Plant Hormones (Phytohormones) – Long Answer Questions

Medium Level (Application & Explanation)

Q1. Explain how auxins control phototropism and gravitropism in plants. Use everyday examples to support your explanation.

Answer: Auxins are growth hormones that mainly cause cell elongation, especially in the tips of shoots and roots. In phototropism, when light comes from one side, auxins move to the shaded side of the stem. Cells on that side elongate faster, so the stem bends towards the light to maximize photosynthesis. In gravitropism, the distribution of auxins helps roots grow downward and shoots upward. Auxins accumulate differently in roots and shoots, which changes the rate of elongation and directs growth. You can observe phototropism by keeping a seedling in a box with a single hole—after a few days it bends toward the hole. This bending is because auxins collect on the darker side, pushing the plant to grow towards light. Thus, auxins make plants smart responders to light and gravity.

Q2. Describe three practical uses of auxins in agriculture and horticulture. Explain the logic behind each use.

Answer: Auxins are used by farmers and gardeners because they control rooting, fruit development, and fruit drop.

For rooting cuttings, dipping a stem cutting (like rose) in IBA solution helps form new roots faster. Auxins stimulate root initials at the cut surface, making propagation quick and reliable.
For fruit setting and seedless fruit (parthenocarpy), spraying auxins on flowers such as tomato can trigger the ovary to develop into fruit without pollination, leading to seedless produce that sells well.
To prevent premature fruit drop, auxin sprays on trees like apple strengthen the fruit stalk attachment, helping fruits stay on the plant until harvest. These uses reduce losses, improve quality, and increase profits by regulating growth at the right place and time.

Q3. What are gibberellins? Explain their major roles in plant growth and give three commercial applications.

Answer: Gibberellins are plant hormones that promote stem elongation and influence seed germination, flowering, and fruit growth. They can convert dwarf plants into tall forms by stimulating internode elongation. In seeds, they help break dormancy by activating enzymes that support germination. Commercially, gibberellins are very useful.

In seedless grapes, spraying gibberellins results in larger, well-spaced berries, making bunches more attractive and market-ready.
In barley malting for brewing, gibberellins speed up sprouting, which prepares grains for processing.
In sugarcane, farmers use gibberellins to increase stalk length, raising total yield. Thus, gibberellins improve plant height, fruit size, and processing quality. They directly translate into higher productivity and better quality in agriculture.

Q4. Explain the role of cytokinins in plant growth. How are they used in tissue culture and post-harvest storage?

Answer: Cytokinins promote cell division and stimulate the growth of shoots and buds. They also delay senescence (aging), keeping leaves green and active for longer. In nature, cytokinins are present in sources like coconut water. In tissue culture, cytokinins work with auxins to control organ formation: a higher cytokinin to auxin ratio promotes shoot formation, while a higher auxin to cytokinin ratio promotes root formation. This principle allows a single cell or tissue piece to regenerate an entire plant. Post-harvest, cytokinins are sprayed on leafy vegetables like lettuce to keep them fresh and green longer, reducing spoilage and improving shelf life. By controlling cell division and delaying aging, cytokinins are essential both in the lab for mass propagation and in the market for longer-lasting produce.

Q5. What is Abscisic Acid (ABA)? Discuss its functions in dormancy, drought response, and leaf fall with examples.

Answer: Abscisic Acid (ABA) is a plant hormone that acts as a growth inhibitor. It promotes dormancy in seeds and buds, helping plants survive unfavorable conditions. For example, wheat seeds have high ABA in winter, preventing early germination; they sprout when conditions improve. ABA is a key stress hormone during drought. It signals stomata to close, reducing water loss through transpiration and protecting the plant from wilting. This conserves precious H₂O, though it also reduces CO₂ entry and photosynthesis. ABA also helps trigger leaf fall (abscission) in autumn or dry conditions, reducing the plant’s water use. These actions ensure survival by saving water, timing germination, and shedding leaves when necessary. In agriculture, understanding ABA helps in seed storage, drought management, and preparing crops for seasonal changes.

