Air Pollutants: Responses on Plants and Its Bio-Monitoring

Read this article to learn about the responses of pollutants on plants and its bio-monitoring.

Responses on Plants:

There are a number of pollutants (gaseous and particulates) found in air. The clear air, which is composed of nitrogen (>77%), oxygen (>20%) and carbon dioxide (< 1%) in the troposphere when moves across the earth’s surface, it collects the products of both natural events (dusts, volcanic gases, decomposition gases) and human activities (emission from car, industries and household).

These potential pollutants of air are called pri­mary pollutants. The primary pollutants mix with air and transform in presence of sunlight to pro­duce secondary pollutants of air.

The major sources of primary air pollutants in global atmosphere are shown in Fig. 13.1.

Effects of Air Pollutants on Vegetation:

During past few decades greater interest has been paid on the effect of air pollutants on vegetation and crops in tropics as newly developing industries and urbanisation result in increased concen­trations of phytotoxic air pollutants. The air pol­lutants contaminate air, water and soil, corrode materials, harm plants and animals and affect hu­man health.

In India alone, extensive field surveys and laboratory investigations have been carried out to study the effect of air pollutions on plants. The nature of interaction of plant canopies with ma­jor gaseous and particulate pollutants in the im­mediate vicinity of plants is complex.

Several en­vironmental factors must be considered as pollut­ant interchanges with plant canopies and adjacent boundary layers which are directly influenced by several environmental factors. Stomata have played important roles in determining the pollutant ab­sorption and response of plants.

However, on the whole, much more evalua­tion of air pollutant effects have to be conducted on a larger scale for better understanding of thresh­old levels and cause-effect relationships of plants and air pollutants.

For detection, quantification and interpreta­tion of plants responses, symptoms of injury, such as chlorosis and necrosis, changes in growth-habit, reduction in quality and yield, biomass con­tent of plants, changes in transpiration, photosyn­thesis and respiration rates, reductions in chloro­phyll, amino acids and ascorbic acid, are deter­mined and correlated with pollutants concentra­tion and exposure dose.

Particulate Matter as Air Pollutants:

Particulate matter is considered to be one of the major air pollutants particularly in urban and in­dustrial areas. The particulates generated from in­dustrial or anthropogenic sources dispersed into the atmosphere, depending on their size and weight, may remain in air for varying period of time.

Normally particulates larger than 10µ in size, settle under forces of gravity on surfaces of veg­etation and soil but the smaller ones remain sus­pended in air for longer period of time and then dispered and diffused by wind current. The sus­pended particulates matter (SPM) gradually gath­ers mass through agglomeration, coalescence and water vapour deposition and eventually settles down on surfaces or may be washed down by rain.

The particulates and dusts in the arbrent air of major Indian cities were surveyed since mid 1970s. The average SPM level alone is always>150µ g/m³ for 24 h period. In 1975, Lerman and Dasley made in exten­sive review on the effects of particulates on plants.

Since then lots of survey were conducted in India and abroad on the said topic. Among particulate pollutants, those that have been investigated with respect to plant life by different workers in India, include pulverized coal dust, petro-coke, cement dust, fly ash, urban dust and auto exhaust. Rao (1971, 79) studied the effect of coal dust on the growth and fruiting behaviour of mango and lemon.

The dust covered mango and lemon leaves showed brown necrotic lesions, starting at the tip and progressing down the lemina. The stigmatic surfaces of open flowers were coated with a thick deposition of coal particulates which inhibited pollen germination and ultimately fruit setting.

Identically Prasad and Rao (1981) reported depo­sition of petro-coke particles on surfaces of plants like green gram (Phaseoius aureus) and noted their phytotoxicity.

Yusuf and Vyas (1982) observed decreased in total chlorophyll of Calotropis procera and Cassia fis­tula growing around Udaipur Cement Works at Bajaj Nagar, Rajasthan. Identically fly ash may af­fect vegetation directly through deposition on leaf surfaces and indirectly through accumulation in the soil medium.

