Why do plants need to respond to their environment
To cope with changing conditions and avoid abiotic stress.
Maximise photosynthesis by obtaining more light, more water and more minerals.
Avoid predation and grazing.
Ensure the germination of its seeds in suitable conditions.
Define the term tropism.
A directional growth response in which the direction of the response is determined by the direction of the external stimulus.
Plant responses coordinated by hormones
The presence of auxin promots the active transport of hydrogen ions through the ATPase enzyme, into the cell walll. This decreases the pH and allows optimum conditions of the wall loosening enzymes to work. These enzymes break bonds in the cellulose, so the walls become less rigid as the cells take in more water.
A shoot bends towards a light source because auxin is transported from the tip of the shoot to the cells in the shade, allowing the cells to take up more water and elongated. Because the cells elongate more on the shaded side than the side in the light, the shoot bends towards the light source.
Auxins and apical dominance, gibberellin and stem
Auxins: Apical dominance is when the growing apical bud at the tip of the shoot inhibits the growth of the lateral buds further down the shoot. Auxins are produced in the tip of the main shoot, and they inhibit the growth of the side shoots. When the tip of the main shoot is removed (e.g. by an animal), the side shoots grow. This shows that auxin produced in the apex of the main shoots and transported to the lateral buds to inhibit their growth.
When there are low concentrations of auxin in the side shoots, their growth is not inhibited, and so they can grow. This is also shown where, as the plant grows taller, the lateral buds at the bottom of the plant start to grow larger - they are further away from the main shoot, so there is a lower concentration of auxin and their growth is less inhibited.
Gibberellins: If genetically dwarf plants are treated with Gibberellic acid, the stems elongate considerably.
Role of hormones in leaf loss in deciduous plants.
Cytokinins stop the leaves of deciduous plants from senescing by making sure that the leaves act as a sink for phloem transport, so the leaves are guarenteed a good supply of nutrients. If cytokinin production drops, the supply of nutrients dwindles and senescence begins. Senescence causes auxin production at the tip of the leaf to drop. This makes the cells in the abscission zone more sensitive to ethene. A drop in auxin concentration causes an increase in ethene production. This increases the production of cellulase, which digests the cell walls of the cells in the abscission zone, eventually spearating the petiole from the stem, so the leaf drops.
Describe how plant hormones are used commercially.
- Produce seedless fruits
- Act as weedkillers
- Act as rooting powder to grow cuttings
- Control fruit ripening
- Control fruit drop
- Restrict hedge growth
- Preserve cut flowers and green vegetables
- Promote sexual maturity in conifers
- Promote sexual maturity in female cucumber plants
- Promote latex flow in rubber plants
- Produce longer nodes in sugar cane
- Produce longer stems in grapes