What is its promise in agriculture?

The long-range objective is more complete understanding of the growth and development of plants. This knowledge will help crop specialists develop more efficient methods of production.

We see that light plays a fundamental role in the regulation of flowering and the production of seed. It is important in germination. Light regulates the habit of growth of seedlings and the adult plants.

Moreover, we find that a single light reaction is concerned with each of these expressions of growth and development probably with others that are still to be studied.

This reaction of plants to light thus gets right to the heart of the regulation of many aspects of plant growth.

IMPROVEMENTS in procedures of plant production came immediately from the discovery of photoperiodism.

One of the first uses made of the information was by cereal agronomists as early as 1922. They used supplemental light over small-grain crops to promote the flowering and fruiting of a winter-grown crop in the greenhouse.

They soon found that they could produce two successive greenhouse crops and still have time to grow a third crop in the field during the normal growing season. This procedure enabled them to complete programs of plant breeding in much shorter time than formerly.

In these first applications, the artificial light was turned on before sundown and continued for several hours to obtain a long-day response. The assumed necessity of following this procedure was based on the feeling that the light, not the dark, was the controlling time period of the daily cycle.

The discovery in 1937 that the reverse was true and that a brief period of light in the middle of the night was about as effective as continuous light from sundown until midnight resulted in modification of lighting procedures.

Agronomists and others found that a few minutes or an hour of light in the middle of winter nights promoted flowering of small grains, for example, as well as did the former method of prolonged lighting.

Florists also were quick to use light to extend the productive period of chrysanthemums. They, too, originally gave the supplemental light in conjunction with the daily period of natural light. Dark-period interruptions proved to be as effective as prolonged lighting, however, and avoided objectionable stein elongation that prolonged lighting often brings about.

Breeders of sugarbeets began almost at once to use supplemental light to induce flowering. They found that incandescent-filament lamps effectively induced flowering. The fluorescent lamps were almost without effect. This difference in response apparently was connected with differences in the wavelength composition of the two kinds of light incandescent-filament light contains much more far red in proportion to red than does fluorescent light.

We now know that this response of beets is in some way a result of the red, far-red reaction.

The results with sugarbeets emphasize the importance of knowledge of the detailed effects of different wavelengths.

The most extensive commercial application of control of daylength in the United States is made by growers of chrysanthemums. They supply cut flowers in a range of varieties and colors throughout the year.

During periods when natural nights are long enough to induce flowering, the growers use supplemental light to delay blossoming until the plant attains proper size and to time the harvest for dates of their choice. They bring the plants into bloom by discontinuing the light treatments several weeks before the desired harvest date, thus allowing the natural long nights to induce flower formation. The procedure is practiced widely in greenhouses and out of doors in places where winter temperatures are warm enough for chrysanthemums.

Several hundred acres of chrysanthemums were grown under lights out of doors in the United States in 1961.

During periods of the year when daily night length is too short to promote the flowering of chrysanthemums, the growers cover the plants with black cloth for a few hours morning or evening, or both, to create adequately long daily dark periods. This procedure is practiced in the greenhouse and sometimes out of doors.

The practices of the chrysanthemum growers are applied commercially but less extensively by growers of orchids, asters, tuberous-rooted begonias, Kalanchoe blossfeldiana, feverfew, and Stevia.

Poinsettia, a short-day plant, usually is grown in the greenhouse in periods of natural long nights. It would seem that no special attention need be paid to its daylength requirements. In practice, however, poinsettias are lighted during the second half of September and the first third of October. Lighting is then discontinued, and often the plants are given artificially lengthened dark periods for a week or two so that the flower-inducing reactions will begin promptly. After that, the natural dark periods are long enough to promote flowering.

Poinsettias are so sensitive to light that special care must be used to avoid low intensities, such as from the watchman's flashlight, a nearby street lamp, or passing automobiles, which would delay or inhibit flowering.

The use of artificial light on field crops presents difficulties.

