ENVS Midterm Condensed Notes PDF - Pesticides and the Environment

lOMoARcPSD|38059268 ENVS Midterm - condensed notes Pesticides and the Environment (University of Guelph) Scan to open on Studocu Studocu is not sponsored or endorsed by any college or university Downloaded by Smella tk ([email protected]) ...

lOMoARcPSD|38059268 ENVS Midterm - condensed notes Pesticides and the Environment (University of Guelph) Scan to open on Studocu Studocu is not sponsored or endorsed by any college or university Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Unit 03 Pesticide Selection: Choose based on end use and application method; consider specific pests, like cockroaches in food pantries. Active Ingredients (a.i.s): Effective for pest control; selection depends on specific circumstances. Formulants: Mixed with a.i.s to enhance characteristics; include: Adjuvants: ○ Spray Activators: Improve pesticide efficacy. ○ Utility Modifiers: Maintain spray solution integrity and broaden application conditions. Preservatives: Protect a.i.s from degradation. Chemical Modifications: Altering a.i.s (e.g., changing functional groups) can enhance properties; example: 2,4-D's solubility improved through formulation. Formulation Types: Available in dry (granules, powders, dusts) or liquid forms (emulsifiable concentrates, sprays). Some are converted to gas during application. Application Methods: Includes traps, misters, and aerial sprays; requires expertise in pest biology and environmental impact. Training and Certification: Essential in industrialized nations; often lacking in developing nations, leading to pesticide poisonings. Pesticide Labels: Must be read before use; enforceable documents in Canada. Provide critical information on a.i.s, storage, hazards, application rates, and required personal protective equipment (PPE). PPE Importance: Reduces contact and exposure; includes chemical-resistant clothing and respirators. - Definition of Pesticide: Agents (chemical or otherwise) used to control pests, named after the pests they target. - Importance of Selection: Choosing the correct pesticide is crucial for effectiveness and safety; reading labels is essential. - Considerations Before Purchase: - Ensure it is registered for the specific pest and crop. - Check registration in both the country and local jurisdiction. - Confirm compatibility with crop management strategies (e.g., reentry times). - Verify acceptable pre-harvest interval. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 - Choose the least toxic option for applicators and others. - Minimize impact on beneficial species and the environment. - Discourage pest resistance. - Ensure control of secondary pests. - Select the right formulation for available application equipment. - Check compatibility with other pesticides needed for the crop. - Pesticide Formulation: Pesticides are rarely used in pure form; they are combined with other ingredients (formulants) to create ready-to-use products. Pesticide Active Ingredients: Approximately 800 active ingredients exist. Formulated Products: There are thousands of formulated pesticide products derived from these active ingredients. Example: Some chemicals, like 2,4-D, have numerous formulated product variations Pesticide Application: Most pesticides need to be formulated for easy and safe application. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Goals of Formulation Chemists: 1. Facilitate application with common equipment. 2. Enhance delivery and uptake by pests. 3. Ensure safety for applicators and the environment. 4. Provide a good shelf life for products. Solubility Challenges: Many pesticides, like 2,4-D, are sparingly soluble in water, making them difficult to apply as pure active ingredients. Example with 2,4-D: Typical concentration for spraying is 1 kg in 200 L of water (5,000 mg/L). 2,4-D acid’s solubility is only 311 mg/L, requiring multiple applications for effective coverage. Formulating 2,4-D as a dimethylamine salt increases its water solubility, making it easier to use. 2,4-D can also be converted to esters (e.g., butoxy-ethyl ester) for oil solubility, allowing for applications in oil or emulsions. Advantages of Formulation: Properly formulated products reduce the need for multiple applications of the active ingredient. Importance of Formulation Selection: Choosing the safest and most effective pesticide formulation is crucial for specific situations. 2,4-D Ester Formulations: Volatility: Esters can vaporize from plant leaves, potentially harming nearby sensitive broadleaf plants. Usage Restrictions: Non-volatile 2,4-D amine formulations are preferred for landscapes near sensitive crops (e.g., soybeans, cotton, tomatoes, grapes). Preferred Applications: Esters are favored for controlling woody plants in remote areas. Dicamba Concerns: Similar to 2,4-D, dicamba is volatile and can cause off-target damage to sensitive crops under hot conditions. Common Types of Pesticide Formulations: Solids: Includes dusts, wettable powders, granules, pellets, tablets, and dry flowables. Liquids: Comprises solutions, emulsifiable concentrates, and flowable suspensions. Gases: Typically sold as volatile liquids or solids that release gas when mixed with water or other substances. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Adjuvants Definition: Substances (surfactants or others) added to pesticide sprays to enhance biological activity and application characteristics. Role of Adjuvants: Can be pre-mixed by manufacturers as formulants or added during spraying for optimal performance based on specific conditions (e.g., sprayer type, weather, crop, pest). Types of Adjuvants: Spray Activators: Improve pesticide efficacy and performance. ○ Examples: Surfactants for better wetting, spreading, and emulsifying. Wetting agents for mixing wettable powders with water. Spreaders for uniform spray coverage. Water conditioning agents to neutralize hard water cations that can inactivate pesticides. Oil concentrates to enhance contact by disrupting waxy cuticles. Stickers to reduce wash-off from rain or irrigation. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Utility Modifiers: Help maintain the integrity of the spray solution and broaden the pesticide's effectiveness under varying conditions. Examples of Utility Modifiers Buffering Agents: Adjust the pH of the spray solution to improve pesticide solubility. Compatibility Agents: Enhance the mixing of multiple pesticides or pesticides with fertilizers. Spray Thickeners: Reduce spray drift by increasing viscosity and droplet size. Usage and Market Trends Adjuvant Proportions: Typically mixed at 1-3% of total spray volume, with newer concentrated adjuvants used at lower rates (0.25 to 1.0%). Market Growth: The global agricultural adjuvants market was valued at approximately USD 3.1 billion in 2020 and is projected to reach USD 4.4 billion by 2026, growing at 6.1%. Cost: Adding adjuvants costs between $2.00 and $20.00 per hectare; although they may be more expensive per unit than pesticides, their efficacy often justifies their use. Future of Pesticide Formulations Microencapsulation: Future pesticides may be developed as "Triggered Release Products" to release active ingredients at optimal times for pest control. Innovative Applications: Pesticides could be applied via biodegradable carriers, such as impregnated strings in crop furrows or nets over trees. Ready-to-Use Packaging Variety of Containers: Homeowner products come in various ready-to-use formats (e.g., traps, aerosol sprays, spray bottles) designed to minimize exposure and ensure proper application rates. Agricultural Packaging: Includes "lock and load" containers for sprayers and water-soluble bags for easy use, emphasizing applicator safety. Applicator Training and Responsibility Importance of Training: Effective application relies on proper training and understanding of agrochemical use, as highlighted by historical quotes emphasizing that application techniques are critical for pesticide effectiveness. Legislation and Enforcement: Supporting training through legislation can help mitigate problems in pesticide handling at low cost. Applicator Accountability: No single application method guarantees proper pesticide use; responsibility lies with the applicator. