6.3.1 Plant hormones and tropisms

Plant hormones coordinate growth

1. A plant hormone is a chemical messenger made by the plant that changes the activity of target cells.
2. Plant hormones coordinate growth, development and responses to environmental stimuli.
3. Unlike a nervous response, a plant response is usually slow because it often depends on cells growing at different rates.

Tropisms direct plant growth

1. A tropism is a directional growth response in which a plant organ grows towards or away from a stimulus.
2. A positive tropism is growth towards the stimulus, whereas a negative tropism is growth away from it.
3. Phototropism is growth in response to light, while gravitropism is growth in response to gravity.
4. Shoots are usually positively phototropic and negatively gravitropic, which places leaves where they can receive more light.
5. Roots are usually positively gravitropic, which directs them into soil for anchorage and access to water and mineral ions.

Auxin creates unequal growth

1. Auxin is produced in the shoot tip and moves into the growing region immediately behind the tip.
2. Auxin changes the rate of cell elongation, so it controls how much individual cells increase in length.
3. When auxin is distributed evenly, cells on opposite sides elongate at the same rate and the organ grows straight.
4. A directional stimulus such as light or gravity can produce an uneven auxin distribution across the organ.
5. Cells on the two sides then elongate at different rates, making one side longer and causing the organ to curve.
6. The effect depends on the organ: a higher auxin concentration promotes elongation in shoot cells but inhibits elongation in root cells.

Shoots bend towards light

1. When light reaches a shoot from one side, more auxin becomes distributed on the shaded side of the shoot.
2. In shoots, the higher auxin concentration stimulates cell elongation on the shaded side.
3. The shaded side therefore becomes longer than the illuminated side, so the shoot curves towards the light in positive phototropism.

Roots respond differently to auxin

1. When a root is placed horizontally, gravity causes more auxin to collect on its lower side.
2. Root cells are more sensitive to auxin than shoot cells, so the higher auxin concentration inhibits cell elongation on the lower side.
3. Cells on the upper side elongate faster, which makes the root curve downwards and produces positive gravitropism.
4. In a horizontal shoot, auxin on the lower side stimulates elongation, so the lower side grows faster and the shoot curves upwards in negative gravitropism.
5. The same hormone can therefore produce opposite growth patterns because roots and shoots respond differently to its concentration.

Compare shoots and roots

1. In a shoot, more auxin means faster elongation, so the side with more auxin becomes longer.
2. In a root, more auxin means slower elongation, so the opposite side becomes longer.
3. In both organs, bending results from unequal cell elongation, not unequal cell division.

6.3.2 Commercial uses of plant hormones

Auxins support crop production

1. Synthetic auxins are used in selective weedkillers that affect broad-leaved weeds more than narrow-leaved grasses and cereal crops.
2. The auxin disrupts normal growth and makes susceptible weeds grow uncontrollably, so they die while the crop remains.
3. Killing weeds reduces competition with the crop for light, water, mineral ions and space, which can increase crop yield.
4. Auxin in rooting powder stimulates root formation at the cut end of a stem cutting.
5. Once roots form, the cutting can grow independently and produces a new plant genetically identical to the parent.

Gibberellins control development

1. Gibberellins can end seed dormancy and start germination when growers want a crop to develop.
2. During germination, gibberellins stimulate enzymes that break stored starch into sugars, which provide a substrate for respiration and growth.
3. Growers can use gibberellins to stimulate flower formation and to promote fruit formation.
4. Applying gibberellins before pollination can start fruit development without fertilisation, producing seedless fruit.
5. Gibberellins can also increase fruit size, which is useful when producing larger seedless grapes.

Ethene controls fruit ripening

1. Ethene is a gaseous plant hormone that triggers and coordinates changes during fruit ripening.
2. Fruit can be harvested while it is unripe and firm, so it is less likely to be bruised during storage and transport.
3. The fruit is exposed to ethene in a ripening room shortly before sale, which produces a more even and predictable stage of ripeness.
4. Controlled ripening lets growers and retailers time supply, extend transport time and reduce losses from fruit becoming overripe too early.

Link each hormone to an outcome

Match the hormone to the use

1. Auxins are used for selective weed control and root formation in cuttings.
2. Gibberellins are used for germination, flowering, fruit development and seedless fruit.
3. Ethene is used to control when harvested fruit ripens.