The effect of a wide spectrum of environmental pollutants and industrial effluents on the growth, biochemistry and productivity of a number of plant species has been reported extensively by research workers all over the world. A number of chemicals like pesticides (including insecticides, acaricides, nematicides, rodenticides, piscicides molluscicides, etc.), fungicides, antibiotics, herbicides, hormones, fertilizers, seed hardening agents, micronutrients, and anti-transpirants are used in agriculture. These chemicals may be applied onto the crop plants as foliar spray or as soil application around the root zone of the crops. Hormones, fertilizers and micronutrients are targeted on the crop plants to increase the crop yield.
Seed hardening agents are used to hasten seed maturity and loss of moisture from the seed. Anti-transpirants are sprayed on the leaves to improve the ability of the plants to withstand water stress. The biochemical and molecular mechanisms of absorption and utilization of these chemicals are fully known. All other chemicals are targeted against the .organisms, which affect the crop plants adversely.
Pesticide consumption in India has increased from 15.4 g per ha in 1960-61 to 450 g per ha in 1989-90. Cotton consumes 44% of the pesticides used while rice consumes 23% by value. The plants are exposed to pesticides by direct application, through the uptake from soil and water, and from atmospheric drift. It has been shown that about half of the pesticides applied by aircraft land outside the target crop land or forest and fall out either on adjoining ecosystems or drift into distant ecosystems.
While pesticides and fungicides are sprayed directly on crop plants or applied to the soil around the plants for the control of different pests and pathogens, herbicides are sprayed on weeds growing in the field. No doubt these chemicals ultimately lead to increase in the yield of the crops by controlling the weeds, pests and diseases, which affect the growth and development of crop plants. However, the metabolic reactions and other biochemical pathways in plants may be affected. In addition, the crop plants are also exposed to spray drifts of herbicides sprayed on weeds leading to injury/damage to them.
Though the harmful effects of all these agrochemicals on human beings and other non-target organisms including soil microflora and fauna, fishes, birds and wild animals, has been extensively studied, not much is known regarding their effects on host plants on which the chemicals are applied. In fact this aspect has been completely ignored by the agricultural scientists as well as environmentalists.
Agrochemicals and Biodiversity:
The use of agrochemicals affects the biodiversity and alters the ecological balance. Differential sensitivity of plant species to toxic and genotoxic effects of pesticides has been shown to cause overall changes in species ratios, both among weeds in a crop field and in natural plant communities, due to the reduced abundance of susceptible species with concurrent increases in naturally tolerant species. This may have further consequences for the entire ecosystem. An important factor in this situation is the occurrence of herbicide-resistant forms within susceptible plant species.
The soil harbours several organisms beneficial to the crop plants like earthworms, vesicular arbuscular mycorrhiza (VAM), nitrogen fixing bacteria, phosphate-solubilising organisms, organic matter—decomposing organisms, parasites and predators of pests and pathogens, etc.
The honeybees are effective pollinators for a number of plant species. The silkworm and lac insects are important sources of economic products. The pesticides and fungicides may affect these beneficial organisms and thereby influence pollination, seed set, soil nutrient status and the growth of plants indirectly.
Extensive use of pesticides and fungicides affects the biodiversity of the environment by selectively controlling one type of organisms in the environment. Control of pests may lead to disappearance of the natural predators and parasites of the pests from the environment, thereby, leading to pest/pathogen resurgence immediately after stopping the chemical spray. Control of one type of pest may lead to resurgence of another pest which was hitherto of minor importance.
For example, control of leaf eating caterpillars through the introduction of transgenic crop varieties containing Bt genes leads to resurgence of sap sucking pests like aphids and thrips. Use of phosphamidan and monocrotophos led to resurgence of leafhoppers due to increased supply of nitrogen and phosphorus present in the pesticides. There was a spurt in red spider and white fly incidences due to large scale use of DDT on cotton in 1980s.
