Baruch Rubin

Clay-based formulations of metolachlor with reduced leaching

The current research in herbicide application aims to develop formulations that reduce leaching of the herbicide to deep layers of the soil and to concentrate its biological activity at the top layers. Adsorption of metolachlor on clay minerals, their organic derivatives or pillared forms provides the best possibility to develop slow-release formulations. Metolachlor is a selective pre-emergence herbicide widely used in irrigated crops to control annual weeds. It is adsorbed by bentonites and montmorillonites, but the amount adsorbed strongly depends on the type of bentonite and possible pretreatment reactions. Wyoming bentonites adsorbed considerable amounts of metolachlor but other bentonites did not bind this herbicide. An acid-activated pillared montmorillonite was also an effective adsorbent of metolachlor. Modification of this sample by preadsorbing different amounts of benzyl trimethylammonium ions did not influence the level of herbicide adsorption. The biological efficiency of the formulations was tested with bioassay soil columns. Slow-release formulations could be prepared with raw bentonites and the acid-activated pillared montmorillonite. A formulation, prepared by adsorbing metolachlor from aqueous solution on the acid-activated pillared montmorillonite, showed high herbicide activity at the top 10 cm, and did not diffuse significantly to greater depths. This formulation should allow a better weed control than the commercial formulations.

Organo-clay formulations of pesticides: reduced leaching and photodegradation

Adsorption of organic cations on several clay minerals is reviewed with an emphasis on the effect of ionic strength and modeling. The clay exchanged with suitable organic cations forms a basis for ecologically acceptable formulations of herbicides with reduced leaching, ground water contamination and enhanced weed control efficacy. Incomplete neutralization of the clay surface charge by an organic cation may be advantageous in achieving maximal adsorption of hydrophobic herbicides. One conclusion from these studies is that optimization of clay-based herbicide formulations requires a selection of structurally compatible organic cations preadsorbed on the clay at optimal coverage. New experimental results are presented for alachlor formulations, which significantly reduce herbicide leaching under conditions of heavy irrigation. We were able to demonstrate that organo-clay formulations of alachlor and metolachlor can increase crop yields in a 1-year field experiment. The photostabilization of pesticides is reviewed and improved organo-clay formulations of the herbicides trifluralin and norflurazon are described. A pillared clay, nanocomposite micro- and/or meso porous material, was effective in reducing leaching and in conferring photostabilization, without added organic cations.

Reduction of photodegradation and volatilization of herbicides in organo-clay formulations

The use of commercially available emulsifiable concentrate (EC) formulations of alachlor and metolachlor may be an environmental hazard because of their volatilization to the atmosphere and photodecomposition, which requires increased applied amounts. The objectives of this study were to develop organo-clay based formulations which would be less volatile and better protected from photodegradation. Bioassays have shown that the use of organo-clay formulations improves photoprotection, reduces volatilization and maintains herbicidal activity in the soil under laboratory and field conditions. Largest adsorption of herbicides by organo-clays correlates with an optimal reduction of photodecomposition and volatilization. It appears that the role of the organic cation, e.g., benzyltrimethylammonium (BTMA) is to enhance the adsorption of the non-polar herbicides to the organo-clay complex, whereas the actual photoprotection is mainly provided by the clay.

Microtubules are an intracellular target of the plant terpene citral

Citral is a component of plant essential oils that possesses several biological activities. It has known medicinal traits, and is used as a food additive and in cosmetics. Citral has been suggested to have potential in weed management, but its precise mode of action at the cellular level is unknown. Here we investigated the immediate response of plant cells to citral at micromolar concentrations. It was found that microtubules of Arabidopsis seedlings were disrupted within minutes after exposure to citral in the gaseous phase, whereas actin filaments remained intact. The effect of citral on plant microtubules was both time- and dose-dependent, and recovery only occurred many hours after a short exposure of several minutes to citral. Citral was also able to disrupt animal microtubules, albeit less efficiently. In addition, polymerization of microtubules in vitro was inhibited in the presence of citral. Taken together, our results suggest that citral is a potent, volatile, anti-microtubule compound.

