Effect of 2,4-Dichlorophenoxyacetic Acid on Root Development in Young Corn Plants

Abstract

1. Seven varieties of sweet corn were planted in the garden. When the plants were approximately 9 inches tall and showed four leaves expanded, they were sprayed with a 1% concentration of the butyl ester of 2,4-dichlorophenoxyacetic acid. A second and third series with several of the same varieties were grown in the greenhouse. They were similarly sprayed. Two stages of growth were treated: young plants with four leaves expanded as in the garden, and older plants with seven leaves expanded. Gross observation and histological collections of the three series were made over a period of 5 weeks. 2. As a result of treatment the leaves showed spray injury, and the youngest exposed leaf in the older plants often remained rolled in the bud and checked the growth of inclosed leaves. Bending of the stem at the nodes was noted in both young and older plants. The major effect of treatment was swelling at the nodes, with stimulation and malformation of associated adventitious roots which in the lower internodes normally form the permanent system of brace roots of the corn. In young plants the third, fourth, and fifth whorls of adventitious roots above the fourth, fifth, and sixth nodes, respectively, were affected. Response varied from extreme fasciation to slight fasciation and the formation of numerous roots. In older plants the lower whorls of roots were unaffected, but at higher nodes-the sixth, seventh, and eight-swelling and incipient root formation occurred (see table 1). 3. Histological study showed that tissues lying in the meristematic zone at the periphery of the stelar area were stimulated to excessive proliferation. In early stages meristematic activity was diffuse, but in later stages three regions became evident. These regions corresponded with tissues derived from the three histogens found in a corn root tip but lacked the precise organization of a primordium. Root fasciation appeared to be a consequence of this lack of organization. Response was histologically similar in the young and older plants, but proliferation occurred in the fourth, fifth, and sixth internodes of young plants and in the sixth, seventh, and eighth internodes of older plants. 4. A comparison of the results in the two treatments leads to the conclusions that (a) 2,4-D induces a major disturbance in root formation in the young corn plant, (b) the response is localized in correlation with the leaves sprayed, and (c) young structures are critically more sensitive than older structures. 5. These three conclusions, determined under the conditions of this experiment, have some practical implication. The corn plant is highly susceptible to 2,4-D injury to the permanent root system when foliar sprays are applied to the first to fifth foliage leaves in early stages of growth. Older plants are less susceptible to this injury. 6. Response in the corn plant is discussed in relation to response in other monocotyledons and in dicotyledons.