BIOLOGY 11

1
BIOLOGY 11

• P
• L
• A
• N
• T

H O R M O N E S
2
Introduction

• Like all living things, plants must be able to
  detect their surroundings and respond to stimuli
  in their environment in an appropriate fashion.
• In higher animals, this is accomplished with the
  nervous and endocrine systems.
• Plants only use a hormonal system as their
  responses to stimuli need not be rapid.

3
Hormones

• Hormones, in either plant or animal, are powerful
  control chemicals.
• Hormone a substance produced in one tissue and
  transported to another tissue where they have a
  specific effect.
• Typically function in minute quantities.
• In plants, hormones regulate growth, development,
  and environmental responses.

4
Auxins

• Auxins (chemically called Indolacetic Acid or
  IAA) was the first plant hormone discovered.
• Its discovery is a good example of the
  scientific method of inquiry in action.
• Auxins are growth regulators in plants and are
  also involved in various plant behaviors.

5
Auxins

• A. Phototropism
• This is a familiar plant behavior that involves
  the plant growing towards light.
• The first scientific investigations of this, by
  Charles Darwin in 1880, led to the discovery of
  auxins.

6
Phototropism

• Darwin was the first to observe the photo -
  sensitivity of emerging coleoptiles (first shoot
  in grasses)

7
Phototropism

• Darwins Experiments
• 1. Control coleoptiles are positively
  phototropic
• 2. Decapitated coleoptiles are no longer
  phototropic
• 3. If coleoptile tip is covered with an opaque
  cap, its no longer phototropic

8
Phototropism

• 4. Coleoptile tip covered by a transparent cap
  remains phototropic.
• 5. Cover the base of the coleoptile with an
  opaque shield, it remains phototropic.
• Conclusion
• The tip of the coleoptile is responsible for its
  phototropic abilities.

9
Phototropism

• Some diffusable substance was produced in the tip
  that alone is enough to cause bending (rest of
  cell structures of tip not required). Therefore
  mechanism isnt electrical or mechanical - must
  be chemical.
• He named this substance
• Auxin (to increase).

10
Darwins Experiment
11
Phototropism

• 1. Auxin is a growth hormone that stimulates cell
  elongation.
• 2. When light strikes one side of the coleoptile,
  auxin is transported to the opposite side -
  therefore auxin concentration higher on side
  opposite light.

12
Phototropism

• 3. The difference in auxin concentration results
  in different rates of cell elongation (greatest
  away from light) which causes the coleoptile to
  bend.
• Auxin is normally produced in the apical
  meristem. It diffuses down into the zone of
  elongation. If light is from on top, the auxin
  diffuses evenly and growth is straight up.

13
Phototropism

• If the light comes from any other direction, the
  concentration of auxin in the zone of elongation
  will be uneven resulting in bending towards the
  light.
• Many herbicides such as 2,4,D are chemically very
  similar to auxin (IAA) - these cause uncontrolled
  growth in the plant which results in their death.

14
(No Transcript)
15
Geotropism

• This is the response of plants to gravity
• Roots show positive geotropism, while stems show
  negative geotropism.
• This is particularly significant in seed
  germination.
• Seeds are not planted in an orientation that
  places the embryonic stem in an up position and
  the embryonic root in a down position.

16
Geotropism
17
Geotropism

• So, when a seed germinates and the stem and root
  emerge, how do they determine which way they
  should grow?
• Auxins are again involved.
• In roots, auxin acts as a growth inhibitor while
  in stems it is a stimulator.
• When the two shoots emerge from the seed, starch
  granules in them settle out due to gravity.

18
Geotropism

• These granules effect the response of auxin.
• In the embryonic root, the bottom cell layers are
  inhibited while the upper layers arent (due to
  the starch granules).
• This differential growth of cell layer turns the
  root downwards.

19
Geotropism

• In the emerging stem, the starch granules on the
  lower cell surfaces stimulate auxin to cause the
  cells to elongate more than the upper cell
  layers.
• As a result, the stem bends upwards.
• Why auxin has a different effect on different
  cells has to do with the genetic differentiation
  that occurs as cells mature.

