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Plant Growth and Development Exercise Solutions | KalviGavani

Samacheer Kalvi 11th Botany Chapter 15 Plant Growth and Development exercise solutions — growth phases, plant hormones, photoperiodism, vernalisation, MCQs, full answers.

Updated 15 June 202614 min read

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Step-by-step answers for all textbook questions in this chapter

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How to Use This Page

Solve each Samacheer Kalvi exercise question on your own first, then compare your answer with the solutions below. Plant Growth and Development covers growth phases, phytohormones, photoperiodism, and vernalisation — focus on the functions of each hormone and the conditions for flowering.

Key Concepts (Quick Recap)

Growth

  • Definition: Irreversible increase in size, volume, and mass of a cell, organ, or organism
  • Growth is measurable: Height, weight, area, volume, cell number
  • Plant growth is indeterminate: Continues throughout life (due to meristems)
  • Animal growth is determinate: Stops at maturity

Phases of Growth

1. Lag phase: Slow growth (initial adjustment) 2. Log phase (Exponential/Geometric): Rapid growth 3. Stationary phase: Growth slows, plant matures

Sigmoid Growth Curve

Growth | | _________ Stationary phase | / | / Log phase (exponential) | / | / | / | / Lag phase | / | / | / |/___________________________ Time

Types of Growth

  • Arithmetic growth: One daughter cell divides, other differentiates. Growth rate is constant. Example: Root elongation. Formula: Lt = L0 + rt
  • Geometric growth: Both daughter cells divide. Growth rate increases exponentially. Example: Early embryo. Formula: Wt = W0e^rt
  • Absolute growth rate: Total growth per unit time
  • Relative growth rate: Growth per unit initial size per unit time

Conditions for Growth

1. Water (turgor pressure for cell expansion) 2. Oxygen (for respiration and energy) 3. Suitable temperature (enzyme activity) 4. Nutrients (minerals and organic nutrients) 5. Light (for photosynthesis and photomorphogenesis)

Plant Growth Regulators (Phytohormones)

HormoneDiscoveryFunctionsExamples/Applications
Auxin (IAA)Went (1928) — coleoptile experimentCell elongation, apical dominance, root initiation, tropisms, fruit developmentUsed in rooting powders, parthenocarpy, weed control (2,4-D)
Gibberellin (GA)Kurosawa (1926) — foolish seedling diseaseStem elongation, seed germination, fruit growth, bolting in rosettesUsed to increase sugarcane yield, increase grape size, break dormancy
CytokininSkoog and Miller (1955) — coconut milkCell division, delay senescence, shoot initiation, lateral bud growthUsed in tissue culture, keeping cut flowers fresh
EthyleneNiejubov (1901) — fruit ripeningFruit ripening, senescence, abscission, stress responseUsed in fruit ripening, rubber latex production
Abscisic Acid (ABA)Addicott (1963) — abscissionSeed dormancy, stress response, stomatal closure, growth inhibitionCalled the stress hormone; helps plants survive drought

Auxin Details

  • Natural auxin: IAA (Indole-3-Acetic Acid)
  • Synthetic auxins: NAA (Naphthalene Acetic Acid), 2,4-D (2,4-Dichlorophenoxyacetic acid), IBA
  • Discovery: Charles Darwin and Francis Darwin (1880) — phototropism in canary grass coleoptile; Frits Went (1928) — isolated auxin from oat coleoptile
  • Functions:
  • Promotes cell elongation (acid growth hypothesis)
  • Maintains apical dominance (suppresses lateral bud growth)
  • Promotes root initiation in cuttings
  • Controls tropisms (phototropism, geotropism)
  • Promotes parthenocarpy (seedless fruit)
  • Used as a selective weed killer (2,4-D kills dicot weeds in monocot fields)

Photoperiodism

  • Definition: Response of plants to the duration of light and dark periods for flowering
  • Discovered by: Garner and Allard (1920) — Maryland Mammoth tobacco
TypeConditionExamples
Long-day plants (LDP)Flower when day length exceeds critical periodSpinach, Radish, Sugar beet, Wheat
Short-day plants (SDP)Flower when day length is less than critical periodChrysanthemum, Poinsettia, Soybean, Tobacco
Day-neutral plants (DNP)Flower regardless of day lengthTomato, Cotton, Sunflower, Cucumber
  • Critical point: It is actually the dark period (not the light period) that is critical for flowering
  • LDP: Short dark period (long light)
  • SDP: Long dark period (short light)
  • Interrupting the dark period with a flash of light: inhibits flowering in SDP, promotes flowering in LDP

