Growth Regulators, Stimulants, and Seeds

PAUL C. MARTH AND JOHN W. MITCHELL.

SOME KINDS of woody plants have been made to grow six times faster than normal by treating them with one of the chemicals we call plant-regulating compounds or plant regulators.

Minute amounts of the compounds can alter the growth and behavior of plants in many ways. They can increase or restrict or otherwise influence the production of flowers and seeds. Many of the effects are as striking as those produced on animals by animal hormones.

A number of the compounds have been used beneficially and safely for years. Others have been discovered recently, and methods of correctly using them are being developed.

It takes 10 to 18 years to produce flowers and fruits on some slow-growing plants, such as fruit, nut, and forest trees. That long period makes a problem for growers and plant breeders who work with them to produce new, good, disease-resistant varieties. Seeds of some wild desert plants, on the other hand, germinate, grow, and produce flowers in a few days after a light rain.

We have various theories as to why plants flower when they do. The discovery of a natural, universally occurring, flowering hormone, or "florigen," would offer a simple explanation.

A great difficulty in conducting research in this field has been the lack of methods of detecting and identifying natural hormones that we now know occur in tiny, changing amounts in different parts of plants.

Modern refined methods, such as chromatography (a technique that permits a scientist to remove, purify, and identify infinitesimal quantities of organic compounds that occur in plant tissues and which may be the causal agents for regulating growth); the use of radioisotopes (which are of aid in tracing the movement and chemical behavior of small quantities of organic compounds); and the adaptation of newly developed physical equipment (such as the ultra centrifuge, the electron microscope, and improvement in the spectrophotometer), have facilitated this research and are opening new approaches to the problem.

By the use of the older methods, which were relatively crude, scientists learned that naturally occurring plant regulators reached a low concentration in the young vegetative tips of plants just before they changed to the flowering state.

Scientists all over the world have long been searching for a simple, effective chemical method of altering growth so that the plant flowers.

The research has been especially successful with pineapple. Growers of pineapples now load their sprayers with a dilute solution of plant-regulating chemical and thoroughly wet the foliage. The plants show visible flowerbuds within a few weeks. The flowering of pineapples, which usually requires a year or more, has been shortened as much as 11 months by the spray treatment. It is not desirable to apply the plant regulator too early after planting, because small plants, chemically induced to flower too soon, ultimately produce small, undesirable fruits. Naphthaleneacetic acid (NAA) once was considered effective, but scientists have found that low concentrations of 2,4-D, which also is used widely as a herbicide, is less costly and more effective in causing pineapples to flower.

Many hundreds of acres of pineapple in Hawaii and Puerto Rico were sprayed commercially in 1959 to regulate flowering. The sprays are applied carefully so that the fruit is of a size to fit processing machines. If the sprays are applied too early, the fruit does not develop to full size. If they are applied too late, flowering is delayed and the fruit matures late and is too big.

In other words, one growth regulator can bring about opposite flowering effects, depending on the concentration and the time it is applied.

Plant regulators that differ chemically also produce opposite effects sometimes. N-meta-tolylphthalamic acid (a plant regulator that affects flowering), for instance, has induced numerous terminal flowers to develop in the greenhouse on varieties of tomato that ordinarily blossom laterally on the plant and never at the top. This chemical, applied under certain conditions, causes unusual fasciated, or doubled, flowers. The development of individual flower parts apparently can be drastically changed by the chemical.

THE GIBBERELLINS, potent growth regulators produced as a metabolic byproduct by the fungus Gibberella fujikuroi, a serious disease of growing rice, have stimulated research and widespread interest. They bring about a wide variety of responses on many different kinds of plants.

Rapid production of seedstalks and flowers within a few weeks after treatment occurred in numerous species of biennial plants, such as beets, carrots, radishes, celery, and cabbage, which require 2 years to flower. It is believed generally that under natural conditions of development of a seedstalk, bolting, or stem elongation, in these plants is due to the production of a natural plant hormone produced by the developing flowers.

Gibberellins, applied externally, apparently can substitute effectively for this hormone.

To demonstrate this phenomenon, F. Lona, of Parma, in Italy, placed the developing seedstalks of bolting radish plants in alcohol. He applied this extract to young radish plants and induced them to bolt in a manner and at a rate like those of comparable plants sprayed with gibberellin.

