Carbon-nitrogen balance (C/N)

1.1. From Chemical Analysis to Physiological Meaning

Let’s start with the simplest fact. In any plant cell, two fundamental flows of substances occur simultaneously. The first is the carbon (C) flow. It enters the plant through stomata as carbon dioxide (CO₂), is incorporated into organic molecules during photosynthesis (Taiz et al., 2023), and ultimately becomes the building material for all organic compounds: carbohydrates, lipids, organic acids, and, of course, amino acids.

The second flow is nitrogen (N). It enters the plant from the soil, mostly as nitrate (NO₃⁻) or ammonium ion (NH₄⁺), and is incorporated into amino acids, proteins, nucleic acids, and other vital compounds (Morot‑Gaudry et al., 2012).

At first glance, the ratio of these two elements may indeed seem like a simple chemical indicator. However, imagine two situations.

Situation one. We take a plant and give it plenty of light, water, and carbon dioxide, but restrict nitrogen supply. Photosynthesis proceeds actively; the plant synthesises many carbohydrates – sucrose, starch. Nitrogen is insufficient for protein synthesis. The C/N ratio will be high.

Situation two. We give the plant plenty of nitrogen fertiliser but limit light. The plant actively absorbs nitrogen, but photosynthesis is slow. Carbohydrates are lacking to build the carbon skeletons needed to incorporate nitrogen into amino acids. The C/N ratio will be low (Marschner, 2012).

In both cases, we get different numerical C/N values. But that is not the main point. The main point is that the plant does not simply accumulate these two elements. It constantly “evaluates” their ratio and makes physiological decisions based on this evaluation.

1.2. C/N as a Signalling System

The plant cannot be viewed as a passive reservoir into which carbon and nitrogen flow. It is an active, self‑regulating system that continuously monitors its internal status. And the C/N ratio is one of the most important signals for the plant (Hopkins & Hüner, 2009; Lambers & Oliveira, 2019).

Why did this particular ratio become a signal? Because it reflects the fundamental balance of two key resources:

• Carbon – a source of energy (in the form of ATP and reducing equivalents) and building material for the carbon skeletons of all organic molecules (Taiz et al., 2023).
• Nitrogen – a key element for building proteins – enzymes, structural proteins, transport proteins, as well as nucleic acids (Morot‑Gaudry et al., 2012).

The balance between these resources is essentially the balance between the plant’s energy supply and its construction potential.

Imagine a construction site. You have a team of builders (this is nitrogen – proteins, enzymes) and building materials (this is carbon – carbohydrates, organic acids). If there are plenty of materials but few workers, construction proceeds slowly (high C/N, growth restricted). If there are many workers but no materials, workers stand idle, their energy is wasted (low C/N, nitrate accumulation, risk of toxicity). The ideal situation is when there are just enough workers and materials for the construction to proceed at an optimal pace.

That is why it is critical for the plant to maintain balance. Too high C/N – the plant accumulates carbohydrates but cannot use them for growth due to nitrogen deficiency. Too low C/N – the plant cannot use nitrogen efficiently due to a shortage of carbon skeletons and energy; nitrogen accumulates as nitrates or ammonium, which can be toxic (Taiz et al., 2023).

1.3. C/N – a Regulator of Growth and Development

But the most surprising thing is that C/N does not merely reflect the state – it regulates physiological processes. The plant uses information about C/N to make decisions: to grow or not to grow, where to direct resources – into roots or shoots, to store reserves or to spend them on growth (Boote et al., 1994; Marschner, 2012; Третьяков, 2000).

This happens because the concentrations of carbohydrates and nitrogen compounds serve as signals for gene expression. Sucrose and glucose, accumulating at high C/N, trigger cascades of signalling reactions that change the activity of genes responsible for growth, development, and storage (Taiz et al., 2023). Nitrogen compounds, in turn, act as signals regulating the activity of enzymes responsible for carbon assimilation (Morot‑Gaudry et al., 2012).

Thus, the plant finds itself at the centre of a complex regulatory system where C/N is the key integrative indicator on the basis of which it adapts its physiology to changing environmental conditions.

1.4. The Dynamic Nature of the Balance

It is important to understand that C/N is never constant. It continuously changes depending on:

• time of day (during the day – photosynthesis, carbohydrate accumulation – C/N rises; at night – carbohydrates are consumed by respiration – C/N falls);
• developmental stage (during active vegetative growth, the need for nitrogen is high – C/N is low; during ripening, when carbohydrates are stored in seeds or tubers – C/N is high) (Boote et al., 1994; Третьяков, 2000);
• environmental conditions (drought, salinity, low temperatures – all alter the C/N balance) (Pessarakli, 2020; Третьяков, 2000).

Unlike an animal, a plant cannot escape unfavourable conditions. Its only way is to adapt. And C/N is one of the key tools of this adaptation.

1.5. From Theory to Practice: Why This Matters for the Agronomist

Understanding C/N as a physiological regulator has direct practical implications for agronomy. Here are a few examples.

