CBSE Class 11 Biology Chapter 15 Notes: Plant Growth And Development

Key Definitions

Growth

An irreversible, permanent increase in size, dry weight, or volume of a cell, organ, or whole organism.

Differentiation

The process by which cells originating from the meristem undergo changes in their structure and function to become specialized for specific roles.

Dedifferentiation

The process where differentiated cells lose their specialization and regain the capacity to divide, e.g., formation of interfascicular cambium.

Redifferentiation

The process where dedifferentiated cells further differentiate to form new cell types or tissues.

Plasticity

The ability of plants to follow different pathways in response to environmental changes or phases of life to form different kinds of structures (e.g., heterophylly).

Photoperiodism

The physiological response of plants to the relative lengths of day and night, primarily affecting flowering.

Vernalisation

The promotion of flowering by a period of low temperature treatment.

Seed Dormancy

A state in which seeds are prevented from germinating even under favourable environmental conditions, often due to internal factors.

Phases and Types of Plant Growth

Plant growth is a complex process encompassing increase in size, mass, and volume. It involves cell division, cell enlargement, and cell differentiation. Growth can be measured by various parameters like increase in fresh weight, dry weight, length, area, volume, and cell number.

Phases of Growth:

1. Meristematic Phase: Characterised by continuous cell division at the root and shoot apices. Cells are rich in protoplasm, have large nuclei, and lack vacuoles.
2. Elongation Phase: Cells proximal to the meristematic zone increase in size significantly through vacuolation and new cell wall material deposition.
3. Maturation Phase: Cells attain their maximum size in terms of wall thickening and protoplasmic modifications, leading to differentiation into specific tissues.

Growth Rates:

• Arithmetic Growth: Only one daughter cell continues to divide, while the other differentiates. A linear curve is obtained (Lₜ = L₀ + rt).
• Geometric Growth: Both daughter cells continue to divide. This leads to a slow initial growth, followed by a rapid exponential phase, and then a slowing phase due to limiting nutrients, forming a S-shaped (Sigmoid) curve. This is typical for most organisms and cells grown in vitro. The formula is W₁ = W₀eʳᵗ, where W₁ is final size, W₀ is initial size, r is relative growth rate, t is time, and e is base of natural logarithms. Relative Growth Rate (RGR) is the growth per unit initial size over a given period.

Overview of Plant Growth Regulators (PGRs)

Photoperiodism, Vernalisation, and Seed Dormancy

Beyond internal regulators, environmental cues significantly impact plant development.

Photoperiodism: The response of plants to the relative lengths of day and night. The site of perception of light/dark duration is the leaves. A hormonal substance (florigen) is then transmitted to the floral bud to initiate flowering.

• Short-Day Plants (SDP): Flower when day length is shorter than a critical photoperiod (e.g., Xanthium, soybean, tobacco). Require a continuous dark period.
• Long-Day Plants (LDP): Flower when day length is longer than a critical photoperiod (e.g., wheat, barley, spinach).
• Day-Neutral Plants (DNP): Flowering is unaffected by photoperiod (e.g., tomato, cucumber, corn).

Vernalisation: The phenomenon where plants require a period of low temperature for flowering. It prevents precocious reproductive development late in the growing season. The stimulus is perceived by the apical meristem. Examples: biennial plants (carrot, cabbage, sugarbeet) and some winter varieties of wheat and barley.

Seed Dormancy: A state of metabolic arrest where seeds fail to germinate even under favourable conditions. It is an adaptive feature that ensures germination occurs under optimal conditions for seedling survival.

Causes of Dormancy:

1. Impermeable and hard seed coat: Restricts water and oxygen uptake, and prevents embryo expansion.
2. Chemical inhibitors: Presence of abscisic acid (ABA), phenolic acids, para-ascorbic acid, etc., in seed coat or embryo.
3. Immature embryo: Embryo is not fully developed at the time of seed dispersal.

Methods to Overcome Dormancy:

• Scarification: Mechanical or chemical (acid) treatment to break the seed coat.
• Stratification: Treating seeds to moist cold conditions (mimicking winter) to break dormancy.
• Chemical treatments: Using gibberellic acid, nitrates, or ethylene.
• Leaching: Washing away inhibitory chemicals with running water.

Exam Tip: Differentiating PGR Functions

Students often confuse the specific roles of different Plant Growth Regulators. For exams, focus on one or two unique, hallmark functions for each PGR. For instance, auxins for apical dominance, gibberellins for stem elongation/bolting, cytokinins for cell division, ethylene for fruit ripening, and ABA for dormancy/stress response. Be ready to explain how these hormones interact (synergistically or antagonistically) for specific plant processes. For example, auxin promotes growth while ABA inhibits it. Also, remember specific plant examples for photoperiodism and vernalisation.

Key Points to Remember

• Growth is irreversible and can be arithmetic (linear) or geometric (sigmoid curve). Most growth in nature follows a sigmoid pattern.
• Differentiation, dedifferentiation, and redifferentiation are crucial for forming complex plant structures and in tissue culture.
• Plasticity allows plants to adapt their growth form to environmental conditions, exemplified by heterophylly in cotton, coriander, and larkspur.
• Auxin was the first PGR discovered (by F.W. Went) and is primarily involved in cell elongation and apical dominance.
• Gibberellins are widely used in agriculture to increase fruit size (e.g., grapes) and promote bolting in rosette plants.
• Cytokinins are essential for cell division and delaying senescence; they act antagonistically to auxins in apical dominance.
• Ethylene is the only gaseous hormone, critical for fruit ripening and abscission. It's often used commercially for ripening fruits.
• Abscisic Acid (ABA) acts as a general plant growth inhibitor and a stress hormone, promoting dormancy and stomatal closure.
• Photoperiodism and vernalisation are environmental controls on flowering, demonstrating plant sensitivity to light duration and temperature.
• Seed dormancy is an evolutionary adaptation; it can be broken by scarification, stratification, or specific chemical treatments.

Practice Questions with Solutions

• Q: Which plant growth regulator is primarily responsible for fruit ripening and is gaseous in nature? A: Ethylene.
• Q: Define photoperiodism and name a type of plant that flowers only when the day length is shorter than a critical duration. A: Photoperiodism is the physiological response of plants to the relative lengths of day and night. Short-Day Plants (SDP) flower when day length is shorter than a critical photoperiod (e.g., soybean).
• Q: What is the significance of seed dormancy, and how can it be overcome in seeds with a hard seed coat? A: Seed dormancy ensures germination occurs under optimal conditions. It can be overcome in seeds with hard coats by scarification (mechanical or chemical abrasion).
• Q: Name two functions of Gibberellins in plants. A: Two functions of Gibberellins are stem elongation (bolting) and promoting seed germination.

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