A Review of Some Different Effects of Air Pollution on Plants
Received: June 09, 2010;
Accepted: July 29, 2010;
Published: October 19, 2010
Air pollution is one of the severe problems world is facing today. It deteriorates
ecological condition and can be defined as the fluctuation in any atmospheric
constituent from the value that would have existed without human activity (Tripathi
and Gautam, 2007). In recent past, air pollutants, responsible for vegetation
injury and crop yield losses, are causing increased concern (Joshi
and Swami, 2007). Urban air pollution is a serious problem in both developing
and developed countries (Li, 2003). The increasing number
of industries and automobile vehicles are continuously adding toxic gases and
other substances to the environment (Jahan and Iqbal, 1992).
All combustion release gases and particles into the air. These can include sulphur
and nitrogen oxides, carbon monoxide and soot particles, as well as smaller
quantities or toxic metals, organic molecules and radioactive isotope (Agbaire
and Esiefarienrhe, 2009). Over the years, there has been a continuous increase
in human population, road transportation, vehicular traffic and industries which
has resulted in further increase in the concentration of gaseous and particulate
pollutants (Joshi et al., 2009). Adverse effects
of air pollution on biota and ecosystems have been demonstrated worldwide. Much
experimental work has been conducted on the analysis of air pollutant effects
on crops and vegetation at various levels ranging from biochemical to ecosystem
levels. It has been observed that ozone concentrations are higher in suburban
and rural areas as compared to the urban areas, whereas SO2 and NO2
concentrations are higher at urban sites (Tiwari et al.,
Environmental stress, such as air pollution, is among the factors most limiting
plan productivity and survivorship (Woo et al., 2007).
It is a major problem arising mainly from industrialization. Pollutants could
be classified as either primary or secondary. Pollutants that are pumped into
the atmosphere and directly pollute the air are called primary pollutants while
those that are formed in the air when primary pollutants react or interact are
known as secondary pollutants (Agbaire, 2009). It has
been observed that plants particularly growing in the urban areas affected greatly
due to varieties of pollutants [oxides of nitrogen and sulphur, hydrocarbon,
ozone, particulate matters, hydrogen fluoride, peroxyacyl nitrates (PAN) etc.]
(Jahan and Iqbal, 1992). Air pollution can directly
affect plants via leaves or indirectly via soil acidification. When exposed
to airborne pollutants, most plants experienced physiological changes before
exhibiting visible damage to leaves (Liu and Ding, 2008).
The atmospheric SO2 adversely affects various morphological and physiological
characteristics of plants. High soil moisture and high relative humidity aggravated
SO2 injury in plants (Tankha and Gupta, 1992).
Industrialization and the automobiles are responsible for maximum amount of
air pollutants and the crop plants are very sensitive to gaseous and particulate
pollutions and these can be used as indicators of air pollution (Joshi
et al., 2009). In urban environments, trees play an important role
in improving air quality by taking up gases and particles (Woo
and Je, 2006). Vegetation is an effective indicator of the overall impact
of air pollution and the effect observed is a time-averaged result that is more
reliable than the one obtained from direct determination of the pollutant in
air over a short period. Although, a large number of trees and shrubs have been
identified and used as dust filters to check the rising urban dust pollution
level (Rai et al., 2010).
Plants provide an enormous leaf area for impingement, absorption and accumulation
of air pollutants to reduce the pollutant level in the air environment, with
a various extent for different species (Liu and Ding, 2008).
The use of plants as monitors of air pollution has long been established as
plants are the initial acceptors of air pollution. They act as the scavengers
for many air borne particulates in the atmosphere (Joshi
and Swami, 2009).
EFFECT ON LEAF MORPHOLOGY
Pollutants can cause leaf injury, stomatal damage, premature senescence, decrease
photosynthetic activity, disturb membrane permeability and reduce growth and
yield in sensitive plant species (Tiwari et al.,
2006). Reductions in leaf area and leaf number may be due to decreased leaf
production rate and enhanced senescence. The reduced leaf area result in reduced
absorbed radiations and subsequently in reduced photosynthetic rate (Tiwari
et al., 2006).
Dineva (2004) and Tiwari et
al. (2006) recorded reduction of leaf area and petiole length under
pollution stress conditions.