Example one: fertilisation. The application of nitrogen fertilisers shifts C/N downwards, stimulating vegetative growth. But if there is too much nitrogen (low C/N), the plant may “overgrow” – forming thin, loose tissues that easily lodge and are susceptible to diseases (Marschner, 2012; Третьяков, 2000).

Example two: drought resistance. At high C/N (plenty of carbohydrates, little nitrogen), the plant directs resources to the roots – they grow more actively, become more developed, allowing the plant to better extract water from deep soil layers and increasing its drought resistance. Low C/N (high nitrogen) stimulates shoot growth at the expense of roots, which can be fatal under drought conditions (Третьяков, 2000).

Example three: crop quality. To obtain grain with high protein content, it is necessary that during grain filling the C/N ratio be balanced – sufficient carbohydrates for starch synthesis and sufficient nitrogen for protein synthesis (Boote et al., 1994; Sadras & Calderini, 2015).

1.6. Summary: C/N – a Regulator, Not Just an Indicator

Thus, we arrive at the key conclusion:

The carbon‑nitrogen balance (C/N) is not merely the chemical ratio of two elements in plant tissues. It is a fundamental physiological indicator that the plant uses as a signal to regulate its vital activities, resource allocation, growth, and adaptation to changing environmental conditions. C/N is the physiological compass that directs plant development in accordance with available resources.

In the following sections, we will examine in detail how exactly the plant evaluates its C/N, which physiological mechanisms are involved in this process, and how changes in C/N affect the appearance, growth, development, and productivity of plants.

2.1. Energy Dependence of Nitrogen Metabolism on Photosynthesis

Let us start with the most obvious. Nitrogen enters the plant from the soil in an oxidised form – nitrate (NO₃⁻). To incorporate this nitrogen into organic molecules, it must first be reduced to ammonium (NH₄⁺) and then built into carbon skeletons. This requires enormous energy expenditure.

Nitrate reduction is an endergonic process – it requires energy. To reduce one nitrate molecule to ammonium, 8 electrons are needed, supplied either by NADPH or reduced ferredoxin (Morot‑Gaudry et al., 2012). The source of these reducing equivalents is photosynthesis:

• In leaves, nitrate reduction occurs mainly via ferredoxin, reduced during the light reactions of photosynthesis (Taiz et al., 2023).
• In roots, NADPH is used, but it ultimately is formed from carbohydrates supplied from the leaves (Morot‑Gaudry et al., 2012).

But nitrate reduction is only half the story. The reduced ammonium must be incorporated into organic molecules – amino acids. The key reaction of this step – the synthesis of glutamine and glutamate – requires not only energy (ATP) but also carbon skeletons (α‑ketoglutarate). And α‑ketoglutarate is an intermediate of the Krebs cycle, which in turn is replenished from carbohydrates supplied by photosynthesis (Morot‑Gaudry et al., 2012).

Thus, every step from nitrate to protein requires either ATP, reducing equivalents, or carbon skeletons – all supplied by photosynthesis.

2.2. Carbon Dependence of Photosynthesis on Nitrogen Metabolism

But this link is not one‑way. If photosynthesis supplies nitrogen metabolism with energy and carbon skeletons, then nitrogen metabolism, in turn, supplies photosynthesis with its proteinaceous machinery.

Recall the key enzyme of photosynthesis – Rubisco (ribulose‑1,5‑bisphosphate carboxylase/oxygenase). It is the most abundant protein on Earth. In leaves of C₃ plants, Rubisco accounts for up to 20‑30% of total leaf nitrogen (Taiz et al., 2023). Without this protein, photosynthesis is impossible. But Rubisco is a protein – therefore, its synthesis requires nitrogen. Without nitrogen, the plant cannot synthesise Rubisco, and photosynthesis stops.

But Rubisco is only one example. Photosynthesis requires a whole complex of proteins: Calvin cycle enzymes, electron‑transport chain proteins (cytochromes, ferredoxin), chlorophyll‑protein complexes of light‑harvesting antennae, regulatory enzymes (e.g., thioredoxins). All of them contain nitrogen (Taiz et al., 2023; Morot‑Gaudry et al., 2012).

Thus, for photosynthesis to work, the plant must constantly synthesise new proteins, replacing worn‑out ones. And for protein synthesis, nitrogen is needed.

2.3. A Closed Loop: From CO₂ to Protein and Back

Now we can construct this cycle as a whole and see its closed nature:

1. Light energy is absorbed by chlorophyll (protein + pigment, requires nitrogen).

2. During the light reactions, ATP and reduced ferredoxin/NADPH are formed.

3. ATP and reducing equivalents are used to reduce nitrate and incorporate ammonium into glutamine and glutamate (requiring carbon skeletons from the Krebs cycle).

4. From glutamate and glutamine, all amino acids are synthesised.

5. From amino acids, proteins are synthesised, including photosynthetic enzymes – Rubisco, Calvin cycle enzymes, electron‑transport chain proteins.

6. These proteins enable a new round of photosynthesis – CO₂ uptake, fixation, and formation of carbon skeletons (which again go into amino acid synthesis).

This cycle clearly shows that photosynthesis and nitrogen metabolism are two sides of the same coin. Without photosynthesis, the plant would have no energy or carbon skeletons for amino acid and protein synthesis. Without nitrogen, there would be no proteins, including the enzymes of photosynthesis. They support each other, forming a system with feedback.

