Effect of pyridazinone herbicides on lipid metabolism in groundnut (Arachis hypogaea) leaves

The effect of five substituted pyridazinones (pyrazon, San 133-410H, San 9774, norflurazon, and San 6706) on lipid metabolism in groundnut (Arachis hypogaea) leaves was investigated under nonphotosynthetic conditions. In experiments with leaf disks, the uptake of [1-¹⁴C]acetate, [³²P]orthophosphate, and [³⁵S]sulfate was significantly inhibited by these herbicides and the magnitude of inhibition varied, depending on the substituents. When the incorporation of these precursors into lipids was measured and expressed as percentage of total uptake, no effect was observed in the case of [1-¹⁴C]acetate but there was significant inhibition in the incorporation of the other two precursors, suggesting that pyridazinones interfere with the metabolism of the phospholipids and the sulfolipid. None of these compounds affected the uptake of [methyl-¹⁴C]choline but all inhibited its incorporation into phosphatidylcholine indicating that phosphatidylcholine metabolism is vulnerable to pyridazinones. The fatty acid synthetase of isolated chloroplasts assayed in the absence of light was inhibited 20–50% by the pyridazinones at 0.1–0.5 mM concentrations. San 9774 showed the most potent inhibition. In addition, the pyridazinone herbicides significantly inhibited sn-glycerol-3-phosphate acyltransferase(s) in both chloroplast and microsomal fractions but showed no effect on phosphatidic acid phosphatase. The magnitude of inhibition of fatty acid synthetase and acyltransferase(s) is related to the nature of the substituent groups on the herbicide. Trifluorophenyl substitution at position 2 or amino substitution at position 5 of the pyridazinone molecule caused the maximum inhibitory effect.     

Influence of herbicides on photosynthetic activity and transpiration rate of intact plants

The effect of various herbicides on photosynthesis, respiration and transpiration of intact plants has been studied in a routine assembly. Simultaneous measurements were made under different experimental conditions in four small plant chambers, in which the shoots of various plant species can be accommodated. The herbicide is applied during measurement, so that the effect can be related to the photosynthetic activity of the same plants before treatment. The selectivity of various herbicides was studied by determining the capacity of a plant species to inactivate a herbicide absorbed by the roots. These and other differential effects of various herbicides on photosynthetic activity of different plant species coincide with the selective properties in the field. Such differences are also observed after leaf sprayings. The duration of the experiments is kept short. Bean plants were studied under various experimental conditions of air humidity, light intensity and temperature resulting in different transpiration rates. The decrease in photosynthetic activity owing to the presence of a herbicide in the nutrient solution at a standard concentration was more rapid at the higher transpiration rates. The total transpiration during treatment up to 50% inhibition of photosynthesis was constant under the various experimental conditions. Specific inhibitors of the photosynthetic process had a more pronounced effect on the photosynthetic activity than on transpiration rate. Some other herbicides affect transpiration as well as photosynthesis.     

Phytotoxic sites of action for molecular design of modern herbicides (Part 1): The photosynthetic electron transport system

A bird's eye review was tried to select the bio‐rational targets from known and novel plant‐specific ones for the molecular design of modern herbicides, which exhibit efficient phytotoxicity at a low‐use rate and preserve a good environment in the 21st century. In phytotoxic sites in the photosynthetic electron transport (PET) system discussed in the present article (Part 1), the generally called bleaching herbicides interfering with the biosynthesis of photosynthetic pigments, chlorophylls and carotenoids, and the biosynthesis of plastoquinone, were considered to be good models for the molecular design of modern herbicides. The PET itself was still considered as an interesting target site for new herbicides, although they need to exert their action in all green leaves of weeds to achieve herbicidal efficacy. Because these herbicides never form a tight binding with D1‐protein, their use‐rate cannot be expected to be as low as the herbicides inhibiting chlorophyll or branched amino‐acid biosynthesis. Other herbicidal targets found in chloroplasts, namely ATP and NADPH formations, have already been omitted from the worldwide biorational molecular design program of herbicides targeting the PET system.     

