Plant Hormones (Phytohormones) – CBSE Class 10 Science

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1. Auxins

Elaboration:

• Auxins are plant hormones that mainly promote cell elongation.
• They are found mostly in the tips of stems and roots.
• Auxins also help plants respond to light (phototropism) and gravity (gravitropism).
  • In phototropism, auxins gather more on the shaded side of the stem. This side grows faster, making the plant bend towards light.
  • In gravitropism, roots grow downward due to how auxins are distributed.

Important Points:

• Auxins are used in plant propagation to help new roots grow from cuttings.
• Synthetic auxins like IAA, IBA, and NAA are manufactured for use in agriculture and gardening.

Examples:

1. Cutting Propagation: Dipping a rose cutting in IBA solution helps it form roots faster.
2. Fruit Setting: Spraying auxin on tomato flowers makes them develop into seedless fruits.
3. Preventing Fruit Drop: Farmers spray auxin on apple trees to keep apples from falling off too early.

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2. Gibberellins

Elaboration:

• Gibberellins help plants grow taller by stimulating stem elongation.
• They also help seeds germinate, flowers bloom, and fruits to set and grow.
• Gibberellins are naturally found in many plants but are also produced by fungi.

Important Points:

• Gibberellins can change dwarf (short) plants into tall plants if treated properly.
• They help in commercial agriculture by improving the quality and yield of some crops.

Examples:

1. Seedless Grapes: Spraying gibberellins on grape flowers makes the grapes larger and separate, becoming seedless.
2. Barley Malting: In beer brewing, gibberellins are used to speed up the sprouting of barley grains.
3. Tall Sugarcane: Farmers use gibberellins to make sugarcane stalks longer and increase yield.

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3. Cytokinins

Elaboration:

• Cytokinins promote cell division and stimulate growth of shoots and buds.
• They delay the aging (senescence) of leaves and help tissues stay fresh.
• Cytokinins work alongside auxins to control growth patterns in plant tissue culture.

Important Points:

• Cytokinins are naturally found in places like coconut water.
• They are useful in laboratories to grow many plants from small samples (tissue culture).

Examples:

1. Tissue Culture: In labs, cytokinins encourage shoot formation in plantlets from a single cell.
2. Keeping Lettuce Fresh: Spraying cytokinins on lettuce leaves keeps them green for longer in storage.
3. Determining Root or Shoot Growth: In tissue culture, the ratio of auxin to cytokinin decides if roots or shoots will grow.

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4. Abscisic Acid (ABA)

Elaboration:

• Abscisic Acid (ABA) slows down plant growth and promotes dormancy in seeds and buds.
• ABA makes leaves close their stomata to save water during drought, helping the plant survive.
• It also triggers leaf fall (abscission) as winter approaches or in dry conditions.

Important Points:

• ABA acts as a stress hormone in plants, especially when there is not enough water.
• It is important for timing when seeds sprout, avoiding early germination.

Examples:

1. Seed Dormancy: Wheat seeds have high ABA in winter to avoid sprouting too soon.
2. Water Conservation: During drought, ABA signals the plant to close stomata, preventing wilting.
3. Leaf Fall in Autumn: ABA helps trees shed leaves before winter.

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Activity: Demonstrating Phototropism (Auxin Action)

Aim:

To show how auxins cause plants to grow towards light (phototropism).

Materials Needed:

• Two identical potted seedlings (e.g., mustard or bean)
• A cardboard box with a hole on one side
• Sunlight or lamp

Steps:

1. Place one seedling in normal light (control).
2. Place the other seedling inside the box. Ensure the only light enters from the hole.
3. Water both seedlings equally.
4. Observe the direction of plant growth over 3–4 days.

Observations:

• The seedling in normal light grows straight up.
• The seedling in the box bends towards the hole (towards light).
• This is because auxins moved to the shaded side, making those cells grow faster, causing the plant to bend.

Conclusions:

• Auxins help the plant grow towards light.
• Phototropism is useful for the plant to maximize photosynthesis by capturing more light.

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Summary Table: Plant Hormones

Hormone	Main Function	Example/Use
Auxins	Cell elongation, tropisms	Root cuttings, seedless fruits
Gibberellins	Stem elongation, germination	Tall crops, seedless grapes
Cytokinins	Cell division, delay senescence	Tissue culture, fresh vegetables
Abscisic Acid	Growth inhibitor, dormancy	Seed dormancy, drought resistance

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Why Are Plant Hormones Important in Agriculture?

• They help farmers produce better crops by controlling plant growth.
• Plant propagation and tissue culture become easier with hormones.
• Fruit quality (size, seedlessness) is improved.
• Post-harvest losses are reduced when hormones prevent premature aging or spoilage.

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Scenario-Based Questions

1. Scenario: A farmer wants to grow seedless watermelons for better sales in the market.

   • Question: Which plant hormone should be used and how does it help?
   • Answer: The farmer should use auxins. Spraying auxins promotes fruit development without seeds (parthenocarpy), leading to seedless watermelons.

2. Scenario: A scientist is trying to increase sugarcane production by making the stems longer.

   • Question: Which hormone would be most helpful?
   • Answer: Gibberellins will help. They cause the internodes to elongate, increasing stem length and thus higher crop yield.

3. Scenario: You find a packet of wheat seeds labeled “ABA-treated for winter sowing.”

   • Question: Why has abscisic acid been used on these seeds?
   • Answer: Abscisic acid keeps seeds dormant so they do not germinate in cold conditions. They will only sprout in spring when conditions are right.

4. Scenario: A gardener wants to keep lettuce leaves fresh and green for a longer period after harvest.

   • Question: What should be sprayed on them?
   • Answer: Cytokinins should be sprayed. They delay leaf aging, so lettuce remains fresh.

5. Scenario: You want to show your classmates how plants are affected by hormones. You set up an experiment with light coming from one direction.

   • Question: What do you expect to see and what hormone causes this effect?
   • Answer: The plant will bend toward the light. Auxins redistribute to the shaded side, causing that side to grow faster and the plant to lean towards the light.

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Keep learning with curiosity and use examples around you! Plant hormones are nature’s secret helpers for healthy, productive plants.

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