The Effects of Plant Hormones

  • Created by: amyquince
  • Created on: 04-06-19 16:19

APICAL DOMINANCE

- Shoot tip at top of plant called apical bud

- auxin stimulates growth of apical bud and inhibits growth at side shoots from lateral buds (apical dominance)

- apical bud dominant over lateral buds

- prevent side shoots from growing so saves energy and prevents side shoots from competing with tip for light

- because energy isnt being used to grow lateral buds it allows plants in busy area to grow tall and fast to reach sunlight

- if you remove apical bud, auxin stops being produced so lateral buds with grow by cell elongation

- if you replace tip with source of auxin, side shoot development is inhibited, auxin controls apical dominance

-auxins become less concentrated as they move away from apical bud

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INVESTIGATING APICAL DOMINANCE

1. plant 30 pea plants that are similare height, age and weight in pots

2. count and record number of side shoots growing from mean stem of each plant

3. for 10 plants remove the topf od shoot and apply paste with auxins to top of stem

4. for another 10 plants remove tip of shoot and apply paste with no auxins in to top of stem

5. leave final 10 plants as they are - this is the controls and are needed for comparison

6. let grow for 6 days, keep all plants at same conditions (light intesity, water etc)

7. after 6 days count the number of side shoots growing from main stem of plant

8. removing tips caused side shoots to grow but applying auxin paste to open tips prevented extra side shoots from growing

9. show auxin inhibits the growth of side shoots and therefore is involved in apical dominance.

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GIBBERELLINS

- produced in young leaves and seeds

- stimulate seed germination, stem elongation, side shoot formation and flowering

- stimulates the stems of plants to grow by stem elongation, unlike auxins, dont inhibit growth in anyway.

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

- inhibited by ABSCISIC ACID

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INVESTIGATING GIBBERELLINS

- genetically altered seeds without gibberellins unable to germinate without being given gibberellins

STEM ELONGATION

- plant 40 plants (dwarf pea plants) that are similar age, height and mass in pots

- leave 20 plants as they grow, watering them all the same way and keeping them all in the same conditions

- leave other plants to grow in same condition except water them with dilute concentration of gibberellins

- let plants grow for 28 days and measure the lengths of all the stems once each weeks

- stems should grow more when watered with gibberellin

- show gibberellins stimulates cell elongation

- calculate rate of growth (tallest - shortest height = average, average / no of days= average grow)

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GIBBERELLINS AND AUXIN TOGETHER

- often synergistic - work together to have a really big effect (eg: help plants to grow really tall)

- Antagonistic - oppsoe each others actions (eg: gibberellins stimuates growth of side shoots but auxin inhibits it)

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ABSCISSION

- deciduous plants lose leaves in winter

- helps to conserve water as it cannot be as easily absorbed from soil, also less light for photosynthesis

- triggered by shortening day length in autumn

- controlled by hormones:

- auxin inhibits leaf loss- auxin produced in young leaves and as leaves grow older less auxin is produced

- ethene stimulates leaf loss- ethene produced by ageing leaves. abscission layer develops at bottow of leaf stalk where it joins to the stem and causes the leaf to separate from the plant. ethene stimulates cells in abscission layer to expand, beaking cell walls and causing leaf to fall off.

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STOMATAL CLOSURE

- plants need to be able to lose stomata to reduce water loss

- guard cells control stomata

- when guard cells full of water they are plump and turgid and the pore is open

- when guard cells lose water they are flaccid making the pore close

- abscisi acid (ABA) is able to trigger stomatal closure

- ABA binds to receptors on guard cell membranes causing specific ion channels to open, which allows calcium ions to enter the cytosol from vacuole. increased conc causes other ion channels to open, allowing other ions such as potassium to leave guard cells, raising water potential of cells. water then leaves the guard cells by osmosis. guard cells become flaccid and stomata close.

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COMMERCIAL USES

- ethen - ripens fruit (stimulates enzymes that break down cell wall, break down chlorophyll and starch into sugars) Eg; bananas harvested before they are ripe because they are less likely to be damaged

- auxins - used in weedkillers (herbicides) - auxins make weeds produce long stems instead of lots of leaves, weeds go too fast so can't get enough nutrients or water

- auxins - used as rooting hormones, making a cutting of the plant grow roots. the cutting can then be planted and grown into a new plant. many cuttings can be taken from original plant and treated with rooting hormones so lots of the same plants can be grown quickly and cheaply. 

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