Photoperiodism, Gravitropism, and Thigmotropism

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Photoperiodism, Gravitropism, and Thigmotropism
AP Biology Unit 5
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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/plan
ts/garden_radish.htm
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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)

Stem placed on its side
Root 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.

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Image taken without permission from
http//images.botany.org/set-08/08-004v1.jpg