Use of 2, 4-D for the control of weeds in maize field resulted in increased incidence of corn aphids and corn borer. Many weeds serve as alternate hosts for crop pests and pathogens. Continuous use of herbicides may lead to more pest attack on the crop since the weeds on which the pests/pathogen survive, have been killed by herbicides. Continuous use of chemical pesticides/fungicides may lead to development of resistance to the chemical by the pest or the pathogen.
Some of the chemicals are genotoxic and may induce mutations in plants as well as in the pest/pathogen. Resistance of houseflies to DDT has been reported as early as 1950. A strain of housefly with 300-fold resistance to DDT has been reported when the chemical was applied in acetone.
The herbicides are extensively used in developed countries for the control of weeds in the field and for clearing forests for cultivation. Human labour is expensive and scarce in these countries. The farm size is very large and therefore mechanized farming is practised to a large extent.
In India, big estates plantations and large farms use herbicides for weed control and for clearing forests for planting with new crop plants. Fall of spray drifts onto the crops and accidental fall of herbicides on crops and/or soil occur frequently in the field.
The pre-emergence herbicides are sprayed on the soil to kill the germinating seeds of weeds. They may affect the germination of seeds of crop plants as well. Herbicides affect the biodiversity by causing preferential death of certain plant species, which would otherwise harbour beneficial organisms.
Transgenic crop varieties containing herbicide resistance genes artificially introduced into them are now available. Though these cultivars are not visually affected by herbicides it is possible that their biochemical and metabolic reaction may be affected leading to draining of energy of the plants.
Foliar application of 2, 4-D above 100 mg m1–1 suppressed flowering permanently in leguminous crops. At lower doses (less than 25 mg m1–1) it delayed flower initiation by two days in Cyamopsis tetragonoloba. The herbicides paraquat, propyzamide and 2, 4-D reduced grass dry matter production in the field and in pot culture experiments. The bacterium Burkholderia (Pseudomonas) capacia has the ability to degrade 2, 4-D. Inoculation of barley seeds with this bacteria resulted in better growth of seedlings in 2, 4-D treated soil.
The herbicide atrazine increased the root length, shoot length and leaf area per plant at 5 ppm level while 10 ppm concentration decreased all the parameters significantly. The growth of Pisum sativum plants was decreased by cyanazine herbicide at 0.05 and 0.1 mg 1–1 concentration and VAM activity of the roots was reduced at 0.1 mg 1–1. The herbicide simazine increased water and nitrate uptake in barley, rye, and oat seedlings, resulting in increased plant weight and total protein content. The herbicides sethoxydim, alachlor, fluazifop and methachlor did not affect the growth and yield of soybean. But paraquat significantly reduced nitrogen fixation by the root nodulating bacteria Bradyrhizobium japonicum and growth of soybean plants.
Pollen tube length decreased from 385.16 µm in control to 265.14 µm at 400 µg m1–1 of maleic hydrazide (MH) in cluster beans, Cyamopsis tetragonoloba. Acrolein at the lowest concentration used was found to be toxic to the pollen of Catharanthus roseus. Complete suppression of pollen germination was observed in F48 and F72 series of flowers. The author has suggested that pollen of these series may be an ideal indicator for detecting herbicide presence in the environment.
Maleic hydrazide (MH) is used to prevent sprouting in potatoes, onion and stored root crops, to delay flowering, and to prevent sucker formation in tobacco. It is also used as a herbicide and growth retardant. It has clastogenic activity on plant chromosomes. Picloram and 2, 4-D induced a larger proportion of lagging chromosomes as compared to 0.4% chromosomal aberrations in unsprayed plants.
In tissue cultured Nicotiana glauca cells 2, 4-D at 0.4 ppm concentration produced 55.28% aberrant anaphase as compared to 33.65% in control. Simazine and Diuron produced multipolar spindles that were not observed in untreated cells.
The community structure of the leaf surface (phyllosphere) microorganisms have a role as decomposers and antagonists to plant pathogens. Under normal condition the microbial population on phyllosphere is held in a dynamic balance by interaction between host plant and saprophytic microflora.