Sites of action of photosynthetic inhibitor herbicides; Experiments with trypsinated chloroplasts

The effect of four photosynthetic inhibitor herbicides, bromacil, ioxynil, metribuzin and monuron, on chloroplast electron transport was investigated. All four compounds completely inhibited electron flow with tripotassium hexacyanoferrate as oxidant, but the inhibition caused by bromacil, metribuzin and monuron was almost totally reversed by trypsin treatment. With ioxynil, only a partial degree of reversability was shown. With a molybdosilicate as oxidant, electron transport was not completely inhibited by any of the herbicides. Whereas the partial inhibition was reversed by tryptic digestion in the presence of bromacil, metribuzin and monuron, there was virtually no reversal in the presence of ioxynil. The results suggest a common site of action for all four herbicides and an additional site for ioxynil nearer to photosystem II.     

Studies into the action of the diphenyl ether herbicides acifluorfen and oxyfluorfen. Part II: The interaction with photosynthetic electron transport reactions

The effects of acifluorfen and oxyfluorfen on photosynthetic electron transport reactions of pea chloroplasts were compared with those induced by paraquat and monuron. Monuron inhibited electron flow between photosystems I and II, and paraquat acted as an electron acceptor for photosystem I, promoting superoxide formation by illuminated chloroplasts. Neither acifluorfen nor oxyfluorfen at concentrations up to 50 μM affected non-cyclic electron flow or promoted superoxide formation. Both herbicides were shown to repress ferredoxin-dependent NADP⁺ reduction by illuminated chloroplasts. Further experiments showed that, in the presence of ferredoxin-NADP⁺ reductase and chloroplast membranes maintained in the dark, p-nitro diphenyl ether (DPE) herbicides promoted the rate of ferredoxin-dependent oxidation of NADPH, implying that these herbicides can accept electrons from reduced ferredoxin. The interaction between acifluorfen, ferredoxin and chloroplast membranes was examined further by following the effect of this herbicide on the peroxidation of illuminated thylakoids. Lipid peroxidation was promoted by acifluorfen, although this effect was abolished if thylakoids were washed prior to use. The effect of washing could be reversed by adding exogenous ferredoxin. These data demonstrate that interaction of DPE herbicides with photosynthetic electron transport in the vicinity of ferredoxin is necessary for light-dependent herbicide activation.     

Effect of atrazine and methabenzthiazuron on oxygen evolution and cell growth of Chlorella pyrenoidosa

Cultures of Chlorella pyrenoidosa Pringsheim were grown photoautotrophically in the presence of two concentrations (0·25 and 0·50 μm ) of atrazine and methabenzthiazuron. The cell density and chlorophyll content were greatly reduced in presence of both herbicides. These herbicides affected net photsynthesis equally without affecting oxygen uptake. However, cell division was more inhibited by atrazine than by methabenzthiazuron, but chlorophyll content per cell was more inhibited by methabenzthiazuron than by atrazine.     

Photosynthesis and growth responses of grapevine to acetochlor and fluoroglycofen

Acetochlor and fluoroglycofen are herbicides used in vineyards to eradicate weeds. This present study characterized the effects of these chemicals on photosynthetic characteristics and the antioxidant enzyme system in non-target grape leaves. The results showed that acetochlor and fluoroglycofen reduced net photosynthetic rate in a dose-dependent manner, but also reduced or increased pigment contents, respectively. According to chlorophyll fluorescence measurements, acetochlor and fluoroglycofen decreased the photochemical efficiency of photosystem II in the light and increased non-photochemical quenching. These herbicides enhanced malondialdehyde contents and accelerated the superoxide anion production rate in dose-dependent manners, which might be associated with lower antioxidant enzyme activities, especially at higher concentrations of the herbicides. Acetochlor and fluoroglycofen inhibited grapevine growth in the growth season one-year after herbicide treatment, and stem height was inhibited by up to 55.4% and 88.0%, respectively. Taken together, these results suggest that both herbicides are detrimental for grape photosynthesis and this might be associated with increased oxidative stress in the first year, while growth inhibition in the second year might be due to after effects of herbicide treatment.     

THE USE OF THREE SIMPLE,RAPID BIOASSAYS ON FORTY-TWO HERBICIDES*

Three rapid bioassays were tested on forty-two herbicides having several different modes of action. A 50% or greater inhibition of growth was found at 1 ppm with thirty-one herbicides in one or more of the bioassays. Of the remaining eleven herbicides, seven were detected at 10 ppm, two at 20 ppm and two at 30 ppm. The techniques used were a Chlorella bioassay, a root bioassay with sorghum, oat and cucumber and a shoot bioassay with sorghum and oat. The duration of the bioassays was 1, 2 and 4 days respectively. As a general rule, the Chlorella bioassay was especially sensitive to photosynthetic and respiratory inhibitors but not sensitive to herbicides with other modes of action, whereas the root and/or shoot bioassays were sensitive to most of the herbicides except the photosynthetic inhibitors. The use of the three bioassays simultaneously is suggested as a possible method for primary screening of herbicides.     

