Plant Growth and Development — NCERT Class 11 Biology

• Growth: An irreversible increase in size, volume, and/or mass of a cell, organ, or organism, typically accompanied by a protoplasmic increase.
• Differentiation: The process by which cells derived from meristems mature and develop specific structures and functions to perform specialised roles (e.g., tracheary elements develop lignified cell walls for water transport).
• Development: The sum of all changes that an organism goes through from germination to senescence — it includes both growth and differentiation.
• Dedifferentiation: The process by which differentiated living cells that had lost the capacity to divide regain the ability to divide under certain conditions (e.g., formation of interfascicular cambium and cork cambium from differentiated parenchyma cells).
• Redifferentiation: The process by which dedifferentiated cells lose the capacity to divide again and mature to perform specific functions (e.g., secondary xylem and secondary phloem formed from cambium cells).
• Determinate growth: Growth that ceases after the organism or organ reaches a certain size (e.g., leaves, flowers, fruits have determinate growth).
• Meristem: A region of actively dividing cells in plants that retains the capacity for continuous cell division (e.g., apical meristem, lateral meristem, intercalary meristem).
• Growth rate: The increase in growth per unit time. It can be expressed as absolute growth rate (total increase) or relative growth rate (growth per unit initial parameter per unit time).

No single parameter is sufficient to demonstrate growth throughout the life of a flowering plant because:

1. Different organs grow at different rates — a root, stem, leaf, and flower all grow differently, so one measurement (e.g., height) cannot capture all. 2. Growth involves multiple dimensions — increase in length, width, area, volume, and mass occur at different times and in different organs. 3. Some growth is invisible externally — e.g., increase in protoplasm, cell division in internal tissues, or thickening of stem (secondary growth). 4. Mass can change without true growth — water uptake increases mass temporarily; dry weight is more reliable but requires destructive sampling. 5. Growth phases vary — during the lag phase, cell division occurs without noticeable size increase; during the log phase, cell elongation dominates. 6. Differentiation and development occur alongside growth — a cell may divide (growth) and then differentiate (specialise), and a single parameter cannot capture both.

Therefore, a combination of parameters — height, leaf area, fresh weight, dry weight, cell number, and volume — is needed to comprehensively measure plant growth.

The five main groups of natural plant growth regulators (PGRs) are:

1. Auxins (e.g., IAA — Indole-3-Acetic Acid):

• Promote cell elongation (by loosening cell wall via acid growth hypothesis).
• Maintain apical dominance (suppress lateral bud growth).
• Promote root initiation in cuttings.
• Prevent premature abscission of leaves and fruits.
• Promote parthenocarpy (seedless fruit formation).
• Used in herbicides (2,4-D kills broadleaf weeds).

2. Gibberellins (e.g., GA₃):

• Promote stem elongation (bolting in rosette plants like cabbage).
• Break seed dormancy and promote germination.
• Promote fruit growth (used to produce seedless grapes).
• Induce flowering in long-day plants under short-day conditions.
• Delay senescence in leaves and fruits.

3. Cytokinins (e.g., zeatin, kinetin):

• Promote cell division (cytokinesis) — essential for tissue culture.
• Delay senescence (ageing) in leaves — keep leaves green longer.
• Help overcome apical dominance (antagonistic to auxins).
• Promote nutrient mobilisation — attract nutrients towards the cytokinin-rich region.
• Promote opening of stomata.

4. Abscisic Acid (ABA):

• Inhibits growth — acts as a growth inhibitor.
• Promotes seed dormancy — prevents premature germination.
• Induces stomatal closure during water stress (reduces transpiration).
• Acts as a stress hormone — increases during drought, salinity, and cold.
• Promotes senescence and abscission of leaves and fruits.

5. Ethylene:

• Promotes fruit ripening (widely used commercially).
• Promotes senescence and abscission of leaves and flowers.
• Promotes transverse growth (horizontal expansion) and inhibits longitudinal growth.
• Breaks seed and bud dormancy.
• Promotes apical hook formation in dicot seedlings (helps the shoot emerge from soil).
• Only gaseous plant hormone.

Auxins play several important roles in plant growth and development:

1. Cell elongation: Auxin (IAA) promotes elongation of cells by activating proton pumps (H⁺-ATPase) in the plasma membrane, lowering cell wall pH, and loosening cell wall bonds (acid growth hypothesis). This allows the cell to expand under turgor pressure.

