1. Growth Responses and Regulation of Growth
Chapter 11

3. LEARNING OBJECTIVES
1 Discuss genetic and environmental factors that affect plant growth and development .
2 Describe phototropism, gravitropism, and thigmotropism .
3 List several ways in which each of the following hor-mones affects plant growth and development: auxin, gib-berellin, cytokinin, ethylene, abscisic acid.
4 Relate which hormone or hormones is/ are involved in each of the following biological processes: leaf abscission, seed germination, apical dominance.
5 Explain how varying amounts of light and darkness induce flowering, and describe the role of phytochrome.
6 Explain how temperature affects flower induction in certain plants.
7 Define circadian rhythm, and give an example.
8 Give an example of a turgor movement, and distinguish between turgor movements and tropisms.

4. LEARNING OBJECTIVE 1
Discuss genetic and environmental factors that affect plant growth and development

5. Internal Genetic Factors
The location of a cell in the young plant body affects gene expression during development
Causes some genes in that cell to be turned off and others to be turned on

6. External Environmental Factors
Factors in the physical environment determine gene expression, affect plant growth and development
changing day length
variation in precipitation
temperature

7. Spring Flowers

8. LEARNING OBJECTIVE 2
Describe phototropism, gravitropism, and thigmotropism

9. Tropisms
Are directional growth responses
Are permanent

10. KEY TERMS PHOTOTROPISM GRAVITROPISM THIGMOTROPISM
Directional growth of a plant caused by light
GRAVITROPISM
Plant growth in response to direction of gravity
THIGMOTROPISM
Growth in response to contact with a solid object

11. Phototropism

12. Gravitropism

13. Fig. 11-3a, p. 223 Figure 11.3: Gravitropism.
(a) A corn (Zea mays) seedling was turned on its side 3 days after germinating. In 1 hour, the root and shoot tips had curved. (b) In 24 hours, the new root growth was downward (positive gravitropism), and the new shoot growth was upward (negative gravitropism). A control seedling germinated at the same time is on the left.
Fig. 11-3a, p. 223

14. On day 3, turned on its side One hour later Fig. 11-3a, p. 223
Figure 11.3: Gravitropism.
(a) A corn (Zea mays) seedling was turned on its side 3 days after germinating. In 1 hour, the root and shoot tips had curved. (b) In 24 hours, the new root growth was downward (positive gravitropism), and the new shoot growth was upward (negative gravitropism). A control seedling germinated at the same time is on the left.
Fig. 11-3a, p. 223

15. Fig. 11-3b, p. 223 Figure 11.3: Gravitropism.
(a) A corn (Zea mays) seedling was turned on its side 3 days after germinating. In 1 hour, the root and shoot tips had curved. (b) In 24 hours, the new root growth was downward (positive gravitropism), and the new shoot growth was upward (negative gravitropism). A control seedling germinated at the same time is on the left.
Fig. 11-3b, p. 223

16. Darwins’ Experiments

17. Figure 11.4: The Darwins’ phototropism experiments.
Fig. 11-4a, p. 224

18. Figure 11.4: The Darwins’ phototropism experiments.
Fig. 11-4b, p. 224

19. (a) (b) (c) (d) Light rays Fig. 11-4b, p. 224
Figure 11.4: The Darwins’ phototropism experiments.
Fig. 11-4b, p. 224

20. Auxin in Coleoptiles

21. Coleoptile tip (a) Agar block (b) (c) Fig. 11-5, p. 225
Figure 11.5: Isolating auxin from coleoptiles.
Fig. 11-5, p. 225

22. LEARNING OBJECTIVE 3
List several ways in which each of the following hormones affects plant growth and development: auxin, gibberellin, cytokinin, ethylene, abscisic acid

23. KEY TERMS
HORMONE
An organic chemical messenger that regulates growth and development in plants and other multicellular organisms

24. Communication Molecules
Five major classes of plant hormones
auxin, gibberellin, cytokinin, ethylene, abscisic acid
A variety of signaling molecules

25. KEY TERMS
AUXIN
Plant hormone involved in growth and development, including stem elongation, apical dominance, and root formation on cuttings

26. Animation: Phototropism
CLICK TO PLAY

27. Auxin and Phototropism

28. Shaded side of coleoptile Auxin Light rays Illuminated side of
Figure 11.6: Unequal distribution of auxin causes phototropism.
Auxin travels down the side of the stem or coleoptile away from the light (black arrow), causing cells on the shaded side to elongate. Therefore, the stem or coleoptile bends toward light.
Fig. 11-6, p. 226

