Hormonal coordination of ontogenesis and the production process

1.1. Ontogenesis as a Succession of Hormonal Programs

Consider the plant's life cycle. From seed germination to senescence, it is not a monotonous process but a sequence of clearly alternating stages. Each stage is characterized by its own unique set of ongoing processes and requires a specific "hormonal status"—certain concentrations and ratios of phytohormones.

Take leaf development, for instance. A young leaf is an active sink for nutrients. It does not photosynthesize but consumes assimilates from outside. The hormonal status of a young leaf is characterized by high levels of cytokinins (stimulating cell division) and auxin, which attract nutrients (Marschner, 2012). However, as it matures, the leaf becomes a source of photoassimilates. Its hormonal status changes: the levels of gibberellins and auxins may decrease, and the signal ratio changes, switching its program from "consumption" to "production and export." This switch is not sudden—it is accompanied by changes in the activity of key enzymes, such as invertase and sucrose phosphate synthase, which are themselves regulated by hormonal signals (Egli, 1994).

An even more striking example is seed development. Egli (1994) describes the changing phases in a developing soybean seed. In the early stages, cytokinin dominates, stimulating cell division. As storage substances accumulate, gibberellins and auxins come into play, controlling growth and mass accumulation. Finally, at the maturation stage, when the seed enters dormancy, abscisic acid (ABA) plays a key role (Taiz et al., 2023). This is not merely a fluctuation in concentrations—it is a shift in the dominant hormonal signals that sequentially "switch on" and "switch off" entire gene expression programs, guiding the fate of the organ.

1.2. A Hormone Seldom Acts Alone

It's important to understand that hormones rarely act in isolation. Their effect is often the result of interactions between multiple signals. A classic example is the germination of barley seeds. Gibberellin, released by the embryo, triggers the synthesis of α-amylase in the aleurone layer, leading to the mobilization of storage reserves. But gibberellin does not act in a vacuum. If ABA is present in the seed, it blocks this process. The ratio of gibberellin to ABA, as well as the sensitivity to these hormones, determines whether the seed will germinate (Taiz et al., 2023). It is the ratio, not the absolute amount of a single hormone, that is the decisive factor.

This brings us to the crucial concept of hormonal balance. There is no single "master" hormone. There is a complex, multi-component hormonal "cocktail" that continuously changes in time and space. It is this balance, this dynamic equilibrium, that is the tool through which the plant manages its development and adapts to environmental conditions (Schopfer & Brennicke, 2016). This balance, in turn, is regulated at several levels: the rates of hormone synthesis and degradation, their transport, and even the sensitivity of target cells.

Thus, in answering the question "Why does no single hormone act continuously?", we conclude that the final physiological effect is always the result of integrating several signals in a specific place and at a specific time. This is why there is a constant succession of hormonal programs throughout the plant's life, each aimed at fulfilling a specific task of the current developmental stage.

This understanding leads us to the next crucial question: how do these changing hormonal programs manage the distribution of resources and the flow of substances—that is, what we call source-sink relations? We will dedicate the next chapter to this.

2.1. Hormones as Conductors of Flow: Attracting Ability

The key concept here is attracting ability or sink strength. This is not just the size of the organ. It is its ability to "attract" assimilates. Sink strength is determined by two components (Egli, 1994; Marschner, 2012):

1. Sink capacity—the maximum rate at which the organ can utilize assimilates. For a seed, this is the number of endosperm cells or the amount of starch synthesized; for a tuber, it's the volume of storage parenchyma.

2. Sink activity—the actual rate of assimilate utilization, which depends on metabolic activity and hormonal stimulation.

It is hormones that are the main signals that increase both sink capacity and activity.

