Day Length and Flowering

by H. A. BORTHWICK

ABOUT 30 years ago a problem of the kind that has concerned farmers for centuries was put up to two scientists in the Department of Agriculture: How could one get the new Maryland Mammoth tobacco to flower and produce seed in the latitude of Washington, D. C.?

The answer, found by W. W. Garner and H. A. Allard, was that the length of darkness and light each 24 hours regulates the flowering of this variety of tobacco and of many other kinds of plants as well. This fundamental law of nature tells us why some plants blossom in the spring, or in the Tropics, or in specific zones, and not in others. It is an example of basic research with wide economic uses.

Garner and Allard submitted the tobacco to all the known factors that govern the growth of plants. One by one, they eliminated differences in soil moisture, temperature, fertilization, and intensity of light as reasons for the failure of the plants to mature properly. But, regardless of treatment, the tobacco grew vegetatively that summer and fall.

To save the tobacco plants from freezing, several were cut back, potted, and moved into the greenhouse for further study. At the same time, tobacco seedlings were also started in the greenhouse. In a few weeks, old plants and seedlings alike began to bloom. Blooming continued through the winter, but, with the beginning of spring, nonflowering shoots were produced again, and these continued to grow vegetatively—without producing seed—throughout the summer. To the two scientists, the fact that old and young plants bloomed simultaneously meant that neither size nor condition of the plants was the limiting factor.

Because blooming started in late autumn and ended in the early spring, there was little doubt that the length of winter days in Washington was what limited the blooming period of these plants. Allard and Garner tested this conclusion further the following season, the summer of 1918, by growing the tobacco and the Biloxi soybean, another plant that did not mature its seed in the latitude of Washington, D. C., in pots, and exposing them to 7 hours of sunlight a day. This was done by placing the plants in dark chambers from 4 p. m. to 9 a. m. daily. Both tobacco and soybean promptly bloomed. This simple experiment answered their problem.

The two men, who worked in what is now the Bureau of Plant Industry, Soils, and Agricultural Engineering, extended their studies to include other kinds of plants. They discovered that some flowered sooner when the days were shortened, and others blossomed only when the days were lengthened. The flowering of still others was apparently not influenced at all by the length of day. They identified these groups of plants as short-day, long-day, and indeterminate types. They also observed that many other responses were influenced---the formation of bulbs and tubers, the coloration and abscission of leaves, and the branching and growth habit of plants. In brief, they recognized, and in 1920 reported, that the variable environmental factor of day length profoundly influenced plant development. This response that an organism makes to relative length of day and night the discoverers called photoperiodism.

Studies of photoperiodism since have been numerous in every part of the world where plants are studied scientifically. In increasing the economy of crop production the studies have been profitable. From the theoretical viewpoint, they have stimulated much basic research in plant physiology that has given new insight into many physiological processes.

How can agriculture profit from studies of photoperiodism? Even though day length does control plant development, man cannot control day length over any considerable area—so what practical advantage could result from his studies of photoperiodism? Such questions are often the first ones asked when people are introduced to the subject.

It is certainly true that it is impractical or impossible to attempt to change the length of day in a region in which field crops are to be grown, but is is quite possible to change the crop to suit the day length of such a place. Farmers have followed this procedure for generations with respect to temperature requirements of their crops.

They know, for example, that winter cereals will winterkill if grown too far north, and will fail to flower because it is not cold enough if grown too far south. Peaches need different amounts of low temperature in the winter to break their rest periods so they can resume growth when spring temperatures are right. Peach varieties requiring the least low temperature in the winter are therefore grown farthest south, while those requiring most low temperature must be kept in the north.

The same principles apply with even better results to crops that are affected by photoperiod, because seasonal variations in length of photo-period are exactly the same from year to year, while the seasonal variations of temperature are always different.

Some of the most important practical results have thus come about through understanding that day-length requirements of a variety often determine whether it can be grown successfully in a particular region. If the day-length requirements of a variety do not fall within the range of day lengths that occur in the locality, there is little use in attempting to modify other environmental factors to compensate. Such attempts seldom succeed. Since the natural day length cannot be changed, the only solution is to select a variety whose day-length requirements are satisfied by those prevailing in the locality in question.