Plant Indicators: Characteristics, Type and Physiological Changes

Plant indicators can be helpful to determine local soil, thus it can be decided which crops should be cultivated in a particular soil and which soil should be left for pasture or other purposes.

Plant indicators are also used to determine optimum use of land resources for forest, pasture, and agricultural crops.

The heredity and environment both are equally important in the expression of phenotypic characters. Heredity performs its action through environment. Species differ in their environmental requirements and establish themselves where conditions are favourable. It is found that certain species of plants, animals and micro-organisms have one or more specific requirements which very much limit their distribution.

The occurrence, character and behaviour of a plant are thus indicator of the combined effect of all factors prevailing in a habitat. Since a plant species or plant community acts as a measure of environmental conditions, it is referred to as biological indicator or bio-indicator or phyto-indicator. In other words, plants which indicate some very specific conditions of environment are called plant indicators.

The knowledge of relationship between plants and ecological factors can be used as an indicator of environment. Many plants are used as indicators of environment. In a plant community some plants are dominant and found in abundance. These plants are important indicators because they bear full impact of habitat. It has been seen, in general, that plant communities are better indicators than individual plants. Individual plants or plant communities are used to determine the types of soil and other conditions of the environment. Sometimes these also indicate past or future conditions of the environment.

The knowledge of plant indicators can be helpful to determine local soil, thus it can be decided which crops should be cultivated in a particular soil and which soil should be left for pasture or other purposes. Plant indicators are also used to determine optimum use of land resources for forest, pasture, and agricultural crops. Many plants also indicate the presence of particular mineral or metal. So the presence of precious metal can be detected by the growth of the specific plant in an area.

Characteristic Features of Plant Indicators:

The characteristic features of plant indicators are as follows:

1. On the basis of distribution the indicators may be ‘steno’ species or ‘eury’ species. The ‘steno’ is used to indicate narrow limits of tolerance and ‘eury’ is used to indicate wide limits of tolerance. A plant may show wide limits of tolerance for certain conditions and narrow limits of tolerance for other conditions. For example, a plant may be indicator of wide limits of tolerance for heat but of narrow limits of tolerance for water. Plants with wide limits of tolerance of heat are called eurythermal and those with narrow limits of tolerance for water are called stenohydric.

2. Plants of large species are better indicator than the plants of small species.

3. Before relying on a single species or group of species as indicators, there should be abundant field evidence.

4. Numerical relationships between species, population and whole communities often provide more reliable indicators than single species

Different Types of Plant Indicators:

Different types of plant indicators have different roles in different aspects which are described below:

Plant indicators for agriculture:

Many plant indicators decide whether soil is suitable for agriculture or not. The growth of a particular crop plant is seen under different environmental conditions and if growth is satisfactory in a particular soil that soil is considered to be suitable for agriculture. For example, growth of the short grasses indicates that water is less in the soil. A natural growth of tall and short grasses indicates that soil is fertile and is also suitable for agriculture. Dhawan and Nanda (1949, 50) and some other workers have recorded plant indicators for different types of soils as given in Table 12.1.

Plant indicators for groundwater:

Certain plant communities indicate the depth of ground water. Central Arid Zone Research Institute, Jodhpur has made the use of certain plant communities to indicate the depth of groundwater and salinity level in the groundwater. Chatterjee and Bhaskar (1977) have listed the plant communities as ecological indicators for ground water in Indian deserts (Table 12.2)

Plant indicators for Over-grazing:

Many plants are over grazed which result in modification of grassland. It has been seen that grasses are removed by overgrazing while others are disturbed and forage production is considerably reduced. Some plants which are vigorous and undisturbed, remain viable and become distinct from rest of the plants.

Some plants show characteristic indication of overgrazing which can be recognized. The predominance of annual weeds and short-lived impala-table perennials indicate severe grazing. Examples of such plants are Polygonum, Chenopodium, Lepidium and Verbena. Some plants are less pronounced and show poor or no over-grazing. Examples of these plants are Opuntia, Grindelia, Vernonia etc.

