Commercial uses of plant hormones

What you'll learn

This topic covers how plant hormones are exploited commercially in agriculture and horticulture. You'll learn about the practical applications of auxins, gibberellins and ethene, understanding how these growth regulators improve crop production, enhance propagation techniques and control the timing of fruit ripening. This knowledge builds on your understanding of plant responses and phototropism.

Key terms and definitions

Auxin — a plant hormone that controls growth near the tips of shoots and roots, used commercially in rooting powders and selective weedkillers

Gibberellin — a plant hormone that promotes stem elongation, seed germination and flowering, widely used to control fruit development

Ethene — a plant hormone in the form of a gas that controls cell division and fruit ripening

Rooting powder — a powder containing auxins applied to plant cuttings to stimulate rapid root development

Selective weedkiller — a herbicide containing synthetic auxins that kills broad-leaved plants (dicots) but not narrow-leaved plants like grasses and cereals

Tissue culture — a technique for growing new plants from small groups of cells taken from the parent plant

Dormancy — a period when seeds or buds are alive but not actively growing, often broken by gibberellins

Propagation — the production of new plants from parent plants, either sexually (from seeds) or asexually (from cuttings)

Core concepts

Auxins in horticulture and agriculture

Auxins are the most commercially exploited plant hormones. Their primary applications include promoting root growth and controlling unwanted plants.

Rooting powders and plant cuttings

Gardeners and commercial growers use auxins to propagate plants from cuttings:

• Plant cuttings are sections of stem cut from a parent plant
• The cut end is dipped in rooting powder containing synthetic auxins
• Auxins stimulate rapid root development from the cut stem
• This produces genetically identical plants (clones) more quickly than growing from seed
• Particularly useful for plants that are difficult to grow from seed or to maintain desirable characteristics

Advantages of using auxins for propagation:

• Faster root development than natural growth
• Higher success rate for cuttings
• Produces plants identical to the parent (preserving desirable traits)
• Enables mass production of ornamental plants and fruit trees

Selective weedkillers

Synthetic auxins are used as selective herbicides in agriculture:

• Applied to cereal crops and lawns to kill broad-leaved weeds
• Broad-leaved plants (dicotyledonous plants) absorb more of the auxin
• The high auxin concentration causes uncontrolled, rapid growth
• This disrupts normal plant processes and kills the weed
• Narrow-leaved plants (monocotyledonous plants) like grasses, wheat and barley are less affected
• They absorb less auxin and are more resistant to its effects

This selective action allows farmers to:

• Protect cereal crops while removing competing weeds
• Increase crop yields by reducing competition for light, water and nutrients
• Maintain productive grassland and lawns
• Reduce manual weeding labour

Gibberellins in crop production

Gibberellins have multiple commercial applications in agriculture and horticulture, particularly controlling fruit production and ending seed dormancy.

Controlling fruit development

Gibberellins are applied to fruit crops to improve commercial value:

• Increase fruit size, particularly in seedless grapes
• Seedless grapes naturally grow smaller without seeds stimulating growth
• Gibberellin application compensates, producing larger, more marketable fruit -延长 flowering period, extending the harvest season
• Improve fruit uniformity and quality

Ending seed dormancy

Many seeds have natural dormancy periods that delay germination:

• Gibberellins break dormancy and promote germination
• Seeds germinate at predictable times, ensuring uniform crop establishment
• Particularly useful when natural cold periods (stratification) are impractical
• Enables year-round production in controlled environments
• Seeds germinate even under unfavourable conditions

Benefits for commercial growers:

• Predictable germination timing
• Uniform crop growth and maturity
• Reduced time between sowing and harvest
• Better scheduling of labour and resources

Promoting growth without flowering

Gibberellins can stimulate stem elongation:

• Useful for plants grown for stems or leaves rather than flowers
• Sugar beet plants treated with gibberellins grow larger without flowering
• Increases biomass and crop yield
• Delays bolting (premature flowering) in vegetables like lettuce and spinach

Ethene in fruit ripening

Ethene controls fruit ripening and is extensively used in the fruit industry to manage harvest and distribution.

Natural role of ethene

Ethene is produced naturally by ripening fruit:

• Stimulates enzymes that break down cell walls
• Converts starch to sugars, making fruit sweeter
• Causes colour changes (e.g., green to red in tomatoes)
• Softens fruit texture
• One ripe fruit releases ethene that triggers ripening in nearby fruit

Commercial control of ripening

Fruit distributors manipulate ethene levels to control ripening timing:

During transport:

• Fruit picked when unripe and firm (less damage during transport)
• Stored in ethene-free environments during shipping
• Low temperatures and controlled atmospheres slow ripening
• Bananas, tomatoes and avocados commonly transported this way from Caribbean and tropical regions to UK markets

Before sale:

• Fruit exposed to ethene gas in ripening rooms
• Controlled temperature and ethene concentration ensure uniform ripening
• Timing adjusted so fruit reaches shops at optimal ripeness
• Reduces waste from over-ripe fruit

Advantages of ethene control:

• Extends shelf life during international transport
• Reduces bruising and damage during shipping
• Fruit arrives in shops at perfect ripeness
• Reduces food waste throughout the supply chain
• Enables global fruit trade (e.g., Caribbean bananas to UK supermarkets)

Tissue culture and cloning

While tissue culture primarily uses nutrient media, plant hormones play crucial supporting roles in this advanced propagation technique.

