Air pollutants - Effects on plants
Particulate matter such as cement, coal, petro coke, dust, and fly-ash screen the light and thus change its quantity and quality. Dust plugs stomata and decreases exchange of gases and transpiration. On wet plant parts dust gives alkalinity to the surface. However, some studies indicate that low concentration of fly ash can enhance the plant growth and may be considered desirable supplement to fertilisers. Ozone causes Chlorosis, spotted leaves due to oxidation, burning of leaf tips, weakening, early leaf fall, reduction in growth and yield. Ozone harms leafy vegetables, field crops, fruits and forests trees.
Tobacco is highly sensitive to ozone. Oxides of nitrogen oxidise chlorophyll resulting in Chlorosis, leaf damage. They may cause reduction in growth, yield, and fruit fall. Since air gets contaminated with several pollutants evolved from a single source, damages result from the cumulative effect of two or more gases. Moreover, the observed symptoms cannot be distinguished from those due to physiological stress factors such as light, temperature, humidity, nutrients etc. Mixtures of gases, SO2 and O3 SO2 and HF show synergetic effects. We have mentioned earlier that particulate matter potentiates the effect of SO2. Acid deposition can leach vital plant nutrients such as calcium from the soil by forming insoluble salts. It can also release aluminium ions bound to the soil into soil water. Aluminium ions damage fine root hairs, thus reducing absorption of water. Increase in pH kills essential soil microorganisms.
Acid deposition can change ion uptake in plants. However, certain species of plants respond to pollutants more than others. For instance, mosses and lichens are highly sensitive to air pollutants and get severely damaged by low levels that do not induce responses in human and other animals. The highly sensitive nature of lichens, mosses and other plants have been used to indicate pollution levels. Such plants are often referred to as bio indicators of air pollution.