plant hormones

Question 1

What are the roles of auxins?
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Answer 1

• maintain apical dominance
• ethene release stimulation
• tropisms
• cell elongation
• prevent leaf fall
• fruit ripening

Question 2

What are the roles of gibberellin?
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Answer 2

• stem elongation
• mobilisation of food stores in a seed at germination stimulated
• pollen tube growth stimulation in fertilisation

Question 3

What are the roles of ethene?

Answer 3

• ripening of fruit
• promote abscission in deciduous trees

Question 4

What are the roles of ABA (abscisic acid)?

Answer 4

• maintains dormancy of seeds and buds.
• stimulates cold protective responses (antifreeze production, stimulates stomatal closing).

Question 5

What hormone does a plant embryo is activated by the seed absorbing water?

Answer 5

• Gibberellins

Question 6

What do gibberellins do in a germinating seed?

Answer 6

• stimulate the production of enzymes which break down the food stores in the seed.

Question 7

What does the seed/embryo use the broken down food stores to do?

Answer 7

• produce ATP for building materials so it can grow and break out through the seed coat.

Question 8

what enzymes might gibberellins switch on genes for?

Answer 8

• proteases and amylases

Question 9

What hormone may work as an antagonist to gibberellins, determining when a seed will germinate?

Answer 9

• ABA

Question 10

What experimental evidence is there to support the role of gibberellins in the germination of seeds?

Answer 10

• Mutant varieties lacking gib genes only germinate if gibs applied externally.
• If gib biosynthesis inhibitors are applied to seeds, they do not germinate/cannot break dormancy. Only when gibs added/inhibitors removed do they germinate.

Question 11

What auxin is a growth stimulant in plants?

Answer 11

• IAA

Question 12

Where is IAA made?

Answer 12

• cells at tips of roots and shoots.
• meristems

Question 13

How can auxins move down the stem/up the root?

Answer 13

• transport tissue and from cell to cell

Question 14

Which hormone is responsible for apical dominance?

Answer 14

• auxins

Question 15

What aspect of the cell wall is affected by auxins?

Answer 15

its plasticity; becomes stretchy.

Question 16

How do auxins increase the plasticity of cell walls/allow for cell elongation?

Answer 16

• Bind to specific receptors and bring about a pH decrease to 5.
• This pH is optimal for the enzymes needed to keep the cell walls flexible.
• As the cells mature, auxin is destroyed by enzymes.
• Lower auxin concs = higher pH = less enzyme action.
• fixed shape cell.

Question 17

How do high concs of auxin affect lateral shoots?

Answer 17

• suppress their growth
• results in apical dominance

Question 18

Why do lateral shoots grow better further down the stem?

Answer 18

• the auxin conc is lower

Question 19

How can the effect on auxins on apical dominance/lateral shoot inhibition be shown?

Answer 19

• If the apical shoot is removed the auxin producing cells are removed and so there is no auxin.
• Lateral shoots grow rapidly unless auxin is applied artificially.

Question 20

How can auxins promote root growth?

Answer 20

• Low concs of auxins promote root growth.

Question 21

How does removal of the apical shoot affect root growth?

Answer 21

• Reduces the amount of auxins which are reaching the roots and thus slows /stops growth

Question 22

Aside from seed germination, what is another role of gibberellins (plant shape)?

Answer 22

• Stem elongation

Question 23

What specific regions of the stem are affected by gibberellins?

Answer 23

• internodes

Question 24

What is an advantage of producing dwarf varieties of plants where gibberellins synthesis is interrupted?

Answer 24

• Less vulnerable to damage as a result of weather and harvesting

Question 25

What are the two ways plant hormones can work together?

Answer 25

- Synergism and antagonism

Question 26

What is synergism between plant hormones?

Answer 26

- Complement each other; give a greater response than they would individually

Question 27

What is antagonism?

Answer 27

- If the substances have opposite effects the balance between them will determine the response of the plant.

Question 28

what are examples of abiotic stress which affect plants?

Answer 28

- day length - cold and heat - lack of water - excess water - high winds - changes in salinity

Question 29

What is photoperiodism?

Answer 29

- Plants' sensitivity to a lack of light in their environment.

Question 30

What is the trigger for plants to respond via photoperiodism?

Answer 30

- lack of light

Question 31

What plant responses are affected by the photoperiod?

Answer 31

- breaking of dormancy of leaf buds so they open up - timing of flowering - when tubers are formed in preparation for overwintering

Question 32

What pigment is responsible for plants' light-sensitivity?

Answer 32

- phytochrome

Question 33

What can trigger leaf abcission?

Answer 33

shortening of the day, lowering temps, water stress

Question 34

What is the result of reducing light intensities/levels in winter? (leaf abcission)

Answer 34

- Reduced/falling concs of auxin

Question 35

What hormone is produced in response to falling auxin concs as winter approaches? (leaf abcission)

Answer 35

- ethene

Question 36

What is the region at the base of the leaf stalk called?

Answer 36

- abcission zone

Question 37

What is the abcission zone made up of?

Answer 37

- two later of cells sensitive to ethene

Question 38

What does ethene do to the abcission zone?

Answer 38

- Initiates gene switching in these cells, resulting in the production of new enzymes which digest and weaken cell walls in the separation layer (outer layer of AZ)

Question 39

How does ethene cause leaf drop/abcission?

Answer 39

- Ethene sensitive cells in the abcission zone are stimulated to produce enzymes which digest the outer layer (separation zone). This occurs by gene switching. - Vascular bundles sealed off. Fatty material is deposited in the cells on the stem side of the separation layer; acts as a protective scar when leaf falls off. - Cells deep in the separation zone respond to hormonal cues by retaining water and swelling. Adds strain to weak outer layer. - Further abiotic factors finalise the process.

Question 40

Why do cells die when they freeze?

Answer 40

- Membranes become disrupted and they will die

Question 41

How does altering the composition of the cytoplasm and sap (in vacuole) affect freezing point?

Answer 41

- Higher solute concs lower the freezing point.

Question 42

What types of molecules can act as antifreeze/protect cells from damage if they do freeze?

Answer 42

- sugars, polysaccharides, amino acids, proteins

Question 43

Which hormone controls the opening and closing of the stomata?

Answer 43

ABA

Question 44

What factors cause ABA release?

Answer 44

abiotic stress.

Question 45

How does ABA act as a warning to the rest of the plant about water stress?

Answer 45

- Soil water falls and ABA is transported from roots (where it is produced) to the leaves. - ABA binds to receptors on the plasma membrane of the guard cells, activates changes in water potential and thus decreases the turgor of the guard cells. - causes them to close and water loss is thus reduced

Question 46

Explain why gibberellin (or any other) is classed as a plant hormone.

Answer 46

1. (gibberellin is) a chemical messenger ✓ 2. produced in one part of plant but has effects in another part / AW✓ 3. affects activity / AW , of target , cells / tissues ✓ 4. long-lasting / acts over long period of time ✓ 5. wide-spread effect