Tropic Responses: How Plants React to the World

Hello Biologists! This chapter is all about how plants, even though they look stationary, are constantly responding and coordinating with their environment. Unlike animals, which use muscles and nerves to move quickly, plants use slow, controlled growth movements called tropic responses.

Understanding tropisms is key to appreciating how plants survive and thrive, ensuring their leaves catch enough light and their roots find essential water and minerals. Let's dive into how these slow, steady movements happen!


1. Defining Tropism: Growth in Response to a Stimulus (Core)

A plant's ability to respond to its surroundings is called sensitivity. When this response involves growth towards or away from a direction stimulus, we call it a tropism.

What is a Tropism?

A Tropism is a growth movement of a plant part in response to a directional external stimulus.

  • The movement must involve growth (a permanent increase in size).
  • The response is directional (it depends on where the stimulus is coming from).

Positive vs. Negative Tropisms

We describe tropisms using two key words:

1. Positive Tropism:

This means the plant part grows towards the stimulus.

Example: A sunflower stem growing towards the Sun is a positive response to light.

2. Negative Tropism:

This means the plant part grows away from the stimulus.

Example: A root growing downwards, away from the light, is a negative response to light.

🔑 Quick Review: Two Main Tropisms

In the IGCSE syllabus, you need to know two main tropisms:

  • Phototropism: Response to light (Photo = Light).
  • Gravitropism (or Geotropism): Response to gravity (Gravi/Geo = Earth/Gravity).

2. Phototropism: Following the Light (Core & Supplement)

Phototropism is the growth response of a plant part to the direction of light.

The Core Response

In most plants:

  • Shoots show positive phototropism (they grow towards the light). This maximizes light absorption for photosynthesis.
  • Roots usually show negative phototropism (they grow away from the light), preferring darkness where there is more likely to be water.

The Role of Auxin (Chemical Control)

The secret behind this growth is a powerful chemical messenger called a plant hormone. The most important hormone here is auxin.

How Auxin Works (Supplement Detail)

Auxin controls growth by causing cells to lengthen (cell elongation). Here is the step-by-step process:

Step 1: Auxin Production
Auxin is primarily made in the tip of the shoot (the apex). This is the part that detects the light stimulus.

Step 2: Diffusion Downwards
Auxin diffuses downwards from the tip through the rest of the shoot.

Step 3: Unequal Distribution due to Light

If light hits the shoot from one side (unilateral light):

  • The auxin moves away from the light source.
  • This results in a higher concentration of auxin accumulating on the shaded side of the shoot.

Step 4: Unequal Growth

  • The high concentration of auxin on the shaded side stimulates cell elongation much more effectively than the lower concentration on the light side.
  • The cells on the shaded side grow longer and faster than the cells on the lighted side.

Step 5: The Bend
Because one side grows faster than the other, the shoot bends towards the light source. This is positive phototropism.

💡 Analogy: The Traffic Jam

Imagine auxin molecules are tiny cars. Light acts like a policeman, directing most of the cars (auxin) to the shaded lane. More cars (more auxin) on the shaded side means those cells press the accelerator (grow faster), causing the whole shoot to curve towards the light.


3. Gravitropism: The Response to Gravity (Core & Supplement)

Gravitropism (also called geotropism) is the growth response of a plant part to the direction of gravity.

The Core Response

This response ensures the plant is anchored and correctly oriented:

  • Shoots show negative gravitropism (they grow upwards, away from gravity). This pushes the stem towards the light.
  • Roots show positive gravitropism (they grow downwards, towards gravity). This helps anchor the plant and ensures access to soil water and minerals.

Auxin's Dual Role in Gravitropism (Supplement Detail)

The mechanism is also controlled by auxin, but here's the crucial twist: Auxin affects roots and shoots differently!

Mechanism when horizontal (e.g., if a plant falls over):

Step 1: Auxin Redistribution
Gravity causes auxin to collect or pool on the lower side of both the shoot and the root.

A) The Shoot (Negative Gravitropism)

The shoot needs to grow up, away from gravity.

  • Lower side: Has a very high concentration of auxin.
  • Shoot response: Shoots are very sensitive to auxin. The high concentration on the lower side causes maximum cell elongation.
  • Result: The lower side grows faster, causing the shoot to bend upwards.
B) The Root (Positive Gravitropism)

The root needs to grow down, towards gravity.

  • Lower side: Has a very high concentration of auxin.
  • Root response: Roots are extremely sensitive to auxin. The high concentration on the lower side INHIBITS (slows down or stops) cell elongation.
  • Result: The upper side, with less auxin, grows faster than the lower side, causing the root to bend downwards.
🧠 Memory Aid: Auxin Concentration

Don't worry if this seems tricky at first! Remember this simple rule:

Auxin loves shoots, but hates roots (when concentrated).

  • In Shoots, high auxin = fast growth.
  • In Roots, high auxin = slow/stopped growth.

4. Investigating Tropic Responses (Core)

You may be asked about simple experiments designed to demonstrate these tropisms.

Investigating Phototropism

To investigate phototropism in a shoot, you compare:

  1. A control group placed in all-around light (should grow straight).
  2. An experimental group placed in a box with light entering from one side only (unilateral light).

Observation: The shoot in unilateral light will bend towards the light source (positive phototropism).

Investigating Gravitropism

To investigate gravitropism, you can:

  1. Place seedlings or young shoots in the normal vertical position (control).
  2. Place seedlings or young shoots in a horizontal position (experimental).

Observation:

  • The experimental shoot will curve upwards (negative gravitropism).
  • The experimental root will curve downwards (positive gravitropism).

🌟 Key Takeaway

Tropisms are essential plant survival strategies driven by the hormone auxin. Phototropism helps leaves capture maximum light, while gravitropism ensures roots grow securely into the ground. Remember that auxin promotes growth in shoots but inhibits growth in roots at high concentrations, explaining why shoots and roots have opposite responses to the same stimuli.