29. Auxin and Root Development

30. Animation: Auxin’s Effects
CLICK TO PLAY

31. KEY TERMS
GIBBERELLIN
Plant hormone involved in growth and development, including stem elongation, flowering, and seed germination

32. Effects of Gibberelin

33. KEY TERMS
CYTOKININ
Plant hormone involved in growth and development, including cell division and delay of senescence

34. Hormones and Tissue Culture

35. without differentiation
Cell division
without differentiation
Cell division
with differentiation
Figure 11.10: Hormones and organ formation when propagating tobacco by tissue culture.
(a) A fragment of tissue explant from the center of a tobacco stem is placed in a culture medium. A complete plant can form from the tissue fragment because each cell of the fragment contains all the genetic information for the entire organism. Varying amounts of auxin and cytokinin in the culture media produce different growth responses. (b) Nutrient agar containing a moderate amount of both auxin and cytokinin caused cells to divide and form a callus (a clump of disorganized, undifferentiated cells). (c) When callus is transplanted to a medium with a higher relative amount of auxin, roots differentiate. (d) Shoot growth is stimulated by a medium with a higher relative amount of cytokinin. Plants grown using tissue culture techniques can be transferred to soil and grown normally.
(a) Initial
explant
b) Callus
(c) Roots
d) Shoots
Fig , p. 229

36. Cytokinin and Senescence

37. KEY TERMS
ETHYLENE
A gaseous plant hormone involved in growth and development, including leaf abscission and fruit ripening

38. Ethylene and Fruit Ripening

39. KEY TERMS ABSCISIC ACID
A plant hormone involved in growth and development, including dormancy and responses to stress

40. Other Signaling Molecules

41. Day-neutral plant grafted to long-day plant Both plants flower
induction
Graft
Figure 11.14: Evidence of a flower-promoting substance.
When a long-day tobacco (Nicotiana silvestris) is grafted to a day-neutral tobacco (N. tabacum) and both plants are exposed to a long-day, short-night regimen, they both flower. The day-neutral plant flowers sooner than it normally would, presumably because a flower-promoting substance passes from the long-day plant to the day-neutral one through the graft. In 2005, after years of searching, biologists identified florigen as a messenger RNA molecule.
Day-neutral plant grafted
to long-day plant
Both plants flower
Fig , p. 232

43. Plant cell responses to infection by fungi, bacteria, or viruses. As a
result of an initial infection and a subsequent signal-transduction pathway, plants produce a variety of antimicrobial molecules

44. Signal Transduction

45. 2 Ubiquitin is attached to proteins that inhibit certain genes.
Plasma membrane
Cell wall
Inhibited genes
are turned on.
Ubiquitin 4
Nucleus
Receptor
Auxin
DNA (contains genes) 3
Proteins are
destroyed.
Protein 1
Auxin binds
to receptor.
Figure 11.15: How auxin works.
The numbered steps are explained in the text.
Cytoplasm
Nuclear envelope
Fig , p. 233

46. Ubiquitin 2
Ubiquitin is tagged to proteins that inhibit certain genes.
Protein
Cytoplasm
Plasma membrane
Cell wall
Nuclear envelope
Nucleus
Previously repressed genes are activated and expressed.
Transcription
DNA 4
Auxin 1
binds to TIR1 receptor.
Receptor
Proteins are targeted for destruction. 3
Figure 11.15: How auxin works.
The numbered steps are explained in the text.
Stepped Art
Fig , p. 233

47. LEARNING OBJECTIVE 4
Relate which hormone or hormones is/are involved in each of the following biological processes: leaf abscission, seed germination, apical dominance

48. Leaf Abscission Ethylene and auxin
As a leaf ages, auxin level in the leaf decreases, and ethylene level increases

49. Seed Germination Gibberellins involved in seed germination
in certain plants (cereals, grasses)
substitutes for low-temperature or light requirements in some seeds (lettuce, oats, tobacco)
Ethylene and abscisic acid
Ethylene promotes seed germination; abscisic acid inhibits seed germination

50. Abscisic Acid and Germination

51. Apical Dominance 1
Inhibition of axillary bud growth by the apical meristem
Auxin
Produced in shoot apical meristem
Inhibits axillary buds near apical meristem from developing into actively growing shoots