2.4. Physiological Meaning of Inseparability

Why did nature “close” these two processes into a single system? The answer is simple – it ensures efficiency and adaptability.

• Efficiency. The plant obtains carbon and nitrogen from different sources (air and soil). If these processes were not linked, the plant could not coordinate their rates. For example, too much nitrogen could enter while carbon skeletons were scarce. Then nitrogen would accumulate as nitrates or ammonium, which is toxic. Or vice versa – photosynthesis could be active while nitrogen was insufficient for protein synthesis, and the photosynthetic apparatus would not operate at full capacity. Integration allows synchronisation of resource uptake and utilisation (Hopkins & Hüner, 2009; Lambers & Oliveira, 2019).
• Adaptability. Because the processes are linked, the plant can rapidly respond to changing conditions. For instance, under drought, stomata close, photosynthesis slows, and the flow of carbon skeletons decreases. In response, the plant reduces the activity of nitrogen metabolism enzymes (e.g., nitrate reductase) so as not to waste energy on nitrate reduction that cannot be used due to carbon skeleton shortage (Pessarakli, 2020; Третьяков, 2000).

2.5. Where Does Integration Occur?

Now that we understand the principle of inseparability, let us look one level deeper and ask: where exactly do the flows of carbon and nitrogen meet? Where does the “encounter” of carbohydrates and ammonium occur that gives rise to amino acids?

This question is important because it shows that integration does not occur “somewhere” in the cell, but at strictly defined nodes of metabolism. Such nodes are key physiological junctions where carbon and nitrogen pathways converge and where their coordination takes place.

One such key node is the synthesis of glutamine and glutamate. As we mentioned, it is through these amino acids that ammonium is first incorporated into organic matter. This process is catalysed by two enzymes: glutamine synthetase (GS) and glutamate synthase (GOGAT) (Morot‑Gaudry et al., 2012; Taiz et al., 2023). But for us, the names of the enzymes are less important than the fact that this process requires the simultaneous presence of:

• ammonium (product of nitrate reduction),
• ATP (product of photosynthesis and respiration),
• α‑ketoglutarate (a carbon skeleton coming from the Krebs cycle, which in turn is fed by photosynthetic carbohydrates).

Thus, GS/GOGAT is not just a biochemical reaction – it is a physiological node where flows of nitrogen, energy, and carbon meet. If even one of these components is absent or scarce, glutamine/glutamate synthesis slows down, and the entire plant receives a signal that the balance is disturbed.

But there are other such nodes. For example, the synthesis of aspartate and asparagine – another key point where carbon (oxaloacetate from the Krebs cycle) and nitrogen (glutamine) meet. Or the synthesis of alanine from pyruvate. All these reactions are integration points.

It is important to emphasise: these nodes are not just chemical reactions. They are regulatory centres. Their activity determines how much nitrogen will be incorporated into organic molecules and how much carbon will be directed to amino acid synthesis rather than to storage as starch or sucrose.

2.6. What Happens When the Balance Is Disrupted?

If integration is disrupted, the consequences for the plant can be severe.

High C/N (much carbon, little nitrogen). Photosynthesis proceeds actively, but nitrogen is insufficient for protein synthesis. Carbon skeletons cannot be used for amino acid synthesis, and they accumulate as carbohydrates (starch, sucrose). This leads to:

• accumulation of carbohydrates in leaves,
• slowed growth (no proteins for building new tissues),
• activation of anthocyanin synthesis (a protective stress response),
• enhanced root growth (in search of nitrogen).

Low C/N (much nitrogen, little carbon). Nitrogen is abundant, but carbon skeletons are lacking. The plant cannot incorporate all the nitrogen into amino acids. Excess ammonium and nitrates can be toxic. Moreover, the shortage of carbohydrates limits the energy supply for nitrogen metabolism. The plant:

• actively synthesises proteins but cannot supply them with carbon skeletons,
• forms loose, thin tissues (poor development of mechanical tissues),
• expends energy on ammonium detoxification,
• becomes more vulnerable to diseases and lodging (Третьяков, 2000; Marschner, 2012).

2.7. Summary: Interdependence – the Basis of Life

Thus, we see that photosynthesis and nitrogen metabolism cannot be separated for three reasons:

1. Energy dependence. Nitrogen metabolism requires ATP and reducing equivalents supplied by photosynthesis.

2. Substrate dependence. Nitrogen metabolism requires carbon skeletons formed during photosynthesis (via the Krebs cycle).

3. Protein dependence. Photosynthesis requires continuous synthesis of proteins (enzymes), which requires nitrogen.

Thus, photosynthesis and nitrogen metabolism form a closed, self‑sustaining cycle, where the products of one process become the substrates for the other. This cycle makes the plant an autonomous and self‑regulating system. Disruption of this cycle (e.g., due to nitrogen or light deficiency) leads to imbalance and, ultimately, to reduced productivity. Understanding this interrelationship is key to managing plant growth and development in agricultural practice.