Steep reduction in saprophytic microflora was detected in the phyllosphere of potato due to the herbicides benthiocarb, fluchloralin and 2, 4-D. Both the fungal and bacterial populations decreased immediately after herbicide spray and started recovering after 15 days.
Butachlor and 2, 4-D at 100, 200 and 500 ppm concentrations in the nutrient medium affected the growth of several soil microflora. Nitrosomonas was most sensitive to the chemicals while Pseudomonas was resistant at 100 and 200 ppm only. Butachlor was more fungistatic than 2, 4-D. Normal concentration (2.6 ppm) of dalapon did not affect soil microflora. But al higher concentrations, which may occur due to dumping or mixing or washing of the herbicide or its container, the soil bacterial population was drastically reduced.
Actinomycetes and fungi were not affected even at such high concentrations. Low concentrations of glyphosate (1-10 µg g–1 soil) had no effect on soil biota at 24 hours after application. Thereafter, the fungal population was found to increase. The soil bacterial population increased at 50 µg concentration. The herbicides diquat, dalapon and simazine inhibited soil denitrification process.
The autotrophic blue green algae and cyanobacteria fix atmospheric nitrogen and are used as biofertilisers. The chlorophyll and carotenoid contents, chlorophyll a/b ratio and cell number of Chlorella protothecoides decreased significantly when grown in a medium containing SANDOZ9789 a pyridazinone herbicide.
The herbicide atrazine did not affect acetylene reduction by blue green algae and cyanobacteria. But the photosynthetic rate of the algae was affected by 50% at 0.1 to 0.5 ppm concentrations. Butachlor and benthiocarb at 5 ppm level reduced the biomass and chlorophyll content greatly in cyanobacteria Anabaena, Aulosua, Nostoc, Scytonema and Tolypothrix. Pandemethlin was harmful even at 1 ppm concentration.
Pesticides have also produced changes in plant metabolism and in nutritional patterns that may have secondary effects on the ecology. Pesticides can alter the chemical composition of plants. The changes that occur appear to be specific for both the plant and the pesticides involved. For example, certain organochlorine insecticides have increased the amounts of some macro- and micro-element constituents (Al, B, Ca, Cu, Fe, K, Mg, Mn, N, P, Sr and Zn) of corn and beans, and decreased the amounts of others. Quinalphos had deleterious effects on chlorophyll and carotenoid content of rice leaves.
Soil treatment of carrot with insecticides nexion, birlane, and dyfonate increased the concentration of free sugars in roots, whereas an opposite effect was observed after the action of the herbicide dosanex. Malathion at 400 ppm concentration in soil decreased the root length, protein content of the leaves, and the activities of phytase and proteases enzymes of four day old wheat seedlings. Phosphatases, DNase, RNase and ATPase activities were found to be increased due to the insecticide.
In another study, DDT, aldrin, endrin, and lindane were found to stimulate synthesis of important amino acids arginine, histidine, leucine, lysine, proline, and tyrosine in corn, but decrease 1 the content of tryptophan over control causing an imbalance in nitrogen metabolism. Sumithion caused impaired nitrogen metabolism in mungbean (Vigna radiata). Methyl parathion caused disturbances in Hill reaction and electron transport system of photosynthesis.
Reduced pollen fertility and seed setting are assumed to be the consequences of chromosome aberrations induced in the meiotic stage. Pesticides that induce chromosomal aberrations in meiosis also reduce pollen viability. Genotoxic pesticides are potentially able to increase mutations in DNA, controlling the expression of various qualitative and quantitative traits that could cause genetic instabilities of natural plant populations and in crop varieties.
The pesticides DDVP and Malathion are mutagenic and are capable of alkylating DNA (Table 31.1). Endosulfan decreased the mitotic index in the root tips of onion. C-mitotic effects, non-orientation and multipolarity increased due to endosulfan. Stickiness, fragments and bridges were observed at higher frequency in pesticide-treated roots.
