Growth, development and hormonal regulation

What is growth?

Growth is an irreversible quantitative increase in the size, mass, and volume of cells, tissues, and organs of a plant (Hopkins & Hüner, 2009; Lambers & Oliveira, 2019). It is the process of forming new structural elements of the organism (Kuznetsov & Dmitrieva, 2006).

Growth manifests as:

• an increase in cell number (through division);
• an increase in cell size (through expansion);
• an accumulation of dry matter.

It is important to understand: growth is a quantitative process. When a seedling pushes through the soil, it is growing. When a leaf increases in size, it is growing. When the root system penetrates new soil horizons, that is also growth.

But growth does not always involve an increase in dry mass. For example, during seed germination before emergence, the energy for growth is derived from the seed's stored reserves in the endosperm, and the total dry mass of the seed and seedling may even decrease due to respiration (Hopkins & Hüner, 2009). Nevertheless, growth is occurring — size increases, and new structures are formed.

What is development?

Development refers to qualitative changes in the structure and functions of the organism, its organs, tissues, and cells during ontogenesis (Medvedev, 2012; Tretyakov et al., 2000). It is the transition from one stage of the life cycle to another, accompanied by the emergence of new organs, changes in physiological functions, and ultimately, a change in the overall appearance of the plant.

Development is a qualitative process. It is development that transforms a seed into a seedling, a seedling into an adult plant that flowers, bears fruit, and senesces.

A classic definition of development was given by D. A. Sabinin: it is qualitative changes in living structures, determined by the organism's progression through its life cycle (cited in Tretyakov et al., 2000).

What is the difference? Key examples

The difference between growth and development is best illustrated by examples where one process occurs while the other does not.

Example 1. Winter and spring crops.

Winter wheat sown in spring grows — it increases in size, tillers, and accumulates biomass. But it does not develop — it does not transition to the reproductive phase, does not head, and does not produce grain. Why? Because for development (the transition to flowering), it requires vernalization — exposure to low positive temperatures, which it did not receive (Tretyakov et al., 2000). In autumn, it undergoes vernalization under natural conditions; in spring, it develops, flowers, and forms a yield. This classic example shows that growth and development are regulated by different mechanisms and can be temporally uncoupled (Sláfer & Kantolic, 2015).

Example 2. Etiolated plants.

Seedlings grown in darkness grow very rapidly — internodes elongate, and the plant becomes long and thin. This is intensive growth. But development is delayed: leaves do not unfold, chlorophyll is not synthesized, and chloroplasts do not form. The plant remains at a juvenile stage, not transitioning to maturity. As soon as light becomes available, photomorphogenesis processes are triggered — development "catches up" with growth, and the plant acquires a normal structure (Taiz et al., 2023; Schopfer & Brennicke, 2016).

Example 3. Determinate and indeterminate plants.

In determinate tomato varieties, stem growth ceases after the formation of a certain number of trusses, and all resources are directed to fruit development and ripening. In indeterminate varieties, growth continues indefinitely, and fruit development is spread out over time. These are genetically determined differences in the balance of growth and development (Connor et al., 2011).

What is ontogenesis?

Ontogenesis is the complete cycle of individual development of an organism from the zygote (or primordium) to natural death (Medvedev, 2012). It encompasses the entire sequence of changes a plant undergoes during its life.

Several key stages are distinguished in ontogenesis (Tretyakov et al., 2000; Medvedev, 2012):

1. Embryonic stage — from zygote to seed maturation. During this time, the basic structures of the embryo are formed.

2. Juvenile (youth) stage — from seed germination to the appearance of the first floral primordia. Vegetative mass grows, but reproductive organs are not yet initiated. The plant is not sensitive to factors that induce flowering during this period.

3. Maturity stage — from the initiation of floral primordia to flowering. The plant acquires the capacity for reproduction.

4. Reproductive (fruiting) stage — from fertilization to seed and fruit maturation.

5. Senescence stage — from the cessation of fruiting to death. Degradative processes predominate over synthesis.

Each stage is characterized by specific morphological, physiological, and biochemical features (Tretyakov et al., 2000).

Growth and development are closely interrelated: development is impossible without growth, but growth can occur without development (Tretyakov et al., 2000). Rapid growth is often accompanied by slowed development, and vice versa. This is important to understand when managing the production process in agronomy.

2.1. The Genome — The Conductor Setting the Theme

Imagine a symphony. Before a single note is played, the composer has already created the score. In it, everything is written: which instruments will enter, when, in what key, and with what dynamics.

For the plant, this "score" is the genome (Medvedev, 2012). This is the hereditary information encoded in DNA molecules. The genome determines the fundamental architecture of the plant: whether it will be tall or short, what type of branching it will have, how many leaves will form before flowering begins, and how much time is needed to transition to the reproductive phase.

