1. Plant Growth & Response
CO 27
Plant Growth & Response

2. Plant Responses Organisms Respond to Stimuli
One defining characteristic of life is an ability to respond to stimuli.
Adaptive organisms respond to environmental stimuli because it leads to longevity and survival
Animals have nerves and muscles; plants respond by growth patterns.

3. How A Plant Responds to Stimuli
Reception – perception of the stimulus
Transduction – the stimulus is changed into a form that is meaningful to the plant
Response – reaction by the plant to a stimulus

4. perception of the stimulus
How A Plant Responds to Stimuli
Pg 479
reaction by the plant to a stimulus
perception of the stimulus
the stimulus is changed into a form that is meaningful to the plant

5. Tropisms
A tropism is plant growth toward or away from a directional stimulus.
Direction is important; the stimulus comes from only one direction instead of many.
Growth toward a stimulus is a positive tropism
Growth away from a stimulus is a negative tropism
By differential growth, one side elongates faster; result is curving toward or away from a stimulus.

6. Tropisms
Three well-known tropisms are named for the stimulus that causes the response.
Phototropism is growth of plants in response to light; stems show positive phototropism.
Gravitropism is response to earth’s gravity;
positive gravitropism – roots
negative gravitropism – stems
Thigmotropism is unequal growth due to touch (e.g., coiling of tendrils around a pole).

7. Tropisms
Plants in Motion Website

8. Phototropism
Phototropism occurs because cells on shady side of stems elongate.
It is believed that a yellow pigment related to riboflavin acts as a photoreceptor for light.
Following reception, plant hormone auxin migrates from bright side to shady side of a stem.
How reception of stimulus causes the production of auxin is not yet known.
Auxin is also involved in gravitropism, apical dominance, and root & seed development

10. Gravitropism (Geotropism)
Negative gravitropism - plant grows upward, opposite gravity
Charles Darwin and his son were first to show that roots display positive gravitropism.
If root cap is removed, roots no longer respond to gravity.
Later researchers showed root cap cells contain statoliths, starch grains within amyloplasts.
Due to gravity, amyloplasts settle to lowest part of cell.

12. Gravitropism (Geotropism)
The hormone auxin underlies both positive and negative gravitropisms.
The two tissues respond differently to auxin, which moves to lower side of both stems and root.
Auxin inhibits growth of root cells; cells of upper surface elongate and root curves downward.
Auxin stimulates growth of stem cells; cells of lower surface elongate and stem curves upward.

13. Thigmotropism
Unequal growth due to contact with solid objects is thigmotropism
Coiling of morning glory or pea tendrils around posts, etc., is a common example.
Cells in contact with an object grow less while those on opposite side elongate.
This process is quite rapid; tendrils have been observed to encircle an object in ten minutes.
Hormones auxin and ethylene are involved; they induce curvature of tendrils in absence of touch.

14. Unequal growth due to contact with solid objects is thigmotropism
Ex. Coiling of morning glory or pea tendrils around posts, etc., is a common example.

15. Cells in contact with an object grow less while those on opposite side elongate.
*This process is quite rapid; tendrils have been observed to encircle an object in ten minutes.
*Hormones auxin and ethylene are involved; they induce curvature of tendrils in absence of touch.

16. Thigmotropism
Thigmomorphogenesis is a touch response involving the whole plant.
Entire plant responds to presence of wind or rain.
A plant growing in a windy location has a shorter, thicker trunk.
Even rubbing of a plant inhibits cellular elongation and produces a shorter, sturdier plant.

17. Nastic Movements
In contrast to tropisms, nastic movements are independent of the direction of stimulus.
Seismonastic movements result from touch, shaking, or thermal stimulation.
This response takes only a second or two, is due to a loss of turgor pressure within cells.
Ex. Venus’s-flytrap - 3 sensitive hairs triggers an impulse-type stimulus. Turgor pressure in leaf cells propel the trap.
Mechanisms are potassium ions that move out of the cell; water follows by osmosis

18. When a Mimosa pudica leaf is touched, leaflets fold because petiole droops. A pulvinus is a thickening at base where turgor pressure can rapidly drop.

19. Sleep Movements
Sleep movements are nastic responses to daily changes in light level.
Movement is due to changes in turgor pressure of motor cells in a pulvinus
Circadian rhythm is a biological rhythm with a 24-hour cycle.
Biological clock is an internal mechanism maintaining biological rhythms in absence of stimuli. Synchronized by external stimuli to 24-hour rhythms.
Photoperiod is more reliable an indicator of seasonal changes than temperature change

20. Prayer Plant – Before dark and after dark when its leaves fold up

21. Plant Hormones
For plants to respond to stimuli, activities of plant cells and structures have to be coordinated.
Most plant communication is done by hormones
Hormones are low concentration chemical messengers active in another part of an organism.
A response is influenced by several hormones; requires a specific ratio of two or more hormones.
Hormones are synthesized in one part of a plant; they travel in phloem after plant receives appropriate stimulus.

