Introduction to Plant Hormones

Have you ever wondered how a houseplant "knows" to lean toward a sunny window, or how a seed "knows" which way is up when it’s buried in the dark soil? Unlike animals, plants don't have a nervous system to make decisions. Instead, they use chemical messengers called hormones to coordinate their growth and development. In this chapter, we will explore how these hormones work and how humans have learned to use them to grow food more efficiently.

Note: This topic is for the "Biology Only" (1BI0) qualification. If you are studying Combined Science, you do not need to learn this specific chapter!

1. Auxins and Plant Tropisms

A tropism is a growth response of a plant toward or away from a stimulus (like light or gravity). The main hormone responsible for this is called auxin.

Key Facts about Auxins:

  • They are produced in the tips of shoots and roots.
  • They dissolve in water and move backwards from the tip to the rest of the plant.
  • In shoots: Auxins stimulate (speed up) cell elongation (the cells get longer).
  • In roots: Auxins inhibit (slow down) cell elongation.

Phototropism: Response to Light

Plants need light for photosynthesis. Phototropism ensures that shoots grow toward the light.

How it works step-by-step:

  1. When a shoot tip is exposed to light from one side, the auxins move to the shaded side (the side away from the light).
  2. Because there is more auxin on the shaded side, the cells on that side elongate faster than the cells on the sunny side.
  3. This uneven growth causes the shoot to bend toward the light.

Gravitropism (Geotropism): Response to Gravity

Gravitropism ensures that roots grow down into the soil (to find water/minerals) and shoots grow up (to find light).

In the Shoot (Negative Gravitropism):

If a shoot is growing sideways, gravity pulls the auxins to the lower side. In the shoot, auxins stimulate growth, so the lower side grows faster, bending the shoot upwards, away from gravity.

In the Root (Positive Gravitropism):

If a root is growing sideways, gravity pulls the auxins to the lower side. However, in the root, auxins inhibit growth. This means the upper side grows faster than the lower side, bending the root downwards, toward gravity.

Quick Review Box:
Shoots: Auxin = More Growth. Bends toward light (Positive Phototropism) and away from gravity (Negative Gravitropism).
Roots: Auxin = Less Growth. Bends toward gravity (Positive Gravitropism).

Common Mistake Alert!

Students often forget that auxins behave differently in roots than in shoots. Remember: Auxins = Growth in Shoots, but Auxins = No Growth in Roots!

2. Commercial Uses of Plant Hormones (Higher Tier Only)

Don't worry if this seems tricky at first! Just remember that humans have "hacked" plant hormones to help us in farming and gardening. We focus on three main hormones: Auxins, Gibberellins, and Ethene.

A. Uses of Auxins

1. Weedkillers: Most weeds in a field of grass or wheat are "broad-leaved" plants. Selective weedkillers made of auxins only affect these broad-leaved plants. The auxins make the weeds grow so rapidly that they use up all their resources and die, leaving the narrow-leaved crops (like wheat) unharmed.

2. Rooting Powders: If you take a "cutting" (a piece of a plant stem) and put it in the ground, it might die. If you dip the end in rooting powder containing auxins, it stimulates the stem to grow roots quickly, allowing gardeners to clone plants easily.

B. Uses of Gibberellins

Gibberellins are hormones that naturally trigger germination (when a seed starts to grow).

1. Ending Seed Dormancy: Farmers use gibberellins to make seeds germinate at times of the year they normally wouldn't, or to ensure all seeds in a field grow at the same time.

  • 2. Promoting Flowering: Gibberellins can make plants flower without the need for specific day lengths or temperatures. They can also result in bigger flowers.
  • 3. Increasing Fruit Size: Some fruits (like seedless grapes) would naturally stay quite small. Spraying them with gibberellins makes them grow much larger.

    • C. Uses of Ethene

    Ethene is a gas produced by aging parts of a plant. It controls cell division and ripening.

    1. Fruit Ripening: This is very important for supermarkets! Fruit (like bananas) is often picked while it is green and unripe. This makes it "harder" so it doesn't get bruised during shipping. When the fruit reaches its destination, it is sprayed with ethene gas to make it ripen just before it goes on the shelves.

    Summary Table: Commercial Uses

    Hormone: Auxins
    Uses: Selective weedkillers; Rooting powder for cuttings.

    Hormone: Gibberellins
    Uses: Germination (ending dormancy); Flowering; Increasing fruit size.

    Hormone: Ethene
    Uses: Controlling fruit ripening for transport.

    Key Takeaways for Exam Success

    • Auxins are the main characters for tropisms; they move to the shaded side or the lower side.
    • In shoots, auxins cause more growth (elongation).
    • In roots, auxins cause less growth.
    • Higher Tier: Be ready to explain why we pick fruit unripe (ethene allows us to control the timing so it isn't mushy when it arrives!).

    Memory Trick: "G" is for Gibberellins and Germination. "E" is for Ethene and Eating (because we wait for fruit to ripen before we eat it!).