Plant Coordination » Plant Response to Stimuli

What you'll learn this session

Study time: 30 minutes

How plants respond to light (phototropism)
How plants respond to gravity (gravitropism)
The role of plant hormones in controlling responses
How auxin influences plant growth
Plant responses to water and touch
Practical applications of plant responses

Introduction to Plant Responses to Stimuli

Just like animals, plants need to respond to their environment to survive. While plants can't move around like we do, they have developed amazing ways to respond to different stimuli in their environment. These responses help plants grow towards light, send roots down into soil and even react to being touched!

Key Definitions:

Tropism: A growth response in which a plant grows towards or away from a stimulus.
Stimulus: A change in the environment that causes a response in an organism.
Auxin: A plant hormone that controls growth responses.
Phototropism: The growth response of a plant to light.
Gravitropism: The growth response of a plant to gravity.

🌱 Why Plants Need to Respond

Plants need to respond to their environment to:

Capture sunlight for photosynthesis
Anchor themselves in soil
Find water and nutrients
Reproduce successfully
Survive changing conditions

💡 Types of Plant Responses

Plants respond to various stimuli including:

Light (phototropism)
Gravity (gravitropism)
Water (hydrotropism)
Touch (thigmotropism)
Chemicals (chemotropism)

Phototropism: Reaching for the Light

Phototropism is the way plants grow towards light. This response is crucial for plants because light is needed for photosynthesis. Shoots typically show positive phototropism (growing towards light), while roots may show negative phototropism (growing away from light).

How Phototropism Works

When light hits a plant shoot from one side, a fascinating chain of events occurs:

🔍 Step 1: Detection

Light is detected by photoreceptors in the tip of the shoot.

🔮 Step 2: Hormone Movement

Auxin (the growth hormone) moves away from the light to the shaded side.

📈 Step 3: Uneven Growth

More auxin on the shaded side causes those cells to elongate faster, bending the plant towards light.

Case Study Focus: Darwin's Experiments

Charles Darwin and his son Francis conducted groundbreaking experiments on phototropism in the 1880s. They discovered that the tip of a seedling was responsible for detecting light. When they covered the tip with a light-proof cap, the seedling didn't bend towards light. However, when they covered the stem below the tip, the plant still bent towards light. This showed that the tip detects light and sends a signal to the lower parts of the plant to grow differently.

Gravitropism: Responding to Gravity

Gravitropism (also called geotropism) is how plants respond to gravity. Roots show positive gravitropism, growing downwards with gravity, while shoots show negative gravitropism, growing upwards against gravity.

The Mechanism of Gravitropism

Gravitropism helps ensure that roots grow down into the soil (where water and nutrients are found) and shoots grow upwards (towards light).

👇 Root Gravitropism

In roots, gravity is detected by special cells called statocytes. These cells contain starch grains called statoliths that sink to the bottom of the cell due to gravity. This triggers auxin to accumulate on the lower side of the root. In roots, high concentrations of auxin inhibit cell elongation, causing the lower side to grow more slowly than the upper side. This makes the root bend downwards.

👆 Shoot Gravitropism

In shoots, a similar detection system exists, but the response is opposite. Auxin accumulates on the lower side of a horizontal shoot, but in shoots, auxin promotes cell elongation. This causes the lower side to grow faster than the upper side, making the shoot bend upwards against gravity.

The Role of Auxin in Plant Responses

Auxin is the key plant hormone that controls tropisms. Its uneven distribution in plant tissues causes differential growth, which results in bending.

How Auxin Works

Auxin's effects depend on:

The concentration of auxin
The type of plant tissue
The age of the plant

🚀 Auxin Effects in Shoots

In shoots, auxin promotes cell elongation by making cell walls more flexible. This allows water to enter cells, causing them to stretch. Higher concentrations of auxin on one side of a shoot cause those cells to elongate more, resulting in the shoot bending.

💧 Auxin Effects in Roots

In roots, auxin has the opposite effect. High concentrations of auxin inhibit cell elongation. When auxin accumulates on the lower side of a horizontal root, those cells grow more slowly, causing the root to bend downwards.

Other Plant Responses

Plants respond to many other stimuli besides light and gravity:

💦 Hydrotropism

This is the response of plant roots to water. Roots grow towards areas of higher moisture, helping plants find water in soil. This response can sometimes override gravitropism if water is scarce.

✋ Thigmotropism

This is the response to touch or contact. Climbing plants like ivy and tendrils of peas show thigmotropism, growing around objects they touch. This helps them climb and find support.

🌳 Chemotropism

This is the response to chemicals. For example, pollen tubes grow towards the ovule in flowers due to chemical signals, ensuring fertilisation can occur.

Practical Application: Plant Hormones in Agriculture

Understanding plant hormones has revolutionised agriculture. Farmers and gardeners use synthetic plant hormones to:

Promote root growth in cuttings (rooting powders contain auxin)
Control fruit ripening (ethylene can be used to ripen fruit after harvest)
Prevent fruit drop (spraying auxin keeps fruit on trees longer)
Create seedless fruits (gibberellins can induce fruit development without pollination)
Control weeds (some herbicides are synthetic auxins that cause abnormal growth in weeds)

Investigating Plant Responses

Scientists and students can investigate plant responses through simple experiments:

Experimental Techniques

Here are some ways to observe plant responses firsthand:

🔬 Phototropism Experiment

Grow seedlings in a box with a small hole on one side to let light in. Observe how the shoots bend towards the light source. You can measure the angle of bending over time to quantify the response.

🔭 Gravitropism Experiment

Place seedlings horizontally and observe how the roots and shoots respond over 24-48 hours. Roots should bend downwards while shoots bend upwards. You can try this with different plant species to compare their response times.

Summary: The Importance of Plant Responses

Plant responses to stimuli are essential survival mechanisms. They allow plants to:

Optimise light capture for photosynthesis
Secure anchorage in the soil
Access water and nutrients efficiently
Adapt to changing environmental conditions
Reproduce successfully

These responses are controlled by complex interactions between environmental signals and plant hormones, particularly auxin. By understanding these mechanisms, scientists have developed ways to manipulate plant growth for agricultural and horticultural purposes.

Remember that while plants can't move around like animals, they are far from passive. They actively sense and respond to their environment in sophisticated ways that ensure their survival and success.

🧠 Test Your Knowledge!