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