Plant responses

Question 1

Why do plants need to respond to the external environment ?

Answer 1

helps plant survive long enough to reproduce

Question 2

What chemical defences are used in response to threat of herbivores?

Answer 2

• tannins
• alkaloids
• pheromones

Question 3

Function of tannins

Answer 3

• toxic
• make leaf taste bitter
• prevent infiltration by pathogenic microorganisms

Question 4

Function of alkaloids

Answer 4

feeding deterrent as they’re bitter and also kill herbivores by affecting their metabolism

Question 5

Function of pheromones

Answer 5

• affect behaviour or physiology of other plants
• used by plants to communicate about danger

Question 6

What are tropisms?

Answer 6

directional responses of plants

Question 7

physical plant defenses

Answer 7

• folding of leaves in response to touch
• thorns
• spines

Question 8

What are the types of tropisms?

Answer 8

• phototropism
• geotropism
• chemotropism
• thigmotropism
• hydrotropism

Question 9

What’s phototropism?

Answer 9

directional growth response to light. shoots growing towards light (+vely phototropic), enables photosynthesis.

Question 10

what’s geotropism?

Answer 10

directional growth response to gravity. Roots grow towards the pull of gravity, helps anchor them in soil.

Question 11

What is chemotropism?

Answer 11

directional growth response to chemicals. On flowers, pollen tubes grow down the style attracted by chemicals, towards the ovary where fertilisation occurs.

Question 12

What’s thigmotropism?

Answer 12

directional growth response to mechanical stimulation. shoots of climbing plants (ivy), wind around other plants or solid structures for support.

Question 13

What are positive tropic responses?

Answer 13

plant responds towards a stimulus

Question 14

What are negative tropic responses?

Answer 14

plant responds away from a stimulus

Question 15

What are nastic responses?

Answer 15

non-directional responses to external stimuli - sensitive plants folding leaves when touched (thigmonasty) in Mimosa Pudica

Question 16

What’s the role of plant hormones?

Answer 16

coordinate plant responses to environmental stimuli, transported from site of manufacture to target cells/tissues.

Question 17

How are plant hormones different from animal ones?

Answer 17

not produced in endocrine glands, produced in a variety of tissues in plants.

Question 18

What processes can plant hormones influence?

Answer 18

• cell division, elongation, differentiation.

Question 19

Examples of plant hormones

Answer 19

• cytokinins
• abscisic acid
• auxins
• gibberellins
• ethene

Question 20

effect of cytokinins?

Answer 20

• promote cell division and expansion
• delay leaf senescence
• OVERCOME APICAL DOMINANCE

Question 21

effect of abscisic acid

Answer 21

• inhibits seed germination and growth
• stomatal closure when the plant is stressed by low water availability

Question 22

effect of gibberellins

Answer 22

• promote seed germination
• promotes elongation of stems by stimulating cell division

Question 23

effect of ethene

Answer 23

• promotes fruit ripening
• stimulates the breakdown of cell walls in the abscission layer, causing the leaf to drop off

Question 24

effect of auxin

Answer 24

• regulates plant growth
• promotes cell elongation and division
• inhibits growth of side shoots (causes apical dominance)
• inhibits leaf fall as it prevents production of ethene
• inhibits root growth at high conc

Question 25

how do hormones travel around plant?

Answer 25

- diffusion - active transport - mass flow by in the phloem sap or in xylem vessels

Question 26

what's apical dominance

Answer 26

inhibition of the **lateral bud growth** further down the shoot by chemicals produced by the **apical bud** at the tip of a plant shoot.

Question 27

Experimental evidence for the role of auxins in the control of apical dominance

Answer 27

- when **shoot tip (apex) is removed**, auxin levels in shoot drop and lateral buds grow. - when **apex removed and paste containing auxin** applied to the cut end, buds didn't grow - upon cutting, a hormone could have been produced at cut end due to exposure to oxygen that promotes growth. - Due to this, **auxin transport inhibitors** applied below apex and buds still grew. - Based on results - high auxin levels inhibit lateral bud growth whereas low levels promote it.

Question 28

what 2 hormones are now thought to be involved in apical dominance?

Answer 28

- abscisic acid (**high levels of auxin maintain its levels high**) - cytokinins (**high auxin levels make shoot apex a sink for cytokinins, when apex is cut, it spreads evenly around plant**)

Question 29

experimental evidence for the role of gibberellin in the control of stem elongation

Answer 29

1) If you graft a plant with a dominant **Le gene** onto the **homozygous le** plant, the plant grows tall. 2) tall pea plants had higher conc of gibberellin 3) adding gibberellins to short plants made them grow tall 4) **Le codes for an enzyme involved in synthesising gibberellins (converts GA20 to GA1)** 5) Scientists found that short plants were homozygous for recessive allele and big plants were homozygous for a dominant allele (Le). 6) Scientists compared gibberellin conc in dwarf pea plants and tall pea plants.

Question 30

what stimulates release of gibberellins (GA)?

Answer 30

when seed **absorbs** water, **embryo** releases gibberellins **into the aleurone layer in the endosperm region of the seed**

Question 31

Explain how gibberellins promote germination

Answer 31

activates genes that code for **amylase** enabling its production, amylase can break down starch to glucose which provides a substrate for **RESPIRATION**, causing growth.

Question 32

What's geotropism?

Answer 32

directional growth response to gravity

Question 33

What's phototropism?

Answer 33

directional growth in response to light

Question 34

How is auxin distributed?

Answer 34

- If a plant is laid on its side, auxin gathers in the lower half of the stem and root. - Auxin slows growth in the root, so the root curves downwards. - roots are **negatively phototrophic**. - Auxin stimulates growth (cell elongation) in the shaded side of shoot, so the stem curves upwards - shoots are **positively phototropic**

Question 35

What's hydrotropism

Answer 35

- direstional growth in response to water. - roots tips growing towards damper areas, increasing their access to water.

Question 36

where does growth occur in a plant?

Answer 36

meristem

Question 37

How can geotropic responses be investigated?

Answer 37

- control plant is spun very slowly by **klinostat** so **gravity is applied equally to all sides of the plant**. Roots and shoots grow horizontally. - other plants are on klinostat (switched off) so shoots grow upwards and roots grow downwards.

Question 38

mechanism of auxins effect

Answer 38

- promotes active transport of H+ into cell wall. - low pH provides optimum conditions for **expansins** to work which break bonds within cellulose (H+ also disrupts these bonds). - cell wall becomes less rigid and can expand as cell takes in water

Question 39

what's the commercial use of ethene?

Answer 39

- stimulates enzymes that break down cell walls making fruit soft and ripe.

Question 40

what's the commercial use of auxin?

Answer 40

- used as **herbicides**, makes plants grow too fast so they can't get enough water or nutrients so they die - used a **rooting hormone** encouraging root growth, lots of the same plant can be grown quickly and cheaply from one plant via cuttings. - promotes growth of seedless fruits

Question 41

what did Boysen-Jensen's work confirm and what were his tests

Answer 41

- that water and/or solutes need to be able to move backwards from the shoot tip for phototropism to happen - when a permeable gelatin block inserted behind shoot tip, the shoot still showed +ve phototropism - when an impermeable mica block was inserted, there was no phototropic response. - tests: - tip separated by gelatin block - tip separated by mica (impermeable)

Question 42

what did Darwin's work confirm and what were his tests

Answer 42

- confirmed that the shoot tip was responsible for phototropic responses - tests: control tip removed tip covered by opaque cap tip covered by transparent cap base covered by opaque shield