Many chemicals, fertilizers, pesticides combustion of coal, and waste treatments release toxic substances into the environment that are taken up by the plants from air, water and soil. Atmospheric pollutants particularly SO2, halides (HF, HCI), ozone, carbon monoxide and per-oxiacetyl nitrate (PAN) produced from automobiles, industrial fumes and strong radiations are dangerous to plants. Harmful substances that reach to plants through the air are SO2, nitrogen oxides, hydrocarbons, dusts and smokes. Plants growing in water are severely affected by toxic chemicals like cyanides, chlorine, hypo-chlorate, phenols, benzyl derivatives and heavy metal compounds of sewage.
Effects of different kinds of pollution can be determined by the nature of pollutants, their concentration and the period of exposure. Under exposure to high concentration, plants suffer acute injury with externally visible symptoms, such as chlorosis, discolouration, necrosis and death of entire plant. Besides morphological changes, chemical, biochemical, physiological and fine structural changes also occur in plants.
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The damaging effects of air pollutants, on vegetation have been recognized by many workers. In general, air pollution decreases the yield of all crops by affecting their photosynthetic activity and growth. Pollution damage can also be recognized by the accumulation of toxic material in the plant, changes in pH, reduced or increased activity of certain enzymes, increase in compounds with SH groups and phenols, lowered ascorbic acid level in the leaves, depression of photosynthesis, stimulation of respiration, low dry matter production, changes in permeability, disturbances in water balance and reduced fertility under prolonged exposure. The disturbances in metabolism are due to chronic injury with irreversible consequences.
Plants show reduced productivity and yield and quality is lowered and ultimately they die. The symptoms of pollution affected plants are varied and unspecific. A particular pollutant affects different plants in very different ways and a particular symptom can be produced by a variety of substances. The influence of external factors (pollutants) on plants depends upon the species, stage of development and the organ and tissue involved. Morphological alteration of a plant and floristic composition of a plant community are commonly used to indicate changes in the environment.
According to Van Haut and Stratmann (1970), visible plant symptoms are most commonly used to indicate the responses of plants to pollutants. Jacobson and Hill (1970) have studied the effects of common pollutants on plants. It is possible that any part of a plant body, if it responds specifically or characteristically to any pollutant, can be used for its indication. M.U. Beg (1980) from Industrial Toxilogy Research Centre, Lucknow has reported the responses to air pollutants as a biological indicator, taking several parameters into consideration such as, seed germination, growth of plant, development of lateral branches, expansion and colour changes in leaf, flower and fruit formation, decolouration of flower, loss of physiological control, mineral composition, chemical constituents of cells, enzymatic activity and pollen germination.
Germination and General Growth:
Seed germination has been used by many workers to monitor pollution responses. Several growth parameters such as percentage of germination, seedling survival, seedling height, cotyledonary expansion and fresh and dry weight have been taken as criteria to assess plant response to a specific pollutant. Phaseolus vulgaris has been grown in smoke free and smoke affected region by Sorauer (1899). The toxic effect of thiosulphate has been indicated as germination inhibition in many plants. Houstan and Dochinger (1977) have evaluated germination inhibition in relation to pollution by sulphur dioxide and ozone. The effects of lead, cadmium, NO and CO have been studied on many plants.
Some plant species are sensitive to pollution e.g., Polygonum, Rheum, Vicia, Phaseolus and Capsella. Generally, the plants’ response to pollutants is characteristic rather than specific Stunting Of com, sweet potato and lye has been reported due to high toxicity. Reduction in root length, shoot length, numbers of tillers, leaves; ears and grains in wheat have been reported under conditions of cement dust pollution.
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Similarly plant height, number of leaves and bolls per plant are reduced in cotton exposed to particulate pollution. Inhibition of lateral growth of forest trees IS caused by lime stone dust. Pine trees do not flourish in SO2 polluted areas. It has been noticed that leaf is the most sensitive organ to pollution.
The pollution indicator value of leaf has been exploited by many workers in response to a variety of conditions. Leaf injuries are a characteristic symptom to various pollutants. The characteristic symptoms on leaf include pigmentation, chlorosis, yellowing, necrosis etc. The leaves of dicotyledons generally exhibit spotted markings between the veins while monocotyledons usually show necrotic streaks between parallel veins. Injury may also occur along the margin and tips.
