The plasma membrane as a barrier to herbicide penetration and site for adjuvant action

Adjuvants are traditionally thought to exert their main effect on the cuticle or spray droplet to enhance foliar-applied herbicide penetration. However, considerable evidence exists indicating that the plasma membrane is a barrier to intracellular penetration of herbicides and a site of action for adjuvants. Surfactants may penetrate through the cuticle and into the region of the plasma membrane. Insertion into the membrane causes a general “ loosening” to provide greater penetration by highly polar herbicides such as glyphosate. Weak acid herbicides typically have a lipophilic moiety and, therefore, can move more easily through the membrane but the rate and accumulation is dependent on pH conditions across the membrane. Ammonium salts have been shown to affect the pH of the apoplast in a manner which allows faster penetration and greater accumulation of weak acid herbicides. Examination and understanding of the plasma membrane as a barrier to herbicide penetration will aid in defining the mechanisms of adjuvant action and improve the efficiency of agrochemical use.     

Cuticular uptake of xenobiotics into living plants. Part 2: influence of the xenobiotic dose on the uptake of bentazone, epoxiconazole and pyraclostrobin, applied in the presence of various surfactants, into Chenopodium album, Sinapis alba and Triticum aestivum leaves

This study has determined the uptake of three pesticides, applied as commercial or model formulations in the presence of a wide range of surfactants, into the leaves of three plant species (bentazone into Chenopodium album L. and Sinapis alba L., epoxiconazole and pyraclostrobin into Triticum aestivum L.). The results have confirmed previous findings that the initial dose (nmol mm(-2)) of xenobiotic applied to plant foliage is a strong, positive determinant of uptake. This held true for all the pesticide formulations studied, although surfactant concentration was found to have an effect. The lower surfactant concentrations studied showed an inferior relationship between the amount of xenobiotic applied and uptake. High molecular mass surfactants also produced much lower uptake than expected from the dose uptake equations in specific situations.     

Surfactant-enhanced foliar uptake of some organic compounds: Interactions with two model polyoxyethylene aliphatic alcohols

Interactions occurring during the surfactant-enhanced foliar uptake of seven model organic compounds were examined using two homogeneous surfactants, hexaethylene glycol monotridecyl ether (C13E6) and hexadecaethylene glycol monododecyl ether (C12E16). Surfactant–compound and compound–surfactant interactions were detected by measurement of their relative uptake rates following application of c. 0·2 μl droplets of the corresponding radiolabelled formulations. The magnitude of surfactant–compound interaction was found to vary according to the physicochemical properties of both the compound and the surfactant, and was influenced by surfactant concentration and target plant species. Interactive and non-interactive mechanisms, both leading to substantial enhancement of compound uptake, could be identified, but their precise nature could not be elucidated. Although penetration of C13E6 into the site of application appeared to be essential in order to activate the uptake of a compound, substantial absorption of C12E16 was not always required to produce the same effect. The results are discussed in the light of possible sites and modes of action for activator polyoxyethylene surfactant adjuvants.     

Adjuvants and herbicidal efficacy – present status and future prospects

The influence of adjuvants on spray liquid behaviour and herbicide performance is reviewed. Total formulation efficacy can be expressed as a function of [deposition:retention:uptake: translocation:a.i. toxicity]. Adjuvants influence the physico-chemical and plant interactions involved for each factor. Deposition efficiency of spray droplets on to a target is dependent largely on the droplet spectrum, whereas retention performance is dependent on plant leaf surface character, orientation and canopy architecture, as well as droplet volume, velocity and dynamic surface tension effects. Uptake into plant foliage is affected by the leaf surface wax, cuticle age and composition and species variability. Uptake can be improved through appropriate formulation to provide either stomatal infiltration or much greater and faster cuticular absorption of the active ingredient. The inherent translocation capability of the a.i. is not affected directly by adjuvants, which are relatively immobile, but they can increase the mass of absorbed a.i. translocated, as a consequence of improved uptake or may reduce it as a result of localized contact phytotoxicity. Considerable progress has been made in developing models of spray droplet deposition, adhesion and retention, as well as uptake. In future, individual models may be combined to provide an integrated formulation efficacy decision support system.     

Relationships between lipophilicity and root uptake and translocation of non-ionised chemicals by barley

The uptake by roots from solution, and subsequent translocation to shoots in barley, of two series of non-ionised chemicals, O-methylcarbamoyloximes and substituted phenylureas, were measured, Uptake of the chemicals by roots was greater the more lipophilic the chemical, and fell to a lower limiting value for polar chemicals. Translocation to the shoots was a passive process, and was most efficient for compounds of intermediate polarity. Both processes had reached equilibrium within 24h of treatment. The reported behaviour of many pesticides in various plant species agrees with the derived relationships, but the detailed mechanisms of these processes are unknown.     

