Selective weedkillers

1. A selective herbicide contains a synthetic auxin, a chemical that behaves like the plant's own auxin.
2. The herbicide is sprayed across a whole field, so it lands on the crop and the weeds together.
3. Broad leaved weeds such as dandelions and thistles have a large leaf surface area, so they absorb far more of the spray.
4. The high auxin concentration inside them makes them grow uncontrollably fast.
5. Growing that fast uses up their stored food faster than photosynthesis can replace it, so the weeds die.
6. Narrow leaved crops such as wheat, barley and grass catch far less spray on their upright leaves and survive.
7. Removing the weeds reduces competition for light, water, mineral ions and space, so the crop yield rises.
8. Repeated use can select for resistant weeds, and spray drifting onto hedgerows reduces the biodiversity around the field.

Rooting powder for cuttings

1. A cutting is a short length of stem cut from a parent plant.
2. The cut end is dipped in rooting powder, which contains auxins at the concentration that promotes root growth.
3. The auxin stimulates the cells at the cut end to develop into new roots.
4. Once rooted, the cutting can absorb water and mineral ions and grow into a complete plant.
5. Every plant produced this way is a clone of the parent, so a desirable variety is reproduced exactly.
6. The method is faster and more reliable than growing from seed, and it works on species that root poorly on their own.
7. Because the plants are genetically identical, a whole crop of them can be wiped out by the same disease.

Producing seedless fruit

1. A flower normally has to be pollinated and fertilised before its ovary develops into a fruit.
2. The developing seeds release hormones, and those hormones make the ovary wall grow into the fruit.
3. Spraying auxins or gibberellins onto unpollinated flowers supplies those hormones from outside.
4. The ovary develops into a fruit even though no fertilisation has happened, which is parthenocarpy.
5. Because there was no fertilisation, the fruit contains no seeds.
6. Seedless grapes, and some tomatoes and cucumbers, are produced this way.
7. Customers will pay more for fruit without seeds, so the grower earns a higher price for the same crop.
8. No seeds also means no way of growing the next generation from them, so growers must buy new stock or take cuttings.

Altering seed dormancy

1. Many seeds stay in dormancy until they have had a period of cold or several months in storage.
2. Dormancy protects a wild seed from germinating in autumn, when the seedling would be killed by winter.
3. For a grower, dormancy is a nuisance, because a sown batch germinates over several weeks rather than all at once.
4. Treating the seeds with gibberellin ends dormancy, so they germinate straight away.
5. The whole batch then germinates at the same time, giving an even crop that can be harvested on one date.
6. Gibberellin is used the same way on barley grains in malting for the brewing industry.
7. It makes every grain produce amylase together, and that amylase converts the stored starch into sugars.
8. The sugars are then fermented by yeast, so treating the grain shortens the whole process.

Controlling fruit ripening

1. Fruit is picked while it is still unripe and hard, because ripe fruit bruises easily in transit.
2. It is then stored in cool air that is low in oxygen and from which ethene has been removed.
3. Those conditions slow respiration and stop the ripening reactions, so the fruit stays unripe for weeks.
4. Ethene gas is released into the store a few days before the fruit is needed.
5. The ethene switches on the ripening enzymes, so the fruit softens, sweetens and changes colour together.
6. Timing the ripening this way means fruit reaches the shelves ready to eat rather than already spoiled.
7. It also lets fruit be shipped long distances, so supermarkets can sell it all year round.
8. The stores need refrigeration and gas control, so the method costs energy and money.