2. Apical dominance: Auxin produced in the apical bud is transported basipetally (downward) and inhibits the growth of lateral (axillary) buds. Removing the apical bud (decapitation) removes the auxin source and allows lateral buds to grow, making the plant bushy.

3. Root initiation: Auxin promotes the formation of adventitious roots from stem cuttings. This property is exploited in horticulture for vegetative propagation. NAA (naphthalene acetic acid) and IBA (indole butyric acid) are synthetic auxins used for this purpose.

4. Prevention of abscission: Auxin prevents premature falling of leaves, flowers, and fruits (abscission). A decrease in auxin level in older leaves and fruits leads to the formation of an abscission layer.

5. Parthenocarpy: Application of auxin to unpollinated flowers can induce seedless fruit formation (parthenocarpy). Examples: seedless tomatoes, seedless apples.

6. Tropic movements: Auxin is responsible for phototropism (growth towards light) and gravitropism (growth with respect to gravity). In phototropism, auxin accumulates on the shaded side, causing cells to elongate more on that side, bending the shoot towards light.

7. Flowering: Auxin promotes flowering in pineapple and some other plants.

Photoperiodism is the physiological response of plants to the relative lengths of light and dark periods (photoperiod) for flowering. It was first studied by Garner and Allard (1920).

Based on photoperiodic response, plants are classified as:

1. Short Day Plants (SDP): Flower when the day length is shorter than a critical photoperiod (or when the night length exceeds a critical duration). Examples: chrysanthemum, tobacco (Maryland Mammoth), soybean, Xanthium, strawberry.

2. Long Day Plants (LDP): Flower when the day length exceeds a critical photoperiod (or when the night length is shorter than a critical duration). Examples: spinach, radish, sugar beet, wheat, henbane (Hyoscyamus).

3. Day Neutral Plants (DNP): Flower regardless of day length — they are not influenced by photoperiod. Examples: tomato, sunflower, maize, cucumber, cotton.

Key point: It is actually the dark period (night length) that is critical for flowering. Short day plants need a long, uninterrupted dark period, while long day plants need a short dark period. If the dark period is interrupted by a brief flash of light, SDP will not flower but LDP will flower.

The phytochrome pigment (Pr and Pfr forms) is the photoreceptor involved in detecting the photoperiod.

Vernalisation is the cold treatment of plants (usually at 0–5 °C for several days to weeks) that promotes or induces flowering. It is the low-temperature induction of flowering.

Key features of vernalisation:

1. Biennial plants like cabbage, carrot, beet, and radish require vernalisation to flower in the second year of their life cycle. They grow vegetatively in the first year, overwinter (experience cold), and then flower in the second year.

2. Winter varieties of wheat, barley, and rye also require vernalisation. They are planted in autumn, overwinter, and flower the next spring.

3. The meristematic cells of the shoot tip (apical meristem) perceive the cold stimulus. The site of perception is the shoot tip.

4. The effect of vernalisation can be reversed by subsequent high-temperature treatment — this is called devernalisation.

5. Vernalisation ensures that plants flower only after experiencing winter, preventing flowering during a warm autumn which would be followed by a killing frost.

6. A hypothetical substance called vernalin has been proposed as the chemical mediator of vernalisation, though it has not been isolated.

(See Intext Page 247 Q1 — answered above.)

(See Intext Page 247 Q2 — answered above.)

(a) Arithmetic growth:

In arithmetic growth, one of the daughter cells continues to divide while the other differentiates and matures. The growth proceeds at a constant rate — the increase in size per unit time is the same. Mathematically:

Lₜ = L₀ + rt

where Lₜ = length at time t, L₀ = length at time zero, r = growth rate, t = time.

When plotted, arithmetic growth gives a straight line. Example: elongation of a root at a constant rate.

(b) Geometric growth:

In geometric growth, both the daughter cells retain the ability to divide after mitosis, so the number of cells increases exponentially. The initial growth is slow (lag phase), then becomes rapid (exponential/log phase). Mathematically:

Wₜ = W₀eʳᵗ

where Wₜ = size at time t, W₀ = initial size, r = growth rate, t = time, e = base of natural log.

When plotted, geometric growth gives an exponential curve. However, due to limited resources and space, geometric growth cannot continue indefinitely and eventually slows down, forming an S-shaped (sigmoid) curve.