Surfactants in plant disease management: A brief review and case studies

In plant disease management, surfactants are generally used in combination with pesticides to facilitate delivery of the pesticide chemicals to the target sites in plants, vectors, or pathogens. Their intended use is rarely for direct effect on plant pathogens. However, trends in recent studies have revealed an appreciable increase in their use for direct management of plant diseases. In this paper, we briefly review the progress made in the use of surfactants for disease management, as adjuvants and when applied for direct effect. We carried out experiments to evaluate the effect of selected synthetic surfactants (Cohere, Kinetic, Silwet-77, and Induce) on powdery mildew in squash and bacterial spot in tomato. Results from this study showed that the effect of surfactants on these pathogens is dependent on the type of plant pathogen or disease. All tested surfactants exhibited in vitro bactericidal activity against Xanthomonas perforans, a causal pathogen of bacterial spot of tomato. In the greenhouse, Cohere significantly reduced disease severity of powdery mildew in squash compared to the untreated control. Cohere also showed promise in field trials against bacterial spot of tomato, but the other surfactants tested promoted this disease. Our studies demonstrate the importance of evaluating surfactants for their use in management of plant diseases both for direct effect and as adjuvants in tank mix.     

Entry of pesticides into the plant symplast as measured by their loss from an ambient solution

A method was developed to determine the extent of penetration of substances into the plant symplast. The known apoplastic dye trisodium 3-hydroxy-5,8,10-pyrenetri-sulphonate (PTS) and various systemic pesticides were studied using potato tuber tissue. The dye penetrated only 5% of the volume of living tissue while those pesticides which display an apoplastic pattern of transport in plants penetrated the entire tissue volume. The pesticides diffused freely out of the tissue when it was transferred to fresh medium. It is proposed that the term euapoplastic be used to describe chemicals that behave like the dye, and that the term pseudoapoplastic be used to describe chemicals that behave like atrazine, carbendazim, carboxin and diuron. 2,4-D, which displays a symplastic transport pattern in the plant, was concentrated by the tissue and did not diffuse out freely. It appears that the property which allows a pesticide to be transported in the symplast is not its ability to penetrate the plasmalemma but rather its ability to be retained by the symplast after entry.     

Penetration of triolein and methyl oleate through isolated plant cuticles and their effect on penetration of [14C]quizalofop-ethyl and [14C]fenoxaprop-ethyl

The penetration of two model seed oil compounds, [¹⁴C]triolein (TRI) and [¹⁴C]methyl oleate (MEO) through plant cuticles and their effects on the penetration of [¹⁴C]quizalofop-ethyl and [¹⁴C]fenoxaprop-ethyl were investigated. Experiments were carried out using isolated cuticles from rubber plant (Ficus elastica Roxb.) leaves and from tomato (Lycopersicon esculentum Mill,) and pepper (Capsicum annuum L.) fruits. Chemicals were deposited in droplets on to cuticle discs maintained on agar blocks under controlled conditions. TRI and MEO were used at 1% (V/V). The transfer of radiolabel through cuticles was negligible for TRI and varied from 6 to 13% after 72 h, according to species, for MEO, The penetration results obtained for quizalofop-ethyl (0.084 mg mL^-1) and fenoxaprop-ethyl (0.189 mg mL^-1) were very similar and varied according to species. The greatest diffusion intoagar was observed for pepper (12.8% and 10.7% after 72 h, for quizalofop-ethyl and fenoxaprop-ethyl respectively), the lowest for rubber plant cuticles (1.4 and 1.3% respectively). Addition of MEO produced significant increases in the penetration of quizalofop-ethyl and fenoxaprop-ethyl through rubber plant and tomato cuticles. TRI had an enhancing effect on the two herbicides only with rubber plant cuticles. Results are discussed with particular consideration of the variations between plant species and the possible mode of action of seed oil adjuvants.     

