The development of a complex plant body with organs like roots, stems, leaves, flowers, fruits, and seeds from a single-celled zygote involves an ordered sequence of events.

Development is the sum of two main processes: growth and differentiation.

The life cycle of a plant begins with seed germination under favourable environmental conditions. If conditions are unfavorable, the seed may enter a period of suspended growth called dormancy. Upon return of favourable conditions, metabolic activities resume, and growth begins.

Plant development is regulated by factors that are both intrinsic (internal, e.g., genetic control, plant growth regulators) and extrinsic (external, e.g., light, temperature, water, oxygen, nutrients).

Growth

Growth is a fundamental characteristic of living organisms. It is defined as an irreversible permanent increase in size of an organ, its parts, or an individual cell. This increase is generally accompanied by metabolic processes that require energy.

For example, the expansion of a leaf is growth. Swelling of wood in water is reversible and due to imbibition, not growth.

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Plant Growth Generally Is Indeterminate

Plant growth is considered indeterminate because plants retain the capacity for unlimited growth throughout their life. This is possible due to the presence of meristems (regions of actively dividing cells) at specific locations (Figure 15.2).

Meristematic cells can divide repeatedly and perpetuate themselves. Some daughter cells from meristematic divisions differentiate and form the plant body, while others remain meristematic.

This continuous addition of new cells by meristematic activity is called the open form of growth.

Primary growth: Increase in length of roots and stems due to the activity of root apical meristem and shoot apical meristem.

Secondary growth: Increase in the girth or diameter of roots and stems in dicotyledonous plants and gymnosperms. This is caused by the activity of lateral meristems, namely vascular cambium and cork cambium, which appear later in the plant's life.

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Growth Is Measurable

At the cellular level, growth is fundamentally an increase in the amount of protoplasm. Since protoplasm measurement is difficult, growth is usually measured using parameters proportional to it:

• Increase in fresh weight
• Increase in dry weight
• Increase in length (e.g., pollen tube)
• Increase in area (e.g., dorsiventral leaf)
• Increase in volume (e.g., fruit, cell)
• Increase in cell number (e.g., maize root apical meristem producing 17,500+ cells/hour)

Growth in a watermelon cell can be expressed as an increase in cell size (up to 350,000 times).

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Phases Of Growth

Growth is typically divided into three phases, observable, for instance, in a root tip (Figure 15.3):

1. Meristematic phase: Located at the root and shoot apices. Cells are constantly dividing, rich in protoplasm, have large nuclei, thin primary cellulosic cell walls, and abundant plasmodesmatal connections.
2. Phase of elongation: Located just proximal (away from the tip) to the meristematic zone. Cells in this region undergo increased vacuolation, cell enlargement, and new cell wall deposition, leading to the increase in length.
3. Phase of maturation: Located further proximal to the elongation phase. Cells here attain their maximal size, undergo wall thickening, and protoplasmic modifications to perform specific functions (differentiation). Tissues and cell types described in Chapter 6 belong to this phase.

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Growth Rates

Growth rate is the increased growth per unit time. It can be expressed mathematically and measured in various ways.

Patterns of growth rate (Figure 15.4):

1. Arithmetic growth: Involves mitotic division where only one daughter cell continues to divide, while the other differentiates and matures. A plot of the length of an organ (like a root) against time yields a linear curve (Figure 15.5).

   Mathematically expressed as: $\textsf{L}_\textsf{t} = \textsf{L}_0 + \textsf{r} \times \textsf{t}$

   Where: $\textsf{L}_\textsf{t}$ = length at time 't', $\textsf{L}_0$ = length at time 'zero', $\textsf{r}$ = growth rate (elongation per unit time).
2. Geometric growth: Involves mitotic division where both daughter cells retain the ability to divide and continue to do so, resulting in a rapid, exponential increase in cell number or size. This phase follows an initial slow (lag) phase. With limited resources, growth slows down, leading to a stationary phase. A plot of growth parameter (size, weight, etc.) against time yields a typical sigmoid or S-curve (Figure 15.6).

   The sigmoid curve is characteristic of living organisms or organs growing in a natural environment and is typical for cells, tissues, and organs.