22. Plant Hormones Growth Promoter *(Florigen – May induce flowering)
Auxins – Promotes cell growth
Giberellins - Promotes cell growth
Cytokinetins – Stimulate Cell Division
Growth Inhibitors
Abscisic Acid – Growth Inhibitor
Ethylene – Promotes ripening of fruit
*(Florigen – May induce flowering)

23. Auxins
Indoleacetic acid (IAA) is most common naturally-occurring auxin.
Produced in shoot & root apical meristem & found in young leaves, flowers, and fruits.
Promotes Cellular Elongation
Promotes softening of cell wall
Involved in Phototropism, Gravitropism,
Apical dominance - prevents growth of axillary buds
Roots to develop from ends of cuttings.
promotes growth of fruit.

24. Auxin in the apical bud inhibits lateral bud development & plant exhibits apical dominance
When the apical bud is removed, lateral branches develop.

25. Stem cutting placed in a solution of IAA
Stem cutting placed in pure water

26. Auxins
Auxin-controlled cell elongation is involved in gravitropism & phototropism.
When gravity is perceived, auxin moves to lower surface of roots and stems.
Darwin discovered with oat seedlings, phototropism would not occur if tip of a seedling is cut off or covered by a cap;
they concluded cause of curvature moved from coleoptile tip to rest of shoot.

27. What is the significance of this experiment?
Frits W. Went experimented with coleoptiles in He cut off tips and placed them on agar. Auxin diffuses into agar block.
Auxin’s Animation
Agar block was placed to one side; coleoptile would curve away from that side regardless of light. He deduced a chemical caused curved growth and named it auxin after Greek word for “to grow”
What is the significance of this experiment?

28. Auxins Animations – The Life Wire Chp 37.1 – Tropisms
Chp 37.2 – Went’s Experiment
Chp 37.3 – Auxin affect on Cell Wall

29. How Auxins Work
In a plant exposed to unidirectional light, auxin moves from bright side to shady side of a stem.
Auxin binds to receptors and activates the ATP-driven proton (H+) pump.
As hydrogen ions are pumped out of the cell, cell wall becomes acidic, breaking hydrogen bonds.
Cellulose fibrils are weakened and activated enzymes further degrade the cell wall.
Electrochemical gradient established causes of uptake of solutes; water follows by osmosis.
Turgid cell presses against cell wall, stretching it so that elongation occurs.

30. Auxin binds to receptors and activates the ATP-driven proton (H+) pump
Cellulose fibrils are weakened & activated enzymes further degrade the cell wall
As hydrogen ions are pumped out of the cell, cell wall becomes acidic, breaking hydrogen bonds

31. Turgid cell presses against cell wall, stretching it so that elongation occurs
Electrochemical gradient established causes of uptake of solutes; water follows by osmosis

32. Figure 27.8

33. Gibberellins
Gibberellins are a group of 70 plant hormones that chemically differ. GA3 (gibberellic acid) is the most common of natural gibberellins.
Gibberellins: Produced in young roots, stems & leaves
growth promoters that elongate cells.
Produces bolting (rice)
Stimulates production of starch digestion enzymes
Release buds from apical dominance
Promotes fruit development & seed germination

34. Gibberellins are often used to promote stem elongation
Plant on the right treated with Gibberellins

35. Gibberellins Mode of action
Hormone GA3 binds to a receptor; a second messenger (Ca2+) inside cell combines with the protein calmodulin.
Ca2+-calmodulin complex activates a gene coding for the enzyme amylase.
Amylase acts on starch to release sugars used as a source of energy by the growing embryo.

36. Reception
Transduction
Response

37. Ca2+-calmodulin complex activates a gene coding for the enzyme amylase.
Amylase acts on starch to release sugars used as a source of energy by the growing embryo.
Hormone GA3 binds to a receptor; this opens up Ca2+ channels. Ca2+ acts as a second messenger inside cell & combines with the protein calmodulin.

38. Calmodulin may act as some sort of transcription factor … causing the production of Amylase that breaks down starch causing a breakage of seed dormancy.

39. Cytokinins
Cytokinins are a class of plant hormones that promote cell division (cytokinesis)
Cytokinins are derivatives of purine base adenine
zeatin - a natural cytokinin
kinetin - a synthetic cytokinin.
Produced in roots (& elsewhere)
Promotes growth in size of leaves
Stimulates cell division
Works with auxin to control cell differentiation -
Releases buds from apical dominance

40. Senescence
Aging processes are senescence; large molecules break down and transported elsewhere.
Cytokinins prevent senescence of leaves; they also initiate development of leaf growth.
Cytokinins initiate growth of lateral buds despite apical dominance.

