Air Pollutants: The Mode Action of Air Pollution
INTRODUCTION
Air pollutants are volatiles that is not found in high concentrations in natural atmospheres and are often produced in urban settings. These pollutants can cause injury to plants, distorting their metabolism, altering their appearance, and lowering their agricultural productivity. As these pollutants move into rural areas, they generate the same problems that, in the total ecosystem, alter plant and animal interactions and distributions.
There are many types of pollutants, but the most common include sulfur dioxide, fluoride, oxides of nitrogen, and oxidative species, such as ozone. All pollutants generate their effects by different mechanisms but most must enter the tissues via the stomata to cause their specific injuries. The injuries are best described by the visible injury or tissue damage symptoms but also plant metabolism tends to be shifted away from the more productive state.
Ozone creates more of the most troublesome conditions by inhibiting and disrupting many photosynthetic processes. Air pollutants can be controlled and their concentrations lowered, but that requires political and economic motivations. Alteration of the atmospheric environment existed as long as Earth has existed. The question of how far the atmosphere can deviate from the norm and still support life will be argued, but in the modern era, that question is more defined.
In the United States, acceptable alternations to the urban atmosphere are defined politically as being small enough not to cause harm primarily to humans and secondarily to plants. The balance between the protection of life and the lack of economic disruption is continuously being argued in the scientific halls, political chambers, and courtrooms. For each pollutant, a balance must be struck in governmental discussions of the scientific issues.
The pollutants that are understood as to their modes of action are shown in Table 1. To be sure, there are many others that are formed by commercial activities but are deemed to be in such low concentrations or are so poorly understood that they are not regulated. As our ability to detect smaller concentrations of these unusual compounds continues to increase, the pressure to regulate those other compounds will grow, especially if they are found to cause problems with human health or to injury, normal growth, development, and productivity of plants.
Presently, as we understand how plants respond to the environment, there is a band of adaptability of the individual to any environmental condition. Each individual integrates its response to the total environment and that generates a general response of the species within that environment—some individuals will perish but the majority will survive if that species is to continue within that particular ecosystem. Our problem is to manage the total ecosystem to maintain diversity (at least as defined by our society) and to generate a concept of risk assessment—that is, what band of adaptability is acceptable to us? For the most part, the appearance of visible injury on the surface of the leave is the test of varied air pollutants.
Yet, it is still the method of choice to monitor in the field what air pollutants are present and how much. Plants respond to air pollutants similarly to other stresses on several levels: exclusion, tolerance, and repair. The response mechanism depends upon the concentration of the air pollutant, environmental conditions, and the developmental and metabolic state of the plant. Any response is detrimental to plant productivity because it costs the plant metabolic resources. For example, the stomata can close under pollutant exposure to exclude the pollutant from the interior of the plant, thus preventing damage. However, stomata closure lowers photosynthetic CO2 fixation, and plant productivity will suffer.
FLUORIDE, SULFUR DIOXIDE, AND AMMONIA
The first three pollutants (fluoride, sulfur dioxide, and ammonia) can be controlled, generally at the source of the pollutants (smelter plants and livestock paddocks). While they have been historical problems, their modes of action are generally understood and their levels are now well controlled. Yet they have an interesting mode of action that revolves around their charge under different pHs and their ease of entry into the cell.
NITROGEN OXIDES AND OZONE
Again the oxides of nitrogen have the same type of responses as does SO2. It was thought that they can add excess nitrogen (N) to the plant, which is similar to fertilizer. Unfortunately, in a typically N-poor ecological setting, this shift in N can cause an alteration in the dominant plants. In southern California, native plants seem to be overwhelmed by grasses, brought in by humans, and fertilized by the atmospheric NOx. While a problem, the mode of action of NOx was deemed to be understood. However, recently it has been suggested strongly that NO could be involved in signal transduction and so extra NO from the atmosphere could shift normal metabolism.
NOx compounds include HNO3 and that acid seems to erode the leaf cuticle leading to excess water loss. The depletion of stratospheric ozone is currently a principal concern of humankind; the depletion can lead to high levels of biologically damaging UV reaching the Earth’s surface. However, the production of tropospheric ozone is different but likewise has a serious impact on life. The combustion of gasoline produces varied forms of NOx (multiple forms of oxides of nitrogen), which cannot be totally diluted by large masses of clean air. One of the principal forms of NOx is NO2. NO2 (being dark brown) absorbs light and decomposes into NO and [O] (atomic oxygen), which reacts vigorously with O2 to produce O3.
As the O3 builds up, it can react with NO to produce NO2 (and O2) leading to an equilibrium of NO/NO2/O3, which is shifted toward excess ozone during the day. This series of reactions lead to the production of the ozone that badly impacts our cities’ atmospheres. Ozone induces visible injury to the surface of leaves and a loss of productivity of plants; both have been used to monitor ozone’s presence and reactions. Generally, the visible injury patterns—such as ‘‘water-logging,’’ lower surface ‘‘bronzing,’’ upper surface ‘‘silvering’’—can be used to assess injury by persons who are well versed in their identification. Yet there are three sequential processes that combine to trigger ozone stress from the movement of gases from the atmosphere into the sites of action within the leaf.
