Herbicides are chemical substances used in agriculture, landscaping, and other areas to control or eliminate weeds. These compounds are widely used to manage plant growth, contributing to healthier crops and ecosystems. Understanding how these chemicals function is important for effective weed management.
General Principles of Herbicide Action
Herbicides operate by interfering with essential biological processes. These chemicals are engineered to disrupt specific metabolic pathways or cellular functions unique to plants, or more pronounced in specific species. By targeting these processes, herbicides can prevent a plant from performing basic life-sustaining activities, leading to its decline or death. The effectiveness of a herbicide often depends on its ability to exploit differences between target weeds and desirable plants, ensuring selective control.
They broadly disrupt plant life by inhibiting energy production, altering growth patterns, or damaging cellular structures. Some compounds might block the plant’s ability to convert sunlight into energy, while others interfere with the synthesis of vital building blocks for growth. Other herbicides can mimic natural plant hormones, causing uncontrolled and distorted development. These disruptions prevent normal physiological functions, leading to eventual demise.
Specific Modes of Action
Herbicides employ diverse biochemical mechanisms to control plant growth, each targeting a specific plant process. Understanding these interactions provides insight into their molecular effects.
Photosynthesis inhibitors
Photosynthesis inhibitors block a plant’s ability to convert light energy into chemical energy. These herbicides typically bind to specific sites within Photosystem II, preventing the electron transport necessary for photosynthesis. This disruption leads to a rapid accumulation of harmful reactive oxygen species and a cessation of food production, causing starvation and cellular damage. Symptoms include yellowing (chlorosis) and tissue death (necrosis) on older leaves.
Amino acid synthesis inhibitors
Amino acid synthesis inhibitors interfere with the formation of essential amino acids, essential for plant proteins. Herbicides in this category often target enzymes like acetolactate synthase (ALS) or 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). By blocking these enzymes, the plant cannot produce amino acids for new growth, leading to stunted growth and eventual death.
Plant growth regulators
Plant growth regulators, also known as synthetic auxins, mimic natural plant hormones that control growth. They cause an overstimulation of growth processes, leading to uncontrolled and distorted patterns. This hormonal imbalance disrupts cell elongation, protein synthesis, and cell division, resulting in abnormal root and shoot development, twisting stems, and misshapen leaves.
Cell division inhibitors
Cell division inhibitors prevent plants from producing new cells, halting root and shoot growth. Many act by disrupting the formation of microtubules, essential for cell division (mitosis). While existing cells may function, the plant cannot generate new tissue, leading to stunted or abnormal development, particularly in meristematic regions like root tips and emerging shoots.
Lipid synthesis inhibitors
Lipid synthesis inhibitors block the production of fatty acids, crucial for cell membranes and other structures. These herbicides typically target the acetyl-CoA carboxylase (ACCase) enzyme, key in fatty acid biosynthesis. Without new lipids, the plant cannot build or repair cell membranes, leading to impaired growth and eventual death, especially in grasses.
How Herbicides Reach and Target Plants
Herbicides interact with plants in different ways based on their chemical properties and how they are applied. This interaction determines where and how broadly the herbicide acts within the plant and the surrounding environment.
Contact herbicides
Contact herbicides work by killing only plant parts they touch. They are typically fast-acting, causing visible damage like wilting or browning within hours or days of application. Because they do not move throughout the plant, thorough foliage coverage is necessary. They are less effective on perennial weeds with deep root systems that can regenerate.
Systemic herbicides
Systemic herbicides are absorbed and translocated throughout the plant’s vascular system. They enter through leaves, stems, or roots and move via the xylem (water transport) and phloem (sugar/nutrient transport) to reach all plant parts, including roots, rhizomes, and growing points. This internal movement allows them to kill the entire plant, making them effective against perennial weeds.
Selective or non-selective
Herbicides are classified as selective or non-selective based on their target range. Selective herbicides kill specific plant types, such as broadleaf weeds, while leaving desirable plants like turfgrass unharmed. This selectivity exploits biochemical or physiological differences between plant species, making the target weed susceptible while the desired plant metabolizes or tolerates the chemical. Non-selective herbicides kill almost all plants they contact, suitable for clearing areas where no vegetation is desired.
Herbicide Behavior After Application
Once applied, herbicides undergo various environmental processes. These processes determine how long a herbicide remains active and its movement within soil and water.
Degradation
Degradation is a significant process where herbicides break down into simpler compounds. This breakdown occurs through microbial action, as soil microorganisms like bacteria and fungi metabolize herbicides as a food source. Sunlight also causes photodegradation, especially for herbicides exposed on soil surfaces or plant foliage. Chemical reactions, such as hydrolysis (interaction with water), further contribute to their breakdown.
Movement
Herbicides also move in soil. Some herbicides adsorb, or bind, to soil particles like clay and organic matter. This adsorption can temporarily reduce availability for plant uptake or microbial degradation. Other herbicides might leach downward through the soil profile with water, potentially reaching groundwater. Surface runoff can transport herbicides away from the application site.
Persistence
Persistence refers to how long a herbicide remains active. Different herbicides have varying persistence rates, ranging from weeks to months, depending on chemical structure and environmental conditions like soil moisture, temperature, and pH. Ideally, a herbicide persists long enough to control target weeds but then degrades rapidly, minimizing its environmental presence. However, very low microbial activity can sometimes prolong persistence.