Reduced expansion of cotyledonary eaves in response to pollution has been observed in several cases. Decrease in dry weight of leaf decrease in leaf thickness, cell size, loss of leaves and early senescence may be due to smoke and SO pollution. Yunus and Ahmad (1980) have observed that leaves in the polluted area of cement factory showed higher stomatal and trichome densities, smaller epidermal cells and trichomes as compared to leaves obtained from unpolluted atmosphere.
Biochemical and Physiological Changes:
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Among the biochemical aspects, the most important parameter is pigment analysis. Chlorophyll a and b have been measured as index of response to different types of pollution. In Cassia and Cynodon, 50 per cent reduction of chlorophyll has been observed while in Saccharum the pigment is least affected. Chemical estimation like proteins, amino acids, soluble sugars, sucrose starch, reducing sugars, vitamin C, riboflavin, thiamine and carbohydrate are used to indicate foliar sensitivity to air pollution.
Physiological activities as opening of stomata and rate of photosynthesis can also be used as indicator of pollution. Photosynthesis as a parameter has been used for mixed exposure to SO2, NO2 and dust. Enzymatic parameters are also used to indicate the presence of particular pollutant.
Peroxidase was found to be most sensitive indicator of pollution in the absence of visible injury. Thus on the basis of enzyme activity, the susceptible species of plants can be identified. Many workers have reported that enzymatic activity is retarded due to air pollution. Other common enzymatic parameters used are ribulose diphosphate carboxylase, glutamate-pyruvate transaminase, glutamate- oxaloacetate transammase etc.
Effects of Different Pollutants on Vegetation:
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Air pollutants are absorbed by the plants in appreciable amount. The absorption is affected by the nature, concentration of pollutant, exposure time, plant species, and age of the plant, nutrient supply and other environmental factor.
The effect of common air pollutants on plants are described below:
Sulphur dioxide:
SO2 is a major air pollutant in the areas where large quantities of fuel are burnt. Leaves of green plants are able to absorb SO2 through their stomata. The absorbed sulphur is incorporated into various organic compounds. SO2 is one of the most phytotoxic pollutants. Foliar injury is a prominent symptom recognized in the leaves.
Its effect on plants varies from species to species depending on genotype and physiological stages of the plants, their age, climatic condition, concentration of pollutant and duration of exposures. The most recognised symptoms are interveinal chlorosis, necrosis in plants brownish colouration in the tip of pine needle. Lower concentration over long period leads to chrome leaf injury such as gradual chlorosis. Trees are injured followed by shrubs and herbs.
Some of the plant species, especially the sensitive ones are unable to regenerate in the area and gradually get eliminated from the soil. SO2 also reacts with cell through stomata causing injury or death of tissues. Flower buds of Gladiolus are very sensitive and they exhibit necrosis at the tips and margins. SO2 also affects photosynthesis, water relations and enzymatic system of plants. Beg and his team (1982, 1985) at ITRC, Lucknow have tried to investigate the mechanism of phytotoxicity of SO2, and suggested certain morphological and physiological traits in plants as monitors of air pollution.
Scientists of ITRC, Lucknow have evolved a simple and quick technique of screening the forest species according to their reactivity. On the basis of their experiment they concluded that the plant spp. like Eucalyptus, Ficus infectoria (Paker), Dalbergia sissoo (Shisham) and Syzygium cuminii (Jamun) are resistant where as Ficus bengalensis. (Bargad), Ficus religiosa (Peepal), Bombax melabaricum (samel) and Terminalia arjuna (Arjun) are susceptible to SO2. Characteristic symptoms were noticed on Aegle marmelos (Bel) and Holoptelea integrifolia (Chilbil) when trees are exposed to 6.5 PPM SO2 for four hours. Interveinal necrosis, margmal necrosis and leaf curling were common visible symptoms. These symptoms appeared after 48 hrs. of exposure.
At low concentrations of SO2 no significant reduction in chlorophyll and carotenoid contents were noticed in forest trees. But at higher concentration of SO2 the reduction in total chlorophyll and carotenoid contents was noticed. Thus, it can be inferred that forest species can be proved as an effective measure for purifying the environment.
Oxides of nitrogen:
Oxides of nitrogen are produced in internal engines where oxygen and nitrogen are subjected to heat and pressure and also by incineration of organic wastes and in petroleum refining. These pollutants produce marked injury on affected plants. Sometimes there is a combined effect of SO2 and oxides of nitrogen which produce more injury than either of them alone. Besides leaf injury, reduction of growth occurs in sensitive plants. NO2 causes leaf injury and reduction of growth in several sensitive plants.
