Plant hormones: auxins, gibberellins and ethene

What you'll learn

Plant hormones (also called plant growth substances) coordinate growth and responses to environmental stimuli in plants. This topic examines three key hormones — auxins, gibberellins and ethene — and their commercial applications. You need to understand how each hormone works, where it's produced, and how it affects plant growth and development for your AQA GCSE Biology exam.

Key terms and definitions

Auxin — a plant hormone that controls cell elongation and coordinates directional growth responses (tropisms), mainly produced in shoot tips and young leaves

Phototropism — a growth response where a plant grows towards or away from light; shoots show positive phototropism (towards light) while roots show negative phototropism (away from light)

Gravitropism (geotropism) — a growth response to gravity; shoots show negative gravitropism (grow upwards, away from gravity) while roots show positive gravitropism (grow downwards, towards gravity)

Gibberellins — plant hormones that promote seed germination, stem elongation and flowering

Ethene — a gaseous plant hormone that controls cell division and ripening of fruits

Tropism — a directional growth response by a plant to an environmental stimulus such as light or gravity

Cell elongation — the process where plant cells increase in length, causing growth in shoots and roots

Core concepts

How auxins control phototropism

Auxins are produced in the tips of growing shoots and move backwards from the tip to control cell elongation in the region behind the tip.

Unequal distribution of auxin causes unequal growth:

• When a shoot tip is exposed to light from one side, auxin accumulates on the shaded side
• Higher auxin concentration on the shaded side stimulates greater cell elongation on that side
• The shaded side grows faster than the illuminated side
• The shoot bends towards the light source

This mechanism ensures shoots grow towards light, maximizing photosynthesis. The bending response is caused by unequal growth rates on opposite sides of the shoot, not by the shoot physically moving.

Key experimental evidence:

Charles Darwin and his son Francis conducted early experiments showing that:

• Covering the shoot tip prevents phototropic response
• Covering the region below the tip does not prevent the response
• The tip must detect light, but growth occurs below the tip

Later research by Boysen-Jensen demonstrated that:

• Auxin is a chemical substance that can diffuse through gelatin blocks
• Placing a barrier on one side of the shoot tip blocks auxin movement and prevents bending

How auxins control gravitropism

Auxins also coordinate gravitropic responses in both shoots and roots, though the effects differ between these organs.

In shoots:

• When a shoot is placed horizontally, auxin accumulates on the lower side due to gravity
• Higher auxin concentration stimulates cell elongation on the lower side
• The lower side grows faster, causing the shoot to bend upwards
• This is negative gravitropism (growth away from gravity)

In roots:

• When a root is horizontal, auxin also accumulates on the lower side
• In roots, high auxin concentrations inhibit cell elongation
• The upper side (with lower auxin) grows faster than the lower side
• The root bends downwards, showing positive gravitropism (growth towards gravity)

The crucial difference is that auxins promote cell elongation in shoots but inhibit it in roots at high concentrations. This single hormone can produce opposite effects in different plant organs.

Gibberellins and their effects

Gibberellins are a group of plant hormones with several important functions in plant growth and development.

Seed germination:

• Gibberellins trigger the breakdown of food stores in seeds
• They activate enzymes that convert starch into glucose
• Glucose provides energy for the embryo to grow
• This initiates germination when conditions are suitable

Stem elongation:

• Gibberellins promote cell division and elongation in stems
• Plants treated with gibberellins grow taller with longer internodes (sections between leaves)
• Dwarf varieties of plants often have genetic mutations affecting gibberellin production or response

Flowering:

• Gibberellins can promote flowering in some plant species
• They interact with other environmental signals like day length

Ending seed dormancy:

• Some seeds remain dormant even in suitable conditions
• Gibberellins can break dormancy and trigger germination
• This is particularly important for seeds requiring cold periods (stratification) before germinating

Ethene and fruit ripening

Ethene (ethylene) is a gaseous plant hormone with distinct effects on plant tissues, particularly in fruit development.