In onion root tips, monocrotophos showed chromosomal breakage at 0.4 ppm level and endosulfan at 0.6 ppm concentration. Other abnormalities like stickiness, laggards, aberrant cells and unequal distribution of chromosomes were also noticed to varying degrees depending on the concentration of the pesticide.
Mitotic index of onion root tips decreased due to methyl parathion at 0.1 and 0.2 per cent concentrations. This was alleviated by including selenium salt in the medium (Table 31.2), Onion root tips exposed to metacid 50 at 0.01-0.10 per cent concentrations for 8 hours had decreased mitotic index and increased percentage of abnormal cells.
Endosulfan upto 25 ppm concentration in the growth medium was found to be beneficial to the growth of Azolla pinnata; but above 50 ppm it decreased heterocyst frequency, nitrogen content, chlorophyll content and growth rate. Nitrogenase activity of the cyanobacteria was inhibited by endosulfan and malathion. The chlorophyll content and biomass of the green manure Azolla pinnata were significantly affected at 5 mg sumithion/ml or more. No detectable carbon assimilation occurred in algal population at 1.5 ppm concentration of the pesticide aminocarb.
However, the algal productivity was restored within 72 hours. DDE, the metabolite of DDT inhibited marine algal cell division and photosynthesis per cell. The pesticide diazinon has no effect on nitrogen fixation in rice soil; but benomyl, carbofuran, parathion, nitrofen and BHC significantly increased nitrogen fixation.
Pentachlorophenol, lindane, fonofos and malathion at 50 µg g–1 soil enhanced denitrification. Soil treated with the nematicides oxamyl and fenamiphos at 600-900 µg active ingredient decreased carbon dioxide production indicating decreased microbial activity.
Juvenile earthworms of Drawida willsi were highly susceptible to the herbicide butachlor and the pesticides malathion and carbofuran as compared to immature and adult worms. Fifty per cent mortality of juvenile worms was recorded with 8 mg butachlor, 15 mg malathion and 12 mg carbofuran per kg soil. Adult earthworms required around 20% more chemicals for 50% mortality. The activity of cholinesterase of earthworms Lampito mauritii and Drawida calebi was inhibited by 6 ppm concentration of monocrotophos applied to the soil. Chlorothaloniol and thiram applied to Pinus carbaea seedlings inhibited mycorrhiza development.
Nitrogenous, phosphatic and potassium fertilizers applied in large quantities may have heavy metals like Cu, Zn, Pb, Cr, As, Se, Mg, Mn, Al, etc., as impurities depending upon the raw material used for the manufacture of the fertilizer and the type of manufacturing process followed. Many micronutrients like iron, calcium, zinc, magnesium, manganese, boron, etc., are also advocated as soil or foliar application to different crop plants to increase the yield and to overcome the deficiency of these micronutrients in the soils of selected regions.
Very often it is claimed that these fertilizers including micronutrients are nontoxic to plants at the dosages tried since no external toxic symptoms are observed. Most often, these micronutrients are applied through the foliage since soil application makes the nutrient unavailable to the plant by getting fixed in the soil. Therefore, it is possible that the metabolic reactions may be affected in plants by interaction of heavy metals with the enzymes or other proteins.
Some of the pesticides, fungicides and herbicides also have heavy metals in them. Copper oxychloride is the most effective chemical used for the control of blister blight disease of tea. Bordeaux mixture containing copper sulphate and lime was the first cheapest form of fungicide used for the control of various plant diseases and is effective even today for the control of many fungal pathogens. Aluminium phosphide is used as a rat poison. Sodium arsenite is used as an herbicide.
The heavy metals may accumulate in the agricultural produce causing toxic symptoms to animals and human beings who consume them. The heavy metals can bind with different metabolic proteins of the plants and thereby inactivate them. If sewage water or industrial effluents are used for irrigating crop plants heavy metal contamination occurs very often. Some of the energy of the plants will have to be utilized in transport of these heavy metals or their ions across the bio-membranes and through cells.