Fluorides:
Fluorides are emitted largely from aluminium and phosphate plants. It is widely spread in the earth’s crust as a natural component of soil, rocks and minerals. When these substances are heated or treated with acid, toxic fluoride are released into the atmosphere. This becomes phytotoxic above the critical levels. In recent years, the losses to agriculture from fluoride have greatly increased with the vast expansion of industries.
This pollutant damages plants if deposited on the leaf surfaces. Thus reduces the photosynthesis. It also blocks stomatal pore thereby reducing respiration, photosynthesis and transpiration and finally plants become weak and ultimately die. On account of fluoride pollution, leaves show marginal necrosis as well as interveinal chlorosis. The fluoride accumulated in plants enters the food chain through herbivorous animals and passes into the soil through animal wastes.
Ozone:
Ozone is a natural component of atmosphere where it filters out dangerous ultraviolet radiation. In urban localities where there is high population density, hydrocarbons and oxides of nitrogen are emitted into the atmosphere from automobile exhaust which in presence of sunlight interact to form large amounts of ozone which causes serious injury to vegetation.
Ozone injury on plants range from collapse of extensive areas of the leaf blade to discrete necrotic spots on upper surface, chlorosis, abscission of leaves, fruits and suppression of growth without visible symptoms of injury. Extent of injury and characteristics symptoms vary widely with type of plant, concentration of toxicant, duration of exposure and environmental conditions. Visible injury may occur on the most sensitive plants.
Ammonia:
It has been observed that fumigating trees with ammonia gas increases the ammonium content. Leaves of soybean, sunflower, cotton and maize are able to absorb ammonia from the atmosphere. In maize, a part of absorbed ammonia is metabolized into amino acids. It is experimentally demonstrated in rye grass and several other crops too. Some hydrophytes e.g., Ceratophyllum demersum, Potamogeton pectinalis and Elodea spp. also absorb NH3 through their roots because ammonia is highly soluble in water. The absorbed ammonia is trans located to the leaves for assimilation.
Carbon monoxide:
Carbon monoxide is a product of incomplete combustion. It is deadly poisonous at high concentration and is one of the important gaseous air pollutants. Human activities result in the production of nearly 250 million metric tons of CO annually. It is taken up by plants including algae.
Particulates:
Particulates or dusts come under major pollutants which remain suspended in the air for a short time. Particulates are produced from many sources as cement factory, lime processing plant, refuse disposal and agricultural practices. Heavy metals arise from factory affluent and combustion. Rao and his associates (1980, 1981, 1985) have studied the responses of certain crops to particulates and developed certain methods to mitigate pollutant including its injury to plants.
These particles become deposited on the plants especially on the leaf surfaces. Thus leaves become coated with dust which block the stomata causing the reduction of photosynthesis followed by reduction in food production. Transpiration and respiration processes are also regulated.
In the coal dust polluted areas mango and lemon trees show morphological changes leading to loss in productivity. It has been seen that the fallout of fine coal particles especially at the time of flowering significantly hampers the process of pollen germination and fertilization which are essential for fruit setting. Trees in such areas show decortication of bark, defoliation of branches and stunting of growth. Similarly, cement dust released from cement factories leads to irreparable degradation of soil characteristics and plant’s structure and function. It is likely that cement dust affects plants through leaf encrustation, stomatal plugging, and changes in leaf reaction.
Control of Pollution through Vegetation:
(Plants as Ameliorator of Environmental pollution):
Plants, being the most important component of the ecosystem must be given top priority in the environmental planning and management. Trees have ability to reduce the amount of pollution in air. Plantation is extremely important in polluted and water catchment areas including urban and industrial areas. Many instances are known which indicate that the trees are capable of making our environment more pleasing and livable. Trees also provide an aesthetic link between man and his environment. Trees play an important role in enhancing the quality of environment by influencing the life supporting systems.
They restore ecological balance of all ecosystems, maintain biological diversity, act as catchment for soil and water, prevent floods and check or reduce the noise pollution. The capacity of plants to absorb aerosol (very small particles of solid or liquid that remain suspended in the air) or gaseous pollutants from the air has not been thoroughly investigated but few studies have shown that trees can effectively filter certain aerosols.
Researches based on the foliar accumulation of chloride and fluoride have also stressed the ability of plant leaves to retain atmospheric aerosols. Plants also improve the quality of air by absorbing or altering gaseous pollutants from the atmosphere. For example, leaves exposed to low level of SO2 may transform the gas to SO4. SO4 is less injurious to plants. Thus plants, in general, act as mitigators of air pollution.