The oxides of the nitrogen/ozone cycle occur within urban atmospheres. The generation of ozone relies upon light during the day and the presence of oxides of nitrogen. Nitrogen dioxide absorbs visible light, which acts to disassociate the NO2 species into NO and atomic oxygen. Atomic oxygen is very unstable and reacts immediately with oxygen to produce ozone. NO can react with ozone to produce nitrogen dioxide again. During the day the equilibrium is toward higher levels of ozone while at night the equilibrium shifts to ‘‘mop up’’ ozone and produce more NO2. The ozone level at night actually drops to near zero in urban settings.
Process 1.
Entry of the pollutant into the leaf. The entry of gases into a leaf is a well-defined path, which includes gaseous diffusion through the leaf boundary layer and stomate into the substomata cavity approximately following a linear flux law in which the flux (j) into the internal space of a leaf is related to the conductance (g) through the boundary layer and stomata, and the gradient of concentration (C) of gas from the outside (Co) to the inside (Ci).
For both water and CO2, this formulation has been used for years. For ozone, internal concentration has been found to be very close to zero, most probably because ozone is extremely reactive with cellular biochemicals. Thus, the effective delivery rate is (g Co) with stomata conductance being the major regulatory control. The flow of gaseous pollutants is from the substomata cavity within the leaf into the cell, through the wall, where varied charges can influence its rate of decomposition and the products formed. An equilibrium between the gas and aqueous phase occurs at the interface where the gaseous species dissolve into the water, according to Henry’s Law.
Process 2.
Biochemical reactions of the gases with the cell. Many reactions of the gas in the water phase at the cell’s surface and the reaction of each species thus generated generates diverse components within the wall region of the cell. These chemical reactions are poorly understood although there are some data suggesting that free radicals and oxygen species do form.
Ozone does react with organic molecules at double bonds (such as ethylene and unsaturated fatty acids) to form carbonyl groups and, under certain circumstances, peroxides. Sulfhydryls are particularly easy targets, with the formation of disulfide bridges or sulfones. In water, the reactions become more confusing, but some oxidative products that can be linked to metabolic changes are hydrogen peroxide (H2O2), hydroxyl radical (HO), and superoxide (O2).
Effective detoxification reactions can occur within the wall and inside the cell via antioxidant metabolites and enzymes, such as ascorbate, glutathione, and superoxide dismutase if they are present at high enough concentrations. Certainly, chemical modification of wall-specific biochemicals is possible, such as ascorbate, glucan synthase, peroxidases, and diamine oxidase
Process 3.
Movement of reaction product(s) and their enzymatic or chemical transformations within the cell. Currently, there are several major theories of how ozone or its toxic products alter varied processes within the cell.
Membrane dysfunction.
The membrane is altered by ozone, principally via protein changes not involving the lipid portions of the membrane (except at extremely high levels of ozone where the fatty acids of lipids are involved). These alterations involve increased permeability with less selectivity, a decline in active transport, and changes in the trigger mechanisms of signal transduction pathways such that the signals are no longer in the correct state of the cell. Changes in the cellular pools of Ca2þ/ Kþ/Hþ are the primary suspects.
Antioxidant protectants.
The varied antioxidants (both as metabolites and enzyme systems) can eliminate the oxidant or its products if present at the time of fumigation and in high abundance. Too rapid entry of the oxidant can overwhelm the antioxidant response.
Stress ethylene interactions.
Visible injury is caused by the interaction of ozone with stress-induced ethylene, either by direct chemical transformation of ethylene to a toxic product or by alteration of the biochemical relations at the ethylene binding and sensing sites.
Impairment of photosynthesis.
At least two distinct processes inhibit photosynthesis. Some product of ozone entry causes a decline in the mRNA for the primary CO2 fixing enzyme, Ribulose 1,5- bisphosphate carboxylase/oxygenase (Rubisco) such that the level of Rubisco slowly declines within the chloroplast, leading to a lowered rate of CO2 fixation and lowered productivity. This process appears to be early senescence and may be linked to general senescence.
Also, ozone alters the normal ionic and water relations of guard cells and subsidiary cells, such that the stomata close and so limit CO2 fixation. Thus, the response of the stomata to the current environment is incorrect for efficient photosynthesis.
Translocation disruption.
One of the most sensitive biochemical systems to ozone exposure seems to be the translocation of sugars, such that even a mild exposure will lower the ability to transform and move carbohydrates within the plant and cause the loss of sugar transport out of the cell to sinks necessary for efficient growth and productivity.
General impairment/disruption of varied pathways of metabolism
A seemingly wide range of impairment of metabolism was noted in early work on ozone injury, including disruption of metabolic pools and changes in enzymatic activity. These results lead to the vaguest concept of how ozone alters metabolism; it is based upon earlier work in which enzymes and metabolites, which could be assayed, were measured. This concept was based upon what could be done rather than a testing of any coherent hypothesis or following interactions of genes and enzymes.
CONCLUSIONS
In conclusion, the pollutants generated in the urban areas remain the most problematic ecological abuse since they are linked to economic productivity and growth. Aside from human health many of these pollutants have been shown to lower plant productivity, induce a visible injury, generate conditions of early senescence, and disrupt many normal metabolic pathways. For these reasons, pollutants must be carefully regulated to prevent the occurrence of high levels in urban air basins.