Diagram of M5: B16.1: Plant Hormones & Growth (unf. gibberellins) | Quizlet

state 4 reasons why plants must respond to abiotic conditions

. cope with changing conditions / avoid abiotic stress

. maximise photosynthesis or to obtain more light / water / minerals

. avoid, herbivory / grazing

. to ensure germination is in suitable conditions / pollination / seed set / seed dispersal ;

Chemical coordination: what are the 2 main limitations of plants?

. rooted (therefore immobile)

. no rapidly responding nervous system

Chemical coordination: main difference between plant respones and animal responses

plant responses are slower

Chemical coordination: name 4 important plant hormones

auxins

gibberellins

ABA (abscisic acid)

ethene

TERM

6 functions of auxins

DEFINITION

. control cell elongation

. prevent abscission (leaf fall)

. maintain apical dominance

. involved in tropisms

. stimulate release of ethene

. involved in fruit ripening

TERM

4 functions of gibberellins

DEFINITION

. control cell elongation

. trigger mobilisation of food stores in a seed at germination

. stimulate pollen tube growth in fertilisation

TERM

2 functions of ethene

DEFINITION

. causes fruit ripening

. promotes abscission in deciduous trees

TERM

2 functions of ABA

DEFINITION

. maintains dormancy of seeds and buds

. stimulates cold protective responses (e.g. antifreeze production, stomatal closing)

Chemical coordination: why are chemicals essential in plant responses?

. signal to other species (e.g. protecting themselves from pest attacks)

. chemical defenses against herbivores

. communicate with other plants?

. seed germination

. LT growth of a tree

etc.

why are scientists still unsure about many plant responses?

. very low concentrations, so hard to isolate plant hormones and measure changes because they work at

. multiple interactions between the different chemical control systems, so hard to isolate single chemicals in a specific response

Auxins: what are they?

growth stimulants produced in plants

Auxins: example of an auxin

IAA (indoleacetic acid)

Auxins: where are they produced?

. tips of roots

. tips of shoots

. meristems

Auxins: where can they move? how?

. down the stem

. up the root

via transport tissue and between cells

Auxins: what does the effect of auxins depend on?

. concentration

. interactions with other hormones

Auxins: what are the 3 main effects on plant growth?

. stimulate growth of apical shoot

. apical dominance

. low conc = root growth promoted

what is the apical shoot?

the shoot growing upwards

what are lateral shoots?

shoots that extend horizontally from the plant stem

Auxins: functions - apical shoot growth: how does it stimulate apical shoot growth?

affect the plasticity of the cell walls, making it stretchier

1. auxin molecules bind to specific receptor sites in the plant cell membrane

2. causes pH to fall to app 5 pH (optimum for the enzymes needed to keep the walls flexible and plastic)

3. as cells mature, auxin is destroyed

4. pH rises, making the enzymes inactive

5. cell wall becomes rigid and more fixed in shape/size

6. cell can no longer expands and grow

Auxins: functions - apical shoot growth: graph to show the effect of the application of auxin on pH in the cell wall and on shoot growth

Auxins: functions - apical dominance: how does it result in apical dominance?

high concentrations suppresses growth of lateral shoots

1. auxin produced in the tip

2. this stimulates quick growth of the main shoot

3. auxin moves down the stem

4. lateral shoots are inhibited by the auxin

auxin concentration is lower farther down the stem, so the lateral shoots at the bottom grow more strongly

Auxins: functions - apical dominance: describe experimental evidence showing that auxins are involved in apical dominance

1. apical shoot is removed

2. auxin-producing cells are removed = no auxin

3. lateral shoots grow faster

4. auxin applied artificially to the cut apical shoot = apical dominance reasserted and lateral shoot growth is suppressed

Auxins: functions - apical dominance: there were some issues with the evidence for the role of auxins in apical dominance

. what were they?

. how were they overcome?

correlation doesn't equal causation as there may have been unexpected effects of cutting off the apical bud

. exposure to oxygen? - cells at the cut end of the stem could have produced a hormone to promote lateral bud growth?

1. a ring of auxin transporter inhibitor could be added below the apical bud

2. auxin couldn't travel to lateral buds

3. lateral buds grew

therefore... low levels of auxin promote growth of lateral shoots

Auxins: functions - apical dominance: there is still another issue with the evidence for apical dominance. what was it?

3rd variable could have been involved

Auxins: functions - apical dominance: name 2 other chemicals that could have caused the results in the experiments

. abscisic acid

. cytokinins

Auxins: functions - apical dominance: how could abscisstic acid be involved in apical dominance?

abscisic acid inhibits bud growth

high auxin levels may keep the abscisic acid levels high in the bud... if the apical tip is removed, the levels of abscisic acid drop, and the lateral buds start to grow

so it may be the abscisic acid causing it, not the auxin levels?

Gibberellins: what are they involved in?

. the germination of seeds

. elongation of plant stems

what are the food stores found in dicot seeds?

cotyledons

what are the food stores found in monocot seeds?

endosperm

Gibberellins: germination - evidence suggests that gibberellins switch on genes which code for ___________ and ___________

amylases, proteases

Gibberellins: germination - what other hormone is thought to be involved? describe

ABA - may act as an antagonist to gibberellins

Gibberellins: germination - what determines whether a seed will germinate?

the relative levels of gibberellin and ABA

Gibberellins: germination - give 2 sources of experimental evidence that supports the role of gibberellin in germination

. mutant varieties have been bred

. gibberellin biosynthesis inhibitors

Gibberellins: germination - evidence: mutant varieties

these lack the gene which enables them to make gibberellins

. they don't germinate

. if they're applied to the seed externally, they germinate normally

Gibberellins: germination - evidence: gibberellin biosynthesis inhibitors

if they're applied to the seeds, they don't germinate as they can't produce gibberellins

. if the inhibition is removed, they germinate

. if gibberellins are applied, the seeds germinate

Gibberellins: germination - diagram to show the role of gibberellins in germination

Gibberellins: elongation of plant stems - how do they do this?

they affect the length of the internodes

Gibberellins: elongation of plant stems - what are internodes?

the regions between the leaves on a stem

Gibberellins: elongation of plant stems - how were gibberellins discovered?

rice seedlings infected with the fungus from the genus Gibberella grew extremely tall and thin

define tropism

a directional growth response in which the direction of the response is determined by the direction of the external stimulus

Auxins: how much is needed?

very small quantities have a large effect

Auxins: functions -

promotes root growth, up to a point

although auxins are produced in the roots, they are also transported from the apical shoots

if you remove the apical shoots, less auxin reaches the roots, stopping root growth - replacing the auxin artificially at the cut apical shoot restores growth of the roots

if auxins are too high, root growth is inhibited

Gibberellins: germination - how does this happen?

1. seed absorbs water

2. embryo activates

3. embryo produces gibberellins

4. gibberellins stimulate the production of enzymes, which break down the food stores found in the seed

5. food stores used to produce ATP for building materials, so it can grow and break through the seed coat

why is it good for plant hormones to work with other substances?

very fine control over the responses of the plant can be achieved

what is synergism?

when different hormones work together to give each other a greater response than they would have independently

what is antagonism?

when different hormones oppose the actions of each other. the balance determines the response of the plant