Crop air pollution assessment methodology
By M. Tariq
Javed, Dr Shahzad M.A. Basra & Dr Irfan Afzal
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
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
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
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
(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
(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