Reviews Section: The Motax 33 -- motorized, portable, air- assisted, CDA sprayer

The Motax 33, a portable air-assisted controlled droplet application (CDA) sprayer, offers a new approach to the application of pesticides within coffee and other bush crops. It uses a rotary atomizer to produce small, uniformly sized droplets which are propelled into the crop foliage by a wide and turbulent air blast, allowing the use of low spray volumes. Compared with traditional high-volume techniques the sprayer offers the possibility of improved spray coverage and penetration at low spray volumes, typically 30--70 l ha–1, allowing more timely spray treatments with the potential to adopt more responsive integrated insect pest and disease management strategies     

A scanning electron microscope study of glyphosate deposits in relation to foliar uptake

The effects of several formulations on foliar uptake of glyphosate, and on the morphology of glyphosate deposits on leaves, were examined in Tartary buckwheat (Fagopyrum tataricum (L.) Gaertn.). [¹⁴C]glyphosate, in the form of the free acid or the isopropylamine salt (IPAS), was applied to foliage alone or with various adjuvants. Uptake of all glyphosate IPAS formulations was greater than that of the corresponding acid formulation. Addition of ‘Tween 20’ enhanced the uptake of glyphosate IPAS compared to glyphosate alone, but had no effect on the uptake of glyphosate acid. Ammonium sulfate and the ‘Roundup’ formulation blank increased the uptake of glyphosate acid and IPAS to 2-3 times that of herbicide alone. Surface deposits, as observed by scanning electron microscopy, varied with the formulation of the herbicide, although there were no differences between the acid and IPAS formulations. Glyphosate alone initially formed a deposit with both crystalline and smooth, amorphous areas. Later in the treatment period (48 and 72 h after application), the deposit was almost entirely crystalline. The addition of ‘Tween 20’ or of formulation blank resulted in the formation of a more amorphous, non-crystalline deposit. Herbicide solutions containing ammonium sulfate dried to form a highly crystalline deposit. However, crystals similar to those of glyphosate alone were not visible in these deposits. The ability of these adjuvants to prevent or delay crystal formation may play a role in their enhancement of herbicide uptake.     

Uptake and metabolism of hexazinone in Rubus hispidus L. and Pyrus melanocarpa (Michx.) Willd

Rooted cuttings of Pyrus melanocarpa (Michx.) Willd. had a 3.5-fold greater tolerance to soil-applied hexazinone than those of Rubus hispidus L. in a greenhouse trial. R. hispidus accumulated four times more ¹⁴C-label in the foliage following root uptake of ¹⁴C-hexazinone than P. melano-carpa . Here, the greater uptake and susceptibility of R. hispidus was related to its greater root:fol-iage (weight) ratio compared to the tolerant P. melanocarpa . However, in whole plant metabolism studies employing younger plants there were no differences in ¹⁴C accumulation in the leaves. Here ¹⁴Chexazinone was converted to a number of hydroxylated and/or demethylated triazinone metabolites in both species following vacuum infiltration into leaf disks and root sections, or following root uptake in whole plants. A major difference in metabolism between the two species was the greater formation of the mono demethylated metabolite, B, [3-cyclohexyl-6-methylamino-l-methyl-l,3,5-triazine-2,4-dione] in P. melanocarpa which may also contribute to its greater tolerance. A loss of total ¹⁴Clabel from all plant parts with time may reflect cleavage of the ¹⁴C-ring-labelled herbicide.     

Mechanisms of symptom production by foliar bacterial pathogens

Most foliar bacterial pathogens are pathovars ofPseudomonas syringae andXanthomonas campestris. Many of them live during the greater part of their life-cycle on the upper parts of the plant. In order to survive and develop in this environment, the pathogens have elaborated an array of mechanisms which enable them to penetrate the plant foliage, produce a substantial endophytic population, and use the host tissue as a nutrient source. These activities result in the formation of different categories of symptoms,e.g. necrotic and chlorotic lesions, wilting, gall formation, leaf abscission and inhibition of plant growth. The operating mechanisms of foliar phytopathogenic bacteria can be divided into four major groups: (i) production of toxins (mostly non-specific), mainly byP. syringae pathovars; (ii) excessive production of plant growth hormones; (iii) enzymatic hydrolyzing activity; and (iv) inhibition of seedling growth by unknown mechanism(s). More than one pathogenic mechanism may operate in a given leaf disease or by a single pathogen. Despite the large amount of literature describing visual, biochemical and biophysical aspects of any leaf disease, basic knowledge is fragmental and comprehensive understanding of the mode of action of any disease is lacking. It is proposed that in order to close up these gaps in our knowledge other approaches, such as molecular biology technology and more intensive use of both bacterial and plant mutants, should be incorporated in pathological studies of foliar bacterial diseases.     