   The exponential phase can be expressed as: $\textsf{W}_1 = \textsf{W}_0 \times \textsf{e}^{\textsf{rt}}$

   Where: $\textsf{W}_1$ = final size, $\textsf{W}_0$ = initial size, $\textsf{r}$ = relative growth rate, $\textsf{t}$ = time, $\textsf{e}$ = base of natural logarithms.

   The rate 'r' is the relative growth rate, indicating the growth per unit initial size per unit time. It is also called the efficiency index, measuring the ability of the plant to produce new material. The final size ($\textsf{W}_1$) depends on the initial size ($\textsf{W}_0$).

Quantitative comparison of growth can use:

• Absolute growth rate: Measurement and comparison of the total growth per unit time (e.g., increase in area in cm$^2$ per week).
• Relative growth rate: Growth per unit time expressed per unit initial parameter (e.g., increase in area per unit initial area per week). This is often more informative when comparing growth in systems of different initial sizes (Figure 15.7).

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Conditions For Growth

Several conditions are necessary for plant growth:

• Water: Essential for cell enlargement (turgidity) and provides the medium for enzymatic activities. Plant growth is closely linked to its water status.
• Oxygen: Required for respiration to release metabolic energy (ATP) needed for growth activities.
• Nutrients: Macro- and micro-essential elements (from soil) are required for the synthesis of protoplasm and as sources of energy.
• Temperature: Each plant species has an optimum temperature range for growth. Deviations can be detrimental.
• Light and Gravity: Environmental signals affecting certain growth phases/stages.

Differentiation, Dedifferentiation And Redifferentiation

Differentiation: The process by which cells derived from meristems (apical meristems, cambium) mature to perform specific functions. This involves structural and functional changes in cell walls and protoplasm.

Example: Tracheary elements differentiate to transport water. They lose protoplasm and develop strong, lignocellulosic secondary walls for transport under tension.

Dedifferentiation: The phenomenon where living differentiated cells (which had lost the capacity to divide) regain the ability to divide under certain conditions. This occurs to form meristems in certain situations.

Example: Formation of interfascicular cambium and cork cambium from differentiated parenchyma cells during secondary growth.

Redifferentiation: The process where the cells produced by dedifferentiated meristems once again lose the capacity to divide and mature to perform specific functions.

Differentiation in plants is considered open because cells originating from the same meristem can differentiate into different cell types at maturity. The final structure of a cell/tissue is also influenced by its position within the plant organ.

Example: Cells pushed away from the root apex differentiate into root-cap cells, while those at the periphery become epidermis.

Development

Development is a comprehensive term encompassing all the changes that an organism undergoes throughout its life cycle, from seed germination to senescence (aging and death).

Figure 15.8 provides a diagrammatic representation of the developmental processes in a plant cell:

Plants exhibit plasticity in their development. This is the ability to follow different pathways in response to the environment or different phases of life to form different kinds of structures.

Example: Heterophylly (presence of different leaf shapes on the same plant). This is seen in cotton, coriander, and larkspur, where juvenile leaves differ from mature leaves. Buttercup shows environmental heterophylly, producing different leaf shapes in air and water.

Development is considered the sum of growth and differentiation. Both growth and differentiation in plants are controlled by intrinsic factors (genetic factors, plant growth regulators) and extrinsic factors (light, temperature, water, oxygen, nutrition).

Plant Growth Regulators

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Characteristics

Plant Growth Regulators (PGRs), also known as plant growth substances, plant hormones, or phytohormones, are small, simple organic molecules that influence plant growth and development.

PGRs have diverse chemical compositions:

• Indole compounds (e.g., indole-3-acetic acid, IAA, a natural auxin)
• Adenine derivatives (e.g., kinetin, a synthetic cytokinin)
• Derivatives of carotenoids (e.g., abscisic acid, ABA)
• Terpenes (e.g., gibberellic acid, GA$_3$)
• Gases (e.g., ethylene, C$_2$H$_4$)

Based on their functions, PGRs are broadly categorized 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 (e.g., Auxins, Gibberellins, Cytokinins).
2. Plant growth inhibitors: Involved in growth-inhibiting activities like dormancy and abscission, and responses to stresses (biotic and abiotic) (e.g., Abscisic acid, ABA).

