The nature of growth and cellular basis of development

1.1. Growth—A Quantitative Change

What is growth in the physiological sense? Growth is an irreversible increase in the size, volume, or mass of a cell, organ, or entire plant (Hopkins & Hüner, 2009; Kuznetsov, 2006). This is a quantitative characteristic.

Two key points must be understood here:

First, irreversibility. If a plant wilts, loses turgor, and decreases in volume, this is not a reduction in growth but a loss of water. Once we restore water supply, the plant returns to its former size—this was a reversible process.

Second, growth is not only an increase in mass. Recall seed germination in darkness. A pea seedling can elongate by 10–12 centimeters, but its dry mass actually decreases during this time! Why? Because reserve substances in the seed are consumed in respiration, losing carbon as carbon dioxide (Hopkins & Hüner, 2009). We intuitively sense that the plant is growing, even though its dry mass is declining. This means growth can also be measured by increases in length or volume—the choice of parameter depends on the researcher's objectives.

1.2. Development—Qualitative Changes

What is development? Development is a qualitative, irreversible change in the structure and functions of an organism (Hopkins & Hüner, 2009; Tretyakov et al., 2000).

This includes the emergence of new organs, cell differentiation, and transitions from one stage of the life cycle to another. Development always involves a change in program—the activation or deactivation of specific genes.

A classic example we often see in the garden is fruit ripening. At a certain point, the fruit ceases to actively increase in size—growth slows or stops. Yet inside, dramatic changes are occurring: sugars and aromatic compounds accumulate, color and texture change, enzymes that soften cell walls become active. This is development, and it proceeds against the background of halted growth (Tretyakov et al., 2000).

Another example: seedling emergence. When a seedling breaks through the soil layer and reaches light, processes of photomorphogenesis are triggered: the hypocotyl hook straightens, cotyledons unfold, and chlorophyll synthesis begins. Stem growth may even slow down (light inhibition of stem growth, discovered by J. Sachs in the 19th century) (Tretyakov et al., 2000), but development proceeds at full speed.

1.3. The Interrelationship of Growth and Development

Of course, growth and development are inseparable. They often occur simultaneously, but they are not the same thing. We can measure growth (increase in length or mass), but we cannot determine whether a plant is developing from these numbers alone. We need to look at form, structure, and the appearance of new organs.

Thus, to our first question—"What does it mean for a plant to 'grow' from a physiological perspective?"—we respond as follows: "It is an irreversible quantitative increase that may occur alongside qualitative changes (development), or without them, although in nature these processes are usually closely intertwined."

But to understand by what means this increase occurs, we must move to the cellular level.

2.1. What Is a Meristem and Why Is It "Eternal"?

Meristematic cells fundamentally differ from mature cells. They remain in the embryonic phase of development: small, isodiametric (equal in all directions), with thin walls, a large nucleus, and numerous small vacuoles (Kuznetsov, 2006). They exhibit intense protein synthesis, RNA synthesis, and cell division.

But the main property of meristematic cells is totipotency: they retain the full set of genetic information and are capable, when needed, of giving rise to any type of tissue or even an entire plant (Hopkins & Hüner, 2009; Tretyakov et al., 2000). This property underlies regeneration and biotechnological methods.

It is important to understand: a meristem is not simply a cluster of dividing cells. Within it, there are initial cells (the "youngest," which divide indefinitely) and their derivatives, which after several divisions enter the phases of elongation and differentiation. The initial cells remain embryonic throughout the plant's life, ensuring the continuity of growth (Kuznetsov, 2006).

2.2. Types of Meristems and Their Strategic Significance

Depending on location and function, several types of meristems are distinguished. Let us examine three main types that vividly illustrate different growth strategies.

2.3. Different Strategies—Different Meristems

Thus, the presence of meristems is not merely "the ability to grow for life." It represents a diversity of strategies that allows plants to occupy different ecological niches:

Conclusion: A plant grows throughout its life because its meristems retain embryonic activity. But it is important not merely to state this fact but to see behind it the adaptive strategies: the intercalary meristems of grasses are not an anatomical curiosity but a key adaptation to open habitats where plants are constantly subjected to grazing or mowing.

Now that we understand where new cells come from, we can move to the next question: how do these cells transform into fully functional organs? This will be the topic of the next section—the cellular basis of growth.