The genome contains developmental regulatory genes — special genes that control entire programs of morphogenesis (Medvedev, 2012). These include, for example:

• Homeobox-containing genes (e.g., the KNOX family) — they maintain cell division in meristems, preventing premature differentiation;
• MADS-box genes — they control flower formation, determining which organ (sepal, petal, stamen, or carpel) will develop at each specific location within the floral meristem.

Mutations in these genes lead to dramatic changes. A classic example is the knotted1 mutation in maize, where the KN1 gene, which should only be active in the meristem, becomes expressed in differentiated leaf cells. As a result, "knots" — groups of dividing cells — appear along the leaf veins, disrupting the normal structure of the organ (Medvedev, 2012).

However, the genome is merely a blueprint. The implementation of this blueprint depends on how genes are "read" in each specific location and at each specific moment in time. This, in turn, is determined by the two other levels of regulation: the site of action (meristems) and the signaling systems (hormones and environmental factors).

2.2. Meristems — The Orchestra Performing the Symphony

If the genome is the score, then meristems are the orchestra. They are the ones that create the living tissue from which all plant organs are formed.

2.3. Phytohormones — The Conductor Leading the Orchestra

So, we have a score (the genome) and an orchestra (meristems). But someone must direct the orchestra, indicating when each instrument should enter, when to play louder, when to play softer.

This role is performed by phytohormones (Schopfer & Brennicke, 2016; Taiz et al., 2023).

Peculiarities of Phytohormone Action

It is important to understand several fundamental aspects of phytohormone action.

First: Hormones act through receptors.

Every cell that can respond to a specific hormone contains receptors — protein molecules that specifically bind that hormone. Hormone binding to the receptor initiates a cascade of signaling reactions that ultimately alter gene activity or the function of membrane transporters (Schopfer & Brennicke, 2016; Taiz et al., 2023).

Remarkably, in many cases, the signaling pathway is structured such that the hormone causes not activation, but degradation of repressor proteins (Taiz et al., 2023). That is, the hormone does not "switch on" a process but "switches off" its inhibition. This allows for a very rapid response: removing the repressor is sufficient for the process to start automatically.

Second: Hormones interact.

In plants, no hormone acts in isolation. Complex relationships exist between them (Tretyakov et al., 2000; Schopfer & Brennicke, 2016):

• Synergism — when two hormones enhance each other's effects. For example, auxin and cytokinin together stimulate cell division in callus.
• Antagonism — when hormones act in opposing ways. A classic example is the ratio of ABA to gibberellins during seed germination: ABA inhibits germination, while gibberellins stimulate it. The outcome depends on their ratio.
• Cascade interactions — when one hormone stimulates the synthesis of another. For example, auxin stimulates ethylene synthesis.

Third: Hormones can act at a distance.

Many hormones are transported from their site of synthesis to their site of action. Specifically:

• Auxins are transported strictly polarly — from the tip to the base (polar transport) (Taiz et al., 2023; Schopfer & Brennicke, 2016).
• Cytokinins are transported via the xylem from roots to shoots.
• Gibberellins can move in both directions via phloem and xylem.
• Ethylene, being a gas, diffuses through intercellular spaces.

This transport ensures communication between distant parts of the plant and coordinates their growth and development.

Comparison of Plants and Animals in Hormonal Regulation

It is important to understand the fundamental difference between hormonal regulation in plants and animals (Schopfer & Brennicke, 2016; Taiz et al., 2023):

2.4. The External Environment — Both Musician and Audience

The genome is the score, meristems are the orchestra, hormones are the conductor. But where does the performance take place? In which hall? With what acoustics? Who is listening?

This is a metaphor for the external environment — the third key factor determining plant growth and development (Sláfer & Kantolic, 2015; Taiz et al., 2023).

The external environment influences growth and development in two ways.

2.5. Interaction of All Three Factors

Now that we have met all the "controllers," it's important to understand how they work together.

The process looks roughly like this:

1. An external signal (e.g., a change in photoperiod) is perceived by specialized systems (particularly phytochrome for perceiving red and far-red light) (Tretyakov et al., 2000).

2. Signal perception causes a change in hormonal status: for example, under long days, gibberellin content increases in long-day plants.

3. The altered hormone ratio is perceived by meristem cells via receptors.

4. A cascade of signaling reactions is initiated, altering gene activity in the nucleus (Schopfer & Brennicke, 2016; Taiz et al., 2023).

5. The new program of gene activity leads to a change in the nature of growth and differentiation of meristem cells (e.g., the apical meristem begins to form flowers instead of leaves).