Vernalisation

  • Definition: Low-temperature treatment (0–5°C for weeks) to induce flowering
  • Discovered by: Lysenko (1928)
  • Significance:
  • Ensures flowering occurs in spring (after winter cold), not in autumn
  • Prevents precocious reproductive development
  • Important for biennial plants that flower in the second year
  • Example: Winter wheat, rye, sugar beet, cabbage, carrot

Senescence

  • Definition: The process of ageing in plants — deterioration of structure and function
  • Types: Whole plant senescence, organ senescence (leaf fall), cellular senescence
  • Hormonal control: Ethylene and ABA promote senescence; Cytokinin delays senescence

I. Choose the Best Answer (MCQs)

Q1. Apical dominance is caused by (a) Gibberellin (b) Cytokinin (c) Auxin (d) Ethylene

Answer(c) Auxin — Apical dominance is the suppression of lateral bud growth by the apical bud. Auxin produced by the apical bud inhibits the growth of lateral buds. Removing the apical bud (decapitation) removes the auxin source, allowing lateral buds to grow.

Q2. The hormone that delays senescence is (a) Auxin (b) Gibberellin (c) Cytokinin (d) Ethylene

Answer(c) Cytokinin — Cytokinin delays senescence (ageing) in leaves and flowers. It promotes cell division and maintains the vitality of plant tissues. This is why cytokinin is used to keep cut flowers fresh.

Q3. Short-day plants flower when the day length is (a) Longer than critical period (b) Shorter than critical period (c) Any day length (d) Exactly 12 hours

Answer(b) Shorter than critical period — Short-day plants (SDP) flower when the day length is shorter than the critical period (and the dark period is longer than the critical dark period). Examples: Chrysanthemum, Poinsettia, Soybean.

Q4. Vernalisation is the treatment of plants with (a) High temperature (b) Low temperature (c) Light (d) Hormones

Answer(b) Low temperature — Vernalisation is the low-temperature treatment (0–5°C) of plants to induce flowering. It ensures that flowering occurs in spring after the cold of winter.

Q5. The plant hormone that promotes fruit ripening is (a) Auxin (b) Gibberellin (c) Cytokinin (d) Ethylene

Answer(d) Ethylene — Ethylene is the gaseous plant hormone that promotes fruit ripening, senescence, and abscission. It is commercially used to ripen fruits like bananas and mangoes.

II. Fill in the Blanks

1. The growth curve of plants is sigmoid (S-shaped). 2. Plant growth is indeterminate (continues throughout life). 3. Animal growth is determinate (stops at maturity). 4. The hormone responsible for apical dominance is auxin. 5. The hormone that delays senescence is cytokinin. 6. The hormone that promotes fruit ripening is ethylene. 7. ABA is also called the stress hormone. 8. Gibberellin was discovered from the foolish seedling disease of rice (caused by Gibberella fujikuroi). 9. Long-day plants flower when the day length is longer than the critical period. 10. Short-day plants flower when the dark period is longer than the critical dark period. 11. Vernalisation requires low temperature (0–5°C). 12. The site of perception of photoperiod is the leaves. 13. 2,4-D is a synthetic auxin used as a weed killer. 14. The absolute growth rate measures total growth per unit time. 15. The relative growth rate measures growth per unit initial size per unit time.

III. True or False (with corrections for false statements)

Q1. Plant growth is determinate.
AnswerFalse — Correction: Plant growth is INDETERMINATE — it continues throughout the plant's life due to the presence of meristematic tissues. Animal growth is determinate (stops at maturity).
Q2. Cytokinin promotes cell elongation.
AnswerFalse — Correction: Cytokinin promotes CELL DIVISION, not cell elongation. Auxin promotes cell elongation.
Q3. Ethylene is a gaseous plant hormone.
AnswerTrue — Ethylene (C₂H₄) is the only gaseous plant hormone. It diffuses through air and promotes fruit ripening, senescence, and abscission.
Q4. Short-day plants require a long dark period for flowering.
AnswerTrue — Short-day plants flower when the dark period exceeds the critical length. The dark period is actually the critical factor, not the light period.
Q5. Vernalisation promotes flowering by treating plants with high temperature.
AnswerFalse — Correction: Vernalisation promotes flowering by treating plants with LOW temperature (0–5°C for several weeks). High temperature treatment is not vernalisation.