Previous researches reported significant reduction in different leaf variables
in the polluted environment in comparison with clean atmosphere (Jahan
and Iqbal, 1992). In their study on Platanus acerifolia showed changes
in leaf blade and petiole size in the polluted air. Significant reduction in
length and area of leaflets and length of petiole of G. officinale of
polluted plants was recorded. Reduction in dimension of leaf blade of five tree
species in the vicinity of heavy dust and SO2 pollution was also
observed (Jahan and Iqbal, 1992). Significant effects
of automobile exhaust on the phenology, periodicity and productivity of roadside
tree species was also reported (Bhatti and Iqbal, 1988).
Decrease in leaf area in drought stress had been observed because tolerance
of water content of tissue possible by decrease in leaf area (Hale
et al., 1987).
Increase in length, breadth of leaflets and decrease in area of leaf had demonstrated
in leaves of Albizia lebbeck under the stress of air pollution (Seyyednejad
et al., 2009a, b). Moreover, study on leaves
of Callistemon citrinus planted in industrial region clears that length,
breadth of leaf and also leaf area decreased (Seyyednejad et al., 2009).
Effect of industrial air pollution on leaf morphology of Prosopis juliflora
was investigated (Koochak and Seyyed Nejad, 2010). Cassia
siamea plants growing at two different sites (polluted and non-polluted)
on two important roads of Agra city exhibited significant differences in their
flowering phonology and floral morphology (Chauhan et
al., 2004). Researchers showed that stimulation of photosynthetic rates
in elevated CO2 was nullified by decreased total leaf area (Noormets
et al., 2001). Totally describe of air pollution is related to morphology
of area of leaf visible damage including reduction of leaf area, changes in
morphology as compare to unpolluted condition, necrosis and chlorosis (Heath,
1980). Naido and Chricot (2004) showed that by the
effect of air pollutant exchange of gases on area of leaf of Avicenia
marine decreased. One way to increase tolerance in contrast with stress is to
balance the water content of tissue by decrease the leaf area (Hale
et al., 1987). It seems that this species use this way as defense
EFFECT ON THE PIGMENTS CONTENT
Air pollution stress leads to stomatal closure, which reduces CO2
availability in leaves and inhibits carbon fixation. Net photosynthetic rate
is a commonly used indicator of impact of increased air pollutants on tree growth
(Woo et al., 2007). Plants that are constantly
exposed to environmental pollutants absorb, accumulate and integrate these pollutants
into their systems. It reported that depending on their sensitivity level, plants
show visible changes which would include alteration in the biochemical processes
or accumulation of certain metabolites (Agbaire and Esiefarienrhe,
Sulphur dioxide (SO2¯), nitrogen oxides (NOx) and
CO2 as well as suspended particulate matter. These pollutants when
absorbed by the leaves may cause a reduction in the concentration of photosynthetic
pigments viz., chlorophyll and carotenoids, which directly affected to the plant
productivity (Joshi and Swami, 2009).
A relationship between traffic density and photosynthetic activity, stomatal
conductance, total chlorophyll content and leaf senescence has been reported
(Honour et al., 2009). One of the most common
impacts of air pollution is the gradual disappearance of chlorophyll and concomitant
yellowing of leaves, which may be associated with a consequent decrease in the
capacity for photosynthesis (Joshi and Swami, 2007).
Chlorophyll is found in the chloroplasts of green plants and is called a photoreceptor.
Chlorophyll itself is actually not a single molecule but a family of related
molecules, designated as chlorophyll "a", "b", "c" and "d". Chlorophyll "a"
is the molecule found in all plant cells and therefore its concentration is
what is reported during chlorophyll analysis (Joshi et
al., 2009). Chlorophyll is the principal photoreceptor in photosynthesis,
the light-driven process in which carbon dioxide is "fixed" to yield carbohydrates
and oxygen. When plants are exposed to the environmental pollution above the
normal physiologically acceptable range, photosynthesis gats inactivated. The
distribution of plant diversity is highly dependent on presence of air pollutants
in the ambient air and sensitivity of the plants. Chlorophyll measurement is
an important tool to evaluate the effects of air pollutants on plants as it
plays an important role in plant metabolism and any reduction in chlorophyll
content corresponds directly to plant growth (Joshi and
Swami, 2009). Chlorophyll is an index of productivity of plant. Whereas
certain pollutants increase the total chlorophyll content, others decrease it
(Agbaire and Esiefarienrhe, 2009). Changes in concentration
of pigments were also determined in leaves of six tree species expose to air
pollution due to vehicle emissions (Joshi and Swami, 2009).