6. As a result, the entire course of the plant's ontogenesis changes (Sláfer & Kantolic, 2015).

And all of this is an interconnected, continuous process. The plant does not simply react to external conditions; it constantly integrates them, processes information, and adjusts its growth and development according to an optimal strategy for survival and reproduction.

Summary: Who Controls Growth?

So, the answer to who controls growth is a complex, multi-level system, comprising:

1. The Genome — the hereditary developmental program.

2. Meristems — the sites of growth, where cells retain the capacity for division and where new organs are initiated.

3. Phytohormones — chemical signals that coordinate the growth and development of the entire plant, regulating meristem activity and gene function.

4. The External Environment — a source of resources and information that modulates and corrects the implementation of the genetic program.

All these components do not work in isolation but as a unified system. The genome creates possibilities, meristems realize them, hormones coordinate, and the environment directs and corrects. And it is precisely this systemic understanding that is the key to learning how to manage the production process in agronomy.

Why are we structuring the study this way?

Imagine we are going to study a car. We could start with the engine, then move to the transmission, then to the wheels... Or we could start with the question: "Why does the car move?" — and gradually break down all the systems that answer that question.

In our module, we will take the second approach. We will start with the most fundamental question — how does a cell even begin to grow? — and gradually build up the picture, adding new levels of regulation: from cell to tissue, from tissue to organ, from organ to the whole plant.

Here are seven key questions we will answer.

Question 1. Why do cells begin to grow?

At first glance, the question seems naive: well, they just grow. But if we think about it, this is one of the most complex questions in developmental biology.

Imagine this: there are two neighboring cells in the same meristem. They are nearly identical. But one begins to actively divide and grow, while the other does not. Why?

To answer this, we will dissect the cellular basis of growth (Kuznetsov & Dmitrieva, 2006; Tretyakov et al., 2000):

1.1. Cell division (mitosis).

We will revisit the cell cycle, its phases (G1 → S → G2 → M), and how this cycle is regulated. Key molecules are cyclin-dependent protein kinases (CDKs). They determine whether a cell will progress from G1 to S phase (initiation of DNA synthesis) or from G2 to mitosis. CDK activation depends on the presence of cyclins, whose synthesis, in turn, is regulated by hormones — primarily cytokinins and auxins (Taiz et al., 2023). We will analyze why both hormones are needed to induce cell division in tissue culture and what happens when they are imbalanced.

1.2. Cell elongation.

This is a specifically plant mode of growth (Kuznetsov & Dmitrieva, 2006). Unlike animal cells, which grow mainly through cytoplasmic synthesis, plant cells grow hundreds or thousands of times by increasing their vacuole. The vacuole fills with water, and the cell expands. But water cannot enter just by itself — it is drawn in by an osmotic gradient: osmotically active substances (ions, sugars, organic acids) accumulate in the vacuole, and water enters following the water potential gradient (Taiz et al., 2023).

There is a subtlety: the cell wall is a rigid structure. It cannot simply stretch. For the cell to grow, the wall must become more plastic. This is where auxin plays a key role. It activates H⁺-ATPase on the plasmalemma, which pumps protons into the cell wall. The wall acidifies (pH drops to 4.5–5.0), activating expansins — proteins that loosen the bonds between cellulose microfibrils. The wall becomes pliable, and the vacuole's pressure (turgor) stretches it (Taiz et al., 2023; Schopfer & Brennicke, 2016). This is the so-called "acid growth hypothesis" of elongation.

But auxin action is not the only mechanism. We will also discuss the role of gibberellins, which stimulate the synthesis of enzymes hydrolyzing starch into osmotically active sugars, and the role of cytokinins, which increase the number of cells ready to divide.

1.3. Cell differentiation.

After the cell has grown (or even during growth), it begins to specialize. It acquires the features that determine its belonging to a specific tissue: epidermis, xylem, phloem, parenchyma. We will examine how a cell "chooses" its path and what role positional information (the cell determines its neighbors) and hormonal gradients play in this choice (Taiz et al., 2023; Medvedev, 2012).

Question 2. How does the plant determine "up" and "down"?

This question is about polarity, one of the fundamental properties of living organisms (Medvedev, 2012; Kuznetsov & Dmitrieva, 2006).

Take a willow cutting. If you place it in water, roots will appear at the lower end, and shoots at the upper end. Turn the cutting over — and roots will still appear at the end that was originally lower (Medvedev, 2012). The plant "remembers" where its morphological top and bottom are.

Why does this happen?

The answer lies in polar auxin transport (Taiz et al., 2023; Schopfer & Brennicke, 2016). Auxin (IAA) is synthesized in the shoot apical meristems and young leaves. From there, it is transported strictly in one direction — from the tip to the base. This transport is not mere diffusion, but an active, energy-consuming process mediated by specific carrier proteins (PIN proteins and ABCB proteins) (Taiz et al., 2023).