Ethylene can fit into either group but is largely considered an inhibitor of growth activities, especially promoting senescence and abscission.

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The Discovery Of Plant Growth Regulators

The discovery of each major class of PGRs has been quite serendipitous:

• Auxins: Charles Darwin and his son Francis Darwin observed that the coleoptiles of canary grass seedlings bent towards unilateral light (phototropism). They concluded that the tip of the coleoptile produced a transmissible influence causing the bending (Figure 15.10). F.W. Went later isolated auxin from the tips of oat seedlings coleoptiles.
• Gibberellins: E. Kurosawa studied the 'bakanae' (foolish seedling) disease in rice, caused by the fungus *Gibberella fujikuroi*. He found that sterile filtrates of the fungus caused similar symptoms (excessive elongation). The active substance was later identified as gibberellic acid.
• Cytokinins: F. Skoog and co-workers found that tobacco stem pith callus could proliferate in culture only if supplemented with extracts containing substances that promoted cell division, in addition to auxins. Miller et al. later identified and crystallized the active substance as kinetin.
• Abscisic acid (ABA): During the mid-1960s, three different research groups independently purified and characterized three inhibitors related to abscission and dormancy (inhibitor-B, abscission II, and dormin). They were found to be chemically identical and named abscisic acid.
• Ethylene: H.H. Cousins observed that a volatile substance released from ripe oranges hastened the ripening of unripe bananas stored nearby. This volatile substance was later identified as ethylene.

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Physiological Effects Of Plant Growth Regulators

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Auxins

First isolated from human urine. The term 'auxin' applies to IAA (Indole-3-acetic acid) and other compounds with similar growth-promoting properties.

Synthesis: Primarily produced in growing apices of stems and roots.

Natural auxins: IAA, IBA (indole butyric acid).

Synthetic auxins: NAA (naphthalene acetic acid), 2,4-D (2,4-dichlorophenoxyacetic acid).

Physiological effects and applications:

• Initiate rooting in stem cuttings (widely used in plant propagation).
• Promote flowering (e.g., in pineapples).
• Prevent fruit and leaf drop at early stages; promote abscission of older leaves/fruits.
• Apical dominance: Inhibit the growth of lateral (axillary) buds (Figure 15.11). Removing the shoot tip (decapitation) reduces apical dominance, promoting lateral bud growth (used in tea plantations, hedge making).
• Induce parthenocarpy (development of fruit without fertilization, resulting in seedless fruits) (e.g., in tomatoes).
• Used as herbicides (e.g., 2,4-D selectively kills dicotyledonous weeds without affecting monocots, used in lawns).
• Control xylem differentiation and promote cell division.

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Gibberellins

Another class of growth-promoting PGRs. Over 100 forms (GA1, GA2, GA3, etc.), found in fungi and higher plants. Gibberellic acid (GA3) is the most studied.

All GAs are acidic.

Physiological effects and applications:

• Increase length of axis (e.g., grapes stalks, sugarcane stems - increasing yield by 20 tonnes/acre).
• Cause fruits like apples to elongate and improve shape.
• Delay senescence, extending market period for fruits.
• Speed up malting process in brewing.
• Hasten maturity in juvenile conifers, leading to early seed production.
• Promote bolting (sudden elongation of internodes before flowering) in beet, cabbages, and rosette plants.

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Cytokinins

PGRs with specific effects on cytokinesis (cell division). Kinetin (a synthetic form) was discovered first. Natural cytokinins (like zeatin from corn kernels and coconut milk) are found.

Synthesis: Occur in regions of rapid cell division (root apices, shoot buds, young fruits).

Physiological effects and applications:

• Promote cell division.
• Help produce new leaves and chloroplasts.
• Promote lateral shoot growth and adventitious shoot formation.
• Help overcome apical dominance.
• Promote nutrient mobilization, delaying leaf senescence.

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Ethylene

A simple gaseous PGR.

Synthesis: Produced in large amounts by senescing tissues and ripening fruits.

