Herbicides are widely used for weed management due to their effectiveness and cost-efficiency, especially since the Green Revolution. While they effectively control weed populations, herbicides pose significant risks to non-target crops, environmental health, and human safety. Various chemical categories of herbicides exist, some of which have been banned due to their harmful effects. Residues from herbicides contribute to air, water, and soil pollution, and herbicide-resistant weeds are becoming increasingly challenging. The need to boost food production for a growing global population has led to higher pesticide use. Since their introduction in the 1940s, herbicides like 2,4-D and atrazine have been crucial in modern agriculture. Their usage surged from the 1960s to the early 1980s and continues to rise, especially in American and Asian countries. Currently, herbicides account for approximately 40% of the pesticide market.
Herbicides can also cause phytotoxicity, which is the harmful effect of herbicides on plants. This can be a particular problem for non-target crops and crops that are sensitive to certain herbicides. Phytotoxicity can lead to reduced crop yields, and in some cases, it can even kill plants.
Phytotoxicity and Resistance Development
The extensive use of herbicides has led to the emergence of herbicide-resistant weeds, reducing their effectiveness and leading to overuse of the products hoping to control these new resistant weeds. Herbicide-resistant crops have exacerbated this issue by promoting monoculture and reducing biodiversity. The repetitive application of pesticides contributes to developing resistant weed species, posing a significant challenge to sustainable agriculture.
Phytotoxicity, Plants, and Environmental Impact
Herbicides, while essential for controlling weeds, can cause phytotoxicity, severely hindering plant growth and development. High doses, improper application, and poor timing can lead to leaf malformations, stunted growth, and necrosis. Herbicides also negatively impact soil microorganisms, aquatic life, and non-target vegetation, raising substantial environmental concerns. Herbicides like glyphosate (roundup) and paraquat can impair plant health by affecting water uptake, nutrient absorption, and overall vigor. Other herbicides, such as Fluroxypyr and Isoproturon, induce oxidative stress, disrupting photosynthesis and nutritional metabolism. This stress results in lipid peroxidation, membrane degradation, and enzyme malfunction, ultimately degrading crop quality and yield.
Impact of Herbicides on Plant Growth and Development
Impact on Soybean
- Glyphosate significantly reduces water absorption, nitrogen fixation, and mineral concentrations in soybean tissues.
- Paraquat decreases relative water content (RWC) in soybean plants one day after treatment.
- Fluroxypyr induces oxidative damage in rice cell membranes
Impact on Maize
- Roundup herbicides reduce maize root growth and elongation by decreasing auxin production.
General Plant Responses
- Various herbicides, including glyphosate and Isoproturon, disrupt photosynthesis, chlorophyll biosynthesis, and overall plant physiology.
- Glyphosate and similar herbicides lower photosynthetic rates and inhibit essential enzyme functions.
Physiological Disorders & Herbicides
Herbicides interfere with photosynthetic electron transport, reducing chlorophyll content and efficiency. Glyphosate affects carbon metabolism, reducing net carbon exchange and altering sugar metabolism. It also impairs nitrogen metabolism by affecting rhizobial symbionts and decreasing nitrogen nutrition. As a metal chelator, glyphosate can immobilize essential nutrients, causing deficiencies. Additionally, herbicides disrupt hormone synthesis, particularly auxin, and impair growth and development. They generate reactive oxygen species (ROS), leading to lipid peroxidation, membrane damage, and enzyme inhibition (Aravind & Prasad, 2005). Paraquat induces oxidative stress, increasing malondialdehyde (MDA) levels, and antioxidant enzyme activities. Herbicides like Isoproturon and ZJ0273 affect antioxidant enzymes, altering activities such as superoxide dismutase (SOD) and peroxidase (POD).
How does phytotoxicity impact the quality of the crop yield?
Herbicides can negatively affect crop yields and quality. For instance, Isoproturon significantly reduces wheat grain yield. Glufosinate ammonium reduces grain numbers and weight in wheat. Some herbicides increase carbohydrate and protein content in crops like maize). Conversely, herbicides such as pendimethalin can suppress nutritional quality by reducing protein and carbohydrate content.
How does phytotoxicity aggravate soil pollution?
