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A directional growth response in which the direction of the response is determined by the direction of the external stimulus.
Why do plants need to respond to their environment?
Plants respond to biotic and abiotic stimuli, because by responding to the environment they manage to avoid predation and abiotic stress (such as cold temperature of drought), so that they can survive long enough to reproduce.
Give 4 different types of tropisms.
- Phototropism: directional growth in response to light stimuli.
- Geotropism: directional growth in response to gravity. Roots grow towards pull of gravity so that it anchors them in the soil and help them to take up water and minerals.
- Chemotropism: directional growth in response to chemical stimulus. Eg. pollen tubes grow towards ovary of a flower.
- Thigmotropism: directional growth in response to touch or contact stimuli.
If a plant responds towards a stimulus, it is a ___ tropic response. If a plant responds away from a stimulus, it is a ___ tropic response.
Plant hormones are often referred to as ___ ___ ___ because, unlike animal hormones, they are not produced in ___ ___, but by cells in a ___ of ___ in the plant.
- plant growth regulators
- endocrine glands
- variety of tissues
But why can plant growth regulators also be called hormones even though they aren't produced in endocrine glands?
- Because they are chemical messengers that can be transported away from their site of manufacture to act in other parts (target cells/tissues) of the plant.
- When they reach their target cells, they bind to receptors on the plasma membrane.
Plant growth regulators can move around the plant in 3 main ways. What?
- Active transport
- Mass flow in the phloem sap or in xylem vessels.
When different hormones amplify each other effects, its called ___. When they cancel each other's effects, we call it ____.
Give 4 examples of plant hormones/growth regulators and their effects.
- Auxins: promote cell elongation, inhibit growth of side shoots, inhibit leaf fall.
- Cytokinins: promote cell division
- Gibberellins: promote seed germination and growth of stems.
- Ethene: promotes fruit ripening.
The ___ ___ around a plant cell limits its ability to __ and ___. Because of this, growth only happnes in particular places in the plant, where there are a group of immature cells that are capable of __. These are called ___.
- plant cell
- divide and expand
1) Where are auxins produced? 2) What is it also called? 3) How do they travel 4) What effect does it have on places it travels?
- At the apex - the tip of shoots
- IAA (Indoleacetic acid)
- By diffusion or active tranport
- elongates the cells, making the shoot grow.
How do auxins work to elongate plant cells?
- Auxin increases the stretchiness of the cell wall by promoting the active transport of hydrogen ions, by an ATPase enzyme on the plasma membrane, into the cell wall.
- The resulting low pH provides optimum conditions for wall-loosening enzymes to work.
- These enzymes break bonds between cellulose microfibrils and the surrounding matrix, making the walls less rigid and allowing them to expand as the cell takes in water.
How do auxins coordinate phototropism in a plant, making it move towards the light?
- Light shining on one side of the shoot causes the auxins to be transported to the shaded side, where they promote an increase in the rate of elongation.
- The shaded side elongates faster than the illuminated side, so this bends the shoot towards the light.
- [Remember that the auxins are broken down by enzymes as it goes down the shoot, so it has no effect lower down.]
Why is losing leaves advantageous to a deciduous plant?
Help plants to conserve water (lost from leaves) during the cold part of the year, when it might be difficult to absorb water from the soil (the soil water may be frozen) and when there's less light for photosynthesis.
Outline the role of hormones in leaf loss in deciduous plants.
- Auxins are antagonistic to ethene.
- Auxins inhibit leaf loss - they are produced by young leaves, but as the leaf ages, less auxin is produced.
- Ethene stimulates leaf loss - ethene is produced by aging leaves, therefore they increase when auxins decrease.
- A layer of cells called the abscission layer develops at bottom of the leaf stalk, and separates the leaf from rest of plant. Ethene increases production of enzyme cellulase, which break the walls of the cells in the abscission zone, eventually separating the petiole from the stem.
What is apical dominance, what is involved and why is it advantageous to a plant?
- The growing apical bud at the tip of the shoot inhibits growth of lateral buds further down the shoot.
- Auxins stimulate the growth of the apical bud, but inhibits the growth of side shoots.