41. Inhibitory Hormones Abscisic acid (ABA) - Growth inhibitor
“Stress hormone”; it maintains seed and bud dormancy (plant organ readies itself for adverse conditions by stopping growth)
Promotes closure of stomates.
Promotes seed and bud dormancy
Promotes resistence to water stress
Formerly thought to promote abscission

42. The hormone abscisic acid (ABA) is believed to bring about the closing of the stomates when water stress occurs.

45. Stomatal closing Stomatal opening
Potassium ions move out of the vacuole and out of the cells.
Water moves out of the vacuoles, following potassium ions.
The guard cells shrink in size.
The stoma closes.
Stomatal opening
Potassium ions move into the vacuoles.
Water moves into the vacuoles, following potassium ions.
The guard cells expand.
The stoma opens.

46. Inhibitory Hormones Ethylene - A gaseous plant hormone
Promotes the ripening of fruit by activity of enzymes that soften fruit.
Involved in leaf abscission (the dropping of leaves, fruits, or flowers)
Lower levels of auxin in these areas (compared to stem) probably initiate abscission.
Once abscission begins, ethylene stimulates production of enzymes such as cellulase that cause leaf, fruit, or flower drop
Working with auxin, inhibits elongatin of roots, stems and leaves

47. Inhibitory Hormones Ethylene
The presence of ethylene in air inhibits growth of plants in general
Ethylene is involved in abscission, the dropping of leaves, fruits, or flowers
Lower levels of auxin in these areas (compared to stem) probably initiate abscission.
Once abscission begins, ethylene stimulates production of enzymes such as cellulase that cause leaf, fruit, or flower drop

48. “A Rotten Apple Spoils the Whole Bunch”
In the presence of an ethylene producing ripe apple abscission of the holly leafs occur.
“A Rotten Apple Spoils the Whole Bunch”
No abscission when a holly twig is placed under glass jar for a week

49. Photoperiodism
Many physiological changes in plants are related to a seasonal change in day length.
seed germination
the breaking of bud dormancy
flowering
onset of senescence

50. Phytochrome and Plant Flowering
U.S.D.A. scientists discovered phytochrome, a blue-green leaf pigment that exists in two forms.
Pr=(phytochrome red) absorbs red light (wavelength of 660 m); it is converted to Pfr.
Pfr=(phytochrome far-red) absorbs far-red light (wavelength of 730 m); it is converted to Pr.

51. Phytochrome and Plant Flowering
Pfr appears to reset the circadian-rhythm clock
Pr is the form of phytochrome synthesized in plant cells
Pr & Pfr are in equilibrium during daylight
Pr accumulates at night
At daybreak, light rapidly converts Pr to Pfr
Night length is responsible for resseting the circadian-rhythm clock

52. The inactive form Pr is prevalent during the night
The inactive form Pr is prevalent during the night. At sunset or in the shade, when there is more fare-red light Pfr is converted to Pr.
Pfr, the active form of of phytochrome, is prevalent during the day because at that time there is more red light than far-red light.
Also, during the night, metabolic processes cause Pfr to be converted to Pr.

54. Other Functions of Phytochrome
The Pr ® Pfr conversion cycle controls other growth functions in plants.
Pfr promotes seed germination and stem branching.
Following germination, presence of Pr dominates; stem elongates and grows toward sunlight while leaves remain small

55. Grown in shade (Far-red light)
Grown in sunlight (red light)
Grown in shade (Far-red light)

56. Photoperiodism Animation #1
Photoperiodism is a physiological response to relative lengths of daylight and darkness (called a photoperiod)
Plants can be divided into 3 groups, based on photoperiodism.
Short-day Plants
Long-day Plants
Day-neutral Plants
Animation #1

57. Photoperiodism & Flowering

58. Photoperiodism Short-day Plants
These flower when day length was shorter than a critical length.
In effect, they require a period of darkness that is longer than a critical length to flower.
Ex. cocklebur, poinsettia, and chrysanthemum.

60. Photoperiodism Long-day Plants
These flower when the day length is longer than a critical length.
In effect, they require a period of darkness that is shorter than a critical length to flower.
Ex. wheat, barley, clover, and spinach.

61. Photoperiodism Day-neutral Plants
These are plants for which flowering is not dependent on day length.
Ex. include tomato and cucumber.

62. Photoperiodism Animation

64. Photoperiodism
In 1938, K. C. Hammer and J. Bonner experimented with artificial lengths of dark and light periods.
Cocklebur, a short-day plant, flowers as long as the dark period lasts over 8.5 hours.
If dark period is interrupted by a flash, it does not flower; darkness amidst day cycle has no effect.
Long-day plants require a dark period shorter than a critical length regardless of length of light period.
Therefore, length of the dark period controls flowering, not length of the light period.