1. Plant Responses to Internal and External Signals 植物對內在與外在訊息的反應
Chapter 39
Plant Responses to Internal and External Signals
植物對內在與外在訊息的反應

2. Key Concepts
Concept 39.1: Signal transduction pathways link signal reception to response
Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli
Concept 39.3: Responses to light are critical for plant success
Concept 39.4: Plants respond to a wide variety of stimuli other than light
Concept 39.5: Plants defend themselves against herbivores and pathogens

3. Stimulus and response (刺激與回應)
Overview: Stimuli and a Stationary Life (刺激與穩定)
Plants, being rooted to the ground must respond to whatever environmental change (環境變遷) comes their way
For example, the bending of a grass seedling (幼苗的彎曲) toward light begins with the plant sensing the direction, quantity, and color of the light
Figure 39.1

4. Stimulus and response (刺激與回應)
Concept 39.1: Signal transduction pathways link signal reception to response
Plants have cellular receptors (細胞接受器)
That they use to detect important changes in their environment
For a stimulus to elicit a response (為了誘發對一個刺激的回應)
Certain cells must have an appropriate receptor

5. A potato left growing in darkness (馬鈴薯在黑暗中的生長) will produce shoots that do not appear healthy, and will lack elongated roots
These are morphological adaptations (形態適應) for growing in darkness collectively referred to as etiolation (白化作用)
(a) Before exposure to light. A dark-grown potato has tall, spindly stems and nonexpanded leaves—morphologicaladaptations that enable the shoots to penetrate the soil. The roots are short, but there is little need for water absorption because little water is lost by the shoots.
Figure 39.2a

6. After the potato is exposed to light
The plant undergoes profound changes called de-etiolation (去白化作用), in which shoots and roots grow normally
(b) After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome (光敏素).
Figure 39.2b

7. Green and etiolated plants (綠化與白化植物)
Green plant 玉米
乳斑榕

8. Chloroplast and etioplast (葉綠體與白化體)
Prolamellar body (PLB)
Grana/stacked thylakoid
light

9. The potato’s response to light is an example of cell-signal processing (細胞-訊息加工過程)
Cell wall
Cytoplasm
1. 接收
2. 傳遞
3. 反應
1. Reception
2. Transduction
3. Response
Activation
of cellular
responses
Relay molecules
備用分子
(第二傳訊者)
Receptor
細胞反應
的活化
Hormone or
environmental
stimulus 受體
(接受器)
賀爾蒙或
環境刺激
Plasma membrane
Figure 39.3

10. Reception (接收或接受)
Internal and external signals are detected by receptors (接受器偵測內在與外在訊息)
Proteins that change in response to specific stimuli
Reception protein (受體蛋白)

11. Transduction (傳遞/傳導) Second messengers (第二傳訊者)
Transfer and amplify signals (轉移與擴大訊息) from receptors to proteins that cause specific responses

12. Plant signal transduction (Reference only)
An example of signal transduction in plants
1 Reception
  Transduction
3 Response
CYTOPLASM
Transcription
factor 1
NUCLEUS
Specific
protein
kinase 1
activated
cGMP
Plasma
membrane P
Second messenger
produced
Transcription
factor 2
Phytochrome
activated
by light
2 One pathway uses cGMP as a second messenger that activates a specific protein kinase.The other pathway involves an increase in cytoplasmic Ca2+ that activates another specific protein kinase. P
Cell
wall
Specific
protein
kinase 2
activated
Transcription
Light
Translation
3 Both pathways lead to expression
of genes for proteins that function in the de-etiolation (greening) response.
1 The light signal is detected by the phytochrome receptor, which then activates at least two signal transduction pathways.
De-etiolation
(greening)
response
proteins
Ca2+ channel
opened
Ca2+
Figure 39.4

13. Response (回應或反應) Ultimately, a signal transduction pathway
Leads to a regulation of one or more cellular activities
In most cases
These responses to stimulation involve the increased activity of certain enzymes
Protein kinases

14. Transcriptional Regulation (轉錄調節)
Transcription factors bind directly to specific regions of DNA
And control the transcription of specific genes

15. De-Etioloation (“Greening”) Proteins 去白化(綠化)蛋白
Many enzymes that function in certain signal responses are involved in photosynthesis directly
While others are involved in supplying the chemical precursors (化學前驅物) necessary for chlorophyll production

