Understanding plant growth and development is fundamental to mastering plant physiology, a key unit in NEET Biology. Chapter 13 introduces Class 11 students to the intricate physiological processes that drive plant development—from cell division and elongation to differentiation, maturation, and senescence. This chapter also explores the role of intrinsic factors like plant hormones and extrinsic factors such as light and temperature in influencing growth patterns.
As part of Unit IV: Plant Physiology, this chapter has moderate NEET weightage, especially with questions on plant hormones (PGRs), photoperiodism, vernalisation, and growth curves. Clear conceptual understanding and the ability to apply these principles in objective NEET-style questions are critical for scoring in this section.
Overview of Chapter 13 Based on NCERT Structure
Chapter 13 in the NCERT textbook is divided into the following key topics:
- Growth
- Differentiation, Dedifferentiation, and Redifferentiation
- Development
- Plant Growth Regulators (PGRs)
- Photoperiodism
- Vernalisation
Each of these subtopics plays a critical role in explaining how plants grow and adapt to their surroundings. NEET often tests these as standalone or integrated concepts. The chapter is especially rich in terminology and process-based mechanisms, making diagrams and flowcharts useful for last-minute revision.
Growth
Growth in biological terms refers to an irreversible permanent increase in size, volume, and dry mass of an organism. In plants, growth is typically indeterminate, meaning they can grow throughout their life. This is because of the presence of meristematic tissues at specific regions like root and shoot tips.
Characteristics of Plant Growth:
- Irreversibility: Once a cell expands or divides, it cannot revert to its previous state.
- Quantitative: Growth can be measured in terms of parameters like length, surface area, volume, and dry weight.
Phases of Growth:
Plant growth is typically divided into three distinct but overlapping phases:
- Meristematic Phase:
- Occurs in apical meristems at root and shoot tips.
- Cells divide actively, have dense cytoplasm, large nuclei, and thin cell walls.
- Elongation Phase:
- Cells increase in size, especially in length.
- Vacuole formation, new cell wall deposition, and enhanced metabolic activity occur.
- Maturation Phase:
- Cells attain maturity and structural specificity.
- Cell enlargement ceases; tissues become functionally specialized.
Growth Rate:
Growth can be measured in terms of rate, which can be:
- Arithmetic Growth: One daughter cell continues to divide, the other differentiates (e.g., root elongation). Represented as:
Lt = L₀ + rt
Where Lt = length at time t, L₀ = initial length, r = rate of growth - Geometric Growth: Both daughter cells continue to divide. Results in exponential growth.
Represented as: W₁ = W₀eʳᵗ
Where W₁ = final size, W₀ = initial size, r = growth rate, t = time, e = base of natural logarithm
Growth Curve:
- When growth rate is plotted against time, it results in a sigmoid (S-shaped) curve with three phases:
- Lag Phase: Slow growth
- Log Phase: Rapid, exponential growth
- Stationary Phase: Growth rate slows due to limited resources
Conditions for Growth:
- Water: For turgidity and enzymatic activity
- Oxygen: For cellular respiration and ATP production
- Nutrients: Both macronutrients and micronutrients
- Light and Temperature: Affect enzyme activity, photosynthesis, and hormone regulation
NEET Focus:
- Graph-based questions on growth curves
- Numerical problems involving arithmetic and geometric growth models
- Questions on meristem location and function
Differentiation, Dedifferentiation, and Redifferentiation
Differentiation
Differentiation is the process by which meristematic cells become structurally and functionally specialized. As cells divide, they begin to assume specific shapes and roles (e.g., xylem, phloem, parenchyma). It is a genetically programmed developmental process.
- For instance, procambial cells differentiate into elements like tracheids and vessel elements.
- Differentiation involves changes in protoplasmic content and cell wall composition.
Dedifferentiation
Dedifferentiation is the phenomenon in which mature, differentiated cells regain their ability to divide under certain conditions. These cells behave like meristematic cells again and contribute to secondary growth.
- Example: Interfascicular cambium and cork cambium arise from permanent tissues.
- It is a reversible process and crucial for regenerative abilities in plants.
Redifferentiation
Redifferentiation is the process in which dedifferentiated cells again lose their capacity to divide and become specialized in function.
- Example: Cells from dedifferentiated cork cambium forming cork cells.
These three processes show that plant cells retain the ability to switch roles depending on developmental needs and external stimuli.
