Plant responses

Question 1

Why do plants need to respond to their environment?

Answer 1

1. To maximise their chance of survival.
2. To avoid predation.
3. To avoid abiotic stress.
4. This ensures they survive long enough to reproduce.

Question 2

How can plants respond to maximise their chance of survival?

Answer 2

• They respond to changes in environment.
• Plants grow towards the light to maximise their light exposure and thus rate of photosynthesis.
• Plants detect direction of gravity, ensuring that roots and stems grow in right direction, allowing these to carry out their duties.
• Plants climb objects in order to reach sunlight.

Question 3

How can plants respond to avoid predation?

Answer 3

• Some plants can change their shape in order to avoid detection or to prevent insect predators from landing.
• Some plants can produce toxins that repel predators (e.g. onions).

Question 4

How can plants respond to avoid abiotic stress?

Answer 4

• Some plants are able to produce natural anti-freeze substances in order to avoid freezing in low temperatures.
• Most plants are able to close their stomata when rate of transpiration too high to avoid excess water loss.

Question 5

What is a tropism?

Answer 5

A plant directional growth response in which the direction of the response is determined by the direction of the external stimuli.

Question 6

What is phototropism?

Answer 6

• Directional growth response determined by light.
• Plant shoots grow towards the direction light (positive phototropic), which maximises their light exposure, thus maximising their rate of photosynthesis.

Question 7

What is geotropism?

Answer 7

• Directional growth response determined by gravity.
• Roots grow in the direction of gravity (positive geotropic), allowing them to anchor plant to ground as well as reach water and minerals (such as magnesium and nitrates) deep underground.
• Shoots from germinated seeds grow against the direction of gravity (negative geotropic), allowing them to surface from underground and reach light, starting photosynthesis.

Question 8

What is chemotropism?

Answer 8

• Directional growth response determined by chemicals.

- Pollen tubes grow towards chemicals released by ovum, allowing them to reach ovum.

Question 9

What is thigmotropism?

Answer 9

• Directional growth response determined by touch.
• Allows climbing plants to wrap around a stems of another plant or solid structures and grow upwards. This maximises their exposure to light.

Question 10

What controls plant responses?

Answer 10

Hormones (or growth regulators) that are produced mainly by growing tissue (e.g. shoot tips) control the variety of plant responses.

Question 11

What is the mechanism of action of plant hormones?

Answer 11

Receptors on specific target tissue have complementary shape to a specific hormone. This ensures that the hormone only binds to cells in target tissue and bring about changes associated with the action of the hormone.

Question 12

How are plant hormones transported from production site to target tissues?

Answer 12

• Very often in plants, the hormones are produced at their site of production, so do not need to be transported.
• From their production site, cell to cell by action of diffusion or active transport, until their target tissues are reached.
• By mass flow in the xylem or phloem.

Question 13

What are the roles of auxins in plants?

Answer 13

• Promotes cell elongation.
• Inhibits side-shoot growth (apical dominance).
• Inhibits leaf abscission.

Question 14

What are the roles of cytokinins in plants?

Answer 14

Promotes cell division.

Question 15

What are the roles of gibberellins in plants?

Answer 15

• Promotes seed germination.
• Promotes stem elongation.
• Stimulates side-shoot formation.
• Promotes flowering.

Question 16

What are the roles of abscistic acid?

Answer 16

• Inhibits seed germination.
• Inhibits growth.
• Causes stomatal closure during water stress.

Question 17

What are the roles of ethene in plants?

Answer 17

• Promotes fruit ripening.

Question 18

What are the locations of meristems in plants?

Answer 18

• Apical meristems: Located at the tips of shoots and roots and are responsible for their growth.
• Lateral bud meristems: Found in lateral buds and are responsible for side-shoot elongation.
• Lateral meristems: Found in rings around the inside of shoots and roots, responsible for their widening.
• Intercalary meristems: Found in nodes where side-shoots branch off of stem and are responsible for stem elongation.

Question 19

How do auxins stimulate shoot elongation?

Answer 19

1. Auxins promote the active transport of protons into the cell wall.
2. High proton concentrations in cell walls results in low pH, creating optimum pH for cell wall loosening enzymes to work.
3. Enzymes (expansins) break bonds in cell wall, making it more elastic, allowing cell to expand when under the turgor pressure of water intake.
4. Cells elongate and become bigger.

Question 20

What causes phototropism?

Answer 20

• Phototropism is the response whereby a growing shoot will bend and grow towards the direction of light.
• This is caused by the elongation of cells on the dark side of the shoot.
• Although the mechanism is still uncertain, links are being made with enzymes phototropin 1 and 2.
• Phototropin 1 concentration decreases moving from light to dark side.
• The result is a redistribution of auxins from apex to dark side of shoot, promoting cell elongation and causing bending.

Question 21

What are the events associated with leaf abscission?