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17. Because of their immobility (不動性)
Concept 39.4: Plants respond to a wide variety of stimuli other than light
Because of their immobility (不動性)
Plants must adjust to a wide range of environmental circumstances through developmental and physiological mechanisms

18. Roots show positive gravitropism Stems show negative gravitropism
Gravity (重力/地心引力)
Response to gravity
Is known as gravitropism (向地性)
Roots show positive gravitropism
Stems show negative gravitropism

19. Plants may detect gravity by the settling of statoliths (平衡石)
Specialized plastids containing dense starch grains
Statoliths
20 m
Figure 39.25a, b
(a)
(b)

20. Mechanical Stimuli (機械性刺激)
The term thigmomorphogenesis (接觸形態發生)
Refers to the changes in form that result from mechanical perturbation (機械性干擾)

21. Rubbing (觸摸) the stems of young plants a couple of times daily (每天觸摸二次)
Results in plants that are shorter than controls
Figure 39.26

22. Growth in response to touch
Is called thigmotropism (向觸性/接觸性)
Occurs in vines (藤蔓/藤本值物) and other climbing plants (攀爬植物)

23. 敏感植物葉的快速膨壓運動
Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses (電脈衝) called action potentials (動作電位)
(a) Unstimulated
未受刺激
(b) Stimulated
受刺激後
Side of pulvinus with flaccid cells
葉枕側面具皺縮細胞(失水)
Leaflets after stimulation
受刺激後的小葉
Side of pulvinus with turgid cells
葉枕側面具膨脹細胞
Pulvinus (motor organ)
葉枕(運動器官)
Vein葉脈
(c) Motor organs 運動器官
0.5 m
Figure 39.27a–c

24. Environmental Stresses (環境逆境)
Have a potentially adverse effect (相反的) on a plant’s survival, growth, and reproduction
Can have a devastating impact on crop yields in agriculture

25. Drought (乾旱) During drought
Plants respond to water deficit (水份缺少) by reducing transpiration
Deeper roots (深根) continue to grow

26. Enzymatic destruction of cells
Flooding (淹水逆境)
Enzymatic destruction of cells
Creates air tubes that help plants survive oxygen deprivation during flooding
Vascular cylinder
Air tubes
Epidermis
100 m
100 m
Figure 39.28a, b
(a) Control root (aerated)
(b) Experimental root (nonaerated)

27. Salt Stress (鹽分逆境)
Plants respond to salt stress by producing solutes tolerated at high concentrations
Keeping the water potential of cells more negative than that of the soil solution

28. Heat-shock proteins (熱休克蛋白)
Heat Stress (高溫逆境)
Heat-shock proteins (熱休克蛋白)
Help plants survive heat stress

29. Altering lipid composition of membranes
Cold Stress (低溫逆境)
Altering lipid composition of membranes
Is a response to cold stress

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31. Classification of plant hormones
Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli
Hormones
Are chemical signals (化學訊息) that coordinate the different parts of an organism
Classification of plant hormones
Auxin, Cytokinins, Gibberellins, Abscisic acids, Ethylene, Brassinosteroids

32. The Discovery of Plant Hormones
Any growth response
That results in curvatures of whole plant organs toward or away from a stimulus is called a tropism (向性)
Is often caused by hormones
Charles Darwin and his son Francis
Conducted some of the earliest experiments on phototropism (向光性), a plant’s response to light, in the late 19th century

33. Photo-signal is a light-activated mobile chemical
Eperiment: In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile(芽鞘) senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted.
Results:
Control
Darwin and Darwin (1880)
Boysen-Jensen (1913)
Light
Shaded
side of
coleoptile
Illuminated
Tip
removed
Tip covered
by opaque
cap
covered
by trans- parent cap
Base covered
by opaque shield
Tip separated
by gelatin block
by mica
芽鞘照光面
芽鞘背光面 雲母
Conclusion: In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin) but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical.
Figure 39.5

34. The discovery of Auxin (植物生長素)
In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments
Experiment: In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others, he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side.
Results: The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark.
Excised tip placed
on agar block
Growth-promoting chemical diffuses into agar block
Agar block with chemical stimulates growth
Offset blocks cause curvature
Control
Conclusion: Went concluded that a coleoptile curved toward light because its dark side had a higher concentration of the growth-promoting chemical, which he named auxin.
Control (agar
block lacking chemical) has no effect
Figure 39.6

