Plants have evolved in a gaseous environment. When the composition of this environment exceeds the capacity of plants to tolerate or adapt to the change, some stress is imposed, and the sensitive components of the ecosystem start malfunctioning. Guard cells and cuticle are the first to be exposed to the pollutant gas. When the gas reaches inside the leaf, the cell membranes are affected. Alterations occur in SH/SS ratio. There can be permeability dysfunction as well.
Free radicals are formed and ion transport can be inhibited and normal functions disrupted. Once within the cell, the pollutant interacts with cytoplasm, organelle membranes and cellular metabolites. Disulphide bonds in proteins can be split up. The overall effect of the above disturbances is that plant growth is impaired and productivity declines. Plants growing in polluted environment often show symptoms of injury,’ general debility and premature ageing. In temperate countries, alfalfa has bee, found to be susceptible to SO2 gladiolus to hydrogen fluoride, tobacco to ozone and petunia to photochemical smog. In the tropics, mango (Mangifera indica) has been reported to be especially sensitive to polluted air.
The aerial parts of the trees growing near thermal power stations, brick kilns, steel factories and smelters (habitats polluted with SO2 and other pollutants) exhibited such symptoms as necrosis of leaves, defoliation of young terminal branches, hardening of floral buds, necrosis of stigmatic surfaces, necrosis of fruit tips, and reduction in fruit size (see Rao, 1972). Recently the leaves of several plants such as Phaseolus vulgaris, Coleus blumei, Daucus carota, Ficus variegatus and others have been found to be capable of fixing carbon monoxide (Bidwell and Bebee, 1974). Some workers are of the opinion that vegetation can constitute a sort of global sink for air pollutants such as carbon monoxide (see Hill, 1971). The average CO concentration in air over major land masses ranges between 0.1 to 1.0 ppm, and Bidwell and Bebee have estimated that about 3 x 1014 grams of it may be fixed by various plants per year. Sulphur oxides are highly toxic.
Their effects include interveinal necroses and yellowing of broad-leaved plants. Nitrogen oxides cause defoliation, chlorosis, necrotic spots, tip and margin burn, and general growth retardation. Excessive dusting of plants with particulate matter can clog stomata thereby preventing gas and water exchange.
Most plants are insensitive to carbon monoxide levels known to affect man, but at high concentrations, CO produces the following symptoms: leaf curling, ageing, and reduction in leaf size.