IV. Match the Following

Column AColumn BAnswer
1. Auxin(a) Delays senescence(b) Apical dominance, cell elongation
2. Gibberellin(b) Apical dominance(c) Stem elongation, seed germination
3. Cytokinin(c) Stem elongation(a) Cell division, delays senescence
4. Ethylene(d) Stress hormone, dormancy(e) Fruit ripening, senescence
5. ABA(e) Fruit ripening(d) Stress hormone, dormancy

V. Answer Briefly (2-mark questions)

Q1. What are phytohormones? Name the five major groups.
Answer

Phytohormones (plant growth regulators) are naturally occurring organic substances that regulate plant growth and development in very small concentrations. The five major groups are: 1. Auxins (e.g., IAA) — cell elongation, apical dominance 2. Gibberellins (e.g., GA₃) — stem elongation, seed germination 3. Cytokinins (e.g., Zeatin) — cell division, delays senescence 4. Ethylene — fruit ripening, senescence 5. Abscisic Acid (ABA) — dormancy, stress response

Q2. Distinguish between arithmetic and geometric growth.
Answer

| Feature | Arithmetic Growth | Geometric Growth | |---|---|---| | Growth rate | Constant | Increases exponentially | | Cell division | One daughter cell divides, other differentiates | Both daughter cells divide | | Example | Root elongation | Early embryo development | | Formula | Lt = L0 + rt | Wt = W0e^rt | | Graph | Linear (straight line) | Exponential (J-shaped) |

Q3. What is photoperiodism? Name the three types of plants.
Answer

Photoperiodism is the response of plants to the duration of light and dark periods for flowering. It was discovered by Garner and Allard (1920). Three types: 1. Long-day plants (LDP): Flower when day length exceeds critical period — e.g., Spinach, Radish 2. Short-day plants (SDP): Flower when day length is less than critical period — e.g., Chrysanthemum, Soybean 3. Day-neutral plants (DNP): Flower regardless of day length — e.g., Tomato, Cotton

Q4. What is vernalisation? Why is it important?
Answer

Vernalisation is the low-temperature treatment (0–5°C for several weeks) of plants to induce flowering. Importance: 1. Ensures flowering occurs in spring after winter cold, not in autumn 2. Prevents precocious reproductive development before the plant is mature enough 3. Important for biennial plants (cabbage, carrot) that need cold to flower in the second year 4. Helps in crop improvement by synchronising flowering

VI. Answer in Detail (5-mark questions)

Q1. Describe the five major plant growth regulators with their functions.
Answer

1. Auxins: - Natural: IAA (Indole-3-Acetic Acid); Synthetic: NAA, 2,4-D, IBA - Discovery: Darwin (1880) — phototropism; Went (1928) — isolated auxin - Functions: Cell elongation, apical dominance, root initiation, tropisms (phototropism, geotropism), parthenocarpy, fruit development - Applications: Rooting of cuttings, weed control (2,4-D), preventing premature fruit drop 2. Gibberellins: - Discovery: Kurosawa (1926) — foolish seedling disease of rice (Bakanae disease) caused by Gibberella fujikuroi - Functions: Stem elongation (internode elongation), seed germination (breaks dormancy), fruit growth, bolting in rosette plants - Applications: Increasing sugarcane yield, increasing grape size (GA₃), malting in brewing industry 3. Cytokinins: - Discovery: Skoog and Miller (1955) — coconut milk factor - Functions: Cell division (cytokinesis), shoot initiation, delays senescence, promotes lateral bud growth (antagonistic to auxin) - Applications: Tissue culture (shoot proliferation), keeping cut flowers fresh, increasing shelf life of vegetables 4. Ethylene: - Discovery: Niejubov (1901) — identified as a gaseous hormone from fruit ripening - Functions: Fruit ripening, senescence, abscission (leaf and fruit fall), stress response, promotes female flowers in cucumber - Applications: Artificial fruit ripening (bananas, mangoes), rubber latex production, synchronising flowering in pineapple 5. Abscisic Acid (ABA): - Discovery: Addicott (1963) — isolated from cotton bolls - Functions: Seed dormancy, stomatal closure (water stress), growth inhibition, stress response, abscission - Applications: Called the "stress hormone"; helps plants survive drought by closing stomata