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Training Programs: In Western Europe and North America, there are extensive training and licensing programs for commercial pesticide applicators and growers. Misuse can lead to revoked privileges. Developing Countries: Many agricultural pesticide applicators lack training, and there is limited information on training in major pesticide-user countries like China, despite recent licensing law revisions. Homeowner Training: Most countries do not offer training for homeowners using domestic pesticide products. Application Equipment Variety of Equipment: Pesticide application equipment ranges from simple aerosol cans to advanced computer-controlled sprayers. Selection depends on area size, pest type, pesticide formulation, and application method. Hand-Operated Sprayers Usage: Suitable for small areas; typically operate under compressed air. Types: ○ Pressurized Cans: Small, non-reusable cans for garden and household pest control. ○ Trigger Pump Sprayers: Non-pressurized; pesticide is dispensed by squeezing the trigger, commonly used for garden pests and weeds. ○ Hose-End Sprayers: Attach to garden hoses; mix pesticide with water at a predetermined rate, reducing exposure to concentrated materials. ○ Hand-Held Hydraulic Sprayers: Require a tank, pressure source, and nozzle system for uniform spray application. Motorized Sprayers Differences from Hand-Held Sprayers: Motorized sprayers use power-driven pumps for pressure and are typically mounted on vehicles or equipment like trucks, trailers, or tractors. Low-Pressure Hydraulic Boom Sprayers Common Use: Predominantly used for applying pesticides (especially herbicides) in large fields, golf courses, and open areas. Specifications: ○ Delivery capacities: 50 to 500 L of spray solution per hectare. ○ Boom length: Typically 6 to 10 m, with nozzles spaced 50 to 100 cm apart. ○ Equipped with agitation devices for uniform mixing of formulations. Advancements: Larger, computerized sprayers with air-conditioned cabs are preferred for large-scale farming and custom application. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 High-Pressure Hydraulic Boom Sprayers Application Need: Used when dense foliage or tree tops require thorough pesticide coverage. Pressure Capability: Can develop pressures up to 7,000 kPa, ensuring even spraying and penetration. Air Blast Sprayers Functionality: Combine air and liquid to deliver pesticides to tree canopies. Mechanism: Pesticide is pumped through nozzles and mixed with air from a high-speed fan, creating fine droplets for better penetration. Versatility: Adjustable for high or low spray volumes and equipped with agitation. Aerial Application Usage: Suitable for large field crops and forests, especially for ultra-low volume applications. Advantages: Faster and more fuel-efficient than ground application, but may raise health and environmental concerns due to noise and potential drift. Limitations: Weather-dependent and may not penetrate canopies as effectively as ground methods. Ultra-Low Volume (ULV) Sprayers Application Rates: Require little or no liquid carrier, with rates typically around 5 to 6 L/ha. Risks: Higher exposure risk to applicators due to concentrated formulations; only a few pesticides are registered for ULV use. Special Types of Pesticide Application Equipment Dust Applicators: Used for spot treatments; drift can be a significant issue. Granular Applicators: Previously used for applying granule fertilizers and herbicides; their use has declined due to regulatory changes. Herbigation Equipment: Injects pesticides into center-pivot irrigation systems; concerns about water contamination necessitate backflow prevention measures. Greenhouse Applicators: Developed for low-volume applications in enclosed environments, where fine droplets are beneficial. Pesticide Application Methods 1. Foggers: ○ Thermal Pulse-Jet Foggers: Use heat to generate fog for pesticide delivery. ○ Cold Foggers: Mechanical aerosol generators that produce a fine mist without heat. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 ○ Air-Assisted Rotary Mist Generators: Combine air and liquid for efficient pesticide application. ○ Air-Assisted Electrostatic Sprayers: Utilize electrostatic forces to enhance pesticide coverage. 2. Integrated Pest Management (IPM): Greenhouses are ideal for IPM, promoting the use of beneficial insects. Careful selection of insecticides is essential to avoid harming non-target insects. Calibration of Application Equipment Purpose: Ensures pesticides are applied at the rate specified on the product label. Sprayer Output: Measured in liters per hectare, requiring careful measurement and adjustments for consistency throughout the application process. Environmental Conditions to Consider Temperature: Affects pest activity; applying pesticides when pests are inactive can be ineffective. Humidity: Essential for plant diseases; conditions must be optimal for fungicides to work. Rainfall: Can wash pesticides off leaves, reducing efficacy; check rain-fastness on labels. Wind: High winds can cause drift, leading to non-target damage. Other Factors Influencing Efficacy 1. Water Quality: ○ Sediment-free and appropriate pH levels are crucial for effectiveness. ○ Hard water can deactivate some pesticides (e.g., glyphosate) due to binding with divalent cations. 2. Plant Growth Stage: ○ Herbicides should be applied during the germination or active growth stages for best results; thicker cuticles in mature or stressed plants hinder penetration. 3. Application Timing: ○ Herbicides can be applied pre-plant, pre-emergence, or post-emergence, with timing critical for crop selectivity. ○ Protectant fungicides must be applied before infection, while systemic fungicides may require specific placement for effectiveness. Unit 04 Over 700 pesticide active ingredients exist today. Pesticides can be classified based on the user’s purpose or goal. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Farmers classify pesticides by target pests and application method (e.g., pre-plant or post-emergence herbicides). Agricultural scientists classify them by chemical families, as similar pesticides share characteristics like crop tolerance and environmental impact. Environmentally, pesticides are best classified by mechanism of toxic action and chemical structure to predict risks to non-target organisms. Understanding different modes of action is key to managing pesticide resistance, which is a growing concern. Rotating pesticides with different mechanisms of action helps prevent pest resistance. Organizations like FRAC, HRAC, IRAC, and RRAC monitor resistance to pesticides. Oral toxicity dose (AOLD50) for rats and acute toxicity concentration (LC50) for sensitive fish are key indicators of toxicity. Additional toxicity and environmental data can be found in the Pesticide Properties Database (PPDB) and other scientific sources. Pesticides affecting energy production primarily target photosynthesis, mostly functioning as herbicides. Photosynthesis involves converting light energy into biologically useful energy (sugars) through chlorophyll in plant chloroplasts. Chloroplasts, found in plant cells, consist of thylakoid membranes and grana, where photosynthesis occurs. Chlorophyll A is the main pigment, with Chlorophyll B and β-carotene aiding in light absorption and protecting chlorophyll from damage. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Two important protein complexes, Photosystem II (PS II) and Photosystem I (PS I), are located in the thylakoid membranes of chloroplasts. PS II is found in the densely packed grana, while PS I is on the outer surface of less dense membranes. The fluids inside the thylakoid membranes (lumen) and in the surrounding stroma contain enzymes for ATP synthesis and carbon dioxide fixation. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 PS II and PS I are connected through a transport chain of electron and hydrogen carriers. Electrons flow from water to NADP+ via a process called non-cyclic electron transport, producing NADPH. Electron transport in photosynthesis starts when chlorophyll in the Photosystem II (PS II) complex absorbs light at a wavelength of 680 nm. The donor side of PS II contains a manganese-dependent water-splitting enzyme (molecular weight: 34 kD) that donates an electron from P680, generating an excited state (P680*). The excited electron is transferred through a series of carriers: From P680* to pheophytin, Then to plastoquinones, Next to cytochromes, Finally, to plastocyanin, which carries it to the receptor side of Photosystem I (PS I). Each PS I complex has a molecular weight of 67 kD and is designed to accept the electron transferred from plastocyanin. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 In Photosystem I (PS I), an electron is accepted from plastocyanin when PS I absorbs light at a wavelength of 700 nm, transitioning from its oxidized state (P700*) to its ground state (P700). The electron from P700 is then transported through multiple iron-sulfur (Fe-S) proteins to ferredoxin-NADP+ reductase, which catalyzes the formation of NADPH. Oxidation of quinones and ferredoxins releases protons into the thylakoid lumen, creating a proton gradient that drives ATP synthesis on the membrane’s outer surface, linking photophosphorylation and electron transport. The oxidation of ferredoxins marks the end of non-cyclic photophosphorylation, but there is also a cyclic process where cytochromes can accept electrons from ferredoxin, further contributing to the proton gradient for ATP production. This cyclic system can produce ATP without NADPH production or oxygen evolution, helping to balance ATP/NADPH ratios to meet cellular metabolic needs. During carbon dioxide fixation, five-carbon sugar molecules are phosphorylated using ATP from photophosphorylation, and CO2 is reduced with NADPH to form two three-carbon sugars, which can then combine to create a six-carbon sugar. Various methods exist to measure photosynthesis rates or their inhibition, including: Measuring oxygen evolution in light-treated plants. Measuring carbon dioxide fixation in light-treated plants. Monitoring fluorescence energy loss in chloroplasts due to blocked electron transport (PAM fluorimetry). Conducting the “Hill Reaction” with isolated chloroplasts and an artificial electron acceptor (e.g., DCIP) to measure the rate of color loss in the presence of Photosystem II inhibitors. Numerous herbicides targeting Photosystem II have been identified, as indicated in related tables and sample structures. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Notable families of herbicides include symmetrical-triazines (e.g., atrazine) and ureas (e.g., linuron), which contain functional groups (-C-O- or -N=C-N-) that bind strongly to the plastoquinone Qb serine site in Photosystem II (PS II). Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Bromoxynil and ioxynil are post-emergence herbicides effective against broad-leaved weeds in crops such as maize, sorghum, and onions. Transgenic research aims to transfer nitrolase genes from bacteria to resistant crops like cotton for selective weed control. Bromoxynil and ioxynil have moderate toxicity in mammals, with AOLD50s between 100 to 200 mg/kg, and are non-persistent in soil, with half-lives of 1 and 10 days, respectively. Atrazine: Inhibitor of PS II Atrazine and similar herbicides primarily inhibit photosynthesis by targeting PS II. They compete with plastoquinone for binding at the electron transfer site, blocking electron flow to cytochromes and PS I. This inhibition results in the cessation of photophosphorylation (ATP production) and NADP+ reduction to NADPH, halting carbon dioxide fixation and sugar synthesis. Plants treated with PS II inhibitors die faster than those placed in darkness, as blocked PS II continues to harvest light but causes the production of toxic free radicals due to untransferred electrons. These free radicals damage chloroplast proteins and membranes, leading to membrane destruction, cell leakage, chlorosis (loss of chlorophyll), and cell death. Symptoms of chlorosis vary with the herbicide type, with early symptoms often appearing at the leaf margins. Chlorosis Symptoms: Atrazine and other triazines cause chlorosis at the leaf margins. Urea herbicides, such as diuron and linuron, cause chlorosis near leaf veins. Atrazine Overview: Developed in the 1950s by Geigy Chemical Co. (now Syngenta). Widely used as a selective soil-active herbicide for controlling germinating weeds in maize and related crops. Maize is a major field crop in the USA and globally, contributing to atrazine’s prevalence since the 1960s. Close analogue simazine is more selective for use in orchards, while cyanazine is less persistent and often used when atrazine residues are a concern. Terbuthylazine is used as an atrazine substitute where restrictions exist (e.g., EU). Regulatory Context: EU restrictions on atrazine usage stem from its potential to leach into shallow aquifers, risking drinking water contamination. The Drinking Water Directive mandates pesticide concentrations in drinking water not exceed 0.1 μg/l for a single pesticide and 0.5 μg/l for total pesticides. Environmental Impact and Toxicology: Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Atrazine is moderately persistent and mobile in soil, leading to carry-over residues that complicate crop rotation and increase pest issues. Continuous use has resulted in triazine-resistant weeds and atrazine contamination in watersheds. Recommended application rates have been reduced to 1.5-2.0 kg/ha for effective weed control. Field half-lives range from 16 to 77 days in soil and 10 to 100 days in water. Low toxicity to mammals, with AOLD50s in mice and rats ranging from 1,300 to 4,000 mg/kg. Acute LC50 for most fish species is greater than 4 mg/L. Controversy and Research: Atrazine has been the subject of debate regarding its ecological impact, with mixed findings on its effects on aquatic animals and mammalian reproductive systems. Some studies reported adverse effects, while others found no significant evidence. A Weight of Evidence analysis concluded adverse effects in aquatic animals are unlikely at environmentally relevant concentrations. High doses in the 1990s were linked to increased mammary tumors in female rats, but these findings were deemed not relevant to humans. Other Inhibitors of Photosynthesis: Besides triazines and ureas, at least eight other herbicide groups also inhibit PS II. Most of these herbicides are relatively non-toxic to non-target organisms (e.g., rats, bees, fish), except for benzonitriles, which uncouple photophosphorylation. Photosynthesis inhibitors could indirectly affect aquatic organisms by reducing oxygen production from aquatic plants. Some chemicals in these groups are toxic to algae, with phytotoxicity variations potentially linked to uptake differences from water. HRAC Class 22 Herbicides: These herbicides kill plants by diverting electrons from Photosystem I (PS I). They are toxic to other organisms because they can also disrupt electron transport in mitochondria, affecting energy production. Examples: Specific members of this class are detailed in Table 6-4. Two examples of herbicides from this class are discussed in further detail in the following sections. 