Hormonal communication in plants

Question 1

examples of nastic responses

Answer 1

Non-directional eg:
- photonasty - flowers/leaves opening due to light
- hydronasty - folding of leaves in response to driught
- thigmonasty - response to touch/vibration

Question 2

examples of tropison responses

Answer 2

Directional eg:
- phototropism - growth towards/away from light
- chemotropism - growth towards/away from chemicals
- geotropism - growth towards/away gravity
- thigmotropism - growth towards/away contact stimulus

Question 3

synergy

Answer 3

hormones amplifying each others’ effect or working together

Question 4

antagonism

Answer 4

hormones cancelling out each others’ effects

Question 5

action of gibberellin in seed germination

Answer 5

• water absorbed and embryo activated
• embryo starts to produce gibberellins which switch on gene for enzyme production such as protease and amylase
• these enzymes break down food stores in seed eg. starch to maltose to glucose
• energy available for growth

Question 6

abscisic acid action

Answer 6

• opposite effect to gibberellins
• maintains seed dormancy by inhibiting amylase production

Question 7

evidence of gibberellin action

Answer 7

• seeds with mutation to not make gibberallin don’t germinate
• when gibberellins added to seeds externally, they germinate
• if chemicals that inhibit gibberellins applied to seeds, they don’t germinate

Question 8

action of gibberellin in stem elongation

Answer 8

• stimulate cell division and elongation in stem
• alters properties of cell wall, lowering water pot of cell and allowing water uptake and therefore increase in cell volume

Question 9

evidence of gibberellin in stem elongation

Answer 9

• dwarf plants have been found to have low levels of gibberellin
• if dwarf plants treated with gibberellins, they grow to same height as normal plants

Question 10

action of auxins on growth of shoots

Answer 10

• auxin molecules bind to receptor site in plant cell membrane, causing pH to fall to 5
• pH 5 is optimal for enzymes needed to pump protons into cells wall causing bonds between microfibres to loosen - cell wall flexible
• K+ channels open and K+ enters cytoplasm
• water moves down water pot grad annd enters cytoplasm
• when cells mature, auxin levels fall
• therefore pH rises so enzymes stop working so the cell wall becomes more fixed and can no longer grow and expand

Question 11

apical dominance meaning and reasoning

Answer 11

• apical dominance - auxins produced at growing tip of apex causing stem to grow upwards
• high conc auxin - suppresses lateral growth
• further down stem - less auxin so buds can grow laterally
• if apical shoot removed - auxin producing cells removed and apical dominance stops
• best for plants to grow upwards towards light to maximise energy for photosynthesis
• sideways growth not so useful - apical dominance caused by auxins ensures growth is upwards

Question 12

synergism

Answer 12

2 or more growth regulators act together to reinforce or amplify the effects
eg. auxins, gibberellins increase cell elongation