Plant Response to Air Pollution

Air, water and soil pollution influences vegetation. Primary air pollutants such as reactive compounds of sulphur (sulphur oxides), nitrogen (nitrogen oxides), carbon monoxide and hydrocarbons, are emitted from fossil fuel combustion. Secondary pollutants, primarily ground level zone have severe implications on plants. These pollutants enter through stomata and adversely affect plant functions. On exposure to the automobile pollution, Sida cordifolia L. and Catharanthus roseus L. showed significant increase in the levels of catalase and peroxidase and evident alteration in chlorophyll, carotenoid and phaeophytin. The stomatal indices, number of epidermal cells per unit area and the number of stomata reportedly decreased due to exposure, while increase in the size of stomata and epidermal cells was found. Bisphenol A, another common industrial pollutant induced dose dependent and stage specific response in soyabean roots, causing only membrane peroxidation at low doses. On the contrary, at higher doses, in addition to membrane peroxidation, the BPA also activated antioxidant systems and enhanced reactive oxygen species levels. Low level exposure did not affect membrane peroxidation at growth stages other than seedling stage.Plant response to pollutants varies with species, age, variety and other abiotic factors. Ozone, among all the pollutats, has been documented to have the most detrimental effects on the plants. Ozone causes damage to almost all levels of plant functions, such as leaf injury, stomatal conductance, photosynthesis, carbon allocation and respiration. Phaseolus vulgaris varieties Irai and Fepargo were studied to demonstrate the effect of high concentration of ozone on enzymatic and non-enzymatic antioxident system of the plant. Ozone increased total thiol levels in Irai while the levels were unchanged in Fepargo 26. Chlorophyll a and b levels were fund to be reduced along with reduction in protein concentration indicating compromised photosynthetic apparatus. Irai had higher levels of reactive oxygen species (ROS) and ascorbate (AsA) concentration, but lower total thiol levels and catalase content. Contrarily, aqueous ozone may be used for the control of fungal pathogens especially against the conidia of A. flavus with effective dose depending upon initial spore inoculum. Certain catagories of agrochemicals such as strobilurin group fungicides, growth regulators such as "Emistym C" demonstrate protective effects against ozone injury in ozone sensitive cultivars of clover and tobacco. Such agrochemicals may possible be used for increased ozone tolerance in sensitive crops. Elevated levels of CO2, however, increase photosynthetic rate and demonstrate positive impact on crop productivity. Elevation of CO2 levels in atmosphere stimulates the light-saturated photosynthetic CO2 assimilation rate in C3 and C4 plants with varying degrees ranging from maximum stimulation for trees and C3 grasses, moderate for shrubs, C3, C4 crops and legumes, and negative for C4 grass. Under elevated CO2, carbohydrate accumulations, such as starch size and number of chloroplasts, can be enhanced, partially due to the carbon substrate increase. However, the excessive carbohydrate accumulation may cause feedback inhibition or physical damage at the chloroplast level, reducing the photosynthetic capacity. Certain measures taken to reduce emission of primary pollutants, such as, using fuels low in sulphur content, introduction of catalytic converters to reduce exhaust pollutants, reducing application of nitrogen-based fertilizers, and switching to cleaner energy sources, may reduce air pollution and its deleterious effects on vegetation. High Frequency non-ionizing Electromagnetic Fields emitted from electronic devices were found to cause altered gene expression in plants. In a study, metabolic activities such as reactive oxygen species metabolism, ?- and ?-amylase, Krebs cycle, pentose phosphate pathway, chlorophyll content, terpene emission, etc. were reportedly modified as a result of the exposure. Heavy metal pollution causes growth reduction, poor performance and low yield in plants growing on heavy metal polluted soil. In hydroponics culture, Boehmeria nivea L. at 20 mg/l showed changes in chlorophyll, electrolyte leakage and hydrogen peroxide levels. Certain plants such as Boehmeria nivea L. and micro-organisms accumulate heavy metals and are suitable for bioremediation which has been accepted as an effective method of treating soils carrying heavy metal pollution. Boehmeria nivea L. can uptake various heavy metals, such as Sb, Cd, Pb, and Zn. The plant can accumulate arsenic catalase and malondialdehyde in plant parts, which indicated that it may be used for phytoremediation. In plants, antioxidant defenses such as ascorbate-glutathione cycle, which is one of the major hydrogen peroxide detoxifying system in plant cells, protect plants from the reactive oxygen species (ROS), produced on exposure to pollutants or environmental stress. Ascorbate peroxidase enzymes, present in chloroplasts, mitochondria, peroxisomes and cytosol, catalyze conversion of H2O2 into water using ascorbate as a specific electron donor.