3.1. Three Phases of Cell Ontogeny

The classical scheme, proposed by J. Sachs in the 19th century and confirmed by modern research, identifies three sequential stages in the life of a plant cell (Kuznetsov, 2006; Tretyakov et al., 2000; Schopfer & Brennicke, 2016):

1. Embryonic Phase (Division Phase)

At this stage, the cell actively divides but hardly increases in size. This is the typical state of meristem cells. Their dimensions are usually 5–10 µm; they are small, isodiametric (i.e., roughly equal in all directions), with thin cellulosic walls, a large nucleus, and numerous small vacuoles (Kuznetsov, 2006).

Physiological features of the embryonic phase:

• Intense synthesis of proteins, RNA, and nucleotides.
• High RNA/protein ratio.
• Active respiration, with significant involvement of anaerobic processes (glycolysis), which is an adaptation to oxygen deficiency in dense meristem tissues.
• Relatively low activity of oxidative phosphorylation.
• Accumulation of reserve substances (starch, lipids) to support subsequent elongation.
• Hormone content: high in cytokinins, low in auxin and ABA (Tretyakov et al., 2000).

Duration: The cell may undergo 3–5 divisions, after which it either remains in the meristem as an initial cell or transitions to the elongation phase.

2. Elongation Phase (Growth by Expansion)

This is the decisive stage, in which the cell increases in volume 50–100 times (Kuznetsov, 2006; Schopfer & Brennicke, 2016). It is elongation that provides the main increase in stem, root, and leaf blade length. The rate of elongation can reach 100% per hour in rapidly growing cells (Kuznetsov, 2006).

How does this happen?

• Formation and growth of the vacuole. In meristematic cells, vacuoles are small. At the onset of elongation, they fuse, forming a single large central vacuole that can occupy up to 90% of the cell volume (Schopfer & Brennicke, 2016).
• Water uptake. Water enters the cell osmotically due to the accumulation of osmotically active substances (sugars, organic acids, K⁺ ions) in the vacuole. This creates turgor pressure, which stretches the cell wall.
• Cell wall biosynthesis. To prevent wall rupture, synthesis of new cellulose microfibrils, hemicelluloses, and pectins proceeds simultaneously, deposited on the inner surface of the wall (Schopfer & Brennicke, 2016).

Physiological changes in the elongation phase (Kuznetsov, 2006; Tretyakov et al., 2000):

Critical vulnerability: Cells in the elongation phase are most sensitive to adverse factors—drought, salinity, low temperatures, nutrient deficiency (Tretyakov et al., 2000). This is because their metabolism is maximally active, and their cell walls are not yet fully strengthened. Therefore, any stress during the elongation stage leads to irreversible reduction in the final size of organs.

Duration: Varies from several hours to several days depending on species and conditions.

3. Differentiation Phase

When the cell reaches its final size, it enters the differentiation phase—acquiring specialized features characteristic of a particular tissue (parenchyma, collenchyma, sclerenchyma, vessels, sieve tubes, etc.) (Hopkins & Hüner, 2009; Kuznetsov, 2006).

In this phase:

• Cell growth ceases.
• Active synthesis of specific proteins and enzymes begins.
• The cell wall may thicken, lignify, or suberize.
• Final differentiation of organelles occurs (e.g., chloroplast formation, development of large vacuoles).

An important nuance: Differentiation does not mean loss of genetic potential. Many differentiated cells retain totipotency—the ability, under certain conditions (injury, tissue culture), to dedifferentiate and recommence division (Hopkins & Hüner, 2009; Kuznetsov, 2006). This phenomenon underlies regeneration and biotechnological methods.

3.2. Why This Specific Sequence: First Division, Then Elongation?

This is not accidental but a strict necessity.

First, division increases the number of cells, creating a "reserve" for future growth. If division is slowed (e.g., due to nutrient deficiency or low temperatures), the organ will contain fewer cells, and its final size will be smaller, even if each cell elongates normally (Lambers & Oliveira, 2019). Example: under nitrogen deficiency, leaf cell numbers decrease, and leaves remain small.

Second, elongation requires enormous energy and biosynthetic investments. The cell must synthesize many new wall components, membranes, and enzymes. If the cell has not accumulated sufficient reserves during the embryonic phase, it cannot fully elongate.

Third, elongation and division are often separated in time and space. For instance, in grasses, cell division occurs at the internode base (intercalary meristem), while elongation occurs further up the stem. This allows precise control of both processes (Schopfer & Brennicke, 2016).

3.3. Growth Characteristics in Different Organs

Root:

• The root grows via the apical meristem located beneath the root cap.
• The elongation zone in the root is short (a few millimeters), but cells elongate very intensively to penetrate soil resistance.
• Root hairs are outgrowths of individual epidermal cells that grow exclusively by elongation (apical growth).

