Controlling plant growth

Auxins - Higher tier

Auxins are mostly made in the tips of the shoots and roots, and can diffuse to other parts of the plant. Auxins change the rate of elongation in plant cells, controlling how long they become. Shoots and roots respond differently to high concentrations of auxins: cells in shoots grow more cells in roots grow less. Phototropisms In a shoot, the shaded side contains more auxin. This means that the shaded side grows longer, causing the shoot to bend towards the light. The diagram shows the typical results seen when growing oat seedlings in a box, with a light at one side. Auxins have the opposite effect on root cells. In a root, the shaded side contains more auxin, but this time the shaded side grows less than the light side. This causes the root to bend away from the light. Geotropisms Auxins are also involved geotropisms. In a root placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow less than the top side, causing the root to bend in the direction of the force of gravity. In a shoot placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow more than the top side, causing the shoot to bend and grow against the force of gravity.

Responses to stimulus of different parts of the plant

Responses to stimulus of different parts of the plant part of plant light gravity shoot-positive phototropism (grow towards the light) negative geotropism (grow against the force of gravity) root -negative phototropism (grow away from the light) positive geotropism (grow in the direction of the force of gravity) The tropisms of shoots mean that they are likely to grow into the air, where there is light for photosynthesis. The tropisms of roots mean that they are likely to grow into the soil, where there is moisture. Tropisms are controlled by plant hormones called auxins. These water-soluble chemicals move through the plant in solution

Controlling plant growth

A 'tropism' is a growth in response to a stimulus. Plants grow towards sources of water and light, which they need to survive and grow. Auxin is a plant hormone produced in the stem tips and roots, which controls the direction of growth. Plant hormones are used in weedkillers, rooting powder and to control fruit ripening.

Uses of plant hormones

Plant hormones can be used in agriculture to control the rate of plant growth. Weed killers Selective weed killers attack some plants but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass, or getting rid of thistles in a field without killing the wheat plants. The selective weed killer contains growth hormone that causes the weeds to grow too quickly. The weed killer is absorbed in larger quantities by the weeds rather than the beneficial plants. Rooting powder Rooting powder makes stem cuttings quickly develop roots. Rooting powder contains growth hormones. Controlling fruit ripening Some hormones slow the ripening of fruits and others speed it up. These hormones and their inhibitors are useful for delaying ripening during transport or when fruit is displayed in shops. Dormancy Dormancy stops seeds germinating until conditions are ideal for growth. Hormones can be used to remove the dormancy of a seed so it can germinate at all times of year. Buds and flowers can also be naturally dormant. Hormones can also be used to make plants grow bushier, make them flower or control the growth of hedge plants.

Tropisms

Plants need light and water for photosynthesis. They have developed responses called tropisms to help make sure they grow towards sources of light and water. There are two main types of tropisms: positive tropisms - the plant grows towards the stimulus negative tropisms - the plant grows away from the stimulus. Phototropism is a tropism where the stimulus is light. A geotropism is a tropism where the stimulus is gravity. The roots and shoots of a plant respond differently to the same stimuli.