Soil health relies on microbial populations that aid in decomposing organic matter, nutrient availability, and maintenance of physical properties. Certain microbes degrade toxic substances like heavy metals, xenobiotics, pesticides, and herbicides. Environmental pollution, including herbicides, negatively affects soil microorganisms. Factors such as herbicide type, application rate, frequency, and weather conditions influence soil health. Research shows herbicides like pendimethalin, oxyfluorfen, and propaquizafop hinder soil microbial development. Atrazine reduces weed and microbial populations, while glyphosate and its metabolite AMPA cause soil toxicity, persisting in clay soils and affecting soil microbes, nutrition, and crop productivity. Glyphosate reduces earthworm biomass and populations, with studies reporting decreased earthworm numbers following prolonged glyphosate use. Isoproturon, diflufenican, and metribuzin also impact soil microorganisms and properties, altering enzyme activities and microbial biomass, which affects soil health.
Phytotoxicity affects human health
Herbicides are crucial for weed management, but their increased use poses environmental and health risks, especially in developing nations with inadequate safety measures. Exposure to herbicides can cause both acute and chronic health issues, including skin rashes, respiratory problems, cancer, and neurological damage.
If you want to read more about phytotoxicity, read the relevant article; Factors to Consider Before Tank Mixing Agrochemicals to Prepare the Spray Solution.
MCs can build up in crops over time, posing a risk to human health through the food chain. The number of MCs (cyanotoxins, microcystins) in edible parts of vegetables grown in soil affected by CBs (cyanobacterial blooms) can reach up to 382 μg/kg (fresh weight), with more than 60% of these vegetables posing a moderate or high health risk if consumed. Epidemiologic research revealed a link between the rising incidence of esophageal and liver cancer and long-term consumption of aquatic food and water contaminated with mercury (MCs). In line with this, over the past few decades, many studies have been conducted to determine the environmental fate and health risks of MCs in aquatic environments. (Read the full scientific paper here)
Vulnerable groups, such as children and farm workers, are particularly at risk and may often face misdiagnosis due to symptom similarity with common illnesses. Herbicides like paraquat and Agent Orange are linked to severe respiratory issues and cancers. Chlorophenoxy acid herbicides, triazines, and glyphosate have been associated with endocrine disruption, DNA damage, and increased risks of diseases such as autism, Parkinson's, and Alzheimer's. Many countries are implementing measures to reduce herbicide-related health and environmental impacts. Continued research is essential to understand the long-term effects and to educate the farming community on safe herbicide use.
Cover photo: https://www.canr.msu.edu/news/plant_phytotoxicity_in_the_greenhouse Photo credit: Erik Runkle, MSU
References
Aravind, P., & Prasad, M. N. V. (2005). Modulation of cadmium-induced oxidative stress in Ceratophyllum demersum by zinc involves ascorbate–glutathione cycle and glutathione metabolism. Plant Physiology and Biochemistry, 43(2), 107-116.
Áy, Z., Mihály, R., Cserháti, M., Kótai, É., & Pauk, J. (2012). The effect of high concentrations of glufosinate ammonium on the yield components of transgenic spring wheat (Triticum aestivum L.) constitutively expressing the bar gene. The Scientific World Journal, 2012(1), 657945.
Chandrakar, C., Shrivastava, G., & Anjum Ahmad, A. A. (2014). Impact of water management, weed, and integrated nutrient management on weed parameters and potato yield (Solanum tuberosum).
Chaudhary, S. U., Iqbal, J., Hussain, M., & Ali, M. A. (2010). Comparison of different herbicidal application methods for weed control in wheat. J. Agric. Res, 48(2), 193-200.
Ding, W., Reddy, K. N., Zablotowicz, R. M., Bellaloui, N., & Bruns, H. A. (2011). Physiological responses of glyphosate-resistant and glyphosate-sensitive soybean to aminomethylphosphonic acid, a metabolite of glyphosate. Chemosphere, 83(4), 593-598.
Duke, S. O., & Powles, S. B. (2008). Glyphosate: a once‐in‐a‐century herbicide. Pest Management Science: formerly Pesticide Science, 64(4), 319-325.
Gomes, M. P., Richardi, V. S., Bicalho, E. M., da Rocha, D. C., Navarro-Silva, M. A., Soffiatti, P., Garcia, Q. S., & Sant'Anna-Santos, B. F. (2019). Effects of Ciprofloxacin and Roundup on seed germination and root development of maize. Science of the Total Environment, 651, 2671-2678.
Hamim, H., Violita, V., Triadiati, T., & Miftahudin, M. (2017). Oxidative stress and photosynthesis reduction of cultivated (Glycine max L.) and wild soybean (G. tomentella L.) exposed to drought and paraquat.
