plant hormones

Question 1

Tropisms

Answer 1

Directional growth responses in plants

Question 2

Phototropisms

Answer 2

Growth in response to light
positve phototropism shoots grow towards light
negative phototropism grow away from light

Question 3

explain phototropism

Answer 3

1. Unilateral light causes auxins to move laterally in shoot tips to shaded side
2. higher auxin conc on shaded side
3. higher auxins conc promotes cell elongation on shaded side which grow faster
4. shoot bends towards light

Question 4

benefits of phototropism

Answer 4

ensures max rate of photosynthesis as shoots grow towards the light (+ve phototropism)

roots grow away from light (-ve phototropism)
get water and minerals in soil

Question 5

Geotropism

Answer 5

Growth is response to gravity
positive geotropism roots grow towards gravity
negative geotropism shoots grow away from gravity

Question 6

explain geotropism

Answer 6

auxins fall to the bottom parts of shoots and roots due to gravity

shoots negative geotropism
auxins prompt shoot growth. bottom part grows faster

roots positive geotropism
auxins inhibit root growth
top part of roots grow faster

Question 7

benefits of geotropism

Answer 7

ensures shoots grow upwards for more light
ensures roots grow downwards to absorb more water and mineral ions

Question 8

Thigmotropism

Answer 8

Influenced by touch

Question 9

Chemotropism

Answer 9

Influenced by chemicals

Question 10

Plant hormones

Answer 10

Cytokinins
Abscisic acid (ABA)
Auxins
Giberellins
Ethene

Question 11

Effects of cytokinins

Answer 11

Promote cell divison
Delay leaf senescence - increases shelf life
Overcome apical dominance - lateral growth
Promote cell expansion

Question 12

ABA

Answer 12

Maintain dormancy of seeds and buds ( prevent germination)
works against gibberellins
stimulate protective measures
antifreeze chemical release to prevent freezing
stimulate stomata closure to prevent water loss for example

Question 13

auxins

Answer 13

inhibits fruit ripening and abscission
controls cell elongation
maintain apical dominance
produced by meristem produced by shoots
auxins favour the growth of apical shoots but inhibit the growth of lateral shoots

Question 14

Effect of gibberellins

Answer 14

Promote seed germination - break bud dormancy (works against ABA)
Promote growth of stems - elongation of internodes
Develop seedless fruit and fruit setting
Acts synergistically w/ auxin

Question 15

Effect of ethene

Answer 15

Promote fruit ripening - starch to sugar and breaks down chlorophyll and cell wall
Stimulates cells in abscission zone to expand and breaks cell wall causing leaf to fall off
Opp to auxin

Question 16

How do plants avoid herbivores
5 chemical
3 physical

Answer 16

physical: thorns, spikes, hairy leaves

chemical
1. Tannis- bitter taste, toxic to insects
2. Alkaloids- bitter taste, poisonous to animals ( affect metabolism)
3. Terpenoids- toxic to insects and fungi ( neurotoxins, repellents)
4. pheromones- affect social behaviour of other members of the same species
5. Volatile organic compounds- same as pheromones but affect other species

Folding response in Mimosa pudica

Question 17

Abscission in deciduous plants

Answer 17

1. Phytochromes detect falling light levels
2. decrease in production of auxins
3. ethene levels increase
4. ethene activates genes in abscission zone to produce digestive enzymes
5. enzyme digest cell wall in superstition zone and vascular bundles are sealed

Question 18

benefits of abscission

Answer 18

1. reduce energy needed to maintain levels in winter
2. fallen leaves insulate roots
3. rotten leaves provide minerals to plants

Question 19

Mechanism of seed germination

Answer 19

gibberellins stimulate production. of digestive enzymes to release glucose from food stores
the glucose can be used in respiration for energy
energy released for seedling growth
the plant embryo in the seeds use the glucose to begin respiring and release energy it needs to grow

Question 20

Mechanism of stomatal closure

Answer 20

roots detect lack of water and produce ABA
ABA travel to leaves and bind to receptors on guard cells
ions move out of guard cells and affect water potential gradient
water leaves by osmosis
guard cells become flaccid and close stomata to decrease transpiration

Question 21

Proof of gibberellins causing seed germination

Answer 21

Mutant varieties that lack gibberellin do not germinate but w/ external gibberellin they do
When gibberellin inhibitors are addeed to normal seeds they dont grow

Question 22

Apical Dominance Effect

Answer 22

Auxin produced at the apex, inhibits growth of lateral buds

Question 23

Experimental evidence for apical dominance

Answer 23

Removal of apical buds allows lateral bud to grow
Auxin/synthetic auxin placed on cut tip continues to inhibit the growth of side shoots

Plant 30 plants of same type, age, genotype and weight in same soil
Remove tip of 10 and apply auxin paste
Remove tip of another 10 and add paste w/out auxin
Leave last 10 as control
Sig. increase of no. of side shoots grown in first 10

Question 24

Recent research on apical dominance

Answer 24

Auxin stimulates production of ABA (inhibits growth)
When apex is removed as is the source of auxin, ABA levels decrease
Most cytokinins go to tip so when tip is removed cytokinins spread evenly around plant promoting growth

Question 25

Where does growth occur in plants

Answer 25

Apical meristems Lateral bud meristems

Question 26

Mechanism of cell elongation by auxin

Answer 26

1. Meristematic cells produce auxins which diffuse down the shoot down a concentration gradient 2. auxins bind to receptors in the cell which open protein channels on the plasm membrane that allows H+ ions to enter the cell 3. low Ph causes cell wall to become more flexible 4. cell absorbs water which expands the cell wall and the cell elongates 5. cell wall becomes rigid when auxins are broken down

Question 27

Confirming auxin as the hormone that causes growth

Answer 27

Impregnated agar blocks w/ diff conc. of auxin Placed them on cut shoot tips Same effects as in reg. shoots Curvature is directly proportional to conc of auxin used

Question 28

Research supporting geotropism in shoots

Answer 28

Plants are grown on a slowly rotating drum (clinostat) so gravitational stimulus is applied evenly Plants grow straight in both light and dark

Question 29

Research supporting geotropism in roots

Answer 29

Seeds are placed in petri dishes w/ moist cotton wool that are rotated 90 degrees as seedlings grow Cover lid w/ oil - ensure no light is coming in All petri dishes should be in same environment Geotropic response in the roots can be seen every 2 hours

Question 30

Investigating role of gibberellin in stem elongation

Answer 30

Plant 40 plants Water 20 plants normally Water other 20 w/ diluted solution of gibberellins Let all 40 grow for 28 days, measuring height every 7 Calculate rate at which plants grew

Question 31

IAA

Answer 31

Natural form of auxin responsible for +ve phototropism so plant bends towards light to phostosynthesise and grow taller and grows roots to reach water and nutrients Light stimulus detected by tip of plant