Plant hormones play vital roles in agriculture and horticulture. Understanding how auxins, gibberellins, ethene, and other hormones are exploited allows humans to manipulate plant growth and development for economic and environmental benefit.

Hormones in Commercial Plant Management

The Role of Plant Hormones

Plant hormones, also known as plant growth regulators, are chemical messengers that influence cell growth, division, differentiation, and organ development. By applying these hormones synthetically, humans can control growth patterns, enhance yield, and improve storage or transport characteristics of crops.

Auxins in Commercial Use

Rooting Powders

Auxins, particularly indole-3-acetic acid (IAA) and synthetic analogues such as indole-3-butyric acid (IBA) and naphthaleneacetic acid (NAA), are used to stimulate root growth in cuttings.

When plant cuttings are dipped into rooting powder containing auxin, root initiation is stimulated in the stem base, increasing the likelihood of successful propagation. This technique is crucial for commercial production of plants that do not easily produce roots naturally, such as woody ornamentals and fruit trees.

Key points:

• Auxin increases cell wall plasticity, allowing elongation of root cells.

• Synthetic auxins are preferred because they are more stable and cost-effective than natural IAA.

• Optimal auxin concentration is critical — excess levels inhibit root growth.

Tissue Culture and Micropropagation

Auxins also work synergistically with cytokinins in tissue culture. The auxin:cytokinin ratio determines organ formation:

• High auxin, low cytokinin → root formation

• Low auxin, high cytokinin → shoot formation

This manipulation allows rapid cloning of genetically identical plants.

A simplified workflow of plant tissue culture from explant to regenerated plant. Commercial labs adjust the auxin:cytokinin ratio to induce roots or shoots for rapid micropropagation of elite cultivars. The diagram includes general steps beyond the OCR syllabus but remains directly relevant to commercial hormone use. Source.

Auxins as Weed Killers (Herbicides)

Synthetic auxins, such as 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), are used as selective herbicides. They are particularly effective against dicotyledonous (broad-leaved) weeds while leaving monocotyledonous (grassy) crops such as wheat and maize unharmed.

Mechanism of action:

• Mimic natural auxin but are non-biodegradable and persist longer.

• Cause uncontrolled cell division and elongation in susceptible plants.

• Lead to distorted growth, nutrient depletion, and plant death.

This selectivity allows efficient weed control in cereal fields and lawns.
However, misuse or high doses can damage non-target plants and ecosystems, so environmental regulation governs herbicide application.

Gibberellins in Agriculture

Controlling Stem Elongation and Growth

Gibberellins are used to promote stem elongation in dwarf varieties of plants, such as sugarcane, by stimulating cell division and elongation in internodes. This leads to greater biomass and sugar yield per plant.

Fruit Development

Gibberellins can induce seedless fruit (parthenocarpy) in species like grapes, enhancing size and commercial value. They also delay senescence, improving shelf life during transport.

Gibberellin treatment produces longer, better-spaced grape bunches suitable for premium table-grape markets. Applications at defined stages promote elongation and reduce compactness, improving appearance and reducing post-harvest losses. The image also implies effects on berry spacing, which aligns with commercial GA₃ use. Source.

Brewing and Malt Production

Gibberellins are used to trigger amylase enzyme synthesis in barley seeds during malting. This converts stored starch into maltose, an essential sugar for brewing.

Commercial advantages:

• More consistent malting process.

• Controlled timing for large-scale beer production.

Ethene and Control of Fruit Ripening

Regulating Ripening in Storage and Transport

Ethene (C₂H₄) is a gaseous plant hormone that stimulates fruit ripening by promoting enzyme activity involved in softening and colour change.

Commercially, ethene or ethylene-releasing compounds (such as ethephon) are used to control ripening schedules.

Practical applications:

• Bananas and tomatoes are harvested green and firm to prevent damage during transport.

• Exposure to ethene in storage triggers uniform ripening just before sale.

• Controlled atmosphere storage (reducing ethene concentration and oxygen) slows ripening, extending storage time.

Bananas in multiple ripeness stages demonstrate how ethene accelerates the transition from green to sale-ready yellow. Commercial systems expose stored fruit to controlled ethene to achieve uniform ripening on demand. This visual aligns with the practice of harvesting firm, green fruit for transport and ripening near market. Source.

Synchronising Flowering and Harvest

Ethene also promotes flower induction in some species (e.g., pineapples), allowing uniform fruiting for efficient mechanical harvesting.

Cytokinins in Commercial Production

Though not explicitly required by the OCR specification for this subsubtopic, cytokinins are often used alongside auxins in horticulture. They delay leaf senescence, promote bud formation, and increase post-harvest freshness in cut flowers and vegetables.

Example uses:

• Spraying cytokinins on lettuce or broccoli maintains green pigmentation.

• Extends shelf life, reducing food waste and increasing profitability.

Integration of Hormonal Applications

Combining Hormonal Effects

Commercial plant production often involves integrated use of several hormones to achieve specific goals:

• Auxins and gibberellins may be combined to improve fruit set.

• Ethene and gibberellins coordinate fruit ripening and growth.

• Adjusting hormone ratios allows precise control over development stages.

Economic and Ethical Considerations

Hormonal applications enhance yield and reduce losses, but must be managed responsibly. Overuse can:

• Disrupt ecosystems through runoff or residue accumulation.

• Influence consumer perception of ‘natural’ food production.

• Lead to resistance in target weeds if herbicides are misused.

Regulatory frameworks such as EU pesticide directives and the UK Chemicals Regulation Division ensure safe application and environmental monitoring.

Summary of Key Commercial Roles

Auxins:

• Rooting powders for propagation.

• Selective weed killers in agriculture.

• Organogenesis in tissue culture.

Gibberellins:

• Stem elongation in dwarf crops.

• Induction of seedless fruit.

• Enzyme activation in brewing.

Ethene:

• Controlled fruit ripening and storage management.

• Synchronised flowering and harvesting.

These hormonal applications represent powerful tools in modern agriculture, allowing efficient, predictable, and profitable plant production aligned with the OCR A-Level Biology syllabus.