Rapid industrialization, urbanization, and congestion of population in a few pockets, in most part of the world, are giving rise to pollution caused by emission of gases such as carbon monoxide, carbon dioxide, sulfur dioxide and nitrogen peroxide which may play havoc with the human population. In Indian culture, neem has been referred as an “air purifier” so it may be an avenue tree of choice in thickly populated areas, by its capacity to survive in adverse conditions, absorb some of the environmental pollutants, and act as an “air freshener” by releasing oxygen and mild odorous principles.
Tannery is one of the industries responsible for pollution of river water. In third world countries, in some areas, the cattle population exceeds that of humans, so an appreciable amount of animal hide is available which is treated with tanning materials to turn it into leather. The whole process requires repeatedly washing with water, so the water requirement is very high; after washing, this water becomes heavily contaminated and is drained back to the rivers.
Chaturvedi () tested neem as one of the trees for tolerance to tannery waste water. The survival rate of the tree was 22–94 percent. Ramanuja and Misra () did culture experiments over 12 weeks, with 8–9 month old seedlings. Plants were irrigated weekly with 2 liters of effluent from a tannery settling tank. Neem was found quite tolerant to this waste water.
The other major polluting industries are thermal plants and chemical factories, such as those for fertilizers and pesticides, which release carbon dioxide, sulfur dioxide and nitrogen peroxide, in addition to suspended particles like dust or fly ash.
Devi and Patel () and Patel and Devi () studied morphological variation in the vegetation around a fertilizer complex. The variation noted was reduced foliage or defoliation, cracking and peeling of bark, reduced leaf and leaflet area and petiole length, mutilation of leaves in various ways and the total absence of flowering.
In the case of leaves in normal plants, the cell wall was undulating but in the polluted area it was straight, with reduced stomatal frequency and some variation in stomatal width and pore area. Starch, insoluble polysaccharides and lipids also varied.
On 3 December 1984, large quantities of methyl isocyanate (MIC) escaped from a pesticide plant in central India. Ram Prasad and Pandey () studied the effect of this poisonous gas on the neem tree. The tree was sensitive to MIC. There was defoliation and blackening of the foliage, but the tree revived two months after the injury with the emergence of new leaves. Farooq et al. () studied the sulfur dioxide resistance of trees and also visible symptoms of sulfur dioxide absorption. Seedlings of 12 species, including neem, were exposed to sulfur dioxide to various concentrations and it was found that neem could tolerate this gas to a major extent. Rao and Dubey (), in their study to find differential responses to sulfur dioxide, analyzed the neem tree along with others for stomatal conductance, sulphate, protein, superoxide dismutase and peroxides for one year in an ambient environment with varying concentration of sulfur dioxide. The results indicated that trees under sulfur dioxide stress developed phytotoxicity by undergoing certain biochemical changes. To find out the effect of sulfur dioxide exposure to tree saplings, Krishnnayya and Bedi () noted that this gas damaged chloroplasts and cytoplasm in palisade cells, followed by rupturing of the outer envelope of the chloroplasts and extrusion of plastoglobuli and starch into the cytoplasm. Thinma Raju et al. () observed that a neem tree growing in a highly polluted area was not affected by various gases, whereas some other trees exhibited symptoms of defoliation, die-back, poor flowering and fruiting.
In one experiment, fly ash-induced injury to leaves and proline metabolism in plants growing at two different distances away from a thermal powerhouse was studied. Trees closer to the source of pollution had higher dust deposition, leaf injury and proline accumulation at a lower pH of cell sap as compared to neem trees away from the source. Proline accumulation was present at both sites, throughout the period of study of four months. It indicated the greater ability of neem to adapt to stress from exposure to air pollution ().
Beg et al. () studied the performance of neem trees at five selected sites. In the vicinity of a power station, gases like nitrogen peroxide and sulfur dioxide were below the permissible levels, but the total suspended particles were higher. Air pollution had a major effect on chlorophyll. It was seen in this study that there was some destruction of chlorophyll, showing thereby that neem is moderately sensitive when exposed to sulfur dioxide in various concentrations but could tolerate this gas to a major extent.
Exhaust from Automobiles
Pollution due to the exhaust from automobiles in some congested areas is fairly high. The effect of this exhaust on the survival of neem, if planted as an avenue tree, is very important. Keeping this point in view, Bhatti and Iqbal () studied the leaf length area, dry weight, etc. of these trees. It was noted that neem tolerated this type of pollution very well and can be planted as a roadside tree in thickly populated areas.
When dust loading of leaves in an automobile polluted area was studied, it was seen by Satyanarayana et al. () that the neem tree accumulated a thick, sticky crust. In the leaves, as compared to the control, the epidermal cell size was smaller and stomatal frequency was higher. Sharma and Roy () also observed the same features on leaves when subjected to automobile exhaust.
The phytotoxic effects of aerial discharge from the steel industry were evaluated for neem by Kumawat and Dubey (). In the neem trees growing in the area, the chlorophyll pigments, carotenoids and leaf pH level decreased, while the leaf injury index, leaf area/dry weight ratio, conductivity of leaf disk water, sulfate content and total chlorophyll: sulfate ratio increased. The authors observed that the pollution injury was maximum in winter, followed by summer and the rainy season.
Detection of Heavy Metals
The neem bark was tested to monitor heavy metals in polluted sites, and compared with those absorbed by moss. Lead, zinc and iron content in both materials were higher at most sites ().
Johri et al. () developed a process of flocculation of water pollutants by using extract of seeds of neem along with Moringa oleifera and Madhuca latifolia. This extract formed a floe with the contaminants and the suspended particles settled down, giving rise to clean water.
Before neem products could be used against insects and pests, quite exhaustive studies were carried out on the safety aspects, particularly with regard to warm-blooded animals. Concern was also expressed over its effect on honey bees. Schmutterer and Holst () studied this aspect. The neem-treated flowers were not repulsive to bees and the neem preparations did not cause any serious damage to their systems. On the other hand, neem controlled the tracheal mites of the bee ().
Neem-based products were not only found safe for earthworms, but for young salmon also ().
Neem trees in a grove are eco-friendly, but pollens have been known to cause allergy in some cases. Karmakar and Chatterjee () isolated and characterized IgE-reactive proteins for pollens. They have shown that AIaI and AIaIVb are the major allergens. Amino acid analysis of these, the effect of pH on them and cross-activity has also been carried out by these authors.
Selections from the book: “Neem: The Divine Tree Azadirachta indica”. Edited by H.S.Puri. Series “Medicinal and aromatic plants – industrial profiles”. 1999.