Why does this determine polarity? Because auxin is a signal. Where there is a lot of it (the tip), a shoot develops. Where there is little (the base of the cutting), auxin accumulates — and this stimulates root formation. The auxin gradient along the axis creates a spatial blueprint: where the shoot will be, where the root, where the lateral buds (Medvedev, 2012).

But polarity is not limited to the shoot-root axis. It also manifests in individual organs: the leaf has an upper and lower side, the root has zones of division, elongation, and absorption. We will examine how polarity is established at the earliest stages of development — during embryogenesis, when the first division of the zygote creates apical and basal cells, from which the shoot and root, respectively, develop (Taiz et al., 2023).

And of course, we will discuss how polarity is used in practice: why cuttings need to be planted "tip up," why root suckers form in certain places, and how polarity can be altered using hormones.

Question 3. Why do lateral branches appear?

This question is about growth correlations, specifically apical dominance (Medvedev, 2012; Tretyakov et al., 2000).

Look at a young plant. The tip of the main shoot is growing actively, while lateral buds (in the leaf axils) are inhibited. If the tip is removed (pinched off), the lateral buds awaken and begin to grow. This is apical dominance: the apical bud suppresses the growth of axillary buds.

How does this work? The classical model: auxin, synthesized in the tip, is transported downward along the stem and inhibits the awakening of lateral buds (Taiz et al., 2023). But the mechanism is actually more complex. It turns out that auxin does not act directly on the bud, but through intermediaries: it stimulates the synthesis of strigolactones in the shoot, and strigolactones then inhibit lateral bud growth (Taiz et al., 2023). Meanwhile, cytokinins, synthesized in the roots, stimulate the awakening of lateral buds. Thus, apical dominance is the result of an interaction between three hormones: auxin (inhibits), strigolactones (inhibit), and cytokinins (stimulate).

We will examine how this interaction works under different conditions, why apical dominance is expressed differently in different plant species, and how it can be used in agronomy (e.g., in shaping fruit tree canopies or in tomato suckering).

Question 4. Why does the plant elongate?

This question deals with the regulation of stem growth, which is determined by the action of gibberellins and light (Tretyakov et al., 2000; Taiz et al., 2023).

Why do plants grown in shade have long, thin internodes? This phenomenon is called etiolation (from the Greek ethos — custom, character — the plant "adapts" to darkness). In darkness, the plant tries to reach light as quickly as possible, so it elongates maximally. The mechanism: in darkness, gibberellins are active, stimulating cell elongation. As soon as light appears, photomorphogenesis is triggered: light suppresses gibberellin action, stem growth slows down, but leaves begin to unfold, and chlorophyll is synthesized (Taiz et al., 2023; Tretyakov et al., 2000).

How does the plant "sense" the presence of neighboring plants? Plants perceive the ratio of red to far-red light (R:FR). Under shade from neighbors, the proportion of far-red light (FR) increases. This signal is perceived by phytochrome (Tretyakov et al., 2000; Taiz et al., 2023). Phytochrome exists in two forms: Pr (inactive, absorbs red light) and Pfr (active, absorbs far-red light). Upon red light illumination, Pr converts to Pfr, and Pfr converts to Pr upon far-red illumination. In shade, where there is a lot of FR, Pr predominates, which releases the inhibition of the gibberellin pathway — the plant elongates. This is the so-called "shade avoidance syndrome" (Lambers & Oliveira, 2019).

We will also examine how temperature and water status influence stem growth, why lodging occurs with excess nitrogen (activates gibberellins), and how retardants (inhibitors of gibberellin synthesis) help combat lodging (Tretyakov et al., 2000).

Question 5. Why do leaves senesce?

This question addresses senescence and programmed cell death (Taiz et al., 2023; Tretyakov et al., 2000).

Each leaf has a limited lifespan. Eventually, it yellows and falls off. But senescence is not simply decay; it is an active, genetically programmed process (Taiz et al., 2023). In senescing leaves, genes are activated (SAGs — senescence-associated genes) whose products are hydrolytic enzymes (proteases, nucleases, lipases). These enzymes break down proteins, DNA, RNA, and membranes. The breakdown products (amino acids, nucleotides, ions) are reutilized — transported to young organs (growing leaves, flowers, fruits). Thus, senescence is not a loss, but a relocation of resources (Tretyakov et al., 2000).

Who controls senescence? Key hormones are cytokinins (delay senescence) and ethylene (accelerates it). Cytokinins are synthesized in roots and transported to leaves via the xylem. When a leaf senesces, cytokinin supply decreases, and degradative processes are initiated. Ethylene, conversely, stimulates senescence by activating hydrolytic enzymes (Taiz et al., 2023).