3.1. Integration at the Cellular Level: Carbon Skeletons as a “Bridge”

Imagine a plant cell as a bustling city. Through its streets flow two main transport streams. One stream – carbon molecules: sugars, organic acids arriving from “factories” – chloroplasts (photosynthetic products) or from storage organs. The second stream – nitrogen‑containing compounds: nitrates, ammonium, amino acids coming from the soil through roots or synthesised within the plant itself.

But these two streams do not simply run parallel. At certain points in the city – at “squares” and “crossroads” – they meet and merge. Such “crossroads” are metabolic nodes where a carbon skeleton (usually an organic acid) combines with an amino group to form an amino acid.

What is a carbon skeleton? It is a chain of carbon atoms that serves as the “backbone” for an amino acid. For example, for alanine synthesis, pyruvate (C₃) is needed; for aspartate – oxaloacetate (C₄); for glutamate – 2‑oxoglutarate (C₅) (Morot‑Gaudry et al., 2012; Taiz et al., 2023). All these organic acids are products of carbohydrate oxidation (glycolysis, Krebs cycle). Hence, they come from photosynthesis.

3.2. The Main Crossroad: GS/GOGAT – the Node Where All Paths Meet

The most famous and perhaps the most important “crossroad” in plant metabolism is the GS/GOGAT system (glutamine synthetase and glutamate synthase). Here, ammonium (NH₄⁺), reduced from nitrate, is first incorporated into organic matter. This node works as follows:

• Glutamine synthetase (GS) links ammonium with glutamate (C₅‑amino acid) using ATP. Glutamine (C₅‑amide) is formed (Morot‑Gaudry et al., 2012).
• Glutamate synthase (GOGAT) transfers the amide group from glutamine to 2‑oxoglutarate (C₅‑keto acid), forming two molecules of glutamate (Taiz et al., 2023; Morot‑Gaudry et al., 2012).

For us, the names of the enzymes are less important than the fact that this node requires the simultaneous presence of three components:

1. Ammonium (NH₄⁺) – product of nitrate reduction.

2. 2‑Oxoglutarate – a carbon skeleton coming from the Krebs cycle. The Krebs cycle is replenished by carbohydrates formed during photosynthesis.

3. ATP – energy also derived from photosynthesis (and respiration).

If even one of these components is absent or scarce, the GS/GOGAT operation slows, and glutamine/glutamate synthesis decreases. This, in turn, affects the entire nitrogen metabolism, since glutamine and glutamate serve as donors of amino groups for the synthesis of all other amino acids (via transamination reactions) (Morot‑Gaudry et al., 2012).

Thus, GS/GOGAT is not just a biochemical reaction. It is a physiological node that evaluates the availability of three key resources: nitrogen, carbon skeleton, and energy. If all three resources are sufficient – amino acid synthesis proceeds actively, and the plant can build new proteins and tissues. If a resource is lacking – the node is inhibited, and the entire synthesis slows.

3.3. Other Crossroads: Synthesis of Aspartate, Alanine, and Other Amino Acids

But GS/GOGAT is not the only meeting point for carbon and nitrogen flows. Besides glutamate and glutamine, there are other amino acids whose synthesis also requires carbon skeletons. Each of these syntheses is its own “crossroad”:

• Aspartate synthesis occurs from oxaloacetate (C₄‑keto acid) by adding an amino group from glutamate (Taiz et al., 2023).
• Alanine synthesis occurs from pyruvate (C₃) by adding an amino group.
• Asparagine synthesis occurs from aspartate by adding a second amino group.
• Synthesis of proline, arginine, histidine – all use carbon skeletons from the Krebs cycle and glycolysis.

All these reactions are integration points where carbon skeletons derived from carbohydrates combine with nitrogen to form amino acids.

But there is another important point. Amino acids, in turn, can serve as sources of carbon skeletons for respiration and other processes. For example, deamination of amino acids (removal of the amino group) yields keto acids that can enter the Krebs cycle and be used for energy production or for synthesis of other organic compounds. Thus, carbon and nitrogen flows can switch depending on the plant’s needs.

3.4. Integration at the Whole‑Plant Level: Transport of Metabolites

But integration points exist not only inside the cell. They also occur at the whole‑plant level. Carbon and nitrogen synthesised in one organ must be delivered to others.

• Leaves – the main suppliers of carbohydrates (sucrose) via phloem flow (Boote et al., 1994; Morot‑Gaudry et al., 2012).
• Roots – the main suppliers of nitrates and amino acids (asparagine, glutamine) via xylem flow (Morot‑Gaudry et al., 2012).

These two flows meet in growing organs – meristems, young leaves, developing fruits and seeds. There, integration of carbon skeletons (arriving from leaves) and amino acids (arriving from roots and old leaves) takes place. In these organs, new proteins are synthesised and tissues are formed.

This means that the integration point can be far removed from the site of carbon synthesis or nitrogen assimilation. For example, carbohydrates produced in upper leaves may be transported to roots, where they combine with nitrogen absorbed from the soil to give rise to amino acids, which then return to the leaves (Lemaire et al., 2008).

3.5. Regulatory Role of Integration Points

Now we come to the most important point: integration points are not just sites of synthesis. They are regulatory centres.

