Angiosperms VIII
Growth and Development:
External Factors
Environmental Influences
• Tropisms
– growth responses to unidirectional stimuli
• Natsic Movements
– growth responses to non-directional stimuli
• Seasonal Responses
– photoperiodism, temperature (cold)
treatments
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?
Brigg’s
Experiment
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
Tropisms:
Gravitropism
• Roots are
positively
gravitropic
• Stems are
negatively
gravitropic
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
Gravitropism (cont.)
• Ca+ ions accumulate on lower side, more
IAA accumulates as a result, which in
turn inhibits cell elongation (in roots)
Gravitropism (cont.)
• Recent data also suggests that cytokinin is involved
in early gravitropic responses near the apex
High cytokinin inhibits
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
Other Tropisms
• Thigmotropism
– “touch response”
in plants
– special epidermal
cells (in tendril)
– probably
influenced by IAA
and ethylene
– ATP also needed
Other Tropisms
• Heliotropism
– “sun tracking”
as as the the lab
video with the
Arctic rose
(Dryas)
– sunflowers are
also good “sun
trackers”
Nastic Movements
• Seismonasty or Thigmonasty
– Shaking response of plants like the “sensitive plant”
– Also includes movements of Venus fly trap, and
sundew
Thigmonasty in Venus Fly Trap
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)
Nastic Movements (cont.)
Oxalis
Thigmomorphogenesis
• (I love this one!)
• Elongation of the stem
(as in tomatoes) is
influenced by the
amount of “shaking”
the plant experiences
“Shaken not
stirred”
– the more periods of
“shaking” the plant
experiences, the less
stem elongation will
occur over time
Arabidopsis plants
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)
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)
Photoperiodism
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?
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
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
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 (farred)
– P730 found to revert to P660 in the dark
The Phytochrome System
Red light
660 nm
Biological
Response
Synthesis
Precursors
Pr
Pfr
Destruction
Far-red light
730 nm
Long Dark
Reversion
Breakdown
Products
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 longnight plants
– and long-day plants are short-night plants
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”?)
Daylength in North America
•Photoperiodism helps explain ecological
distributions of many species
Other Phytochrome Responses
• Seed germination
in light-sensitive
species (Grand
Rapids lettuce)
• Etiolation
• Sun and shade
leaves
• many others …
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