6.1.2.1 Paraquat and Diquat Types: Bipyridilium herbicides (e.g., paraquat and diquat). Uses and Mode of Action: These herbicides interact with Photosystem I (PS I) but do not block electron transport. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Paraquat and diquat cations compete with ferredoxin for electrons emitted by PS I, diverting the flow of electrons. This diversion prevents NADP+ from being reduced to NADPH. Reactions and Effects: When paraquat cations intercept an electron, they become reduced paraquat free radicals. The free radicals react with oxygen, regenerating paraquat cations and producing superoxide (O2*). Some superoxide can react with hydrogen ions (H+) to form hydrogen peroxide (H2O2), which interacts with paraquat to produce highly reactive hydroxyl radicals ( OH). Hydroxyl radicals attack fatty acids in thylakoid and cell membranes, causing lipid peroxidation, membrane destruction, and rapid cell death. Toxicity: Paraquat itself is not directly toxic but acts as a catalyst for the production of harmful oxidants, leading to significant damage in plant cells. Basic Processes of Respiration The process of respiration is fundamentally similar across all living organisms, making pesticides that inhibit respiration highly toxic to a diverse array of organisms, including plants, animals, insects, fungi, and bacteria. Here’s an overview of the respiration process: 1. Overview of Respiration: Respiration can be viewed as the opposite of carbon dioxide fixation. It involves the oxidation of carbohydrates, leading to the production of carbon dioxide (CO2), water (H2O), and reducing compounds like NADH, along with high-energy compounds such as ATP. 2. Key Processes in Sugar Respiration: Glycolysis: Occurs in the cytoplasm. Converts glucose into pyruvate, producing a small amount of ATP and NADH. Krebs Cycle (Tricarboxylic Acid Cycle): Takes place on the surfaces of the inner mitochondrial membranes (cristae). Processes pyruvate to generate CO2, ATP, NADH, and FADH2. Electron Transport System (Oxidative Phosphorylation): Occurs within the membranes of the cristae in the mitochondrion. Involves a series of redox reactions that lead to the production of a large amount of ATP through chemiosmosis. These processes work together to efficiently convert the energy stored in carbohydrates into usable energy forms for the cell, underscoring the importance of respiration in all life forms. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Glycolysis and the Krebs Cycle Glycolysis 1. Hydrolysis of Starch: Starch is hydrolyzed to form sugars. 2. Conversion to Pyruvic Acid: The sugars are then broken down and oxidized to produce pyruvic acid (or pyruvate). During this process, hydrogen ions (H⁺) and electrons are released. 3. Production of NADH: The liberated hydrogen ions and electrons are utilized in the reduction of NAD⁺ to form NADH, an important energy carrier. Krebs Cycle (Citric Acid Cycle) 1. Entry of Pyruvic Acid: Pyruvic acid produced in glycolysis undergoes a dehydrogenase reaction with Coenzyme A (CoA-SH), resulting in the release of carbon dioxide (CO₂) and the formation of acetyl-CoA and NADH. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 2. Combination with Oxaloacetic Acid: Acetyl-CoA combines with oxaloacetic acid to enter the Krebs cycle. 3. Cyclic Metabolic Reactions: The Krebs cycle is a series of cyclic metabolic reactions that convert various organic acids into one another, primarily to extract energy from stored carbohydrates or lipids. 4. Regeneration of Oxaloacetic Acid: As the cycle progresses, oxaloacetic acid is regenerated to allow the cycle to continue. 5. Dehydrogenase Reactions: Throughout the cycle, several dehydrogenase reactions occur, producing NADH, FADH₂, and ATP. The two carbon atoms that entered the cycle as part of the acetyl-CoA are released as two molecules of carbon dioxide (CO₂). Summary The glycolysis and Krebs cycle together play a crucial role in cellular respiration, breaking down carbohydrates and lipids to release energy and producing important energy carriers (NADH and FADH₂) that will be used in the electron transport chain for further ATP production. Utilization of Reduced Pyridine Nucleotides (NADH and FADH₂) ATP Production 1. Electron Transport Chain: The reduced pyridine nucleotides (NADH and FADH₂) donate their electrons and protons (hydrogens) to the electron transport chain (ETC), located in the inner mitochondrial membrane. As electrons pass through a series of protein complexes (cytochromes), energy is released, which is used to pump protons (H⁺) across the mitochondrial membrane, creating a proton gradient. 2. Role of Oxygen: Oxygen serves as the ultimate electron acceptor in the ETC. At the end of the chain, electrons combine with oxygen and protons to form water (H₂O), which is a crucial step for maintaining the flow of electrons through the chain. 3. Chemiosmosis and ATP Synthesis: The proton gradient established across the inner mitochondrial membrane drives protons back into the mitochondrial matrix through ATP synthase, a process known as chemiosmosis. The flow of protons through ATP synthase provides the energy required to convert ADP and inorganic phosphate (Pi) into ATP, the high-energy phosphate compound. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Direct Utilization in Reductase Reactions In addition to ATP production, reduced pyridine nucleotides (NADH and FADH₂) can participate directly in various reductase reactions within the cell. These reactions involve the transfer of electrons and protons to reduce substrates, playing a key role in biosynthetic pathways and metabolic processes. Summary Reduced pyridine nucleotides like NADH and FADH₂ are essential for energy production through the electron transport chain, where they facilitate ATP synthesis via chemiosmosis. They also serve important roles in various biochemical reactions by providing reducing power for the synthesis of cellular components. Phosphorylation in the Electron Transport Chain Electron Transport System Overview 1. Oxidation and Electron Transport: In the electron transport chain (ETC), electrons from reduced pyridine nucleotides (NADH and FADH₂) are transferred through a series of protein complexes. As electrons flow through these complexes, they release energy used to pump protons (H⁺) across the inner mitochondrial membrane, creating a proton gradient. 2. Formation of Water: At the end of the chain, oxygen acts as the final electron acceptor, combining with electrons and protons to form water (H₂O). This reaction is crucial for maintaining the flow of electrons through the ETC. Coupled Phosphorylation At three key points in the electron transport chain, energy from the oxidation of electron carriers is coupled to the phosphorylation of ADP to form ATP. This coupling occurs through intermediates and ensures efficient energy conversion. ATP Yield Based on Electron Source 1. NADH vs. FADH₂: When electrons are supplied to the ETC by NADH, approximately 2.5 molecules of ATP are produced for each pair of electrons introduced. In contrast, electrons from FADH₂ yield around 1.5 molecules of ATP. This difference arises because FADH₂ enters the electron transport chain at a later point, resulting in fewer protons being pumped across the membrane. 