THE LIFE PROCESSES OF SEEDS

Light, Flowering, and the Production of Seed

HARRY A. BORTHWICK.

ONE MINUTE of artificial light each night holds back the flowering of certain plants, promotes the flowering of some, and has no measurable effect on others.

Our understanding of how we can use light to control flowering; a matter that has practical applications and a direct bearing on the organs in which seeds are produced has grown in the past few years.

For countless generations people have known that external conditions influence the flowering of plants.

They knew, for example, that the flowering dates of fruit trees vary somewhat from year to year. They attributed this variation largely to seasonal differences in temperature.

They also knew, however, that despite these relatively small differences in time of flowering, their fruit trees always bloomed in early spring, their cereals in early summer, and many of their ornamentals, such as chrysanthemums and cosmos, in late summer or early autumn. These plants thus were able to recognize the onset of the various seasons and to synchronize their development with the change of season.

The mechanism by which certain plants are able to time the events of their lives in such a way that they always bloom at a particular time of year remained undetected until 1919.

In that year Dr. W. W. Garner and Dr. H. A. Allard, plant physiologists in the Department of Agriculture, discovered that Maryland Mammoth tobacco and Biloxi and other varieties of soybean, which normally bloom in autumn, could be made to flower in June or July by subjecting them to artificially shortened days and lengthened nights.

This discovery was one of the most significant advances in botanical science in this century. It marked the recognition of a hitherto unsuspected feature of the environment, the daily duration of light and darkness, as a most important factor regulating plant growth and development.

Dr. Garner and Dr. Allard called the phenomenon photoperiodism, an appropriate name because it recognized the importance of both a light-requiring (photo-) reaction and a time-measuring one (period) in the response.

Their discovery was confirmed promptly by countless investigators. The phenomenon was found to occur among many flowering plants.

Many kinds, the short-day ones, flower only when the daily light periods are short and the dark periods are long. Examples include numerous fall- flowering - plants, such as cosmos, chrysanthemum, and cocklebur.

Others, the long-day ones, flower only when days are long and the dark periods are short. Sugarbeet, garden beet, spinach, wheat, oats, and barley are examples.

In still others, the day-neutral or indeterminate ones, flowering apparently is not influenced by daylength over a wide range of daylengths. Among such plants are tomato and many kinds of garden beans.

THE PROCESS by which plants form flowers is complex and takes time. One can measure the time in some plants by transferring them at a given moment from daylength conditions that prevent flowering to daylength conditions that lead to flowering.

The date of transfer thus gives us a known starting point, which is important because we cannot see the first steps of flowering. Under natural conditions, therefore, one never knows when the flowering process really starts.

We should keep in mind that before we can see any microscopic evidence of flower formation, some most important biochemical changes must occur to cause this shift from vegetative development to floral development.

It is to the initial causal reactions of this change that we give special attention. When we learn more about these first reactions, we should be in a better position to study the remaining steps in flowering.

The entire flowering process is completed in some species in several days. In others it may require many weeks or even months. We can recognize the earliest visible stages of flower formation in a soybean or cocklebur 3 or 4 days after the plants receive short-day treatment if we examine the growing points with a microscope. In another week or more we might be able to see them without a microscope.

The start of flower formation in the chrysanthemum can be seen microscopically a week or 10 days after the beginning of short-day treatment. The flowers are not ready for harvest, however, until 8 to 10 weeks later, or more,. depending on the variety.

In some plants, such as apple, the flower primordia are formed during the summer. They develop throughout the rest of the growing season, remain in the bud over winter, and open during the following spring. The entire process takes 8 or 9 months.

We know that the entire process of flower formation and in some instances the formation of fruit and seed are influenced profoundly by daylength.

In some plants, however, the initiation of flowering is so clearly under control of the daylength reaction that we can advantageously restrict the observations to that step in the flowering process. For example, a cocklebur that receives only one short day in its entire life may flower.

The chain of reactions leading to flowering, once they are started by a short-day treatment, can proceed to completion in long days. This does not mean that these reactions might not go faster if more short days were given.

It permits us, for experimental convenience, however, to deal with this initial effect of light without becoming involved with the complexities of the many reactions that make up the flowering process itself.

THE MECHANISM by which light acts to control flowering received attention immediately after Garner's and Al-lard's discovery of photoperiodism.

Scientists soon learned that the day-length stimulus is received by the leaves and that its controlling action on flowering is transmitted in some way through the leafstalks to the growing points of the stem where the flowers are formed. They found they could bring about flowering in some short-day plants by subjecting a single leaf to short days even though all other leaves receive long days. That fact indicates that a flower-promoting stimulus is produced in the short-day leaf not a flower-inhibiting one in the long-day ones.

Many workers have searched without success for a flower-inducing hormone in leaves of photo-periodically induced plants.

Because plants flowered on some photoperiods and not on others, it was evident that plants were able to measure time. Whether they measured duration of darkness or light, however, was not apparent until experiments were performed in which each long dark period of daily short-day cycles was broken into two short dark periods by insertion of a few minutes of light near the middle. The effect, which was equivalent to that of a treatment with long days and short nights, inhibited the flowering of short-day plants and promoted that of long-day ones.

The opposite kind of experiment, in which a short period of darkness was placed in the middle of a long light period, resulted in no detectable difference in plant response.

These experiments showed that the time measured was the dark period. The effectiveness of the brief period of light during a long dark period depends markedly on whether the light period is placed in the middle of the dark period or elsewhere.

It has maximum effect if it is placed near the middle. It may have no observable influence on flowering if it comes near the beginning or end of the dark period.

Flowering of many short-day plants, such as soybean, chrysanthemum, and Japanese morning-glory, can be completely inhibited checked and held up by less than a minute of light of 25 to 50 foot-candles in the middle of dark periods at least 12 hours long. Long-day plants, such as barley and other small grains, are induced to flower by similar light treatments in the middle of 12-hour dark periods.

The discovery of this remarkable responsiveness of plants to irradiances of such brief duration and low energy suggested that a further way to investigate the nature of the light reaction would be to interrupt the dark periods with light of narrow wavelength limits and of known energies.

Such experiments done quantitatively show, for example, whether the photo-period reaction depends on light absorption by chlorophyll or by some other pigmented substance that has light-absorbing characteristics different from those of chlorophyll. The method thus permits one to learn whether responses of plants to light that seem quite different superficially are controlled by the same photo-reactions or by different ones.

The procedure involves measuring the minimum light energy required at each wavelength position to cause a particular response, such as promotion of flowering of long-day plants or inhibition of flowering of short-day ones.

Such experiments require special equipment to obtain light that is sufficiently pure and has enough energy to cause the plant to react. This is sometimes done by the use of light filters that permit passage of only the wavelengths desired, or it may be done by passing a strong beam of light through a spectrograph, an instrument containing a system of mirrors and prisms arranged to produce a spectrum. Such an instrument, when illuminated with very high-intensity light from a carbon-arc or other high-intensity source, produces a spectrum of sufficient size that small plants or whole leaves of larger ones can be irradiated with enough light of the desired wavelength range to cause the plant to exhibit some developmental response that we can measure.

EFFECTIVENESS of the different colors of light was measured with such an instrument for several long- and short-day plants. The results were remarkably similar.