Direct effects of fly ash on plants may include changes in the cuticular pattern of leaves, decreases in number and size of stomata and increases in length and density of trichomes. But indirectly it helps in better growth of plants by amending the soil nutrient level.

Several workers also studied the phytotoxic effects of dust originating from urban and indus­trial areas. The dust may be considered as a mix­ture of heterogenous particulate matter consist­ing of heavy metal particulates, tarry deposits and other kinds of particles related to the day to day activities in the area.

Srivastava (1980), Bhirava Murthy and Kumar (1985) Sahu and Santra (1989) studied the effects of particulates on foliar traits of several plants in the field conditions. On the whole stomatal number, pore size and trichome size were decreased in polluted area in contrast to the con­trol site.

On the whole, the reasons for growth reduc­tion and un-favourable alterations in different plant parameters under particulate pollution can be des­cribed as follows:

(a) Quantitative and qualitative changes in solar radiation impringing on the leaf surface and alterations in the energy exchange processes of leaf due to dust layer;

(b) Produces chloroplast injury and decreases chlorophyll content;

(c) Interruption in gaseous exchange due to shad­ing by particles and clogging of stomata by dust;

(d) Dust induces the alterations in pH and other physicochemical properties of soil support­ing the plant growth.

Sulphur Dioxide as Air Pollutants:

Small quantities of SO₂ have always been avail­able in the atmosphere as a result of biological and natural oxidation of sulphides. The excessive quantities of SO₂ have, however, been added to the atmosphere by man, through his activities of burning fuels since time immemorial and later from industrial processes.

On a national scale, out of a total of 5750 thousand ton of air pollutants emit­ted in India during 1956-66, 1,350 thousand ton was only SO₂. For the past two de­cades NEERI had made a comprehensive survey on the SO₂ level for major Indian cities. The de­tails are given in Table 13.1 and 13.2.

SO₂ is a highly phytotoxic gas combined with particulate matter suspended in the air, it proves more toxic to all the organisms. Summary of the effects of SO₂ in plants is given in Fig. 13.2.

During past two decades or so, there are a num­ber of fumigation studies as well as field survey was conducted to understand the effect of SO₂ on plants of varied categories. Various kinds of foliar symp­toms including chlorotic changes, necrosis, drying, loss of chlorophylls; retardation of growth, flow­ering, and fruit setting were noticed. On the basis of such studies several plants were screened for their sensitivities and tolerance (Table 13.2).

Exposure to gaseous pollutants such as SO₂ at even low concentrations has several damaging consequences:

(a) Alteration of membrane permeability due to peroxidation of the membrane by the pro­cess as stated below:

(b) Degradation of chlorophylls and inhibition of chlorophyll synthesis;

(c) Net reduction of photosynthesis (as much as 20-25%)

(d) Water – use efficiency adversely affected due to rise of transpiration rate, and

(e) Stomatal conductance increased.

Oxides of Nitrogen as Air Pollutants:

Nitrous oxide (NO) and Nitrogen dioxide (NO₂) are two important gases in the nitrogen oxide group (NO_X) of pollutants which are pro­duced primarily during high temperature combus­tion of fossil fuels due to oxidation of atmospheric nitrogen.

Normally oxides in the atmosphere remain always too low to cause plant injury. Benedict and Breen (1955) found that most sensitive weeds required at least 20 ppm and resistant ones about 50 ppm NO_x for producing injury. The high con­centration NO_x exposure produces foliar necrotic lerions similar to magnesium deficient necrosis and in severe cases there may be excessive defoliation of plants.

In contrast, it is sometime suggested that NO_x after entering the plant, may get reduced into NH₄, which in turn may combine with oxidation prod­ucts of SO₂, to form NH₂S0₄, which is a nutri­ent.

However, another importance of nitrogen ox­ides (NO₂) is pollutant because of their participa­tion in photochemical reactions giving rise to Ozone (O₃) and peroxyacetyl nitrate (PAN), two highly phytotoxic secondary pollutants.

