Plant Growth and Development

Growth: Growth is an irreversible permanent increase in size, shape, number, volume or dry weight of a cell, organ or whole organism. It involves an increase in protoplasmic material and is expressed in parameters such as length, area, volume, cell number, etc.

Differentiation: It is the process by which cells derived from the meristem (which are initially similar) undergo structural and functional changes to become specialised for performing specific functions. For example, cells differentiate to form xylem vessels, sieve tubes, etc.

Development: Development is the sum total of all changes that an organism goes through during its life cycle — from germination of seed to senescence. It includes both growth and differentiation:
$$\text{Development} = \text{Growth} + \text{Differentiation}$$

Dedifferentiation: It is the process by which living differentiated cells that have lost the capacity to divide regain the capacity to divide under certain conditions. For example, formation of meristems (interfascicular cambium, cork cambium) from differentiated parenchyma cells.

Redifferentiation: After dedifferentiation, when the cells that have regained the capacity to divide lose it again and mature to perform specific functions, the process is called redifferentiation. For example, secondary xylem and phloem formed from vascular cambium.

Determinate Growth: Growth that ceases after reaching a certain size or stage is called determinate (or limited/closed) growth. It is characteristic of most animals and leaves, flowers, and fruits in plants.

Meristem: Meristems are regions of active cell division in plants. They are composed of undifferentiated, actively dividing cells. Based on position, they are classified as:
- Apical meristem (at root and shoot tips)
- Intercalary meristem (at internodes/leaf bases)
- Lateral meristem (vascular cambium, cork cambium)

Growth Rate: Growth rate is the increase in growth per unit time. It can be expressed as arithmetic or geometric growth rate:
$$\text{Growth Rate} = \frac{\text{Increase in parameter (size, number, etc.)}}{\text{Time}}$$
It can be absolute (total increase) or relative (increase per unit of existing material per unit time).

Given/Concept: Growth is a complex process and can be measured using various parameters such as fresh weight, dry weight, length, area, volume, cell number, etc.

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

1. Different stages require different parameters: During germination, increase in fresh weight or length may be appropriate. During vegetative growth, increase in height or leaf area is measured. During seed formation, dry weight is a better indicator.

2. Conflicting results: Fresh weight may increase due to water absorption (which is not true growth), while dry weight may decrease during germination (as stored food is consumed). Hence, fresh weight alone or dry weight alone cannot represent growth at all stages.

3. Organ-specific growth: Different organs grow differently — roots grow in length, leaves grow in area, fruits grow in volume, and seeds accumulate dry matter. No single parameter captures all these changes.

4. Metabolic changes: During senescence, a plant may lose fresh weight but cell number may remain the same.

Conclusion: Therefore, a combination of parameters is needed to accurately demonstrate growth throughout the life of a flowering plant.

(a) Arithmetic Growth:

Definition: In arithmetic growth, following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. The rate of growth is constant and the increase in length/size occurs at a constant rate.

Mathematical expression:
$$L_t = L_0 + rt$$
Where:
- $L_t$ = length at time $t$
- $L_0$ = length at time zero
- $r$ = growth rate / elongation per unit time

Example: Elongation of a root at a constant rate. When plotted, it gives a linear curve.

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(b) Geometric Growth:

Definition: In geometric growth, both the daughter cells produced after mitotic division retain the ability to divide. Initially the growth is slow (lag phase), then it increases rapidly. The growth is proportional to the existing material (nutrients, space are not limiting).

Mathematical expression:
$$W_1 = W_0 \cdot e^{rt}$$
Where:
- $W_1$ = final size (weight, height, number, etc.)
- $W_0$ = initial size at the beginning
- $r$ = growth rate (relative growth rate)
- $t$ = time
- $e$ = base of natural logarithm

Example: Early embryo development, microbial growth in unlimited nutrient medium. When plotted, it gives a J-shaped (exponential) curve.

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(c) Sigmoid Growth Curve:

Definition: When the growth of a living organism (or its organ) is plotted against time, it gives an S-shaped (sigmoid) curve. This is the typical growth curve for most organisms growing in a natural environment with limited resources.

