1. Biology, 9th ed, Sylvia Mader
Ch Plant Responses
Chapter 27
Control of Plant Growth/Response

2. Biology, 9th ed, Sylvia Mader
Ch Plant Responses
Chapter 27
Control of Plant Growth/Response
Tropisms
Plant growth toward or away from a unidirectional stimulus is called a tropism
Positive is towards stimulus
Negative is away from stimulus
Due to differential growth - one side of organ elongates faster than the other
Three types of tropisms:
- Phototropism - movement in response to light
- Gravitropism - movement in response to gravity
- Thigmotropism - in response to touch

3. Biology, 9th ed, Sylvia Mader
Chapter 27
Phototropism
Control of Plant Growth/Response

4. Biology, 9th ed, Sylvia Mader
Phototropism
Chapter 27
Control of Plant Growth/Response
Positive phototropism:
Studied by Charles & Francis Darwin
Occurs because cells on the shady side of the stem elongate
A pigment related to riboflavin thought to act as a photoreceptor when phototropism occurs
Plant hormone called auxin migrates from lighted side of stem to shady side of stem
Cells on the shady side elongate faster than those on the bright side, causing stem to curve toward the light

5. Biology, 9th ed, Sylvia Mader
Gravitropism
Chapter 27
Control of Plant Growth/Response
When a plant is placed on its side, the stem grows upward, opposite of the pull of gravity. This is an example of negative response called gravitropism
Roots, in contrast, show positive gravitropism, as they grow downwards.
Roots without root caps don’t respond to gravity
Root cap cells contain sensors called statoliths, which are starch grains located within amyloplasts, a type of plastid.
- Amyloplasts settle to lower part of cell & cause bending of root.

6. Biology, 9th ed, Sylvia Mader
Chapter 27
Gravitropism
Control of Plant Growth/Response

7. Biology, 9th ed, Sylvia Mader
Chapter 27
Gravitropism
Control of Plant Growth/Response
Negative gravitropism of stems
Positive gravitropism of roots
Sedimentation of statoliths

8. Biology, 9th ed, Sylvia Mader
Gravitropism
Chapter 27
Control of Plant Growth/Response
Auxin is responsible for:
Positive gravitropism of roots, and
Negative gravitropism of shoots
How does auxin do this:
Amyloplasts come in contact with ER which releases stored calcium ions.
This leads to activation of auxin pumps & auxin enters the cells
Roots & stems respond differently to auxin:
- Auxin inhibits growth of root cells, so cells on upper surface elongate so root curves downward
- Auxin stimulates growth of stem cells, so cells on lower surface elongate so stem curves upward

9. Biology, 9th ed, Sylvia Mader
Chapter 27
Gravitropism
Control of Plant Growth/Response
Negative gravitropism of stems
Positive gravitropism of roots
Sedimentation of statoliths

10. Biology, 9th ed, Sylvia Mader
Thigmotropism
Chapter 27
Control of Plant Growth/Response
Unusual growth due to contact with solid objects is called thigmotropism
Ex: Coiling of tendrils
The plant grows straight until it touches something.
Cells in contact with object grow less while those on the opposite side elongate.
Response can be quite rapid; within 10 minutes
Sometimes it seems to need light which might be a need for ATP for the response.

11. Biology, 9th ed, Sylvia Mader
Coiling Response
Chapter 27
Control of Plant Growth/Response

12. Biology, 9th ed, Sylvia Mader
Nastic Movements
Chapter 27
Control of Plant Growth/Response
Nastic movements:
Do not involve growth and
Are not dependent on the stimulus direction
Seismonastic movements result from:
Touch, shaking, or
Thermal stimulation
Due to loss of turgor pressure within a few cells located in a thickening, called a pulvinus, at the base of each leaflet. Touch causes K+ to flow out of cells & then water follows.
•Ex: Mimosa leaves & Venus flytrap

13. Seismonastic Movement
Biology, 9th ed, Sylvia Mader
Seismonastic Movement
Chapter 27
Control of Plant Growth/Response
Mimosa pudica

14. Biology, 9th ed, Sylvia Mader
Nastic Movements
Chapter 27
Control of Plant Growth/Response
Sleep movements:
Occur daily in response to light and dark changes
Ex: Prayer Plant
Movement due to changes in turgor pressure of motor cells in a pulvinus located at the base of each leaf.