Bleaching activity and chemical constitution of phenylpyridazinones

The bleaching effect of 2-phenylpyridazinones substituted at the 4 or 5 position of the pyridazinone moiety or at the phenyl ring (position 9) was assayed using a greening Scenedesmus mutant after its transfer from heterotrophic to autotrophic growth. The following relationship between bleaching activities and Hammett electronic parameters of the various substituents could be demonstrated. The biological activity of the pyridazinone skeleton was enhanced with substituents showing (a) increasing σ_m values in position 4, (b) increasing σ_m or σ_p values in position 9, and (c) decreasing σ_p values in position 5. These findings could be corroborated by data on pigment bleaching and decrease of photosynthetic oxygen evolution of autotrophic (green) wild-type Scenedesmus after growth in the presence of sublethal concentrations of pyridazinones. There is no structure/activity relationship with direct inhibition of photosynthetic electron transport. Based on electronic parameters, the construction of phenylpyridazinone derivatives with bleaching activity is proposed.     

Mechanisms of herbicide sorption in microalgae and the influence of environmental factors

The sorption kinetics of photosystem II herbicides in the unicellular microalgae Ankistrodesmus braunii are independent of the herbicide concentration. While diuron-type herbicides attain an equilibrium state of sorption after at most 5 min, it takes up to 60 min for phenol-type herbicides. The kinetics of the inhibition of photosynthetic electron transport display a good correlation to the kinetics of sorption. For both types of herbicides, a phase of prevalent partitioning into the lipid phase of the cell membranes preceeds binding to thylakoid membranes. With phenol-type herbicides this phase is much more extended than with diuron-type compounds. As a consequence, there is no displacement of previously bound herbicides of this type from algal cells by addition of an excess of other photosystem II herbicides. The sorption of phenol-type herbicides to microalgae is under strong influence of environmental factors, such as light, temperature, pH, or oxygen concentration. Thus, besides the lipophilicity of these biocides, the response of the target cells to environmental factors is at least as important for herbicide sorption in the cells. From these results it is concluded that a prediction of the biocide accumulation in aquatic microorganisms can only be partly deduced from the properties of the pesticide molecule. The target cells and their complex responses to their environment strongly relativize correlations of chemical properties, e.g., lipophilicity and biological activity.     

Relationship between dew presence and Bassia dasyphylla plant growth

Dew has been recognized for its ecological significance and has also been identified as an additional source of water in arid zones.We used factorial control experiment,under dew presence in the field,to explore photosynthetic performance,water status and growth response of desert annual herbage.Bassia dasyphylla seedlings were grown in contrasting dew treatments (dew-absent and dew-present) and different watering regimes (normal and deficient).The effects of dew on the water status and photosynthetic performance of Bassia dasyphylla,grown in a desert area of the Hexi Corridor in Northwestern China,were evaluated.The results indicated the presence of dew significantly increased relative water content (RWC) of shoots and total biomass of plants in both water regimes,and enhanced the diurnal shoot water potential and stomatal conductance in the early morning,as well as photosynthetic rate,which reached its maximum only in the water-stressed regime.The presence of dew increased aboveground growth of plants and photosynthate accumulation in leaves,but decreased the root-to-shoot ratio in both water regimes.Dew may have an important role in improving plant water status and ameliorating the adverse effects of plants exposed to prolonged drought.     

Pyridazinones as bioregulators in crop protection

Substituted pyridazinones induced specific decreases in the linolenic acid content accompanied by increases in the linoleic acid content of plant membranes. The ratio of linoleic acid to linolenic acid was used to establish structure-activity correlations for a series of about 50 different substituted pyridazinones. The most active compound found was 4-chloro-5-dimethylamino-2-phenylpyridazin-3-one. X-ray structural analysis of this compound yielded three types of crystals differing in their molecular parameters. Application of computerised cluster analysis confirmed the specific biological activity of selected groups of compounds. Electron-density calculations yielded substituents with specific activity on the linoleic acid/linolenic acid ratio. Correlation of this ratio, produced by different substituted pyridazinones, can be used to search for potentially more active compounds. Treatment of plants, moulds and insects with pyridazinones led to interesting results of importance to agriculture, and confirmed the bioregulator activity of some of these compounds.     