Aluminium is the third most abundant mineral in earth’s crust. The ability of different plant species-to tolerate aluminium varies very widely. Aluminium content of acidic soils is high. It lowers phosphorus availability and blocks the uptake of calcium and magnesium ions causing imbalance in mineral nutrition of the plants. Aluminium produces rigidity in actin cytoskeleton of the cells. Aluminium binds to nucleic acids and thereby inhibits cell division.
Root nodule biomass of legumes was decreased by 26-72 per cent by 100 µm concentration of aluminium sulphate and by 59-87 per cent by the same concentration of mercuric chloride. Root growth of barley and Vicia faba were inhibited at 1.85 and 9.3 µm concentrations of aluminium. But rye and Lupinus luteus required 222 µm to bring about the same inhibition of root growth. Aluminium at 440 and 880 µm concentrations decreased the nodulation, nodule dry weight and dry weight of roots by Frankia on Casuarina.
Aluminium tolerant plants release malic acid and citric acid from the root hairs to immobilize it. Maize plants treated with aluminium for 24 hours had decreased root elongation rates and this decrease was reversed by pretreatment of the roots with 1.0 µm silicon. The ameliorative effect of silicon was due to lowering of aluminium uptake and exclusion of aluminium from the root tips. Tolerance of sorghum plants to aluminium toxicity was inherited as a dominant character.
Dry matter and protein content of the seedlings of Pisum sativum decreased by 8% and 51%, respectively over control due to arsenic. Zinc inhibited electron transport during photosynthesis in isolated barley chloroplast. Photosynthesis, cellular respiration and plant-water relationship were affected by cadmium. Nitrate reductase activity, leaf nitrate content and potassium uptake of P. sativum were reduced by cadmium. Potatoes were tolerant to manganese and mature leaves accumulated upto 700 µg g–1 without any toxic symptoms.
Chromosomal abnormality in the root tips of Helianthus annuus was observed 24 hours after treatment with heavy metals aluminium, copper, lead, cadmium, nickel and zinc. Manganese caused increased laggards, multipolar formation, chromosomal breakage and disturbed anaphase in sunflower. Hexavalent chromium acted as a prophase poison for onion root tips.
Cadmium decreased the growth, nucleic acid content, photosynthesis and nitrogen fixation by the blue green alga Nostoc muscorum. Cadmium caused significant reduction in surface area of thylakoids of the alga Anabaena flosaquae. Anabaena azollae showed growth inhibition above 4 ppm concentration of lead and zinc while Chlorogloea fritschii and Synechocystis showed growth inhibition above 3 ppm concentration of cadmium, lead and zinc.
The order of toxicity was Hg > Ag > Cd > Pb > Cu for Chlorella while it was Hg > Cd > Ag > Pb for phytoplankton. A reduced active mycorrhizal root tip count was found in soil rich in copper, lead and zinc. Cenococcum graniforme mycorrhiza was susceptible to aluminium at all the concentrations (0.508 meq mg–1) tried.
Methyl bromide is a broad-spectrum biocide used as a fumigant. It is very efficient against fungal pathogens. The bromide content of the soil increased to 5-10 mg kg–1 soil after fumigation. But this higher concentration of bromide had no effect on crop plants because of possible interaction with chloride and nitrate ions present in soil. Fumigation of wheat field with chloropicrin or with methyl bromide increased the yield of the crop.
The soil nitrate nitrogen increased at 15 and 25 weeks after fumigation. The mycelial dry weight of Pisolithus tinctorius (VAM) in Pinus seedlings was found to be drastically reduced by the fungicide brassicol but not by captan at normal recommended doses. The height and dry weights of seedlings were affected greatly by brassicol only (Table 31.3). Benomyl, carbendazim and zineb at 25 ppm or lower concentrations increased the germination of megasporocarps of Azolla filiculoides.
