Effects of 2,4-Dichlorophenoxyacetic Acid Upon the Nitrogen Metabolism and Water Relations of Soybean Plants Grown at Different Nitrogen Levels

Abstract

1. High- and low-nitrogen soybean plants grown in nutrient solutions were treated by introduction of sodium 2,4-dichlorophenoxyacetate (2,4-D) in the nutrient solutions at a concentration of 4 p.p.m. of the acid equivalent. After exposure to 2,4-D for 22-23 hours the plants were placed in newly prepared solutions similar to those used before the treatment. Harvests were made at the start of 2,4-D treatment and 1, 4, and 7 days later. Plants of each group were divided into leaf, stem and petiole, and root fractions, and fresh and dry weights of each fraction were taken. Analyses for total, soluble, amide, alpha-amino, ammonium, and nitrate nitrogen were made on the tissue of each fraction. Changes in distribution of the different forms of nitrogen in the organs following treatment were similar for both high-N and low-N plants. For brevity, the nitrogen analyses for the high-N plants only are reported. 2. Twisting of stem tips and epinasty of petioles, later followed by yellowing of leaves and swelling of stems and petiole bases, and ultimately by wilting and drying of leaves, were visible responses to 2,4-D treatment and appeared sooner in high-N than in low-N plants. 3. Neither high-N nor low-N plants showed any appreciable change in total dry weight after exposure to 2,4-D, in contrast with respective untreated control plants which continued to accumulate dry matter. 4. By the end of the 23-hour period following the start of exposure to 2,4-D, the percentage dry matter of leaves of treated plants was very significantly less than that of corresponding controls. Some days later, however, leaves of the treated plants wilted and showed a much higher percentage of dry matter. Possible factors affecting the water relations of the treated plants have been mentioned. 5. Treatment with 2,4-D apparently changed the absorbing capacity of roots as indicated by failure of treated plants to increase their content of total nitrogen, including nitrates, after exposure to 2,4-D. 6. Plants treated with 2,4-D showed a decrease in the percentage of protein nitrogen in the leaves and an increase in the concentration of this fraction in stems and roots as compared with untreated controls. At the same time, all organs of treated plants showed increases in soluble organic nitrogen fractions, particularly in undetermined forms classed as residual soluble organic nitrogen, over corresponding controls. Apparently some of the protein in the leaves of treated plants was hydrolyzed, and the products were translocated to the stems and roots where more protein was resynthesized. These results suggest that 2,4-D may affect proteolytic enzymes differently in leaves than in stems or roots.