B30: Plant Hormones
Auxins, phototropism, geotropism, gibberellins, ethylene, commercial uses of plant hormones as weedkillers and rooting powder, and the required practical for GCSE Biology.
Key Definitions
- Auxin - a plant hormone that controls cell elongation, tropisms, and apical dominance. Produced in the tips of shoots and roots.
- Phototropism - the growth response of a plant to light. Shoots grow towards light (positive phototropism).
- Geotropism (gravitropism) - the growth response of a plant to gravity. Roots grow downwards (positive geotropism).
- Gibberellins - plant hormones that promote seed germination and stem growth.
- Ethylene (ethene) - a gaseous plant hormone that promotes fruit ripening.
Auxins and Tropisms
Auxins are the most important plant hormones for controlling tropisms (growth responses to stimuli). They are produced in the tips of shoots and roots and move through the plant by diffusion and active transport.
How Auxins Work
- Auxins promote cell elongation in shoots - they cause cells to grow longer, making the shoot bend.
- Auxins inhibit cell elongation in roots - high concentrations of auxin slow down root cell growth.
- Auxins move away from light - they accumulate on the shaded side of a shoot.
- Auxins move downwards in response to gravity - they accumulate on the lower side of a root.
Phototropism in Shoots
- Light shines on one side of the shoot tip.
- Auxin moves to the shaded side (away from the light).
- Higher auxin concentration on the shaded side causes greater cell elongation on that side.
- The shoot bends towards the light (positive phototropism).
Geotropism in Roots
- Gravity causes auxin to accumulate on the lower side of the root.
- In roots, high auxin concentration inhibits cell elongation.
- The cells on the upper side (less auxin) grow longer than those on the lower side.
- The root bends downwards (positive geotropism) towards gravity.
| Feature | Shoot | Root |
|---|---|---|
| Response to light | Positive phototropism (grows towards light) | Not sensitive to light for tropism |
| Response to gravity | Negative geotropism (grows upwards) | Positive geotropism (grows downwards) |
| Effect of auxin on cell elongation | Promotes elongation | Inhibits elongation |
| Result of unequal auxin distribution | Grows towards side with MORE auxin | Grows towards side with LESS auxin |
Worked Example 1 - Explaining Phototropism
Question: A plant is placed near a window. After a few days, the shoot has bent towards the window. Explain how auxins cause this response.
Solution: When light shines on one side of the shoot tip, auxin moves away from the light and accumulates on the shaded side of the shoot. In shoots, auxin promotes cell elongation, so the cells on the shaded side grow longer than the cells on the lit side. This unequal growth causes the shoot to bend towards the light. This is called positive phototropism and it benefits the plant by ensuring its leaves receive maximum light for photosynthesis.
Worked Example 2 - Explaining Geotropism in Roots
Question: Explain how auxins cause a root to grow downwards (positive geotropism).
Solution: Gravity causes auxin to accumulate on the lower side of the root. In roots (unlike shoots), high concentrations of auxin inhibit cell elongation. So the cells on the lower side (more auxin) grow less than the cells on the upper side (less auxin). This unequal growth causes the root to bend downwards towards gravity. This is called positive geotropism and it benefits the plant by ensuring roots grow into the soil where they can absorb water and mineral ions.
Gibberellins
Gibberellins are plant hormones with two main functions:
- Seed germination: Gibberellins stimulate the production of amylase in seeds, which breaks down starch into glucose. The glucose provides energy for the growing embryo. This is why gibberellins are essential for seeds to germinate.
- Stem growth: Gibberellins promote elongation of stems, helping plants grow taller.
Ethylene (Ethene)
Ethylene is a gaseous plant hormone that plays a key role in fruit ripening. As fruit ripens, it produces more ethylene, which accelerates the ripening process. This is a positive feedback mechanism - ripe fruit produces ethylene, which makes nearby fruit ripen faster too.