• Auxin (IAA) is perhaps the most well-known "organizer" of the sink. Classic experiments by Nitsch (1950) showed that if seeds are removed from a developing strawberry, fruit growth stops. However, if auxin is applied to the receptacle, growth resumes, and a parthenocarpic (seedless) fruit develops (Egli, 1994). Auxin, synthesized in the seeds, makes the fruit a powerful center for assimilate attraction. A similar effect is observed in bean experiments: removing fruits reduced the accumulation of radioactive phosphorus in the pedicel, while applying IAA and kinetin restored this influx (Marschner, 2012). Auxin "switches on" the attracting ability of the organ.

• Cytokinins—their role is often antagonistic to auxin in terms of apical dominance, but they are powerful activators of sinks. In roots, they stimulate cell division, and in leaves, they attract amino acids, delaying senescence. Together with auxin, they create a hormonal "center of attraction." In tissue culture, with a specific auxin-to-cytokinin ratio, it is possible to induce root formation (high IAA/cytokinin ratio) or shoot formation (low ratio)—direct evidence that the hormonal balance determines the direction of morphogenesis and assimilate flow (Tretyakov, 2000; Schopfer & Brennicke, 2016).

• Gibberellins also enhance attracting ability. They stimulate cell elongation, increasing the size of the sink. For example, gibberellin treatment of grapes increases berry size by promoting sugar influx. However, their main role is not just "attracting" but creating "capacity" for accumulation (Egli, 1994; Marschner, 2012).

• Abscisic acid (ABA)—often associated with growth inhibition, it is vital for the accumulation of storage substances. In a maturing seed, high ABA levels switch metabolism towards the synthesis of storage proteins and lipids and promote assimilate flow from the mother plant into the seed (Taiz et al., 2023). Thus, ABA plays a key role in the final stages of sink formation.

2.2. Hormonal Regulation of the Source: Feedback

It is important to understand that source-sink relations are not a one-way street. The sink, via hormonal signals, influences the work of the source. This mechanism is called feedback regulation (Marschner, 2012).

• Stimulation of the source: When a seed starts accumulating starch intensively, it consumes a lot of sugars. This lowers the sugar concentration in the phloem and in the source leaves, which serves as a signal to increase the rate of photosynthesis. This signal is often mediated by hormones. For example, removing part of the fruits or ears in cereals leads to a sharp drop in photosynthesis in the leaves (Marschner, 2012; Tretyakov, 2000). Why? Because the absence of a strong sink (the fruits) means there is no "demand" for photoassimilates. The accumulation of sugars in the leaves triggers repression of photosynthesis genes, and the leaf "reduces its output." This phenomenon is a classic example of how sink strength determines source activity.

• Inhibition of the source: The flip side is an excess of assimilates. If there are few sinks, carbohydrates (sucrose, hexoses) accumulate in the leaves. This, in turn, inhibits photosynthesis and may accelerate leaf senescence (Marschner, 2012; Lambers, 2019). How exactly does the plant "sense" sugar accumulation and respond? Through sugar signaling, which interacts with hormonal signals (particularly the cytokinin signaling pathway). Thus, maintaining a balance between "production" and "consumption" is a continuous regulation process where hormones act as integrators of signals from both ends of the system.

2.3. A Practical Example: Why Nitrogen Delays Tuberization

The most illustrative example of hormonal regulation of source-sink relations is the effect of nitrogen nutrition on tuberization in potato (Marschner, 2012). High nitrogen levels stimulate vigorous vegetative growth (shoots). This is associated with increased synthesis of gibberellins and cytokinins in actively growing points. Gibberellins, as we recall, inhibit tuberization. A tuber is a powerful sink, but its formation requires a shift in the hormonal balance: a decrease in gibberellin levels and an increase in ABA (which stimulates tuberization) and cytokinins (which cause stolon thickening).

When we supply a lot of nitrogen, we maintain high gibberellin levels in the shoots. This makes the shoots the dominant sink. All assimilates are directed to them, and tubers, as sinks, receive fewer resources. If nitrogen is limited, gibberellin levels drop, the ABA/gibberellin ratio increases, and the plant switches to developing tubers—they become the main sink. This is why balanced nutrition is so crucial for obtaining a high tuber yield (Marschner, 2012).