15. Biology, 9th ed, Sylvia Mader
Chapter 27
Sleep Movement
Control of Plant Growth/Response
Prayer plant

16. Biology, 9th ed, Sylvia Mader
Circadian Rhythms
Chapter 27
Control of Plant Growth/Response
Biological rhythms with a 24-hour cycle
Tend to be persistent
Rhythm is maintained in the absence of environmental stimuli
Caused by a biological clock
Without environmental stimuli, circadian rhythms continue but the cycle extends to 25 or 26 hours
Believed that the clocks are synchronized by external stimuli such as length of daylight compared to length of darkness. This is called the photoperiod.

17. Biology, 9th ed, Sylvia Mader
Plant Hormones
Chapter 27
Control of Plant Growth/Response
Almost all communication on a plant is done by hormones
Chemical signals produced in very low concentrations in one part of plants and then active in another part of the plant
Hormones travel within phloem, or from cell to cell, in response to the appropriate stimulus
Each hormone has a specific chemical structure

18. Biology, 9th ed, Sylvia Mader
Auxins
Chapter 27
Control of Plant Growth/Response
The most common naturally occurring auxin is indoleacetic acid (IAA).
It is produced in shoot apical meristem and is found in young leaves and in flowers and fruits
Auxins affect many aspects of plant growth & development

19. Biology, 9th ed, Sylvia Mader
Effects of Auxin
Chapter 27
Control of Plant Growth/Response
Apical Dominance
Apically produced auxin prevents the growth of axillary buds (side buds)
When a terminal bud is removed, the nearest lateral buds begin to grow, and the plant branches
Pruning the top of a plant generally achieves a fuller look by removing the apical dominance
Weak solution of auxin applied to woody cutting causes rapid growth of adventitious roots
Promotes fruit growth

20. Biology, 9th ed, Sylvia Mader
Apical Dominance
Chapter 27
Control of Plant Growth/Response

21. Biology, 9th ed, Sylvia Mader
Effects of Auxin
Chapter 27
Control of Plant Growth/Response
Auxin production by seeds also promotes the growth of fruit.
As long as auxin is concentrated in leaves or fruits rather than in the stem, leaves and fruits do not fall off.
Trees can be sprayed with auxin to keep mature fruit from falling to ground
Auxin is sprayed on tomatoes to induce development of fruit without pollination creating seedless tomatoes

22. Phototropism Experiments
Biology, 9th ed, Sylvia Mader
Chapter 27
Control of Plant Growth/Response
Phototropism Experiments
Darwin & Darwin (1880s)
1. Used coleoptiles (grass shoots)
2. Found that shoots bend if:
a. Tips of shoots are present &  Normal
 Covered with clear cap
 Opaque base
3. No bending if:
a. Tip covered with cap
b. Tip was removed
4. Concluded tip senses light

23. Phototropism Experiments
Biology, 9th ed, Sylvia Mader
Phototropism Experiments
Chapter 27
Control of Plant Growth/Response

24. Peter Boysen-Jensen (1913)
Biology, 9th ed, Sylvia Mader
Chapter 27
Peter Boysen-Jensen (1913)
Control of Plant Growth/Response
1. Removed tips of shoots
2. Placed gelatin on stump
3. Replaced tip on top of gelatin:
a. Shoots bent towards light
4. Put piece of impermeable mica between shoot and tip:
a. No phototropic response
5. Concluded that some mobile chemical is responsible for the phototropic response

25. Phototropism Experiments
Biology, 9th ed, Sylvia Mader
Phototropism Experiments
Chapter 27
Control of Plant Growth/Response

26. Biology, 9th ed, Sylvia Mader
Chapter 27
Control of Plant Growth/Response
A. Paal (1918)
1. Removed tips of shoots
2. Put shoots in the dark
3. Replaced tips back on stumps but put them off-center on stumps
4. Tip placed on right side:
a. Shoots bent towards left
5. Tip placed on left side:
a. Shoots bent towards right
6. Suggested tip produces chemical that moves down shoot & causes cells below it to grow
a. Light must alter its amount

27. Phototropism Experiments
Biology, 9th ed, Sylvia Mader
Phototropism Experiments
Chapter 27
Control of Plant Growth/Response

28. Biology, 9th ed, Sylvia Mader
Chapter 27
Control of Plant Growth/Response
Fritz Went (1926)
1. Removed tips & placed them on blocks of agar for an hour
2. Put blocks of agar only on cut ends of stumps
3. If placed in center of stump:
a. Shoots grew straight upward
4. If placed off-center of stump
a. Shoots grew & bent to opposite side
5. Blank agar blocks didn’t grow
a. Definitive evidence of a hormone
He named the hormone auxin.

