In which plant was the importance of day length for flowering first observed?

The area of photoperception in plants is

If a tree flowers thrice in a year $(October, January, \text{and } July)$ in Northern India, it is said to be

$A$ long-day plant will show flowering when the critical photoperiod duration is as follows:
Duration of light period $= A$
Duration of dark period $= B$

Assertion: Dark period plays a more important part in flowering than the light period.
Reason: Flowering occurs in short-day plants if the dark period is interrupted by a light break.

What is the effect on phytochrome in plants following continuous red light exposure?