Answer 21

1. Cytokinins ensures that leaves act as sinks for phloem. They have a steady supply of nutrients, preventing senescence.
2. When cytokinin concentration drops, less nutrients are supplied to leaves, resulting in senescence.
3. Senescence results in auxin production at leaf tips to drop.
4. Fall in auxin concentration results in cells of the abscission zone to become more sensitive to ethene, as well as increasing ethene production.
5. Cellulase production increases.
6. Cellulase breaks down cell walls of cells in abscission zone, which eventually results in the leaf separating from the stem.

Question 22

What is apical dominance?

Answer 22

The growing of the apical bud at shoot tip inhibits the growth of lateral buds further down shoot.

Question 23

What evidence is there to suggest apical dominance?

Answer 23

When vertical shoot tip is cut, lateral bud activity increases and lateral shoots grow longer.

Question 24

What are possible explanations for this phenomenon?

Answer 24

• Auxins inhibits lateral buds and lateral shoot growth. Removing the apex removes source of auxins. Auxin concentration decreases, allowing lateral shoots to grow.
• Increased exposure to oxygen at cut end of the stem may promote activity of hormones responsible for lateral shoot growth.
• There’s an indirect link between auxins and inhibition of lateral buds.

Question 25

What evidence supports the theory of auxin inhibition?

Answer 25

When a ring of auxin transport inhibitor placed directly below apex, lateral shoots grew more. This supports the idea that high auxin levels are involved with lateral bud inhibition.

Question 26

What is actually responsible for apical dominance?

Answer 26

• Abscisic acid is responsible for inhibiting lateral bud growth.
• High levels of auxins possibly maintains high abscisic level in the lateral buds.
• When apex removed, source of auxin removed and auxin concentration drops, so abscisic acid concentration also drops.
• Cytokinins also responsible for bud growth.
• High levels of auxins at apex makes it a sink for cytokinins.
• Most cytokinins go to apex to promote vertical shoot growth and very little goes to lateral buds.
• When apex removed, cytokinins more evenly distributed throughout stem, so more available to promote lateral bud growth.

Question 27

How did scientists conclude that gibberellins were responsible for plant growth?

Answer 27

• When GA3 (gibberellic acid) was applied to various varieties of dwarf plants, they grew tall.
• Despite GA3 stimulates stem elongation in plants does not mean it does so in nature.
• Another gibberellin, GA1 was studied.
• It was found that dwarf varieties of a pea plant with lele genotype had lower concentrations of GA1 compared to tall varieties with LeLe genotype, indicating that there was some relationship between GA1 and stem growth.
• The Le gene was responsible for converting GA20 to GA1.
• The shoot of a dwarf pea plant with mutation that prevented in from synthesising GA1 was grafted onto the shoot of the dwarf lele pea plant.
• The shoots grew tall because it contained enzymes that were able to convert GA20 in lele pea plant into GA1.
• This proved that GA1 was directly linked to stem elongation.

Question 28

How do gibberellins cause stem elongation?

Answer 28

• Stimulates cell elongation in internode regions by loosening cell walls.
• Stimulates cell division intercalary meristems.

Question 29

What are the commercial uses of auxins?

Answer 29

• Used to make rooting powder for stem cuttings.
• Promotes ovule growth in unpollinated flowers and formation of seedless fruits via positive multiplier effect.
• Artificial auxins (modified) are used as selective herbicides. They cannot be broken down by auxin enzymes as they don’t have complementary shape. They promote stem elongation until plant can no longer support itself, falls over and dies.

Question 30

What are the commercial uses of gibberellins?

Answer 30

• Delays senescence (ripening) of citrus fruits, increasing shelf life.
• Works together with cytokinins to elongate fruits (e.g. apples) and improve shape.
• Elongates grape stalks so grapes not as tightly packed and can increase in size.
• Causes stem (internode) elongation in sugar cane plants so that more sugar can be produced and stored per plant.
• Stimulates biennial plants to produce seeds in the first year, speeding up process of artificial selection by plant breeders.
• Induces seed formation in young plants.
• Gibberellin inhibitors are used to inhibit stem elongation and keep plants short, preventing lodging.

Question 31

What are the commercial uses of cytokinins?

Answer 31

• Used with gibberellins to encourage fruit elongation.

- Used to delay senescence (yellowing) of leafy vegetables, such as lettuce; prolonging shelf life.

Question 32

How can ethene be applied to plants commercially?

Answer 32

2-chloroethylphosphoric acid sprayed onto plants and releases ethene gas slowly over time.

Question 33

What are the commercial uses of ethene?

Answer 33

• Speeds up fruit ripening.
• Promotes fruit drop in cotton, cherries and walnut.
• Promotes female sex expression in cucumbers, preventing self pollination and increases yield.
• Promotes lateral growth in certain plants.
• Restricting plant ethene production by storing under low temperatures, low oxygen and high carbon dioxide levels delays fruit ripening, allowing for long distance shipping.