Srivastava, (1975) working on bean leaves under NO₂ funigation in illuminated condition found that transpiration rate is reduced due to partial closure of stomata. NO₂ injury in plants is also mediated by acid formation through photo- oxidation. Several other workers have found that even at low concentration this gas reduces the pho­tosynthesis without any visible injury.

The reduc­tion in the rate of photosynthesis is possible through an increase in photorespiration as well as NO induced competition for NADPH used in nitrate reduction and carbon assimilation. Chloroplast membrane is also affected. NO_x also re­duced to NH₃ and then to amino acids and pro­tein.

Ozone and Other Air Pollutants:

Ozone is present in traces in the air and its con­tent increases in upper atmosphere. Several pol­lutant gases viz. CFC’s are able to convert O₃ to O₂ and thus causes O₃ depletion. In several in vitro studies O₃ alone or in combination affect plant growth and produces considerable necrotic symptoms.

Peroxyacetyl nitrate (PAN) is a secondary pollutant forming smog by the action of light on hydrocarbons and NO_x in air. Injury symptoms in several plants first observed in Los Angeles (USA) due to unpleasant hazy atmosphere around indus­tries were actually due to PAN. Mature leaves are more susceptible than young and immature leaves. PAN enters into leaves through stomata and re­duces photosynthesis through injury to chloroplast, inhibition of electron transport, and inter­ference with enzyme systems connected with pho­tosynthesis.

Fluoride is another important air pollutant which is released in combustion process of fossil fuels and from aluminium industries and phos­phate reduction plants. Different plants have dif­ferent levels of sensitivity. Conifers and lichens were most sensitive plant species. Necrosis, chlo­rosis and de-colouration are most important foliar symptoms of fluoride injury.

Air Pollutants Combined Effect on Ecosystems:

A brief exposure to single air pollutant or a com­bination of pollutants showed symptomatic effects on individual plants, but for longer periods are necessary to detect any ecosystem changes due to air pollutant effect. Air pollutants affect the veg­etation in three important ways—as gaseous diffusible pollutants, as particulate deposition or as acid deposition.

Plants are continuously exposed to fluctuat­ing concentrations of mixed air pollutants and develops characteristics foliar injury. When plants are exposed to a high concen­tration of pollutants for a short period, acute ne­crosis of leaf tissue, bleaching and discolouration of leaf were observed.

Chronic injury, however, results from exposure of plants to low concentra­tions of a pollutant over a long period of time. Symptoms of chronic injury are bronzing, chlo­rosis and premature senescence. In nature, both acute and chronic injury often occurs on the same plant. Symptoms of typical foliar injury, in re­sponse to various air pollutants are presented in the Table 13.3.

Air Pollutants and Plant Productivity:

A large number of gaseous and particulate air pollutants are known to cause undesirable effects on plants and ultimately the gross productivity is affected either directly or indirectly. The produc­tivity was estimated either by chlorophyll assay or by dry matter accumulation. Several crops, trees and wild herbs were tested in vitro and in the field in this regards. On the whole the gross productiv­ity is seriously affected at higher level of different pollutants.

Vegetation as Sink of Air Pollutants:

It is well-known for a considerable period that vegetation may acts as sink for atmospheric con­taminants. It can filter out dust, soot, smoke and many other fine particles present in air. In the past, several studies have been made for measuring the particulate scavenging ability of plants in Hyde Park, London; in Soviet Union; in India.

The overall dust collecting effi­ciency of street trees varies from 3 x 10² to 3.5 x 10² tons yr^-1. Different species have differential dust collection efficiency. In general, broad hairy leaved plant species which are tolerant to pollutants are best suitable for dust filter on road sides.

In this connection, Peepal, Pakur, Teak, Sal, Arjun, Ashok, Mango, and Jarul were most suitable tree species in the tropical environment. However, over all ef­fects of air pollutants on plant is influenced by other environmental factors (Fig. 13.3).