The activity of enzymes working in these nodes (GS, GOGAT, aspartate aminotransferase, alanine aminotransferase, etc.) depends on the availability of substrates. If carbon skeletons are abundant but ammonium is scarce, amino acid synthesis slows. If ammonium is abundant but carbon skeletons are scarce, ammonium accumulates, which can be toxic.

But besides substrate regulation, there is also signalling regulation. Concentrations of carbohydrates (sucrose, glucose) and nitrogen metabolites (glutamine, asparagine) serve as signals that affect the expression of genes encoding enzymes at these nodes (Taiz et al., 2023). Thus, the plant can adapt the activity of integration points to current needs.

For example, at high C/N (plenty of carbohydrates, little nitrogen), genes responsible for mobilising carbon to roots and enhancing their growth (in search of nitrogen) are activated (Третьяков, 2000). At low C/N (plenty of nitrogen, little carbon), the plant instead stimulates photosynthesis to obtain more carbon skeletons.

3.6. A Practical Example: How Integration Works Under Nitrogen Deficiency

Let us consider a concrete example. Imagine a plant suffering from nitrogen starvation. Nitrate in the soil is low. Roots cannot supply sufficient ammonium for GS/GOGAT. Synthesis of glutamine and glutamate slows. This leads to:

• less amino acid synthesis, fewer proteins;
• slower renewal of the photosynthetic apparatus (Rubisco, electron‑transport proteins);
• photosynthesis gradually declines (Hopkins & Hüner, 2009);
• carbohydrates that are still synthesised accumulate in leaves;
• the high C/N triggers signalling pathways: the plant directs carbohydrates to roots, stimulating their growth in search of nitrogen (Marschner, 2012; Третьяков, 2000).

In this situation, the integration point (GS/GOGAT) acts as a sensor of nitrogen deficiency. It “reports” to the plant that nitrogen is lacking and initiates adaptive changes.

3.7. Summary: Integration Points – the Key to C/N Regulation

Thus, the flows of carbon and nitrogen meet at several key points of metabolism:

1. Inside the cell – mainly in reactions that synthesise glutamine, glutamate, aspartate, and other amino acids from carbon skeletons (pyruvate, 2‑oxoglutarate, oxaloacetate) and ammonium (or amino groups).

2. Between organs – in growing tissues where carbohydrates from leaves and nitrogenous metabolites from roots converge.

These integration points are regulatory centres. Their activity depends on substrate availability and on signals reflecting C/N balance. Through these centres, the plant regulates protein synthesis, growth, and resource allocation.

Significance for the agronomist: Understanding integration points explains why applying nitrogen fertilisers without ensuring sufficient light or carbon dioxide does not lead to proportional growth. And conversely – why under intense lighting the plant needs more nitrogen so that the photosynthetic apparatus can operate at full capacity. Integration is the key to optimising plant nutrition.

4.1. Appearance as a Projection of C/N Balance

Let us think of a plant as a complex system that must constantly allocate limited resources among different “expenditure items”. Simplifying, all resources can be reduced to two categories: carbon (energy and building material) and nitrogen (enzymes, proteins, nucleic acids). And on which resource is more abundant depends on what the plant will invest its efforts in.

This is like a family budget. If you have plenty of money (carbon) but few workers (nitrogen), you will invest money in tools and equipment (roots) rather than hiring new workers. If you have plenty of workers but little money, you will use all workers for simple, quick tasks (shoot growth), but you cannot afford complex long‑term projects.

It is this ratio that determines morphogenesis – the shaping of the plant’s external structure.

4.2. High C/N: The “Conservative” Strategy

When the plant has plenty of carbon (carbohydrates) and little nitrogen, C/N is high. This situation often arises with good illumination, adequate moisture, but nitrogen fertiliser deficiency, or during natural ageing when photosynthesis is still active but nitrogen demand decreases.

What do we see externally?

1. Slowed shoot growth. The plant cannot synthesise enough proteins to build new cells. Therefore, stem elongation, new leaf emergence, and branching slow. The plant becomes compact, dwarfed (Marschner, 2012; Третьяков, 2000).

2. Dark green leaf colour (sometimes with a bluish tinge). With excess carbohydrates and nitrogen deficiency, chlorophyll synthesis may continue, but leaf blade expansion is inhibited. As a result, chlorophyll is concentrated in a smaller tissue volume, and the leaf becomes darker. Also, carbohydrate accumulation may lead to increased leaf thickness (higher specific leaf weight) (Taiz et al., 2023; Lambers & Oliveira, 2019).

3. Appearance of anthocyanin pigmentation (reddish, purple hues). This is one of the most striking signs of high C/N. Anthocyanins are pigments synthesised from carbohydrates and serve as protection against excess light and oxidative stress. With excess carbohydrates and insufficient nitrogen, the plant cannot use all the sugar for growth, and the surplus is diverted to anthocyanin synthesis (Hopkins & Hüner, 2009; Третьяков, 2000). This often appears on young leaves and on leaves experiencing light stress.