Stem:

• In dicots, the stem elongates uniformly along the entire internode length.
• In grasses, via intercalary growth.
• Light inhibits stem elongation (photoinhibition), resulting in compact, lodging-resistant plants under light conditions (Tretyakov et al., 2000).

Leaf:

• In dicots, the leaf grows through uniform cell division and elongation across the blade.
• In grasses, via a basal intercalary meristem, ensuring linear leaf elongation.

Conclusion: Organ enlargement is not simply "cells getting bigger." It is a strictly determined sequence: first, the meristem produces new cells (division); then these cells elongate (sharp volume increase); and finally, they differentiate into specialized tissues. Elongation is the main "engine" of volumetric growth, while division provides its structural foundation.

Now that we know how a cell transitions from division to elongation, a logical question arises: how does the cell wall, being rigid and strong, suddenly become plastic and allow the cell to elongate? This is the subject of the next section—the molecular mechanism of cell elongation.

4.1. The Biophysical Basis of Elongation: The Lockhart Equation

Before delving into molecular details, we need to understand the basic physics of the process. In 1965, J. Lockhart proposed a simple but very fruitful equation that describes the growth rate of a cell (Schopfer & Brennicke, 2016; Lambers & Oliveira, 2019; Taiz et al., 2023):

Where:

• dV/Vdt — the relative rate of volume change (growth rate).
• m — the coefficient of extensibility of the cell wall. This is a measure of how easily the wall deforms under pressure.
• P — turgor pressure (internal hydrostatic pressure).
• Y — the yield threshold. This is the minimum turgor pressure required to cause irreversible (plastic) deformation of the wall.

The Lockhart equation tells us two key things:

1. Growth is possible only when turgor pressure (P) exceeds the yield threshold (Y). If P < Y, the cell does not grow, even if it is turgid.

2. The growth rate is determined by two factors: the magnitude of effective turgor (P - Y) and the extensibility of the wall (m).

Key conclusion: For a cell to grow, one must either increase turgor (P), lower the yield threshold (Y), or increase wall extensibility (m). In most cases, experiments show that growth regulation occurs precisely through changes in wall extensibility (m) and yield threshold (Y), rather than through changes in turgor (Van Volkenburgh, 1994; Schopfer & Brennicke, 2016; Taiz et al., 2023).

This was experimentally demonstrated through stress-relaxation measurements (Fig. from Schopfer & Brennicke, 2016). If water access to a growing cell is blocked, turgor begins to fall, but the wall continues to "relax"—this is stress relaxation. From the rate of this decline, the extensibility coefficient (m) can be calculated. It was found that auxin treatment increases m but does not affect Y (Schopfer & Brennicke, 2016).

4.2. The Molecular Mechanism of Elongation: From Signal to Water

Now let us move to the molecular chain of events that leads to changes in m and Y. This chain was elucidated through the work of R. Cleland, D. Rayle, A. Hager, and many other researchers and became known as the "acid-growth hypothesis" (Van Volkenburgh, 1994; Taiz et al., 2023; Medvedev, 2012).

Let us examine it step by step:

4.3. Additional Mechanisms: Hydroxyl Radicals

In recent years, it has been discovered that, in addition to expansins, hydroxyl radicals (•OH) participate in wall loosening (Schopfer & Brennicke, 2016). These are formed in the apoplast through the reaction between hydrogen peroxide and metal ions (e.g., Cu²⁺) or with the involvement of peroxidases. Hydroxyl radicals can cleave wall polysaccharides (pectins, hemicelluloses), making the wall more relaxed.

Experiments show: if •OH formation is blocked with radical scavengers, growth is inhibited. This opens new perspectives for understanding growth regulation (Schopfer & Brennicke, 2016).

4.4. Limitations: When Does the Wall Stop Growing?

The wall cannot elongate indefinitely. As the cell ages, irreversible changes occur:

• Lignification—lignin deposition makes the wall rigid and inextensible.
• Suberization—suberin deposition (in cork tissue).
• Cross-linking between wall components (e.g., formation of dityrosine bridges in extensin proteins).

Additionally, with age, the cell loses sensitivity to hormones (Tretyakov et al., 2000). This is related to changes in receptor composition and signaling pathways.

4.5. What Have We Learned About Cell Elongation?

Main conclusion: Cell elongation is not a passive "swelling" process driven by water pressure. It is an active, strictly regulated physiological process that includes hormone signal perception, H⁺-ATPase activation, wall acidification, expansin activation, weakening of calcium bonds, plastic wall deformation under turgor pressure, and finally, synthesis of new wall components.