20
Other Effects of Auxin

• Auxins are also involved in the following plant
  functions
• Budding
• Fruit development
• Leaf falling
• Secondary growth in stems and roots

21
Gibberellins

• The Japanese observed a disease in rice that
  caused them to grow very rapidly and die before
  sexual maturity (and thus produce no rice).
• They called this the Foolish Seedling Disease.
• Discovered that it was caused by a fungus named
  Gibberella fujikuroi that grew on the seedlings.

22
Gibberellins

• This secreted a substance that caused the
  disease.
• It was soon discovered that almost all plants
  also produce this chemical which is a hormone and
  is now called Gibberellin.
• Gibberellins cause cell elongation, so are they
  just another auxin?
• No. Heres why.

23
Gibberellins

• 1. Gibberellins can make genetic dwarf plants
  tall - auxins cant.
• 2. Gibberellins have no effect on decapitated
  coleoptiles.
• 3. Gibberelins do not cause stem or root bending.
• 4. Gibberelins do not prevent leaf abscission.

24
Gibberellins

• 5. Gibberelins do not inhibit root cell
  elongation.
• 6. Gibberellins stimulate phloem growth, auxins
  stimulate xylem.
• 7. Gibberelins do not inhibit lateral budding in
  stems, auxins do (apical dominance).

25
Gibberellins

• 8. Gibberellins do
• Help seeds break dormancy.
• Induce biennials to flower in one year
  (significant in horticulture).
• Can induce early flowering in annuals.
• Therefore, gibberellins and auxins are
  complimentary to each other in regulating plant
  growth and development.

26
Ethylene

• A unique hormone in that it is a gas.
• It diffuses from the tissues of the fruit.
• Promotes ripening, senescence (aging), and leaf
  abscission.

27
Abscisic Acid

• A growth inhibitor that slows plant growth,
  especially primary stem growth.
• Stimulates bud scales over buds to prepare them
  for dormancy.
• Stops growth of seed embryo to prepare it for
  dormancy.

28
Cytokinins

• Promote cell division in plant embryonic tissue
  without causing cell differentiation.
• A certain ratio of cytokinins to auxins result in
  organized growth leading to cell differentiation.
• If ratio of auxin is higher, tissue becomes
  roots, if cytokinin is higher, it becomes stems.
• Also contribute to Apical Dominance.

29
Photoperiodism

• This is the response of plants to varying amounts
  of light which control some aspect of behavior.
• This is especially significant with regards to
  flowering times and germination times.
• When these behaviors occur is governed by the
  length of day vs. night.

30
Photoperiodism

• In nature, some plants are short-day plants
  they flower when the day length is short, ie
  spring or fall.
• Other plants are long-day plants that flower when
  days are long and nights are short.
• How do the plants tell the length of day?

31
Photoperiodism

• Experiments show that if short day plants are
  exposed to light (even for a few seconds) during
  its normal night, it will not flower.
• If long-day plants are exposed to light in the
  night, they flower but in the wrong season.
• If you shield short-day plants from light for a
  few hours, they flower as normal.

32
Photoperiodism

• From this, it can be determined that flowering
  time is determined by the length of the night,
  not the length of day. Conclusion plants respond
  to the last type of light they are exposed to.
• They postulate the existence of a yet
  undiscovered hormone they called phytochrome
  (since discovered).

33
Circadian Rhythms

• Response of a plant to the length of day-night
  can also operate independently.
• Ex certain flowers bloom at dawn and close at
  dusk - bring them inside with 24 hr. of light and
  they still open and close on schedule.
• This is a circadian rhythm or a biological clock.

34
Circadian Rhythms

• These clocks can be re-set.
• If you expose the plant to new dawn/dusk times,
  it will reset its blooming and closing within a
  few days.
• Therefore, circadian rhythms must be genetically
  inherited, but capable of re-setting.
• Phytochromes are likely involved.