Plant Growth and Development — NCERT Solutions

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.

(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.

(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.

(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

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.