16.1 Plant Hormones And Growth In Plants Flashcards
What is a tropism?
Directional growth in response to environmental cues
What are the two key limitations on plants?
Immobile
No nervous system
What is the role of auxin in plants?
CONTROL cell elongation
PREVENT abscission
Apical dominance maintenance
Fruit ripening
Stimulate the release of ethene
Involved in tropisms
What are the names of some plant hormones?
Auxin
Gibberelin
ABA Abscisic Acid
Ethene
What is the role of gibberellins in plants?
Causes STEM ELONGATION
Stimulate POLLEN TUBE GROWTH in fertilisation
Mobilisation of FOOD STORES in a seed at GERMINATION
What is the role of ethene in plants?
Causes FRUIT RIPENING
PROMOTES abscission in deciduous trees
What is the role of ABA in plants?
Maintains DORMANCY of seeds and buds
Stimulates STOMATAL CLOSURE
Stimulates cold protective responses
E.g. antifreeze production
Why are scientists still unsure about the details of many plant responses?
Plant hormones work at very low concentrations SO
isolating them and measuring changes in concentration is not easy
Multiple interactions between different CHEMICAL CONTROL systems makes it difficult to isolate role of a single chemical in a specific response
How does a seed germinate because of plant hormones?
- Seed absorbs water
- Embryo is activated and begins to produce gibberellins
- Gibberllins stimulate PRODUCTION OF ENZYMES that break down food stores found in seed
- Embryo plant uses food stores to produce ATP
- Build materials
- Grows and breaks out through seed coat
What determines whether a seed will germinate?
Relative levels of gibberellins and ABA
Where is the food store found in dicot seeds?
Cotyledons
Where is the food store found in monocot seeds?
Endosperm
What does evidence suggest about gibberellins involved in seed germination?
Switch on genes coding for digestive enzymes for germination
E.g. amylases and proteases
What is an antagonist?
Substance which interferes with action of another
What is an example of an antagonist in plant hormones?
ABA is an antagonist of GIBBERELLINS
What is ABA?
Abscisic acid
What is experimental evidence supporting the role of gibberellins in the germination of seeds?
Mutant varieties of seeds lacking genes which produce gibberellin do not germinate
They germinate normally if gibberellins are applied externally
Seeds with GIBBERELLIN BIOSYNTHESIS INHIBITITOR applied to not germinate
Germinate if inhibitor is removed OR gibberellins applied
What is an example of an auxin?
IAA indoleacetic acid
What does IAA stand for?
Indoleacetic acid
Wher are auxins made in a plant?
Tips of roots and shoots
Meristems
How does auxin move through a plant?
Down the stem
Up the roots
TRANSPORT TISSUE and from CELL TO CELL
What does auxin do?
Stimulates growth of main, apical shoot
What does evidence suggest about auxin and cell walls?
Auxin causes cell walls to stretch more easily
How does auxin cause cell elongation?
- Auxin molecules bind to receptors in plant cell membrane
- pH falls to around 5
- Optimum pH for enzymes to keep cell flexible and plastic
- Cells mature
- Auxin destroyed
- pH rises and plasticity enzymes become inactive
- Cell wall becomes rigid again
What does high concentrations of auxin do?
Suppress growth of LATERAL shoots
What is something else high leves of auxin does?
Stimulates growth in MAIN shoot
What happens if the apical shoot is removed?
- Auxin-producing cells removed so NO AUXIN
- Lateral shoots freed from dominance of apical shoot
- Lateral shoots grow faster
- Applying auxin artificially to apical shoot reasserts apical dominance and lateral shoot growth is suppressed
What does low concentrations of auxin do?
Low concentrations of auxin a promote root growth
What happens to the roots UP TO A GIVEN CONCENTRATION of auxin?
The more auxin that reaches the roots, the more they GROW
How does the root get auxin?
Low concentrations from growing shoots
What happens to the ROOTS if the apical shoot is removed?
- Apical shoot is removed
- Amount of auxin reaching roots is greatly REDUCED
- Root growth slows and STOPS
- Replacing auxin artificially AT CUT APICAL SHOOT restores root growth
What does high auxin concentrations do to the roots?
High auxin concentration inhibits root growth
What is something else gibberellins are responsible for in plants?
Elongation of plant stems during growth
What are ‘internodes’?
Regions between leaves on a stem
What do gibberellins affect?
Length of plant internodes
What do plants that have short stems produce?
FEW or NO gibberellins
What does interrupting the gibberellin synthesis pathway do?
Makes plant stems much shorter
What does shorter plant stems allow?
Reduces waste
Makes plants LESS VULNERABLE to DAMAGE by WEATHER and harvesting
What does growing seedlings ‘hydroponically’ mean?
In nutrient solution rather tha soil
How should the data from each experimental group measuring the effects of the concentrations of plant hormones be measured?
Standard deviation
What is synergism?
When two factors’ actions are greater than the sum of their total actions
What is antagonism in plants (compared to synergism)?
Substances have opposite effects
