Growth

**Growth** is one of the most fundamental characteristics of living organisms. In plants, it is defined as an **irreversible permanent increase in size** of an organ, its parts, or even a single cell. Growth is generally accompanied by metabolic processes (both anabolic and catabolic) that require energy. For example, the expansion of a leaf is considered growth.

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Plant Growth Generally Is Indeterminate

Plant growth is unique because plants possess the capacity for **unlimited growth throughout their life**. This is due to the presence of **meristems** (regions of actively dividing cells) at specific locations in the plant body (e.g., root and shoot apices, vascular cambium). Cells in meristems divide continuously. One daughter cell remains meristematic (self-perpetuates), while the other differentiates and matures to form part of the plant body. This pattern, where new cells are constantly added by meristematic activity, is called the **open form of growth**.

• **Primary growth:** Responsible for the elongation of roots and stems along their axis, due to the activity of root apical meristem and shoot apical meristem.
• **Secondary growth:** In dicots and gymnosperms, the lateral meristems (vascular cambium and cork-cambium) appear later in life and cause an increase in the girth (diameter) of the stem and root.

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Growth Is Measurable

Growth, at the cellular level, is primarily an increase in the amount of protoplasm. Since measuring protoplasm directly is difficult, growth is measured by various parameters that are proportional to it:

• Increase in fresh weight, dry weight, length, area, volume, or cell number.

Examples:

• Growth in maize root apical meristem is measured as an increase in cell number.
• Growth in watermelon cells is measured as an increase in cell size (volume).
• Growth of a pollen tube is measured in terms of its length.
• Growth in a dorsiventral leaf is measured as an increase in surface area.

These parameters quantify growth, making it measurable.

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Phases Of Growth

Growth period is generally divided into three phases, based on the characteristics of cells in the region behind the root or shoot apex:

• **Meristematic phase:** Located at the root and shoot apices. Cells divide constantly, are rich in protoplasm, have large nuclei, thin primary cell walls, and abundant plasmodesmatal connections.
• **Elongation phase:** Proximal to the meristematic zone. Cells undergo rapid enlargement (increase in vacuolation, cell enlargement) and new cell wall deposition, responsible for growth in length.
• **Maturation phase:** Further proximal to the elongation phase. Cells attain maximal size, wall thickening, and protoplasmic modifications, becoming mature specialized cells and tissues.

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Growth Rates

**Growth rate** is the increased growth per unit time. It can be expressed mathematically.

Growth rates can be:

• **Arithmetic growth:** One daughter cell from mitotic division continues to divide, while the other differentiates and matures. The rate of growth is constant. A plot of length (or a similar parameter) against time gives a **linear curve**.

  Mathematical expression: $\text{L}_t = \text{L}_0 + rt$

  Where, L$_t$ = length at time t, L$_0$ = length at time zero, r = growth rate (elongation per unit time).

• **Geometric growth:** Both daughter cells retain the ability to divide and continue to do so. Growth is slow initially (lag phase), then increases rapidly (exponential or log phase) as the population grows exponentially. With limited resources, growth slows down (stationary phase). A plot of growth parameter against time gives a **sigmoid or S-curve**. This is characteristic of organisms or cells growing in a limited environment.

  Mathematical expression (for exponential growth): $\text{W}_1 = \text{W}_0 \text{e}^{rt}$

  Where, W$_1$ = final size, W$_0$ = initial size, r = growth rate (relative growth rate), t = time, e = base of natural logarithms. The relative growth rate (r) is an efficiency index, measuring the plant's ability to produce new material.

Quantitative comparisons of growth can also be made using:

• **Absolute growth rate:** Total growth per unit time (e.g., 5 cm$^2$ increase in area per week).
• **Relative growth rate:** Growth per unit time expressed per unit of the initial parameter (e.g., increase in area per unit initial area per week). This is often a better comparison for systems starting at different sizes.

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Conditions For Growth

Plant growth requires specific conditions:

• **Water:** Essential for cell enlargement (turgidity), enzymatic activities, and nutrient transport. Plant growth is closely linked to the plant's water status.
• **Oxygen:** Needed for metabolic energy release (respiration) essential for growth activities.
• **Nutrients:** Macro and micro essential elements are required for protoplasm synthesis and act as energy sources.
• **Temperature:** An optimum temperature range is necessary for enzymes involved in growth processes. Deviations can be detrimental.
• **Light and Gravity:** Environmental signals that affect specific phases or directions of growth (tropisms), as discussed in previous chapters.