The surface of vegetation provides a major Alteration and functions as sink to mitigate pollutants from air. At present there is great crisis of CO2-oxygen balance in atmosphere. Only green plants have capacity to utilize CO2 in the process of photosynthesis and release free oxygen in the atmosphere. Green plants not only take up CO2 and release O2 but their leaves retain some pollutants.
Dense and tall trees also filter out dust. Data collected from Lucknow showed high dust trapping potential of different plants. In this regard peepal (Ficus religiosa), mango (Mangifera indica) and ashok (Polyalthia longifolid) trees were found to be better dust collector than amaltas (Cassia fistula), Kachanar (Bauhinia variegata) and gulmohar (Delonix regia).
Enough evidences are now there which show the role of plants as sinks for gaseous pollutants. For, example, an oak (Quercus suber) tree has been found to reduce about 20 per cent ozone concentration. It has been shown that alfalfa canopy removed 1/4 ton of nitrogen oxide or SO2 per square mile per day. Tree plantation is economically important for protection against environmental pollution. According to an estimate a peepal (Ficus religiosa) tree is reported to replace 2,252 kg polluted air with 1,713 kg of O2 during its life time which is sufficient for 60,000 people. Hence real value of medium size tree comes to about Rs. 15.7 lac in 50 years of life time.
The use of vegetation in filtering out the dust, soot and particulates from the atmosphere has been widely accepted.
Ahmad et al (1988) at National Botanical Research Institute, Lucknow have initiated studies on the impact of air pollution on vegetation with following objectives:
1. To study the nature and extent of damage caused to vegetation by common air pollutants.
2. To determine the role of plants in indicating the level of pollution in a particular area.
3. To identify pollution tolerant/resistant plant species to be used as green belts around industrial areas to mitigate air pollution.
4. To determine the dust trapping/filtering potential of common trees and shrubs.
Thus pollution is a problem which is going to increase with the increase of population in the coming years but vegetation can reduce it up to some extent if it may not altogether eliminate.
Pollution can be checked through vegetation by adapting the following measures:
1. Green belts should be planted around urban and industrial areas with plants that are pollution resistant to help in purifying the atmosphere by regulating temperature, humidity and air movement and also to reduce damage caused by air pollutants. Green belt around the cities will also increase the aesthetic sense.
2. Screening of Indian plants should be done in order to replace pollution tolerant species.
3. Research laboratories and organisations should carry out regular monitoring of environmental pollution through proper vegetation in metropolitan cities and industrial towns.
4. Many forest tree species have been proved as an effective measure for purifying the environment.
Based on the vegetational survey in response to pollution some tolerant species have been enlisted as suggested by Ahmad et al, 1988:
Herbs:
Achyranthes aspera (Latjira)
Amaranthus graecizans (A variety of Chaulai)
Argemone mexicana (Sialkanta)
Solarium xanthocarpum (Bhatkataiya)
Shrubs:
Bougainvillea (Baganvilas)
Calotropis procera (Madar)
Cassia sophera (Kasunda)
Clerodendron infortunatum (Bhant)
Croton sparsiflorus
Ipomoea fistulosa (Behaya)
Lantana camara (Ghaneri)
Murraya exotica (Kamini)
Nerium odorum (Kaner)
Ricinus communis (Arand)
Tabernaemontana coronaria (Wild Chandni)
Trees:
Acacia arabica (Kateria Babul)
Aegle marmelos (Bel)
Albizzia lebbeck (Siris)
Syzygium cuminii (Jamun)
Tamarindus indica (Imli)
Zizyphus mauritiana (Ber)
Alstonia macrophylla (Chatian)
A. scholaris (Chitaman)
Anthocephalus cadamba (Kadamb)
Artocarpous heterophylla (Kathal)
Azadirachta indica (Neem)
Butea frondosa (Palas)
Casuarina equisetifolia (Jhau)
Citrus medica (Lemon)
Dalbergia sissoo (Shisham)
Delonix regia (Gulmohar) ,
Diospyros melanoxylon (Tendu)
Eucalyptus citriodora
Ficus bengalensis (Bargad)
F. infectoria (Pakar)
F. religiosa (Peepal)
Holoptelea integrifolia (Papri)
Lagerstroemia sp. (Sawani)
Madhuca indica (Mahua)
Mimusops elengi (Maulsri)
Moringa oleifera (Sahjan)
Phyllanthus districhus (Harfarwari)
P. emblica (Amla)
Pithaclobium dulce (Jangal Jalebi)
Polyalthia longifolia (Ashok)
Psidium guajava (Guava)