Plant Responses Flashcards
What are the different types of tropisms?
- phototropism - response to light
- geotropism - response to gravity
- chemotropism - response to certain chemicals
- thigmotropism - response to touch
Why do plants need to be able to respond to their environment?
• plants need to increase their chances of survival by responding to changes in their environment
- e.g. They sense the direction of light and grow towards it to maximise the light adsorption for photosynthesis
• plants are more likely to survive if they respond to the presence of
predators to avoid being eaten
- e.g. White clover can produce substances that are toxic to cattle
• plants are more likely to survive if they respond to abiotic stress
- e.g. Some plants respond to extreme cold by producing their own form of antifreeze
What causes a response?
• plant responses are coordinated by growth regulators (hormones) - they speed up or slow down plant growth
• they are produced only in the growing regions of the plants
- apical meristems (located in the tips of roots and shoots)
- lateral bud meristems (located in the buds)
- lateral meristem (found in a cylinder near the outside of roots and shoots, responsible for stems getting wider)
• they move around the whole plant
• they are specific and act on particular target tissues which have specific complementary receptors in their cell surface membranes
What happens if light is shining in one direction onto the shoot?
- indoleacetic acid (IAA) is an important auxin involved in phototropism, it is produced in the tip of the shoot (apical buds)
• if light is shining from one direction IAA is transported to cells on the shaded side
• cells on the shaded side now have a higher concentration of IAA so elongate more
• so the shoot grows towards the light
Explain how hormones contribute to leaf loss in deciduous plants
- cytokinins stop the leaves of deciduous trees senescing (turning brown and dying) by making sure the leaf acts a sink for phloem transport - leaf has a good supply of nutrients
- auxin produced in the leaf prevents abscission (shedding) by making the cells insensitive to ethene
- when the concentration of cytokinins in the leaf falls leaf senescence occurs which causes the production of auxin in the tip to decrease
- the makes the abscission zone become more sensitive to ethene
- the drop in auxin concentration also causes an increase in ethene concentration
- this causes the production of the enzyme cellulose to increase which digests the cells in the abscission zone eventually separating the leaf from the stem
Define the term tropism
- it is a directional growth response in which the direction of the response is determined by the direction of the external stimulus
- positive tropism - growth towards the stimulus
- negative tropism - growth away from the stimulus
What is apical dominance?
- auxins stimulate the growth of the apical bud and inhibit the growth of side shoots, this is called apical dominance as the apical bud is dominant over the side shoots
- apical dominance helps to save energy and prevents side shoots from the same plant competing with the tip for light
What experimental evidence is there to prove the role of auxins in the control of apical dominance?
- if the apical bud is removed or an auxin transport inhibitor is applied below the apes of the main shoot then side shoots will start to grow
- this shows that auxin is produced in the apex of the main shoot and transported to lateral buds to inhibit their growth
- when there are low concentrations of auxin in the side shoot 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, this is because they are further away from the main shoot, so there is a lower concentration of auxin and their growth is less inhibited
What do gibberellins do?
- gibberellins are produced in young leaves and in seeds and they stimulate seed germination, stem elongation, side shoot formation and flowering
- gibberellins stimulate the stems of plants to grow by cell elongation
How can plant hormones be used commercially in the fruit industry?
• Ethene stimulates the ripening of fruit
- it stimulates enzymes that break down cell walls, break down chlorophyll and convert starch to sugars
- this makes fruit soft, ripe and ready to eat
• auxins and gibberellins make fruit develop
- auxins and gibberellins are sprayed onto unpollinated flowers which makes the fruit develop without fertilisation
• auxins can prevent or tigger fruit drop
- applying a low concentration of auxins in the early stages of fruit production prevents the fruit from dropping off the plant
- but applying a high concentration of auxins at a later stage of fruit production triggers the fruit to drop
How can plant hormones be used commercially by farmers and gardeners?
• auxins are used in selective weed killers (herbicides)
- auxins make weeds produce long stems instead of lots of leaves, this makes weeds grow too fast so they can’t get enough nutrients or water so they die
• auxins are used as rooting hormones
- auxins make a cutting grow roots, the cutting can then be planted and grown into a new plant
What experimental evidence is there to prove the role of gibberellins in the control of stem elongation
• if genetically dwarf plants are treated with gibberellic acid, the stems elongate considerably