2.4. Source-Sink as the Key to Productivity

So, we see that plant productivity is, essentially, the efficiency of managing carbon flows. A high yield is possible when:

1. The source (leaves) operates at maximum efficiency.

2. The sink (grain, fruit, tuber) possesses sufficient capacity and activity.

3. There is good "transport connection" between them (phloem), also regulated by hormones (particularly auxin, which stimulates the differentiation of conducting tissues).

An efficient variety is, first and foremost, a balanced variety with an optimal distribution of assimilates between vegetative and reproductive organs. This is achieved precisely through the fine-tuning of hormonal relations (Lambers, 2019; Connor et al., 2011). This is why modern productivity physiology pays increasing attention not just to photosynthesis, but specifically to source-sink relations and the hormonal control of these relations.

Thus, hormones are not just local "growth regulators." They are global conductors that, by managing source-sink connections, determine which organ will be the dominant sink at any given moment and, consequently, where resources will go and what the final productivity will be.

Now that we've seen how hormones distribute the already baked "pie" (carbon), a logical question arises: does the ingredient from which this pie is made—that is, nutrition—influence the hormonal conductor composition itself? The next chapter will address this.

3.1. Nitrogen and Cytokinins: Regulation by Root Signals

It is this nexus—"roots → cytokinins → shoot"—that is the key to answering the question about the influence of nutrition on hormonal status. The amount of nitrogen available directly determines the rate of cytokinin synthesis and export from the roots (Marschner, 2012; Lambers, 2019).

High nitrogen nutrition levels: Roots are active, absorbing nitrate intensively, and large amounts of cytokinins are synthesized in the apical meristems. This powerful flow of cytokinins reaches the shoots, where it:

• Stimulates cell division and elongation in leaves and stems.
• Enhances protein and nucleic acid synthesis.
• Delays leaf senescence (note that here cytokinins act as antagonists of ABA and ethylene).
• Increases the attracting ability of growing shoot meristems.

As a result, we observe vigorous vegetative growth, strong tillering (in cereals), and powerful development of the leaf surface. The plant becomes "shoot-centric" (Marschner, 2012). Recall the potato example from the previous chapter: it is precisely this high level of cytokinins (and gibberellins) that makes shoots the main sink, suppressing tuberization.

Low nitrogen nutrition levels (nitrogen deficiency): Root activity decreases, cytokinin synthesis drops sharply, and their export to the shoots is reduced. The consequences include:

• Inhibition of cell division and elongation in shoots.
• Accelerated leaf senescence.
• Reduced attracting ability of apical meristems.
• Shift in the hormonal balance in favor of ABA and ethylene, which, unlike cytokinins, are enhanced under nitrogen deficiency (Marschner, 2012).

The plant restructures: above-ground growth slows down, while the roots, receiving excess assimilates (which have no use in the inhibited shoot), begin to grow more intensively in relative terms. This phenomenon—an increase in the root/shoot ratio under N-deficiency—is a classic illustration of feedback regulation, with cytokinin signaling at its core. It is a survival strategy: the roots "search" for nitrogen, while the shoot reduces its "appetite."

3.2. Nitrogen and Other Hormones: Interaction of Metabolic Networks

But nitrogen affects not only cytokinins. It has other "targets" in hormonal metabolism as well:

• Gibberellins and ABA: Nitrogen deficiency is often accompanied by a decrease in the content of active gibberellins and, conversely, an increase in ABA concentration. It is precisely this shift towards ABA that contributes to accelerated leaf senescence under N-starvation. Here we observe antagonism: cytokinins and gibberellins are growth activators; ABA and ethylene are inhibitors. The balance between these hormone groups depends heavily on nitrogen availability (Tretyakov, 2000; Schopfer & Brennicke, 2016).
• Ethylene: Interestingly, under nutrient deficiency (not only nitrogen but also phosphorus, potassium), ethylene production often increases. For example, K-deficiency is accompanied by elevated ethylene levels, which can serve as a stress signal and induce senescence and organ abscission processes (Marschner, 2012). Thus, nutrient deficiency is a stress, and ABA and ethylene are classic "stress hormones."

