Plant Hormones and their Commercial Uses

Auxin

• Auxin is a hormone that controls growth near the tips of shoots and roots.
• It controls the growth of a plant in response to light (phototropism) and gravity (geotropism or gravitropism).
• Auxin is produced in the tips and moves backwards to stimulate the cell elongation process that occurs in the cells just behind the tips.
• If the tip of a shoot is removed, no auxin is available and the shoot may stop growing.
• Extra auxin promotes growth in the shoot but inhibits growth in the root.
• When a shoot tip is exposed to light, more auxin accumulates on the side that's in the shade than the side that's in the light.
• This makes the cells grow (elongate) faster on the shaded side, so the shoots bend towards the light.
• When a shoot grows sideways, gravity produces an unequal distribution of auxin in the tip, with more auxin on the lower side.
• This causes the lower side to grow faster, bending the shoot upwards.
• A root growing sideways will also have more auxin on its lower side.
• But in a root, the extra auxin inhibits growth. This means the cells on top elongate faster, and the root bends downwards.

Commercial uses of Plant Hormones

• Auxins are useful for controlling plant growth.
• Most weeds growing in fields of crops or in a lawn are broad-leaved, in contrast to grasses and cereals which have very narrow leaves. 
• Selective weedkillers have been developed using auxins, which only affec the broad-leaved plants. They totally disrupted their normal growth patterns, which soon kills them, whilst leaving…