We will analyze why senescence is beneficial for the whole plant (resource redistribution), why it can be delayed by cytokinin treatment, and how this is used in agriculture (e.g., to prolong the life of sugar beet or vegetable crop leaves).

Question 6. How does flowering begin?

This is one of the most important and enigmatic questions in plant physiology. The transition from vegetative growth to reproductive development is a pivotal moment in a plant's life (Sláfer & Kantolic, 2015; Tretyakov et al., 2000).

Plants do not flower "just because." They flower when conditions are favorable for it. How do they determine the right time?

Two main mechanisms are involved:

6.1. Photoperiodism — the response to the ratio of light and dark periods (Tretyakov et al., 2000; Sláfer & Kantolic, 2015).

• Short-day plants (soybean, rice, millet) flower when the day becomes shorter than a critical length (e.g., < 12 hours).
• Long-day plants (wheat, barley, spinach) flower when the day becomes longer than a critical length.
• Day-neutral plants (tomato, cucumber) flower regardless of photoperiod.

The photoperiod is perceived not by the meristem, but by the leaf (Tretyakov et al., 2000). In the leaf, under the appropriate photoperiod, florigen is synthesized — a hormonal signal transported via the phloem to the apical meristem, where it triggers flower formation. For a long time, the nature of florigen was a mystery. It is now known to be a protein encoded by the FT (Flowering Locus T) gene in Arabidopsis and its homologs in other plants (Taiz et al., 2023). Florigen is not the only signal, but it is one of the key ones.

6.2. Vernalization — the requirement for prolonged exposure to low positive temperatures (Tretyakov et al., 2000; Sláfer & Kantolic, 2015).

Winter plants (wheat, rye, biennial crops) will not flower without vernalization. They must overwinter to transition to flowering. Vernalization is perceived by shoot meristems, and it is not transmitted throughout the plant (each meristem must undergo vernalization independently) (Sláfer & Kantolic, 2015). The molecular mechanism of vernalization involves the epigenetic regulation of the flowering repressor gene FLC (Flowering Locus C) in Arabidopsis and its analogs in other plants (Taiz et al., 2023; Medvedev, 2012). Low temperatures cause chromatin methylation in the region of this gene, it is "silenced" — and flowering becomes possible.

We will also discuss how photoperiodism and vernalization interact, why these requirements differ among plant varieties, and how this is used in breeding and agricultural practices.

Question 7. How do hormones work together?

The final question is the most integrative. We have already mentioned hormone interactions in each of the previous topics. Now we will assemble everything into a unified system (Schopfer & Brennicke, 2016; Taiz et al., 2023).

We will examine:

• Synergism and antagonism of hormones with specific examples (e.g., the ABA:gibberellin ratio during seed germination, the auxin:cytokinin ratio in tissue culture) (Tretyakov et al., 2000).
• Hormonal cascades — where one hormone regulates the synthesis of another (auxin → ethylene, gibberellins → auxin).
• Hormonal gradients — spatial distribution of hormones, determining polarity and differentiation.
• Hormonal "clocks" — circadian rhythms in hormonal regulation (e.g., daily fluctuations in auxin and gibberellin content).
• Reception and signal transduction — how a hormone, upon entering a cell, alters gene function (Taiz et al., 2023; Schopfer & Brennicke, 2016).

We will pay special attention to the fact that in many cases, hormones act through repressor degradation, rather than promoter activation. This allows for very rapid and fine-tuned regulation (Taiz et al., 2023).

Summary: The Logic of Study

So, seven questions we will address:

1. Cellular level: why does a cell begin to grow (division, elongation, differentiation).

2. Organismal level 1: how does the plant determine up and down (polarity, polar auxin transport).

3. Organismal level 2: why do lateral branches appear (apical dominance, role of auxin, strigolactones, and cytokinins).

4. Organismal level 3: why does the plant elongate (role of gibberellins, etiolation, phytochrome).

5. Temporal level 1: why do leaves senesce (programmed death, reutilization, role of cytokinins and ethylene).

6. Temporal level 2: how does flowering begin (photoperiodism, vernalization, florigen).

7. Integrative level: how do hormones work together (interactions, cascades, signaling pathways).

See the logic? We move from simple to complex, from cell to organism, from space to time, from individual hormones to their interactions. This allows us to build a coherent body of knowledge, where each new question builds upon the previous ones.

And, of course, at each stage, we will ask: "How can this be applied in agronomy?" Because, as we said at the end of the first lecture, understanding these mechanisms is the key to managing the production process.

4.1. Why do plants lodge and how to combat it?

Lodging in cereal crops is one of the most serious problems in grain production. When stems are too long and thin, they cannot support the weight of the head, especially after rain or strong wind. Plants lodge, the head falls to the ground, light and air access are disrupted, and grain germinates or rots. Yield losses can reach 30–50% (Tretyakov et al., 2000).

