Plants can effectively be used as cheap and naturally available monitoring systems or bioassays of the level and type of air, soil and water pollution in an area. The type and concentration of a pollutant can be reliably found out by various characteristics damage symptoms produced in the plants because such damage symptoms are pollutant specific as well as concentration specific. For example, in young needles of Pinus, chlorois indicates SO2 pollution, necrosis indicates HF pollution, beaching indicates NO2 pollution while chlorotic mottle indicates Cl2 pollution in the atmosphere. These characteristic symptoms of damage in young pine needles appear only when concentration is 0.3 ppm for SO2, 0.07 ppm for HF and 1.0 ppm for Cl2. Similarly, browning in moss leaves due to fluoride accumulation is 5% in 65 ppm dry weight accumulation but rises to 90% in 4500 ppm dry weight accumulation. However, certain precautions have to be taken while using plants as pollution indicators.
Precautions in use of plants as pollution indicators
The damage symptoms in plants should preferably be studied in the local native species. Cultivated and introduced species should be avoided.
The species sensitive to pollutants should be first identified in the local flora and then used for pollution monitoring. Tolerant species should be identified and avoided in such work.
Damage symptoms for a particular pollutant should be studied in different species sensitive to that pollutant so that presence of the pollutant in the area may be cross-checked. For example, grey necrosis in Geranium, ivory necrosis in Zinnia, brown necrosis in Chrysanthemum and reddish necrosis in Azalea indicates absolutely certain presence of SO2 pollution in the area.
Many types of damage symptoms viz. morphological, anatomical, ultra-structural physiological, biochemical etc. should be studied in one or more sensitive plant species to ascertain the presence of a particular pollutant in the area.
Samples should be taken from as many different sites in the area as possible. From such data the extent of pollution can be determined and the possibility of symptoms being due to some pathogen is also excluded because the intensity of damage symptoms due to pollution varies in different sites according to the distance from the source of pollution while it is same in all sites in case of a pathogenic disease.
The possibility of damage symptoms in plants occurring due to some cause other than pollution e.g. due to pathogen, environmental condition or nutritional deficiency/excess should be thoroughly checked and ruled out.
Important characteristics of plant species used in pollution monitoring
The plant species used to monitor pollution in an area should have certain important features for the success of such programme. Most important such features are:
Species should be easy to identify in the field and easy to handle for damage analysis.
Species should have a wide range of distribution so that it can be used in different areas.
Species should be sensitive to many types of pollutants so that it can be used to monitor different types of pollutants in the area.
Species should produce highly specific damage symptoms in response to particular types and concentrations of pollutants.
Plants commonly used as pollution indicators
Though all types of sensitive species can be used in monitoring pollution, most useful and commonly used plants include sensitive species of lichens, mosses, plankton algae, aquatic ferns and angiosperms, other ferns, conifers oaks and many crop plants. Mosses, lichens, ferns algae and aquatic plants are generally more useful in pollution monitoring because their range of pollutant specificity is usually much higher than that of higher vascular plants. Examples of some common types of plants useful as pollution monitors are given below.
AIR & SOIL POLLUTION
Benthic marine algae
Methods of using plants as pollution indicators/monitors
Decrease in the densities of sensitive species, increase in the density of tolerant species. Absence of highly sensitive species, changes in the species composition of vegetation and distribution pattern of populations in the area are studied. Such studies indicate the type and concentration of pollutant(s) as well as the spread of the pollution problem in the area.
In freshwater bodies:
- Changes in diatom community, decrease in plankton algae and aquatic hydrophytes and spread of Sphagnum moss indicate increased water acidity. Specific changes in the aquatic flora can indicate the pH of the water quite correctly.
- Eutrophication and water blooms indicate sewage, organic matter and chemical fertilizer pollution of water.
- Abundance of Eichhornia indicates sewage and heavy metal pollution of water.
- Increase in E. coli and aerobic decomposer bacteria indicates water pollution due to organic sewage.
In terrestrial areas:
- Decrease in the populations of mosses (Sphagnum, Bryum) and lichens (Parmelia) generally indicates air pollution by SO2, NO2, fluorides and HCl. Absence of most bryopytes , particularly Sphagnum and Bryum indicates atmospheric SO2 pollution of 0.17 ppm or more. Poikilohydrous mosses are particularly useful as pollution indicators.
- Changes in sensitive species of herbs and grasses occur much earlier than in shrub and tree populations. Generally, the degree of ‘Crown die-back’ and death of trees is directly related to the level of SO2, NO2 HF and HCl pollution of air.
