Plant hormones are widely used in agriculture and horticulture to control plant growth. Higher tier students should be able to describe the effects of some plant hormones and the different ways people use them.
Auxins are used in three main ways. As weed killers, they make broad-leaved weeds grow too fast and die, which leaves the narrow-leaved crop plants unharmed. As rooting powders, they help cuttings grow roots quickly, so a gardener can grow many new plants from one parent. They are also used to promote growth in tissue culture, where tiny pieces of plant are grown into new plants. Using weed killers affects biodiversity, because the number of different plant species in an area is reduced.
Ethene is used in the food industry to control the ripening of fruit during storage and transport. Fruit can be picked while it is unripe and firm, so it is not damaged on the journey. Ethene is then added to ripen it just before it reaches the shops.
Gibberellins can be used to end seed dormancy, to promote flowering and to increase fruit size.