Plant Responses

Plants need to respond to their environment in order to:

• Cope with changing conditions and avoid abiotic stress
• Maximise photosynthesis by obtaining more light, more water and more minerals
• Avoid grazing
• Ensure the germination of its seeds in suitable conditions

Tropism: A directional growth response in which the direction of the response is determined by the direction of the external stimulus.

Plant responses to environmental changes are coordinated by hormones.
The presence of auxin promots the active transport of hydrogen ions throug the ATPase enzymes into the cell wall. This decreases the pH and allows optimum conditions for the wall loosening enzymes to work. These enzymes break bonds within the cellulose, so the walls become less rigid and can expand as the cells take in 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 elongate. Because the cells elongate more on the shaded side than the side in the light, the shoot bends towards the light source.

When a plant becomes overcrowded, auxin is released and causes phototropism as shoots bend towards the light. Etiolate means that the plant grows taller. Climbing plants will climb up and over other plants, a response due to thigmotropism. Roots will grow towards water and therefore minerals. Some plants (e.g. dandelions) will secrete chemicals from their roots that kill other plants.

Experimental evidence for the…