Plant Responses

• A stimulus is anything that causes a reaction in an organism. E.g. loght, gravity, temperature
• External factors in responses in flowering plants are: light (energy for photosynthesis), day length (influences fruit formation), gravity (causes roots to grow downwards), and temperature (affects rate of enzyme activity)
• an Internal factor for responses in flowering plants is that growth regulators are produced in the meristematic zone.
• A tropism is a change in the growth of a plant in response to an external stimulus.
• Phototropism is the change in growth of a plant towards light.
• Geotropism is the change in growth of a plant in response to gravity.
• Thigmotropism is the change in growth of a plant in response to touch.
• Hydrotropism is the change in growth of a plant in response to water.
• Chemotropism is the change in growth of a plant in response to chemicals.
• Growth regulators are chemicals that control the growth of a plant.
• Growth promoters increase the rate of growth e.g. Auxins and growth inhibitors slow down the growth of a plant e.g. Ethylene
• Indoleacetic acid is made in the shoot tips, young leaves and seeds. Auxins cause stem root and growth as well as fruit formation.
• Auxin stimulation root growth and development of fruit.
• Auxins loosen cell walls which allows them to expand and elongate. If one side of the stem is exposed to light, IAA will diffuse down the shaded side, these elongate and the stem bends toward the light.
• Ethene causes fruit to ripen and color to form. It causes leaves to fall in autumn. It is used to commercially ripen bananas.
• Abscisic acid is produced in the leaves of plants and causes them to respond to harmful conditions. It causes stomata to close in dry conditions.
• Plath growth regulators can be produced artificially outside of the plant e.g. Rooting powder and tissue culturing.
• In tissue culturing, small pieces of plants are used to produce many more new plants. Auxins are used to produce a mass of cells called a callus.
• Some plants have epidermis or bark; this prevents pathogens to enter the plant and also prevents the loss of water. Some plants have thorns to prevent them from being eaten.
• Phytoalexins prevent against micro-organisms and pathogens by attacking their cell walls.