High Complexity (Analytical & Scenario-Based)

Q6. Design an experiment to demonstrate phototropism in seedlings. Identify variables, controls, observations, and the role of auxins in your analysis.

Answer: Set up two identical seedlings (e.g., mustard). Keep one in uniform light (control). Place the other inside a box with a single side hole, so light enters from one direction. Keep water, soil, and temperature the same for both. The independent variable is light direction; the dependent variable is the direction of growth. Over 3–4 days, the control grows straight up, while the boxed seedling bends toward the hole. Analysis: In directional light, auxins move to the shaded side of the stem, causing faster cell elongation there. The unequal growth makes the stem curve toward light, showing positive phototropism. Possible errors include unequal watering, moving the box, or variable light intensity. This experiment proves that auxin redistribution underlies bending toward light to maximize photosynthesis.

Q7. A farmer’s wheat crop remains short and the apple orchard shows early fruit drop. Propose a hormone-based plan to solve both issues, explaining benefits and precautions.

Answer: To increase plant height in wheat, the farmer can apply gibberellins at appropriate stages to promote stem elongation. This can improve light capture and potentially increase yield. However, overuse may cause lodging (plants falling), so dosage and timing must be carefully controlled. For apple trees with premature fruit drop, the farmer can spray auxins to strengthen fruit attachment and delay abscission until harvest. This reduces losses and improves marketable yield. The plan should include:

Use gibberellins on wheat during vegetative growth.
Use auxin sprays on apples after fruit set to prevent early drop.
Monitor weather and plant condition to avoid stress interactions.
Avoid mixing hormones in one tank unless recommended, and follow label instructions. Together, this targeted approach improves both crop stature and fruit retention.

Q8. In a plant tissue culture lab, how would you use the auxin-cytokinin ratio to mass-produce plantlets? Explain steps, decisions, and expected outcomes.

Answer: Start with a sterile explant (leaf, stem, or meristem) placed on a nutrient medium. Use the auxin to cytokinin ratio to direct development.

For shoot multiplication, use a higher cytokinin level. Cytokinins promote cell division in shoot meristems, producing many shoots from a small tissue piece.
Once shoots form, transfer them to a medium with higher auxin to induce root formation. Auxins stimulate root initials, turning shoots into complete plantlets.
After robust roots develop, hardening is done by gradually moving plantlets to soil-like conditions with controlled humidity. This approach yields many genetically identical plants quickly. The same principle explains why coconut water (rich in cytokinins) can help growth. By tuning the hormone ratio, you choose whether the plant forms roots or shoots, enabling mass propagation of healthy, uniform plants.

Q9. A semi-arid region faces frequent droughts. Evaluate how ABA-mediated stomatal closure compares with irrigation and mulching in conserving water. Propose a balanced strategy.

Answer: ABA triggers stomatal closure, which reduces transpiration and conserves H₂O during drought. This protects plants from wilting but also lowers CO₂ uptake, which may reduce photosynthesis and yield if prolonged. Irrigation directly supplies water but may be limited or costly. Mulching reduces evaporation from soil, keeps roots cooler, and improves moisture retention without affecting gas exchange. A balanced plan could include:

Use of ABA-based treatments or stress-priming to prepare plants for short drought spells.
Mulching with straw or plastic to cut surface water loss.
Efficient irrigation (drip) timed for critical stages like flowering and fruit set.
Choice of drought-tolerant varieties and proper spacing to reduce competition. This integrated approach leverages ABA’s protective role while maintaining growth through better water management.

Q10. An orchard has three issues: apples drop early, grapes remain small and packed, and winter-sown vegetable seeds fail to sprout. Diagnose the hormone-related causes and suggest remedies.

Answer: These problems point to different hormone needs.

Apples dropping early: Low auxin levels at the fruit stalk can lead to abscission. Remedy: Apply auxin sprays after fruit set to strengthen attachment and prevent premature fruit drop.
Small, crowded grape berries: Insufficient gibberellin action leads to tight clusters. Remedy: Spray gibberellins during flowering to enlarge berries and increase spacing, often producing seedless and market-preferred bunches.
Winter-sown seeds not sprouting: High **A...