3.3. Not Only Nitrogen: The Role of Other Macronutrients

Although nitrogen is the primary regulator of cytokinin status, other elements also play an important role.

• Phosphorus and Cytokinins: P-deficiency, like N-deficiency, reduces cytokinin synthesis in roots (Marschner, 2012). This is not surprising, as nucleotide synthesis (adenine is the base for cytokinins) and energy metabolism (ATP) directly depend on phosphorus.
• Potassium: Although its influence on cytokinin synthesis is less direct, it is critical for the proper functioning of all transport systems and enzymes involved in hormonal metabolism (e.g., for ATPase activity). K-deficiency can disrupt hormone transport and action, although its role as a "secondary messenger" is less obvious (Marschner, 2012).

3.4. Nutrition as a Signal, Not Just "Fuel"

From all this, an important conclusion follows: mineral nutrition is not only "raw material" for the synthesis of organic substances but also a powerful regulatory signal. A change in nitrate concentration in the soil (or nutrient solution) is perceived by the roots as a signal. This signal is transformed into a change in hormonal status (primarily cytokinin status) and, in this form, is transmitted to the shoot, triggering a physiological and morphological response. This is feedforward regulation, where the plant adapts its architecture and functions in advance, even before an acute shortage of resources for protein synthesis occurs.

This leads us to an astonishing conclusion: plant physiology is a unified network in which soil nutrition, through hormonal signaling, controls growth and development as effectively as light. The plant perceives not only light and gravity but also the chemical composition of its environment.

Now that we have seen how tightly nutrition is linked to hormonal balance, we can move on to a more practical question: how are these insights used to manage the plant using exogenous growth regulators? And, most importantly, we will articulate the essence of that hormonal balance we have been discussing.

4.1. Two Main Mechanisms of Growth Regulator Action

To understand why regulators work, one must distinguish between two fundamentally different approaches to managing the plant's hormonal status (Tretyakov, 2000; Schopfer & Brennicke, 2016).

Direct "substitution" or addition of a hormone. We apply a substance externally that acts as an endogenous hormone. These can be natural phytohormones (e.g., gibberellic acid—GA3) or their synthetic analogs. Synthetic analogs (e.g., 2,4-D, naphthaleneacetic acid (NAA), indolebutyric acid (IBA)) have an important advantage over natural hormones: they are more resistant to enzymatic degradation (particularly to IAA oxidase oxidation) and therefore act longer and often more effectively (Hopkins & Huner, 2009; Schopfer & Brennicke, 2016).

• Example: Treatment with IBA or NAA to stimulate root formation. In cuttings of many woody plants, endogenous auxin content is low and insufficient to trigger rhizogenesis. The application of a synthetic auxin analog compensates for this deficiency, "switching on" the program for adventitious root formation (Tretyakov, 2000).
• Example: Gibberellin treatment to increase berry size in grapes. Gibberellin stimulates the elongation of skin and pulp cells. Endogenous gibberellin synthesis in berries may be insufficient to achieve maximum size, and its addition from the outside enhances the fruit's attracting ability and tissue growth (Marschner, 2012).

Blocking biosynthesis or action of a hormone (antihormones, retardants, inhibitors). This approach is equally important. Instead of adding a stimulator, we remove a brake, or conversely, suppress an overly active process. The most prominent example is retardants (antigibberellins), such as CCC (chlormequat chloride), Alar, or phosphon-D. They inhibit key enzymes in gibberellin biosynthesis, for example, at the stage of cyclizing geranylgeranyl diphosphate to ent-kaurene (Hopkins & Huner, 2009; Schopfer & Brennicke, 2016).

