Uptake, translocation, and metabolism 2,4-D in enlist crops and control of drought-stressed waterhemp (Amaranthus tuberculatus) with 2,4-D and glyphosate

The synthetic auxin herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), is one of the oldest and most widely-used herbicides in the world and is a systemic, postemergence (POST) herbicide that is selective in monocots such as corn, wheat, sorghum, and turf, but dicots are sensitive to 2,4-D. Auxin herbicides mimic and induce similar responses as the natural phytohormone, indole-3-acetic acid. Sensitive dicots are controlled through a cascade of events derived from elevated cellular levels of 2,4-D, resulting in increased phytohormone levels, reactive oxygen species, cell membrane injury, and ultimately plant death. Selectivity of 2,4-D is derived from metabolism differences between monocots and dicots. Until recently, 2,4-D could not be used in dicot crops like soybean (Glycine max) and cotton (Gossypium hirsutum); however, genetically-modified varieties are being developed. The Enlist Weed Control System™ developed by Dow AgroSciences will confer 2,4-D resistance in several crops including corn (Zea mays), soybean, and cotton. Resistance is derived from insertion of a transgene from the soil bacteria enzyme family, aryloxyalkanoate dioxygenase (AAD), which can metabolize 2,4-D to the nonherbicidal metabolite, dichlorophenol. During the development of Enlist soybean, injury was observed to treated leaves following certain herbicide applications, which is atypical of synthetic auxins. When the premixed product, Enlist Duo (2,4-D choline, glyphosate, and specific adjuvant package (Adj.)) or the tank-mixture of 2,4-D choline + glyphosate + Adj. was applied to Enlist soybean, small necrotic spots formed on treated leaves, but injury to Enlist corn was less frequent. Injury to Enlist soybean was unexpected due to the previously determined rapid rate of 2,4-D metabolism by the AAD enzyme. To develop a better understanding of how and why this injury occurs in Enlist soybean, research was conducted to measure uptake, translocation, and metabolism of 2,4-D in Enlist soybean, Enlist corn, and non-transformed varieties utilizing radiolabeled 2,4-D in a whole-plant and an excised-leaf assay. It was concluded that enhanced 2,4-D uptake with the Enlist Duo treatment leads to injury in Enlist soybean. In both Enlist crop varieties, the rate and/or amount of 2,4-D metabolism were greater relative to non-transformed varieties and glyphosate did not affect 2,4-D metabolism. In the Enlist Duo treatment, 2,4-D uptake is very rapid and greater than with other treatments in soybean. In corn, 2,4-D uptake levels were much lower than detected in soybean, and surprisingly Enlist Duo resulted in the least amount of uptake. When the concentration of free 2,4-D was reduced in soybean by utilizing the ester formulation of 2,4-D, injury to Enlist soybean was eliminated. By tank mixing a chloroacetamide herbicide with Enlist Duo, injury was observed in Enlist corn and 2,4-D uptake was increased compared to using Enlist Duo alone. Excessive 2,4-D uptake levels derived from Enlist Duo result in injury to Enlist soybean. The rapid influx of 2,4-D into soybean may overwhelm or exceed the metabolic capacity of the AAD enzyme, resulting in a pool of free 2,4-D acid in the soybean plant. Injury is reduced when 2,4-D ester is used instead of 2,4-D choline because 2,4-D ester must be converted to the active form of 2,4-D acid by esterases located within the cuticle and/or apoplast, limiting the influx and amount of 2,4-D that must be metabolized by AAD. Reducing 2,4-D uptake in Enlist crops by altering the adjuvants or changing the formulation of 2,4-D would reduce the risk of crop injury, but 2,4-D uptake in weeds and efficacy may be compromised. Weed control with POST herbicides can be affected by many factors including the growing conditions during the application. An environmental condition known to alter POST herbicide efficacy is drought. Plants under drought stress (water stress) tend to be more difficult to control compared to unstressed plants. With predicted climate changes, a greater potential for periods of less rain or more frequent droughts may drive the need to maintain weed control levels when plants are drought stressed. The objectives of this research were to determine the effect of drought stress on waterhemp (Amaranthus tuberculatus) control POST with 2,4-D and/or glyphosate, and to define potential differences between the two herbicides in relation to drought stress and POST activity. Greenhouse assays and a whole-plant assay utilizing radiolabeled herbicides were conducted to investigate levels of waterhemp control in relation to varying drought stress levels, timing of the stress, and their effect on herbicide uptake and translocation. Levels of waterhemp control were determined in the greenhouse with varying rates of 2,4-D and glyphosate (less than labeled rates; termed “low” and “high”), as well as tank mixtures of these two herbicides, under varying levels of water stress created by watering the plants with 10, 20, or 40 mL of water per day. Another greenhouse assay was utilized to determine waterhemp control with the two herbicides under different timings of water stress. The drought condition occurred either one week before the herbicide application, one week after the herbicide application, or during the full two-week period. Herbicide efficacy increased as the amount of water supplied per day increased. At high-stress levels, the reduction of waterhemp dry matter was greater with 2,4-D-low compared to glyphosate-low and was equivalent to glyphosate-high. Herbicide efficacy was greatest when the drought stress occurred before the herbicide application and when plants were watered to saturation after the application. When the drought stress occurred immediately after the herbicide application, waterhemp dry matter levels were equal to plants held under drought stress both before and after the herbicide application. Uptake and translocation of radiolabeled glyphosate was significantly less in plants under drought stress, while uptake and translocation of radiolabeled 2,4-D was not altered. Significant dry matter reduction in waterhemp plants under drought stress is possible with 2,4-D compared to glyphosate at the rates examined in the greenhouse study. Greater uptake and translocation of 2,4-D in drought-stressed waterhemp plants may have contributed the greater herbicide efficacy achieved with 2,4-D compared to glyphosate. With both herbicides, higher efficacy levels were attained when plants were not stressed after the herbicide application. Whenever possible, timing POST herbicide applications in accordance with rainfall events and selecting the appropriate herbicide-adjuvant combination can increase waterhemp control, even if the plants are stressed prior to the application