1. The Chapter 31 Test has been postponed until April 3

2. Chapter 31
Light Receptors – Part 1

3. You Must Know
How phototropism and photoperiodism use changes in the environment to modify plant growth and behavior.

4. Concept 31.2: Responses to light are critical for plant success
Light triggers many key events in plant growth and development, collectively known as photomorphogenesis 4

5. Plants detect not only presence of light but also its direction, intensity, and wavelength (color).
Light wavelengths below 500nm induce
curvature.
Wavelength (nm)
400
450
500
550
650
600
700
436 nm
Phototropic effectiveness
1.0
0.8
0.6
0.4
0.2
White
light
Refracting prism
A graph called an action spectrum depicts relative response of a process to different wavelengths.
Action spectra are useful in studying any process that depends on light.
Blue light induces the most curvature
of coleoptiles. 5

6. There are two major classes of light receptors
blue-light photoreceptors
phytochromes, photoreceptors that absorb mostly red light 6

7. Various blue-light photoreceptors control
phototropism (movement in response to light)
stomatal opening,
hypocotyl elongation 7

8. Phytochrome Photoreceptors
Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life.
These responses include seed germination, shade avoidance and flowering (which needs to be done at a specific time of the year.) 8

9. Low light because of shade
Leaves in the canopy absorb red light
Low light because of shade
Plants shaded by other plants receive more far-red than red light. When a plant senses a high ratio of far-red light it “knows” it is in a competitive environment.
The ratio of red to far-red light is unaffected for plants that are shaded by non-plants.
Review Slides

10. Why is this adaptive? phytochromes
Many seeds remain dormant until light conditions are optimal.
Dark
Red
Red
Far-red
Dark
Dark (control)
Red light increased germination, while far-red light inhibited germination.
The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome.
Red
Far-red
Dark
Red
Far-red
Why is this adaptive?
phytochromes 10

11. This is how a plant “knows” the ratio of red to far red light it is receiving.
The conversion of Pr to Pfr is relatively fast. Pr
Synthesis
Red light
Pfr
Far-red
light
Enzymatic
destruction
Slow conversion
in darkness
(some species)
Red light triggers the conversion of Pr to Pfr.
Far-red light triggers the conversion of Pfr to Pr .
The conversion to Pfr is faster than the conversion to Pr .
Sunlight increases the ratio of Pfr to Pr and triggers germination.
Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses. 11

12. Finish after spring break

13. These seeds “know” they are being shaded by a plant and so will “wait for another time to start growing.”
Once a seed has germinated, if it “knows” it is being shaded by another plant it will grow tall “as fast as it can to beat the competition.”
Red
Far-red
Phytochromes and shade avoidance: The phytochrome system also provides the plant with information about the quality of light.
Leaves in the canopy absorb red light.
Shaded plants receive more far-red than red light.
In the “shade avoidance” response, the phytochrome ratio shifts in favor of Pr when a tree is shaded.
This shift induces the vertical growth of the plant.

14. Etiolation De-etiolation (b) After a week’s exposure
Figure 31.11
Etiolation
De-etiolation
(b) After a week’s exposure
to natural daylight
A potato left growing in darkness produces shoots that look unhealthy, and it lacks elongated roots.
These are morphological adaptations for growing in darkness, collectively called etiolation.
After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally.
(a) Before exposure to light 14

15. Use the following criteria to evaluate the completeness of your model:
Make a model that demonstrates the role of phytochromes in plant development. Your model should include phytochrome, “light”, a “seed”, and a “plant.”
Use the following criteria to evaluate the completeness of your model:
The phytochrome can transition between Pr and Pfr
The seed germinates when appropriate.
The seed stays dormant when appropriate.
The plant shows vertical growth when appropriate.
The plant shows lateral branching when appropriate.

16. Review

17. Setup for Theodor W. Engelmann’s 1883 experiment.

18. O2 is a waste product of the light reaction of photosynthesis
Filament
of alga
Aerobic bacteria
(c) Engelmann’s experiment
400
700
600
500
Aerobic bacteria
Filament
of alga
The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann.
In his experiment, he exposed different segments of a filamentous alga to different wavelengths.
Areas receiving wavelengths favorable to photosynthesis produced excess O2.
He used the growth of aerobic bacteria clustered along the alga as a measure of O2 production.
Pay particular attention to the homework that walks you through Engelmann’s experiment.
400
500
Engelmann’s experiment 18

19. Far Red
730 nm
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