Why do stems elongate? Gibberellins are primarily responsible for stem elongation. They stimulate cell elongation in the internodes. The higher the gibberellin level, the longer the stem. The gibberellin level depends on the variety (genetically determined) and on conditions: excess nitrogen fertilizers stimulate gibberellin synthesis and enhance growth (Tretyakov et al., 2000).

How to combat lodging? The most effective way is to use varieties with short, strong stems (dwarf varieties). This was a key element of the "Green Revolution" (we will return to this). However, if the variety is medium-tall, retardants — substances that inhibit gibberellin synthesis — can be used (Tretyakov et al., 2000).

For example, CCC (chlormequat chloride) is a classic retardant used on winter wheat, barley, and rye. It blocks one of the early steps in gibberellin synthesis (the cyclization of copalyl diphosphate to ent-kaurene). As a result, the stem becomes 15–25% shorter, but also thicker, and the stem walls become stronger. CCC also increases productive tillering and drought tolerance (Tretyakov et al., 2000).

Another example is triazole-based retardants (paclobutrazol, uniconazole) . They are used in fruit growing and ornamental horticulture to restrict shoot growth.

What should the agronomist know? To use retardants effectively, one needs to understand:

1. When to apply? Usually at the stem elongation stage, when the stem is actively growing. Earlier — the effect will be weaker; later — the stem has already grown.

2. At what dose? Excess retardant can overly inhibit growth, reducing yield.

3. With which varieties does it work? Not all varieties respond equally to retardants.

This knowledge is based on understanding the biochemistry and physiology of gibberellins (Tretyakov et al., 2000; Taiz et al., 2023).

4.2. Why is suckering (pruning) done?

Suckering tomatoes — removing lateral shoots (suckers) from the leaf axils — is one of the classic agronomic practices based on the physiology of apical dominance (Tretyakov et al., 2000).

What is apical dominance? The apical bud of the main shoot synthesizes auxin, which is transported downwards and inhibits the awakening and growth of axillary buds. If the tip is removed (pinched), the auxin flow ceases, and lateral buds begin to grow.

How is this used in suckering? We do not remove the tomato tip (it is needed for further growth). However, we remove the suckers — young lateral shoots that are just beginning to grow. Why does this increase yield? Because each sucker is a competitor for assimilates. If we leave suckers, the plant spends some of its photoassimilates on their growth. If we remove them, all resources are directed to the fruits on the main shoot (Tretyakov et al., 2000).

Furthermore, tomatoes have determinate and indeterminate varieties. In determinate varieties, the main shoot growth is limited to a few trusses, and suckering is done differently (often only partial). In indeterminate varieties, growth is unlimited, and they are formed into a single stem, removing all suckers (Tretyakov et al., 2000).

What should the agronomist know? To sucker properly, one needs to understand:

1. When to remove suckers? Best when they are still small (3–5 cm). At this time, they consume almost no assimilates, and the wound heals quickly.

2. What is the plant type (determinate or indeterminate)? This determines the plant formation scheme.

3. What is the variety? Different varieties have different branching tendencies.

This is a direct practical application of knowledge about growth correlations and the role of auxin and cytokinins.

4.3. Why are growth regulators used?

Growth regulators are chemical substances that influence plant growth and development. Among them are natural phytohormones, their synthetic analogs, and substances that affect hormone synthesis or action (Tretyakov et al., 2000; Taiz et al., 2023).

Here are just a few examples.

Rooting cuttings.

Auxin (and its synthetic analogs — IBA, NAA) stimulates the formation of adventitious roots on cuttings (Tretyakov et al., 2000). How does this work? Auxin activates the division of pericycle cells — the tissue that gives rise to lateral roots. Therefore, cuttings treated with auxin root faster and in greater numbers. This is widely used in nursery and fruit production for rooting difficult-to-root species.

Preventing fruit drop.

Auxins are used to prevent pre-harvest fruit drop in apples and pears (Tretyakov et al., 2000). Mechanism: auxin is synthesized in the fruit, transported to the pedicel, and prevents the formation of the abscission layer. When the fruit ripens, auxin synthesis in it declines, and the formation of the abscission layer is initiated (under the influence of ethylene). Auxin treatment shortly before harvest mimics the presence of endogenous hormone and delays drop.

Fruit ripening.

