁺ and Mg²⁺ is disturbed (competition for cation channels and membrane binding sites), leading to symptoms resembling calcium or magnesium deficiency (Medvedev, 2012).

Thus, ammonium toxicity is often masked as deficiency of other elements, making diagnosis difficult.

How Does the Plant Defend Itself Against Ammonium? Physiological Mechanisms of Tolerance

Plants are not defenceless against ammonium. Through evolution, they have developed mechanisms to avoid toxicity or minimise its consequences:

Species Differences in Ammonium Tolerance

Tolerance to ammonium nutrition varies greatly among species (Britto & Kronzucker, 2002; Marschner, 2012):

Ammonium‑tolerant species: rice (Oryza sativa), many aquatic and wetland plants, some grasses (e.g., Lolium perenne). They have:

• high GS activity in roots;
• efficient delivery of carbohydrates to roots;
• ability to rapidly assimilate ammonium.

Ammonium‑sensitive species: barley (Hordeum vulgare), most dicot crops (tomato, legumes). They:

• assimilate ammonium more slowly;
• suffer more from competition with K⁺;
• excrete more ammonium into the medium, creating a "vicious circle" and depleting carbohydrate reserves.

Interestingly, sensitivity to ammonium often correlates with metabolic type: C₃ plants are generally more sensitive than C₄ plants, because the latter have a more efficient carbohydrate recycling system (Connor et al., 2011).

Connection with Previous Sections: Ammonium and Nitrate in a Unified System

Now we can link all parts of the lecture together:

It is clear that the choice between forms is determined not only by their availability but also by the plant's physiological state—light, carbohydrate supply, need for pH regulation, developmental stage. This is exactly what makes nitrogen nutrition a complex but manageable system.

Practical Implications for Agronomy

Understanding the mechanisms of ammonium toxicity allows optimisation of nitrogen fertiliser use:

• On light, well‑drained soils, nitrate fertilisers are preferred.
• On waterlogged, acidic soils where nitrification is suppressed, ammonium fertilisers may be the only N source, but they require caution (split application, use of nitrification inhibitors) (Harper, 1994; Connor et al., 2011).
• Mixed nitrate‑ammonium nutrition (e.g., NH₄NO₃) is often optimal, as it reduces toxicity risk and exploits the advantages of both forms.
• Species differences must be considered when choosing fertilisers: rice uses ammonium well; barley and wheat use nitrate.
• Potassium nutrition should be balanced with ammonium nutrition to avoid competition.

Conclusion: Ammonium as a Physiological Challenge

Ammonium toxicity is not accidental but a natural consequence of its physicochemical properties. It is a "cheap" form of nitrogen, but this "cheapness" requires the plant to maintain high metabolic activity, a constant supply of carbohydrates, and strict control over pH and membrane gradients. Species that have evolutionarily adapted to these conditions (rice, wetland plants) successfully utilise ammonium. Those that are not adapted (most field crops) suffer from imbalance at high ammonium fertiliser doses.

Thus, ammonium is not "bad" nitrogen but nitrogen with its own rules of the game. And it is the task of the physiologist and agronomist to know these rules and apply them in practice.

Key Concepts and Conclusions (Section 5)