Movement of metolachlor in soil: effect of new organo-clay formulations

The use of commercially available formulations of metolachlor has resulted in its leaching and migration to water sources. Formulations of metolachlor designed to reduce its leaching in soil have been prepared by adding the herbicide dissolved in an organic solvent or in water to organo-clay complexes. Best formulations were made when the organo-clay complex was formed by adsorbing the monovalent organic cations benzyltrimethylammonium (BTMA) or benzyltriethylammonium (BTEA) onto sodium montmorillonite (Mont) at 0.5 or 0.8 mole kg−1 clay. Adsorption of metolachlor to organo-clays followed the sequence Mont-BTMA 0.5 > Mont-BTMA 0.8 > Mont-BTEA 0.8 > Mont-BTEA 0.5 > Mont. Fourier transform infrared (FTIR) analysis demonstrated the occurrence of shifts of several peaks of adsorbed metolachlor relative to the free herbicide, indicating the existence of strong interactions between metolachlor molecules and the organo-clay surface. Leaching studies employing organo-clay and commercial formulations were carried out under greenhouse and field conditions. Metolachlor applied as organo-clay formulations leached less than the commercial formulation. Organo-clay formulations prepared by adding the herbicide as a water solution showed less leaching in the soil profile than those made by using organic solvent. Under greenhouse conditions, the herbicidal activity of organo-clay formulations was similar to that of the commercial one. Under field conditions, leaching from Mont-BTMA 0.5-metolachlor was less than that from the commercial formulation, demonstrating the environmental and agricultural advantages of the organo-clay formulations of metolachlor. © 1999 Society of Chemical Industry

SIGNIFICANCE OF THE LONG RANGE TRANSPORT OF PESTICIDES IN THE ATMOSPHERE

Synopsis:Since the 1960’s there has been a growing body of data regarding the presence of pesticides in the atmosphere. The monitoring results obtained show that traces of pesticides may undergo long range transport and be deposited considerable distances away from the treatment areas, including remote areas such as the Arctic and Antarctic regions. Pesticides have been found in air, rain, cloud water, fog and snow. The appearance and subsequent behaviour of pesticides in the atmosphere are complex processes and the concentrations found depend on several variables such as their volatility, photostability, method of application and extent of use. Whilst volatility of pesticides can be linked to their Henry’s Law constant this is very much a simplification since it is also influenced by the surfaces treated, e.g. soil or leaves, and by the extent to which aerosols are formed during the application. The disappearance of pesticides from the atmosphere is due to hydrolysis, indirect photolysis via OH. radicals and to deposition in rain. Pesticides which are resistant to hydrolysis and photolysis can be transported over great distances, for example, organochlorine insecticides have been detected in the Arctic regions. In general, concentrations in rainwater are, when detected, in the low or sub mg/l range and highest concentrations are found during the time of application. The use of fugacity models has been shown to be a useful approach to predict concentrations in air. Under most conditions the presence of pesticides in air, or rainwater, has no significant effects on non-target systems, including direct and indirect effects. Exceptions to this are damage by auxin-type herbicides to sensitive plants which has resulted on restrictions in their use in certain areas and transient chlorotic spotting thought to be caused by drift of aerosols from application of low rate sulfonyl urea herbicides. For animal species one possible exception has been postulated. This is for persistent organochlorine pesticides in Arctic regions where, due to the very oligotrophic nature of the Arctic ocean, they are more liable to bioaccumulate and be transported in the food web giving enhanced levels in mothers’ milk.

Glyphosate-Induced Anther Indehiscence in Cotton Is Partially Temperature Dependent and Involves Cytoskeleton and Secondary Wall Modifications and Auxin Accumulation

Yield reduction caused by late application of glyphosate to glyphosate-resistant cotton (Gossypium hirsutum; GRC) expressing CP4 5-enol-pyruvylshikmate-3-P synthase under the cauliflower mosaic virus-35S promoter has been attributed to male sterility. This study was aimed to elucidate the factors and mechanisms involved in this phenomenon. Western and tissue-print blots demonstrated a reduced expression of the transgene in anthers of GRC compared to ovules of the same plants. Glyphosate application to GRC grown at a high temperature regime after the initiation of flower buds caused a complete loss of pollen viability and inhibition of anther dehiscence, while at a moderate temperature regime only 50% of the pollen grains were disrupted and anther dehiscence was normal. Glyphosate-damaged anthers exhibited a change in the deposition of the secondary cell wall thickenings (SWT) in the endothecium cells, from the normal longitudinal orientation to a transverse orientation, and hindered septum disintegration. These changes occurred only at the high temperature regime. The reorientation of SWT in GRC was accompanied by a similar change in microtubule orientation. A similar reorientation of microtubules was also observed in Arabidopsis (Arabidopsis thaliana) seedlings expressing green fluorescent protein tubulin (tubulin α 6) following glyphosate treatment. Glyphosate treatment induced the accumulation of high levels of indole-3-acetic acid in GRC anthers. Cotton plants treated with 2,4-dichlorophenoxyacetic acid had male sterile flowers, with SWT abnormalities in the endothecium layer similar to those observed in glyphosate-treated plants. Our data demonstrate that glyphosate inhibits anther dehiscence by inducing changes in the microtubule and cell wall organization in the endothecium cells, which are mediated by auxin.