The shading effects due to deposition of suspended particulate matter on the
leaf surface might be responsible for this decrease in the concentration of
chlorophyll in polluted area. It might clog the stomata thus interfering with
the gaseous exchange, which leads to increase in leaf temperature which may
consequently retard chlorophyll synthesis. Dusted or encrusted leaf surface
is responsible for reduced photosynthesis and thereby causing reduction in chlorophyll
content (Joshi and Swami, 2009). A considerable loss
in total chlorophyll, in the leaves of plants exposed to pollution supports
the argument that the chloroplast is the primary site of attack by air pollutants
such as SO2 and NOx. Air pollutants make their entrance
into the tissues through the stomata and cause partial denaturation of the chloroplast
and decreases pigment contents in the cells of polluted leaves. High amount
of gaseous SO2 causes destruction of chlorophyll (Tripathi
and Gautam, 2007). Several researches have recorded reduction in chlorophyll
content under air pollution (Tiwari et al., 2006;
Tripathi and Gautam, 2007; Joshi
and Swami, 2007, 2009; Joshi
et al., 2009). On the contrary, Several researches have exhibited
increase in chlorophyll content under air pollution, such as Tripathi
and Gautam (2007) reported that Mangifera indica leaves subjected
to air pollution showed an increase (12.8%) in chlorophyll content (Tripathi
and Gautam, 2007). Agbaire and Esiefarienrhe (2009)
in a study have demonstrated that plants from experimental site contain more
chlorophyll compared with those from the control.
Increase in content of chlorophyll a, chlorophyll b, total chlorophyll and
carotenoid in Albizia lebbeck and Callistemon citrinus, has been
reported by Seyyednejad et al. (2009). Investigation proved that chlorosis,
is the first indicator of Flour effect on plant (Kendrickk
et al., 1956). Yun (2007) showed reduction
in photosynthesis because of the PSII function damage, in sensitive species
of tobacco (Yun, 2007). Carotenoids exist in plasma of
plant tissues, photosynthetic or non photosynthetic; the function of carotenoids
in chloroplasts is as pigments to capture the light. But probably more important
role is in protecting the cells and live organisms encounter with damage of
free radical oxidative (Fleschin et al., 2003).
Plants fumigated with 40, 80 and 120 ppbv concentrations of O3 exhibited
significant reduction in total chlorophyll content, RuBP carboxylase activity
and net photosynthesis (Chapla and Kamalakar, 2004).
Carotenoids protect photosynthetic organisms against potentially harmful photoxidative
processes and are essential structural components of the photosynthetic antenna
and reaction center (Joshi and Swami, 2009). Carotenoids
are a class of natural fat-soluble pigments found principally in plants, algae
and photosynthetic bacteria, where play a critical role in the photosynthetic
process. They act as accessory pigments in higher plants. They are tougher than
chlorophyll but much less efficient in light gathering, help the valuable but
much fragile chlorophyll and protect chlorophyll from photoxidative destruction
(Joshi et al., 2009). Joshi
and Swami (2007) showed among four plant species subjected to air pollution,
highest decrease in carotenoid contents was reported for Eucalyptus cirtiodora.
Joshi and Swami (2009) also determined the concentration
of carotenoids in the leaves of six tree species exposed to vehicular emission.
They reported the reduction in concentration of carotenoids in the leaf samples
collected from polluted sites (Joshi and Swami, 2009).
Several researchers have reported reduced carotenoid content under air pollution
(Joshi et al., 2009; Tripathi
and Gautam, 2007; Tiwari et al., 2006).
EFFECT ON SUGAR
Soluble sugar is an important constituent and source of energy for all living
organisms. Plants manufacture this organic substance during photosynthesis and
breakdown during respiration (Tripathi and Gautam, 2007).
Tripathi and Gautam (2007), in their study revealed
significant loss of soluble sugar in all tested species at all polluted sites.
The concentration of soluble sugars is indicative of the physiological activity
of a plant and it determines the sensitivity of plants to air pollution. Reduction
in soluble sugar content in polluted stations can be attributed to increased
respiration and decreased CO2 fixation because of chlorophyll deterioration.
It has been mentioned that pollutants like SO2, NO2 and
H2S under hardening conditions can cause more depletion of soluble
sugars in the leaves of plants grown in polluted area. The reaction of sulfite
with aldehydes and ketones of carbohydrates can also cause reduction in carbohydrate
content (Tripathi and Gautam, 2007).