• Example: Use of retardants to prevent lodging in cereals. Under high nitrogen nutrition, cereals synthesize a lot of gibberellins, stems elongate, become thin, and lodge. Treatment with CCC reduces endogenous gibberellin levels, leading to shorter internodes, thicker stems, and increased lodging resistance. Grain yield often not only does not decrease but increases, as resources are redistributed towards the ear (Marschner, 2012; Tretyakov, 2000). This is a classic example of shifting source-sink relations by suppressing the gibberellin signal.

4.2. Regulators Working through Changes in Hormonal Balance

It is important to understand that many regulators are not direct hormone analogs but act indirectly by altering the ratio of several hormones.

Ethylene-releasing compounds (e.g., Ethephon). These substances decompose in plant tissues, releasing ethylene. They are used for:

• Accelerating fruit ripening (tomatoes, bananas).
• Enhancing fruit abscission (for mechanical harvesting).
• Stimulating the formation of female flowers in cucurbits (a clear example of how ethylene shifts the hormonal balance, altering flower sex).

Cytokinins and their analogs. Their application is not just about stimulating cell division. A classic effect is delaying leaf senescence. If a detached leaf is treated with cytokinin, it does not turn yellow for a long time because cytokinin maintains protein and chlorophyll synthesis, counteracting the action of ABA and ethylene (Marschner, 2012). This allows, for example, prolonging the life of leaves in some vegetable crops.

Synthetic auxins (2,4-D) as herbicides. This is one of the most interesting examples. At low concentrations, 2,4-D acts as an auxin. At high concentrations, it causes uncontrolled, chaotic growth, loss of polarity, epinasty, and ultimately the death of dicot plants. Cereals are much less sensitive to it, allowing 2,4-D to be used as a selective herbicide. Why does this work? Because an excess of the auxin signal "disrupts" the hormonal system of dicots, triggering a program of stress and death, while cereals have different detoxification mechanisms or receptor sensitivity (Hopkins & Huner, 2009; Schopfer & Brennicke, 2016).

4.3. Why Don't Regulators Always Work? Competence and Timing

However, it would be a mistake to assume that any regulator always produces a predictable result. The effectiveness of treatment depends on several critical conditions we discussed in the first chapter:

1. Tissue competence. Gibberellin treatment will cause stem elongation in dwarf peas but will not affect normal ones. This is because dwarf mutants have blocked gibberellin synthesis, and the tissues are "waiting" for the signal. In a normal plant, endogenous gibberellin levels already saturate the receptors, and external application has no effect. This is a classic example (Hopkins & Huner, 2009; Schopfer & Brennicke, 2016).

2. Timing of treatment. To stimulate rooting in cuttings, auxin must be applied at a specific time when cambium cells are competent to divide. If treated earlier or later, there will be no effect. Similarly, retardants are applied before the stem begins to elongate actively.

3. Concentration. The dose-response curve for phytohormones is often bell-shaped. This means a very low concentration is ineffective, an optimal one triggers the desired response, and a high concentration may produce the opposite or toxic effect (Schopfer & Brennicke, 2016). For instance, high auxin concentrations can cause inhibition rather than growth.

Thus, growth regulators work not because they are "magical," but because they allow us to intervene purposefully in the hormonal network, altering the concentration or action of one of its components. The success of this intervention depends entirely on our understanding of the plant's current hormonal status and its competence.

This leads us to the concluding section of our lecture, where we will formulate the main idea that runs through the entire topic of hormonal regulation: What is hormonal balance, and why is it the most important concept in the physiology of plant development?

5.1. Hormonal Balance Is Not Just Concentration

The most common mistake in studying plant physiology is to look for a single hormone responsible for a particular process. For example, trying to state that "gibberellin causes germination" and "ABA causes dormancy." This is a simplification that leads to misunderstanding. In reality, these two hormones always act in tandem, and the result is determined by their ratio (Taiz et al., 2023; Schopfer & Brennicke, 2016).

