Presence of pollution of some sort in the environment may cause physiological changes, biochemical changes, changes in growth pattern and gross morphology of organisms occurring in the locality. Symptoms of toxicity appear. While determination of physiological and biochemical changes involve more sophisticated methods, changes in growth pattern, gross morphology and symptoms of toxicity are easy to observe.
Therefore, these parameters have frequently been used by many workers as a convenient means to assess the environmental quality. For example, in a number of species higher concentration of aluminium causes mottling of leaves, higher concentrations of cobalt causes white dead spots on leaves and that of cadmium causes yellow-white chlorotic spots in between veins.
Higher concentration of iron may cause stunted growth and stubby roots. Manganese toxicity results in chlorotic leaves, curling and dead areas appear on the margin of leaves. White dead spots on leaves are common in plants growing in areas which have a higher concentration of nickel. Molybdenum toxicity causes stunted growth and yellow orange colouration of aerial parts of the plant.
These observations serve as useful indicator of heavy metal pollution. However, symptoms of toxicity caused by toxic heavy metals often overlap. They largely depend on a highly complicated set of interactions within the organisms and therefore could be misleading as well.
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The response of a large number of organisms especially those of plants to different types of stress caused by pollution of environment have not been studied and well documented. In Salvia and Dahlia, for example, ozone causes red or brown streaks on the upper surface of leaves, curling of leaf margins and wilting of the apical part.
In Gladiolus tissues of the tip of the leaves and margins are destroyed by flourine or hydrogen flouride fumes present in air. Sulphur dioxide causes light spots on the margins and destruction of the tissues of leaves of Xenia. Peroxyacetyl nitrate present in higher concentrations in the air results in destruction of chlorophyll and killing of tissues on the lower surface of leaves of Salvia and Chrysanthamum sp.
These plants maintained under controlled conditions may be placed in the environment which is to be monitored and responses observed from time to time. This response could lead to valuable set of information about the status of pollution in the locality. In case of aquatic systems or soils, samples of water or soil may be brought to laboratory.
The test organism is introduced or grown in the medium brought from the locality concerned and the responses recorded. The symptoms of toxicity or responses envinced by the test organisms thus provide more definite information about the state of environment under observation.