Effect of different photosystem II inhibitors on chloroplasts isolated from species either susceptible or resistant toward s-triazine herbicides

In chloroplasts isolated from susceptible and atrazine-resistant Amaranthus retroflexus, the inhibition of photosynthetic electron transport by various classes of herbicides has been investigated. Resistance of mutant Amaranthus is not restricted to s-triazines but also extends to uracils, 1,2,4-triazine-5-ones, and ureas. For 1,2,4-triazin-5-ones and chloroplasts of both biotypes, a correlation between inhibition of photosynthetic electron transport and the partition coefficient could be established. In the case of phenolic herbicides only modestly decreased or even higher sensitivity of chloroplasts from the resistant biotype as compared to the susceptible one could be observed. These results are confirmed by binding of radioactively labeled herbicides to chloroplasts of both plants. Specific binding of atrazine or metribuzin to resistant chloroplasts is completely abolished, and that of diuron or phenisopham diminished as compared to susceptible chloroplasts. In contrast, binding of phenolic herbicides generally is enhanced in resistant chloroplasts. Photoaffinity labeling of thylakoids from both biotypes by 2-azido-4-nitro-6-[2′,3′-³H]isobutylphenol yields almost identical labeling patterns. These results are consistent with a recently proposed model (W. Oettmeier, K. Masson, and U. Johanningmeier, Biochim. Biophys. Acta679, 376 (1982) of two different herbicide binding proteins at the reducing side of photosystem II: a 32- to 34-kdalton protein responsible for binding of triazines, triazinones, ureas, and related herbicides and a photosystem II reaction center protein for binding of phenolic herbicides.     

Detection of triazine and urea herbicide residues by various characteristics of oat seedlings in bioassays

Atrazine, simazine, diuron, and linuron applied to soil increased the percentage moisture of oat (Avena sativa L.) shoots in bioassays at the lowest dose tested of 0·25 ppm. Further increases occurred up to 2 ppm but at higher concentrations the percentage moisture decreased. At all doses of each herbicide, shoot dry weight was decreased. In oats grown on soil collected from a peach orchard which had received repeated annual applications of these herbicides, the percentage moisture of the oat shoots was higher than the control value whenever the oat dry weight was decreased and provided a method of residue detection as sensitive as dry weight measurements. Treatment of oats by soil application of the above herbicides in bioassays also caused increases in the electrical conductivity of an aqueous extract of the oat shoots per mg of dry weight and this characteristic was slightly more sensitive than dry weight in detecting herbicides in orchard soil. The conductivity of the extract per mg of water in the shoots, however, only increased as percentage moisture decreased. The weight of neutral water-soluble material in oat shoots decreased much more rapidly than dry weight in bioassays with standard herbicide concentrations. Determination of the weight of neutral water-soluble material in oat plants grown on orchard soil samples indicated the presence of herbicide residues in 50% of the cases in which residues were not detectable by dry weight. The weight of neutral material as a percentage of dry weight was almost as sensitive. Chemical analysis of soil in which oat plants had a decreased level of neutral water-soluble compounds indicated that this characteristic had a lower limit of detection for herbicide residues of approximately 0.10 ppm.     

Effects of EPTC, fluorodifen and monuron on transpiration and photosynthetic oxygen output in Eupatorium odoratum plants

The influence of foliar sprays of EPTC, fluorodifen and monuron on transpiration and photosynthetic oxygen output of Eupatorium odoratum plants is demonstrated. Foliar sprays with 100 ppm EPTC or monuron led to a significant decrease in transpiration for a continuous period of 14 days. Treatment with 100 ppm fluorodifen led to a noticeable increase in transpiration during the first eight days. All three herbicides significantly reduced photosynthetic oxygen output from the Eupaiorium plants. EPTC and monuron however, gave greatest effect. Reduction in transpiration from the plants was due to stomatal closure while reduction in photosynthetic oxygen output was due to both stomatal closure and probably also inhibition of the Hill reaction.     