35. A Survey of Plant Hormones

36. A Survey of Plant Hormones

37. A Survey of Plant Hormones (1)

38. A Survey of Plant Hormones (2)

39. In general, hormones control plant growth and development
By affecting the division, elongation, and differentiation of cells
Plant hormones are produced in very low concentrations
But a minute amount can have a profound effect on the growth and development of a plant organ

40. Auxin (植物生長素) The term auxin
Is used for any chemical substance that promotes cell elongation (細胞延伸) in different target tissues
Auxin transporters (植物生長素運輸蛋白)
move the hormone out of the basal end of one cell, and into the apical end of neighboring cells

41. Auxin transporters and polar transport of auxin
植物生長素運輸蛋白與其極性運輸
Experimrnt: To investigate how auxin is transported unidirectionally, researchers designed an experiment to identify the location of the auxin transport protein. They used a greenish-yellow fluorescent molecule to label antibodies that bind to the auxin transport protein. They applied the antibodies to longitudinally sectioned Arabidopsis stems.
Results: The left micrograph shows that the auxin transport protein is not found in all tissues of the stem, but only in the xylem parenchyma. In the right micrograph, a higher magnification reveals that the auxin transport protein is primarily localized to the basal end of the cells.
Cell 1
Cell 2
100 m
Epidermis
Cortex
Phloem
Xylem
Pith
Basal end of cell
25 m
Conclusion: The results support the hypothesis that concentration of the auxin transport protein at the basal ends of cells is responsible for polar transport of auxin.
Figure 39.7

42. The Role of Auxin in Cell Elongation
According to a model called the acid growth hypothesis
Proton pumps play a major role in the growth response of cells to auxin

43. Cell elongation in response to auxin
3 Wedge-shaped expansins, activated
by low pH, separate cellulose microfibrils from
cross-linking polysaccharides. The exposed cross-linking
polysaccharides are now more accessible to cell wall enzymes.
Cell wall enzymes
Expansin
胞壁擴張酶
Cross-linking cell wall polysaccharides
4 The enzymatic cleaving of the cross-linking
polysaccharides allows the microfibrils to slide (滑動). The extensibility (延展性) of the cell wall is increased. Turgor (膨壓) causes the cell to expand.
Cell wall
Microfibril
H2O
Cell
wall H+
Plasma
membrane H+
2 The cell wall becomes more acidic. H+ H+ H+ H+ H+ H+
1 Auxin increases the activity of proton pumps.
Nucleus
Cytoplasm
ATP
Vacuole
Plasma membrane H+
Cytoplasm
5 With the cellulose loosened,
the cell can elongate.
Figure 39.8

44. Cell elongation in response to auxin
Plasma
membrane
Cytoplasm
Nucleus
Vacuole
H2O
Cell
wall

45. Physiologic functions of auxin in plant
Auxin and Cell Elongation : according to a model called the acid growth hypothesis, proton pumps play a major role in the growth response of cells to auxin
Lateral and Adventitious Root Formation: Auxin is involved in the formation and branching of roots
Auxins as Herbicides: an overdose of auxins can kill eudicots
Other Effects of Auxin: Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem

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47. Control of Cell Division and Differentiation
Cytokinins (細胞分裂素)
Cytokinins
Stimulate cell division
Control of Cell Division and Differentiation
Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits
Work together with auxin

48. Control of Apical Dominance (頂端優勢)
Cytokinins, auxin, and other factors interact in the control of apical dominance. The ability of a terminal bud to suppress development of axillary buds (腋芽)
If the terminal bud (頂芽) is removed, plants become bushier (茂密)
Figure 39.9

49. Control of Apical Dominance (頂端優勢)
Cytokinins, auxin, and other factors interact in the control of apical dominance. The ability of a terminal bud to suppress development of axillary buds (腋芽)
If the terminal bud (頂芽) is removed, plants become bushier (茂密)
“Stump” after removal of apical bud
Lateral branches
Axillary buds
Figure 39.9

50. Anti-Aging Effects (抗老化效應)
Cytokinins retard (阻礙/延遲) the aging of some plant organs
How?

51. Anti-Aging Effects (抗老化效應)
Cytokinins retard (阻礙/延遲) the aging of some plant organs
By inhibiting protein breakdown
stimulating RNA and protein synthesis,
and mobilizing nutrients from surrounding tissues

52. Such as stem elongation fruit growth, and seed germination
Gibberellins (吉貝林素)
Gibberellins have a variety of effects
Such as stem elongation
fruit growth, and
seed germination

53. Stem Elongation (莖的延伸)
Gibberellins stimulate growth of both leaves and stems
In stems
Gibberellins stimulate cell elongation and cell division

54. Fruit Growth In many plants
Both auxin and gibberellins must be present for fruit to set

55. Gibberellins are used commercially
In the spraying of Thompson seedless grapes
Figure 39.10

56. The major commercial uses of GAs are
to promote the growth of a variety of fruit crops,
to increase sugar yield in sugarcane, and
to stimulate the barley-malting process in the beer-brewing industry.