Gibberellins or substances like gibberellins occur rather widely in many species.

An early demonstration of their existence and potency was given in experiments by John W. Mitchell. In searching for plant hormones of high potency from natural sources, he found that the translucent juice in young developing bean seeds about 7 to 9 days after fertilization is an extremely rich source of what is now called gibberellin-like hormones. This research was done in 1950, long before the purified gibberellins were available in this country. An extract of one tiny bean seed (slightly larger than the head of a pin), applied to four young bean plants, caused them all to develop new stem growth at a rate 25 times greater than the controls. This rate of growth would be comparable to that resulting from a heavy application of gibberellin.

Another scientist of the Department of Agriculture, Frank H. Stodola, of the Northern Utilization Research and Development Division, Peoria, Ill., prepared the first pure gibberellin in the United States.

Biennial plants are not the only ones that produce earlier flowers when treated with gibberellic acid. Neil Stuart and others, in experiments conducted at the Agricultural Research Center at Beltsville, Md., were able to induce early flowering in more or less woody plants, such as hydrangeas and Pfitzer juniper.

Some plants require a period of cold treatment to stimulate production of flowers. Hydrangea plants, for instance, are generally stored 3 months at 40 F. before they are forced in the greenhouse to produce flowers for Easter or Mother's Day. Spraying the plants with dilute solutions of gibberellic acid brought about the flowering of hydrangea bushes without any cold treatment. The best flowers, however, were produced by plants that received a short cold treatment, 2 or 3 weeks, before they were sprayed with the substance.

Pecan trees produce flowers and fruits when they are about 18 years old. Breeding for new varieties of pecans is consequently a slow process. Research by L. W. Martin and S. C. Wiggans, of Oklahoma State University, indicates that this long waiting period may be shortened considerably by stimulating the rapid overall growth of the trees with gibberellic acid.

At Beltsville we increased the size of black walnut trees growing in a greenhouse from an average of 1.5 feet (untreated) to 8.5 feet (treated) by placing a narrow band of lanolin paste containing 1 percent of gibberellic acid around the stems one-half inch from the terminal bud. We applied the paste three times during the summer and spaced the treatments so that the terminal buds never ceased growing until the short days of fall.

Some trees, including the black walnut, produce a flush of spring growth, often less than a foot long. Then the buds become dormant. Before producing additional new growth, the buds generally must be subjected to cold or overwintering temperature. Treatment with gibberellin substitutes for the cold requirement, overcomes summer dormancy, and causes the plants to produce the equivalent of several years' growth in one season.

We do not know yet whether this stimulation in vegetative growth will mean earlier flowering and fruiting.

Plant breeders who work with various annual crops would like to obtain two or more generations in one season, thereby speeding up their program.

On the basis of research in California on barley, that is possible experimentally. Two crops of barley were obtained in one season by accelerating overall plant growth and maturity by applications of gibberellic acid.

CONSIDERABLE RESEARCH has gone into tests to find a chemical means of retarding flowering of crop plants. A few weeks' delay in the flowering of fruit trees, for instance, may avoid damage from frost. Some investigators have effected a delay of a few days by the use of spray applications of such chemicals as naphthaleneacetic acid and malefic hydrazide to apples, peaches, pears, and cherries.

The margin of safety with regard to injury to trees from the chemical is too narrow and the delay in flowering is too small to justify a recommendation of these chemicals for this purpose. The fact that they have been successful in some experiments, however, gives us hope that new and better chemicals or methods of application will be developed for this purpose.

Many vegetables grown for market or home use sometimes flower prematurely, and the edible crop is therefore not usable because of woody seedstalks.

The flowering of celery and cabbage grown for table use in Michigan is delayed by spraying the plants with a growth regulator at the time flower-buds are forming during cold temperatures, which favor flower initiation.

S. H. Wittwer, of Michigan State University, found that alpha-orthochlorophenoxyacetic acid spray at a concentration of 100 p.p.m. (parts per million) was effective on cabbage and celery.

In California the premature bolting of sugarbeets grown for processing has been retarded experimentally by the application of 2,4-dichlorophenoxyacetic acid (2,4-D) at 50 p.p.m. The time of application is critical.