Urea hydrolyzing bacteria Micrococcus spp. and Proteus spp. were inhibited by the fungicide tridemorph at 75 mg kg–1 soil. The fungicide captan was found to inhibit soil denitrification. Forty five days after the soil was treated with 400 ppm concentration of the fungicide bavistin total fungal population of the soil was about 30% less as compared to untreated control; Azotobacter was less by around 25% and Rhizobium by 33% (Table 31.4), Soil nitrification was found to be decreased and ammonification increased by bavistin.
Antibiotics are predominantly used for the control of bacterial diseases occurring in crop plants and in algal culture. Chloramphenicol, cycloheximide and streptomycin sulphate below 25 ppm level increased the germination of megasporocarps of Azolla filiculoides by around 10%. Penicillin and cycloheximide at higher concentrations (25 ppm) caused a decrease in germination of the same. Azolla-Anaebena symbiotic system is used as a green manure particularly in rice fields where it can be grown and then incorporated in situ into the soil.
Megasporocarp germination plays a major role in the perpetuation of the system in subsequent crops. Streptomycin even at very low level of 0.05% decreased the inflorescence length and increased the percentage of sterile florets to a great extent in pearl millet. In transgenic crop varieties, the new gene of interest is introduced along with a reporter gene conferring antibiotic resistance. The energy of the plant is wasted by producing enzymes responsible for antibiotic resistance.
Seed hardening chemicals potassium chloride and calcium chloride at 1.0% concentration were beneficial to Co13 ragi (Eleusine coracana) cultivar while at higher concentrations they decreased the germination. The seed hardening chemical sodium chloride decreased germination and vigour of ragi seedlings. Pre-sowing seed treatment with various forms of potash decreased the yield and yield components of wheat under rainfed condition.
Vigna unguiculata seeds were treated with 10, 50 and 100 ppm concentrations of anti-transpirants CCC, MH and Phosphon D. The length of radicle and hypocotyls were reduced by all the three anti-transpirants. The plumule length was affected by all the concentrations of MH and higher concentration of Phosphon D. Root length, fresh weight and dry weights of seedlings were not decreased by 100 ppm Phosphon D but adversely affected by other treatments.
Agrochemicals on Soil Enzymes:
All biological activities of soil proceed under the influence of soil enzymes of mostly microbial origin. The enzymes may arise from extracellular release from microorganisms or from dead and decaying tissues/organisms. Monocrotophos reduced the activities of cellulase, protease, urease and phosphatase enzymes of soil. The extent of reduction was very high at 100 ppm as compared to 10 ppm.
Cellulase and protease activities were restored in about 21 days after treatment while urease and phosphatase were still significantly affected. Soil dehydrogenase activity is an indirect measure of the microbial load and activity in soil. Soil dehydrogenase activity was inhibited by 31% by the fungicide tridemorph at 100 mg kg–1 soil and 50% inhibition was observed with 250 mg kg–1. Butachlor, oxyflorfen and 2, 4-D application increased soil dehydrogenase activity over control on 7th day after application. Thereafter the enzyme activity decreased significantly upto 35 days.
The effect of different agrochemicals on plant growth and productivity must be routinely observed by the agricultural scientists as well as the environmentalists for all new chemicals before recommending them for use. Basic studies on the harmful effects of herbicides, pesticides and fungicides on plant metabolism must be given importance in future. External symptoms on plants like scorching and/or chlorosis due to chemical application is grossly inadequate.
Analysis of chemical residues and their metabolites in crop produces will not indicate the effect of agrochemicals on biochemical and metabolic activities of the plants. The plant may spend its energy for transporting and metabolizing the chemical. This much energy if conserved would have been used for productive purpose by the plant. The effect of agrochemicals on the bioenergetics of crop plants must be known before it is recommended for use.
The role of chemicals on absorption and transport of nutrients and other energy compounds to the economic part, genotoxic/mutagenic effects and chemical quality of the produce must be thoroughly studied. The cost of reduction in growth and soil fertility by the agrochemicals has to be worked out so that the harmful effects may be quantified in monetary terms.