57. Gibberellic acid (GA3) is the GA most often used commercially, since it can be readily obtained in large quantities from fermentations of the fungus Gibberella fujikuroi. The global (excluding China) use of GA3 per annum is approximately 50 tons.

58. Germination—break seed dorminancy to germinate
After water is imbibed, the release of gibberellins from the embryo signals the seeds to break dormancy and germinate (打破休眠促進萌芽)
1. After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm.
Endosperm
內胚乳
shoot 澱粉
分解酶
Aleurone
糊粉層
-amylase
Sugar GA
seedling GA
Water
root
Radicle 胚根
Scutellum
(cotyledon) 子葉
Figure 39.11

59. Germination—break seed dorminancy to germinate
After water is imbibed, the release of gibberellins from the embryo signals the seeds to break dormancy and germinate (打破休眠促進萌芽)
2. The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is -amylase, which hydrolyzes
starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.)
1. After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm.
Endosperm
內胚乳
shoot 澱粉
分解酶
Aleurone
糊粉層
-amylase
Sugar GA
seedling GA
Water
root
Radicle 胚根
Scutellum
(cotyledon) 子葉
Figure 39.11

60. Germination—break seed dorminancy to germinate
After water is imbibed, the release of gibberellins from the embryo signals the seeds to break dormancy and germinate (打破休眠促進萌芽)
2. The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is -amylase, which hydrolyzes
starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.)
1. After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm.
3. Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.
Endosperm
內胚乳
shoot 澱粉
分解酶
Aleurone
糊粉層
-amylase
Sugar GA
seedling GA
Water
root
Radicle 胚根
Scutellum
(cotyledon) 子葉
Figure 39.11

61. Figure 2   Structure of a germinating barley grain and the biochemical processes that occur during the malting process.

62. Gibberellins from the embryo of germinating grains are necessary for the synthesis of α-amylase by the cells of the aleurone layer,
which, in turn is necessary for the hydrolysis of starch within the endosperm.
Why is this process important in the brewing process—beer production?

63. In the brewing industry, the production of beer relies on this hydrolytic breakdown of starch in barley grains to yield fermentable sugars, principally maltose,
which are then subjected to fermentation by yeast.
During fermentation, glycolytic enzymes from yeast break down the sugars, resulting in ethanol.

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65. Two of the many effects of abscisic acid (ABA) are
Seed dormancy (種子休眠)
Drought tolerance (耐乾旱)

66. Seed dormancy has great survival value
Because it ensures that the seed will germinate only when there are optimal conditions

67. Precocious germination (早熟的萌芽) is observed in maize mutants
That lack a factor ( a gene product) required for ABA to induce expression of certain genes
Coleoptile 芽鞘
Figure 39.12

68. Drought Tolerance (耐乾旱)
ABA is the primary internal signal
That enables plants to withstand drought

69. Plants produce ethylene in response to stresses (逆境) such as
Drought (乾旱)
flooding (淹水)
mechanical pressure (機械性壓破)
injury (受傷)
Infection (感染)

70. Apoptosis: Programmed Cell Death 細胞凋亡：程式化細胞死亡
A burst of ethylene (乙烯的爆發)
Is associated with the programmed destruction (程式化崩壞) of cells, organs, or whole plants

71. Leaf Abscission (葉的離層酸)
0.5 mm
保護層Protective layer
Abscission layer 離層
Stem
Petiole 葉柄
A change in the balance of auxin and ethylene controls leaf abscission
The process that occurs in autumn when a leaf falls
Figure 39.16

72. A burst of ethylene production in the fruit
Fruit Ripening (果實成熟)
A burst of ethylene production in the fruit
Triggers the ripening process

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74. Concept 39.3: Responses to light are critical for plant success
Light cues many key events in plant growth and development
Effects of light on plant morphology
Are what plant biologists call photomorphogenesis (光形態發生)