29. Demonstrating Phototropism - Went’s Experiment
Biology, 9th ed, Sylvia Mader
Chapter 27
Control of Plant Growth/Response
Demonstrating Phototropism - Went’s Experiment

30. Biology, 9th ed, Sylvia Mader
Chapter 27
Phototropism
Control of Plant Growth/Response

31. Biology, 9th ed, Sylvia Mader
How Auxins Work
Biology, 9th ed, Sylvia Mader
Chapter 27
Control of Plant Growth/Response
When a stem is exposed to unidirectional light, auxin moves to the shady side.
Auxins bind to plasma membrane receptors which leads to a series of reactions & the generation of at least three specific second messengers:
1. Activates a proton, H+, pump
Acidic conditions cause cell wall to loosen
Cellulose fibrils are weakened
2. Activates Golgi apparatus
Sends out vesicles laden with cell wall materials

32. Biology, 9th ed, Sylvia Mader
How Auxins Work
Biology, 9th ed, Sylvia Mader
Chapter 27
Control of Plant Growth/Response
3. Stimulates DNA-binding protein
Activates a particular gene
Leads to production of growth factors
Cell walls become extensible & then fill with water by osmosis.
Turgor pressure increases due to the entry of water & the cell elongates.
This occurs on side opposite to the light so the stem lengthens on shady side causing a bending toward the light.

33. Biology, 9th ed, Sylvia Mader
Auxin Mode of Action
Chapter 27
Control of Plant Growth/Response

34. Biology, 9th ed, Sylvia Mader
Chapter 27
Control of Plant Growth/Response mm

35. Biology, 9th ed, Sylvia Mader
Gibberellins
Chapter 27
Control of Plant Growth/Response
Growth promoting hormones
Bring about internode elongation of stem cells
Gibberellic acid (GA3) = most common
Stem elongation
Can cause dwarf plants to grow huge
Sources of gibberellin:
Young leaves, roots, embryos, seeds & fruits
Commercial uses:
Break dormancy of buds & seeds, induce flowering, increase size of flowers, produce larger seedless grapes

36. Effect of Gibberellins
Biology, 9th ed, Sylvia Mader
Effect of Gibberellins
Chapter 27
Control of Plant Growth/Response
Treated
No treatment

37. Biology, 9th ed, Sylvia Mader
Gibberellins
Chapter 27
Control of Plant Growth/Response
How GA3 acts as a chemical messenger:
Embryo produces gibberellins
Amylase, enzyme that breaks down starch, appears in cells just inside seed coat
GA3 is the first messenger
Attaches to receptor in plasma membrane
Second messenger, calcium ions, combines with a DNA-binding protein
Believed to activate the gene that codes for amylase. This acts on starch to release sugars used as source of energy for growing embryo

38. Gibberellic Acid: Structure and Mode of Action
Biology, 9th ed, Sylvia Mader
Gibberellic Acid: Structure and Mode of Action
Chapter 27
Control of Plant Growth/Response

39. Biology, 9th ed, Sylvia Mader
Cytokinins
Chapter 27
Control of Plant Growth/Response
Cytokinins
A class of plant hormones that promote cell division (cytokinesis)
First isolated in 1967 from corn = zeatin
Produced in dividing tissues or roots & in seeds & fruits
Promotes cell division
Prevents senescence (Aging process. Leaves die and fall off)
Initiates leaf growth. Lateral buds will grow when cytokinin is applied to them.

40. Biology, 9th ed, Sylvia Mader
Abscisic Acid
Chapter 27
Control of Plant Growth/Response
Abscisic acid (ABA): (aka stress hormone)
Initiates and maintains seed and bud dormancy
Brings about closure of stomata
Dormancy occurs when a plant readies itself for adverse conditions by stopping growth
ABA moves from leaves to vegetative buds in fall
Buds are converted to winter buds which get covered by thick, hardened scales
In spring, reduction in level of ABA & increases in gibberellins break seed and bud dormancy.
Produced by:
Any “green tissue” with chloroplasts
Monocot endosperm, and
Roots

41. Abscisic Acid: Control of Stoma Opening
Biology, 9th ed, Sylvia Mader
Abscisic Acid: Control of Stoma Opening
Chapter 27
Control of Plant Growth/Response
ABA binding leads to influx of Ca2+ & the opening of K+ channels. Water exits guard cells & stoma closes.

