We know that days are long in summer and short in winter. Not just us, even some plants are aware of this fact and change their behavior accordingly. In the year 1920s, scientists were quite puzzled by the behavior of certain soybean plants. Sunlight, water, fertilizers could not make them flower. Finally varying the factors, they discovered that the beans would not flower in the long days of summer. There were but other varieties that would flower only when the days are long.
As time passed by, it was learnt that many kinds of plants will flower and fruit only when the day is of a particular length. The main factor remains the number of hours of darkness and not those of light. This response to light is called photoperiodism. It is not an arbitrary reaction, but one that enhances the plant’s chances of survival and those of its seeds. There is a critical day-length associated with some plants. These plants flower only if the days are longer than or shorter than the critical day length.
To learn more about photoperiodism, researchers made growth chambers in which the duration of dark and light periods were controlled using timers. Using the wavelength of different colored light, they were able to bring about an atmosphere that could make the plants flower.
Plants in different regions have their own photoperiods. Growers then learnt to get their plants flower at any time by adjusting the amount of light. That’s how certain flowers are available all round the year though they can actually flower only during a particular time of the year.
While flowers like morning glory, chrysanthemums are short-day plants, spinach, radish etc are long day plants. There are also neutral plants like tomato whose flowering doesn’t have any relation to the length of the day.