Ripening process:

• Ethene controls the ripening of fruits
• It triggers colour changes as chlorophyll breaks down and other pigments develop
• It softens fruit tissues by breaking down cell walls
• It converts starch to sugars, making fruit sweeter
• It produces characteristic aromas and flavours

Autocatalytic effect:

• Ripening fruits produce ethene
• Ethene stimulates more ethene production
• This creates a positive feedback loop accelerating ripening
• This explains why "one bad apple spoils the barrel" — ethene from one ripening fruit triggers ripening in nearby fruits

Leaf fall (abscission):

• Ethene also promotes leaf fall in deciduous plants
• It stimulates the formation of an abscission layer at the base of leaf stalks
• This weakens the attachment, allowing leaves to fall

Commercial uses of auxins

Understanding auxin action has led to several important agricultural and horticultural applications.

Rooting powders and cuttings:

• Plant cuttings (stems cut from parent plants) can develop roots and grow into new plants
• Auxin-containing rooting powders promote root development in cuttings
• Gardeners dip the cut end into rooting powder before planting
• This increases success rates for propagation by cuttings

Selective weedkillers:

• Synthetic auxins are used as selective herbicides
• Broadleaf weeds (dicots) are more sensitive to these chemicals than grasses (monocots)
• High doses of synthetic auxin cause uncontrolled, unsustainable growth in broadleaf plants
• Weeds grow rapidly then die from exhaustion of resources
• Cereal crops and lawns (which are grasses) remain largely unaffected

Seedless fruits:

• Applying auxin to unpollinated flowers can trigger fruit development without fertilization
• This produces seedless fruits (parthenocarpy)
• Seedless grapes and some varieties of tomatoes are produced this way

Commercial uses of gibberellins

Gibberellins have various commercial applications in agriculture and the food industry.

Delaying senescence:

• Applying gibberellins to citrus fruits keeps them fresh longer
• This extends shelf life during storage and transport
• Particularly useful for fruit exported from tropical regions to distant markets

Improving fruit size:

• Gibberellin application to seedless grapes increases fruit size
• Larger grapes have higher market value
• This is standard practice in commercial grape production

Uniform germination:

• Adding gibberellins to seeds ensures more uniform germination
• Seeds germinate at the same time, producing uniform crop growth
• This allows synchronized harvesting and improves efficiency

Brewing industry:

• Gibberellins speed up germination of barley during malting
• This produces malt more quickly for beer production
• Reduces production time and costs

Commercial uses of ethene

The properties of ethene are exploited commercially, particularly in the fruit industry.

Controlled ripening:

• Bananas, tomatoes and other fruits are often picked unripe
• Unripe fruits are firmer and withstand transport better
• Fruits are stored in ethene-free conditions during shipping
• Before sale, fruits are exposed to ethene gas in ripening rooms
• This triggers rapid, uniform ripening

Preventing premature ripening:

• During storage and transport, ethene must be removed or blocked
• Low temperature storage slows ethene production
• Good ventilation prevents ethene accumulation
• Some storage facilities use ethene absorbers or inhibitors

Coordinating harvest:

• Spraying ethene on crops can synchronize ripening
• This allows entire fields to be harvested at once
• Reduces labor costs and improves efficiency

Worked examples

Example 1: Phototropism mechanism (4 marks)

Question: A student investigated phototropism by placing cress seedlings in a box with light coming from one side only. After two days, the shoots had bent towards the light. Explain how auxin causes this response.

Mark scheme answer:

• Auxin is produced in the shoot tip (1 mark)
• Auxin moves/accumulates on the shaded side of the shoot (1 mark)
• Auxin promotes/stimulates cell elongation (1 mark)
• Cells on the shaded side elongate more, causing the shoot to bend towards the light (1 mark)

Examiner note: This is a typical 4-mark "Explain" question. You need to provide a clear sequence showing: production → movement → effect on cells → visible response. Each step earns one mark.