Substantial evidence is available to support the potential that plants in general and trees in par­ticular have to function as sinks for gaseous pol­lutants. Once in contact with plants gases may be found to dissolved on exterior surfaces or taken up by the plants via stomata. The relative foliar sorption of SO₂ (fumigated at 1 ppm) and O, (fu­migated at 0.20 ppm) by selected seedlings are pre­sented in Table 13.4.

Efforts to estimate the sink capacity of forest vegetation under natural conditions must consider a complex set of variables including pollutants con­centration and deposition velocity, meteorologi­cal parameters, and dimensions (leaf or canopy area) and conditions of trees. As per calculation of US environmental protection agency (1976), the Table 13.5, showed the estimated gaseous pol­lutant removal by a model forest.

The tolerant species of plants functions as pol­lution ‘sink’ and therefore a number of environ­mental benefits can be derived by planting toler­ant species of plants in polluted areas. For this reason, an evaluation of plants with respect to their tolerance level to air pollution may be essential. To evaluate the tolerance level of plant species to air pollution, Singh and Rao (1983) used four leaf parameters to derive an empirical number indi­cating the air pollution tolerance index (APTI) as shown in the formulae:

APTI = A (T + P)+ R/10

where, A is ascorbic acid content, T is total chlo­rophyll, P is leaf extract pH and R is relative water content of leaf. The whole sum is divided by 10 to reduce it to a small figure. It has been interesting to note the fact that APTI value of a plant species as determined by the above method compares well with its air pollution tolerance level as observed under field conditions.

Biomonitoring of Air Pollution:

The concept of using certain plant species as en­vironmental indicators is fairly well established in the field of ecology. These are termed as plant indicators. Identically some plant species and cultivars are relatively sensitive to certain air pollut­ants. It is therefore, possible to monitor the levels of air pollutants through proper quantification and standardisation of plant responses of sensitive spe­cies.

In recent years, increasing efforts are being made to use plants for detecting air pollutants par­ticularly SO₂, NO_x, H₂S, O₃, HF etc. The use of lichen, moss for air quality mapping in industrial areas of Europe and North America represents one of the finest ex­amples of biomonitoring of air pollutants with the help of plants.

Use of higher plants for monitoring air pollution is, however, a recent development. A number of plant parameters either singly or in combina­tion may be used for evaluating the pollution stress.

For broad categories of plant responses are used as indicators:

(1) floristic composition,

(2) mor­phological survey,

(3) mineral composition analy­ses, and

(4) physiological and biochemical -assay.

Table 13.6 represents the list of commonly used plants for biomonitoring of air pollution.

In India considerable work has been done on the effect of air pollution on plants at BHU, Varanasi; BARC and Institute of Science, Mumbai, JNU, New Delhi; NBRI, Lucknow; University of Calcutta and Kalyani, West Bengal and Andhra University, Waltair.

For past two decades, several countries used the indicator plants for effect measurements of different pollutants: Tobacco variety Bel-W₃ in the Netherland and in the United Kingdom; Li­chen in Canada. There has been some international cooperation in Europe to moni­tor air pollution effects on plants.

The major advantages of plant bio-monitoring are as follows:

(a) Indicator plants provide a direct method of studying the effects of the prevailing air pol­lution on living organisms.

(b) These plants also provide a measure of inte­grated effects of all environmental factors, including air pollutants and weather condi­tions.

(c) It is possible to study the relationships be­tween concentrations of air pollutants and its effects on plants when both are measured at the same site.

(d) It provides possibilities of determining spa­tial and temporal trends in the occurrence and intensity of effects of several air pollutants on natural and cultivated plants.

(e) It is also possible to analyse the pollutants which was accumulated substantially within plants.

(f) It acts as a sensitive early warning system which may stimulate prophylactic measures to prevent or diminish disastrous effects of air pollution.

On the whole, when the selective plants are used for the monitoring of air pollution effect, a high degree of standardisation of the plant mate­rial and of physical and chemical environmental conditions is a prerequisite.