Plant Responses Flashcards
(a) Explain why plants need to respond to their environment in terms of the need to avoid predation and abiotic stress;
Plants respond to external stimuli as well as biotic and abiotic components of the environment to help the plant avoid stress, being eaten, and be able to survive long enough to reproduce. These responses are coordinated by hormones.
Plants, like animal, 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 light absorption for photosynthesis.
- The can sense gravity, so their roots and shoots grow in the right direction.
- Climbing plants have a sense of touch, so they can find things to climb and reach the sunlight.
Plants are more likely to survive if they respond to the presence of predators to avoid being eaten; e.g. some plants secrete toxic substances, with the white clover secreting substances toxic to cattle, to avoid being eaten (cows eat white clover when fields are overgrazed).
Plants are more likely to survive if they respond to abiotic stress; anything that’s non-living, harmful and natural, like a drought. E.g. some plants respond to the extreme cold by producing their own form of antifreeze; carrots produce antifreeze proteins at low temperatures; the proteins bind to ice crystals and lower the temperature that water freezes at, stop more ice crystals from growing
(b) Define the term tropism;
A directional growth response, in which the direction of growth is determined by the direction of the external stimulus. A response towards the stimulus is positive; away, negative.
Stimulus; Growth response:
Light; phototropism (shoots are positively phototropic and roots are negatively phototropic)
Gravity; geotropism (shoots are negatively geotropic and roots are positively geotropic)
Chemical; chemotropism (shoots are negatively chemotropic and roots are positively chemotropic)
Touch; thigotropism.
(c) Explain how plant responses to environmental changes are co-ordinated by hormones, with reference to responding to changes in light direction;
Auxins stimulate the growth of shoots by cell elongation; the cell walls become loose and stretchy, so the cells become longer.
When the shoot is illuminated from all sides, the auxins are evenly distributed and move down the shoot, promoting the active transport of H+ ions through the ATPase enzyme into the cell wall. Lowering the pH of the cell walls allows optimum conditions for the wall loosening enzymes, expansins, to work. These weaken the cell wall, breaking bonds within the cellulose so the walls become less rigid and expand as the cells take in water, elongating.
When the shoot is only illuminated from one side however, auxins move down from the shoot towards the shaded side, so that the cells on the shaded side elongate at a much greater rate taking up much more water, causing the shoot to grow towards the light. The resulting expansion is in length, not width, due to the construction of their cell walls.
(d) Evaluate the experimental evidence for the role of auxins in the control of apical dominance and gibberellin in the control of stem elongation;
Auxins
Auxins stimulate the growth of the apical bud (bud at the top, tip of the main shoot), and inhibit the growth of lateral (side) buds further down the shoot; apical dominance – the apical bud is dominant over the side shoots.
Auxins are produced by the apical bud. Removing the apical bud or applying an auxin transport inhibitor below the apex of the main shoot allows lateral buds to grow; the source of the inhibitory auxin is removed.
This shows evidence that the inhibitory auxin is produced in the apical bud of the main shoot and transported to lateral buds to inhibit lateral growth; saving energy and preventing lateral shoots from the same plant competing with the shoot tip/apical bud for light.
This is also shown when the plant grows taller; the lateral buds furthest away from the tip may start growing, as auxin is much diluted as it passes down the stem from the apical bud, lateral buds at the base of the stem may no longer be inhibited, as the concentration of auxin reaching them is below the threshold value for inhibition. Once active, the lateral buds in turn produce auxins and cytokinins (promote cell division). Apical dominance gives a plant a shape that allows all parts access to light.
Gibberellins
Associated with the promotion of seed germination and the growth of stems, side shoot formation and flowering. If genetically dwarf plants are treated with Gibberellic acid, the stems elongate considerably.
(e) Outline the role of hormones in leaf loss in deciduous plants;
- Cytokinins stops leaves senescing (ageing) by making sure the phloem constantly provides nutrients to the leaf as the leaf acts as a sink (removes sugar) for phloem transport, guaranteeing a good supply. As cytokinin production drops, the supply of nutrients dwindles, the leaves start to sensce. Senescence causes auxin production in the tip of the leaf to decrease; causing an increase in ethene production, as well as causing cells in the abscission zone to be more sensitive to ethene. This leads to an increased production of cellulose, which digest the walls of the cells in the abscission zone, eventually separating the petiole (stalk bit) from the stem; leaf falls off.
- Auxins inhibit leaf loss; auxins are produced by young leaves; as the leaf gets older, less auxin is produced, leading to leaf loss (abscission). Auxins are antagonistic to ethene.
- Abscisic acid (ABA) is a plant hormone that stimulates the production of ethene. Ethene stimulates the fall of leaves; abscission. Ethene is produced by ageing leaves. A layer of cells (the abscission layer) develops at the bottom of the leaf stalk, where the leaf joins the stem. It separates the leaf from the rest of the plant. Ethene stimulates the cells in the abscission layer to expand, breaking the cell walls, where the leaf is broken off by a mechanical force, such as wind, or just falls off.
(f) Describe how plant hormones are used commercially
- Synthetic auxins are used as growth stimulants of roots in cuttings
- A synthetic auxin, 2, 4-D, is used as a selective weedkiller; at appropriate concentrations it kills broadleaved species, but not grass or cereal crops.
- Auxins are used in: the production of seedless fruit, as a herbicide (plants grows like shit and then can’t support itself), treating unpollinated flowers.
- A form of abscisic acid that is not readily broken down by plants is used as an anti-transpirant as it closes the stomata
- Cytokinins are used to prevent the yellowing of plants such as lettuce, and to promote growth of shoots and buds in cuttings
- Gibberellins can be used to delay senescence in citrus fruits, improve the shape of apples, increase grape size (stalks elongate so grape has more space to grow bigger), in brewing, and other shit.
- Ethene is used to speed up fruit ripening; to promote fruit abscission in plants such as cotton, cherry and walnut; to promote female sex expression in cucumbers so they taste less bitter; and to promote lateral growth in some plants to yield compact flowering stems