Although it is used commercially to control flowering of chrysanthemums and a few other ornamentals, artificial light is not used in the commercial production of field crops. It is not probable that extensive field use will be made of it in the foreseeable future. The reason is that the cost of providing an extensive lighting installation makes it impractical.

Sugarcane, as grown in Hawaii and Puerto Rico, illustrates the problems. The yield of sugar is less when cane forms flowers because flowering stops the growth of leaves and stems. The further growth of the plant thus is restricted. Sugarcane begins to form flower primordia about the first of September only at that time of year does the natural daylength become favorable.

Cane, moreover, is unusual in that it is unable to flower when the day-length is longer or shorter than these September days. Therefore it is unnecessary to use light for more than a period of 2 or 3 weeks in September to prevent flowering throughout the entire year. Light applied properly during this period is 100 percent effective in preventing the formation of flowers, and the amount of light needed each night is trivial. Nevertheless, the costs of an adequate lighting installation preclude use of this procedure.

One might therefore reasonably question the wisdom of devoting so much time and money to the study of flower control by light if the knowledge has so little promise of extensive field application.

KNOWLEDGE of the light reactions of plants does have practical applications in other than the direct use of light.

An important one is that it helps us find or breed varieties of crops that are adapted to the natural daylength conditions of an area. If we can do that, we do not have to try to change the daylength conditions of large regions to meet crop requirements.

Soybeans are grown extensively in the United States, but no one variety is widely grown. Instead, certain varieties are restricted to comparatively narrow latitudes 75 to 100 miles wide. In areas either to the north or south, other varieties are sown because they are better adapted to the slightly different daylength conditions and they are therefore more apt to mature at the proper time and have greater yields.

It seems almost incredible that day-length differences such as occur between points only loo or so miles apart north and south could cause measurable differences in plant response. An experiment with soybeans at the Agricultural Research Center at Beltsville, Md., however, demonstrated that this was indeed true.

The durations of natural light, including twilight, were calculated for each day of the growing season at Beltsville, and points in southern Virginia and central North Carolina.

Soybeans grown at Beltsville on these three artificially maintained day-length schedules matured at different times. Those of the southernmost schedule were significantly earlier than the middle one. The middle one was earlier than the northern one. The greatest difference in daylength between neighboring lots occurred on the longest day of the season and was only about 15 minutes. Since all of the lots were subjected equally to all other fluctuating environmental variables except daylength, the differences in maturing must be attributed to the effects of daylength.

Daylength influences the further growth and development of flowers after they are initiated, although I did not stress this point in the earlier part of this discussion.

One long night causes floral initiation in cocklebur, but repeated treatments with long night are necessary for more rapid development of the flowers.

Flowers of soybean plants often drop, unless the plants are given long nights until after the pods are set.

Initiation of flowers by red kidney bean occurs regardless of daylength, but at some temperatures daylength markedly influences the yield of beans.

In the blue-mist spirea, the visible flowerbuds form on any daylength but never open on long days. On short days, however, the young buds grow rapidly and the flowers open in about 3 weeks.

The effects of daylength may thus be expressed at any or all stages in the development of flowers.

THE FUTURE of our knowledge and understanding of the action of light in the control of flowering and many other features of plant growth and development is bright.

At the beginning of 1961, when I prepared this chapter, the photoreactive pigment had been extracted from dark-grown corn seedlings and held for several months without loss of photoreversibility. Its presence had also been detected in a dozen or more other kinds of plants, and initial steps in its purification had been made. Its complete purification and identification are expected, and with identification one hopes may come knowledge of the reaction catalyzed by its active form.

This work leads to understanding of a basic reaction controlling growth and development of plants, but the immediate objectives are not the solution of individual problems of plant production.

When the fundamental principles of light action on plants are understood, specialists will apply them intelligently to many production problems peculiar to their individual crops.

HARRY A. BORTHWICK is Director of the Plant Physiology Pioneering Research Laboratory, Plant Industry Station, Beltsville, Md. This laboratory was chartered in 1957. Its purpose is to investigate the various effects of light on the growth and development of plants and to find and understand the basic photoreactions that control these plant responses.