Differentiation, Dedifferentiation And Redifferentiation

**Differentiation** is the process by which cells derived from meristems mature to perform specific functions. During differentiation, cells undergo structural changes in their cell walls and protoplasm to become specialized.

Example: Formation of tracheary elements (components of xylem). Cells differentiate by losing their protoplasm and developing strong, elastic, lignocellulosic secondary cell walls, enabling them to efficiently transport water under tension.

Plant differentiation is considered **open** because cells from the same meristem can differentiate into different structures depending on their location within the developing tissue/organ. For instance, cells pushed to the periphery of the root apex mature as epidermis, while those positioned away might become root-cap cells.

Plants also exhibit **dedifferentiation**. Living, differentiated cells that have lost the capacity to divide can regain the ability to divide under certain conditions. Example: Formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells during secondary growth.

The meristems formed by dedifferentiation (like cambium) can divide and produce cells that once again lose the capacity to divide but mature to perform specific functions. This process is called **redifferentiation**.

Example: Secondary xylem and phloem formed by the vascular cambium are products of redifferentiation.

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Development

**Development** encompasses the entire sequence of changes an organism undergoes throughout its life cycle, starting from seed germination (or zygote formation) through growth, differentiation, maturation, and senescence, finally leading to death. Development is the sum of growth and differentiation processes.

The development of a mature plant from a zygote follows a precise and highly ordered succession of events, resulting in the formation of complex body organization and all its organs.

Plants exhibit **plasticity** in their development, meaning they can follow different pathways in response to their environment or different phases of life, forming different kinds of structures. This ability to modify form in response to environment is a type of plasticity.

Example of plasticity: **Heterophylly** (different shapes of leaves on the same plant). In cotton, coriander, and larkspur, juvenile leaves have different shapes than mature leaves. In buttercup, leaves produced in air differ in shape from those produced in water, demonstrating environmental influence on leaf shape.

Growth, differentiation, and development are closely interconnected events in a plant's life. Development is broadly seen as the integration of growth and differentiation. Development in plants is controlled by both **intrinsic factors** (internal, like genetic control and plant growth regulators) and **extrinsic factors** (external, like light, temperature, water, oxygen, nutrition, gravity).

Plant Growth Regulators (PGRs)

**Plant growth regulators (PGRs)** are small, simple organic molecules that act as chemical signals influencing various aspects of plant growth, differentiation, and development. They are also called **plant growth substances, plant hormones, or phytohormones**. PGRs are synthesised in various parts of the plant and are transported to other parts where they exert their effects, often by diffusion.

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Characteristics

PGRs are chemically diverse, including indole compounds (like IAA), adenine derivatives (like kinetin), carotenoid derivatives (like abscisic acid), terpenes (like gibberellic acid), and gases (ethylene).

Based on function, PGRs are broadly classified into two groups:

1. **Plant growth promoters:** Involved in growth-promoting activities like cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation. Examples: **Auxins, Gibberellins, Cytokinins**.
2. **Plant growth inhibitors:** Play a role in plant responses to stress, dormancy, and abscission. Example: **Abscisic acid (ABA)**.

**Ethylene** is a gaseous PGR that can fit into either group, but is largely involved in inhibiting growth-promoting activities and promoting senescence and abscission. It also plays a role in fruit ripening.

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The Discovery Of Plant Growth Regulators

The discovery of the main PGR groups was often accidental:

• **Auxins:** Charles Darwin and his son Francis Darwin observed phototropism in canary grass coleoptiles, concluding a transmittable influence from the tip caused bending. F.W. Went later isolated auxin (Indole-3-acetic acid, IAA) from oat seedling tips.
• **Gibberellins:** Discovered from studying the 'bakanae' (foolish seedling) disease in rice caused by the fungus *Gibberella fujikuroi*. E. Kurosawa reported that filtrates of the fungus caused disease symptoms. The active substance was identified as gibberellic acid.
• **Cytokinins:** F. Skoog and co-workers found that cell proliferation (callus) in tobacco stem segments required auxins plus other substances in vascular tissue extracts, yeast extract, coconut milk, or DNA. Miller et al. later identified and crystallized kinetin, a substance promoting cytokinesis.
• **Abscisic acid (ABA):** Purified and characterized independently by three researchers looking for inhibitors causing dormancy and abscission. Named Abscisic acid after finding they were chemically identical.
• **Ethylene:** H.H. Cousins observed that a volatile substance from ripened oranges hastened the ripening of unripened bananas stored nearby. Identified as ethylene, a gaseous PGR.

