A short-day plant has a critical night length of 10 hours. It is kept on 8 hours of light and 16 hours of dark, but the dark period is interrupted in the middle by a one-minute flash of red light. What happens?
AIt flowers, because the total darkness is still 16 hours
BIt does not flower, because no single stretch of darkness reaches 10 hours
CIt flowers faster than usual
DIt dies from the light flash
The flash splits the night into two 8-hour periods. Neither reaches the 10-hour critical length, so the plant responds as if nights were short and does not flower. This night-break result shows that plants measure continuous darkness.
Question 2 True / False
Short-day plants flower when the day length is shorter than a critical value, so the length of the day is what they measure.
TTrue
FFalse
Answer: False
The name is historical. Night-break experiments in the 1930s and 1940s showed that the length of uninterrupted darkness controls flowering. Short-day plants are really long-night plants.
Question 3 Short Answer
A short-day plant receives a night-break flash of red light followed at once by a flash of far-red light. Will it flower under long nights?
Think about your answer, then reveal below.
Model answer: Yes. Red light converts phytochrome to its active Pfr form, but the far-red flash converts it back to Pr. Because the last flash was far-red, the night acts as if uninterrupted, and the short-day plant flowers.
Red and far-red effects are reversible, and the final flash decides the outcome. This reversibility was the key evidence that a single photoreversible pigment, phytochrome, measures the night.
Question 4 Multiple Choice
Where in the plant is the photoperiod detected, and how does the shoot tip learn that it is time to flower?
AIn the roots, which send a hormone through the xylem
BIn the leaves, which send a signal (florigen) through the phloem to the shoot tips
CIn the flower buds themselves
DIn the seeds, which store the information from the previous year
Grafting experiments showed that an induced leaf can make an uninduced plant flower. In Arabidopsis the signal includes the protein FT (FLOWERING LOCUS T), made in leaves and carried in the phloem to the shoot apex.