Biology - 16.1 plant hormones and growth in plants

How have plants evolved to form a system of hormones

Chemicals that are produced in 1 region of the plant are transported both through the tissues and from cell to cell and have an effect in another part of the plant

What are some important plant hormones

Auxins
Gibberellin
Ethene
ABA (abscisic acid)

What is the role of the plant hormones

- auxins - control cell elongation, prevent leaf fall (abscission), maintain apical dominance, involved in tropisms, stimulate the release of ethene, involved in fruit ripening
- gibberellin - cause stem elongation, trigger the mobilisation of food stores in a seed at germination, stimulate pollen tube growth in fertilisation
- ethene - causes fruit ripening, promotes abscission in deciduous trees
- ABA (abscisic acid) - maintains dormancy of seeds and buds, stimulates cold protective responses, for example, antifreeze production, stimulates stomatal closing

What is the apical bud

The shoot tip at the top of a flowering plant

What is apical dominance

When auxins stimulate the growth of the apical bud and inhibit the growth of side shoots from lateral buds
Apical buds are dominant over the lateral buds

What does apical dominance prevent

Side shoots from growing - this saves energy and prevents side shoots from the same plant competing with the shoot tip for light

What does apical dominance allows a plant to do

In an area where there are loads of other plants, it allows the plant to grow tall very fast to reach the sunlight

What happens when you remove the apical bud

The plant wont produce auxins, so the side shoots will start growing by cell division and cell elongation

What happens when you replace the tip of a plant

The source of auxins, side shoot development is inhibited meaning apical dominance is controlled by auxin

What happens to auxin concentration as you move down the plant

Become less concentrated as they move away from apical bud. If plant grows very tall, the bottom of the plant will have a low auxin concentration so side shoots will start to grow near the bottom.
Promote root growth as well

How can you investigate the role of auxins on apical dominance

1) Plant 30 plants (e.g. pea plants) that are a similar age, height and weight in pots.
2)Count and record the number of side shoots growing from the main stem of each plant.
3) For 10 plants, remove the tip of the shoot and apply a paste containing auxins to the top of the stem.
4) For another 10 plants, remove the tip of the shoot and apply a paste without auxins to the top of the stem.
5) Leave the final 10 plants as they are - these are your untreated controls. These are needed for comparison so that you know the effect you see is likely to be due to the hormone and not any other factor.
6) Let each group grow for six days. You need to keep all the plants in the same conditions - the same light intensity, water, etc. This makes sure any variables that may affect your results are controlled,
which makes your experiment valid.
7) After six days, count the number of side shoots growing from the main stem of each of your plants.
8)The results in the table show that removing the tips of shoots caused extra side shoots to grow, but removing tips and applying auxins prevented extra side shoots from growing.
9) The results suggest auxins inhibit the growth of side shoots -
- suggesting that auxins are involved in apical dominance.

What are gibberellins?

Produced in young leaves and in seeds. They stimulate seed germination, stem elongation, side shoot formation and flowering

What do gibberellins do?

Stimulate the stems of plants to grow by stem elongation - helps plant grow very tall

what do gibberellins stimulate

seed germination, by triggering the breakdown of starch into glucose in the seed. the plant embryo in the seed can then use the glucose to begin respiring and release the energy it needs to grow

How can you investigate the role of gibberellins in stem elongation

1) Plant 40 plants (e.g. dwarf pea plants) that are a similar age, height and mass in pots.
2)Leave 20 plants as they are to grow, watering them all in the same way and keeping
them all in the same conditions - these are your controls.
3) Leave the other 20 plants to grow in the same conditions, except water them with a dilute solution of gibberellin (e.g. 100 mg dm-* gibberellin).
4) Let the plants grow for about 28 days and measure the lengths of all the stems once each week.
5) You might get results a bit like these:
6) The results in the table show that stems grow more when watered with a dilute solution of gibberellin.
7)The results suggest gibberellin stimulates stem elongation.
8) You might have to calculate the rate of growth of the plants in your exam, e.g:
In 28 days the plants watered normally grew an average of 9 cm (23 cm - 14 cm), so they grew at an average rate of 9 ÷ 28 = 0.32 cm/day or 0.32 cm day-1. In 28 days the plants watered with gibberellin grew an average of 32 cm (46 cm - 14 cm), so they grew at an average rate of 32 ÷ 28 = 1.14 cm day-1.

what are gibberellins inhibited by

abscisic acid

why are auxins and gibberellins often called synergistic

work together to have a big effect - work together to help plant grow very tall

why are auxins and gibberellins often called antagonistic

they oppose each other's actions - gibberellins stimulate the growth of side shoots but auxins inhibit the growth of side shoots