1. Plants

2. Basic Structures

3. Apical meristems enable the plant to grow in length.
-located in tips of roots and
in the buds.

4. Plants, being rooted to the ground Must respond to whatever environmental change comes their way

5. Plants have cellular receptors
That they use to detect important changes in their environment
For a stimulus to elicit a response
Certain cells must have an appropriate receptor

6. The potato’s response to light
Is an example of cell-signal processing
Figure 39.3
CELL
WALL
CYTOPLASM
  1 Reception
2 Transduction
3 Response
Receptor
Relay molecules
Activation
of cellular
responses
Hormone or
environmental
stimulus
Plasma membrane

7. Plant hormones help coordinate growth, development, and responses to stimuli
Are chemical signals that coordinate the different parts of an organism

8. 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
Is often caused by hormones
Shoots growing towards the light is positive phototropism

9. Charles Darwin and his son Francis
Conducted some of the earliest experiments on phototropism, a plant’s response to light, in the late 19th century
In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted.
EXPERIMENT
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
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.

10. In 1926, Frits Went
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.
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
Control (agar block lacking chemical) has no effect
Control
Offset blocks cause curvature
RESULTS
CONCLUSION
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.
EXPERIMENT
Extracted the chemical messenger for phototropism, auxin, by removing the coleoptile tip & placed it on a block of agar. This allowed the chemical to travel through.

11. A Survey of Plant Hormones

12. 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

13. Auxin
Is used for any chemical substance that promotes cell elongation in different target tissues
Auxin transporters
Move the hormone from shoot tip to base and never in the reverse even if the plant is turned upside down.
Auxin
Is involved in the formation and branching of roots

14. Other Effects of Auxin
Auxin affects secondary growth ms & roots to thicken)
By inducing cell division in the vascular cambium and influencing differentiation of secondary xylem
Developing seeds synthesize auxin
tomatoes grown in greenhouse conditions sprayed with auxin induce fruit development without a need for pollination
This allows for seedless tomatoes

15. Cytokinins Cytokinins Stimulate cell division
Are produced in actively growing tissues such as roots, embryos, and fruits
Work together with auxin

16. Control of Apical Dominance
Cytokinins, auxin, and other factors interact in the control of apical dominance
The ability of a terminal bud to suppress development of axillary buds
Axillary buds
Figure 39.9a

17. It is suggested that auxin and cytokinin counter act each other to control axillary bud inhibition
If the terminal (apical) bud is removed (which is the primary source of auxin
Plants become bushier
If a plant is sprayed with cytokinins the plant becomes bushier
“Stump” after removal of apical bud
Lateral branches
Figure 39.9b

18. Anti-Aging Effects Cytokinins retard the aging of some plant organs
By inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues
Florists use cytokinins on their cut flowers to keep them fresh.

19. Gibberellins Gibberellins have a variety of effects
Such as stem elongation, fruit growth, and seed germination
Gibberellins stimulate growth of both leaves and stems
In stems
Gibberellins stimulate cell elongation and cell division
The release of gibberellins from the embryo of a seed signals the seed to break dormancy and germinate.

20. Fruit Growth In many plants Gibberellins are used commercially
Both auxin and gibberellins must be present for fruit to set
Gibberellins are used commercially
In the spraying of Thompson seedless grapes making them grow larger.

21. Abscisic Acid Two of the many effects of abscisic acid (ABA) are
Seed dormancy (presence of ABA)
Seed dormancy has great survival value
Because it ensures that the seed will germinate only when there are optimal conditions
Prepares it for winter
Drought tolerance
ABA is the primary internal signal
That enables plants to withstand drought

22. Ethylene Plants produce ethylene
In response to stresses such as drought, flooding, mechanical pressure, injury, infection, fruit ripening and programmed cell death (apoptosis).

23. The Triple Response to Mechanical Stress
Ethylene induces the triple response
Which allows a growing shoot to avoid obstacles
Ethylene induces the triple response in pea seedlings, with increased ethylene concentration causing increased response.
CONCLUSION
Germinating pea seedlings were placed in the
dark and exposed to varying ethylene concentrations. Their growth was compared with a control seedling not treated with ethylene.
EXPERIMENT
All the treated seedlings exhibited the triple response. Response was greater with increased concentration.
RESULTS
0.00
0.10
0.20
0.40
0.80
Ethylene concentration (parts per million)
1. Slowing of stem elongation
2. Thickening of the stem
3. Curvature causing stem to grow
horizontally.
Figure 39.13

24. Apoptosis: Programmed Cell Death
A burst of ethylene
Is associated with the programmed destruction of cells, organs, or whole plants
Fruit Ripening
A burst of ethylene production in the fruit
Triggers the ripening process

25. Leaf Abscission
A change in the balance of auxin and ethylene controls leaf abscission
The process that occurs in autumn when a leaf falls
0.5 mm
Protective layer
Abscission layer
Stem
Petiole
Figure 39.16

26. In animals, internal & external signals regulate a variety of physiological responses that synchronize with environmental cycles and cues.

27. Circadian rhythms

29. Photoperiodism
From the results above these plants can be more appropriately called “long night plants” and “short night plants”. Photoperiodism is controlled by the length of the night= critical night length
It is the leaves that give the cue that the night is long enough or short enough and signals the buds to flower.

30. Photoperiodism Day-neutral plants: light has no effect on their growth
Tomatoes
Rice
Dandelions

31. Photoperiodism Vernalization:
Some plants (winter wheat) need to be exposed to cold temperatures for several weeks

32. Plants respond to a wide variety of stimuli other than light

33. Gravitropism Auxin plays a key role
Underground how does a seedling know which way to grow?

34. Thigmomorphogenesis Plants growing on a windy range
Have thicker trunks
Touching leaves can alter plant growth
Climbing vines