Formation of nitrite in plants treated with herbicides that inhibit photosynthesis

Avena sativa and Sinapis alba plants were sprayed with selected herbicides and time was allowed for the herbicides to penetrate into the plants. The plants were then illuminated in a nitrogen atmosphere for 60 min. Nitrite (up to 600 nmol/g fresh weight) was found in plants treated with herbicides that inhibit photosynthesis; these plants died a few hours later. Nitrite (up to 408 nmol/g fresh weight) was found in plants treated with herbicides that uncouple oxidative phosphorylation, but here the highest values were obtained after incubation in air. No nitrite was found in control plants incubated in nitrogen or in plants treated with photosynthesis-inhibiting herbicides and incubated in air. Experiments with plants either grown on different nitrogen sources or adapted to different light intensities rule out the possibility that the damage observed in the plants could be caused by nitrite. Possible reasons for the herbicidally induced damage are discussed.     

Mechanisms of action and structure activity relations of herbicides that inhibit photosynthesis

A survey and chemical classification are given for the most important herbicidal inhibitors of photosynthesis. The photochemical reactions involved in photosynthesis are reviewed. The herbicides discussed here all interfere with photosynthetic electron transport in the light reactions. Comments are made on the redox catalysts and the electron transport chain. The mode of action of the herbicides due to inhibition of the light reactions I and II or of photophosphorylation are described. Structure activity correlations according to the regression analysis (Hansch-approach) are discussed with examples in the classes of acetanilides, benzimidazoles and triazinones. The correlation studies in the series of the Hill-reaction inhibitors have led to a model for the essential structural elements. Some work on the problem of selectivity and its importance is reviewed.     

Target sites for herbicides: entering the 21st century

At present the use-rate of modern herbicides is in the range of 100-300 g AI ha-1, with a tendency to decline. The low use-rate (ca 10 g AI ha-1) of the original sulfonylurea and cyclic imide herbicides prompted agrochemical scientists to look for even more active compounds which led to the successive discoveries of many new herbicidal acetolactate synthase inhibitors and no less than 18 cyclic imides in the class of protoporphyrinogen-IX oxidase inhibitors in the 1990s. In this paper, mechanisms of action related to function and biosynthesis of chlorophylls, carotenoids, plastoquinone, amino acids, fatty acids and photosynthetic electron transport and other metabolic processes are discussed as plant-specific herbicidal target domains.     

CHEMICAL WEED CONTROL IN MAIZE

Summary. From 1959 to 1962 the effects of some triazine and growth regulator herbicides applied pre-emergence and post-emergence to maize grown for silage were studied in field experiments on light to medium loam soils at Harper Adams College, Shropshire.
Atrazine applied pre-emergence at 1–1.5 lb/ac gave good control of annual weeds and was superior to simazine at equivalent rates. Soil moisture and rainfall after spraying were found to influence the activity of the triazines, but both were completely tolerated by maize. In 1960, 1961 and 1962 atrazine was much more effective against annual weeds than post-emergence applications of MCPA and 2,4-D. However MCPA and 2,4-D gave adequate control of a range of annual and perennial weeds under favourable conditions in 1959 and 1960 but results were poor in the two following years due to the presence of resistant and late germinating weeds .
The effect of the herbicides on development and yield of ihe maize are discussed.
Le désherbage chimique dans le mais     

Performance of pyrazosulfuron-ethyl in non-puddled transplanted rainy season rice and its residual effect on growth of the succeeding crop in rice-wheat cropping pattern

Abstract

A study was carried out in sandy clay loam textured soil of Mymensingh, Bangladesh to evaluate weed control efficiency of pre-emergence herbicide pyrazosulfuron-ethyl either alone or in sequential application with post-emergence herbicide in transplanted rainy season rice under non-puddled strip-tilled (NPST) field condition and also to examine the residual effect of those herbicides on germination and growth of the subsequently grown wheat crop. Five treatment combinations of pyrazosulfuron-ethyl were tested against one weedy check and one weed-free check. The study revealed that NPST rice field was mostly infested by grass and sedge weeds and herbicide treatments offered a wide range of control (above 50% to 95%) on all types of weeds. Application of pyrazosulfuron-ethyl followed by (fb) orthosulfamuron fb butachlor plus propanil provided the most effective and economic weed control over two years of the study. Moreover, micro-plot bioassay study claimed germination and growth of subsequently grown wheat were not adversely affected by herbicides that were applied in rice. Therefore, application of pyrazosulfuron-ethyl followed by post-emergence herbicide could be effective and economic to control weeds in NPST rice under rice-wheat system, but proper rate and time of application should strictly be followed.