A major characteristic with dinitroanilines is volatility, with trifluralin being the most volatile because of the addition of the trifluoromethyl groups . Pendimethalin is classified as moderately volatile with a vapor pressure of 1.25 10-3 Pa . Therefore, incorporation in soil after an application is encouraged either by rainfall, irrigation or mechanical incorporation to reduce volatilization and achieve adequate weed control with pendimethalin . The soil residual carryover to the following growing season after application of pendimethalin and other dinitroaniline herbicides can be concerning when practicing crop rotations that include susceptible crop species. As mentioned, soil persistence is relatively lengthy and if the environment does not promote degradation, then the compound can be present for the following growing season. Photodegradation is observed with pendimethalin but is aminor degradation pathway . Angeles et al. observed pre-plant herbicides, including pendimethalin, to persist in the soil longer than intended caused by cultural changes from flood irrigation to drip irrigation in processing tomato fields in the San Joaquin Valley, CA. Hanson and Thill investigated imazethapyr and pendimethalin soil persistence causing winter wheat injury after application on pea/lentil fields; however, vertical grow racks wheat injury and yield reduction was rate and location dependent. Mode of Action.
Excellent reviews evaluating dinitroaniline mode of action and behavior in plants have been performed by Parka and Soper , Appleby and Valverde and Chen et al. . The mode of action of pendimethalin and other dinitroanilines is inhibiting the mitotic pathway of cell division by preventing the assembly of the microtubules. Microtubules are protein-like organelles that are made up of α- and β-tubulin molecules which are heterodimers and form the mitotic spindle which orient cells and direct the cell division . During cell division, the microtubules create the mitotic spindle by polymerizing the tubulin dimers. In the presence of the herbicides the tubulin does not polymerize. The herbicide molecule will bind to the α-tubulin, which prevents the β-tubulin from docking and polymerization cannot be continued. The polymerization inhibition leads to the halt in cell division, which cause the accumulation of incomplete cells in one area and create the anatomical feature of swollen or clubbed root tips and stunted growing points. Dinitroanilines exclusively bind to plant cells and do not bind to animal microtubules . There is evidence which suggests dinitroanilines also inhibit the calcium uptake in the mitochondria. Cytoplasmic calcium is a regulator of cell cycle and redistribute among theorganelles and cytoplasm . Studies by Hertel and Marme demonstrated dinitroaniline compounds caused Ca2+ to accumulate in the cytoplasm at 10-4 M nmol mg-1 protein concentrations, which could cause interference with microtubule assembly and cell division.
However, Morejohn et al. reported oryzalin to depolymerize microtubules at very low concentrations . Therefore, Appleby and Valverde concluded it was unlikely that the effect of calcium regulation would depolymerize microtubules. The calcium deregulation may be a side effect of the dinitroaniline compounds that can affect plant growth when applied post-emergence, but is not the herbicidal mode of action. Additional evidence also suggest dinitroaniline herbicides can affect guard cell functions when applied on the plant foliage in lab studies. Marcus et al. demonstrated microtubule inhibitors prevented stomata guard cells to open, then, re-open after application of drugs that blocked the microtubule-inhibitors. Microtubules remain present in guard cells after cell differentiation and function as a guiding mechanism and signal mechanisms for the opening/closing of guard cells . Marcus et al. tested fusicossin, a drug that induces guard cell opening by activation of the proton pumps, after use of microtubule-inhibitors which caused the guard cells to reopen after being signaled to close by the microtubule inhibitors. These results may indicate the role of microtubules in signal transduction of protons like the Ca2+ activity in guard cells which acts to open and close stomata . The results support the observed regulation of Ca2+ by Hertel and Marme . Other effects from dinitroaniline herbicide applications include oxidative cell damage by reactive oxygen species that are created in response to the stress in treated plants and reduced absorption and translocation of nutrients in treated plants .
However, these effects also are observed in relatively tolerant crops and do not contribute to the primary herbicidal mode of action but can be important in suppressing plant growth. Weed Resistance. While resistance to dinitroaniline herbicides is limited, there have been cases reported on 12 weed species including Alopecurus aequialis, A. myosuroides, Amaranthus palmeri, Avena fatua, Beckmannia syzgachne, Echinochloa crus-galli var. crus-galli, Eleusine indica, Fumaria densiflora, Lolium rigidum, Poa annua, Seteria viridis and Sorghum halepense . The relatively low number of resistance cases may be attributed to lack of documenting or due to the typical practice of applying dinitroanilines in combination with other herbicides. The relatively low resistance could be due to possible fitness costs associated with the resistance mechanisms . In most crops, dinitroanilines are used as part of an herbicide program and suspected herbicide-resistant plants are controlled with the preemergence herbicide mixtures applied early in the season and any surviving plant after the preemergence application are likely to be controlled with a post-emergence herbicide with a different mode of action later in the growing season . Resistance mutations in the α-tubulin genes have been documented in dinitroaniline resistant populations inducing target-site resistance . The resistance-endowing mutation, Thr-239-lle, was initially reported in E. indica and later also in L. rigidum . Other resistance-endowing mutations are presented and explained in the review by Chen et al. . There is evidence of fitness loss from the dinitroaniline-resistance mutation Arg- 243-Met resulting in a severe reduction in plant biomass accumulation . Non-target site resistance mechanisms to dinitroanilines are not common; however, there is not to many research many research on the subject matter probably because of the difficulty in quantifying metabolites in plants . Early research demonstrated degradation of pendimethalin in tolerant plants, but no single metabolite was more abundant and in some species the parent molecule was the majority recovered residue . However, there is some indirect evidence of metabolic pendimethalin degradation in multiple resistant populations . The cytochrome P450 genes which Han et al. identifies as responsible for metabolic resistance have been documented to confer resistant to many herbicides and would not be surprising if they contributed to resistance mechanisms against dinitroanilines . Crop and Weed Tolerance. Lipid content in plant has been associated with tolerance to dinitroaniline herbicides. Hilton and Christiansen and Ndon and Harvey demonstrated that lipid content in the seed or roots of the tolerant species can bind the herbicide and prevent it from reaching the site of action. The results agree with the physico-chemical properties of the dinitroanilines, 4×4 plastic tray which are lipid-soluble and attracted to lipid-rich plant tissues.