On the origin of Near Eastern founder crops and the 'dump-heap hypothesis'

The transition from hunting gathering to a farming based economy – the Neolithic Revolution, was a crucial junction in the human career, attracting the attention of many scholars: archaeologists, anthropologists, geographers, botanists, geneticists and evolutionists among others. Our understanding of this major transformation is rather limited mainly due to the inability to fully reconstruct the cultural, biological and environmental setup of the relevant period and organisms involved. Many students of the subject of plant domestication have seriously entertained the hypothesis that mans first crop plants have originated from weeds associated with the disturbed habitats surrounding pre-agricultural ancient human dwellings and or with human refuse heaps – the so called ‘dump heap hypothesis’. In this paper we re-examine this hypothesis in light of the known biology of the Near Eastern founder crops and the ecological preferences of their wild progenitors. Contrary to the ‘dump-heap hypothesis’, we propose that Near Eastern farming originated as a result of a long term interaction between humans and plants and was mainly driven by the nutritional features of the respective crops and cultural forces.

The relative effect of citral on mitotic microtubules in wheat roots and BY2 cells

The plant volatile monoterpene citral is a highly active compound with suggested allelopathic traits. Seed germination and seedling development are inhibited in the presence of citral, and it disrupts microtubules in both plant and animal cells in interphase. We addressed the following additional questions: can citral interfere with cell division; what is the relative effect of citral on mitotic microtubules compared to interphase cortical microtubules; what is its effect on newly formed cell plates; and how does it affect the association of microtubules with γ-tubulin? In wheat seedlings, citral led to inhibition of root elongation, curvature of newly formed cell walls and deformation of microtubule arrays. Citrals effect on microtubules was both dose- and time-dependent, with mitotic microtubules appearing to be more sensitive to citral than cortical microtubules. Association of γ-tubulin with microtubules was more sensitive to citral than were the microtubules themselves. To reveal the role of disrupted mitotic microtubules in dictating aberrations in cell plates in the presence of citral, we used tobacco BY2 cells expressing GFP-Tua6. Citral disrupted mitotic microtubules, inhibited the cell cycle and increased the frequency of asymmetric cell plates in these cells. The time scale of citrals effect in BY2 cells suggested a direct influence on cell plates during their formation. Taken together, we suggest that at lower concentrations, citral interferes with cell division by disrupting mitotic microtubules and cell plates, and at higher concentrations it inhibits cell elongation by disrupting cortical microtubules.

HERBICIDE RESISTANCE IN WEEDS AND CROPS, PROGRESS AND PROSPECTS

Herbicide resistance has appeared in numerous plant species and to several herbicide classes throughout the world, particularly when monoculture, monoherbicide and minimum tillage are widely practised. Much research has been directed at the elucidation of the resistance mechanisms, their molecular bases and their implications for agriculture and the environment. These studies have significantly contributed to our basic knowledge on the physiological, biochemical and molecular mode of action of herbicides and their selectivity mechanisms. Altered herbicide target site, enhanced detoxification and sequestration of the herbicide away from its target site are the major reported resistance mechanisms. Multi-disciplinary models have been developed to better understand the population dynamics processes involved in the evolution and spread of herbicide resistance, and to evaluate management strategies to overcome and delay its appearance. Reduction of selection pressure, through rational use of lower rates of herbicides, crop and herbicide rotations, mixtures of herbicides, safeners, and synergists are suggested to delay and combat resistance. Classical breeding, in vitro and in vivo selection and genetic engineering techniques have been employed to improve crop selectivity and develop herbicide-resistant crops. Proper utilization of resistant crops and awareness of the potential risks involved in their introduction may improve our ability to develop cost-effective and flexible weed management programmes.