Three phases:
1. Lag phase: Initial slow growth; cells prepare for division (metabolic activity increases).
2. Log phase (Exponential phase): Rapid, exponential increase in growth.
3. Stationary/Senescent phase: Growth slows down and finally stops as nutrients become limiting or the organism ages.

Example: Growth of a pea seedling, growth of a population in a limited environment.

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(d) Absolute and Relative Growth Rates:

Absolute Growth Rate (AGR):
It is the total growth (increase in size or weight) per unit time. It does not take into account the initial size of the organism.
$$\text{AGR} = \frac{\text{Increase in parameter}}{\text{Time}}$$

Relative Growth Rate (RGR):
It is the increase in growth per unit time expressed on a common basis (per unit of initial parameter). It compares the efficiency of growth.
$$\text{RGR} = \frac{\text{Increase in parameter}}{\text{Initial parameter} \times \text{Time}}$$

Example: If leaf A grows from 5 cm² to 10 cm² and leaf B grows from 50 cm² to 55 cm² in the same time:
- AGR of A = 5 cm²/time; AGR of B = 5 cm²/time (same)
- RGR of A = 5/5 = 1.0; RGR of B = 5/50 = 0.1 (A is more efficient)

Conclusion: Relative growth rate is a better measure of growth efficiency.

Five Main Groups of Natural Plant Growth Regulators (PGRs):
1. Auxins
2. Gibberellins
3. Cytokinins
4. Abscisic Acid (ABA)
5. Ethylene

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Note on Gibberellins:

Discovery:
Gibberellins were discovered by Japanese scientist E. Kurosawa in 1926 while studying the 'bakanae' (foolish seedling) disease of rice caused by the fungus *Gibberella fujikuroi*. Infected rice seedlings grew abnormally tall. The active substance was isolated and named gibberellin (GA). Later, more than 100 types of gibberellins (GA₁, GA₂, GA₃, etc.) were identified from both fungi and higher plants. GA₃ (gibberellic acid) is the most studied.

Physiological Functions:
1. Stem elongation: Gibberellins promote cell elongation and cell division, causing internodal elongation. They are responsible for 'bolting' in rosette plants.
2. Fruit development: They promote fruit growth (parthenocarpy) in some plants.
3. Seed germination: They break seed dormancy and promote germination by stimulating the synthesis of hydrolytic enzymes (e.g., α-amylase) in aleurone layer of cereal grains.
4. Flowering: They induce flowering in long-day plants even under short-day conditions.
5. Delay of senescence: They delay senescence in leaves and fruits.
6. Sex determination: They promote maleness in dioecious plants.

Agricultural/Horticultural Applications:
1. Sugarcane: Spraying GA on sugarcane increases the length of the stem, thereby increasing the yield of sugar by up to 20 tonnes per acre.
2. Malting industry: GA₃ is used to speed up the malting process in brewing industry by promoting α-amylase production in barley seeds.
3. Fruit production: Used to produce seedless (parthenocarpic) fruits in grapes, tomatoes, etc.
4. Bolting: Used to induce early flowering and elongation in rosette plants like cabbage, beet.
5. Delay of senescence: Used to delay ripening and extend shelf life of fruits.

Abscisic Acid (ABA) as Stress Hormone:

Abscisic acid (ABA) is called the stress hormone because it plays a crucial role in helping plants respond to various environmental stresses. The reasons are:

1. Stomatal closure: Under water stress (drought), ABA accumulates in leaves and induces the closure of stomata by causing loss of turgor in guard cells. This reduces water loss through transpiration and helps the plant survive drought conditions.

2. Seed dormancy: ABA induces dormancy in seeds, helping them withstand desiccation (drying out) and other unfavourable conditions. It prevents premature germination.

3. Bud dormancy: ABA promotes dormancy in buds during unfavourable seasons (winter), protecting the plant from cold and desiccation.

4. Response to other stresses: ABA also helps plants respond to other abiotic stresses such as cold, heat, salinity, and wounding by regulating gene expression related to stress tolerance.