NEET Focus:
- Conceptual questions involving the cyclic nature of cell specialization
- Examples of cambium formation
- Distinction between primary and secondary growth processes
Development
Development is defined as a sequence of events that results in the formation, growth, and differentiation of cells and organs. In plants, development is a highly plastic process, meaning it can change depending on internal signals (like hormones) and external factors (like light or temperature).
Plant Development = Growth + Differentiation
Every organism goes through a developmental journey from zygote to maturity, and in plants, this journey includes a variety of morphogenetic processes:
- Cell division
- Cell enlargement
- Cell differentiation
- Tissue and organ development
- Reproductive maturity
Plasticity in Plant Development
Plasticity refers to the plant’s ability to modify its growth or development in response to environmental stimuli. For instance:
- Heterophylly in cotton, coriander, and larkspur — the same plant produces different types of leaves based on environmental conditions or stages of development.
This highlights the dynamic and adaptable nature of plant development, which contrasts with the more rigid patterns observed in animals.
Types of Developmental Responses:
- Intrinsic regulation: Controlled by genetic programming and hormones
- Extrinsic factors: Environmental cues like photoperiod, temperature, water availability, etc.
NEET Focus:
- Understanding plant plasticity
- Terminology like growth, development, and differentiation
- Differences between growth and development
- Real-life examples like heterophylly for concept application
Plant Growth Regulators (PGRs)
Plant Growth Regulators (PGRs) are chemical messengers or hormones that significantly influence various aspects of plant growth and development. These substances are often active in very low concentrations and can act promotively or inhibitorily depending on the physiological context.
Characteristics of PGRs:
- They can be naturally occurring or synthetically produced.
- PGRs regulate physiological processes such as cell division, cell elongation, flowering, seed dormancy, and senescence.
- They may act synergistically (promoting together) or antagonistically (opposing each other).
Major Classes of Plant Growth Regulators:
1. Auxins – Growth Promoters
- First PGR discovered; Indole-3-acetic acid (IAA) is a natural auxin.
- Functions:
- Cell elongation (especially in shoots)
- Apical dominance
- Root initiation
- Prevents premature fruit and leaf drop
- Used in horticulture for parthenocarpy (seedless fruits)
- Example: NAA, IBA (synthetic auxins)
2. Gibberellins – Growth Promoters
- Discovered from the fungus Gibberella fujikuroi.
- Functions:
- Stem elongation (overcomes dwarfism)
- Breaks seed dormancy
- Promotes bolting in rosette plants (e.g., cabbage)
- Induces parthenocarpy
- Over 100 types identified (e.g., GA₁, GA₃)
3. Cytokinins – Growth Promoters
- Promote cell division and delay senescence.
- Found in coconut milk and corn kernels.
- Functions:
- Promote lateral bud growth (break apical dominance)
- Delay leaf aging
- Used in tissue culture to promote shoot proliferation
- Examples: Zeatin, Kinetin (natural and synthetic)
4. Abscisic Acid (ABA) – Growth Inhibitor
- Called the stress hormone.
- Functions:
- Induces seed and bud dormancy
- Promotes stomatal closure during water stress
- Promotes leaf abscission and senescence
5. Ethylene – Gaseous Growth Regulator
- The only gaseous PGR.
- Functions:
- Stimulates fruit ripening (used commercially)
- Promotes senescence and abscission of plant parts
- Enhances respiration rate during ripening (climacteric)
- Breaks seed dormancy
- Example: Ethephon is a commercially used ethylene-releasing compound
PGR Interactions:
- Auxin + Cytokinin = balanced growth in tissue culture
- Gibberellin vs. ABA = dormancy vs. germination
NEET Focus:
- Matching-type questions on PGRs and their functions
- Assertion-Reasoning questions on PGR interactions
- Real-life applications (e.g., bolting, parthenocarpy)
Photoperiodism
Photoperiodism refers to the physiological reaction of organisms (particularly plants) to the relative lengths of day and night. This environmental cue is vital for initiating flowering and other seasonal responses.
Discovery
- First observed by Garner and Allard (1920) while working on Maryland Mammoth tobacco.
- It flowered only during short days.
Categories of Plants Based on Photoperiodic Response:
- Short Day Plants (SDP): Flower when day length is shorter than a critical period.
- e.g., Rice, Dahlia, Xanthium
- Long Day Plants (LDP): Flower when day length exceeds a critical period.
- e.g., Pea, Wheat, Radish
- Day-Neutral Plants (DNP): Flowering is not affected by day length.
