Exposure of plants to SO2 can lead to acute injury and may effect stomatal activity. Rapid absorption of a toxic amount of SO2 leads to the appearance of marginal or interveinal necroses, especially in younger leaves. The rather tender leaves of most monocotyledonous plants are quite sensitive to SO2; exposure to low concentrations of this pollutant often results in injury to leaf tips and if the SO2 concentration is higher than the whole leaf may become bleached.
In most leaves, the SO2 injury first appears at the tips and extends downward with increasing concentrations and exposure time. Leaves of alfalfa that have beat exposed to more than 1 ppm of SO2 often have a high percentage of their stomata fully or partly closed. The areas which have actually been injured do not recover but the uninjured portions of the leaf are enabling of regaining their function following an exposure to SO2 and freshly formed leaves develop normally.
A field study of trees, herbaceous plants, agricultural crops and surface soils of 34 sites around an Aluminium Factory at Renukoot, Mirzapur (Varanasi) was undertaken by Pal (1974). The fallout pattern of airborne fluorides and the distribution of fluoride in plant and soil samples collected from the area were determined.
Elevated levels of fluoride were found in all the samples and there was a significant increase in fluoride concentration at distances nearer the factory. The fluoride concentration in a plant was found to be maximum in leaves, followed by root
An examination of stray cattle foraging around the Aluminium Factory revealed typical symptoms of fluorosis. The clinical symptoms of ailment constituted dental lesion of varying intensity and elevated levels of urinary fluoride.
Both gaseous and particulate fluorides are known to be emitted into the air. As far as the effects on vegetation are concerned, the gaseous fluorides are probably the most important. There can be no single criterion for the diverse kinds of effects of atmospheric fluorine on different kinds of plants. Each curve is seen to occupy a different position because of differences among plants in their susceptibility to fluorine and the consequence of fluorine-induced effects (see McCune 1969).
D.N. Rao and his students (personal communication) have developed appropriate techniques for air pollution sampling and monitoring of plant responses to different pollutants. They have also developed methods to fumigate plants, in suitable fumigation chambers, to pollutants of known concentration.
The objective of this work has been to use plants as indicators and monitors of air pollution through analysis of changes in morphological, physiological and biochemical characteristics of plants. Their preliminary findings indicate that phytomass, chlorophyll and ascorbic acid levels may be of help in quantifying plant responses to different air pollutants.
Another objective of their research is to understand the causes underlying the sensitive or resistant response of plants to certain pollutants, with a view to applying this knowledge to screening of various plants for suitability in shelter-belts as well as in revegetating certain barren and derelict areas created by industrial, mining, or other human activities.