Iris is a long-day plant that normally flowers in the spring. Which regimen would be the most effective in making an iris bloom in late fall?
A) interrupting the plant's nights at 2:00 am with a flash of far-red light
B) interrupting the plant's nights at 2:00 am with a flash of red light followed by a flash of far-red light
C) interrupting the plant's nights at 2:00 am with a red flash, then a far-red flash, then a red flash
D) interrupting the plant's days at 2:00 pm by putting it in the dark

Question

Quizplus · Accepted Answer

Long-day plants like iris require long periods of light to flower. Interrupting the night with a red light flash can simulate longer days, as red light promotes flowering in these plants. The sequence of red, far-red, and then red light ensures that the effect of the far-red light (which would inhibit flowering) is canceled out by the final red light flash, effectively mimicking long daylight conditions conducive to flowering.

Iris Is a Long-Day Plant That Normally Flowers in the Spring