- e.g., Tomato, Cotton, Sunflower
Critical Day Length
- Defined as the maximum or minimum duration of light required to induce flowering.
- Specific to each plant species.
Florigen Hypothesis
- Flowering hormone called florigen is believed to be responsible.
- Synthesized in leaves and transported to floral buds.
Site of Perception
- Leaves perceive the photoperiod.
- Signals are transmitted to shoot apices to initiate flowering.
Importance in Agriculture
- Helps in manipulating flowering time for commercial cropping.
- Controlled light exposure can induce off-season flowering in horticulture.
NEET Focus:
- Examples of SDPs, LDPs, and DNPs
- Florigen hypothesis
- Role of leaves in photoperiod perception
- Assertion-Reason questions and matching-based MCQs
Vernalisation
Vernalisation is the acceleration of flowering in plants by exposure to low temperatures. This cold treatment is a crucial environmental trigger, especially for plants that require winter chilling before they can flower during spring.
Definition:
- Vernalisation is the process of inducing flowering in plants by subjecting them to prolonged exposure to cold temperatures.
- This is especially common in biennial and perennial plants.
Examples of Vernalised Plants:
- Wheat, barley, rye, cabbage, carrots, and sugar beet
Sites of Vernalisation:
- The meristematic tissues (like shoot tips or embryos) are the primary sites of perception.
- Seeds or seedlings are commonly treated for vernalisation.
Importance in Agriculture:
- Promotes early flowering and fruiting in biennial crops.
- Prevents bolting in leafy vegetables like cabbage.
- Helps to synchronize flowering in large-scale crop production.
Vernalin Hypothesis:
- A hypothetical hormone called vernalin is believed to be produced during cold treatment and later induces flowering.
- Though not yet chemically identified, this theory is supported by experimental evidence.
Vernalisation vs. Photoperiodism:
| Feature | Vernalisation | Photoperiodism |
| Stimulus | Low temperature | Day length (light period) |
| Site of action | Meristematic regions | Leaves |
| Example | Wheat, Barley | Xanthium, Pea |
NEET Focus:
- Definition and role of vernalisation
- Vernalin and site of perception
- Application-based questions in agriculture
- Comparative reasoning between vernalisation and photoperiodism
NEET Illustrative Questions
- Which plant hormone is known as the “stress hormone”?
- (a) Auxin
- (b) Gibberellin
- (c) Abscisic acid
- (d) Cytokinin
Answer: (c)
- The sigmoid growth curve has:
- (a) Lag phase, log phase, stationary phase
- (b) Only log phase
- (c) Lag phase and stationary phase
- (d) Exponential phase only
Answer: (a)
- Photoperiodism is perceived by:
- (a) Flowers
- (b) Roots
- (c) Leaves
- (d) Stems
Answer: (c)
- The hormone used to delay senescence in leafy vegetables is:
- (a) Ethylene
- (b) Cytokinin
- (c) ABA
- (d) Gibberellin
Answer: (b)
- Vernalisation leads to:
- (a) Delay in flowering
- (b) Early flowering
- (c) Inhibition of bolting
- (d) Dormancy
Answer: (b)
FAQs on Plant Growth and Development
1. What is the significance of plant growth regulators for NEET?
Plant growth regulators are frequently tested in NEET due to their physiological and agricultural importance. Focus on their discovery, functions, and effects on plant tissues.
2. How can I remember which plant responds to which photoperiod?
Use mnemonic devices like “SDR” (Short Day – Rice) and “LPR” (Long Day – Pea, Radish) to associate plants with their flowering responses.
3. What is the difference between growth and development in plants?
Growth is quantitative (e.g., increase in size or mass), whereas development includes all qualitative changes like cell differentiation and organ formation.
4. Are the topics of photoperiodism and vernalisation important for NEET?
Yes, NEET frequently includes conceptual and assertion-reason questions from these areas. Diagrams and charts enhance understanding.
5. How do auxins promote apical dominance?
Auxins produced at the apical bud inhibit the growth of lateral buds, thus promoting apical dominance—a favorite NEET MCQ concept.
Conclusion
Chapter 13: Plant Growth and Development provides a comprehensive understanding of how plants grow, differentiate, and respond to environmental stimuli. NEET aspirants must pay attention to plant hormones, growth curves, and environmental factors like light and temperature. Questions from this chapter often test your application skills and clarity in biological terminology. Make sure to revise definitions, examples, and hormonal effects systematically to score high in NEET’s plant physiology segment.
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