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Physiological Effects Of Plant Growth Regulators

PGRs influence a wide range of physiological processes in plants.

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Auxins

Promoters. Indole-3-acetic acid (IAA) and indole butyric acid (IBA) are natural auxins. NAA and 2,4-D are synthetic auxins, used widely in agriculture/horticulture.

Physiological effects/applications:

• Initiate rooting in stem cuttings (plant propagation).
• Promote flowering (e.g., pineapples).
• Prevent premature fruit and leaf drop, but promote abscission of older mature leaves/fruits.
• Cause apical dominance (inhibition of lateral buds by the apical bud). Removal of shoot tip (decapitation) overcomes this, promoting lateral branching (e.g., in tea plantations, hedge making).

• Induce parthenocarpy (fruit development without fertilisation) (e.g., in tomatoes).
• Used as herbicides (e.g., 2,4-D kills dicot weeds without affecting monocots).
• Control xylem differentiation and aid cell division.

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Gibberellins (GAs)

Promoters. Over 100 types (GA$_1$, GA$_2$, GA$_3$, etc.), mostly acidic. GA$_3$ is well-studied.

Physiological effects/applications:

• Cause increase in axis length (e.g., increase length of grape stalks, elongate apples).
• Delay senescence, extending market period for fruits.
• Speed up malting process in brewing.
• Increase stem length and yield in sugarcane (spraying increases length).
• Hasten maturity period in juvenile conifers, leading to early seed production.
• Promote bolting (internode elongation before flowering) in rosette plants (beet, cabbage).

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Cytokinins

Promoters. Have specific effects on **cytokinesis** (cell division). Kinetin (a modified adenine) discovered from DNA. Natural cytokinins include zeatin (from corn kernels, coconut milk), synthesised in regions of rapid cell division (root apices, developing buds/fruits).

Physiological effects/applications:

• Promote cell division.
• Help produce new leaves, chloroplasts.
• Promote lateral shoot growth and adventitious shoot formation.
• Help overcome apical dominance (antagonistic to auxin).
• Promote nutrient mobilisation, delaying leaf senescence.

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Ethylene (C$_2$H$_4$)

Gaseous PGR. Synthesised in large amounts by tissues undergoing senescence and ripening fruits. Largely an inhibitor of growth-promoting activities.

Physiological effects/applications:

• Promotes senescence and abscission of leaves and flowers.
• Highly effective in **fruit ripening**. Enhances respiration rate during ripening (respiratory climactic).
• Breaks seed and bud dormancy, initiates germination (peanut seeds, potato tubers).
• Promotes rapid internode/petiole elongation in deep water plants (e.g., deep water rice) to keep parts above water.
• Promotes root growth and root hair formation (increasing absorption surface).
• Used to initiate flowering and synchronise fruit-set in pineapples; induces flowering in mango.

Ethephon is a widely used compound that releases ethylene slowly in plants.

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Abscisic Acid (ABA)

Inhibitor. Discovered for its role in abscission and dormancy, but has wide effects. General plant growth inhibitor and metabolism inhibitor.

Physiological effects/applications:

• Inhibits seed germination.
• Stimulates closure of stomata.
• Increases tolerance to various stresses (stress hormone).
• Promotes seed development, maturation, and dormancy.

ABA often acts as an **antagonist** to growth promoters like GAs (e.g., promoting dormancy vs. breaking it). PGRs can also act synergistically (combined effect is greater than sum of individual effects) or individually.

Multiple PGRs often interact to affect a single event (e.g., seed/bud dormancy, abscission, senescence, apical dominance). PGRs are one type of intrinsic control, interacting with genetic control and extrinsic factors (light, temperature, nutrition, oxygen, gravity) to regulate plant growth and development (e.g., vernalisation, flowering, dormancy, seed germination, plant movements).

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