5. Abscission: ABA promotes abscission of leaves, fruits, and flowers under stress conditions.

Conclusion: Since ABA is produced in response to various environmental stresses and helps the plant overcome them, it is appropriately called the stress hormone. It generally acts as a growth inhibitor and is antagonistic to gibberellins.

Concept:
In higher plants, growth and differentiation are said to be 'open' because they are not fixed or predetermined — they can continue throughout the life of the plant and are influenced by the environment and developmental signals.

Growth is open:
- In higher plants, the meristems (apical, intercalary, lateral) are present throughout the life of the plant and continue to produce new cells indefinitely.
- Unlike animals (where growth is determinate/closed and stops after a certain age), plants can grow continuously as long as they are alive.
- New organs (leaves, branches, flowers, roots) are continuously formed.
- This is called indeterminate or open growth.

Differentiation is open:
- In plants, differentiation is also open because the cells retain the capacity to dedifferentiate (revert to meristematic state) and then redifferentiate into new cell types.
- A differentiated cell can become meristematic again under appropriate conditions (e.g., wound healing, tissue culture).
- This property is called totipotency — every living cell has the genetic potential to develop into a complete organism.
- The developmental fate of a plant cell is not irreversibly fixed; it can change depending on signals from the environment or other cells.

Conclusion: Because both growth (through persistent meristems) and differentiation (through dedifferentiation and redifferentiation) are not permanently fixed and can occur throughout the plant's life, both are said to be open in higher plants. This gives plants great plasticity in development.

Concept — Photoperiodism:
Photoperiodism is the response of plants to the relative duration of light and dark periods (photoperiod) for flowering. Based on their photoperiodic requirement:
- Short Day Plants (SDP): Require a light period shorter than a critical day length (or dark period longer than a critical length) to flower. E.g., Chrysanthemum, tobacco.
- Long Day Plants (LDP): Require a light period longer than a critical day length (or dark period shorter than a critical length) to flower. E.g., wheat, spinach, henbane.

Explanation:
The critical day length is different for different plants. It is important to note that:
- A short day plant flowers when the day length is less than its critical photoperiod.
- A long day plant flowers when the day length is more than its critical photoperiod.

If the critical day length of an SDP is, say, 14 hours (flowers when day < 14 h) and the critical day length of an LDP is, say, 10 hours (flowers when day > 10 h), then on a day with 12 hours of light:
- The SDP will flower because 12 h < 14 h (its critical photoperiod).
- The LDP will also flower because 12 h > 10 h (its critical photoperiod).

Thus, both can flower simultaneously at the same place on the same day, provided the actual day length falls between the critical photoperiods of the two plants.

Conclusion: The terms 'short day' and 'long day' are relative, not absolute. What matters is whether the actual photoperiod is above or below each plant's own critical day length. Hence, both an SDP and an LDP can flower simultaneously at a given place.

(a) Induce rooting in a twig:
PGR used: Auxin (IBA — Indole Butyric Acid)

Auxins promote the initiation of roots in stem cuttings/twigs. IBA is the most widely used auxin for this purpose. When the base of a twig is treated with IBA, it stimulates adventitious root formation, helping the cutting to establish itself.

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(b) Quickly ripen a fruit:
PGR used: Ethylene

Ethylene is a gaseous PGR that promotes fruit ripening. It enhances the rate of respiration (climacteric rise) and activates enzymes that break down cell wall components, converting starch to sugars and changing colour and texture. Ethephon (which releases ethylene) is used commercially to ripen fruits quickly.

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(c) Delay leaf senescence:
PGR used: Cytokinin

Cytokinins delay senescence (ageing) of leaves by promoting protein synthesis and retarding the breakdown of chlorophyll. This phenomenon is called Richmond-Lang effect. They keep the leaves green and metabolically active for longer.

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(d) Induce growth in axillary buds:
PGR used: Cytokinin

Cytokinins promote the growth of axillary (lateral) buds by overcoming apical dominance (which is caused by auxins produced at the apical bud). Application of cytokinin to axillary buds stimulates their growth and branching.