Uses of Plant Hormones
| Application | Hormone Used | How It Works |
|---|---|---|
| Weedkillers (herbicides) | Synthetic auxins (e.g. 2,4-D) | Synthetic auxins cause excessive, uncontrolled growth in broad-leaved weeds, killing them. Narrow-leaved crops (grasses/cereals) are unaffected. |
| Rooting powder | Auxins | Applying auxin to a cutting stimulates root growth, allowing gardeners to clone plants cheaply. |
| Seedless fruit production | Gibberellins | Applied to unpollinated flowers, gibberellins cause fruit to develop without seeds (parthenocarpy). |
| Fruit ripening control | Ethylene | Ethylene can be added to speed up ripening (e.g. bananas in transit) or inhibited to delay ripening for storage. |
| Dormancy breaking | Gibberellins | Gibberellins can break seed dormancy, ensuring seeds germinate when planted. |
Worked Example 3 - Synthetic Auxin Weedkillers
Question: Explain how synthetic auxins are used as selective weedkillers and why they kill weeds but not crops.
Solution: Synthetic auxins (such as 2,4-D) are sprayed on fields as selective weedkillers. Broad-leaved weeds (dicots) are very sensitive to these auxins and respond with excessive, uncontrolled growth, which disrupts their normal growth patterns and eventually kills them. Narrow-leaved crop plants (monocots, such as wheat and barley) are not sensitive to the same concentration of synthetic auxin and continue to grow normally. This selectivity allows farmers to kill weeds that would otherwise compete with crops for light, water, and minerals, without harming the crop plants.
Worked Example 4 - Rooting Powder
Question: A gardener wants to produce many identical plants from a single parent plant. Explain how they can use rooting powder to do this.
Solution: The gardener takes cuttings (small pieces of stem) from the parent plant. They dip the cut end of each cutting into rooting powder, which contains auxin hormones. The auxin stimulates root growth from the cut end of the stem. Once roots have developed, the cutting can be planted and will grow into a new, genetically identical plant (a clone). This method is much cheaper and faster than growing plants from seeds, and it ensures the new plants have the same desirable characteristics as the parent plant.
Required Practical: Effect of Light or Gravity on Seedling Growth
Investigating the Effect of Light on Seedling Growth
- Aim: To investigate the effect of light on the growth of seedlings (phototropism).
- Method: Grow cress seedlings in boxes with different light conditions: (a) light from one side only, (b) light from all sides (control), (c) no light (in darkness). After several days, observe and measure the direction of growth.
- Expected results: (a) Seedlings in one-sided light will bend towards the light (positive phototropism). (b) Seedlings in even light will grow straight upwards. (c) Seedlings in darkness will grow tall and spindly (etiolation) with small yellow leaves.
- Variables: Independent = direction/intensity of light. Dependent = direction of seedling growth (bend angle). Control = same type of seed, same number of seeds, same soil, same temperature, same watering.
Practice Questions
Foundation
Q1. Explain how auxins cause a shoot to bend towards light.
Show Answer
When light shines on one side of the shoot, auxin moves away from the light and accumulates on the shaded side. In shoots, auxin promotes cell elongation, so the cells on the shaded side grow longer than those on the lit side. This unequal growth causes the shoot to bend towards the light.
Higher
Q2. Explain how auxins cause a root to grow downwards (positive geotropism), and why this is different from the effect of auxin on shoots.
Show Answer
Gravity causes auxin to accumulate on the lower side of the root. In roots, high auxin concentration inhibits cell elongation (the opposite of shoots, where auxin promotes elongation). The cells on the upper side (less auxin) grow longer, causing the root to bend downwards. This difference occurs because root and shoot cells respond differently to the same concentration of auxin.
Foundation
Q3. Give two commercial uses of plant hormones and explain how each works.
Show Answer
1. Synthetic auxins as weedkillers: They cause excessive, uncontrolled growth in broad-leaved weeds, killing them while leaving narrow-leaved crops unaffected. 2. Rooting powder containing auxins: Applied to cuttings to stimulate root growth, allowing gardeners to clone plants cheaply. Other valid answers include: gibberellins for seedless fruit production, ethylene for controlling fruit ripening.
Higher
Q4. Describe an experiment to investigate the effect of light on seedling growth (phototropism). State the variables that should be controlled.
Show Answer
Method: Grow cress seedlings in three boxes: (a) one-sided light (cut a hole in one side of the box), (b) even light from all sides (control), (c) complete darkness. After 5-7 days, observe the direction of growth and measure the angle of any bending. Expected results: (a) seedlings bend towards the light, (b) seedlings grow straight up, (c) seedlings grow tall and spindly. Control variables: same type and number of seeds, same soil, same temperature, same amount of water, same time period.
Foundation
Q5. Describe the role of gibberellins in seed germination.