Ethylene is the classic fruit ripening hormone (especially for climacteric fruits — apples, bananas, tomatoes, avocados) (Taiz et al., 2023; Tretyakov et al., 2000). Ethylene initiates a cascade of biochemical reactions: starch hydrolysis to sugars, chlorophyll degradation, pigment synthesis (carotenoids, anthocyanins), and tissue softening (pectinesterase activation). Therefore, unripe tomatoes, bananas, or apples are placed in special chambers where ethylene is supplied, and after a few days, they ripen to a marketable state. This allows fruits to be harvested at technical maturity, transported over long distances without loss, and ripened on-site.

Stimulation of parthenocarpy (seedless fruit formation).

Gibberellins and auxins can stimulate fruit formation without fertilization — parthenocarpy (Tretyakov et al., 2000). This is widely used in seedless grape varieties (e.g., sultanas): gibberellin treatment increases berry size and makes clusters more marketable. In tomatoes, auxins are used to stimulate fruit set in greenhouses when pollinating insects are absent.

Stimulation of seed and tuber germination.

Gibberellins break seed and tuber dormancy (Tretyakov et al., 2000). For example, to obtain two potato crops per year, early potato tubers are treated with gibberellin to promote rapid sprouting after harvest. In lettuce, tobacco, and other light-sensitive seeds, gibberellins can substitute for light stimulation.

What should the agronomist know? To use growth regulators effectively, one needs to understand:

1. Which substance to use for a specific task (auxin, gibberellin, cytokinin, ethylene-releasing compound).

2. At what concentration? Too low a concentration will be ineffective; too high may be toxic.

3. At what developmental stage? For example, auxin treatment to prevent drop is effective a few weeks before harvest, while ethylene treatment for ripening is done post-harvest.

4. On which variety? Varieties may differ in their sensitivity to hormones.

4.4. Why do dwarf varieties produce higher yields?

The Green Revolution of the 1960s is one of the most striking examples of applying physiological knowledge in breeding. Its key element was the development of dwarf varieties of wheat and rice with short, strong stems.

What's the essence? Traditional wheat varieties have long, thin stems. When high doses of nitrogen fertilizer are applied (to increase yield), the plant grows vigorously, the stem elongates further, becomes weak, and the crop lodges. Yield is lost. This was a "bottleneck" for intensification.

How was the problem solved? Breeders developed varieties with shortened stems. These have shorter (by 20–40%), thicker, and stronger stems. These varieties do not lodge even under high nitrogen rates. Furthermore, they have a higher harvest index — the ratio of grain mass to total biomass. While old varieties had a harvest index of 0.35–0.40 (i.e., 35–40% of biomass went to grain), new ones had 0.45–0.50, and modern record holders reach up to 0.55–0.60 (Tretyakov et al., 2000; Connor et al., 2011).

How is this related to hormone physiology? Dwarfism in many wheat varieties is caused by mutations in the Rht (Reduced height) genes (Schopfer & Brennicke, 2016). These genes encode repressor proteins of the gibberellin signaling pathway (DELLA proteins). Mutations make these proteins insensitive to gibberellins — they are not degraded in response to the hormonal signal, and gibberellin cannot stimulate stem cell elongation (Taiz et al., 2023). As a result, the stem remains short, but all other processes (including grain protein synthesis) proceed normally.

This is a brilliant example of how knowledge of the molecular mechanisms of hormonal regulation enabled targeted breeding and dramatically increased yields (in India and Mexico, wheat yields rose 3–4 times in a single decade).

What should the agronomist know? In modern cultivar assortments, short-stemmed varieties occupy the main position. However, they also have drawbacks: for example, they often require more careful control of water status and mineral nutrition, as their root systems are less vigorous. Moreover, dwarf varieties may suffer more from drought and heat stress. Therefore, variety selection is always a compromise based on an understanding of physiology.

4.5. Why do plants branch differently?

Branching is a crucial trait determining plant architecture. It affects:

• Light capture (a branched plant has a larger leaf area).
• Water and mineral uptake (the root system can also branch).
• Number of flowers and fruits (in many crops, fruits are produced on lateral shoots).
• Harvestability (e.g., excessive branching in cotton complicates mechanical harvesting).

How is branching regulated? As we have discussed, branching is determined by apical dominance, which itself depends on the balance of auxin, cytokinins, and strigolactones (Taiz et al., 2023). This balance is genetically determined in different species and varieties. Consequently:

• In some plants, we desire strong branching (e.g., cotton, to have more bolls).
• In others, we want to minimize branching to channel all assimilates into the main shoot (e.g., corn, where we don't want extra suckers).

An agronomist can influence branching in several ways (Tretyakov et al., 2000):

1. Variety selection — the simplest method. There are strongly branching and weakly branching varieties.

2. Planting density — at high density, apical dominance is enhanced (due to competition for light), and branching is suppressed. In sparse stands, the opposite occurs.

3. Pinching (topping) — removes apical dominance and stimulates branching. Used in ornamental crops, vegetables (e.g., for shaping pepper plants), and some field crops.

