Comparative Effects of 2,4-Dichlorophenoxyacetic Acid on Nitrate Metabolism in Corn and Cucumber

Abstract

Since the first demonstrations of the herbicidal action of 2,4-D, many possible mechanisms for the toxicity have been proposed. These include changed respiratory metabolism, abnormal cellular prolifer-ation, and interference in nucleotide metabolism. It has further been recorded that nitrate metabolism is altered in response to applied 2,4-D. In many broad leaved weed species (14, 5) and in the leaves of sugar beet plants (2,13) nitrate was reported to accumu-late following 2,4-D application. In contrast no record of nitrate accumulation has been reported for plants of the Gramineae family. The data of Gentner (7) indicates that plant species vary greatly in their resistance to damage by post-emergence treatment with 2,4-D. A comparison of plant species that have been reported to accumu-late nitrate following 2,4-D treatment (5, 14) sug-gested to the authors that plants that accumulate nitrate are most susceptible to damage by 2,4-D and conversely. Of the plant species listed by Gentner (7), corn was one of the most resistant and cucumber one of the most susceptible to damage by 2,4-D treat-ment. As part of a general study into the mechanism of nitrate metabolism a comparison was made between the effects of 2,4-D treatment on nitrate utilization in corn and cucumber plants. Materials and Methods Greenihouse and Growth Chamber Studies. One hundred seeds of either Zea mays (var. Hy2 X Oh7) or Cucumis sativus (var. Long Green) were planted in a fertile soil-peat mixture contained in a flat (20 X 13 X 3 inches). The soil was irrigated as re-quired with tap water adjusted to pH 6.0 with nitric acid. In the greenhouse, 18 hours of supplemental light (1,400 ft-c) were provided by banks of fluo-rescent lamps. Minimum night temperature was 180 while day temperature varied from 240 to 320. The growth chamber was regulated so that the night and day temperatures were 160 and 270 respectively. Banks of fluorescent lamps combined with a few in-candescent lamps provided 4,000 to 4,500 ft-c of light. The photoperiod was 16 hours. Spray treatments were made when the cucumber seedlings had 2 expanded leaves and the corn seed-lings had reached the 3 leaf stage (approximately 2 weeks after sowing). A deVilbis hand atomizer modified by direct attachment of a compressed air line was used to apply 100 ml of 4.5 X 10-4 M 2,4-D to each flat. Comparable flats of seedlings left un-sprayed or sprayed with equivalent amounts of water were used as controls. Leaf tissue was collected for assay 5 hours after treatment. Variations in 2,4-D concentration and harvest time are detailed in the individual experiments. A neutral potassium salt solution of 2,4-D was used in all experiments except the first of the 3 spray treatments of field grown corn, when the amine salt was inadvertently used. Field Studies. Seeds (Hy2 X Oh7) were hill planted at the rate of 32,000 plants per acre on May 24. There were 28 rows, each with 10 hills in the plot. Spray treatments were confined to one-half of the plot in order to minimize drift onto control plants. Rows were arbitrarily selected from respective halves of the plots for treatment or control. Spray treat-ments were made with a hand sprayer and 2 liters of 2,4-D solution were applied as uniformly as possible over the entire leaf surface of the plants in 2 rows. The first application of 2,4-D at 2 concentrations (2.25 X 10-4 and 4.5 X 1O-4M was made at 8:00 AM on June 29 and 2 subsequent treatments were made at 8:00 AM on July 13 and 27. A composite sample was obtained by removing at random the up-permost fully expanded leaf from 3 plants from a single row. One sample was collected from each of 2 treated and control rows at various times after spray treatment. Plants were sprayed once and no plant was sampled more than once. Corn and cucumber seeds were planted in the same field on August 22 to provide seedlings at a comparable stage of growth to those used in the initial greenhouse treatments. Two rows each 10 ft long and 36 inches apart were used for each species. Seeds were spaced at one-half inch intervals in the rows. Germination and growth of corn and cucum-bers was slower than anticipated due to unfavorable environmental conditions. On September 5 one half of one row of each species was sprayed with 9 X 10-4 M 2,4-D at the rate of 100 ml of solution per 675