- It prevents side shoots from growing and this saves energy and prevents side shoots from the same plant competing with the shoot tip for light. It allows the plant to use more energy to grow tall, so to out-compete other plants to reach sunlight.
In taller plants, there are side shoots down below. Why?
Because auxins become less concentrated as they move away from the apical bud to the rest of the plant. If a plant grows very tall, the bottom of plant will have low auxin concentration so side shoots will start to grow near the bottom.
you have to evaluate experimental evidence for the role of auxins in the control of apical dominance and role of gibberellin in control of stem elongation, so you have to talk about controlling variables (such as water, light intensity etc) and repeating and all that, ok?
Explain very briefly the experimental evidence to show that auxins are involved in control of apical dominance.
- When you break the shoot tip (apex, full of auxins) off a plant, it starts to grow side branches from lateral buds that were previously dormant.
- So, for a test, they applied a paste containing auxins to the cut end of shoot, and lateral buds did not grow.
- However, this is weak evidence, as it could be that exposure to oxygen caused cells on cut end of stem to produce hormone that promoted lateral bud growth.
- So, they applied ring of auxin transport inhibitor below the apex of shoot, and lateral buds grew. Based on these results, they suggested that low auxin conc promoted growth in lateral buds.
- However, mustn't assume that auxin directly causes this.
- In fact, later a scientist disproved the direct causative link, and found that 2 other hormones, (abscisic acid and cytokinins) had a role to play.
- High auxin levels made abscisic acid conc to be high in lateral bud, inhibiting growth.
- High auxin levels made cytokinins (which promote growth) come to the shoot apex, but when low conc of auxin, the cytokinin goes to buds too.
Gibberellins ___ the stems of plants to __ by __ __. If a __ variety of a plant is treated with gibberellin, it will grow __.
- cell elongation
Give 2 ways auxins and gibberellins work together to affect plant growth.
- Auxins and gibberellins are often synergistic - they work together to amplify each other's effects. Eg. auxins and gibberellins work together to help plants grow very tall.
- They are also sometimes antagonistic. Eg. gibberellins stimulate growth of side shoots but auxins inhibit growth of side shoots.
Give ways in which auxin is used commercially. (5)
- Artificial auxins can be sued to prevent leaf & fruit 'drop'. Surprisingly, high conc of auxin can promote fruit drop, which is useful if farmer wants to produce larger fruits.
- Promote flowering
- Taking cuttings: Dipping end of cutting in rooting powder (made of auxin, fungicide and talcum) before planting encourages root growth.
- Seedless fruit: treat unpollinated flowers with auxin can promote growth of seedless fruit - because it promotes ovule growth etc..
- Herbicides: Artficial auxins can act in weeds longer because they are not a close fit to enzymes that break them down. They promote shoot growth so much that stem cannot support itself, buckles and dies.
Describe how gibberellins are used commercially. (5)
- Sugar production: spray sugar cane with gibberellins stimulates growth internodes, making stem longer. As sugar is stored in cells of the internodes, more sugar is produced.
- Plant breeding: some plants/trees (eg. conifers) take a long time before becoming reproductively active. Gibberellins help shorten this time by inducing seed formation.
- Fruit production: They can delay ageing in citrus fruit, extending time they can be left unpicked and available in shops.
- Also, with cytokinins, make apples elongate to improve shape.
- Grapes can get bigger because stalks elongate.
- Brewing: Gibberellins switch on genes for amylase production, which is needed to break down starch into maltose needed for beer.
- Also: stopping plants make gibberellins is also useful. Spray gibberellin synthesis inhibitors make them less likely to lodge (stems bend over), making it easier to harvest.
What is another word for ageing when talking about plants?
Give ways in which cytokinins are used commercially. (2)
- Delay leaf ageing, so are used to prevent yellowing of lettuce leaves after they had been picked.
- Also used in tissue culture to help mass-produce plants, because they promote bud and shoot growth.
Give commercial use of ethene.
- They help ripen fruits. (By breaking down cell walls and converting starch into sugars).
- Bananas are harvested before they're ripe because they are less likely to be damaged. When they arrive, they are exposed to ethene so they all ripen at the same time on the shelves and in people's homes.
- (also therefore important to inhibit effects of ethene before shipping).