4.5.4.2 - Use of Plant Hormones

Growth control uses

A plant hormone is a chemical made by a plant that affects growth or development in plant cells. People can apply natural or synthetic plant hormones to control plant growth in agriculture, horticulture and the food industry.

Horticulture is the growing of plants in gardens, nurseries and glasshouses, often for food, flowers or ornamental use. For this specification point, remember the exact hormone-to-use match rather than a vague idea that "hormones make plants grow".

[DIAGRAM: asset_name: plant-hormone-use-map - diagram 1; asset_slug: 069_4_5_4_2_use_of_plant_hormones_diagram1; file: diagram_assets/imagegen_regen_all/069_4_5_4_2_use_of_plant_hormones_diagram1_imagegen.png; recommended_method: codex_image_gen; description: Monochrome concept map with three plant hormone groups. Auxins branch to weed killers, rooting powders and tissue culture growth. Ethene branches to fruit ripening during storage and transport. Gibberellins branch to ending seed dormancy, promoting flowering and increasing fruit size. Include a small biodiversity note from auxin weed-killer use: fewer weeds can mean fewer plant species and less food or habitat.]

Auxins in horticulture

Auxins are plant hormones involved in growth. In this lesson, the assessable uses are weed killers, rooting powders and promoting growth in tissue culture.

Some weed killers contain synthetic auxins. In susceptible weeds, these can cause rapid, uncontrolled growth, which damages or kills the plant. A good GCSE answer says as weed killers, not just "for growth", because "growth" on its own is too vague.

Rooting powders are used when plant cuttings are taken. A cutting is a piece of plant, such as a stem, that can grow into a new plant if it forms roots. Auxin in rooting powder increases the chance that roots will form, so more cuttings survive.

In tissue culture, small pieces of plant tissue are grown on a nutrient medium. Auxins can be added to promote growth of the plant tissue. You do not need a detailed sterile tissue-culture method here; keep the answer to the AQA use.

Ethene and ripening

Ethene is a plant hormone involved in fruit ripening. Many sources call it ethylene; AQA uses the GCSE chemistry name ethene.

Fruit often needs to travel a long distance before it is sold. If fruit ripens too early, it may become soft, damaged or over-ripe before it reaches the shop. If ripening is controlled, fruit can be transported and stored while it is firmer, then ripened closer to sale.

The important exam phrase is control ripening, not simply "make fruit ripe". Control can mean timing ripening so fruit is at a suitable stage for transport, storage and sale.

Gibberellins in crops

Gibberellins are plant hormones involved in seed germination, flowering and fruit development. The detailed mechanism is not required; you need the three AQA uses and when each one makes sense.