The deteriorating biosphere and ecosystem caused by environmental pollution as a result of deforestation, urbanization, rapid industrialization, vehicular traffic and thermal power stations has become an alarming menace to human beings besides a threat to all other fauna and flora. All over the world there is now an awareness of global warming and greenhouse effects caused mainly by environmental pollution. The importance of plants in overcoming the harmful effects of pollutants to a great extent has been realized by the modern landscape architects, biologists, environmentalists, urban development authorities, Civic bodies and social welfare organizations.
The environmental pollution may be of air, water, soil, noise and dust. Air pollution is due to the pollutants released by automobiles, factories and thermal power generators. The oxides of carbon (carbon monoxide, carbon dioxide), sulphur (sulphur dioxide) and nitrogen (NO and NO2), lead (Pb), suspended particulate matters (SPM) and unsaturated hydrocarbons are the major pollutants by automobiles and industry.
Besides these, there are some other pollutants like dust, noise, industrial wastes and polluted water containing heavy metals. Emission of carbon monoxide from vehicles is a major air pollutant in Delhi and other big cities like Mumbai, Calcutta, Chennai, Bangalore etc. In India, automobiles contribute 1.8 million tonnes of pollutant annually.
The permissible levels of carbon monoxide for petrol driven four wheeler vehicles (cars, trucks, buses) is 3 per cent and 4.5 per cent for two- and three wheeler (scooters, motorcycles, auto rickshaws). The standard for diesel vehicles is 65 Hartridge smoke unit (see Table 5.1). Depletion of ozone in the atmosphere is also increasing at an alarming rate. According to a recent estimate, 65 per cent of Delhi’s air pollution is due to automobiles. In 1990, the amount of suspended particulate matter was 780 tonnes per day and increased to 1100 tonnes in 1991.
Pesticides and fertilizers used in agriculture also cause pollution of soil and water. Nowadays in USA. Holland and other countries bio pesticides based on micro organisms and naturally occurring fatty acid compounds compatible with the environment developed by genetic engineering are being used in growing flowers, vegetables, ornamental plants and other plants, particularly in greenhouses.
Biological and integrated pest control are the other two recently adopted methods being used abroad. Integrated pest management with the help of biological control using parasites and predators along with selective chemical pesticides which do not harm the predators and are environment friendly has t he advantages of low cost without pesticide residue.
Similarly bio fertilizers like agro-bacterium, rhizobium. mycorrhiza, blue green algae, Azolla and organic manures such as compost cow dung, oil cakes, leaf-mould, green manuring which are environment friendly have importance in growing plants without the use oft chemical fertilizers. In greenhouse the use of soil less substrates like rock wool. vermiculite, per etc and circulating drain water in a closed system In which water and fertilizer are collected from the substrate and disinfected with heat, ozone and ultraviolet and recirculated is now becoming quite common. The used rock wool is recycled for reuse and biodegradable pots and net trays are used in greenhouse to minimize pollution.
Pollution Tolerant Plants:
In India, there is not much information available on pollution tolerant plants However a truer scientific studies undertaken have suggested that evergreen plants with profuse branching and a large canopy dense foliage may be planted in Industrial areas and cities to overcome the harmful effects of pollution to a great extent.
Trees with broad, large sized and hairy leaves help in trapping the dust. In cities where pollution is mainly by emissions from automobiles it is useful to plant avenue trees along the roads and small and medium sized shrubs in central verge. The traffic islands may be planted with shrubs, seasonal flowers, low hedges and lawn. The depth and density of planting will depend upon the intensity and source of pollution.
Indoor plants help to clean the air inside the house by reducing the carbon monoxide, nitrous oxide, benzene and formaldehyde contents in the air. Many trees thrive well in industrial areas and cities, like Alstonia scholaris, Ailanthus excelsa, Azadirachta indica, Acacia auriculariformis, Cassia siamea, Cordia sebestena, Dalbergia sisoo, Ficus infectoria, Ficus benghalensis, Heterophragma adenophyllurn, Peltophorum ferrugineum, Putranjiva roxburghli, Madhuca Mimusops elengi, Parkinsonia aculeata, Saraca indices, Polyalthia longifolia, Thespesia populinea, Pterospermum acertfolium and Terrninatia aduna.
Among the pollution tolerant shrubs the most successfully grown are Bougainvillea, Lantana, Cassia alata, Murraya exotica, Nerium odorum, Tabernaemotana coronaria and Thevetia nerifolia.
The choice of pollution tolerant plant species to be planted for overcoming the adverse effects of pollution will depend upon the location, climate, soil and extent and kind of pollution. A plant species which is found to be highly pollution tolerant in one place may not be so effective in another place, mainly because of differences in temperature, humidity and type of pollution.