16 - Plant Responses

Question 1

What is a tropism?

Answer 1

The directional growth of a plant in response to a directional stimulus

Question 2

What is an abiotic stress?

Answer 2

Stress caused by a non-living factor i.e. lack of water

Question 3

What is a biotic stress?

Answer 3

Stress caused by a living thing i.e. a pathogen

Question 4

What is a herbivory stress?

Answer 4

Stress caused to a plant by being eaten

Question 5

What is phytochrome?

Answer 5

A light-sensitive pigment in plants which exists in two variants, the balance of which can invoke a change

Question 6

What is photoperiodism?

Answer 6

A plant’s response to seasonal changes in length of night and day due to sensitivity to light availability

Question 7

What are the two key limitations of plants in terms of responses?

Answer 7

They are rooted and do not have a rapidly responding nervous system

Question 8

What are 4 important plant hormones?

Answer 8

1. ABA 2. Ethene 3. Gibberellins 4. Auxins

Question 9

What happens to auxin levels in plants as a result of low light levels?

Answer 9

They are also low

Question 10

What does the plant do in response to low auxin levels caused by low light availability?

Answer 10

Produces the hormone ethene

Question 11

How does ethene cause abscission?

Answer 11

It switches on plant digestive enzymes. This causes a fatty layer to be laid down in the abscission zone, and also causes the cells on the separation side to swell. This weakens the joint of leaf to stem, causing the leaf to fall off

Question 12

What is abscission?

Answer 12

When plants shed fruits, leafs, flowers, and seeds.

Question 13

What is the abscission zone?

Answer 13

Layer of cells at base of petiole that weakens and allows leaf to drop off

Question 14

Why is it important that plants avoid freezing?

Answer 14

Expanding, frozen water can damage cell walls

Question 15

What is an adaptation that most plants have to prevent freezing?

Answer 15

The vacuole and cytosol contain certain solvents which lower the freezing temperature of water

Question 16

What is a specific adaptation which carrots have to prevent freezing?

Answer 16

They produce a protein which binds to ice crystals, stopping them from freezing

Question 17

What hormone causes closure of guard cells?

Answer 17

ABA

Question 18

What causes ABA to be released?

Answer 18

When the roots detect low water concentration

Question 19

How does ABA work?

Answer 19

It causes Ca²⁺ and K⁺ ions to leave the guard cells, increasing the water potential and thus causing water to leave the cells, causing the stomata to close

Question 20

What are 2 known roles of ethene in plants?

Answer 20

Promotes abscission and fruit ripening

Question 21

What hormone can stimulate antifreeze production in plants?

Answer 21

ABA

Question 22

What are 5 known roles of auxins in plants?

Answer 22

1. Stimulate ethene production 2. Involved in tropisms 3. Control cell elongation 4. Prevent abscission 5. Maintain apical dominance

Question 23

What are 3 known roles of gibberellins in plants?

Answer 23

1. Cause stem elongation 2. Trigger mobilisation of food stores in seeds at germination 3. Stimulate pollen tube growth in fertilisation

Question 24

What are 3 known roles of ABA in plants?

Answer 24

1. Stimulate stomatal closure 2. Stimulate cold protective response 3. Maintains dormancy of seeds and buds

Question 25

What is a positive tropism?

Answer 25

Growth towards a stimulus

Question 26

What is a negative tropism?

Answer 26

Growth away from a stimulus

Question 27

What is the stimulus for geotropism?

Answer 27

Gravity

Question 28

What is the stimulus for thigmotropism?

Answer 28

Touch

Question 29

What is the stimulus for electrotropism?

Answer 29

Electric fields

Question 30

What are the only 2 places in a plant where growth can occur?

Answer 30

Shoots and meristems

Question 31

How do gibberellins affect stem length?

Answer 31

More gibberellins mean a longer stem

Question 32

What stimulates a plant seed to start producing gibberellins?

Answer 32

Absorption of water

Question 33

What do gibberellins do when produced in seeds?

Answer 33

Produce proteases and amylases

Question 34

What do the proteases and amylases do in seeds?

Answer 34

Cause a store of protein to be built up, which helps growth and energy release in the seed

Question 35

What acts as an antagonist to gibberellins in seeds?

Answer 35

ABA

Question 36

What determines when a seed will germinate?

Answer 36

The relative levels of ABA and gibberellins

Question 37

What 2 pieces of experimental evidence indicate that gibberellins are involved in seed germination?

Answer 37

1. Mutant seeds with no gibberellin production gene do not germinate 2. If gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate

Question 38

What is an example of an auxin?

Answer 38

Indoleacetic acid (IAA)

Question 39

Where are auxins produced?

Answer 39

Tips of shoots and roots, meristems

Question 40

What shoot do auxins stimulate the growth of?

Answer 40

The main, apical shoot

Question 41

What do auxins affect about the cell wall?

