Air Pollution effects plants also in number of harmful ways just like humans. The various toxic gases which are released in atmosphere triggers harmful chemical reactions. The burning of hydrocarbons in motor vehicle engines gives rise to CO2, CO, SO2 (sulfur dioxide), NOx (NO [nitrogen monoxide]) and NO2 in varying proportions—and C2H4 (ethylene), as well as a variety of other hydrocarbons.
Additional SO2 originates from domestic and industrial burning of fossil fuels. Industrial plants, such as chemical works and metal-smelting plants, release SO2, H2S, NO2, and HF (hydrogen fluoride) into the atmosphere. Tall chimney stacks may be used to carry gases and particles to a high altitude and thus avoid local pollution, but the pollutants return to Earth, sometimes hundreds of kilometers from the original source. Photochemical smog is the product of chemical reactions driven by sunlight and involving NOx of urban and industrial origin and volatile organic compounds from either vegetation (biogenic hydrocarbons) or human activities (anthropogenic hydrocarbons).
Ozone (O3) and peroxyacetylnitrate (PAN) produced in these complex reactions can become injurious to plants and other life forms, depending on concentration and duration of exposure. Hydrogen peroxide, another potentially injurious molecule, can form by the reaction between O3 and naturally released volatiles (terpenes) from forest trees.
The concentrations of polluting gases, or their solutions, to which plants are exposed are thus highly variable, depending on location, wind direction, rainfall, and sunlight. In urban areas, concentrations of SO2 and NOx in the air are typically 0.02 to 0.5 mL L–1, the upper value being within the range that is inhibitory to plant growth. Relatively long-term experiments at appropriate concentrations of pollutants are necessary to establish the real impact of air pollution on vegetation. The reaction of plants to high concentrations of pollutants in short-term experiments may overwhelm the plant′s defense mechanisms and provoke abnormal symptoms. The responses of plants to polluting gases can also be affected by other ambient conditions, such as light, humidity, temperature, and the supply of water and minerals. Experiments aimed at determining the impact of chronic exposure to low concentrations of gases should allow plants to grow under near-natural conditions.
One method is to grow the plants in open-top chambers into which gases are carefully metered, or where plants receiving ambient, polluted air are compared with controls receiving air that has been scrubbed of pollutants.
Polluting Gases and Dust Inhibit Stomatal Movements, Photosynthesis, and Growth.
Dust pollution is of localized importance near roads, quarries, cement works, and other industrial areas. Apart from screening out sunlight, dust on leaves blocks stomata and lowers their conductance to CO2, simultaneously interfering with photosystem II.
Polluting gases such as SO2 and NOx enter leaves through stomata, following the same diffusion pathway as CO2. NOx dissolves in cells and gives rise to nitrite ions (NO2–, which are toxic at high concentrations) and nitrate ions (NO3–) that enter into nitrogen metabolism as if they had been absorbed through the roots.
In some cases, exposure to pollutant gases, particularly SO2, causes stomatal closure, which protects the leaf against further entry of the pollutant but also curtails photosynthesis. In the cells, SO2 dissolves to give bisulfite and sulfite ions; sulfite is toxic, but at low concentrations it is metabolized by chloroplasts to sulfate, which is not toxic.
At sufficiently low concentrations, bisulfite and sulfite are effectively detoxified by plants, and SO2 air pollution then provides a sulfur source for the plant.
In urban areas these polluting gases may be present in such high concentrations that they cannot be detoxified rapidly enough to avoid injury. Ozone is presently considered to be the most damaging phytotoxic air pollutant.
Ozone is highly reactive: It binds to plasma membranes and it alters metabolism. As a result, stomatal apertures are poorly regulated, chloroplast thylakoid membranes are damaged, rubisco is degraded, and photosynthesis is inhibited.
Ozone reacts with O2 and produces reactive oxygen species, including hydrogen peroxide (H2O2), superoxide (O2–), singlet oxygen, and the hydroxyl radical (•OH). These denature proteins and damage nucleic acids (thereby giving rise to mutations), and cause lipid peroxidation, which breaks down lipids in membranes.
Reactive oxygen species form also in the absence of O3, particularly in electron transport in the mitochondria and chloroplasts, when electrons can be donated to O2.
