16.1 plant hormones and growth in plants

limitations of plants responding to their environment

they are not mobile and do not have a rapidly responding nervous system

roles of auxins

• control cell elongation

• prevent leaf fall

• maintain apical dominance

• involved in tropisms

• stimulate release of ethene

• involved in fruit ripening

role of gibberellin

• cause stem elongation

• trigger the mobilisation of food

• stored in a seed at germination

• stimulate pollen tube growth in fertilisation

role of ethene

• causes fruit ripening

• promotes abscission in deciduous trees

role of ABA

• maintains dormancy of seeds and buds

• stimulates cold protective responses eg antifreeze protection

• stimulated stomatal closing 

what happens during seed germination

1. for a seed to germinate the seed needs to absorb water

2. this activates the embryo to produce gibberellins

3. gibberellins stimulates the production of enzymes to breakdown tghe food stored insdie of the seed

4. the food stores are then used to produce ATP for building materials so the embyro can grow and break through the seed coat

evidence for the action of gibberellin

mutated seed produced which lack the gibberellin gene - they don’t germinate and if gibberellin is then artificially added to the mutant seeds they germinate

if gibberellin inhibitors are added to non mutant genes - no germination, if the inhibition is removed or gibberellin is artificially added they germinate

where are auxins made and how do they move around the plant

• made in cells at the tip of the roots and shoots, and in the meritstems

• auxins move down the stem and up the root both in the transport tissue and from cell to cell

• the effect of auxin depends on its concentration and interaction with hormones

what is apical dominance

• auxin is produced in the meristem cells at the top of the apical shoot

• higher levels of auxin stimulates the growth of the apical shoot but inhibits the growth of the lateral shoots

• lateral shoots near the top where there is more auxin will be shorter 

• shoots further down with less auxin will be longer

apical shoot growth

1. auxins bind to receptor sites on the plant cell membrane

2. this causes a fall in PH to PH 5

3. this is the optimum PH for the enzymes that keep the walls very flexible and plastic

4. as the cells mature auxin is destroyed, so auxin levels fall and PH rises

5. the enzyme maintaining plasticity becomes inactive

6. as a result, the wall becomes rigid and more fixed in shape and size and the cells can no longer expand and grow

effect of auxin on the root + experimental evidence for this

• low concentrations of auxin promote root growth

• auxin produced by root tips diffuses down from the growing shoot tops

experimental evidence

remove the apical shoot

• auxin reaching the root reduced so root growth slows

• replace the auxin at the shoot and root growth restarts

synergism

different hormones working together to produce a greater response

antagonism

• different hormones working in opposite directions

• outcome will depend on the relative levels of each hormone