In general, broadleaves have greater lipid content in seeds, roots and shoots than grasses, but a positive correlation of lipid content with relative dinitroaniline herbicide tolerance has been observed in grasses such as corn, foxtail, sorghum, and oats . The lipid binding is a major mechanism of tolerance to dinitroanilines in carrots, Daucus carota . Safening to dinitroanilines has been demonstrated with applications of lipid type substances to seeds or soil in various plant species . Herbicide placement has been an important action for improving crop tolerance. The dinitroaniline characteristics suggests they will bind to organic matter and not be readily leached; therefore, injury occurs based on the proximity of sensitive plant parts to the herbicide . The majority of dinitroaniline herbicides will remain in the upper 7.5 cm of the soil after an application . In green foxtail, Setaria viridis, an application of trifluralin in the soil shoot zone caused similar injury to an application in the seed zone, and greater injury than a root zone application, indicating the early shoot herbicide absorption is important for injury on grasses . Planting the crop seed deeper in the soil can be a management action to prevent contact with the herbicide in the crop’s sites of absorption.The use of dinitroanilines in rice production systems was not widely adopted before the 1980’s because of the potential for significant rice injury . However, various research efforts further expanded their potential use in rice systems . Koger et al. performed studies to understand effects of rice cultivar, planting depth and rainfall on crop safety after a pendimethalin application in dry-seeded rice. A cultivar effect was observed in three long-grain cultivars and was attributed to the varying mesocotyl lengths. An elongated mesocotyl may mean increased herbicide absorption on the soil surface, while a shorter mesocotyl length would reduce herbicide absorption at the seedling growing point on the soil surface. Therefore, it was observed that deeper planting led to greater crop safety to the pendimethalin. Khaliq and Matloob suggested a similar mechanism to pendimethalin tolerance in a dry-seeded system. Awan et al. and Ahmed and Chauhan determined rice injury from pendimethalin is affected by soil moisture and application rate in dry-seeded rice. By delaying the soil saturation time up to 7 days after seeding and pendimethalin application, rice injury can be reduced; however, in comparison with other preemergence herbicides, pendimethalin causedthe greatest injury levels . The decrease in grain yields and increased injury levels reduced adoption of pendimethalin by growers in dry-seeded rice . In drill-seeded rice, pendimethalin is commonly used. The selectivity mechanism is deeper rice seed planting. Koger et al. reported that planting depth is influential in enabling crop safety to pendimethalin. When rice seeds are placed 7 to 10 cm in soil, it prevents the growing points from coming in contact with the herbicide on the soil surface, while the herbicide can control the weeds seeds emerging on the soil surface . Pendimethalin has successfully been incorporated in drill-seeded rice systems of the US Mid-South. Pendimethalin has been a useful herbicide to manage propanil-resistant barnyardgrass in the Mid-South . Pendimethalin can be mixed well with other herbicides and is incorporated as a post-emergence application to overlay soil residual herbicide activity .Barrett and Lavy evaluated pendimethalin dissipation in common aerobic and nonaerobic cropping systems. The systems evaluated included soybeans , upland rice and lowland rice . Barrett and Lavy demonstrated soil half-lives of 3 to 7 days in lowland rice and upland rice, while half-lives were 7 days in the first year and nearly 20 days the second year in soybeans. The results indicated that soil-water content was a significant factor in pendimethalin dissipation and these results were supported by Savage . Barrett and Lavy described the dissipation spectrum as rapid dissipation in lowland rice > upland rice > soybeans, which was most likely caused by the alternate wetting intervals in the rice croppingsystems that accelerated the dissipation. The dry/wet soil cycles would have increased volatilization in the dry soil and reduce the concentration of pendimethalin . Weber suggests volatilization decreases in anerobic or flooded conditions where pendimethalin vapor moves less readily in the wet soil compared to movement in dry soil with more open pore space. Makkar et al. demonstrated pendimethalin soil dissipation in dry-seeded and transplanted rice fields to follow a biphasic first-order dissipation. The biphasic first-order dissipation results in a rapid initial dissipation after application followed by steady a dissipation rate. The behavior is commonly observed with other dinitroaniline herbicides . The dry-seeded rice was in non-flooded conditions and flush irrigated, while the transplanted rice was continuously under flooded conditions . Similar to Barrett and Lavy , Makkar et al. demonstrated total pendimethalin dissipated 1 to 2 days faster at the initial phase and about 10 days faster at the final phase in the transplanted rice field when compared to the dry-seeded rice field . The pendimethalin fate in a flooded rice field is most likely binding to organic compounds in the soil . Microbial and photodegradation are other important pathways that can contribute to degradation ; however, the binding action to organic matter appears to be most significant in many environments . In water-seeded rice, behavior of herbicides in the water is important to study. There are many herbicides that need to be activated with the water and perform best in the flooded conditions, while there are herbicides that are absorbed by the foliage and the flood will decrease efficacy .