2. P/O Ratio: Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 The P/O ratio (phosphate/oxygen ratio) measures the efficiency of ATP production in relation to oxygen consumption. A tightly coupled mitochondrial system typically has a P/O ratio greater than 2, indicating that for each atom of oxygen reduced, more than 2 ATP molecules are synthesized. Summary In the electron transport chain, the coupling of oxidation and phosphorylation occurs at several points, leading to the production of ATP. The amount of ATP generated depends on whether NADH or FADH₂ donates electrons, influencing the overall efficiency of cellular respiration and energy production. 138-141 Most pesticides known to be inhibitors of mitosis or cell division are particularly toxic to plants, thus most are either herbicides or fungicides Symptoms of mitotic cell inhibitors Arrested cell division between prophase and telophase, resulting in an elevated mitotic index (The mitotic index is the number of apparently dividing cells/ the number of resting cells) Disruption of microtubule assembly so that individual chromosomes migrate to form micronuclei Inhibition of cell wall deposition so that numerous multinucleated cells are formed - Normal Cell Division: - Chromosomes double in prophase. - Microtubule fibers (spindle fibers) form, made of tubulin. - These fibers relocate one chromosome of each pair to daughter cells during metaphase, anaphase, and telophase. - Action of Mitotic Inhibitors: - Mitotic inhibitors, like dinitroaniline herbicides, bind to microtubulin proteins, preventing tubulin addition at the synthesis end. - Microtubule degradation continues at the opposite end, causing fibers to shorten. - Eventually, the fibers disappear, preventing chromosome separation and relocation. - Cell division stops, abnormal cells form, and growth ceases. This leads to observable symptoms in affected tissues. Pendimethalin, Trifluralin, and Dinitroaniline Herbicides: Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Uses: ○ Dinitroaniline herbicides (e.g., trifluralin, pendimethalin) are primarily soil-active and effective against germinating weed seedlings, especially grasses. ○ Commonly used in crops like soybeans, canola, cotton, wheat, maize, and lawns (e.g., for crabgrass control). ○ Most are volatile in warm, moist soils and applied as pre-plant, soil-incorporated treatments, except pendimethalin, which can be used pre-emergence without incorporation. ○ They inhibit seedling growth, particularly lateral root formation. ○ Effective in controlling shallow, germinating grassy weeds while allowing deeply seeded crops to thrive. Environmental Impact and Toxicology: ○ Moderately persistent in soil, leading to possible residue carry-over, especially in fall-seeded crops. ○ High toxicity to fish, but their immobility in soil limits contamination of aquatic environments. ○ Dinitroaniline herbicides like trifluralin and pendimethalin have been detected in the Arctic, but at concentrations below toxicological concern. They can undergo long-range transport due to their volatility. Natural Auxins: Plant hormones like indole acetic acid (IAA) regulate growth processes (e.g., cell division, enlargement) throughout the plant life cycle. Auxin concentrations are tightly controlled by synthesis, degradation, and conjugation with amino acids. Low auxin concentrations stimulate growth, while high concentrations cause damage due to uncontrolled growth. Auxin Receptors: 1. Auxin-binding protein 1 (ABP1) – found in the endoplasmic reticulum (ER) and outer cell membrane. 2. Auxin-signaling F-box (TIR1/AFB) – found in the nucleus, regulates cell division. 3. S-phase kinase-associated protein 2 (SKP2) – also in the nucleus, connects auxin signaling with cell division. Regulation of IAA: The tonoplast-bound transport protein WAT1 transports IAA into the vacuole to regulate its concentration within the cell. Auxins influence the expression of genes and interact with other plant hormones like abscisic acid, cytokinin, and ethylene. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Herbicidal Auxin Mimics: Synthetic auxins (e.g., phenoxy herbicides) are less easily metabolized or detoxified than natural auxins. Their accumulation leads to distorted plant growth, including misshaped leaves, twisted stems, seedless fruits, and inhibited roots. Lower concentrations stimulate growth, but high concentrations inhibit it, causing classical symptoms of herbicidal exposure, such as thickened and twisted plant structures. Growth Inhibition in Young Tissues: ○ Auxin herbicides move with sugars to sites of active growth, affecting younger, rapidly growing tissues first. ○ These tissues show the most severe growth inhibition symptoms. Monocot Grass Tolerance: ○ Grasses (monocots) have greater tolerance to auxin herbicides due to differences in uptake, translocation, and metabolism. ○ While auxin herbicides cause typical auxin-related symptoms in broadleaf plants, they can also control some grass weeds. Symptoms in Grasses: ○ Injurious doses in grasses lead to chlorosis (yellowing) of leaves, necrosis (tissue death), and eventual death. ○ These symptoms differ from the distorted growth seen in broadleaf plants. Discovery: 2,4-D and MCPA were discovered during WWII as systemic, foliage-applied herbicides, selectively controlling broadleaf weeds in cereals and turf-grass. Natural auxins like indole acetic acid and indole butyric acid were isolated and synthesized in the 1930s, leading to the development of synthetic auxins like 2,4-D, MCPA, and naphthalene acetic acid. Current Uses: Indole butyric acid is used to stimulate root growth. Naphthalene acetic acid is used to control fruit abscission. Most auxin herbicides are non-persistent in soil and more toxic to deciduous plants than conifers, except for picloram and dicamba, which are highly active in soil and toxic to conifers. Agricultural Importance: 2,4-D and MCPA are post-emergence herbicides, effectively controlling broadleaf weeds in major crops like rice, wheat, and maize. Before the widespread use of 2,4-D in the 1950s, wild mustards caused significant yield losses in Canadian wheat and barley farming. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 REFER TO THE CHEMICAL STRUCTURES OF PESTICIDES IN THIS SECTION Environmental Impact and Toxicology of Synthetic Auxin herbicides: Soil Activity: Most synthetic auxin herbicides degrade easily in soil, except dicamba and picloram, which are moderately persistent and used for soil treatments targeting perennial weeds and woody plants. Spray Drift Hazards: A major concern is spray drift, which can damage nearby sensitive broadleaf crops (e.g., beans, tomatoes, grapes). Amine salt formulations are more water-soluble and less volatile than ester formulations. Ester formulations, though more effective on woody plants, pose risks of vapor drift, especially under high temperatures. Choline salts of 2,4-D, with very low volatility, reduce off-target damage in hot weather. 