(c) Sigmoid growth curve:

The sigmoid growth curve (S-curve) is the typical growth pattern observed in plants and other organisms. It has three distinct phases:

1. Lag phase: Initial slow growth as cells prepare for division (increase in protoplasm but not much increase in size). 2. Log/Exponential phase: Rapid growth due to cell division and cell elongation (geometric growth phase). 3. Stationary phase: Growth slows down as resources become limited, and eventually stabilises. The organism reaches maturity.

A plot of growth rate (dW/dt) against time gives a bell-shaped curve, while a plot of size (W) against time gives the sigmoid curve.

(d) Absolute and relative growth rates:

• Absolute growth rate: The total increase in size per unit time. It measures the actual amount of growth. Example: If a leaf grows from 5 cm² to 10 cm² in one day, the absolute growth rate is 5 cm²/day.

• Relative growth rate: The growth per unit initial size per unit time. It is expressed as a percentage of the initial size. Example: If a leaf of 5 cm² grows to 10 cm² (absolute growth = 5 cm²), the relative growth rate is (5/5) × 100 = 100% per day. If another leaf of 50 cm² grows to 55 cm² (absolute growth = 5 cm²), the relative growth rate is (5/50) × 100 = 10% per day.

Both leaves have the same absolute growth rate (5 cm²/day) but different relative growth rates (100% vs 10%). Relative growth rate is more meaningful for comparing growth of different-sized organs or organisms.

(See Intext Page 252 Q1 — answered above.)

(See Intext Page 252 Q1 — answered above for detailed descriptions. Summary below.)

Plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition that regulate plant growth and development. They are also called phytohormones or plant hormones.

1. Auxins: Cell elongation, apical dominance, root initiation, prevents abscission, parthenocarpy. 2. Gibberellins: Stem elongation, bolting, seed germination, fruit growth, delays senescence. 3. Cytokinins: Cell division, delays senescence, overcomes apical dominance, nutrient mobilisation. 4. Abscisic acid (ABA): Growth inhibitor, seed dormancy, stomatal closure, stress hormone, promotes senescence. 5. Ethylene: Fruit ripening, senescence, abscission, transverse growth, seed dormancy breakage — the only gaseous hormone.

Apical dominance is the phenomenon in which the growing apical (terminal) bud inhibits the growth of lateral (axillary) buds below it. As a result, the plant grows taller rather than bushier.

Role of auxin in apial dominance:

• Auxin (IAA) is synthesised in the apical bud and transported basipetally (downward) through the stem.
• This basipetal auxin transport creates a concentration gradient — higher auxin near the apex, lower away from it.
• The auxin reaching the lateral buds inhibits their growth by diverting nutrients towards the apical bud and by reducing cytokinin levels near the axillary buds.
• When the apical bud is removed (decapitation), the source of auxin is eliminated, and the lateral buds begin to grow, resulting in a bushy plant.
• Application of auxin paste on the cut end of a decapitated shoot restores apical dominance and prevents lateral bud growth.

This principle is used in pruning (removing the shoot tip to promote lateral branching) and in hedge making.

Gibberellins (GAs) play the following roles in plant growth:

1. Stem elongation: Gibberellins promote internode elongation. In dwarf plants (genetically dwarf varieties of pea, maize, etc.), application of GA restores normal height, confirming that dwarfism is caused by GA deficiency.

2. Bolting: In rosette plants (e.g., cabbage, radish, beet), the stem is very short and leaves are clustered at the base. Gibberellins induce rapid stem elongation (bolting) just before flowering. Without GA treatment, rosette plants would not bolt.

3. Seed germination: GA breaks seed dormancy by stimulating the production of hydrolytic enzymes (α-amylase) in the aleurone layer of cereal grains. This mobilises stored food (starch → sugars) for the growing embryo.

4. Fruit growth: Gibberellins promote fruit development, especially in seedless (parthenocarpic) varieties. Used commercially to produce seedless grapes and apples.

5. Flowering: GA can induce flowering in long-day plants even under short-day conditions and in biennials that require vernalisation.

6. Delaying senescence: Gibberellins can delay leaf and fruit senescence, extending shelf life.

Cytokinins play the following roles in plant growth:

1. Cell division (cytokinesis): Cytokinins promote cell division, especially when combined with auxin. This is the basis of tissue culture — the auxin:cytokinin ratio determines whether callus, roots, or shoots are formed.

