5.5 PLANT RESPONSE

Question 1

Describe the two types of response to external stimuli in plants

Answer 1

1) Tropism = a directional growth towards an external stimulus
2) Nastic response = a nondirectional response to external stimuli (usually chemical release)

Question 2

Describe the five physical defences of plants

Answer 2

1) Cellulose cell wall - physical barrier
2) Ligin thickening of cell walls - waterproof and indigestible
3) Waxy cuticle - prevents water from collecting on surface so pathogens are not attracted
4) Callose - deposited in sieve tubes to block flow of pathogens

Question 3

State the three chemicals released to prevent plants being eaten

Answer 3

1) Tannins
2) Alkaloids
3) Phermones

Question 4

Describe the action of tannins

Answer 4

• Toxic to microorganisms and larger herbivores
• Taste bad

Question 5

Describe the action of alkaloids

Answer 5

• Taste bitter
• Found in growing tips and flowers

Question 6

Describe the action of pheromones

Answer 6

• When released affect the behaviour of others

Question 7

State the four types of tropism

Answer 7

1) Phototropism
2) Geotropism
3) Chemotropism
4) Thigmotropism

Question 8

Define phototropism

Answer 8

• Where shoots grow towards light (positive phototropism)

Question 9

Define geotropism

Answer 9

• Where roots grow towards gravity (positive geotropism)

Question 10

Define chemotropism

Answer 10

• On a flower, pollen tubes grow down the style (attracted by chemicals) towards the ovary where fertilization can take place

Question 11

Define thigmotropism

Answer 11

• Shoots of climbing plants wind around other plants or solid structures to gain support

Question 12

Define photoperiod

Answer 12

• The period of time in a day that an organism is exposed to light

Question 13

State the five plant hormones

Answer 13

1) Auxins
2) Cytokinins
3) Gibberellins
4) Ethylene
5) Abscisic acid

Question 14

Describe the location and four effects of cytokinin hormone

Answer 14

(ROOTS but transported from there)
1) Promote cell division
2) Delay leaf senescence (delays aging)
3) Promote cell expansion
4) Overcome apical dominance

Question 15

Describe the location and two effects of abscisic acid hormone

Answer 15

(MATURE LEAVES/ROOTS)
1) Inhibits seed germination and growth
2) Causes stomatal closure when the plant is stressed by lower water availibility

Question 16

Describe the location and three effects of auxin (IAA) hormone

Answer 16

(APICAL SHOOT TIP BUDS)
1) Promotes cell elongation
2) Inhibits growth of side shoots (from lateral buds)
3) Inhibits leaf fall

Question 17

Describe the location and two effects of gibberellin hormone

Answer 17

(APICAL BUDS OF ROOTS AND SHOOTS)
1) Promotes seed germination
2) Promotes stem growth (via cell elongation by loosening cell walls) and (via cell division by stimulating the production of a protein that controls the cell cycle)

Question 18

Describe the location and effect of ethene hormone

Answer 18

(LEAVES/STEMS/YOUNG FRUIT)
1) Promotes fruit ripening

Question 19

State the three ways in which hormones move around the plant

Answer 19

1) Active transport
2) Diffusion
3) Mass flow in the phloem sap or xylem vessel

Question 20

Explain what and where the apical and lateral bud is

Answer 20

Apical bud = on the top
Lateral bud = on the side
- (Bud = arise from meristems, a place that can grow into flower/stem/shoot)
- If the shoot tip (apical bud) is cut off, then plant starts to grow from lateral buds
- This is because inhibitory auxin levels drops in the shoots (as its gone now) so the lateral buds grow as they are no longer inhibited by auxins

Question 21

Define apical dominance

Answer 21

• The inhibition of lateral bud growth (further down the shoot) due to auxins produced by the apical bud (at the tip of the plant shoot) that travel down to the lateral buds and thus prevent side shoot growth

Question 22

Describe the experimental evidence for lateral bud inhibition

Answer 22

1) Apical bud/shoot tip is cut off and replaced with auxin paste
2) Lateral buds did not grow
3) Then an auxin transport inhibitor is apllied below the apex of the shoot to prevent the auxin from the auxin paste from travelling down to the lateral buds
4) Lateral buds grow
5) Therefore, its concluded that normal/high auxin hormone levels in the lateral buds inhibits its side shoot growth but low auxin hormone levels in the lateral buds promote its side shoot growth