Plant indicators of forest:

Some plants indicate the characteristic types of forest and they grow in an area which is not disturbed. Narenga porphyrocoma is a grass which binds the soil. In such a soil sal (Shorea robustd) can be cultivated. Viola species in western Himalayas is a suitable indicator for plantation of Cedrus deodara and Pinus wallichiana. If we know that a particular forest grows better in certain area of specific soil the productivity can be increased. For example, Quercus stellata and Q. mariandica grow on upland, lowland or on sterile sandy soils.

Sometimes forests is destroyed due to fire, overgrazing and other environmental factors and the area is left to reach upto climax. In this, subdominant species get favourable chances for growth and survival. This indicates the future plants to come and establish.

Plant indicators for humus:

Some plants act as humus indicators. Monotropa, Neottia and mushrooms indicate the presence of humus in soil. Strobilanthes and Impatiens indicate the presence of high humus or litter which prevents regeneration of tree species.

Plant indicators for moisture:

Plants which prefer to grow in arid area indicate the poor or very low moisture content in the soil. Sacchamm munja, Acacia nilotica, Calotropis, Agave, Opuntia, Argemone are such plants. Some plants grow in low soil moisture as Citrullus colocynthis. Eucalyptus lowers the water table. Echinops echinatus. Cassia auriculata are found in the area of deep water table. Typha, Phragmites, and Vetiveria grow in water-logged soil. Growth of Typha, Phragmites, Juncus and Carex indicates the swampy condition. Mangrove vegetation and Polygonum are found in water-logged saline soils.

Plant indicators for Soil types:

Many plants indicate the characteristic soils. For example, Casuarina equisetifolia, Ipomoea pes-caprae, Citrullus colocynthis, Calligonum polygonoides, Lycium barbarum and Panicum grow in sandy soil. Sacchamm munja prefers to grow in sandy loams. Imperata cylindrica and Vetiveria zizanioides grow on clayey soils. Cotton prefers to grow in black soil.

Plants indicators for soil reaction:

Many plants indicate whether the soil is acidic or basic. For example, Rumex acetosa. Rhododendron, Polytrichum and Sphagnum indicate acidic soils. Many forest trees as Shorea robusta, Pinus roxburghii are calcium loving. Tectona grandis (teak), Cupressus torulosa, Ixora parviflora and Taxus baccata are calcicoles. Some mosses e.g. Tartulla and Neckera grow on lime stones. Halophytes such as Suaeda fruticosa, Tamarix ariculata, Salicornea, Chenopodium, Salsola foetida grow in salty soil.

Plant indicators for minerals:

Many plants indicate the presence of characteristic minerals in the soils. These plants are called metallocoles or metallophytes.

The following plants grow in the presence of specific metals:

(i) Diamond:

Vallozia Candida grows in presence of diamond in Brazil.

(ii) Gold:

Equisetum arvense, Lonicera confuse, Papaver libonoticum, Alpinia speciosa, Thuja species indicate the presence of gold minerals in the soil.

(iii) Silver:

Eriogonium ovalifolium indicates the presence of silver minerals in soils in U.S.A.

(iv) Mercury:

Stellaria setacea grows in Spain in mercury rich soils,

(v) Uranium:

Astragalus species grows in USA in uranium rich soils,

(vi) Selenium:

Astragalus species, Neptunia amplexicaulis, Stanleya pinnata, Onopsis condensator, etc. grow in selenium rich habitat.

(vii) Copper:

Viscaria alpina in Norway, Gymnolea acutiloba in America, Gypsophila

patrini in USSR grow on the soil rich in copper.

(viii) Zinc:

Viola calaminara, V. lutea in Europe grow on the soil rich in zinc minerals,

(ix) Boron:

Salsola nitrata, Eurotia cerutoides grow in boron rich soils.

(x) Cobalt:

Silene cobalticola in Congo and Nyssa sylvatica grow in America in cobalt rich soils.

(xi) Nickel:

Lychnis alpina grows in Sweden in presence of nickel.

(xii) Sulphur:

Allium, Arabis, Oenothera, and Atriplex grow on soils rich in sulphur minerals.

(xiii) Lithium:

Lycium Juncus, Thalictrum grow on soils containing lithium,

(xiv) Iron:

Damara ovata, Dacrydium caledonicum grow in Scotland on the soils rich in iron.