- Changes in soil microorganism populations indicate soil pollution. Increase in ammonifying bacteria shows NH4 pollution, reduction in nitrate and nitrite bacteria shows NO3 pollution, decrease in decomposer bacterial populations indicates soil acidification and pesticide pollution.
Physiological and yield analysis
The rates of growth, photosynthesis, respiration, enzyme activities, percentage flowering, yield of fruits and seeds, percentage of seedling mortality in sensitive species are important characteristics that are very helpful in determining the type and level of pollution in an area because these show highly characteristic and specific changes even before other morphological or vegetational changes appear. In general, pollution is indicated by increased rate of respiration, decreased rate of photosynthesis, stimulation of the activity of catabolic enzymes, decrease or inhibition of the activities of anabolic enzymes, reduced flowering and seed output and increased seedling mortality.
Analysis of visible injury symptoms
Leaves of sensitive species generally produce highly specific and characteristic visible injury symptoms in response to pollutants. The study of such symptoms can reliably indicate the type and level of pollutant(s) present in the environment. For such analysis, leaves of same age are collected at same time of the day and at different localities in the area from plants of a particular sensitive species. Most common symptoms studied are chlorosis, necrosis, discolouration, tip-burn, bleaching, bronzing, stipples and mottles in the leaves. Characteristic colours and patterns of these symptoms in particular plant species indicate the type and level of pollutant present.
In some plant species, specific pollutants cause highly characteristic damage to cells and tissues. The careful analysis of the type and degree of such cell/tissue damage in the sections of various plant parts is very useful indicator of pollution problem. For example, fluorides cause highly specific injury in the cells and tissues of pine needles.
Pollution can also be monitored by study of specific injury symptoms in the cell organelles, particularly chloroplasts, mitochondria and cell membranes. For example, SO2 pollution causes membrane damage in moss Sphagnum, chloroplast degradation and membrane damage in mosses Grimmia pulvinata and Hypnum cupressiforme while Pb pollution is indicated by presence of electron-dense vesicles containing lead in the cells of moss Rhytidiadelphus squarrosus.
Accumulation of characteristic substances in different plant parts can also indicate the type and level of pollution in the environment.
S/N ratio: Sulphur is accumulated in the pollution of SO2 and H2S and nitrogen content is increased in NOx and NH3 pollution in the leaves and twigs of sensitive plant species. As SO2 and NOx are usually present together as air pollutants, sulphur to nitrogen ratio (S/N ratio) is a very useful indicator of their relative proportion in the atmosphere.
Amino acid content: Increase of amino acids (particularly glutamine and asparagines) in the leaves by a factor of 10 than the normal shows NH3 pollution while increase by a factor of 2 indicates NO2 or SO2 pollution in the atmosphere.
Pigment analysis: The presence of degradation products of certain pigments and absence of others in the leaves damaged by pollution can also indicate pollution. For example, absence of carotenes and specific zones of chlorophyll degradation products in the chromatograms indicates NO2 pollution but the presence of carotenes with chlorophyll degradation products shows SO2 pollution.
Bark acidity: Air pollution by SO2 an HF can usually be related with the level of bark acidity. Dicotyledonous trees are better for such analysis than coniferous trees but Scots pine has been successfully used for bark acidity analysis.
Metal accumulation : Many plants can absorb and accumulate different metals from their environment. Analysis of plant tissues for identifying the type and level of metal accumulated is very useful in study of metal pollution problem in the soil, air and water. For example, Willow, birch and poplar accumulate Zn and Cd, Box-elder accumulates boron and tomato accumulates sulphur from the soil. Peat mosses, particularly Sphagnum acutifolium accumulates Zn, Cd and Pb; terricolous carpet-forming mosses like Hylocomium splendens, Pleurozium schreberi and Hypnum cupressiforme accumulate Hg, Ag, Be and other common metal pollutants. Aquatic mosses like Fontinalis antipyretica, Eurhynchium ripariodes, alage like Cladophora, vascular plats like Eichhornia and Azolla are very useful in identification of water pollution by a variety of heavy metals.
Moss bag technique: This technique has been very useful in identifying atmospheric metal pollution. Ten square centimetre flat bags of nylon nets are filled with acid-washed clean Sphagnum acutifolium moss and are suspended on poles or tied to trees at specified heights. The moss absorbs various metal pollutants from the air. After specified time interval, the moss is again acid washed. The types and amounts of metals absorbed by the moss is found out indicating the metal pollution status of the air. These moss bags can be repeatedly used in such metal monitoring work. Similarly, living aquatic moss plants in perspex cylinders or wrapped in nylon nets are secured in the water body at specified depths. Analysis of the metals accumulated and the level of the decay of moss plants is used to identify water pollution by metals.