Growth Regulators and Greenhouse Tomatoes

Growers who specialize in growing tomatoes in greenhouses in the winter have, besides the usual problems of soil fertility and disease control, the additional problem of how to make the young tomato fruits hang on the vines and grow vigorously. Because there are few insects or air currents in greenhouses in winter to do it for them, the growers must pollinate the tomato flowers by shaking the individual flower clusters. But even carefully hand-pollinated flowers often fail to set a high percentage of fruit, especially when the weather is cloudy. Often that failure is due to the failure of the flowers to produce enough good pollen. Vigorous pollen, the kind that is generally developed during the summer by field plants, serves to fertilize the fruits so that seeds develop, and also apparently contains substances that stimulate the young fruit to grow rapidly—something the weak winter pollen often does not do.

Growth-regulating substances are the answer. With them, the young fruit can be artificially stimulated and often made to stick on the vines and grow vigorously. Not all of the fruits, of course, will develop seeds, because some of the flowers are not pollinated, but in size and flavor the sterile ones often surpass the pollinated fruits. The safest and most widely used way now to apply the chemicals for this purpose is to spray the flowers, when they are fully open, with an emulsion containing 1 to 2 percent lanolin (fat from sheep's wool) and 0.2 percent of indolebutyric acid. If entirely seedless fruit is wanted, it is necessary to cut off the stamens, the male flower parts, at the time of treatment.

Indolebutyric acid is relatively mild in its effect as a growth regulator. A number of compounds (naphthoxy and phenoxy) are many times more active than indolebutyric acid in effecting some kinds of plant responses. These have been tested for use in the production of greenhouse tomatoes. Of the more potent ones, naphthoxyacetic acid holds some promise, especially when mixed with indolebutyric acid. Most of the other active compounds cause outer parts of the fruit to grow faster than the inner parts. As a result, pockets form in the fruit and remain unfilled, and the fruit is of lower quality.

At first, small amounts of a mixture of growth regulator and lanolin were rubbed by hand on each tomato flower. A time-saving improvement is the use of a small atomizer with which an emulsion or solution of the hormone can be sprayed on the flower clusters. The most recent method, still in the experimental stage, is the aerosol bomb, a small metal tank that is-partly filled with the growth regulator together with a liquid (such as dimethyl ether or Freon) that produces high pressure inside the container.

Frequently, the flowers of tomato plants grown in the field also fail to set satisfactorily. That may be due to factors other than poor pollen—such as relatively high night temperatures or cloudy weather that cause Powers of plants grown in very fertile soil to drop. So far we have had no consistent success in using the chemicals for improving, the set under such field conditions.

Of course, the growth regulators, effective as they are, cannot replace sound cultural practices like good soil, proper temperature, and sunlight.

Rooting Cuttings

The stems of many kinds of plants can easily be stimulated to produce roots. For example : If the stem of a tomato vine is bent so that it rests in a horizontal position near moist ground, it will produce many roots along its lower surface. Or, stems of some climbing plants, like ivy, can be made to root by simply fastening them close to a building or tree trunk.

On the other hand, some plants of considerable economic importance, among them apple trees and pines, can be rooted only with great difficulty and under exacting conditions. Because the multiplication of many such plants is most quickly and cheaply done through the use of cuttings, it is important to improve methods of propagating them in this way.

The young stems of many kinds of plants have been stimulated by growth regulators to produce roots. Growth regulators are of considerable value to the plant propagator, since cuttings that root readily can often be made to root even faster by means of such treatments. Those that root with difficulty can often be made to produce a sturdy root system in less time than would be required without the chemical treatment. Indolebutyric acid and naphthaleneacetamide are the compounds most commonly used for the purpose.

Cuttings can be treated by dipping their basal ends in a solution of one of the substances, or by dusting the ends with talc that contains them. Formulations to be used for this purpose are available at most stores that handle farm supplies.

W. D. McClellan and Neil W. Stuart, working at Beltsville, found that growth regulators stimulated the rooting of lily scales, which are used as cuttings. Lily scales, like some other kinds of fleshy cuttings, sometimes rot in the ground. To prevent such loss, succulent cuttings are generally treated before planting with a fungicide, which prevents the growth of some kinds of molds. Drs. McClellan and Stuart added the growth-regulating substances to the fungicide and thereby decreased mold infection and also stimulated root production at the same time.

Growth regulators, as we have seen, can be stimulants when they are used in minute amounts. But when they are applied in larger doses the chemicals create so strong a reaction in many kinds of plants .that they are severely injured. In fact, only a small amount of a growth regulator such as 2,4–D is needed to kill some plants. There are others that show little injury when the chemical is applied to them. For reasons yet unknown, most kinds of grasses are of the resistant type. Scientists are now busy trying to find out how growth regulators kill plants; the more thoroughly the technicians understand the effects of the chemicals, the more intelligently they can put them to work killing weedy plants.