2. Delay of senescence (Richmond-Lang effect): Cytokinins delay the ageing (senescence) of leaves by maintaining protein levels, chlorophyll content, and nutrient mobilisation. This is why cytokinin-treated leaves stay green longer.

3. Overcoming apical dominance: Cytokinins are antagonistic to auxins in promoting lateral bud growth. High cytokinin levels relative to auxin promote branching.

4. Nutrient mobilisation: Cytokinins attract nutrients towards the region where they are concentrated (nutrient sink effect).

5. Stomatal opening: Cytokinins promote the opening of stomata, working in opposition to ABA (which closes stomata).

6. Promotion of fruit growth: Cytokinins, along with auxins and gibberellins, promote fruit development.

Abscisic acid (ABA) plays the following roles:

1. Seed dormancy: ABA induces and maintains seed dormancy, preventing premature germination. It counteracts the germination-promoting effects of gibberellins.

2. Stomatal closure: During water stress (drought), ABA is synthesised in the leaves and triggers the closing of stomata by causing guard cells to lose K⁺ ions and become flaccid. This reduces transpiration and conserves water.

3. Growth inhibition: ABA inhibits plant growth by counteracting the effects of growth-promoting hormones (auxins, gibberellins, cytokinins).

4. Stress response: ABA is known as the stress hormone because its levels increase dramatically during drought, salinity, and cold stress. It helps the plant survive adverse conditions.

5. Senescence and abscission: ABA promotes the senescence (ageing) of leaves and flowers, and promotes abscission (falling) of leaves and fruits.

6. Inhibition of germination: ABA inhibits vivipary (germination of seeds while still attached to the parent plant).

Ethylene plays the following roles:

1. Fruit ripening: Ethylene is the fruit-ripening hormone. It accelerates the ripening of climacteric fruits (e.g., banana, mango, apple, tomato). Commercially, ethylene is used to ripen fruits uniformly.

2. Senescence: Ethylene promotes senescence of leaves, flowers, and fruits.

3. Abscission: Ethylene promotes the abscission (shedding) of leaves, flowers, and fruits by stimulating the formation of the abscission layer.

4. Transverse growth: Ethylene promotes horizontal (transverse) growth and inhibits longitudinal growth. This causes swollen, thick stems — a response seen in waterlogged plants (epinasty).

5. Apical hook formation: In dicot seedlings, ethylene promotes the formation of the apical hook, which protects the shoot tip as it emerges through the soil.

6. Seed and bud dormancy: Ethylene can break seed and bud dormancy and promote germination.

7. Root initiation: Ethylene promotes root initiation and root hair formation, enhancing water and mineral absorption.

8. Sex expression: In monoecious plants (e.g., cucumber, pumpkin), ethylene increases the number of female flowers.

(a) Photoperiodism — see Intext Page 258 Q1 (answered above).

(b) Vernalisation — see Intext Page 258 Q2 (answered above).

Seed dormancy is a state in which viable seeds fail to germinate even under favourable conditions of moisture, temperature, and oxygen. It is a survival strategy that ensures germination occurs only under optimal conditions.

Causes of seed dormancy: 1. Hard, impermeable seed coat (physical barrier to water and O₂) 2. Chemical inhibitors (ABA, phenolic compounds) 3. Immature embryo (embryo needs more time to develop) 4. Requirement of specific light or temperature conditions 5. Presence of thick, hard fruit wall around the seed

Methods to break seed dormancy:

1. Scarification: Mechanical or chemical weakening of the hard seed coat to allow water and oxygen penetration. Methods include scratching with sandpaper, nicking with a knife, or treating with concentrated sulphuric acid.

2. Stratification (chilling): Exposing seeds to low temperatures (0–5 °C) for a period of time to break dormancy. This simulates the natural overwintering process.

3. Gibberellin treatment: Application of gibberellins (GA₃) breaks dormancy by promoting enzyme synthesis (α-amylase) and counteracting the effect of ABA.

4. Cytokinin treatment: Cytokinins can overcome seed dormancy, especially in combination with gibberellins or light.

5. Light exposure: Some seeds (e.g., lettuce) require light to germinate. The phytochrome system detects red light and triggers germination.

6. Temperature fluctuations: Alternating warm and cold temperatures can break dormancy in some seeds.

7. After-ripening: Storing seeds for a period under dry conditions allows the embryo to mature and inhibitors to degrade.

8. Leaching: Washing out chemical inhibitors by soaking seeds in water.