Photoperiodism

Photoperiodism is the response of plants to the relative lengths of day and night, especially in flowering.
It was first described by Garner and Allard (1920) in the 'Maryland Mammoth' variety of tobacco.
This variety did not flower in summer, however vigorously it grew, but flowered when the days were short; it was the first short-day plant to be recognised.
Hence photoperiodism was first characterised in tobacco.

The response of plants to the relative lengths of day and night, especially with regard to flowering, is called photoperiodism.
It was first shown by W.W. Garner and H.A. Allard in 1920.
They worked with a mutant variety of tobacco called Maryland Mammoth, which kept growing vegetatively in summer and flowered only in winter in the greenhouse.
They found that it flowered only when the day length was shorter than a critical value; it is a short day plant.
They made similar observations in the Biloxi variety of soybean.
So the importance of day length in flowering was first shown in tobacco.

Photoperiodism is the response of plants to the relative lengths of day and night. What really matters is the length of the uninterrupted dark period.
Short day plants (long night plants) flower only when they get a continuous dark period longer than a critical length.
If the long night is interrupted even by a brief flash of light, the dark period is broken into two short nights, and a short day plant does not flower.
Here the plant flowered with a 12 hour uninterrupted night, but failed to flower when the night was interrupted by a flash of light. This is the typical behaviour of a short day plant.
A long day plant would be promoted (not inhibited) by a night break, and a day neutral plant would not respond to it at all.
So the plant is a short day plant.

Long day plants (LDP) flower when the day length exceeds a critical value (i.e. when nights are short).
Spinach, wheat, radish, sugar beet and Hyoscyamus are long day plants.
Tobacco (Maryland Mammoth) and soybean (Glycine max) are short day plants, and Mirabilis jalapa is often given as a short day / day neutral example.
So spinach is a long day plant.