Question

Poinsettias are a common ornamental plant bought around the winter holiday season and known for their red flowers. They can be expensive and sometimes people keep them alive from one year to the next. To get the blooms to form in mid-December is tricky though. You need to keep the plants in complete darkness from 5 pm to 8 am every day starting in mid-October and interrupting the darkness by even just a minute or two can interfere with the flowering. Given what you know about flowering explain why these plants would need to be kept in total darkness for this time period and what happens if they get even a very short light exposure.

Answer 1

Poinsettias are short-day plants i.e. long-night plants.
They require uninterrupted darkness for >12 hrs for flowering.

The CONSTANS protein is required for flowering. It is accumulated during the night time and degraded during the day time. Sufficient amounts of CONSTANS protein has to be accumulated in plants in order to flower. i.e. plants need to be kept in dark for sufficient amount of time so that the threshold CONSTANS level can be achieved to trigger flowering.