Plant Growth Regulators

by JOHN W. MITCHELL

ALITTLE more than a decade ago scientists learned that they could change the way some plants grow by putting a chemical on the stems. The substance they used was one that they had just isolated from human urine, but they found out later that some of the crystals they had so laboriously purified were those of indoleacetic acid, a substance that had been known for years and had been made in the laboratory.

They discovered that plants, somewhat like animals and humans, are sensitive to chemical stimulants that can be made readily in the laboratory and factory. Because plants respond by changing their rate or type of growth, the chemicals are called growth-regulating substances, or growth regulators. So far they have been produced synthetically, only; they have not been found in green plants.

There are, however, some regulators of growth that plants do produce. They are properly called plant hormones. They are not secreted by special glands, as animal hormones are; these natural growth regulators are made in certain parts of the plant, such as leaves and buds. They are often moved through the plant from one part to another. Scientists believe they govern growth in plants much as animal hormones regulate growth in animals. The true plant hormones have not yet been made in the laboratory; in fact, only small amounts of them have ever been isolated from plants. But we are trying to extract some of them in larger amounts, so we can learn whether plants treated with true plant hormones will behave in a way that might be useful in the production of some kinds of crops.

Since the first discovery, scientists have found that plants are stimulated to respond in various ways by more than a hundred different organic chemicals that are not found in plants but that can be made in the laboratory. The way plants behave when treated with the chemicals depends on the kind of growth regulator used and on the amount and way it is applied. For instance, if only a speck, about one-millionth of an ounce, of 2,4–D ( chemically called 2,4–dichlorophenoxyacetic acid) is put on one side of the stem of a bean seedling, the cells along the treated side grow faster than those on the untreated side, and the plant will bend sharply in a direction away from the treated surface.

If, however, about 2,000 times that amount of 2,4–D ( about as much of the powder as can be held on the flat side of a toothpick for a distance of an eighth of an inch from the blunt end) is mixed with a little lanolin and the mixture rubbed on a tender section of the stem, the plant responds differently. Food materials within the plant are moved from other parts of the stem, and possibly from some of the leaves, into the treated section, where many new cells are formed. The new cells finally become organized and arranged so that they form new roots inside the stem. The young roots, called root primordia, later push their way to the outer surface of the stem, and if that part of the stem is covered with moist soil the primordia will grow out into it and function as ordinary roots do in supplying the plant with water and nutrients.

If, on the other hand, the above-ground parts of the plant are sprayed or dusted with 2,4–D, the response is yet different, for leaf growth ceases, the rate of respiration of the plant is increased, and its reserve food materials are broken down and subsequently burned up. As a result the plant generally dies within I to 3 weeks after treatment, or the length of time required for its reserve food materials to be depleted.