4. Use of growth regulators — for example, cytokinins stimulate the awakening of lateral buds, while auxins inhibit them.

4.6. Why do flowering times depend on conditions?

This question is directly linked to photoperiodism and vernalization (Tretyakov et al., 2000; Sláfer & Kantolic, 2015).

For the agronomist, knowing when a plant will flower is crucial. Flowering timing affects:

• Maturity dates — earlier flowering generally means earlier harvest.
• Stress tolerance — flowering is the most vulnerable phase. If it coincides with drought or frost, yield will be lost.
• Synchrony of flowering — for cross-pollinated crops, it is important that all plants flower at roughly the same time (for better pollination).

How can an agronomist manage flowering time?

1. Variety selection — varieties differ in photoperiodic and vernalization sensitivity.

2. Sowing dates — flowering can be shifted by changing the sowing date (especially for long-day and short-day crops).

3. Photoperiod manipulation — in greenhouses, supplemental lighting or shading can be used to accelerate or delay flowering. For example, chrysanthemums (a short-day plant) flower when day length is reduced; in greenhouses, this is used to produce flowers by a specific date.

4. Vernalization — in winter crops (wheat, rye), seeds can be artificially vernalized (held at low temperature) and sown in spring. This allows for accelerated breeding (producing two generations per year) and the use of winter varieties in regions with harsh winters where they would not survive (Tretyakov et al., 2000).

A practical example: In southern regions, winter wheat is sown at the optimal time to undergo vernalization in autumn, but not too early to avoid overgrowth. If sown too early, plants may overgrow and die from frost in winter. If sown too late, they may not complete vernalization and will not head. Knowledge of developmental physiology allows the selection of the optimal sowing date.

4.7. How to manage plant senescence?

Senescence seems like the end of a plant's life. But for the agronomist, it is also a tool.

Scenario 1: Seed harvest.

For seed production, it is important that seeds mature uniformly and do not shatter. To accelerate and synchronize maturation, desiccation is used — treatment of plants with substances that accelerate drying and senescence (Tretyakov et al., 2000). For example, treating wheat crops with magnesium chlorate a few days before harvest accelerates drying, allowing harvesting by combine without losses.

Scenario 2: Forage harvest (green mass).

For forage crops, conversely, delaying senescence and prolonging the active growth period is desirable. Cytokinins delay leaf senescence. Timely nitrogen fertilization and irrigation help plants retain green leaves longer, producing more forage mass (Tretyakov et al., 2000).

Scenario 3: Product storage.

Vegetables and fruits continue to senesce after harvest. To extend storage life, senescence processes must be slowed. This is achieved by:

• Lowering storage temperature (slows metabolism, including ethylene synthesis).
• Removing ethylene from the storage atmosphere (ethylene accelerates senescence).
• Treating the product with ethylene inhibitors (e.g., 1-MCP — 1-methylcyclopropene, which blocks ethylene receptors) (Taiz et al., 2023).

What should the agronomist know?

Let's summarize. All the examples above show that knowledge of the physiology of growth and development is not a luxury, but a necessity for the modern agronomist.

Here are the key practical questions our module answers:

1. Why do cereals lodge and how to combat it? → Role of gibberellins, retardants.

2. Why is suckering done? → Apical dominance, role of auxin.

3. Why are growth regulators used? → Specific hormones for specific tasks (rooting, ripening, preventing drop).

4. Why do dwarf varieties produce higher yields? → Gibberellin regulation, harvest index, Green Revolution.

5. Why do plants branch differently? → Hormone balance, variety selection, and agronomic practices.

6. Why do flowering times depend on conditions? → Photoperiodism, vernalization, management of sowing dates.

7. How to manage plant senescence? → Role of ethylene, cytokinins, storage and harvesting methods.

Conclusion

Dear listeners, we conclude this introductory lecture. We have covered four key blocks:

1. Growth is not the only process. We clearly distinguished the concepts of growth (quantitative changes) and development (qualitative changes).

2. Who controls growth? We identified three main "controllers": the genome (hereditary program), meristems (sites of growth), and phytohormones (chemical signals) — and showed that their interaction determines the entire course of ontogenesis.

3. What lies ahead? We outlined seven key questions we will address in the module: from the cellular basis of growth to integrative mechanisms of hormonal regulation.

4. Why is this important for the agronomist? We demonstrated with concrete examples how understanding these mechanisms allows management of the production process: combating lodging, using growth regulators, selecting varieties, regulating flowering and harvest times.

Now you have a framework, a system of coordinates. In the following lectures, we will fill this framework with specific knowledge: we will dissect each hormone, each process, and each regulatory link in detail. But now you know where we are going and why.