4. Enhanced root growth. At high C/N, the plant directs carbohydrates downwards – to the root system. Roots begin to branch vigorously, elongate, and penetrate deeper soil layers in search of nitrogen and water. The root‑to‑shoot ratio increases (Marschner, 2012; Третьяков, 2000). This is a survival strategy under nitrogen deficiency.

5. Early flowering and maturation (in some species). Under stress (including nitrogen starvation), the plant may accelerate reproductive development to leave offspring while resources are still available. This manifests as earlier transition to flowering and seed maturation (Boote et al., 1994; Sadras & Calderini, 2015).

6. Accumulation of reserve substances (starch, sugars) in stems, roots, tubers. The plant “stores” excess carbohydrates for future use when conditions improve (Третьяков, 2000).

Physiological meaning: High C/N is a signal of “economy mode”. The plant does not build new tissues but invests in root system development (searching for nitrogen), storage, and stress protection (anthocyanins). This is a survival strategy under unfavourable conditions.

4.3. Low C/N: The “Expansive” Strategy

The opposite situation occurs when nitrogen is abundant but carbon (carbohydrates) is lacking. This often happens with generous nitrogen fertilisation but insufficient light, or in overly dense stands where plants shade each other.

What do we see externally?

1. Vigorous shoot growth. The plant actively synthesises proteins, and cells divide and elongate. Stems become long, internodes elongated, leaves large but thin. The plant looks robust but “loose” (Marschner, 2012).

2. Light green leaf colour (sometimes with yellowishness). With excess nitrogen and carbohydrate shortage, chlorophyll synthesis may be impaired, or chlorophyll is diluted in rapidly growing tissues. Leaves become pale green. In addition, low C/N often leads to nitrate accumulation in vacuoles, which can also affect colour (Taiz et al., 2023).

3. Absence of anthocyanins. The plant spends all carbohydrates on growth, leaving none for anthocyanin synthesis. Therefore, at low C/N we rarely see reddish hues (Hopkins & Hüner, 2009).

4. Poor root development. Carbohydrates are primarily directed to shoots, and roots receive fewer resources. The root system becomes shallow, poorly branched. This makes the plant sensitive to drought and deficiencies of other nutrients (Третьяков, 2000).

5. Thin, weakened tissues. Due to carbohydrate shortage for cellulose, lignin, and other structural polysaccharides, cell walls become thinner, and mechanical tissues are poorly developed. Stems easily lodge (especially in cereals) (Boote et al., 1994; Marschner, 2012).

6. Increased susceptibility to diseases and pests. Thin tissues, low content of protective substances (phenolics, anthocyanins), and weakened immunity make the plant more vulnerable to pathogens (Marschner, 2012; Третьяков, 2000).

7. Delayed maturation. With excess nitrogen, vegetative growth may continue too long, delaying the transition to flowering and maturation (Boote et al., 1994; Sadras & Calderini, 2015).

Physiological meaning: Low C/N is a signal of “all hands on deck for growth”. The plant invests all resources in rapid space capture to outcompete neighbours and obtain more light. This strategy is effective under resource abundance but makes the plant vulnerable under any stress.

4.4. Transitional States and Adaptive Plasticity

It is important to understand that C/N is not a static characteristic but a dynamic indicator. It changes during the day, depending on developmental stage and under the influence of environmental conditions. Therefore, the plant’s appearance also constantly changes, reflecting its current physiological state.

• In the morning, after the night when carbohydrates were consumed by respiration, C/N may be relatively low. Leaves have a lighter shade.
• During the day, as photosynthesis proceeds, carbohydrates accumulate, C/N rises, and leaves may darken, and anthocyanin colouration may appear (under stress).
• In autumn, during ripening, photosynthesis weakens, and the outflow of carbohydrates to storage organs (seeds, tubers) leads to a decrease in C/N in leaves and triggers senescence processes (Третьяков, 2000; Taiz et al., 2023).

Moreover, the plant itself adapts to the current C/N by changing its morphology. This is manifested in:

• Changes in leaf angle. At high C/N, leaves may become more vertical to improve illumination and enhance photosynthesis.
• Thickening of leaves and accumulation of waxy deposits. These are protective responses at high C/N.
• Accelerated senescence of lower leaves under low C/N, when nitrogen is mobilised from old tissues to young ones.

All these changes are the result of regulatory systems that “evaluate” C/N and trigger corresponding developmental programmes.

4.5. Significance for the Agronomist: Diagnosis and Management

The ability to “read” the plant’s appearance is a key skill for the agronomist. Here are a few practical applications:

1. Diagnosis of nitrogen deficiency. Characteristic signs: pale green colour (under severe deficiency – yellowing) of old leaves (nitrogen is mobilised to young ones), slowed growth, sometimes reddish stem hues (anthocyanins). This is a signal for nitrogen top‑dressing.

2. Diagnosis of nitrogen excess. Dark green, “fat” foliage, elongated stems, weak roots, lodging. This signals the need to reduce nitrogen rates or improve light conditions (thinning).

3. Assessment of fertiliser efficiency. If after nitrogen application plants acquire a characteristic “overgrown” look but do not increase productivity, it likely indicates a lack of other factors (light, water, phosphorus, potassium), and C/N remains low due to carbon deficiency.

