Photoperiodism in plants

The relative length of day and night and the seasons is important to plants. The number of hours of darkness in a 24-hour cycle is an important factor in determining blossoming and growing time. Night length triggers seed germination, tuber and bulb formation, and other growth characteristics such as colour, enlargement of leaves and stem size and shape. This rhythmic characteristic is called photoperiodism and is of great value to growers.

Plants can be classified according to photoperiodicity.

Short day (long night)

Long day (short night)

Indeterminate or day neutral

The Perennial Chrysanthemum is a short day length plant that will not flower when the day is long (short night). To postpone flowering, Chrysanthemum growers, instead of lengthening the day, interrupt the night for about four hours. This makes the night appear short to plants, which then continue to grow vegetatively instead of starting to flower

A more economical method of postponing flowering of chrysanthemums is to apply cycles of light, switching light on for 10 minutes and off for 50 minutes, for four hours during the night, instead of applying light continuously. This is cyclic lighting. It is an effective way of growing flowers. If lighting levels are higher then the grower will see better stem and flower quality and less opportunity for disease.

The China Aster is a typical long-day (short-night) plant. Long-day plants can be brought to flower ahead of the normal time by lengthening the day. Relatively low intensities of light are enough to induce flowering, when applied early in the morning or at the end of the day. A dark-period interruption - from a few minutes to a few hours - as with other long-day plants, effectively induces flowering just as it inhibits flowering of short-day plants.

Poinsettias must have complete and continuous darkness for about 12 hours a day in order to flower. Even 1 minute of light in the middle of the dark period will prevent their flowering.

Tuberous Begonias flower only when daily dark periods are short - less than 12 hours, but they require long dark periods for best production of tubers. Flowering of tomatoes, however, is not influenced by photoperiod.

Artificial light is used to extend natural day-length.The commonest methods are:

In general ± 1,5 µmol m-2 s-1 is enough to let most plants react as if the day has been lengthened. Depending on light source, this value corresponds to 85-110 lux. For this application the following light sources are recommended, depending on the crop:

The use of fluorescent uses only one-fifth the energy compared with incandescent. However, incandescent is better for some plants (strawberry, gypsophila) because of their preference for this spectrum