Plant hormones

Welcome to the World of Plant Hormones!

Did you know that plants are constantly moving? Even though they don't have muscles or a nervous system, they are experts at sensing their environment. In this chapter, we will discover how plants use chemical messengers called hormones to grow toward the light, find water in the soil, and even decide when it's time for their fruit to ripen. This is all part of homeostasis and response—how a living thing reacts to changes to stay healthy and survive.

1. How Plants Sense the World: Tropisms

Plants can’t get up and walk to a sunnier spot, so they do the next best thing: they grow toward it! A plant’s growth response toward or away from a stimulus is called a tropism.

There are two main types you need to know:

• Phototropism: Growth in response to light ("Photo" means light).
• Gravitropism (or Geotropism): Growth in response to gravity ("Gravi" for gravity).

Quick Review:
- Shoots grow toward light (Positive Phototropism) and away from gravity (Negative Gravitropism).
- Roots grow toward gravity (Positive Gravitropism).

2. The Master Hormone: Auxin

The main hormone in charge of these movements is auxin. It is produced in the tips of the shoots and roots. The most important thing to remember is that auxin causes unequal growth because it doesn't spread out evenly.

How Auxin Works in Shoots (Phototropism)

Think of auxin as a chemical that loves the shade. When light shines on one side of a shoot, the auxin moves to the shaded side.

Step-by-Step:
1. Auxin is made in the tip of the shoot.
2. Light hits one side.
3. Auxin moves to the dark/shaded side.
4. In the shoot, auxin stimulates (speeds up) cell growth.
5. The cells on the shaded side grow faster and longer than the cells on the light side.
6. This uneven growth makes the shoot bend toward the light.

How Auxin Works with Gravity (Gravitropism)

Gravity pulls auxin down to the lower side of both roots and shoots. However, auxin acts differently in roots than it does in shoots!

• In the Shoot: Auxin on the lower side makes those cells grow faster, pushing the shoot upwards.
• In the Root: Auxin on the lower side actually stops (inhibits) growth. This means the top side grows faster, bending the root downwards into the soil.

Memory Aid: Think of the "Root" as being "Restrictive." Auxin restricts growth in roots but boosts growth in shoots!

Key Takeaway: Unequal distribution of auxin causes unequal growth rates, which allows the plant to bend.

3. Other Important Hormones (Higher Tier Only)

Don't worry if this seems like a lot of names; just focus on what each one "does" for the plant.

Gibberellins

These are the "starters." Gibberellins are vital for initiating seed germination (making the seed start to grow). They also help the plant grow tall stems and produce flowers.

Ethene

Unlike other hormones, ethene is a gas. It controls cell division and, most importantly, fruit ripening. Farmers use this to make sure fruit is perfectly ripe just as it hits the supermarket shelves.

Quick Review Box:
- Auxin: Growth and tropisms.
- Gibberellins: Starting seeds and flowering.
- Ethene: Ripening fruit.

4. Required Practical 8: Investigating Light and Gravity

In class, you will likely grow seeds (like cress or mustard) in different conditions to see how they react.

• To test light: Place seedlings in a box with a hole on one side. You will see the shoots bend toward the light.
• To test gravity: Place a seedling on its side in the dark. You will see the shoot grow up and the root grow down.

Common Mistake to Avoid: When drawing your results, make sure the bend happens just behind the tip, not at the very base of the plant. The tip is where the "sensing" happens!

5. Using Hormones in Agriculture (Higher Tier Only)

Humans have learned how to use these plant "chemicals" to help us grow food more efficiently. Here is how we use them:

Uses of Auxins

• Weed Killers: Most weeds in wheat fields have broad leaves. Scientists made selective weed killers using auxins. They make the weeds grow so fast they run out of energy and die, while the narrow-leafed crops (like wheat) are left alone.
• Rooting Powders: If you take a "cutting" (a leaf or stem) from a plant, dipping it in auxin powder helps it grow new roots quickly.
• Tissue Culture: Using auxins to stimulate cells to divide and form new plants in a lab.

Uses of Ethene

Farmers often pick fruit while it is still green and hard so it doesn't get bruised during shipping. When the fruit reaches the warehouse, they spray it with ethene gas to trigger it to ripen all at once before it goes to the shop.

Uses of Gibberellins

• Ending Seed Dormancy: Making seeds grow at any time of year.
• Promoting Flowering: Making plants flower on demand, or growing extra-large flowers.
• Increasing Fruit Size: For example, making seedless grapes grow to the same size as normal ones.

Key Takeaway: Plant hormones aren't just for the plant—they are powerful tools used by farmers to control how our food grows and ripens.

Final Summary Checklist

1. Can you define Phototropism and Gravitropism?
2. Do you know that auxin moves to the shaded side?
3. Can you explain why shoots grow up and roots grow down?
4. (HT) Do you know that ethene is used for ripening fruit?
5. (HT) Can you name three uses for Gibberellins?

You've got this! Just remember: Auxin is the "growth juice" that prefers the dark.