(xv) Aluminium:

Ulex aquifolium grows in Italy on soils rich in aluminium. Besides above, the mineral content in a plant tissue can be employed in biogeochemical prospecting. Lyon and Brooks (1969) have found Olearia rani to be valuable for the molybdenum. Similarly, silver has been discovered in certain localities in leaves of plants. Sulphate content of leaf can directly be related to SO₂ concentration in air. Farrar (1977) has suggested that high sulphur content in pine needles indicates high concentration of SO₂ in atmosphere.

Fluoride content in Sorghum vulgare leaves indicates the distance up to which air pollution by a fluoride source can fall out and this distance may be upto 4 km. In some cases higher copper content may be due to high tension copper wires. Mercury concentration in Festuca rubra grass may be due to chloroalkali set-up and lead in leaves may increase due to automobile exhaust.

Indicators of fires:

Some plants are well adapted to grow in burnt and highly disturbed areas as for example, Agrostis hiemalis, Epilobium spicatum, Populus tremuloides. Pteris aquilina, and fungus Pyronema confluens grow in areas subjected to fire. Indicators of petroleum deposits. Some protozoans, as Fusilinds indicate petroleum deposits in the area.

Plant indicators for pollution:

The use of vegetation as biological indicator of environmental pollution has a long history. Knowledge of specific resistance to pollutant is of practical significance when plants grow in industrial or thickly populated areas. Species differ in sensitivity to pollutants. In general, plants are more sensitive to pollutants than human.

Therefore, plants can be used for the bio-indication of environmental pollution. Sensitive species can serve as indicators and resistant species as accumulators which collect large amount of pollutants without damage. Mosses, lichens and some fungi are much sensitive to SO₂ and halides. Even 1% SO₂ concentration is harmful to higher plants. Lichens do not survive in areas exposed to SO₂ for long time.

Many chemicals, fertilizers, pesticides and fossil fuels release toxic substances into the environment that are taken up by the plants from air, water, and soil. Atmospheric pollutants, particularly SO₂ halides (HF, HCl), Ozone and Peroxi-acetyl-nitrate (PAN) produced from automobiles; industrial times and strong radiations are dangerous to plants. Harmful substances that reach plant through the air are SO₂, nitrogen oxides, hydrocarbons, dust, and smoke. Plants growing in water are severely affected by toxic chemicals like cyanide, chlorine, hypochlorate, phenols, benzyl derivatives and heavy-metal compounds of sewage.

The effects of different kinds of pollution can be determined by the nature of pollutants, their concentrations and the period of exposure. Under exposure to high concentration, plants suffer acute injury with externally visible symptoms, such as chlorosis, discolouration, necrosis and death of entire plant. Besides morphological changes, biochemical, physiological and fine structural changes also occur in plants.

Pollution damage can be recognized by the accumulation of toxic material in the plant, changes in pH, reduced or increased activity of certain enzymes, increase in compounds with SH groups and phenols, lowered ascorbic acid level in the leaves, depression of photosynthesis, stimulation of respiration, low dry matter production, changes in permeability, disturbances in water balance, reduced fertility under prolonged exposure.

The disturbances in metabolism develop due to chronic injury with irreversible consequences. Plants show reduced productivity and yield and quality is also lowered. Besides above, the structure of wood is changed, branches dry out and gradually the trees die. The symptoms of pollution affected plants are varied and unspecific. A particular pollutant affects different plants in very different ways and a particular symptom can be produced by a variety of substances. The influence of external factors (pollutants) on plants depends on the species, state of development and the organ or tissue involved.

Morphological alteration of a plant and floristic composition of a plant community are commonly used to indicate changes in the environment. According to Van Haut and Stratmann (1970), visible plant symptoms are most commonly used to indicate the responses of plants to pollutants. Jacobson and Hill (1970) have studied the effects of common pollutants on plants.

It is possible that any part of plant body, if it responds specifically or characteristically to any pollutant, can be used for its indication. Goldstein (1974) emphasized that the number and kind of biological indicators can be subdivided in order of decreasing biological complexity, such as organism, organ, tissue, cell, cell-free preparation and enzymatic studies. M.U. Beg (1980) from Industrial Toxicology Research, Centre, Lucknow has reported the responses of pollutants as a biological indicator taking several parameters into consideration.

