1. Growth and Development: External Factors
Angiosperms VIII
Growth and Development:
External Factors

2. Environmental Influences
Tropisms
growth responses to unidirectional stimuli
Natsic Movements
growth responses to non-directional stimuli
Seasonal Responses
photoperiodism, temperature (cold) treatments

3. Tropisms: Phototropism
Unequal cell elongation due to higher levels of IAA on the dark side via the acid growth hypothesis
But, how does light influence this?
Light causes destruction of IAA on the lighted side?
Light causes IAA to move to dark side?
Light decreases the production of IAA on the illuminated side?

4. Brigg’s Experiment

5. Phototropism (cont.) Light causes IAA to move to dark side!
But, what type (quality) of light and what pigment is involved?
Only BLUE light is effective
The pigments phototropin 1 and 2 (flavoproteins) involved
However, many molecular questions remain

6. Tropisms: Gravitropism
Roots are positively gravitropic
Stems are negatively gravitropic

7. Gravitropism
In roots, the root cap cells are responsible for detecting gravity
Thought that shifting amyloplasts are the “trigger” (starch-statolith hypothesis)
Others models have also been suggested

8. Gravitropism (cont.)
Ca+ ions accumulate on lower side, more IAA accumulates as a result, which in turn inhibits cell elongation (in roots)

9. Gravitropism (cont.)
Recent data also suggests that cytokinin is involved in early gravitropic responses near the apex
High cytokinin inhibits

10. Gravitropism (cont.)
In stems, Ca+ ions accumulates on the upper side, IAA on the lower side
In the presence of IAA, mRNAs on the lower surface produce enzymes which allow for increased cell elongation
This lower surface elongation causes the upward bending in the stem

11. Other Tropisms Thigmotropism “touch response” in plants
special epidermal cells (in tendril)
probably influenced by IAA and ethylene
ATP also needed

12. Other Tropisms Heliotropism
“sun tracking” as as the the lab video with the Arctic rose (Dryas)
sunflowers are also good “sun trackers”

13. Nastic Movements Seismonasty or Thigmonasty
Shaking response of plants like the “sensitive plant”
Also includes movements of Venus fly trap, and sundew

14. Thigmonasty in Venus Fly Trap

15. Nastic Movements (cont.)
Nyctinasty
sometimes called “sleep movements”
caused by daily rhythms of light and dark
includes opening and closing of flowers (African water lily in lab video) and drooping leaves at night (beans, Oxalis)

16. Nastic Movements (cont.)
Oxalis

17. Thigmomorphogenesis
“Shaken not
stirred”
(I love this one!)
Elongation of the stem (as in tomatoes) is influenced by the amount of “shaking” the plant experiences
the more periods of “shaking” the plant experiences, the less stem elongation will occur over time
Arabidopsis plants

18. Seasonal Responses
In the 1920’s Garner and Allard found a large tobacco plant they called “Maryland Mammoth”
It did not flower until December (inside); normal tobacco flowers in the summer
Began to study length of day/night cycles on flowering (photoperiodism)

19. Kinds of Flowering Responses
Short-Day Plants
flower only if light period is shorter than critical photoperiod (Asters, poinsettias, some mums)
Long-Day Plants
flower only if light period is longer than critical photoperiod (lettuce, spinach, corn)
Day-Neutral Plants
no response to photoperiod (roses, tomatoes)
Intermediate-Day Plants
flower only with “intermediate” length of light period (sugar cane)

20. Photoperiodism

21. How do plants measure the length of day/night cycles?
What part of the plant is involved?
What wavelengths of light are effective?
How fast is the response and how does it travel?
Is the response universal?
What pigment is involved?

22. Answers (in part) The leaf is the receptor
actually can be accomplished with only part of one leaf (fully expanded)
Blue light has no effect; red light only needs to be applied to elicit a response
two wavelengths are involved:
660 nm and 730 nm
the response is reversible by alternating these wavelengths

23. Answers (cont.)
Effect (presumably a substance) is transported, within hours in most cases, to the stem
Travels through the phloem
One plant (given the correct photoperiod) can stimulate, through a graft, another plant (exposed to incorrect photoperiod) to flower

24. Answers (cont.)
Pigment, in two forms, was discovered in the 1959 by Borthwick and Hendricks at the USDA
called the pigment phytochrome
comes in two forms P660 (red) and P730 (far-red)
P730 found to revert to P660 in the dark

25. The Phytochrome System
Pfr
Biological
Response
Red light
660 nm
Precursors
Synthesis Pr
Breakdown
Products
Destruction
Long Dark
Reversion
Far-red light
730 nm

26. Does this explain flowering?
At first, scientists thought yes
Long day plants need lots of Pfr to flower and short nights keep levels high
Short day plants need low Pfr and long nights keep levels of Pfr low
Now, not exactly…..
dark reversion more rapid thought, and some think the phytochrome system may just “set” an internal flowering clock
actually, short-day plants are really long-night plants
and long-day plants are short-night plants

27. Flowering (cont.)
No one has yet to find the “flowering hormone” called “florigen”
many have thought that “florigen” does not exist, but there are flower-inhibiting substances which must be deactivated
currently, research on Arabidopsis (2005) indicated that a messenger RNA molecule from the Flower Locus (FT) gene may be the molecule which moves to the shoot apex to initiate the flowering signal (“florigen”?)

28. Daylength in North America
Photoperiodism helps explain ecological distributions of many species

29. Other Phytochrome Responses
Seed germination in light-sensitive species (Grand Rapids lettuce)
Etiolation
Sun and shade leaves
many others …

30. Responses to Cold
Many plants will not flower without a prolonged “cold” period (winter) (carrots, beets, lilacs)
Others will flower earlier (winter wheat, rye, barley) if exposed to cold in seedling stage (planted in the fall)
cold treatment of seedlings, called vernalization, used to get a crop which can be harvested early
Cold-hardiness and dormancy are also influenced by photoperiod and temperature