HUMAN beings probably are the first to experience the harmful effects of air pollution when they lit fires in poorly ventilated caves. Since then they have gone on polluting the atmosphere. Until recently, the problems of environmental pollution were local and minor because of the earth's own ability to absorb and purify minor quantities of pollutants.

Industrialisation of society, introduction of motorised vehicles, and the population explosion, are factors contributing to growing air pollution. Now it has become important to find ways to clean the atmosphere.

In Pakistan, air pollution is caused by automobiles, including aircraft, producing carbon monoxide, hydrocarbons, nitrogen oxides and lead pollution. Stationary sources of air pollution are industrial plants, power generation systems, construction projects and solid wastes. These sources add pollution like sulphur dioxide, nitrogen oxides and dust, ash, soot, metals and various other chemicals. The air-born particulate matter, found in sample collected from Ayub Agriculture Research Institute (ARRI), Faisalabad, is: gypsum six per cent, calcite 10 per cent, chino chlore 13 per cent, albite 16 per cent, quartz 25 per cent, and illite 30 per cent.

Pollutants, like sulphur dioxide, nitrogen oxides, ozone and peroxyacl nitrates (PANs), directly damage plant leaves when they enter leaf pores.

Agricultural crops can be injured when exposed to high concentrations of various air pollutants. Injury ranges from visible markings on the foliage, to reduced growth and yield, to premature death of the plant.

Atmospheric ozone is the main pollutant in the oxidant smog complex. Throughout the growing season, particularly July and August, ozone levels vary significantly.

Localised, domestic ozone levels also contribute to the already high background levels. Injury levels vary annually and white bean, which are particularly sensitive, are often used as an indicator of damage. Other sensitive species include cucumber, grape, green bean, lettuce, onion, potato, radish, rutabagas, spinach, sweet corn, tobacco and tomato. Resistant species include endive, pear and apricot.

Ozone symptoms occur on the surface of affected leaves and appear as a flecking, bronzing or bleaching of the leaf tissues. Although yield reductions are usually with visible foliar injury, crop loss can also occur without any sign of pollutant stress. Conversely, some crops can sustain visible foliar injury without any adverse effect on yield.

Sulphur dioxide enters the leaves mainly through the stomata (microscopic openings) and the resultant injury is classified as either acute or chronic. Acute injury is caused by absorption of high concentrations of sulphur dioxide in a relatively short time. The symptoms appear as two-sided lesions that usually occur between the veins and occasionally along the margins of the leaves. The colour of the necrotic area can vary from a light tan or near white to an orange-red or brown depending on the time of year, the plant species affected and weather conditions.

Recently expanded leaves usually are the most sensitive to acute sulphur dioxide injury, the very youngest and oldest being somewhat more resistant.

Different plant species and varieties and even individuals of the same species may vary considerably in their sensitivity to sulphur dioxide. These variations occur because of the differences in geographical location, climate, stage of growth and maturation.

The following crop plants are generally considered susceptible to sulphur dioxide: alfalfa, barley, buckwheat, clover, oats, pumpkin, radish, rhubarb, spinach, squash, swiss chard and tobacco. Resistant crop plants include asparagus, cabbage, celery, corn, onion and potato.

Fluorides absorbed by leaves are conducted towards the margins of broad leaves (grapes) and to the tips of monocotyledonous leaves (gladiolus). Little injury takes place at the site of absorption, whereas the margins or the tips of the leaves build up injurious concentrations. The injury starts as a grey or light-green water-soaked lesion, which turns tan to reddish-brown. With continued exposure the necrotic areas increase in size, spreading inward to the midrib on broad leaves and downward on monocotyledonous leaves.

Studies of susceptibility of plant species to fluorides show that apricot, barley (young), blueberry, peach, gladiolus, grape, plum, prune, sweet corn and tulip are most sensitive. Resistant plants include alfalfa, asparagus, bean (snap), cabbage, carrot, cauliflower, celery, cucumber, eggplant, pea, pear, pepper, potato, squash, tobacco and wheat.

Complete system expression on affected vegetation usually takes several days to develop, and appears as irregular, bleached, bifacial, necrotic lesions. Grasses often show reddish, inter-veinal necrotic streaking or dark upper surface discoloration. Flowers, fruit and woody tissues usually are not affected, and in the case of severe injury to fruit trees, recovery through the production of new leaves can occur.

Sensitive species include apple, barley, beans, clover, radish, raspberry and soybean. Resistant species include alfalfa, beet, carrot, corn, cucumber, eggplant, onion, peach, rhubarb and tomato.

Particulate matter such as cement dust, magnesium-lime dust and carbon soot deposited on vegetation can inhibit the normal respiration and photosynthesis mechanisms within the leaf. Cement dust may cause chlorosis and death of leaf tissue by the combination of a thick crust and alkaline toxicity produced in wet weather.

The dust coating also may affect the normal action of pesticides and other agricultural chemicals applied as sprays to foliage. In addition, accumulation of alkaline dusts in the soil can increase soil pH to levels adverse to crop growth.

In the current scenario of increasing demand for feed and fibre due to population pressure, the air pollution threats to agricultural crops should be minimised. Research into ways of reducing the air pollution effects on crops made little progress as yet. So there is dire need of long-term studies to check the hypothesis that air pollution not only reduces productivity of crops but also alters the nutritional quality.

The following are some suggestions to deal with this issue urgently in a fruitful way:

(a) There is a need to evaluate effects of air pollutants and other stresses on crops and non-wood plants by monitoring the onset of injury and reductions in the yield/biomass of sensitive species.

(b) Identification of realistic dose-response functions, incorporating modifying factors for a range of economically important crops and for crops at risk from pollution.

(c) Efforts should be made to validate and substantiate critical levels of ozone for crops and non-wood plants.

(d) Facilitation by the government in the preparation of country maps showing where critical levels for ozone are exceeded.

(e) Research assistance in assessing the economic loss due to ozone pollution.

(f) Promotion of advance level research by conducting literature reviews and specific experiments on the accumulation of atmospheric deposition of heavy metals.

(g) Finally, the development of accurate air pollution crop assessment methodology by incorporating chamber studies, field plots and several statistical methods.

(h) Making recommendations to the authorities for selection and adaptation of suitable crops under polluted environment on the basis some pre-determined rules.

Courtesy: The DAWN