Question 23

State and explain the three plant hormones invloved in bud growth

Answer 23

1) AUXINS = apical dominance

2) ABSCISIC ACID = abscisic acid simply inhibit bud growth
- High auxin levels keep abscisic acid levels high
- But when shoot tip/apical bud is cut off, the abscisic acid levels drop and the bud grows

3) CYTOKININS = cytokinins simply promote bud growth
- Directly applying cytokinins can overpower the effect of apical dominance
- High levels of auxin cause cytokinins to move to the shoot apex/tip/apical bud
- When the shoot apex/tip/apical bud is cut off, the cytokinins spread evenly around the plant so buds grow

Question 24

Explain how gibberellins cause seed germination

Answer 24

• When the seed absorbs water, the embryo releases gibberellin
• This stimulates the production of amylase which breaks down starch into glucose ready for respiration/protein synthesis

Question 25

State the **genotypes** of **dwarf plants** and **tall plants**

Answer 25

Question 26

Explain with diagrams how the **presence** and **absence** of the **Le allele** allows plants to be **tall**

Answer 26

- **Active gibberellin GA₁** is required for a **plant** to be **tall**

Question 27

Draw the experiment for **grafting** of plants to prove **stem elongation** is caused by **gibberellins**

Answer 27

Question 28

State where **plant growth** occurs

Answer 28

**Meristems**: 1) **Apical** **buds** (for **length** due to tem elongation) 2) **Lateral buds** (for **width** due to side shoots)

Question 29

State the **four commercial uses** of **gibberellins**

Answer 29

1) **Fruit production** 2) **Brewing** 3) **Sugar production** 4) **Plant breeding**

Question 30

State the **three commercial uses** of **auxins**

Answer 30

1) **Herbicides** 2) **Seedless fruit** 3) **Plant cuttings**

Question 31

State the **commercial use** of **ethene**

Answer 31

1) **Fruit ripening**

Question 32

State the **two****commercial uses** of **cytokinins**

Answer 32

1) **Prevent leaf falling** 2) **Mass plant production**

Question 33

Explain **gibberillins** in **fruit production**

Answer 33

- **Sprayed** onto **fruit** to make their **stems** **elongate** causing them to **grow bigger** - Can act with **cytokinins** to maje **apples** a **elongate/improve shape**

Question 34

Explain **gibberellins** in **brewing**

Answer 34

- During **germination**, the **seed** produces **amylase** which **breaks down starch** into **maltose** - Adding **excess gibberellins** can **speed up** the process - **Maltose** has **malt** for **beer**

Question 35

Explain **gibberellins** in **sugar production**

Answer 35

- **Gibberellins** stimulate **growth** between **nodes** making the **stems** **elongate** - **Sugar canes** store **sugar** in these **cells** - So **sugar yeild increases**

Question 36

Explain **gibberellins** in **plant breeding**

Answer 36

- **Gibberellins** can **speed up** the **plant breeding** by **inducing seed formation** on **young plants**

Question 37

Explain **auxins** in **herbicides**

Answer 37

- **Excess auxins** promote **rapid shoot growth** so the **stem** cannot **support** itself, **buckes** and **dies**

Question 38

Explain **auxins** in **seedless fruit**

Answer 38

- **Excess auxins** spayed onto **unpollinated flowers** can result in **ovule growth** - This **triggers** automatic **auxin production** - This helps the **fruit** in their **developmental process**

Question 39

Explain **auxins** in **plant cuttings**

Answer 39

- **Dipping** the **root/end** of the **plant cutting** before plantation promotes **root growth** - **Plants** can be **clones fast** and **cheap**

Question 40

Explain **ethene** in **fruit ripening**

Answer 40

- **Ethene** activated **enzymes** that help in the **ripening process**

Question 41

Explain **cytokinins** in **delay** of **leaf falling**

Answer 41

- **Delays leaf falling** by the **degradation** of **chloroplasts**

Question 42

Explain **cytokinins** in **mass production** of **plants**

Answer 42

- Can be used in **tissue culture** to help **mass produce plants** by **encouraging cell division**

Question 43

Explain the process and purpose of **prevention** of **ripening**

Answer 43

PROCESS: - **Storing fruit** at **low temperatures**, **little oxygen** and **high CO2** prevents **ethene synthesis** and therefore **prevents ripening** - **Silver salts** are also **ethene inhibitors** PURPOSE: - So the **fruit** remains **hard** and **easier** for **transport**