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(e) 'Bolt' a rosette plant:
PGR used: Gibberellin (GA₃)

Bolting refers to the sudden elongation of the internodes of a rosette plant before flowering. Gibberellins promote rapid internode elongation (bolting) in rosette plants like cabbage, beet, and henbane, even without the required photoperiod or vernalisation.

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(f) Induce immediate stomatal closure in leaves:
PGR used: Abscisic Acid (ABA)

ABA is the stress hormone that induces rapid closure of stomata by causing loss of turgor in guard cells. It promotes the efflux of K⁺ ions from guard cells, leading to stomatal closure. This is the plant's response to water stress/drought.

Answer: No, a defoliated plant would NOT respond to the photoperiodic cycle.

Reason:

1. Site of photoperiodic perception: The perception of photoperiod (relative duration of light and dark) takes place in the leaves, not in the shoot apex or any other part of the plant.

2. Role of leaves: Leaves contain the photoreceptor pigment phytochrome (and possibly cryptochrome) that perceives the light signals. When the appropriate photoperiod is perceived, the leaves produce a flowering hormone called florigen (now identified as the protein product of the *FT* gene), which is translocated to the shoot apex to induce flowering.

3. Defoliated plant: If a plant is defoliated (all leaves removed), there is no organ left to perceive the photoperiodic stimulus. Even if the plant is exposed to the correct photoperiod, no florigen will be produced and the plant will not flower.

Experimental evidence: Classic experiments by Chailakhyan showed that:
- If only the leaves of a plant are exposed to the correct photoperiod (while the shoot apex is kept in darkness), the plant flowers.
- If only the shoot apex is exposed to the correct photoperiod (while leaves are covered), the plant does not flower.

Conclusion: Since leaves are the organs that perceive the photoperiodic signal and produce the flowering stimulus, a defoliated plant cannot respond to the photoperiodic cycle and will not flower.

(a) GA₃ is applied to rice seedlings:

Expected result: The rice seedlings would show excessive elongation of the internodes, resulting in abnormally tall, spindly plants with pale green leaves. This condition is called 'bakanae' disease (foolish seedling disease).

Reason: GA₃ (gibberellic acid) promotes cell elongation and cell division in the internodal region. It was first discovered in the context of this disease caused by the fungus *Gibberella fujikuroi* in rice. The infected/GA₃-treated seedlings grow much taller than normal but are weak and eventually fall over and die without producing grain.

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(b) Dividing cells stop differentiating:

Expected result: The plant would show uncontrolled, abnormal growth — a condition similar to tumour or cancer in plants (e.g., crown gall disease).

Reason: Normal plant development requires a balance between cell division and differentiation. If dividing cells stop differentiating, they will keep dividing indefinitely without forming specialised tissues and organs. This would disrupt the normal organisation of the plant body. Organs like leaves, roots, flowers, and vascular tissue would not form properly, and the plant would not be able to carry out normal physiological functions.

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(c) A rotten fruit gets mixed with unripe fruits:

Expected result: The unripe fruits would ripen faster than normal.

Reason: A rotten/overripe fruit releases large amounts of ethylene gas. Ethylene is a gaseous plant growth regulator that promotes fruit ripening. When a rotten fruit is mixed with unripe fruits, the ethylene released from it diffuses to the surrounding unripe fruits and accelerates their ripening process. This is the basis of the traditional practice of keeping a ripe banana or apple with unripe fruits to hasten ripening. Commercially, ethephon (an ethylene-releasing compound) is used for the same purpose.

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(d) You forget to add cytokinin to the culture medium:

Expected result: In tissue culture, the callus would not differentiate into shoots; instead, it would continue to grow as an undifferentiated mass of cells, or roots may form preferentially.

Reason: In plant tissue culture, the ratio of auxin to cytokinin determines the type of differentiation:
- High auxin : low cytokinin → root formation
- Low auxin : high cytokinin → shoot formation
- Equal ratio → callus formation

If cytokinin is not added, the auxin-to-cytokinin ratio will be very high, favouring root formation or continued callus growth. Shoot differentiation (organogenesis) will not occur. The cells may also show senescence faster, as cytokinins are known to delay ageing and promote cell division.