75. Effects of light on plant morphology-Photomorphogenesis (光形態發生)
(De-etiolation)
(Germination)
(Neighbor perception)
(Shade avoidance)
Color
Flowering
Intensity (photosynthetic adaptation : chloroplast movement)
Direction (Phototropism)
Darkness duration (=Photoperiod)
Seed
Seedling
Juvenile
Adult
Light
Dark
Circadian cycle

76. Light-Regulated Plant Developments
Plant utilizes light as a signal to collect info about the environment. What are some of the Light-regulated plant developments :
Germination (seed)
photo-morphogenesis (seedling)
photo-tropism (seedling)
shade avoidance responses (vegetative growth)
stomatal opening and chloroplast movement
flowering time (flowering stage for next generation)

77. Plants not only detect the presence of light
But also its direction, intensity, and wavelength (color) (植物偵測光質、光量、光度、光向)
A graph called an action spectrum (作用光譜)
Depicts the relative response of a process to different wavelengths of light (光的波長)

78. Phototropic effectiveness
Action spectra (作用光譜) are useful in the study of any process that depends on light
Experiment: Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light.
Wavelength (nm)
1.0
0.8
0.6
0.2
450
500
550
600
650
700
Light
Time = 0 min.
Time = 90 min.
0.4
400
Phototropic effectiveness
relative to 436 nm
Results: The graph below shows phototropic effectiveness (curvature per photon) relative to effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light.
Conclusion: The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.
Figure 39.17

79. Research on action spectra and absorption spectra of pigments
Led to the identification of Three major classes of light receptors: including the phytochromes (藍光光受體與光敏素)

80. Blue-Light Photoreceptors (藍光光受體)
Various blue-light photoreceptors controls
hypocotyl elongation (下胚軸的延伸)
stomatal opening (氣孔的開張)
Phototropism (向光性)

81. Phytochromes as Photoreceptors (光敏素是光受體)
Regulate many of a plant’s responses to light throughout its life
Phytochromes and Seed Germination
Studies of seed germination led to the discovery of phytochromes
In the 1930s, scientists at the U.S. Department of Agriculture (USDA)
Determined the action spectrum for light-induced germination of lettuce seeds

82. Red light effect on germination (紅光效應)
Experiment: During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in the dark, and the results were compared with control seeds that were not exposed to light.
Dark (control)
Dark
Red
Far-
red
Results: The bar below each photo indicates the sequence of red-light exposure, far-red light exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposed to red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right).
Conclusion: Red light stimulated germination, and far-red light inhibited germination. The final exposure was the determining factor. The effects of red and far-red light were reversible.
Figure 39.18

83. A phytochrome is the photoreceptor responsible for the opposing effects of red and far-red light
A phytochrome consists of two identical proteins joined to form
one functional molecule. Each of these proteins has two domains.
Chromophore
Photoreceptor
=peptide+
chromophore
Photoreceptor activity. One domain,
which functions as the photoreceptor,
is covalently bonded to a nonprotein
pigment, or chromophore.
Kinase activity. The other domain
has protein kinase activity. The
photoreceptor domains interact with the kinase domains to link light reception to cellular responses triggered by the kinase.
Kinase
Figure 39.19

84. Phytochromes exist in two photoreversible states with conversion of Pr to Pfr triggering many developmental responses
Synthesis
Far-red
light
Red light
Slow conversion
in darkness
(some plants)
Responses:
seed germination,
control of
flowering, etc.
Enzymatic
destruction
Pfr Pr
Figure 39.20

85. Phytochromes and Shade Avoidance
The phytochrome system
Also provides the plant with information about the quality of light
In the “shade avoidance” response of a tree
The phytochrome ratio (Pr / Pfr) shifts in favor of Pr when a tree is shaded

86. Biological Clocks and Circadian Rhythms 生物時鐘與生理節律
Many plant processes
Oscillate (擺動) during the day

87. Many legumes (豆科植物)
Lower their leaves in the evening and raise them in the morning
Noon
Midnight
Figure 39.21

88. Cyclical responses to environmental stimuli are called circadian rhythms (日週期、生理節律)
And are approximately 24 hours long
Can be entrained to exactly 24 hours by the day/night cycle (日夜循環)

89. The Effect of Light on the Biological Clock 光對生物時鐘的效應
Phytochrome conversion (光敏素的轉換) marks sunrise and sunset
Providing the biological clock with environmental cues (啟示/提示)

90. Photoperiodism and Responses to Seasons 光週期現象與對季節變化的反應
Photoperiod (光週期), the relative lengths of night and day
Is the environmental stimulus plants use most often to detect the time of year
Photoperiodism (光週期現象)
Is a physiological response to photoperiod

