Investigate tropic responses in plants

Plant hormones called auxins control tropic responses. When light hits one side of a shoot:

1. Auxins move to the shaded side of the plant
2. Higher auxin concentration on the shaded side causes faster cell elongation
3. The shaded side grows longer, bending the plant toward the light

This same principle applies to geotropism: gravity causes auxins to accumulate on the lower side, causing differential growth.

Aim

To investigate how plants respond to directional stimuli such as light and gravity.

Materials Needed

• Plant seedlings (bean or maize seedlings work well)
• Small pots or containers
• Soil
• Water
• Cardboard boxes or black paper
• Ruler
• Marker pen

Procedure

Experiment 1: Investigating Phototropism

1. Select three similar seedlings of the same age and height.
2. Plant each seedling in a pot with moist soil.
3. Set up three conditions:
   • Pot A: Place in a well-lit area (control)
   • Pot B: Cover with a box that has a hole on one side (unilateral light)
   • Pot C: Keep in complete darkness
4. Water all pots equally and place them in the same temperature area.
5. Observe and measure the direction of growth every day for 5–7 days.
6. Record your observations in a table:

Experiment 2: Investigating Geotropism

1. Plant seedlings in pots.
2. Lay one pot on its side (horizontal position).
3. Keep another pot upright (control).
4. Observe the direction of root and shoot growth over 5–7 days.

Variables to Control

• Independent variable: Direction of light or position of plant
• Dependent variable: Direction/degree of plant growth
• Controlled variables: Water amount, soil type, temperature, plant age

Expected Results

• In Pot B (unilateral light), the shoot should bend toward the light.
• In the horizontal pot, the root should bend downward (positive geotropism) and the shoot should bend upward (negative geotropism).

Safety Precautions

• Handle soil and plants with clean hands; wash hands after handling plants.
• Use scissors carefully when preparing plant materials.
• Do not eat plant materials used in experiments.

After your experiment:

1. Compare the growth direction in each condition.
2. Explain why the plants responded as they did—relate your observations to auxin distribution.
3. Conclusion: State whether your hypothesis was supported. For example: "The shoot showed positive phototropism because auxins moved to the shaded side, causing faster growth on that side."

In Tanzania, farmers and gardeners apply knowledge of tropic responses when growing crops like maize and beans. For example, understanding phototropism helps farmers position seedbeds to ensure seedlings receive equal sunlight, leading to stronger, more uniform growth. Similarly, knowing how geotropism works helps explain why properly planted seeds produce downward-growing roots that anchor the plant and upward-growing shoots that reach sunlight—critical for successful crop yields in smallholder farms across regions like Morogoro and Mbeya.