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Photoperiodism, Gravitropism,
and Thigmotropism
AP Biology
Unit 5
Photoperiodism
• How a plant responds (with respect to
flowering) to the relative amount of light
(“photoperiod”)
• In reality, plants are responding to the
relative amount of night.
Slide 2 of 15
Photoperiodism: Types of Plants
• 3 different types of plants:
– “Short Day”  flower when days
are short, nights are long (Ex.
poinsettias, chrysanthemums)
– “Long Day”  flower when days
are long and nights are short (Ex.
Spinach, Radish)
– “Day Neutral”  flowering does
not depend on length of day or night
(Ex. tomato)
Images taken without permission from http://www.fernlea.com/xmas/pix/poinsettia.jpg, and
http://www.illinoiswildflowers.info/weeds/plants/garden_radish.htm
Slide 3 of 15
Question…
• Poinsettias are short day plants– how could
nurseries make sure they bloom just before
Christmas?
– Control the amount of light and dark they
experience
Slide 4 of 15
Phytochromes
• Plants absorb light via blue-light
photoreceptors and phytochromes (Pr and
Pfr).
• Pr and Pfr play a significant role in the
flowering and germinating responses
Slide 5 of 15
Phytochromes
• Germination and flowering
occurs in response to red and
far-red light
– effects of both lights are
reversible
– Pr and Pfr are isomers (alternate
forms)
– red light (660 nm) activates Pr
to become Pfr
– far-red light (730 nm) activates
Pfr to become Pr
Slide 6 of 15
Flowering
• Pfr
– inhibits flowering in short day plants
– promotes flowering in long day plants
• Sunlight consists of quite a bit of red light,
not much far red light
• During the day, which form of phytochrome
is in?
– Pfr
Slide 7 of 15
Flowering
• At sunset, most of the phytochrome is in the
Pfr form
• During the night, Pfr gets converted back
into Pr or breaks down
• Whether a plant flowers or not depends on
the amount of Pfr left (which relates to the
amount of night)
Slide 8 of 15
Flowering Hormone?
• There also appears to be
a flowering hormone
called florigen – not
fully understood yet
Slide 9 of 15
Flowering
• Photoperiodism
Animation
Slide 10 of 15
Germination
• Red light stimulates
germination
• Far red light inhibits
germination
• What matters is the last
light the seeds are
exposed to
Slide 11 of 15
Gravitropism
• Response of a plant to
gravity
• Shoots will grow
against gravity
(upwards)
• Roots will grow with
gravity (downwards)
Slide 12 of 15
Stem placed on its side
Root placed on its side
Auxin and Gravitropism
• Auxin is responsible for gravitropism
– inhibits cell elongation in roots
– stimulates cell elongation in shoots
In stems
Slide 13 of 15
Gravitropism & Starch
• Statoliths (dense starch-containing plastids)
may accumulate in the direction of gravity
to assist in gravitropism
Slide 14 of 15
Thigmotropism
• Response of plant to touch.
• Ex. Tendrils of vines wrapping around
things, venus fly traps, “shy plant”
• Response is similar to nervous response in
humans.
Slide 15 of 15
Image taken without permission from http://images.botany.org/set-08/08-004v1.jpg