2,4-D and 2,4,5-T Controversy: Synthetic auxins faced controversy, beginning with their secret discovery during WWII and the possibility of their use in chemical warfare. Rachel Carson's "Silent Spring" criticized these herbicides for increasing nitrate levels in plants, potentially toxic to cattle. The use of Agent Orange (a mix of 2,4-D and 2,4,5-T) during the Vietnam War led to widespread opposition due to dioxin contaminants, known to be teratogenic. Although 2,4,5-T was banned, modern formulations of 2,4-D, with reduced dioxin contamination, are still in use. Re-registration: 2,4-D has been re-registered for lawn and turf use by the US EPA and Health Canada, though some provinces and cities have enacted bans on its cosmetic use. Toxicity: While synthetic auxins are debated, natural auxins (e.g., indole butyric acid) found in plants we eat can be more toxic to mammals than synthetic auxins like 2,4-D. Gibberellic Acids (GAs): Enhance elongation growth of plant stems, with more than 80 known types. Used to promote growth in crops like maize, rice, and for specific purposes such as stimulating sprouting in seed potatoes and improving cherry size and color. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Chlormequat Chloride (CCC): A synthetic plant growth regulator that inhibits gibberellic acid synthesis. Primarily used to reduce stem elongation in cereal crops to prevent lodging (falling over) and to stimulate lateral branching in flowers. Cytokinins: Natural hormones that promote cell division (cytokinesis). The ratio of cytokinins to auxins affects plant development, with higher cytokinin levels maintaining cell division and delaying leaf senescence. Thidiazuron: A synthetic cytokinin used commercially as a defoliant to aid cotton harvesting by promoting cell division in the abscission zone between stems and leaves. Page 170 - Page 190 Low-rate Herbicides (ALS Inhibitors): Benefits: ○ Require low application rates (2-100 g/ha) compared to earlier herbicides (thousands of grams). ○ Lower environmental contamination due to reduced use and quicker biodegradation. ○ Reduced risk of soil and water contamination, making them a safer option for chemical weed control. Problems: ○ Soil residue risks: Some crops like sugar beets and lentils are highly sensitive, and residue carry-over can damage subsequent crops, requiring growers to wait multiple seasons before planting sensitive species. ○ Development of herbicide-resistant weeds: Repeated use of ALS inhibitors can lead to resistant biotypes. These biotypes may become cross-resistant to other herbicides in the same family, posing a challenge for weed management. ○ Sensitive crops may be injured even by very low residues of ALS herbicides. Glyphosate: Uses: ○ A non-selective, systemic herbicide introduced in 1974, used globally for over 100 applications. ○ Inhibits the enzyme EPSPS, disrupting the shikimic acid pathway, reducing aromatic amino acids, and deregulating carbon flow in plants, ultimately leading to plant death. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 ○ Commonly used in agriculture for pre-planting weed control, forestry, and as a burn-down treatment for annual weeds before crop planting. ○ Post-emergence weed control in herbicide-resistant crops like maize, soybeans, and cotton, with some applications in aquatic weed control. Problems: ○ Environmental impact: Binds strongly to soil, limiting leaching beyond 30 cm, but degradation varies (half-lives range from 2 to 200 days). Risk of spray drift damage to nearby sensitive plants, but this risk is lower than for auxin herbicides like 2,4-D. ○ Increasing cases of glyphosate-resistant weeds, with over 53 reported species globally as of 2021. ○ Persistence of glyphosate’s effects in perennial plants and its impact on non-target organisms, especially in forestry and aquatic environments. Glyphosate-Based Herbicides (GBHs): Toxicity of Glyphosate: ○ Glyphosate as an active ingredient has low acute toxicity to both terrestrial and aquatic organisms. However, its poor ability to penetrate plant leaves requires the use of surfactants to aid absorption. ○ Surfactants like POEA are more toxic than glyphosate itself. For example, the toxicity (AOLD50) of glyphosate in rats is >5000 mg/kg, while POEA's toxicity is about four times higher at 1200 mg/kg. Human Toxicity: ○ Glyphosate has low toxicity to humans from environmental or food exposure, but concern grew when the International Agency for Research on Cancer (IARC) classified it as a "probable human carcinogen" in 2015. Regulatory agencies have disagreed, concluding that glyphosate is not carcinogenic, citing differences in data sources and risk assessment methods. Aquatic Toxicity: ○ Glyphosate is much less toxic to aquatic organisms than GBHs containing POEA. Some formulations of GBHs without POEA are less harmful to non-target aquatic organisms. For example, Cúspide 480SL®, formulated with a less toxic alkyl-polysaccharide surfactant, was found to be 10 times less toxic than formulations with POEA. Glufosinate Ammonium (HRAC Group-10): Mode of Action: ○ Glufosinate ammonium inhibits glutamine synthase, an enzyme essential for nitrogen assimilation and amino acid production. It competes with glutamic acid at the enzyme's binding site, leading to plant death. Uses: Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 ○ Glufosinate ammonium is a non-residual, non-selective, contact herbicide. It is used for post-emergence weed control in orchards and plantations and as a desiccant for potatoes and sunflowers before harvest. It is also used in glufosinate-resistant crops like soybeans, maize, and canola. Environmental Impact: ○ Glufosinate undergoes rapid microbial degradation and does not persist in soil, reducing the risk of environmental contamination. It has low toxicity to mammals, with AOLD50s ranging from 430 to 2000 mg/kg, and is relatively less toxic to fish and aquatic organisms. Fatty Acid Biosynthesis: Importance of ACCase: ○ ACCase is located in the chloroplasts of plant cells and plays a vital role in lipid synthesis, which is crucial for membrane development, especially during rapid seedling growth. ○ As a rate-limiting step in fatty acid biosynthesis, ACCase is a target for herbicides, making it sensitive to inhibition. Herbicidal Action: Toxicity to Grasses: ○ Most herbicides that inhibit lipid synthesis tend to be particularly toxic to grasses. However, there is a limited selectivity that allows for use in some cereal crops. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 ○ Crop tolerance to these herbicides often depends on the crop's metabolism. Slower metabolism of the herbicides can lead to increased crop injury, especially in cereals that are marginally tolerant. Translocation and Interaction with Other Herbicides: ○ Many of these herbicides do not translocate well within the plant. Their effectiveness can be reduced if mixed with auxin herbicides like 2,4-D. Symptoms of Injury: Injured Grass Seedlings: ○ The initial symptoms of herbicide injury in grass seedlings include inhibited root and shoot growth, leaf wilting, and chlorosis (yellowing of leaves). ○ As the injury progresses, the leaves may dry out and undergo necrosis (tissue death). Affected leaves can be easily detached, and injured seedlings can be pulled from the soil more easily due to compromised ACCase activity in rapidly growing tissues. Insecticidal and Acaricidal Activity: The text also mentions tetronic and tetramic acid derivatives that exhibit insecticidal and acaricidal activity, which are classified under IRAC Group-23. These newer products work by inhibiting ACCase in mites and some insects, further illustrating the enzyme's importance beyond just plant metabolism. Cyclohexanedione Herbicides (Dims) Example: Sethoxydim ○ Introduced in 1983, sethoxydim is a postemergence herbicide that selectively controls grassy weeds in broad-leaved crops. ○ It is rapidly translocated within plants, both upward (acropetally) and downward (basipetally). ○ Commonly used in various crops, including cotton, canola, soybeans, potatoes, and vegetables. ○ Acts as a potent inhibitor of acetyl-CoA carboxylase, affecting lipid synthesis, particularly in grasses. Related Herbicides: ○ Other dim herbicides share similar uses and mechanisms. Pinoxaden, while structurally different, acts similarly and has a comparable weed control spectrum. Environmental Impact and Toxicology: ○ These herbicides are non-persistent in the soil, with half-lives of less than 10 days. ○ They exhibit low toxicity to mammals (AOLD50 > 2,400 mg/kg for mice and > 1,200 mg/kg for rats) and low to moderate toxicity to fish. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 6.6.2.5.2 Aryl-Propanoic Acid Herbicides (Fops) Example: Diclofop-Methyl ○ Used to control grassy weeds like annual ryegrass and wild oats in cereals (wheat, barley) and other crops (canola, turf). ○ A selective and systemic herbicide that translocates both acropetally and basipetally. Related Herbicides: ○ Other fops include fluazifop-P-butyl and haloxyfop, with similar applications in broad-leaved crops. Environmental Impact and Toxicology: ○ Like dims, diclofop-methyl is non-persistent in soil (half-lives of 10 days or less) and has low toxicity to mammals. ○ However, it can be toxic to fish (with LC50s as low as 0.23 mg/L), but contamination of aquatic environments is rare due to its agricultural use pattern. Weed and Crop Resistance to ACCase Inhibitors Resistance to ACCase inhibitors (both dims and fops) is common among weed biotypes. ○ Many resistant biotypes show cross-resistance to other herbicides within these classes due to an altered ACCase that is less sensitive to inhibition. ○ An example of specific resistance is seen in a diclofop-resistant ryegrass from Australia, where the resistance mechanism may involve differences in the herbicide's metabolism rather than enzyme sensitivity. Transgenic Resistance: ○ Collaboration between chemical companies and DeKalb Genetics led to the development of transgenic maize hybrids resistant to sethoxydim. Some of these hybrids were introduced as early as 1997. Acaricides and Insecticides Derived from Tetronic and Tetramic Acids Example: Spirodiclofen ○ Application: Foliar-applied acaricide effective against various sucking mites and insects, including: Pear sucker Scale insects Earwigs Aphids Whiteflies ○ Crops: Used on apples, pears, grapes, peaches, apricots, nectarines, oranges, currants, tomatoes, cucumbers, and almonds. Related Compounds: Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 ○ Spiromesifen and Spirotetramat: Used on fruits, vegetables, and ornamentals for similar pest control. New Product: Spiropidion ○ Approved by the USEPA in December 2020. Environmental Impact: ○ These compounds are selective, causing minimal harm to beneficial insects but may affect beneficial mites. ○ They have a soil half-life of approximately 10 days and do not bioaccumulate in the food chain. 6.6.2.5.4 Inhibitors of the Synthesis of Very Long Chain Fatty Acids (VLCFAs) in Plants Overview: ○ Classified in HRAC Group-15, these herbicides include eight different chemical groupings. ○ They are soil-active and primarily target germinating plant seedlings, especially grasses. Mode of Action: ○ Affected grassy weed seedlings often fail to emerge from the soil, making these herbicides effective against germinating weeds and nutsedge. Widely Used Herbicides: ○ Chloroacetamides, particularly acetochlor and metolachlor, are among the most widely used globally due to their selectivity for important crops like: Maize Sorghum Soybeans Cotton Potatoes Sugarcane Mechanism of Action: ○ Several chloroacetamides bind strongly to the elongase (VLCFAE) enzyme system at very low concentrations, though the specific isozymes and binding mechanisms are still not fully understood. Biochemical Safeners Purpose: Developed in response to the common crop injury observed at herbicide application rates necessary for controlling significant grass weeds. Examples: ○ Dichlormid ○ Benoxacor Function: Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 ○ These safeners significantly affect the metabolism of herbicides in maize and sorghum, enhancing the tolerance of these crops to the herbicides without diminishing their effectiveness on target weeds. Target Crop Specificity: ○ The efficacy of these safeners is primarily noted in maize and sorghum, with little to no effect on other plant species. Context and Implications The use of biochemical safeners is a critical advancement in agricultural practices, allowing for effective weed control while minimizing damage to desirable crops. This approach has enabled farmers to manage grass weed populations without compromising the health and yield of their crops, ultimately contributing to sustainable agricultural practices. EPTC and Thiocarbamate Herbicides Uses EPTC: A pre-plant, soil-incorporated herbicide for controlling germinating annual grasses and perennial sedges. ○ Absorption: Taken up by young roots or shoots and translocated upwards. ○ Target Weeds: Effective against annual grasses and perennial quack grass (e.g., Cyperus spp.) in various crops, including: Beans Peas Legumes Alfalfa Cotton Maize Sunflowers Selectivity: When combined with chloracetamide safeners like dichlormid, EPTC selectively controls grassy weeds in slightly resistant crops like maize and sorghum. Related Herbicides Butylate, Molinate, Pebulate: Similar in action to EPTC. ○ Butylate: Important for weed control in maize. ○ Molinate: Significant for controlling barnyard grass (Echinochloa spp.) in rice. Environmental Impact and Toxicology Volatility: Must be applied as pre-plant, soil-incorporated treatments for effectiveness. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Degradation: Rapidly degraded by soil microorganisms (half-lives of 1 to 2 weeks). Toxicity: Low toxicity to mammals (AOLD50 in rats) and moderate toxicity to fish. Metolachlor and Acetochlor Uses Introduced in the 1960s and 1970s, these chloracetamide herbicides are used in crops such as: ○ Maize ○ Sorghum ○ Cotton ○ Potatoes ○ Peanuts ○ Soybeans Effectiveness: Especially effective against nutsedges and germinating annual grasses. Formulation: Often mixed with chloracetamide safeners (e.g., benoxacor) to enhance crop tolerance. Environmental Impact and Toxicology Contamination: Common in watersheds where maize and soybeans are grown due to high application rates and moderate persistence in soil (half-lives > 3 weeks). Aquatic Risk: Generally, concentrations in aquatic environments do not pose risks to non-target organisms. Toxicity: Low toxicity in mammals and relatively low toxicity to fish. Inhibitors of Protoporphyrinogen Oxidase (PROTOX) Importance: Inhibitors of PROTOX affect porphyrin biosynthesis, crucial for chlorophyll, hemoglobin, and cytochrome enzymes. Classification: These herbicides fall under HRAC Group-14, with many companies developing or marketing these inhibitors. Protoporphyrinogen Oxidase (PROTOX) Inhibitors Role in Photodynamic Therapy: Knowledge gained from PROTOX inhibiting pesticides can aid in developing cancer treatments, utilizing mechanisms similar to pest control methods. Competitive Inhibition: Herbicides in the HRAC Group-14 act as competitive inhibitors at the protoporphyrinogen IX binding site on PROTOX. ○ Binding Competition: Studies show that protoporphyrinogen IX and various active herbicides can displace each other from this binding site. Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 ○ Hydrophobic Environment: The effectiveness of PROTOX inhibitors correlates with the hydrophobicity of their molecular structures, suggesting that the binding site is located in a hydrophobic area. Mechanism of Action in Plants Localization: PROTOX and other enzymes in the porphyrin biosynthesis pathway are primarily found in chloroplasts, with some located in mitochondria. Light Exposure: Injury from PROTOX inhibitors is often not visible until plants are exposed to light. ○ Feedback Mechanism: Protochlorophyllide acts as a feedback inhibitor, reducing the synthesis of aminolevulinic acid (ALA) in the early stages of porphyrin synthesis. ○ Chlorophyll Conversion: When exposed to light, protochlorophyllide rapidly converts to chlorophyllide, reducing feedback inhibition, which accelerates the pathway and leads to increased accumulation of protoporphyrinogen IX. Light’s Role in Lipid Peroxidation Photooxidation Process: Light mediates the photooxidation of protoporphyrinogen IX, resulting in the generation of free radicals and singlet oxygen. Cell Damage: The accumulation of these reactive species causes lipid peroxidation and disrupts cellular membranes, contributing to plant injury. 6.6.2.6.1 Acifluorfen and Other Diphenyl Ether Herbicides Uses: ○ Acifluorfen is a post-emergence herbicide effective against various broadleaf and grassy weeds, particularly during the seedling stage. ○ It is absorbed by leaves but has limited translocation due to its rapid action. ○ Selectively used in crops such as soybeans and peanuts, with tolerance linked to the cleavage of the ether bond and conjugation with homoglutathione. ○ Symptoms of injury include chlorosis, desiccation, and necrosis of leaves within 1-2 days, with sublethal doses causing foliar bronzing. Environmental Impact and Toxicology: ○ Acifluorfen is photodegradable in water and soil, with half-lives varying (14-60 days for acifluorfen, 35 days for oxyfluorfen, and 100 days for fomesafen). ○ Soil residues of fomesafen can damage sensitive crops in subsequent seasons. ○ It has moderate toxicity to fish, with effects on animal systems due to the importance of the porphyrin biosynthesis pathway being an ongoing research topic. 6.6.2.6.2 Oxadiazon, a Phenyl Heterocycle Herbicide Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 Uses: ○ Oxadiazon is a pre-emergence herbicide applied to control germinating broadleaf and grassy weeds in Bermuda grass turf and ornamental crops. ○ It is absorbed by the shoots of emerging seedlings, causing them to wilt and develop chlorotic and necrotic leaves shortly after emergence. Environmental Impact and Toxicology: ○ Strongly adsorbed to soil colloids, oxadiazon has a moderate to long persistence with a soil half-life averaging 60 days and is immobile in soil. ○ Its effectiveness is improved by rainfall or irrigation. ○ It is relatively non-toxic to mammals (AOLD50 > 5000 mg/kg) and moderately toxic to fish. 6.6.2.7 Herbicides that Inhibit Carotenoid Biosynthesis Mechanism of Action: ○ These herbicides (in HRAC Groups 12, 27, and 34) inhibit the biosynthesis of carotenoid pigments in plants, leading to a secondary loss of chlorophyll due to enhanced photodegradation. ○ Chlorophylls are essential for photosynthesis, while carotenoids protect chlorophyll from oxidative damage caused by free radicals in light exposure. Role of Carotenoids Non-Enzymatic Scavenging: Carotenoids help quench excited chlorophyll and singlet oxygen, preventing the formation of harmful reactive oxygen species like superoxide and hydrogen peroxide. Protection of Chlorophyll: By mitigating oxidative stress, carotenoids play a critical role in protecting chlorophyll from degradation, thus maintaining the integrity of photosynthetic membranes in chloroplasts. Effects of Bleaching Herbicides Mechanism of Action: ○ Herbicides classified in HRAC Group-12 inhibit phytoene desaturase, a key enzyme in carotenoid biosynthesis. ○ Amitrole, the only herbicide in HRAC Group-34, inhibits lycopene cyclase, the last enzyme involved in the production of several carotenoids (e.g., γ-carotene, β-carotene, and torulene). ○ Herbicides in HRAC Group-27 inhibit hydroxyphenyl pyruvate dioxygenase, another enzyme involved in carotenoid synthesis. Symptoms of Toxicity: ○ The first signs of injury, particularly associated with amitrole, include a characteristic whitening of leaf tissue, leading to the term "bleaching herbicide." Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 ○ Initially, affected leaves may appear normal, but they eventually become necrotic and die as chlorophyll is degraded. Consequences of Carotenoid Depletion Photosynthesis Impairment: ○ A decrease in carotenoid biosynthesis leads to the oxidation and degradation of chlorophyll. ○ This results in damage to photosynthetic membranes through lipid peroxidation, ultimately reducing the rate of photosynthesis. Visual Changes in Leaves: ○ Leaves produced after treatment with these herbicides lack both carotenoid and chlorophyll pigments, resulting in a white appearance. Fluridone Uses and Mode of Action Application: Fluridone is primarily used as an aquatic herbicide for controlling submerged and emerged weeds in bodies of water such as lakes, ponds, reservoirs, and irrigation ditches. Application Rates: ○ Concentrations in water should not exceed 90 mg/L after any treatment. ○ A maximum total of 150 mg/L is allowed during one growth cycle. Mechanism: Fluridone acts as a bleaching herbicide by inhibiting phytoene desaturase, a key enzyme in carotenoid biosynthesis. This leads to the destruction of chlorophyll and halts photosynthesis, leaving the plants devoid of pigments. Environmental Impact and Toxicology Soil Behavior: ○ Fluridone is highly adsorbed and immobile in soil, resulting in significant persistence, with a half-life of about one year. Water Degradation: In aerobic pond water, its half-life is approximately 20 days, with degradation occurring through photodecomposition and microbial activity. Toxicity: ○ Fluridone is specifically toxic to plants but shows very low toxicity to mammals, with AOLD50s in mice and rats exceeding 10,000 mg/kg. ○ In water, LC50s for organisms like Daphnia spp. and fish range from 6.3 to 14.3 mg/L. 6.6.2.7.2 Amitrole Uses Downloaded by Smella tk ([email protected])  lOMoARcPSD|38059268 History: Amitrole was one of the first herbicides discovered that inhibits carotenoid production, marketed since the 1950s. Application: It is a non-selective herbicide that is effective against several perennial weeds. Initially used in fruit crops (e.g., cranberries, apples, cherries), its use has now declined, primarily due to the rise of glyphosate, which offers superior herbicidal properties. Systemic Activity: Amitrole exhibits good systemic activity, translocating in both the xylem and phloem, making it effective against perennial weeds, including poison ivy. Environmental Impact and Toxicology Soil Behavior: Amitrole is non-persistent in soil, with a half-life of about two weeks. Toxicity: It is considered essentially non-toxic to mammals, with AOLD50s in rats exceeding 10,000 mg/kg. While Amitrole has been an effective herbicide, its use has come under scrutiny due to potential health risks: 1. Health Risks: ○ Chronic exposure to Amitrole has been linked to: Liver tumors in mice. Thyroid and pituitary tumors in rats. An increase in the incidence of cranial malformations in teratogenicity tests conducted on rabbits. 2. Regulatory Actions: ○ Due to these concerns, Amitrole has been restricted in several countries. ○ In Canada, its use is highly limited, with the only approved application being on spruce barefoot seedbeds. Downloaded by Smella tk ([email protected])