Answer 41

They make it more plastic and stretchy

Question 42

How do auxins make cell walls more plastic?

Answer 42

They bind to receptors and cause a fall in pH to around 5, the optimal pH for enzymes which keep the cell wall plastic

Question 43

What happens to auxins as cells mature and what does this cause?

Answer 43

They are destroyed, causing the enzymes maintaining cell wall plasticity to be destroyed and meaning that the cell wall becomes more rigid and fixed in place

Question 44

High levels of auxins suppress the growth of what?

Answer 44

Lateral shoots

Question 45

How do auxins cause apical dominance?

Answer 45

At the top of the shoot there are high levels of auxins which inhibit lateral shoot growth, causing the apical shoot to grow better and be dominant. As you move further down the plant auxin levels fall and the lateral shoots become more pronounced

Question 46

What experimental evidence is there for the role of auxins in maintaining apical dominance?

Answer 46

If the apical shoot is removed, apical dominance stops due to a lack of auxins, and this can be reversed by applying auxins to the cut shoot tip

Question 47

What do low levels of auxins promote except lateral shoot growth?

Answer 47

Root growth

Question 48

What are 4 types of plant defence against herbivory?

Answer 48

1. Physical defences 2. Chemical defences 3. Pheromones 4. Folding in response to touch

Question 49

What plant is used folding its leaves in response to touch as a defence strategy?

Answer 49

Mimosa pudica

Question 50

How does folding its leaves help defend Mimosa pudica against herbivory?

Answer 50

Can knock insects off and startle larger predators

Question 51

What are 4 examples of plant physical defences against herbivory?

Answer 51

1. Barbs/thorns 2. Hairy leaves 3. Stings 4. Fibrous and inedible tissue

Question 52

What are 3 types of plant chemical defences against herbivory?

Answer 52

1. Tannins 2. Alkaloids 3. Terpenoids

Question 53

How do tannins protect plants against herbivory?

Answer 53

They are very bitter and astringent tasting, and are also toxic to insects

Question 54

How do alkaloids protect plants against herbivory?

Answer 54

Bitter tasting, affecting metabolism of animals

Question 55

How do terpenoids protect plants against herbivory?

Answer 55

Can act as toxins to insects and fungi which attack the plant

Question 56

What is a pheromone?

Answer 56

A chemical made by an organism which affects the social behaviour of other members of the species

Question 57

Why do plants not use pheromones much?

Answer 57

They do not behave socially

Question 58

What are 2 examples of plants using pheromones?

Answer 58

1. Some plants can release chemicals in their root systems to tell their neighbours that they are suffering from water stress 2. When attacked by insects, maple trees release a pheromone which causes other leaves on the tree to prepare chemical defences such as callose

Question 59

What are VOCs and how do plants use them?

Answer 59

Chemicals which act as pheromones for other organisms, particularly insects. Plants can use these in a variety of ways- they can attract their attackers' predator, or create chemicals to repel their attackers

Question 60

What form at nodes on plant stems containing meristems?

Answer 60

Branches

Question 61

What type of phototropism do shoots and roots display?

Answer 61

Shoots positive, roots negative

Question 62

How do auxins cause cell elongation?

Answer 62

The plasticity of cell walls which they create allows the cell to expand as they absorb water

Question 63

What do increased light levels do to auxins?

Answer 63

Cause the auxins to diffuse away from the light source

Question 64

What causes plants to grow more quickly in the dark?

Answer 64

Gibberellins

Question 65

Why does it make sense that plants grow more quickly in the dark?

Answer 65

Would allow them to move towards light more quickly

Question 66

How does ethene cause fruit ripening?

Answer 66

Increased ethene levels cause increased respiration and increased carbon dioxide levels, causing ripening

Question 67

Why is it advantageous for fruit to be unripe during transport?

Answer 67

It is harder, as starch is converted to sugar during ripening

Question 68

What type of plant hormone does rooting powder contain?

Answer 68

Auxins

Question 69

How is rooting powder used?

Answer 69

You dip a cut stem in it and it increases the chances of roots forming

Question 70

What type of plant cloning are hormones especially vital in?

Answer 70

Micropropagation

Question 71

What are 4 advantages of hormonal weedkillers?

Answer 71

1. Cheap 2. Can be plant species-specific 3. Non-harmful to animals 4. Can be used in fields of monocot crops to kill dicot weeds

Question 72

How do hormonal weedkillers often work?

Answer 72

They use synthetic auxins to stimulate unsustainable growth in the weeds

Question 73

What is Synergism

Answer 73

This is where two hormones work together, complementing each other causing a greater response than they would on their own.

Question 74

What is Antagonism

Answer 74

This is where two hormones have an opposite affect on each other e.g. one promoting plant growth and one inhibiting it. The balance between the response will determine the plant response.