Show Answer
Gibberellins stimulate the production of the enzyme amylase in the seed. Amylase breaks down starch (stored in the seed) into glucose. The glucose provides energy for the growing embryo to respire and grow. Without gibberellins, the seed would not be able to access its stored energy and germination would not occur.
Exam Tips
- For phototropism: auxin moves to the shaded side, cells on the shaded side elongate more, shoot bends towards light.
- For geotropism: auxin accumulates on the lower side of roots, but in roots auxin inhibits elongation, so root bends downwards.
- The key difference: auxin promotes elongation in shoots but inhibits elongation in roots.
- Synthetic auxins as weedkillers are selective - they kill broad-leaved weeds but not narrow-leaved crops.
- Remember: gibberellins = germination and growth; ethylene = ripening; auxins = tropisms and rooting.
🔬 Required Practical
Investigating the Effect of Light on Plant Growth (Auxin Phototropism)
Grow cress seedlings in three conditions: (a) light from one side only, (b) light from all sides (control), (c) complete darkness. After 5–7 days, observe and measure the direction of growth and angle of bending. Expected: (a) shoots bend towards the light (positive phototropism), (b) shoots grow straight up, (c) shoots grow tall and spindly (etiolation). Control variables: same type and number of seeds, same soil, same temperature, same watering. Repeat with several boxes per condition for reliability.
🔢 Maths Skills
Mathematical Skills
Calculating percentage growth: measure the initial height of a seedling, then measure again after a set time. Percentage growth = ((final height – initial height) / initial height) × 100. For example, if a seedling grows from 2.0 cm to 3.5 cm in 5 days: ((3.5 – 2.0) / 2.0) × 100 = 75% growth. You may also need to calculate means from repeated measurements and plot growth curves.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Auxin promotes growth in all tissues. Correct: Auxin promotes cell elongation in shoots but inhibits cell elongation in roots at the same concentration — different tissues respond differently to the same hormone.
2. Wrong: Plants choose to grow towards light. Correct: Phototropism is an automatic hormonal response — auxin redistribution causes unequal growth; there is no conscious choice involved.
✍️ 6-Mark Question
Extended Answer
6 marks: Explain how auxin causes phototropism and describe how to investigate it.
Auxin causes phototropism through unequal distribution. When light shines on one side of a shoot tip, auxin moves away from the light and accumulates on the shaded side. In shoots, auxin promotes cell elongation, so the cells on the shaded side grow longer than those on the lit side. This unequal growth causes the shoot to bend towards the light (positive phototropism), which benefits the plant by maximising light absorption for photosynthesis. To investigate phototropism, grow cress seedlings in three separate boxes: one with a hole cut in one side to allow one-sided light, one with even light from all sides (control), and one in complete darkness. Keep all other variables the same (same type and number of seeds, same soil, temperature, and watering). After 5–7 days, measure the angle of bending in each condition. The one-sided light seedlings should bend towards the light, the control should grow straight, and the dark-grown seedlings should be tall and spindly (etiolated). Repeat the experiment several times and calculate mean values to improve reliability.
Mark scheme: 1 mark for auxin moving away from light; 1 mark for accumulation on shaded side; 1 mark for cell elongation causing bending; 1 mark for describing the experimental method; 1 mark for naming control variables; 1 mark for repeating for reliability or stating expected results.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A farmer uses a synthetic auxin (2,4-D) as a weedkiller on a wheat field. Broad-leaved weeds die, but the wheat is unaffected. A neighbouring organic farm complains that the weedkiller has drifted onto their crops. (a) Explain why the synthetic auxin kills broad-leaved weeds but not wheat. (b) Evaluate the safety of using synthetic auxin weedkillers — give one advantage and one risk. (c) Suggest how the farmer could reduce the risk of drift affecting neighbouring farms.
Answers: (a) Broad-leaved plants (dicots) are sensitive to synthetic auxins and respond with excessive, uncontrolled growth that kills them. Narrow-leaved grasses (monocots like wheat) are not sensitive to the same concentration and grow normally. (b) Advantage: selective weedkillers increase crop yield by removing competition for light, water, and minerals without harming the crop. Risk: drift can damage non-target plants (as the organic farm experienced); there may be environmental concerns about chemical run-off into waterways and effects on biodiversity. (c) Use lower spray pressure or add drift-reduction nozzles; spray on calm days with low wind; maintain buffer zones between sprayed fields and neighbouring land; use shielded spraying equipment.
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