Attempts have been made to use certain structures and functions of plants, such as seed germination, growth of plant, development of lateral branches, expansion and colour changes in leaf, flower and fruit formation, discoloration of flower, loss of physiological control, mineral composition, chemical constituents of cells, enzymatic activity and pollen germination as indicators of pollution stresses. Important aspects in response to pollution are summarized in Table 12.3.

Seed germination has been used by many workers to monitor pollution responses. Several growth parameters such as percentage of germination, seedling survival, seedling height, cotyledonary expansion and fresh and dry weight have been taken as criteria to assess plant response to a specific pollutant. Phaseolus vulgaris has been grown in smoke-free and smoke- affected regions by Sorauer (1899).

The toxic effect of thiosulphate has been indicated as germinator inhibition in many plants. Houstan and Dochinger (1977) have evaluated germination inhibition in relation to pollution by sulphur dioxide and ozone. The effects of lead, cadmium, NO and CO₂ have been studied on many plants. Besides seed germination, pollen germination in Nicotiana sylvestris has been used to indicate pollution.

Some plant species are good indicators of pollution. Polygonum, Rheum, Vicia, Phaseolus, and Capsella have been observed as pollution indicators. According to Brandt (1974), a large number of plant species are capable of indicating specific contaminants. Generally, the plants response to pollutants is characteristic rather than specific. Efforts have been made to develop certain plant strains which can specifically be used as indicator for a particular pollutant.

Stunting of com, sweet potato and rye has been reported due to high toxicity. Reduction in root length, shoot length, numbers of tillers, leaves; ears and grains in wheat have been reported under condition of cement dust pollution. Similarly plant height, number of leaves and bolls per plant are reduced in cotton exposed to particulate pollution. Inhibition of lateral growth of forest trees is caused by lime stone dust. Pine trees do not flourish in SO₂ polluted areas. It has been noticed that leaf is the most sensitive organ to pollution.

The pollution indicator value of leaf has been exploited by many workers in response to a variety of conditions. Leaf injury is a characteristic symptom to various pollutants. The characteristic symptoms on leaf include pigmentation, chlorosis, yellowing, necrosis etc. The leaves of dicotyledons generally exhibit spotted markings between the veins while monocotyledons usually show necrotic streaks between parallel veins. Injury may also occur along the leaf margin and tip. Symptoms produced by ozone, oxides of nitrogen and chlorine are almost similar. Reduced expansion of cotyledonary leaves in response to pollution has been observed in several cases.

Recently epidermal morphology has been studied as indicator of different pollutants especially SO₂. Cuticular and epidermal damage can be used to indicate air pollution. Dry weight of leaf, decrease in leaf thickness, cell size, loss of leaves and early senescence may be due to smoke and SO2 pollution. Yunus and Ahmad (1980) have observed that leaves in the polluted area of cement factory showed higher stomatal and trichome densities, smaller epidermal cells and trichomes as compared to leaves obtained from unpolluted atmosphere.

Biochemical and Physiological Changes:

Chemical composition of leaf has widely been used to indicate environmental conditions. Among the biochemical estimations, the most important parameter is pigment analysis. Chlorophylls a and b have been measured as index for response to different types of pollution. In Cassia and Cynodon, 5% reduction of chlorophyll has been observed while in Saccharum the pigment is least affected. Chemical estimation like proteins, amino acids, soluble sugars, sucrose, and starch, reducing sugars, vitamin C, riboflavin, thiamine and carbohydrate are used to indicate foliar sensitivity to air pollution.

Physiological activities as opening of stomata and rate of photosynthesis can also be used as indicators of pollution. Photosynthesis as a parameter has been used for mixed exposure of SO₂, NO₂ and dust. Enzymatic parameters are also used to indicate the presence of particular pollutant. Peroxidase was found to be most sensitive indicator of pollutants in the absence of visible injury. Kellar (1974) and Jager (1975) have reported a differential response of enzymes in areas affected by fluoride, automobile pollution and SO₂.

Thus on the basis of enzyme activity, the susceptible species of plants can be identified. Many workers have reported that enzymatic activity has been related to air pollution. Other common enzymatic parameters used are ribulose diphosphate carboxylase, glutamate-pyruvate transaminase, glutamate-oxaloacetate transaminase and peroxidase for SO₂ pollution.