Use of the scanning electron microscope and cathodoluminescence in studying the application of pesticides to plants

The efficacy of a pesticidal formulation is in part affected by the surface to which it is applied. During the drying process, spray drops can interact with the complex microenvironment of trichomes, secretory glands, stomata, epicuticular wax and cuticle of the leaf epidermis. The subsequent uptake and persistence of the deposit can be further influenced by the action of the microclimate, including humidity, temperature, rain and dew. The scanning electron microscope (SEM), which is an instrument ideally suited for the study of the surface of bulk specimens, has been used to show the spreading and distribution of pesticides applied as sprays and individual drops to leaf surfaces. Particle size, crystal growth and co-precipitation of the active ingredient with surfactants and other adjuvants have also been investigated. The SEM can also show how some pesticides and their adjuvants continue to be taken up by the leaf long after the drops have visibly dried. Phytotoxicity, sometimes associated with the penetration of active ingredient or formulation adjuvant, can also be demonstrated using the microscope, especially if either specifically emits cathodoluminescence. These images can also supply complementary information concerning the persistence of deposits, provided that interpretation of the results is carried out with care and, if possible, in conjunction with residue and biological assays.     

Integrated Pest Management of Pest Mole Crickets with Emphasis on the Southeastern USA

There are at least 70 species of mole crickets (Orthoptera: Gryllotalpidae). Some are rare, others are innocuous, and a few are important pests. These soil-dwelling pests damage underground parts of a long list of cultivated plants. Although tillage and flooding are used successfully in some situations to bring these pests to the soil surface and expose them to vertebrate and other predators, chemical pesticides are widely used against them. Knowledge of their life history is used to time application of chemical treatments to save money, but is not used as widely as it might be. Classical biological control has been used against immigrant mole crickets in Hawaii, Puerto Rico, and the southern USA. In Florida, three Scapteriscus species from South America cause major damage to pastures and turf and are targets of a classical biological control program. Population levels of two of the pest species have been reduced substantially in Florida by establishment of a tachinid fly (Ormia depleta) and a steinernematid nematode (Steinernema scapterisci) from South America. The nematode also functions as a biopesticide. Managers of pastures and turf in Florida have thus far derived benefit from these classical biological control agents without understanding their function: use of chemicals is reduced when mole cricket populations are lower due to action of these organisms. Future enhancement of the action of O. depleta and of a sphecid wasp (Larra bicolor, which also was introduced from South America) probably will demand deliberate planting of nectar sources for adults of these biological control agents, and the advantage will be to managers who adopt such a strategy. Chemical pesticide use is strongly promoted by a large chemical industry, whereas biopesticidal use has thus far been little promoted and sales have been few. Even managers who do not change their simple strategy of pesticide use in response to damage by mole crickets, and have no knowledge of the differing life cycles of the three Scapteriscus species or of the presence and action of the classical biological control agents, will derive benefit as these biological control agents (and a predatory beetle which has not yet been released) increase their distribution.     

高效低风险是农药发展的必由之路

农药是与人类生存活动紧密相关的一类重要的农用化学品,随着人们对环境安全和自身健康日益关注,对农药认识和理解趋于全面和理性.笔者认为,目前对农药发展的要求已由“高效、低毒、低残留”发展和延伸到高效低风险的阶段.农药产业也应调整思路,积极更新产品.高效低风险农药已成为当前农药发展的必由之路.     

Botanical Pesticides and Their Mode of Action

Pest management is facing economic and ecological challenge worldwide due to human and environmental hazards caused by majority of the synthetic pesticide chemicals. Identification of novel effective insecticidal compounds is essential to combat increasing resistance rates. Botanical pesticides have long been touted as attractive alternatives to synthetic chemical pesticides for pest management because botanicals reputedly pose little threat to the environment or to human health. The body of scientific literature documenting bioactivity of plant derivatives to arthropod pests continues to expand, yet only a handful of botanicals are currently used in agriculture in the industrialized world, and there are few prospects for commercial development of new botanical products. Pyrethrum and neem are well established commercially, pesticides based on plant essential oils have entered the marketplace, and the use of rotenone appears to be waning. A number of plant substances have been considered for use as pest antifeedants, repellents and toxicants, but apart from some natural mosquito repellents, a little commercial success has ensued for plant substances that modify arthropod behavior. Several factors appear to limit the success of botanicals, most notably regulatory barriers and the availability of competing products (newer synthetics and fermentation products) that are cost-effective and relatively safe compared with their predecessors. In the context of agricultural pest management, botanical pesticides are best suited for use in organic food production in industrialized countries but can play a much greater role in the production and postharvest protection of food in developing countries. Botanicals have been in use for a long time for pest control. The compounds offer many environmental advantages. However, their uses during the 20th century have been rather marginal compared with other bio-control methods of pests and pathogens. Improvement in the understanding of plant allelochemical mechanisms of activity offer new prospects for using these substances in crop protection. I’m trying in this article to present different kinds of botanical pesticides came from different recourses and their mode of actions as well as I will try to examine the reasons behind their limited use (disadvantages) and the actual crop protection developments involving biopesticides of plant origin for organic or traditional agricultures to keep our environment clean and safer for humankind and animals.     