4. Forecasting stress tolerance. Plants with high C/N (compact, with developed roots) are usually more drought‑ and frost‑tolerant. Plants with low C/N (vigorously growing, with thin tissues) are more sensitive to adverse conditions (Третьяков, 2000).

4.6. Summary: Appearance Is the Mirror of Physiology

Thus, we see that nutrition changes the plant’s appearance because the C/N balance directly determines the direction of metabolic flows:

• High C/N → carbohydrate accumulation → enhanced root formation, storage, synthesis of protective pigments (anthocyanins), slowed shoot growth.
• Low C/N → active use of carbohydrates for protein synthesis → vigorous shoot growth, thin tissues, weakened roots, absence of protective pigments.

Therefore, plant morphology is a direct and clear reflection of its C/N balance. By learning to recognise these signs, the agronomist gains the ability to rapidly assess the physiological state of plants and timely adjust agronomic practices to steer development in the desired direction and achieve maximum productivity.

5.1. Sugar Signals: How the Plant “Measures” Its Carbon Status

The plant cannot simply “count” carbohydrate molecules. It evaluates their concentration through a complex system of sensors and signalling cascades. Key players here are the breakdown products of sucrose – glucose and fructose (Taiz et al., 2023). These molecules, accumulating in the cytosol at high C/N, trigger signalling pathways that change the expression of hundreds of genes.

One of the central sensors is hexokinase (HXK). This enzyme not only catalyses the phosphorylation of glucose (the first step of its metabolic utilisation) but also performs a signalling function. In the nucleus, hexokinase is part of a protein complex that regulates the transcription of genes responsible for photosynthesis, growth, and senescence (Taiz et al., 2023). High glucose concentration, via hexokinase, represses the expression of photosynthetic genes (to avoid excessive carbohydrate accumulation) and stimulates the synthesis of enzymes that mobilise reserves.

Another key regulator is SnRK1 (akin to AMP‑activated protein kinase in animals). This protein kinase is activated at low energy concentration and high AMP/ATP ratio, i.e., under carbohydrate deficiency (low C/N) (Taiz et al., 2023). SnRK1 phosphorylates numerous enzymes and transcription factors, switching metabolism from anabolism (synthesis) to catabolism (degradation). It activates genes that mobilise reserves and represses genes responsible for growth and protein synthesis. Thus, SnRK1 serves as the main “switch” under carbon shortage.

Interestingly, trehalose‑6‑phosphate – a metabolite intermediate in trehalose synthesis – acts as an indicator of high sucrose concentration and inhibits SnRK1, thereby allowing growth to continue when carbohydrate supply is sufficient (Taiz et al., 2023). This is one example of how metabolites directly regulate signalling systems, making them inseparable from metabolism.

5.2. Nitrogen Signals: Nitrate, Ammonium, and Amino Acids as Regulators

Similarly, the plant “measures” its nitrogen status through the concentrations of nitrate, ammonium, glutamine, and other amino acids. Nitrate itself is a powerful signal that induces the expression of nitrate reductase and other nitrogen metabolism genes (Morot‑Gaudry et al., 2012; Marschner, 2012). When nitrate is abundant, the plant activates nitrogen assimilation programmes.

But probably a more important signal is the concentration of glutamine and glutamate – the end products of primary nitrogen incorporation. High glutamine content in tissues, especially in roots, signals adequate nitrogen nutrition. This signal suppresses further nitrate uptake via feedback mechanisms (Marschner, 2012). When glutamine is abundant, roots reduce the activity of nitrate transporters. This prevents nitrogen overload when its assimilation cannot be balanced by carbon skeleton availability.

Thus, the plant continuously “scans” the ratio of carbohydrates to amino acids. This ratio is precisely the C/N that, transformed into hormonal signals, determines the plant’s architecture.

5.3. Hormones – Integrators of C/N Signals

Now we come to the main point: how do metabolic signals translate into changes in growth and development? Here phytohormones play a key role – chemical messengers that transmit information within the plant and between its organs.

Cytokinins – hormones mediating between root and shoot. Cytokinins are synthesised in roots (mainly in root meristems) and transported via xylem to shoots (Marschner, 2012; Taiz et al., 2023). Their synthesis and transport depend on nitrogen supply to the roots. When nitrate concentration in the soil is high, cytokinin synthesis in roots increases. Upon reaching the shoot, cytokinins stimulate cell division, lateral shoot growth, and delay leaf senescence. They essentially “inform” the shoot: “There is enough nitrogen in the roots; you can grow, allocate resources to protein and chlorophyll synthesis.”

Conversely, under nitrogen deficiency (high C/N), cytokinin synthesis in roots decreases, and their delivery to the shoot diminishes. This leads to slowed shoot growth but stimulates root growth (through carbohydrate redistribution) and accelerates senescence of old leaves (from which nitrogen is mobilised) (Marschner, 2012; Третьяков, 2000). Cytokinins, therefore, are direct “translators” of the roots’ nitrogen status into shoot growth programmes.

