Photoperiodism, Photoperiodism, Gravitropism, and Gravitropism, and Thigmotropism Thigmotropism AP Biology Unit 5
Jan 20, 2016
Photoperiodism, Gravitropism, Photoperiodism, Gravitropism, and Thigmotropismand Thigmotropism
AP BiologyUnit 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.
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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
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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
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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
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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
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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)
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Flowering Hormone?
• There also appears to be a flowering hormone called florigen – not fully understood yet
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Flowering
• Photoperiodism Animation
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Germination
• Red light stimulates germination
• Far red light inhibits germination
• What matters is the last light the seeds are exposed to
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Gravitropism
• Response of a plant to gravity
• Shoots will grow against gravity (upwards)
• Roots will grow with gravity (downwards)
Root placed on its side
Stem placed on its side
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Auxin and Gravitropism
• Auxin is responsible for gravitropism – inhibits cell elongation in roots – stimulates cell elongation in shoots
In stems
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Gravitropism & Starch
• Statoliths (dense starch-containing plastids) may accumulate in the direction of gravity to assist in gravitropism
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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.
Image taken without permission from http://images.botany.org/set-08/08-004v1.jpgSlide 15 of 15