52. Apical Dominance 2 Cytokinins and ethylene
Cytokinins promote growth of axillary buds; ethylene inhibits axillary bud development

53. Auxin and Axillary Bud Development

54. LEARNING OBJECTIVE 5
Explain how varying amounts of light and darkness induce flowering
Describe the role of phytochrome

55. KEY TERMS PHOTOPERIODISM
Physiological response (such as flowering) of plants to variations in length of daylight and darkness

56. Photoperiodism
Some plants are short-day plants, some are long-day plants, others are intermediate-day plants
Plant measures length of dark period
In day-neutral plants, photoperiod does not affect flowering

57. Short-Day Plants

58. Photoperiodic Responses

59. Chrysanthemum (short-day/ long-night plant)
(a) Short days
and long nights
(b) Long days
and short nights
(c) Short days
and long nights
(interrupted with
a brief period of
light)
(d) Long days
(interrupted with
a brief period of
dark) and short
nights 1
Chrysanthemum (short-day/ long-night plant)
Figure 11.17: Photoperiodic responses of short-day and long-day plants. 2
Black-eyed Susan (long-day/short-night plant)
Fig , p. 234

60. KEY TERMS
PHYTOCHROME
A blue-green proteinaceous pigment involved in many plant responses to light, independent of photosynthesis

61. Phytochrome There are about five different phytochrome proteins
Each exists in two forms and readily converts from one form to the other after absorption of light of specific wavelengths

62. Phytochrome Forms
Pr strongly absorbs red light with a relatively short wavelength (660 nm)
Changes to the second form (Pfr)
Pfr absorbs red light with a relatively long wavelength (730 nm)
The active form, triggers or inhibits responses such as flowering

63. Phytochrome Conversion

64. Red light (660 nm) Short-lived intermediate forms Inactive formPr
Pfr
Figure 11.18: Phytochrome.
This pigment occurs in two forms, designated Pr and Pfr, and readily converts from one form to the other. Red light converts Pr to Pfr, and far-red light converts Pfr to Pr.
Physiological
response
(such as
flowering)
Far-red light
(730 nm)
Fig , p. 235

65. Animation: Phytochrome Conversions
CLICK TO PLAY

66. LEARNING OBJECTIVE 6
Explain how temperature affects flower induction in certain plants

67. Temperature Requirements
Certain plants have temperature requirements that must be met in order for them to flower

68. KEY TERMS VERNALIZATION
The low-temperature requirement for flowering in some plant species

69. Temperature Requirements

70. LEARNING OBJECTIVE 7
Define circadian rhythm
Give an example

71. KEY TERMS CIRCADIAN RHYTHM
A biological activity with an internal rhythm that approximates the 24-hour day

72. Circadian Rhythms Reset by the rising and setting of the sun
Circadian rhythms in plants affect
gene expression
rate of photosynthesis
opening and closing of stomata

73. Sleep Movements

74. LEARNING OBJECTIVE 8
Give an example of a turgor movement, and distinguish between turgor movements and tropisms

75. KEY TERMS TURGOR MOVEMENT
Temporary plant movement that results from changes in internal water pressure in a plant part
Examples: Leaves of the sensitive plant and Venus flytrap

76. Sensitive Plant

77. Figure 11.21: Turgor movements in the sensitive plant (Mimosa pudica).
Fig a, p. 237

78. Figure 11.21: Turgor movements in the sensitive plant (Mimosa pudica).
Fig b, p. 237

79. Figure 11.21: Turgor movements in the sensitive plant (Mimosa pudica).
Fig c, p. 237

80. Cross-sectional views
Leaflet
open
Leaflet
Pulvinus
Vascular
tissue
Decrease of
turgor in
parenchyma
cells
Leaflet
folded
Parenchyma
cells retaining
turgor
Figure 11.21: Turgor movements in the sensitive plant (Mimosa pudica).
Cross-sectional views
(c) How the folding and drooping occurs. Pulvini occur in three areas: the
base of each leaflet, the base of each cluster of leaflets, and the base of each
leaf. Only changes in the pulvini at the bases of leaflets are shown. (Top right)
A section through two leaflets, showing their pulvini when the leaf is undisturbed.
(Bottom right) A section through the two leaflets, showing how a loss
of turgor produces the folding of the leaves.
Fig c, p. 237

81. Turgor and Tropisms Turgor movements are temporary plant movements
Tropisms are permanent growth responses