Biology - Botany - How to Solve: Plant Growth and Development (Auxins, Gibberellins, Cytokinins, Photoperiodism, Vernalisation) – NEET UG Guide

How to Solve: Plant Growth and Development (Auxins, Gibberellins, Cytokinins, Photoperiodism, Vernalisation) – NEET UG Guide

Introduction

"Mastering plant hormones and growth responses can get you 4-6 marks in NEET UG—enough to push you into the top 1%! These concepts explain why your houseplants bend toward light, how farmers grow seedless grapes, and even how winter wheat survives freezing temperatures."

WHAT YOU NEED TO KNOW FIRST

1. Basic plant anatomy (shoot apex, root tip, vascular tissues).
2. Tropisms (phototropism, geotropism, hydrotropism).
3. Seed germination process (imbibition, enzyme activation, radicle emergence).

KEY TERMS & FORMULAS

1. Plant Hormones (Phytohormones)

2. Photoperiodism

• Definition: Plant response to day/night length (photoperiod).
• Key Terms:
• Short-Day Plants (SDP): Flower when night > critical length (e.g., chrysanthemum, rice).
• Long-Day Plants (LDP): Flower when night < critical length (e.g., spinach, wheat).
• Day-Neutral Plants (DNP): Flower regardless of photoperiod (e.g., tomato, cucumber).
• Formula (Critical Night Length):
• Flowering = Night Length > Critical Dark Period (for SDP)
• Flowering = Night Length < Critical Dark Period (for LDP)
• MEMORISE: It’s the night length, not day length, that matters!

3. Vernalisation

• Definition: Cold treatment required for flowering (e.g., winter wheat).
• Key Points:
• Site of Perception: Shoot apex (meristem).
• Duration: Weeks to months of low temperature (0–10°C).
• De-vernalisation: High temperature reverses the effect.
• MEMORISE: Vernalisation is obligate in some plants (e.g., cabbage) and facultative in others (e.g., winter wheat).

STEP-BY-STEP METHOD

Step 1: Identify the Hormone or Response

• Question Type 1: "Which hormone causes X effect?"
• Match the effect to the hormone (e.g., "cell elongation" → auxin).
• Question Type 2: "What is the role of Y hormone?"
• Recall its primary function (e.g., gibberellins → stem elongation).
• Question Type 3: "What happens if hormone Z is absent?"
• Predict the opposite effect (e.g., no auxin → no apical dominance → bushy plant).

Step 2: Apply Photoperiodism Rules

1. Check if the plant is SDP, LDP, or DNP.
2. Compare night length to critical dark period.
3. SDP: Night > critical → flowers.
4. LDP: Night < critical → flowers.
5. DNP: Ignore photoperiod.
6. If interrupted night (flash of light):
7. SDP: No flowering (night length resets).
8. LDP: Flowers (night length shortened).

Step 3: Solve Vernalisation Problems

1. Check if the plant requires cold treatment.
2. Winter wheat → yes.
3. Spring wheat → no.
4. Determine if vernalisation is obligate or facultative.
5. Obligate: Must have cold to flower.
6. Facultative: Flowers faster with cold but can flower without it.
7. Check for de-vernalisation.
8. High temp after cold → reverses effect.

Step 4: Combine Hormones for Tissue Culture Questions

• High auxin + low cytokinin → Root formation.
• Low auxin + high cytokinin → Shoot formation.
• Equal auxin + cytokinin → Callus (undifferentiated cells).

WORKED EXAMPLES

Example 1 – Basic (Hormone Identification)

Question: Which hormone is responsible for apical dominance in plants? Solution:
1. Identify the effect: Apical dominance = suppression of lateral buds.
2. Recall hormone functions: - Auxin → promotes apical dominance. - Cytokinin → promotes lateral bud growth (opposes auxin).
3. Conclusion: Auxin is responsible. Answer: Auxin (IAA). What we did and why: Matched the effect (apical dominance) to the correct hormone (auxin) using memory recall.

Example 2 – Medium (Photoperiodism)

Question: A short-day plant (SDP) has a critical night length of 10 hours. Will it flower if exposed to: a) 14 hours of light and 10 hours of dark? b) 12 hours of light and 12 hours of dark with a 1-minute flash of light in the middle of the night? Solution:
1. Part a: - Night length = 10 hours. - Critical night length = 10 hours. - SDP rule: Night ≥ critical → flowers. - Conclusion: Yes, it will flower.
2. Part b: - Night length = 12 hours, but interrupted by light. - SDP rule: Interrupted night → no flowering (night length resets). - Conclusion: No, it will not flower. Answer: a) Yes, b) No. What we did and why: Applied the SDP rule (night length ≥ critical) and accounted for night interruption (resets the clock).

Example 3 – Exam-Style (Vernalisation + Hormones)

Question: A farmer grows winter wheat in a region with mild winters. The plants fail to flower. Which of the following could explain this?
1. Lack of gibberellins.
2. Insufficient vernalisation.
3. Excess cytokinins.
4. Short-day photoperiod. Solution:
1. Winter wheat requires vernalisation (cold treatment) to flower.
2. Mild winters → insufficient cold → no vernalisation.
3. Check other options: - Gibberellins → promote stem elongation, not flowering in wheat. - Cytokinins → promote cell division, not flowering. - Photoperiod → wheat is day-neutral (not affected by day length).
4. Conclusion: Insufficient vernalisation is the correct answer. Answer: 2. Insufficient vernalisation. What we did and why: Eliminated incorrect options by recalling wheat’s specific requirements (vernalisation-dependent, day-neutral).

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP (Night Before Exam)

"Listen up! For plant hormones, remember: - Auxin = apical dominance + cell elongation. - Gibberellins = stem growth + seed germination. - Cytokinins = cell division + lateral buds. - Ethylene = fruit ripening + leaf fall. - ABA = stress hormone (closes stomata).

For photoperiodism: - Short-day plants (SDP) flower when night > critical length. - Long-day plants (LDP) flower when night < critical length. - Day-neutral plants don’t care about day length. - A flash of light at night resets the clock for SDP (no flowering).

For vernalisation: - Winter wheat needs cold to flower. - No cold → no flowering. - High temp after cold can reverse it (de-vernalisation).

Tissue culture? - High auxin + low cytokinin → roots. - Low auxin + high cytokinin → shoots. - Equal → callus.

Now go crush that NEET!