Q2. Explain photoperiodism with examples.
Answer

Photoperiodism is the physiological response of plants to the duration of light and dark periods for flowering. It was discovered by Garner and Allard (1920) using Maryland Mammoth tobacco. Critical Dark Period: - The dark period (not the light period) is the critical factor for flowering - SDP need a long dark period; LDP need a short dark period - If the dark period is interrupted by a flash of light: SDP will NOT flower; LDP WILL flower Long-Day Plants (LDP): - Flower when the day length exceeds the critical photoperiod - Need a short dark period - Examples: Spinach, Radish, Sugar beet, Wheat, Lettuce - Flower in summer (long days) Short-Day Plants (SDP): - Flower when the day length is less than the critical photoperiod - Need a long dark period - Examples: Chrysanthemum, Poinsettia, Soybean, Tobacco (Maryland Mammoth) - Flower in autumn/winter (short days) Day-Neutral Plants (DNP): - Flower regardless of day length - Not affected by photoperiod - Examples: Tomato, Cotton, Sunflower, Cucumber, Maize Site of Perception: - The leaves perceive the photoperiod - When the correct photoperiod is perceived, the leaves produce a chemical signal (florigen) that travels to the shoot apex and induces flowering

Q3. What is vernalisation? Explain its significance.
Answer

Vernalisation is the low-temperature treatment (0–5°C) of plants to induce flowering. It was discovered by Lysenko (1928). Process: 1. Seeds or seedlings are exposed to low temperatures (0–5°C) for several weeks 2. The cold treatment converts the winter variety into a spring variety 3. After vernalisation, the plant flowers when favourable conditions return Significance: 1. Ensures flowering in spring: Plants flower only after experiencing the cold of winter, preventing precocious flowering in autumn 2. Prevents reproductive failure: If a biennial plant were to flower in autumn, the seeds might not survive winter 3. Crop improvement: Vernalisation can be used to induce early flowering in crops, reducing the life cycle 4. Biennial to annual conversion: Some biennial plants can be made to flower in one year through vernalisation Examples of plants requiring vernalisation: - Winter wheat, rye (cereal crops) - Cabbage, carrot, beet (biennial vegetables) - Sugar beet Devernalisation: - The reversal of vernalisation by high temperatures (above 30°C) after the cold treatment - Once devernalised, the plant needs to undergo vernalisation again to flower

Common Mistakes to Avoid

  • Saying plant growth is determinate: Plant growth is INDETERMINATE (continues throughout life). Animal growth is determinate (stops at maturity).
  • Confusing auxin and cytokinin functions: Auxin promotes cell ELONGATION and apical dominance. Cytokinin promotes cell DIVISION and delays senescence. They are antagonistic.
  • Confusing short-day and long-day plants: Short-day plants need LONG nights (short days). Long-day plants need SHORT nights (long days). It's actually the dark period that matters.
  • Saying ethylene is a solid hormone: Ethylene is a GAS (C₂H₄). It is the only gaseous plant hormone. It diffuses through air.
  • Forgetting that the dark period is critical for photoperiodism: The dark period, not the light period, is the critical factor. Interrupting the dark period with a flash of light prevents flowering in SDP but promotes it in LDP.
  • Confusing ABA and ethylene: ABA is the stress hormone (drought, dormancy). Ethylene is the ripening hormone (fruit ripening, senescence).
  • Saying vernalisation uses high temperature: Vernalisation uses LOW temperature (0–5°C). High temperature after vernalisation causes devernalisation.

Exam Checklist

  • [ ] Can you define growth and distinguish between arithmetic and geometric growth?
  • [ ] Can you draw and explain the sigmoid growth curve?
  • [ ] Can you list the five phytohormones and their functions?
  • [ ] Can you explain apical dominance and how it is controlled by auxin?
  • [ ] Can you describe the discovery of auxin (Darwin and Went)?
  • [ ] Can you explain photoperiodism with examples of LDP, SDP, and DNP?
  • [ ] Can you explain why the dark period is critical for flowering?
  • [ ] Can you explain vernalisation and its significance?
  • [ ] Can you define and calculate absolute and relative growth rates?
  • [ ] Can you explain the applications of each phytohormone in agriculture?

Frequently Asked Questions

The five major phytohormones are: (1) Auxin — promotes cell elongation, apical dominance; (2) Gibberellin — promotes stem elongation, seed germination; (3) Cytokinin — promotes cell division, delays senescence; (4) Ethylene — promotes fruit ripening, senescence; (5) Abscisic acid (ABA) — inhibits growth, promotes dormancy and stress responses.