Photoperiodism

Light exerts its influence on living organisms through variation in day length called photo-period. In plants, photo-period and temperature affects flowering, fruit and seed production, bud and seed dormancy, leaf fall and germination. Photoperiod affect flowering, when shoot meristem, starts producing floral buds instead of leaves and lateral buds.

In 1920 W.W. Garner and H.A Allard (agronomist in USA) were working with various varieties of tobacco plants. Tobacco plants (Nicotiana tobacum) are self-pollinated and gave flowers in summer. One day it was noticed that a single plant was quite different from other varieties. It had broad leaves, was 3 meter tall and did not flower. It was named Maryland mammoth tobacco plant.

Under field condition during summer when the days were warm and long, all other tobacco plants flowered profusely, but Maryland showed no sign of flowering. At the end of the growing season, they transferred the plant to green house to protect it from frost. In the middle of December the plant flowered. It was then allowed to self-pollinate and seeds were obtained. These seeds produced new Maryland mammoth plants. The plants flowered in winter.

Garner and Allard put seedlings of the mutant i.e. Maryland mammoth plant in special chamber, where day lengths could be regulated. When day lengths were shortened artificially to about 9 hours, the plant flowered.

Experiment on Soybean

Garner and Allard made a series of soybean (soybean) planting over a period of several weeks, in the late summer, they observed the flowering time of the plants in the various groups.

Soybean

Despite age difference due to different planting time, all the soybean flowered surprisingly close to the same time, in late summer as the day shortened.

The critical factor in both tobacco and soybean was the length of the day.

Flowering occurred when the day shortened below a critical length. This phenomenon is called photoperiodism. Photoperiodism is any response by a plant to relative lengths of daylight and darkness.

Plants are classified into three main groups on the basis of how photoperiodism affects their flowering.

Short-day plants

These plants flower when the day length is less than a certain critical length e.g. Maryland mammoth, cocklebur, chrysanthemum.

Long-day plants

They flower, only when the day length exceeds from the critical length period e.g. spinach, sugar beet, clove, lettuce, Henbane (Hyoscyamus Niger), snapdragon, cabbage, spring wheat, spring barley etc.

Phytochrome detects varying periods of day length and darkness 

The critical lengths for both long day and short day plants tend to fall in the 12-14 hours range.

Day-neutral plants

In these plants the flowering is not affected by day length or darkness. Thus the plants flower in response to some other type of stimulus, either external or internal e.g. tomato, pansy, bean, sweet, pea, rose, etc.

Night Interruption experiments on short-day plants using red light interruption and combination of red and far-red (fr) light interruptions 

Now it has been discovered that the actual stimulus for flowering is the uninterrupted dark period rather than the light period. So the short day plants are actually long night plants and long-day plants are short-night plants.