13.3.1 Photoperiodism in Plants: Control of Flowering

Question 1

Photoperiodism in Plants: Control of Flowering

Answer 1

• Hormones are thought to be directly involved in flowering, though specific mechanisms and hormones have yet to be discovered.
• Through flowering, plants exhibit circadian rhythms, which are physiological cycles about 24 hours long.
• Photoperiodism is a physiological response to a light cycle. In 1920, W.W. Garner and H.A. Allard identified photoperiodism in plants when they noted that some plants seemed to require a short light period to flower, while others seemed to require a longer light period to flower. Garner and Allard termed these short-day plants and long-day plants, respectively.
• In the 1940s, researchers discovered that night length, not day length, was the critical factor that determines whether a plant flowers.

Question 2

note

Answer 2

• If a plant has all but one of its leaves covered, it can still flower. If a plant has all of its leaves covered, it can’t flower. This observation shows that leaves somehow detect photoperiods.
• Hormones are thought to carry the signal from leaf to bud for a plant to flower. Evidence points to the existence of hormones, but the hormonal signal has not yet been discovered. When two plants are grafted together and one plant is kept in the dark, both plants will flower. This result suggests the transmission of a flowering-inducing substance (i.e., hormone) from the plant in the light to the plant in the dark.

Question 3

long-day plant

Answer 3

• In the 1940s, researchers found that the length of
  uninterrupted darkness determined flowering in plants, not the length of day. For example, a long-day plant will flower during a short night (see figure on left). But a long-day plant can flower during a long night if a flash of light divides the long night into two short periods of darkness.

Question 4

short-day plant

Answer 4

• In contrast to long-day plants, short-day plants require long nights to flower. If a flash of light interrupts a long night, the plant will not flower.

Question 5

note 2

Answer 5

• The identification of long-day and short-day plants has had an impact on the floral and agricultural industry. For example, flashing a long-day plant like a chrysanthemum in the middle of a long night is a cheaper way to induce
  flowering than lengthening the day with artificial light.
  Flashing a short-day plant like sugar cane in the middle of a long night will prevent flowering, thus helping to conserve the plant’s sugar stores.

Question 6

Garner and Allard studied photoperiodicity in plants. Which of the following is a good description of photoperiodicity?

Answer 6

• A response to the seasonal changes in the hours of dark and light in each 24 hour period

Question 7

Long-day plants will flower only when

Answer 7

• the night length is less than or equal to the critical length

Question 8

If a short day plant has a critical night length of 14 hours, which of the following 24 hour cycles would prevent flowering?

Answer 8

• 4 hours light / 8 hours dark / 4 hours light / 8 hours dark.

Question 9

Flowering in plants is controlled by

Answer 9

• the length of the night.

Question 10

A plant’s flowering rhythm can be disturbed by

Answer 10

• short exposures to light during the night.

Question 11

What internal factor controls flowering in plants?

Answer 11

• hormones

Question 12

A physiological response to the relative lengths of day and night is called

Answer 12

• photoperiodism