Transcript Plant Timing Responses

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Plant Timing Responses
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Biological Plant Rhythms
• Plants show many rhythms, just as
animals do.
• These are controlled by exogenous or
endogenous factors, such as a biological
clock or timing mechanism.
• Some plants show daily or circadian
rhythms e.g. Opening and closing of
stomata, sleep movements of the leaves
http://www.hrt.msu.edu/course/HRT204L/VEG_ID/Limabeanleaf.jpg
of bean plants, and many flowers open
during the day and close at night.
Bean leaf simulation
• http://trc.ucdavis.edu/biosci10v/bis10v/
media/ch17/leaf_movement.html
• Endogenous rhythms are genetically
based.
http://www.bioweb.uncc.edu/1110Lab/notes/notes1/labpics/Stomata.JPG
http://scotthaefner.com/surface/gallery/fullsiz
e/sunflowers.jpg
Annual Rhythms
•Plants can show annual
rhythms such as; seed
germination, flowering, and
leaf fall.
•These may be controlled by
factors such as
temperature and day length.
•A plant will only respond to
changing day length if it has
a mechanism to measure it. http://www.rbgsyd.nsw.gov.au/__data/ass
ets/image/81026/Seed_germination_620.J
PG
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affodils.jpg
http://wagoneers.com/fotos/2003/mt-vernon-daffodils.jpg
Photoperiodism
• The regulation of plant
processes by changing day
length.
• Flowering plants can be broadly grouped into
• 1. Short-day Plants: Length of daylight must
be shorter than a critical length in order for
the plant to flower.
• 2. Long-day Plants: Length of daylight must
be longer than a critical length in order for
the plant to flower.
• 3. Day-neutral: These flower regardless of
day length as long as they have enough light
for normal growth (often native to tropics).
www.theflowerexpert.com
Important points!
• Plants actually measure the length of the
dark period NOT the period of light.
• Even a bright flash of light during the dark
period will break it.
• The leaves actually detect the period of
darkness and then send a message to the
flower bud via the hormone florigen.
• In long day plants Pfr induces flowering.
• In short day plants Pfr inhibits flowering.
Can you explain why??
Photoperiodism
• The regulation of plant processes by changing day
length.
• a pigment call phytochrome measures the time that
has passed.
• Phytochromes exist in two forms
– Pr absorbs red (R; 660 nm) light
– Pfr absorbs far red (FR; 725 nm) light
• Pr absorbs red light and converts into Pfr
• Pfr absorbs far red light and converts into Pr.
• In the dark, Pfr spontaneously converts back to Pr.
• Sunlight is richer in red (660 nm) than far red (730
nm) light so at sundown, all the phytochrome is Pfr.
• During the night, the Pfr converts back to Pr.
• If this process is interrupted by a flash of
660-nm light, the Pr is immediately
reconverted to Pfr and the night's work is
undone .
• A subsequent exposure to far red (730 nm)
light converts the pigment back to Pr and the
steps leading to the release of florigen can be
completed.
Phytochrome has Active (Pfr) and
Inactive (Pr) Forms
Pr is inactive, absorbs
PROMOTES OR
INHIBITS GROWTH
Absorbed (Pfr is active)
Photoperiodism simulation
http://trc.ucdavis.edu/biosci10v/bis10v/media/ch19/day_length.html
www.cartage.org.lb/.../CockleburChart.gif
http://fig.cox.miami.edu/Faculty/Dana/photoperiodism.jpg
poinsettias
http://www.djamas.com/images/HPIM5402.JPG
http://botit.botany.wisc.edu/images/130/Tropisms/p
1.What type of flowering
plants are these?
2. Estimate each examples
critical day length.
http://www.countingthoughts.com/ct/resources/images/biology%20images/photos/Photoperiodism1.png
Other Phytochrome Responses
• Pigment formation.
• Leaf expansion.
• Sleep movements of leaves.
• Some small seeds will only germinate in the light
(Germination is promoted by the Pfr produced in
sunlight or red light).
Why would this response be an advantage?
• Shade avoidance response or etiolation occur in the
dark or in absence of red light. The plant grows long
and thin until they reach light.
Other environmental cues and
plant responses.
Dormancy and its Causes.
• Some Viable seeds do not germinate even
when there is water, oxygen and a suitable
temperature.
• Dormancy can be regulated by the
environment or by the seed.
• Seed regulated dormancy comes in two forms:
1) dormancy caused by external or physical
factors e.g. impervious seed coat
2) Dormancy caused by internal or chemical
factors.
Scarification.
•This is when the impervious seed coat is broken
so that germination can occur.
•Scarification may be done using
•Acid or hot water
•Abrasion
•Passing through the digestive tract of an animal.
•Decomposition of seed coat by soil organisms
•Fire (in some cases).
•Exposure to moist chilling (Stratification). The
seed must spend time at or near freezing
temperature. This ensures that the seed
germinates in spring rather than autumn when it is
produced.
Vernalisation
• Temperature can
also be a cue for
flowering.
• Some plants will
not flower unless
they experience a
cold spell e.g.
Lilac, stone fruit.
http://www.english-countrygarden.com/a/i/flowers/lilac-11.jpg
Key Point Summary
• Plants show many biological rhythms. Some are controlled
exogenously, some endogenously.
• Phytochromes are involved in many plant processes.
• Phytochromes exist in two forms
Pr (absorbs red light) -- Pfr (absorbs far red light)
• In many plants, Photoperiodism determines flowering (night
length). Phytochrome is involved as an on/off switch.
• Other environmental cues affect plants e.g. seed dormancy
and stratification or scarification.
• Some plants need a cold spell to flower (vernalisation)
• Plants show many circadian rhythms such as sleep
movements of flowers and leaves.