Other Effects

Scientists have found other potential uses for growth regulators. Bananas have been made to ripen faster by spraying them with a mixture of 2,4-D and water. Experimentally, scientists have made pears and Grimes Golden apples ripen more evenly, and parsnips to cure in a relatively short time after harvest, by spraying or dipping the fruits and vegetables in water mixtures of the chemical.

It is possible also that the keeping qualities of apples may be improved. The skins of stored apples often develop brown patches soon after they are removed from storage and placed on the market. The disease is known as apple scald. Drs. H. A. Schomer and Marth found that some kinds of apples scalded much less severely after the fruit was dipped in a lanolin emulsion containing naphthaleneacetic acid than when untreated.

Tests have been mad-, to determine whether other kinds of fruit can be stimulated as greenhouse tomatoes are. On an experimental basis, Drs. Marth and E. M. Meader stimulated the growth of fruit of some varieties of blackberries by spraying the flowers when they open with the water mixture of naphthaleneacetic and naphthoxyacetic acids. This practical use of growth regulators is being subjected to further tests, and shows promise as a means of improving yields.

Growers of pineapples find that too many of the fruits ripen at about the same time, and sometimes the facilities for processing are thereby taxed to capacity. It has been found that pineapples can be made to flower and ripen at an early date by spraying them with naphthaleneacetic acid or 2,4-D. Some growers spray plants in part of their fields with these chemicals so as to make the fruits mature at different dates, and they can stagger the harvest periods for the different sections of the planting.

In Puerto Rico, Dr. J. van Overbeek discovered that plant growth regulators can be used to advantage there in the production of pineapples. The Red Spanish variety, the principal one grown in Puerto Rico, produces a large percentage of flowers during the winter when the days are relatively short. Even immature plants of the variety often flower and bear small and inferior fruits. But plants of the Cabezona variety are relatively slow to flower. They may even grow for 5 years before they produce fruit. Puerto Rican scientists found that Cabezona pineapples can be stimulated to produce flowers and fruit during any month of the year by spraying the center of the plant with minute amounts of naphthaleneacetic acid or 2,4-D. Fruits of especially high quality can be produced in this way since the plants can be made to flower after they have produced a full number of leaves and reached the most suitable growth.

In the preparation of Christmas decorations, difficulty is sometimes experienced in shipping wreaths made of holly because the leaves fail to remain on the branches until they reach the market. In experimental work by J. A. Milbrach and H. Hartman this difficulty has been lessened by spraying the leaves with a water mixture of 0.01 percent of naphthaleneacetic acid.

In experiments at the University of Wisconsin by T. C. Allen and E. Fisher, the yield of wax beans and of Refugee beans was increased by dusting the plants with talc containing naphthaleneacetic acid. The reason : The dusted plants retained most of their flowers, while the untreated plants lost many flowers because of insect injury or adverse weather conditions. The experimenters found that the insecticides rotenone and pyrethrum could be added to the dust containing the growth-regulating substances so as to control some insects without impairing the effectiveness of the dust mixture in increasing bean yields.

Natural fertilizers like manure are sometimes used for top dressing on areas seeded to grass. Weed seeds, usually found in manure, are a problem because they germinate and sometimes make it difficult for the grass to become established. Department and other scientists applied 2,4-D to shredded manure on an experimental basis, and thus prevented subsequent growth of most of the weed seeds that it contained. By such a pretreatment, it may be possible to kill or prevent the subsequent growth of some kinds of weed seeds in manure that is used as fertilizer.

In the laboratory scientists have learned that the chemical composition of some kinds of plants changed rapidly after the plants were sprayed with growth-regulating substances. The leaves of bean plants, for instance, contained nearly twice as much sugar on the fourth day after they had been treated with naphthaleneacetic acid as did unsprayed ones. Although no practical use has yet been made of this type of response, scientists are experimenting further in an attempt to learn how growth regulators can be used to control the chemical composition of plants.

In growing useful kinds of molds, it is sometimes difficult to prevent some kinds of bacteria from interfering with the cultures, the growing colonies. Scientists have found that the growth of some kinds of bacteria can be retarded by adding 2,4–D to media upon which the molds are growing. They believe that in this way it may be possible to protect mold cultures against infection from some kind of bacteria.

THE AUTHOR

John W. Mitchell is a physiologist in charge of the work on plant-growth-regulating substances at the Bureau of Plant Industry, Soils, and Agricultural Engineering. Dr. Mitchell has done research on the composition of plants and the effect of growth-regulating chemicals on them. He is a graduate of the University of Chicago.