The tissue culture process

Tissue culture allows rapid production of many identical plants:

1. Small tissue samples removed from parent plant (explants)
2. Explants placed in sterile growth medium containing nutrients
3. Plant hormones (auxins and cytokinins) added to promote cell division and differentiation
4. Cells form callus (unspecialized mass of cells)
5. Hormones adjusted to stimulate root and shoot formation
6. Plantlets transferred to soil when sufficiently developed

Commercial advantages

Tissue culture with hormone control offers several benefits:

• Produces thousands of identical plants from one parent
• Preserves rare or valuable plant varieties
• Disease-free plants when done under sterile conditions
• Year-round production independent of seasons
• Rapid multiplication of plants with desirable characteristics
• Used for orchids, oil palms and disease-free crop varieties

Worked examples

Example 1: Explaining selective weedkillers (4 marks)

Question: A farmer applies a weedkiller to wheat fields to kill dandelions. The weedkiller contains synthetic auxins. Explain why the dandelions die but the wheat plants are not affected.

Mark scheme answer:

• The weedkiller contains synthetic auxins (1 mark)
• Dandelions are broad-leaved plants that absorb more of the auxin / are more sensitive to auxin (1 mark)
• The high concentration of auxin causes rapid, uncontrolled growth in the dandelions (1 mark)
• This disrupts normal processes and kills the dandelions / wheat is narrow-leaved and less affected by the auxin (1 mark)

Examiner note: Ensure you explain both why dandelions die AND why wheat survives. Avoid vague statements like "it affects them differently" — specify the mechanism.

Example 2: Comparing hormone uses (6 marks)

Question: A gardener wants to grow new rose plants from cuttings and also wants to increase the size of grapes. Describe how plant hormones could be used in each case.

Mark scheme answer:

For rose cuttings:

• Use rooting powder containing auxins (1 mark)
• Dip the cut end of the rose stem in the powder (1 mark)
• Auxins stimulate root growth / development (1 mark)

For increasing grape size:

• Apply gibberellins to the grape plant / grapes (1 mark)
• Gibberellins promote cell division / cell elongation (1 mark)
• This increases the size of the fruit (1 mark)

Examiner note: This question tests application of knowledge. You must name the specific hormone and its specific use — don't confuse auxins and gibberellins.

Example 3: Fruit ripening control (5 marks)

Question: Bananas are grown in the Caribbean and transported to the UK. Explain how commercial growers use plant hormones to ensure bananas arrive in supermarkets at the correct ripeness.

Mark scheme answer:

• Bananas are picked when unripe / green / firm (1 mark)
• They are transported in ethene-free conditions / low temperature / controlled atmosphere (1 mark)
• This prevents premature ripening during transport (1 mark)
• Before distribution to shops, bananas are exposed to ethene gas (1 mark)
• This stimulates ripening / the ethene triggers enzyme activity that ripens the fruit (1 mark)

Examiner note: This tests understanding of practical application. Good answers explain the complete process from harvest to shop, not just one stage.

Common mistakes and how to avoid them

• Confusing which hormone does what — Learn specific functions: auxins for roots and weedkiller, gibberellins for fruit size and germination, ethene for ripening. Make a comparison table to memorize these.

• Writing "plant food" or "fertilizer" instead of hormone — Plant hormones are chemical messengers that control growth, not nutrients. Use precise terminology: auxins, gibberellins, ethene.

• Failing to explain WHY selective weedkillers are selective — Don't just say "it kills weeds but not crops." Explain that broad-leaved plants absorb more auxin and that high concentrations cause uncontrolled growth leading to death.

• Saying ethene "makes fruit ripe" — Be more specific: ethene triggers enzyme production that breaks down cell walls, converts starch to sugar, and causes colour changes. This shows deeper understanding.

• Confusing tissue culture with taking cuttings — Tissue culture uses small cell samples grown in sterile conditions with nutrient media. Cuttings are stem sections grown in soil with rooting powder. Both use hormones but are different techniques.

• Not reading the question carefully about marks allocation — A 4-mark question needs 4 distinct points. If you only write 2 sentences, you cannot score full marks regardless of quality.

Exam technique for "Commercial uses of plant hormones"

• Command word awareness — "Describe" requires you to state features or processes; "Explain" requires reasons or mechanisms. For explain questions, use "because," "this causes," or "as a result" to show causation.

• Link to commercial benefits — Questions often ask about practical applications. Connect hormones to real outcomes: increased profit, reduced waste, better yield, extended shelf life, labour savings.

• Use correct hormone names — Write "auxins," "gibberellins," and "ethene" not "plant hormones" or "growth chemicals." Examiners award marks for precision.

• Apply knowledge to unfamiliar contexts — You might see questions about fruits or plants not specifically mentioned in class. Apply your hormone knowledge to the scenario: if it's about roots, think auxins; if about fruit size, think gibberellins; if about ripening, think ethene.

Quick revision summary

Plant hormones have important commercial applications. Auxins promote root growth in cuttings (rooting powder) and act as selective weedkillers by killing broad-leaved plants through uncontrolled growth while leaving narrow-leaved crops unharmed. Gibberellins control fruit size in seedless grapes, end seed dormancy for predictable germination, and promote stem growth. Ethene regulates fruit ripening, allowing fruit to be transported unripe and ripened on demand using ethene gas in controlled environments. These applications improve crop yields, reduce waste and enable global food distribution.