42. Biology, 9th ed, Sylvia Mader
Ethylene
Chapter 27
Control of Plant Growth/Response
Ethylene is involved in abscission, the dropping of leaves, fruits & flowers from a plant
Once abscission has begun:
Ethylene stimulates certain enzymes like cellulase
Causes leaves, fruits, or flowers to drop
Also ripens fruit by increasing activity of enzymes that soften fruit
Uses in agriculture:
To hasten ripening of green fruits
To create pleasing colors before sales
Ethylene is a gas that can induce ripening of nearby fruits

43. Biology, 9th ed, Sylvia Mader
Functions of Ethylene
Chapter 27
Control of Plant Growth/Response

44. Biology, 9th ed, Sylvia Mader
Photoperiodism
Chapter 27
Control of Plant Growth/Response
Photoperiodism is any physiological response prompted by changes in day or night length
Photoperiod
1. The relative lengths of day and night
2. This changes with the seasons
3. Flowering, germination & dormancy all occur at specific times of year
4. Thus, photoperiod is the major environmental factor that needs to be measured by plants

45. Biology, 9th ed, Sylvia Mader
Photoperiodism
Chapter 27
Control of Plant Growth/Response
Three Types of Plants:
1. Short-Day Plants (Long-night)
a. Flower when days are short (fall, winter)
b. Actually controlled by night length: Night length must be longer than a critical value.
c. Continuity of darkness is what matters. A flash of light will disrupt flowering
d. Examples: Chrysanthemums, poinsettias, rice, ragweed

46. Photoperiodism and Flowering
Biology, 9th ed, Sylvia Mader
Photoperiodism and Flowering
Chapter 27
Control of Plant Growth/Response

47. Biology, 9th ed, Sylvia Mader
Photoperiodism
Chapter 27
Control of Plant Growth/Response
2. Long-Day Plants (Short-night)
a. Flower when days are long (late spring, summer)
b. Night must be shorter than a critical value
c. A flash of light during the night can induce flowering during the wrong season
d. Examples: spinach, wheat, lettuce, iris, petunia, mustard

48. Photoperiodism and Flowering
Biology, 9th ed, Sylvia Mader
Photoperiodism and Flowering
Chapter 27
Control of Plant Growth/Response

49. Biology, 9th ed, Sylvia Mader
Photoperiodism
Chapter 27
Control of Plant Growth/Response
3. Day-Neutral Plants
a. Day length doesn’t matter
b. Flower year round
c. Examples: Roses, carnations, dandelions, sunflowers

50. Biology, 9th ed, Sylvia Mader
Photoperiodism
Chapter 27
Control of Plant Growth/Response
How is photoperiod detected?
1. Involves phytochrome, a light-absorbing pigment that exists in 2 inter-changeable forms:
a. Pr strongly absorbs red light ( nm)
b. Pfr absorbs far-red light (700–730 nm)
2. When Pr absorbs red light it is converted quickly to Pfr
3. When Pfr absorbs far-red light it is converted slowly to Pr

51. Phytochrome Conversion Cycle
Biology, 9th ed, Sylvia Mader
Phytochrome Conversion Cycle
Chapter 27
Control of Plant Growth/Response
happens quickly
(happens slowly)

52. Biology, 9th ed, Sylvia Mader
Photoperiodism
Chapter 27
Control of Plant Growth/Response
4. Pfr slowly reverts back to Pr in the dark
5. At sunset, far-red light is common
a. So Pfr begins to convert to Pr
b. This marks end of day; start of night.
c. Pr accumulates slowly all night
6. At sunrise, red-light is common
a. So Pr converts to Pfr relatively quickly
b. This marks end of night; start of day.

53. Phytochrome Conversion Cycle
Biology, 9th ed, Sylvia Mader
Phytochrome Conversion Cycle
Chapter 27
Control of Plant Growth/Response
happens quickly
(happens slowly)