Stem > always grows towards light
Roots > always grow away from light
Auxins (plant hormones controlling its growth) are produced in the growing tips of shoots and roots
- Auxins respond to light, gravity and moisture
- Auxins cause some cells to elongate or grow at a faster rate
Tropism = plant’s response to light, gravity or moisture
- Positive tropism = response towards stimulus
- Negative tropism = response away from stimulus
Geotropism = plant’s response to gravity
- Positive geotropism = roots grow towards direction of gravity
- Negative geotropism = shoots grow away from direction of gravity
This video shows how Geotropism works
Hydrotropism = plant’s response to water
- Positive hydrotropism = shoots always follow direction of water
Phototropism = plant’s response to light
This video shows how Phototropism works
Light destroys auxins. Auxins on sunlit side of plant are destroyed, growth slows. Growth of plant on shaded side continues > plant growth leans towards light
Commercial Ues of Plant Hormones
1. Growing cuttings: cuttings are dipped into powder containing growth hormones. Plants are cloned quickly and cheaply
2. Producing fruit without seeds: Growth hormones sprayed on unpollinated flowers > grow fruit without fertilisation
3. Ripening fruit: Plant hormone ethene ripens fruit. Sprayed onto unripened fruit during transportation to supermarkets
4. Increasing size of fruit: e.g. grapes