Key chain to remember:

Hormone → activation of H⁺-ATPase → apoplast acidification → expansin activation → wall loosening → stretching under turgor pressure → water uptake → volume increase → synthesis of new wall components

Why is this important for the agronomist? Understanding this mechanism explains why:

• Plants are sensitive to drought precisely in the elongation phase (water uptake is disrupted).
• Aluminum and cadmium ions are toxic—they interfere with H⁺-ATPase function and calcium metabolism.
• Hormonal treatments can influence growth (e.g., gibberellins, auxins).
• Stress tolerance is often associated with preserving the wall's ability to elongate.

Now that we know how a single cell grows, we can move to the next level of organization—how to measure and compare the growth of different plants and organs.

5.1. Simple Measurement Methods

Let us begin with the most obvious—what can be measured with a ruler or scales.

5.2. Why Are Simple Measurements Insufficient? The Problem of Comparison

Imagine we compare the growth of two plants:

• An oak seedling with an initial mass of 5 g gains 1 g per day (absolute increase = 1 g/day).
• A weed with an initial mass of 20 g gains 2 g per day (absolute increase = 2 g/day).

Absolutely, the weed grows faster (2 > 1). But if we calculate what fraction of its mass the plant has added, we get:

• Oak: 1/5 = 0.20 (20% per day)
• Weed: 2/20 = 0.10 (10% per day)

Relatively, the oak grows twice as fast! Absolute indicators would mislead us. This is precisely why physiologists have developed parameters that account for the plant's initial size.

5.3. Relative Growth Rate (RGR)—The Key Parameter

Relative Growth Rate (RGR) is the increase in biomass per unit of existing biomass per unit time (Lambers & Oliveira, 2019; Connor et al., 2011).

Formula:

Where:

• W₁ and W₂ — dry mass of the plant at times t₁ and t₂
• ln — natural logarithm

What does RGR show? It is the rate of biomass accumulation per gram of existing biomass. Its dimension is g/(g·day) or day⁻¹.

Example: RGR = 0.1 day⁻¹ means that the plant increases its mass by 10% of the current mass per day.

Why use logarithms? Because growth in the exponential phase is described by an exponential function, and log transformation makes it linear, simplifying calculations (Lambers & Oliveira, 2019).

5.4. Decomposing RGR into Components: What Specifically Determines Growth Rate?

RGR is an integral parameter. To understand what causes one plant to grow faster than another, RGR is decomposed into components (Lambers & Oliveira, 2019; Connor et al., 2011):

Where:

5.5. What Does This Analysis Give Us? Experimental Data

Studies on 24 species of herbaceous plants showed (Lambers & Oliveira, 2019; Connor et al., 2011):

1. SLA is the main factor determining high RGR in fast-growing species. Fast-growing plants have thin leaves (high SLA), providing a large light-capture area per unit of invested biomass.

2. NAR does not correlate with RGR in dicots but correlates in monocots. This shows that different plant groups use different strategies.

3. LMR is a less significant factor than SLA, but also contributes (especially in dicots).

4. Slow-growing species have low SLA because they invest more resources in defensive structures (thick cell walls, lignin, phenolic compounds). This makes them more stress-resistant but reduces their potential growth rate.

5.6. An Alternative Approach: Analysis Based on Nutrient Elements

In addition to the "light" approach (RGR via LAR and NAR), there is an approach based on nutrient element concentration (Lambers & Oliveira, 2019):

Where:

• NP (Nutrient Productivity) — productivity per nutrient element (g biomass per mole of element per day).
• PNC (Plant Nutrient Concentration) — concentration of the element in the plant (moles of element per g biomass).

This approach is particularly useful for analyzing growth under nutrient-limited conditions, where mineral elements rather than light limit growth.

5.7. Practical Recommendations for the Agronomist (Tretyakov et al., 2000)

For field growth assessment, the following parameters are used:

5.8. Summary: Why Are Such Complex Parameters Needed?

We began with the question "How to measure growth?" and saw that simple measurements are only the beginning.

Main point: For the physiologist, it is important not simply to say "the plant grew 5 cm" but to understand how and by what means it grew. The parameters RGR, LAR, SLA, and NAR allow us to:

1. Compare plants of different sizes (through RGR).

2. Identify which specific factor limits growth (SLA, NAR, LMR).

3. Predict responses to changing conditions (light, nutrition, temperature).

4. Assess the growth strategies of different species and varieties.

Key practical conclusion: Thin leaves (high SLA) are the key to rapid growth, but the price is reduced stress tolerance. Understanding this trade-off is the foundation for breeding and selecting varieties for specific conditions.