A classic example is the regulation of seed germination. We have already mentioned that it is determined by the gibberellin-to-ABA ratio. But this ratio, in turn, is under the control of other hormones, as well as environmental factors: moisture, temperature, light. Phytochrome (a photoreceptor) can alter the sensitivity of cells to these hormones and can also influence their synthesis. Thus, hormonal balance is the intersection point of multiple regulatory pathways. It is not static but exists in a state of continuous dynamic equilibrium (Schopfer & Brennicke, 2016).

5.2. Balance as a "Function" of Tissues and Developmental Stages

The second important aspect is tissue and age specificity. The same hormonal balance can cause different effects in different tissues. Recall the classic experiment with tobacco callus culture (Schopfer & Brennicke, 2016; Tretyakov, 2000):

• High auxin/cytokinin ratio → stimulation of root formation.
• Low auxin/cytokinin ratio → stimulation of shoot formation.
• Intermediate ratio → undifferentiated callus growth.

Here, the same "balance" (specifically, the ratio of two hormones) determines completely different morphogenetic programs. However, in each case, it is interpreted by the callus cells as a signal to implement a specific genetic program—the program for root or shoot formation. This is the implementation of the principle of competence, which we discussed in the first chapter: the final response is determined not only by the signal but also by the tissue's "readiness" to receive it.

We observe the same in ontogenesis. In a young, actively growing plant, the balance is shifted towards auxins, gibberellins, and cytokinins—hormones that stimulate cell division and elongation. In a mature, fruiting plant, this balance changes: the relative role of ABA and ethylene increases, promoting the shift towards storage substance accumulation, ripening, and ultimately, senescence (Egli, 1994; Marschner, 2012). This is not just a change in concentrations—it is a change in the hormonal program, where one group of signals replaces another, and the old program is "switched off."

5.3. Hormonal Balance and Productivity: The Key to Crop Management

Understanding hormonal balance has direct, practical significance for agriculture. It underlies all techniques for managing the production process:

1. Nutrition (nitrogen, phosphorus, potassium) is not just "feed" for the plant. It is a tool for shifting the hormonal balance in the desired direction. High nitrogen levels, by stimulating cytokinin synthesis, shift the balance in favor of vegetative growth. This is why plants on nitrogen-rich soils grow so vigorously but may delay flowering and fruiting (Marschner, 2012).

2. Growth regulators (retardants, ethylene-releasing compounds) are tools for artificially shifting the hormonal balance. By applying a retardant (e.g., CCC), we suppress gibberellin synthesis. This shifts the balance towards ABA and ethylene, causing reduced stem elongation but may promote the redistribution of assimilates towards the ear or tubers (Tretyakov, 2000).

3. Managing source-sink relations is essentially managing the balance between signals determining sink strength (auxin, cytokinins, gibberellins) and signals determining senescence and ripening (ABA, ethylene). We know that removing the shoot apex (the source of auxin) can "awaken" lateral buds because the auxin/cytokinin ratio changes locally (Egli, 1994). This is a change in the hormonal balance in a specific tissue area.

5.4. The Main Conclusion: Integration, Not Isolation

So, what is the essence of hormonal balance? It is not merely the sum of concentrations of all hormones. It is:

• A dynamic pattern, changing over time (ontogenesis, circadian rhythms) and space (between tissues and organs).
• An integral indicator, reflecting the interaction of multiple internal (genetic) and external (light, temperature, nutrition) signals.
• A tool for implementing genetic programs. Hormones are not "instructions" but rather "action signals" that trigger or suppress the work of entire gene cascades. The balance of these signals determines which specific program will be implemented at a given moment in a given cell.

Understanding this principle radically changes our view of the plant. We cease to see it as a set of isolated processes managed by individual hormones. We begin to see it as a unified, integrated system where all elements—from photosynthesis to transport and storage—are connected through hormonal signaling.

It is this integral approach that is the main takeaway from today's lecture. And it is the foundation for understanding how we can, by acting on hormonal balance, manage the growth, development, and ultimately, the productivity of agricultural crops.