Some researchers showed that Concentrations of total and soluble sugars decreased
significantly in the sensitive trees to the air pollution. In damaged Q.
cerris leaves the decrease in concentrations of sugars was higher in September.
The decrease in total sugar content of damaged leaves probably corresponded
with the photosynthetic inhibition or stimulation of respiration rate (Tzvetkova
and Kolarov, 1996). Furthermore, increase in amount of soluble sugar is
a protecting mechanism of leaves it has been shown in Pinto bean in exposure
with different concentration of ozone (Dugger and Ting,
Following ozone exposure, soluble sugars in pine needle decreased (Wilkinson
and Barnes, 1973). Subsequently they increased, frequently in association
with foliar injury (Dugger and Ting, 1970; Miller
et al., 1969). The increase of soluble sugars was also observed following
chronic exposure (Miller et al., 1969). The increase
in soluble sugar was reported in Albizia lebbeck and Callistemon citrinus
grown in industrial land (Seyyednejad et al., 2009a).
Investigations revealed that the more resistant species plants to the air pollution
as compare to sensitive species showed more concentration of soluble sugar (Kameli
and Losel, 1993; Ludlow, 1993). Study on resistance
of Dodonea viscosa and Prosopis juliflora to industrial air pollution
were done and results showed increase in soluble sugar (Abedi
et al., 2009a, b; Koochak
and Seyyed Nejad, 2010).
EFFECT ON PROLINE
Some workers has been published the increase in free proline content in response
to various environmental stresses in plants (Levitt, 1972).
Thypical environmental stress (high and low temperature, drought, air and soil
pollution) can cause excess Reactive Oxygen Species (ROS) in plant cells, which
are extremely reactive and cytotoxic to all organisms (Pukacka
and Pukacki, 2000). High exposure to air pollutants forces chloroplasts
into an excessive excitation energy level, which in turn increases the generation
of ROS and induces oxidative stress (Woo et al.,
2007). The deleterious effects of the pollutants are caused by the production
of Reactive Oxygen Species (ROS) in plants, which cause peroxidative destruction
of cellular constituents (Tiwari et al., 2006).
It has been reported that proline act as a free radical scavenger to protect
plants away from damage by oxidative stress. Although, the scavenging reaction
of ROS with other amino acids, such as tryptophan, tyrosine, histidine, etc.
are more effective compared with proline, proline is of special interest because
of its extensive accumulation in plants during environmental stress (Wang
et al., 2009).
Tankha and Gupta (1992) showed the increase in content
of proline with increasing SO2 concentration. According to existence
of SO2 and CO in the industrial area as the result of chemical activities,
these results probably indicate that it has been clearly inconceivable to designate
a harmless threshold toxic SO2 concentration for level of particular
species since other environmental factors during pollution profoundly affect
the degree of damage (Seyyednejad et al., 2009b).
Significant increase in content of proline in Albizia lebbeck grown
in polluted area has been reported (Seyyednejad et al.,
2009b) the concentration of proline increased in leaves of Callistemon
citrinus planted round petrochemical site in comparison with control site
(Seyyednejad et al., 2009b). Proline is a universal
osmolytic accumulated in response to several stress and may have a role in plant
defense reactions (Khattab, 2007). Obviously proline
has main role in protection in different kinds of stress. Accumulation of proline
in plants is a physiological response to osmotic stress (Szekely,
The effects of pollutants on plants include pigment destruction, depletion
of cellular lipids and peroxidation of polyunsaturated fatty acid (Tiwari
et al., 2006). There appears to be a relationship between lipid peroxidation
and proline accumulation in plants subjected to diverse kinds of stress (Wang
et al., 2009). If such a relationship exists, proline accumulation
might play an important role in inhibiting air pollution-induced lipid peroxidation.
proline accumulation often occurs in a variety of plants in the present of different
stresses. For example, proline accumulation in leaves of plants exposed to SO2
fumigation (Tankha and Gupta, 1992), heavy metals (Wang
et al., 2009) and salt (Woodward and Bennett,
2005) stress has been reported (Tankha and Gupta, 1992;
Wang et al., 2009; Woodward
and Bennett, 2005).
The authors wish to thanks the Vice Chancellor for Research of Shahid Chamran University of Ahvaz.
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