91. Photoperiodism and Control of Flowering 光週期現象與開花的調空
Some developmental processes, including flowering in many species
Requires a certain photoperiod

92. Critical Night Length
In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length
Experiment: During the 1940s, researchers conducted experiments in which periods of darkness were interrupted with brief exposure to light to test how the light and dark portions of a photoperiod affected flowering in “short-day” and “long-day” plants.
Figure 39.22
Conclusion: The experiments indicated that flowering of each species was determined by a critical period of darkness (“critical night length”) for that species, not by a specific period of light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day” plants are really “short-night” plants.
24 hours
Darkness
Flash of light
Critical dark period
Light
Experiment:
(a) “Short-day” plants flowered only if a period of continuous darkness was longer than a critical dark period for that particular species (13 hours in this example). A period of darkness can be ended by a brief exposure to light.
(b) “Long-day” plants flowered only if a period of continuous darkness was shorter than a critical dark period for that particular species (13 hours in this example).

93. Action spectra and photoreversibility experiments show that phytochrome is the pigment that receives red light, which can interrupt the nighttime portion of the photoperiod
Experiment： A unique characteristic of phytochrome is reversibility in response to red and far-red light. To test whether phytochrome is the pigment measuring interruption of dark periods, researchers observed how flashes of red light and far-red light affected flowering in “short-day” and “long-day” plants. 24 20 16 12 8 4
Hours
Short-day (long-night) plant
Long-day (short-night) plant R FR
Critical dark period
Result
Conclusion: A flash of red light shortened the dark period. A subsequent flash of far-red light canceled the red light’s effect. If a red flash followed a far-red flash, the effect of the far-red light was canceled. This reversibility indicated that it is phytochrome that measures the interruption of dark periods.
Figure 39.23

94. A Flowering Hormone? (開花賀爾蒙？)
The flowering signal, not yet chemically identified
Is called florigen (開花賀爾蒙？), and it may be a hormone or a change in relative concentrations of multiple hormones

95. Figure 39.24
To test whether there is a flowering hormone, researchers conducted an experiment in which a plant that had been induced to flower by photoperiod was grafted to a plant that had not been induced.
EXPERIMENT
RESULTS
CONCLUSION
Both plants flowered, indicating the transmission of a flower-inducing substance. In some cases, the transmission worked even if one was a short-day plant and the other was a long-day plant.
Plant subjected to photoperiod
that induces flowering
that does not induce flowering
Graft
Time (several weeks)

96. From Signal perception …to…. Responses

97. Meristem Transition and Flowering
Whatever combination of environmental cues and internal signals is necessary for flowering to occur
The outcome is the transition of a bud’s meristem from a vegetative to a flowering state

98. 報告完畢
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99. Plants counter external threats (植物克服外來威脅)
Concept 39.5: Plants defend themselves against herbivores (草食性動物) and pathogens (病源菌)
Plants counter external threats (植物克服外來威脅)
With defense systems (防禦系統) that deter herbivory and prevent infection or combat pathogens

100. Defenses Against Herbivores (ref) (反抗草食性動物的防禦)
Herbivory (草食性動物), animals eating plants
Is a stress that plants face in any ecosystem
Plants counter excessive herbivory
With physical defenses (物理性防禦) such as thorns (刺針)
With chemical defenses (化學性防禦) such as distasteful or toxic compounds (有毒物質)

101. Some plants even “recruit” (招引) predatory animals (掠食性動物) that help defend the plant against specific herbivores
4. Recruitment of parasitoid wasps
that lay their eggs within caterpillars
3. Synthesis and release of volatile attractants (合成與釋放揮發性物質)
2. Chemical in saliva
1. Wounding
2. Signal transduction pathway
招引寄生蜂產卵
於毛蟲體內
Figure 39.29

102. Defenses Against Pathogens (反抗病源的防禦)
A plant’s first line of defense (第一道防線) against infection
Is the physical barrier (物理性障礙物) of the plant’s “skin,” the epidermis and the periderm (表皮與內皮)
Once a pathogen invades (侵入) a plant
The plant mounts a chemical attack (化學攻擊) as a second line of defense (第二道防線) that kills the pathogen and prevents its spread

103. The second defense system (第二道防線)
Is enhanced by the plant’s inherited ability (遺傳能力) to recognize certain pathogens

104. 報告完畢
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