Example 2: Comparing tropisms (6 marks)

Question: Compare and contrast the roles of auxin in phototropism and gravitropism in plant shoots.

Mark scheme answer:

Similarities:

• Both involve auxin produced in the shoot tip (1 mark)
• Both involve unequal distribution of auxin across the shoot (1 mark)
• In both cases, auxin promotes cell elongation (1 mark)
• Both result in directional growth responses (1 mark)

Differences:

• Phototropism is a response to light, gravitropism is a response to gravity (1 mark)
• In phototropism, auxin moves to the shaded side; in gravitropism, auxin moves to the lower side (1 mark)

Examiner note: "Compare and contrast" requires both similarities and differences. Allocate time appropriately — don't just list differences. Use comparative language: "whereas," "both," "similarly," "in contrast."

Example 3: Commercial application (3 marks)

Question: A banana grower in the Caribbean picks bananas when they are green and unripe. Explain why the grower stores the bananas in well-ventilated, cool conditions during shipping to the UK.

Mark scheme answer:

• Unripe bananas are firmer/less damaged during transport (1 mark)
• Cool conditions slow down ethene production/ripening process (1 mark)
• Good ventilation prevents ethene accumulation, which would cause premature ripening (1 mark)

Examiner note: This question tests application to a real-world scenario. Connect the biology (ethene causes ripening) to the practical problem (preventing ripening during transport). The Caribbean-UK context is relevant to Kramizo users.

Common mistakes and how to avoid them

• Confusing the effects of auxin in shoots versus roots. Remember: auxin promotes growth in shoots but inhibits growth in roots at high concentrations. This is why shoots and roots show opposite gravitropic responses despite both having auxin on the lower side.

• Saying plants "move" towards light. Plants don't move — they grow directionally due to unequal cell elongation on different sides. Always use growth-related terminology: "bends," "grows towards," "cell elongation."

• Mixing up gibberellins and auxins. Auxins primarily control tropisms and cell elongation; gibberellins mainly promote germination, stem elongation and flowering. Keep their functions separate.

• Writing "ethylene" instead of "ethene." The AQA specification uses "ethene" (the IUPAC chemical name). Use this term in your exam answers to match the specification language.

• Failing to explain mechanisms fully. When asked to "explain," provide the complete sequence: where the hormone is produced → how it moves → what it does to cells → what visible effect this causes. Don't skip steps.

• Not applying knowledge to commercial contexts. Questions often involve real agricultural or horticultural scenarios. Practice linking hormone functions to practical uses like rooting powders, selective weedkillers, or controlled fruit ripening.

Exam technique for "Plant hormones: auxins, gibberellins and ethene"

• Command words matter. "Describe" requires you to state what happens; "Explain" requires you to say how/why it happens using causal links. For "Explain" questions about tropisms, always include: hormone production, movement, effect on cells, and resulting growth pattern.

• Use precise terminology. Write "cell elongation" not just "growth"; "accumulates on the shaded side" not just "moves away from light"; "positive phototropism" not just "grows towards light." Precision earns marks.

• Extended response questions (4-6 marks) need structure. Start with hormone production/location, then describe movement/distribution, then explain effects on cells, finally link to the visible response. This logical sequence helps you avoid missing steps.

• Read commercial application questions carefully. Identify which hormone is involved, what effect it has, and why this is commercially useful. Questions may require you to apply your knowledge to unfamiliar scenarios — use your understanding of hormone functions to work out the answer.

Quick revision summary

Plants use hormones to control growth and responses to stimuli. Auxins control tropisms by promoting cell elongation; they accumulate on shaded sides (phototropism) or lower sides (gravitropism), causing directional growth. In roots, high auxin concentrations inhibit growth. Gibberellins promote seed germination, stem elongation and flowering. Ethene triggers fruit ripening and leaf fall. Commercial applications include rooting powders, selective weedkillers (auxins), controlling fruit ripening during transport (ethene), and promoting uniform germination (gibberellins). Understanding hormone production, movement and cellular effects is essential for exam success.