Physiological effects and applications:

• Influences horizontal growth of seedlings, swelling of the axis, and apical hook formation in dicot seedlings.
• Promotes senescence and abscission (shedding) of plant organs (leaves, flowers, fruits).
• Highly effective in fruit ripening; enhances respiration rate during ripening (respiratory climactic).
• Breaks seed and bud dormancy; initiates germination (e.g., in peanut seeds, potato tubers).
• Promotes rapid internode/petiole elongation in deepwater rice, keeping parts above water.
• Promotes root growth and root hair formation, increasing absorption surface.
• Used to initiate flowering and synchronize fruit-set in pineapples; induces flowering in mango.

Ethephon: A widely used compound that is readily absorbed and releases ethylene slowly inside the plant. Used to hasten fruit ripening (tomatoes, apples) and accelerate abscission (thinning of cotton, cherry, walnut). Promotes female flowers in cucumbers, increasing yield.

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Abscisic Acid

Discovered for its role in abscission and dormancy. ABA is a general plant growth inhibitor and metabolism inhibitor.

Physiological effects and applications:

• Inhibits seed germination.
• Stimulates stomatal closure.
• Increases tolerance of plants to various stresses (drought, cold, etc.), earning it the name stress hormone.
• Plays important role in seed development, maturation, and dormancy (helps seeds survive desiccation).
• Often acts as an antagonist to gibberellins (promoting dormancy vs. breaking it).

PGRs interact in complex ways; their roles can be complimentary, antagonistic, individual, or synergistic, influencing every phase of plant growth and development. Many extrinsic factors influence plant development by affecting PGR activity.

Photoperiodism

Photoperiodism is the response of plants to the relative durations of light and dark periods, particularly concerning the induction of flowering.

Some plants require a specific periodic exposure to light for flowering. They can measure the duration of light (and equally importantly, dark).

Based on their photoperiod requirement for flowering:

• Long day plants: Require exposure to light for a period exceeding a specific critical duration to flower.
• Short day plants: Require exposure to light for a period less than a specific critical duration to flower.
• Day-neutral plants: Flowering is not dependent on the duration of light exposure.

The critical duration is different for different species (Figure 15.12).

It is known that the duration of the dark period is also of crucial importance in photoperiodism.

Site of perception: Although shoot apices are where flowering occurs, the perception of light/dark duration takes place in the leaves.

Hypothesis: A hormonal substance(s) responsible for flowering is synthesized in the leaves under appropriate photoperiod conditions and then transported to the shoot apices to induce flowering.

Vernalisation

Vernalisation is the phenomenon where flowering is quantitatively or qualitatively dependent on exposure to a period of low temperature.

Significance: Vernalisation prevents premature flowering late in the growing season, giving the plant sufficient time to mature before reproduction.

Examples:

• Winter varieties of wheat, barley, and rye: Planted in autumn, they germinate, survive winter as seedlings, resume growth in spring, and flower/produce grain in summer. If planted in spring, they fail to flower or mature within that season due to lack of cold exposure.
• Biennial plants: Monocarpic plants that flower and die in the second season (e.g., sugarbeet, cabbage, carrots). Exposure to a cold treatment stimulates subsequent flowering.

Seed Dormancy

Seed dormancy is a state where seeds fail to germinate even when external conditions (water, oxygen, suitable temperature) are favorable. This is due to endogenous control or conditions within the seed itself.

Reasons for seed dormancy:

• Impermeable and hard seed coat, preventing water uptake or gas exchange.
• Presence of chemical inhibitors (e.g., abscisic acid, phenolic acids, para-ascorbic acid).
• Immature embryo.

Breaking seed dormancy can occur through natural means or artificial treatments:

• Mechanical abrasion: Breaking the seed coat barrier using knives, sandpaper, vigorous shaking. In nature, this is done by microbial action or passage through animal digestive tracts.
• Chilling conditions (stratification): Exposing seeds to low temperatures for a period can remove inhibitory effects (often effective for seeds from temperate climates).
• Chemical treatments: Application of substances like gibberellic acid and nitrates can break dormancy.
• Changing environmental conditions: Manipulating light and temperature can also overcome dormancy in some seeds.

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