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Dinitroanilines

Introduction

General Information about the Dinitroanilines.
The herbicides in the dinitroaniline family were originally discovered in evaluations of dyes and dye chemical synthesis intermediates (don't forget the debt we owe Jack the Ripper for starting this industry).

Chemicals in this family are typically bright yellow in color due to the two nitro (-NO2) groups on the phenyl ring, and are often refered to as the "yellow compounds".

The first, and most important, herbicide in this family is trifluralin which was introduced first in 1964. Trifluralin is a major herbicide in soybean production, and it had a big impact in that crop.

Herbicides such as trifluralin, pendimethalin, oryzalin, etc. in this family generally are used to control grassy weeds, as well as some dicot weeds like pigweeds and lambsquarters. Other dicots are not controlled, such as ragweeds and smartweeds.

Herbicides in this family also have some soil fungicidal activity.

Chemistry

This herbicide family is divided into two subfamilies:
-Methylaniline herbicides include trifluralin, pendimethalin, benefin, dinitramine, fluchloralin, profluralin, etc.

-Sulfonylaniline members include oryzalin, nitralin, etc.

Physiology and Metabolism of the Dinitroanilines in Plants

Mode of Dinitroaniline Action

Dinitroaniline herbicides act by inhibiting cell division (mitosis). Specifically, they inhibit microtubulin synthesis necessary in the formation of cell walls and in chromosome movement to daughter cells during mitosis. The cell does not complete division and affected cells remain as single cells with multiple nuclear chromosomes: multi-nucleated cells.

Mode of Dinitroaniline Lethality:

Dinitroaniline herbicides kill susceptible plants by inhibiting cell division in root cells, which arrests normal root growth. This inhibition leads to plant dehydration due to severely restricting the root system size and function.

Uptake and Movement of Dinitroanilines in Plants

Dinitroaniline herbicides are absorbed somewhat by plant root systems, and to a greater extent by young seedling shoot organs such as the hypocotyl or coleoptile.

Little or not translocation of these herbicides occurs in plants.

These herbicides are very lipophilic (oil loving) and tend to concentrate in high lipid areas of the shoot and root symplast. It is in these areas they exert thier toxic action.

Basis of Selectivity between Plant Species

Metabolism. Dinitroaniline herbicides are not metabolized by plants, susceptible or resistant.

There are three primary bases of selectivity between susceptible and resistant plant species.

Uptake Differences.
The first is greater uptake of dinitroaniline herbicides by susceptible species. This greater uptake can result in greater injury to roots developing at the site of uptake. Differential protection, and resistance, is afforded by differences in lipid content of species. Because these herbicides are very lipophilic, they can be sequestered and rendered unavailable for plant injury, by partioning into this lipid component.

Placement Selectivity.
A second basis of selectivity between susceptible and resistant species is physically separating the herbicide layer in the soil from the crop plant but not from the weeds. This system is used in growing small grains such as wheat in the prairie regions of Canada and the U.S. Two methods are used.
-Trifluralin is applied to 7-10 cm (3-4 in) tall wheat and shallow incorporated into the soil with a drag harrow. The herbicide is in contact with surface germinating weeds yet the established wheat plant has sufficient root system below the herbicide to survive and grow.
-A second method is fall application of trifluralin. Weeds are controlled in the early spring by residual herbicide in the soil yet there is not enough left to injure spring planted small grain crops.
Neither of these systems is perfect and some injury is possible.

Trifluralin Resistance.
Resistance to trifluralin has been discovered in goosegrass (Eleucine sp.; southern USA) and green foxtail (Manitoba, Canada). Resistance in these new variants was 1000 to 10,000 fold greater than that in the susceptible wild type. Resistant variants looked the same as the susceptible type. Three different resistant patterns in species was discovered: resistant, intermediate resistance and susceptible. Early research revealed the basis of resistance was not due to differences in translocation or to lipid content (sequestration). The basis of resistance was found to be due to hyperstabilization of microtubules, rendering them immune to dinitroaniline inhibition. Resistant cells form microtubules in the presence or absence of dinitroaniline herbicides. The mechanism of resistance in the intermediate variant has not been determined, it is not the same mechanism as that in the resistant type. Carrots (Daucus carota) naturally has this hyperstabilized microtubule form and high levels of dinitroaniline resistance.

Fate of Dinitroanilines in the Environment

Soil.

Dinitroaniline herbicides are strongly adsorbed to soil colloids. Use rates increase with increasing organic matter content and on heavy, clay soils.

Many of the herbicides in this family must be mixed, incorporated, into the soil to avoid volatilization losses, therefore preplant incorporation (PPI) is the most common method of field application. Once incorporated into the soil, the herbicides volatilize and fill the soil air spaces. It is most likely in the gas phase that these chemicals are taken up by plants.

Trifluralin is fairly long-lived in the soil and can persist for 4-6 months after application. Some concern exists that trifluralin used in soybeans can persist in the soil until the following year and cause early season injury to corn. Factors such as the rate used, time of herbicide application, soil type, herbicide degradation rate, soil moisture, sensitivity of the corn genotype, etc. can lead to greater or lesser corn injury from this carryover. Trifluralin also can carryover and harm a subsequent winter wheat crop (below):

Water.

Dinitroaniline herbicides are lipophilic and fairly insoluable in water. They pose little threat as ground- and surface-water contaminants in the environment

Air.

Most dinitroaniline herbicides are volatile and will evaporate into the air if not incorporated into the soil fairly soon (hours) after application. Herbicides such as pendimethalin and oryzalin are exceptions to this.

Dinitroaniline herbicies are subject to decomposition due to photodegradation. This factor is probably not of agronomic importance owing to the volatility losses occuring before light degradation. It probably is an important degradation mechanism in the air environment after evaporation.

Animal & HumanToxicology.

The synthesis of trifluralin can result in the synthesis of nitrosamines, a harmful and undesirable byproduct. Nitrosamines are highly reactive chemical species. They are metabolized to acid (HNO2), which acts in biological systems by deaminating molecules. They have been implicated as carcinogens by removing amino groups from DNA chromosome nucleotide bases (i.e. cytosine, adenine, tyrosine, guanine). They also can act as toxic alkylating agents.

Plant Injury Symptomology of Dinitroanilines in Plants

The most typical injury symptoms are stunting and restricted root development (below).

Root systems can be inhibited, roots may appear swollen (left). Root tips often appear short, swollen and "club-shaped" due to inhibition of root growth.

Shoots may also be affected. Dicot shoots often will have a swollen or cracked hypocotyl region, near the soil surface (left).

Injured corn can appear stunted (below) and leaves sometimes have a purplish color around the margins.

Plant emergence can be delayed or stopped entirely.

Injury can also occur to susceptible plants due to persistence in the soil, excessive application rates, poor soil incorporation and cool spring weather leading to slow plant growth.

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