Abscisic acid (ABA) – stress hormone and “conservative” strategy. ABA is synthesised in leaves in response to water deficit and other stresses (salinity, high temperatures), but its level also rises under high C/N, especially under nitrogen deficiency (Marschner, 2012; Третьяков, 2000). ABA causes stomatal closure (reducing water loss but also limiting photosynthesis), inhibits shoot growth, and stimulates root growth. It also promotes storage substance accumulation and transition to dormancy. At high C/N, when carbohydrates are abundant but nitrogen is scarce, ABA helps the plant “conserve” – reducing metabolism, strengthening roots, and preparing for adverse conditions. This is a survival strategy.

Ethylene – hormone of senescence and adaptation. Ethylene is involved in the regulation of leaf and fruit senescence, as well as in responses to stresses (flooding, injury). At high C/N, when growth is slowed, ethylene production may increase, accelerating the abscission of old leaves and mobilisation of nitrogen from them to young tissues or storage organs (Sadras & Calderini, 2015). Ethylene also interacts with other hormones, e.g., it suppresses the action of cytokinins and auxins. Under low C/N, when the plant is actively growing, ethylene production may be low.

Gibberellins and auxins – growth activators. Gibberellins stimulate stem elongation and cell division; auxins promote apical dominance and root formation. Their synthesis and activity are strongly dependent on the C/N balance. At low C/N (nitrogen excess, much carbohydrate consumed for growth), gibberellin and auxin levels are usually high, leading to vigorous shoot growth (Третьяков, 2000). At high C/N (nitrogen deficiency), their synthesis may decrease, favouring compact habit and enhanced root formation (on which, as we saw, cytokinins and ABA also act).

5.4. A Complex Network of Interactions

It is important to emphasise that hormonal regulation is not a linear chain but a complex network with many cross‑connections. The same hormone can act differently in different tissues and at different developmental stages. Moreover, hormones interact with each other – synergistically (enhancing) or antagonistically (suppressing). For example, cytokinins and ABA often act as antagonists: cytokinins stimulate growth, ABA inhibits. Auxins and cytokinins together control the root‑to‑shoot ratio and tissue differentiation (Marschner, 2012).

Furthermore, the hormones themselves can influence carbohydrate and nitrogen metabolism. For instance, cytokinins activate enzymes involved in chlorophyll synthesis and nitrogen assimilation. ABA, conversely, can inhibit nitrate reductase and stimulate protein degradation. Thus, hormones not only transmit information about C/N status but also actively participate in shaping it, closing feedback loops.

5.5. How Does This Work in Real Conditions?

Let us consider two classical examples to see the entire system in action.

Example 1: Nitrogen starvation. Soil is poor in nitrates. Roots absorb little nitrogen. Cytokinin synthesis in roots decreases, and their delivery to the shoot falls. The concentration of glutamine and glutamate in the plant decreases. Carbohydrates still produced in leaves accumulate, causing high C/N. This activates the hexokinase signalling pathway and raises ABA levels. As a result: shoot growth slows, leaves acquire a dark green hue (chlorophyll concentrated in smaller volume), anthocyanin colouration appears (excess sugars), roots begin to grow actively (carbohydrates directed downwards), old leaves yellow and die (mobilisation of nitrogen). The plant switches to “survival mode”, saving resources and searching for nitrogen.

Example 2: Nitrogen excess. Soil is rich in nitrates. Roots actively absorb nitrogen. Cytokinin synthesis increases, and they flood into the shoot. The concentration of glutamine and glutamate is high. Carbohydrates are insufficient for synthesising all amino acids and proteins, so C/N is low. Sugar signals are weak (hexokinase not activated). Gibberellins and auxins promote growth. As a result: vigorous stem and leaf growth, bright green colour (much chlorophyll but thin tissues), roots poorly developed, plant prone to lodging, disease resistance lowered. The plant invests all resources in space capture but becomes vulnerable.

5.6. Summary: Unity of Metabolism, Signalling, and Growth

Thus, we see that maintaining C/N balance is not the work of a single mechanism. It is the result of the coordinated activity of an entire integrative regulatory system, which includes:

1. Metabolic signals – concentrations of sugars, amino acids, nitrate, which directly reflect C/N status.

2. Hormonal signals – cytokinins, ABA, ethylene, gibberellins, auxins, which arise as a consequence of the metabolic state and transmit information between organs.

3. Signalling cascades – the hexokinase pathway, SnRK1, which translate metabolic signals into changes in gene expression.

4. Morphogenetic programmes – changes in root and shoot growth, leaf senescence, which are the ultimate output of this regulatory network.

It is crucial to realise that hormones are not “commanders” but rather “translators” and “integrators”. They provide the linkage between metabolism and development. It is this integration that allows the plant to adapt to continuously changing environmental conditions, using available resources efficiently and directing them toward growth, reproduction, or survival depending on the situation.

For the agronomist, this means that by managing plant nutrition (applying fertilisers), we influence not merely the chemical composition of tissues but this entire complex regulatory network. We can deliberately “shift” the C/N balance to stimulate either vegetative growth (low C/N) or root formation and storage (high C/N). Understanding these mechanisms allows not only to diagnose plant condition but also to predict their behaviour and manage it.