Jiang, L.-X., Jin, L.-G., Guo, Y., Tao, B., & Qiu, L.-J. (2013). Glyphosate effects on the gene expression of the apical bud in soybean (Glycine max). Biochemical and Biophysical Research Communications, 437(4), 544-549.
Liu, X., Qi, C., Wang, Z., Li, Y., Wang, Q., Guo, M., & Cao, A. (2014). Effect of picloram herbicide on physiological responses of Eupatorium adenophorum Spreng. Chilean journal of agricultural research, 74(4), 438-444.
Mendes, I. D. S., & Rezende, M. O. O. (2014). Assessment of the allelopathic effect of leaf and seed extracts of Canavalia ensiformis as postemergent bioherbicides: A green alternative for sustainable agriculture. Journal of Environmental Science and Health, Part B, 49(5), 374-380.
Orcaray, L., Zulet, A., Zabalza, A., & Royuela, M. (2012). Impairment of carbon metabolism induced by the herbicide glyphosate. Journal of plant physiology, 169(1), 27-33.
Puig, C. G., Reigosa, M. J., Valentão, P., Andrade, P. B., & Pedrol, N. (2018). Unravelling the bioherbicide potential of Eucalyptus globulus Labill: Biochemistry and effects of its aqueous extract. PloS one, 13(2), e0192872.
Rajashekar, N., Prakasha, P., & Murthy, T. (2012). Seed germination and physiological behavior of maize (cv. NAC-6002) seedlings under abiotic stress (pendimethalin) condition.
Serra, A.-A., Nuttens, A., Larvor, V., Renault, D., Couée, I., Sulmon, C., & Gouesbet, G. (2013). Low environmentally relevant levels of bioactive xenobiotics and associated degradation products cause cryptic perturbations of metabolism and molecular stress responses in Arabidopsis thaliana. Journal of experimental botany, 64(10), 2753-2766.
Travaini, M. L., Sosa, G. M., Ceccarelli, E. A., Walter, H., Cantrell, C. L., Carrillo, N. J., Dayan, F. E., Meepagala, K. M., & Duke, S. O. (2016). Khellin and visnagin, furanochromones from Ammi visnaga (L.) Lam., as potential bioherbicides. Journal of agricultural and food chemistry, 64(50), 9475-9487.
Wei, C., Zhou, S., Li, W., Jiang, C., Yang, W., Han, C., Zhang, C., & Shao, H. (2019). Chemical composition and allelopathic, phytotoxic and pesticidal activities of Atriplex cana Ledeb.(Amaranthaceae) essential oil. Chemistry & Biodiversity, 16(4), e1800595.
Wu, G. L., Cui, J., Tao, L., & Yang, H. (2010). Fluroxypyr triggers oxidative damage by producing superoxide and hydrogen peroxide in rice (Oryza sativa). Ecotoxicology, 19, 124-132.
Xu, L., Zhang, W., Ali, B., Islam, F., Zhu, J., & Zhou, W. (2015). Synergism of herbicide toxicity by 5-aminolevulinic acid is related to physiological and ultra-structural disorders in crickweed (Malachium aquaticum L.). Pesticide Biochemistry and Physiology, 125, 53-61.
Yanniccari, M., Tambussi, E., Istilart, C., & Castro, A. M. (2012). Glyphosate effects on gas exchange and chlorophyll fluorescence responses of two Lolium perenne L. biotypes with differential herbicide sensitivity. Plant Physiology and Biochemistry, 57, 210-217.
Yin, X. L., Jiang, L., Song, N. H., & Yang, H. (2008). Toxic reactivity of wheat (Triticum aestivum) plants to herbicide isoproturon. Journal of agricultural and food chemistry, 56(12), 4825-4831.
Zablotowicz, R. M., & Reddy, K. N. (2007). Nitrogenase activity, nitrogen content, and yield responses to glyphosate in glyphosate-resistant soybean. Crop Protection, 26(3), 370-376.
Zhou, Q., Liu, W., Zhang, Y., & Liu, K. K. (2007). Action mechanisms of acetolactate synthase-inhibiting herbicides. Pesticide Biochemistry and Physiology, 89(2), 89-96.
Zobiole, L., Oliveira Jr, R., Kremer, R., Constantin, J., Yamada, T., Castro, C., Oliveira, F., & Oliveira Jr, A. (2010). Effect of glyphosate on symbiotic N2 fixation and nickel concentration in glyphosate-resistant soybeans. Applied Soil Ecology, 44(2), 176-180.