Cells are protected, at least in part, from reactive oxygen species by enzymatic and nonenzymatic defense mechanisms (Bowler et al. 1992 ; Elstner and Osswald 1994). Defense against reactive oxygen species is provided by the scavenging properties of molecules, such as ascorbic acid, α-tocopherol, phenolic compounds, and glutathione.
Superoxide dismutases (SODs) catalyze the reduction of superoxide to hydrogen peroxide. Hydrogen peroxide is then converted to H2O by the action of catalases and peroxidases. Of particular importance is the ascorbate-specific peroxidase localized in the chloroplast. Acting in concert, ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase remove H2O2 in a series of reactions called the Halliwell–Asada pathway, named after its discoverers.
Glutathione is a sulfur-containing tripeptide that, in its reduced form, reacts rapidly with dehydroascorbate and becomes oxidized in the process. Glutathione reductase catalyzes the regeneration of reduced glutathione (GSH) from its oxidized form (GSSG) in the following reaction:
GSSG + NADPH + H+ → 2 GSH + NADP+
Exposure of plants to reactive oxygen species stimulates the transcription and translation of genes that encode enzymes involved in protection mechanisms. In Arabidopsis, exposure for 6 hours per day to low levels of O3 induces the expression of several genes that encode enzymes associated with protection from reactive oxygen species, including SOD, glutathione S-transferase (which catalyzes detoxification reactions involving glutathione), and phenylalanine ammonia lyase (an important enzyme at the start of the phenylpropanoid pathway that leads to the synthesis of flavonoids and other phenolics).
In transgenic tobacco transformed with a gene from Escherichia coli to give additional glutathione reductase activity in the chloroplast, short-term exposure to high levels of SO2 is much less damaging than for wild-type tobacco. Environmental extremes may either accelerate the production of reactive oxygen species or impair the normal defense mechanisms that protect cells from reactive oxygen species. In water-deficient leaves, for example, greater oxygen photo reduction by photosystems I and II increases superoxide production, and the pool of glutathione, as well as the activity of glutathione reductase, increase—presumably as part of the cell defense mechanism. In contrast, levels of ascorbate, another antioxidant, generally decline with mild water stress.
Transgenic plants overexpressing mitochondrion superoxide dismutase (Mn-SOD), the isozyme localized in the mitochondrial matrix, show less water-deficit damage and, significantly, improved survival and yield under field conditions (McKersie et al. 1996). In other experiments, transgenic alfalfa overexpressing Mn-SOD, was found to be more tolerant of freezing. Conversely, winter rye, wheat, and barley acclimated at 2 °C for several weeks, were found to have developed resistance to the herbicides, paraquat and acifluorfen, which generate reactive oxygen species.
Such investigations support the hypothesis that tolerance of oxidative stress is an important factor in tolerance to a wide range of environmental extremes. Many deleterious changes in metabolism caused by air pollution precede external symptoms of injury, which appear only at much higher concentrations.
For example, when plants are exposed to air containing NOx, lesions on leaves appear at an NOx concentration of 5 mL L–1, but photosynthesis starts to be inhibited at a concentration of only 0.1 mL L–1. These low, threshold concentrations refer to the effects of a single pollutant.
However, two or more pollutants acting together can have a synergistic effect, producing damage at lower concentrations than if they were acting separately. In addition, vegetation weakened by air pollution can become more susceptible to invasion by pathogens and pests.
Polluting Gases, Dissolved in Rainwater, Fall as “Acid Rain” . Unpolluted rain is slightly acidic, with a pH close to 5.6, because the CO2 dissolved in it produces the weak acid, H2CO3. Dissolution of NOx and SO2 in water droplets in the atmosphere causes the pH of rain to decrease to 3 to 4, and in southern California polluted droplets in fog can be as acidic as pH 1.7. Dilute acidic solution can remove mineral nutrients from leaves, depending on the age of the leaf and the integrity of the cuticle and surface waxes.
In soils that lack free calcium carbonate, and therefore are not strongly buffered, such additions of acid can be harmful to plants. Furthermore, the added acid can result in the release of aluminum ions from soil minerals, causing aluminum toxicity.