13.3.2 Phytochromes and the Photoperiodic Response Flashcards Preview

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Flashcards in 13.3.2 Phytochromes and the Photoperiodic Response Deck (9)
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1
Q

Phytochromes and the Photoperiodic Response

A
  • Red is the wavelength of light with the strongest influence on the photoperiod of a plant.
  • Although a flash of red light shortens a plant’s perception of night length, a subsequent flash of far-red light negates the effect.
  • Phytochromes are pigments involved in the flowering response of plants to light. Phytochromes function as photoreceptors in plants by isomerizing between two different forms (Pr and Pfr) in response to red and far-red light, respectively.
  • The interconversion of Pr and Pfr during night and day is involved in setting the biological clock of a plant.
2
Q

note

A
  • Research that involved the control of flowering in plants showed that red wavelengths of light are the most effective.
  • When short-day plants are exposed to short nights, they will not flower (top portion, column one). In addition, if short-day plants are exposed to long nights interrupted by a flash of red light, they also will not flower (top of column 3). If a flash of far-red light immediately follows the flash of red light, the plant will flower just as if no light had been flashed at all (top
    of column four).
  • Conversely, long-day plants will flower if a long night is interrupted by a flash of red light (bottom of column 3). If
    a flash of far-red light follows the flash of red light, then the plant will not flower (bottom of column 4).
3
Q

phytochromes

A
  • Pigments called phytochromes are at least partly responsible for a plant’s photoperiodic response to light. Light striking a phytochrome causes a chemical change that induces a response. Phytochrome has two isomeric forms: Pr, the red-light absorbing form and Pfr, the far-red light absorbing form. Red light isomerizes Pr to Pfr, and far-red light converts Pfr to Pr
  • Because white light contains both red and far-red light, the interconversions of Pr and Pfr will occur simultaneously, establishing equilibrium between the two isomers.
  • At night, plants make more Pr phytochrome and degrade more Pfr phytochrome; thus, the phytochrome ratio shifts in favor of Pr in the dark.
  • A few hours after sunset, Pfr is almost completely converted to the Pr form. In the morning, the build-up of Pr is converted to Pfr, and eventually equilibrium is reestablished.
  • As a marker of day and night, phytochrome conversion may help a plant’s biological clock distinguish sunrise and sunset
4
Q

Which of the following negates the effect of exposing a plant to red light during the night?

A
  • Exposure to far red light during the night
5
Q

Which of the following is not true?

A
  • Long-day plants will not flower if a long night is interrupted by a flash of light.
6
Q

The phytochrome system helps set the biological clock by indicating to a plant that light is present when

A
  • Pr is rapidly converted to Pfr.
7
Q

Which type of light is the most effective at interrupting night length?

A
  • Red light
8
Q

Under which of the following conditions will a long-day plant flower? Assume its critical period is 13 hours.

i. 14 hours of light; 10 hours of dark
ii. 11 hours of light; 4 hours of dark; flash of red light; 9 hours of dark
iii. 11 hours of light; 4 hours of dark; flash of red light; 2 hours of dark; flash of far red light; 7 hours of dark
iv. 11 hours of light; 4 hours of dark; flash of red light; 2 hours of dark; flash of far red light; 2 hours of dark; flash of red light; 5 hours of dark

A
  • i, ii, iv
9
Q

The signal for flowering could be artificially released in a long-day plant experimentally exposed to flashes of

A
  • red light during the night.

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