Modelling penetration of plant cuticles by crop protection agents and effects of adjuvants on their rates of penetration

A theory of cuticular penetration of crop protection agents (CPAs) is presented, which incorporates properties of cuticles and cuticular waxes as well as properties of active ingredients and adjuvants. Based on this theory, two models are developed which are analytical in the sense that they help to quantify and understand (i) differences in permeability among cuticles from different species, (ii) effects of properties of CPAs on permeabilities of cuticles and rates of uptake and (iii) the effects of adjuvants on properties of cuticles and rates of uptake of CPAs. The models can be used to predict rates of uptake of CPAs as affected by properties of cuticular waxes, active ingredients and adjuvants. However, before this can be done, a constant, two parameters and at least two variables must be estimated. Properties of cuticles are accounted for by the constant D⁰/Δx and the parameter β′. The former, the ratio of the mobility of a hypothetical molecule having zero molar volume (D⁰x) divided by the path length (Δx) across the cuticle, has the dimension of velocity (ms^?1) and is independent of the solubility of the CPA. The latter is a measure of size selectivity of the cuticle. Differences in permeabilities of cuticles from different species increase with increasing size of active ingredients due to size selectivity (β′). Removing cuticular waxes from Citrus cuticles increased D⁰/Δx by a factor of 2042, while β was not affected. Differential solubility of CPAs is considered part of the driving force and at least two different partition coefficients are needed to account for differences in solubilities in cuticular waxes, cutin, water and the formulation residue on the surface of the cuticles. Adjuvants are solvents in the formulation residue on the leaf surface once the carriers (water and other volatile solvents) have evaporated and certain adjuvants also act as accelerators; they penetrate the cuticle and increase D⁰/Δx. Thus, accelerators increase rates of uptake and this effect depends on two factors, (i) the intrinsic activity of the accelerator and (ii) rate of penetration into the cuticle, because the active ingredients follow the accelerator front across the cuticle. Since accelerators penetrate from the formulation residue into the cuticle, the volume of the formulation residue decreases with time. This maintains high concentrations of CPAs in the formulation residue and, thus, maximum driving forces and rates of penetration. To utilise fully this dual accelerator effect, it is necessary to match velocities of penetration of accelerators and active ingredients accurately.     

Genetically engineered viral insecticides–A progress report 1986 1989

Among the viruses that are pathogenic for insect species, baculoviruses have been shown to be useful as insecticides for pest control. In some cases they have been used as cost-effective and environmentally acceptable alternatives to chemical insecticides. However, because viruses need to be ingested and replicate extensively in their host before they kill it, baculovirus insecticides are much slower than chemicals or other reagents that kill insects either on contact or shortly after ingestion. The objective of the programme of genetic engineering of baculovirus insecticides is to improve their speed of action while maintaining their host specificity and other attributes that make them desirable alternatives to chemical pesticides. Since 1986 four field releases have been undertaken involving genetically engineered baculovirus insecticides. The first release used a genetically marked Autographa californica nuclear polyhedrosis virus (AcNPV). The study began in 1986 and was terminated in 1987. The results demonstrated that an innocuous piece of DNA, appropriately positioned in the AcNPV genome, was an effective means to tag the virus without affecting its phenotype, allowing it to be identified in bioassays of plant and soil samples. The second release, in 1987, involved a genetically marked virus from which the gene coding for the protective polyhedrin protein of the virus had been removed. The field data obtained with this virus showed that it did not persist in the environment, neither in soil, nor on vegetation, nor in the corpses of caterpillars. The third and fourth releases were undertaken in 1988. For one of these studies the marked, polyhedrin-negative virus was again used. In the other study a polyhedrin-negative virus that contained a junk' (/?-galactosidase) gene was employed.     

农药内吸性的研究现状与改善策略

合理利用和改善农药在植物中的内吸传导特性，可大幅提高农药在靶标部位的积累并减少农药对环境的污染。通过对农药内吸性的研究，已逐步揭示了植物吸收和传导农药的基本原理，对内吸性农药的研发具有重要意义。随着农药改造和纳米载体的出现及发展，利用不同方法或手段来改善农药内吸性的研究报道也逐渐增加。本综述主要从农药在植物中的内吸性研究进展、研究方法以及改善农药内吸性的策略3个方面进行了阐述，可为内吸性农药及其剂型的开发提供参考。