A4 Polluting Effects

Topic

Chemical pollution disrupts three distinct plant equilibria simultaneously: the soil interface equilibrium, the chemical exchange equilibrium between the plant and its environment, and the leaf-atmosphere interface equilibrium. Whether chemical pollution enters through the soil or the air, the contaminant travels along a pathway until it reaches a site of damage within the plant tissue; it is precisely this flow of the contaminant that the plant's detoxification responses must interrupt before the pollutant reaches its target site of damage.

When chemical pollution enters through the soil, it does so in the form of soil contaminants—such as heavy metals, salinity-increasing salts, or herbicides—which alter the soil interface equilibrium and the chemical exchange equilibrium between the root and the soil. The higher the concentration of soil contaminants, the greater the disruption to the root's uptake of water and nutrients across the soil interface; in this scenario, that impairment of water and nutrient uptake constitutes the site of damage reached by the flow of the contaminant.

Conversely, when chemical pollution enters through the air, it arrives as airborne pollutants—such as ozone, sulfur dioxide, nitrogen oxides, or fine particulate matter—that cross the leaf-atmosphere interface equilibrium by entering through the stomata, the passageways connecting the leaf-atmosphere interface to the leaf's interior. Once inside the leaf, airborne pollutants reach the chloroplasts and leaf tissue membranes directly, causing direct oxidative damage; in this aerial pathway, that direct oxidative damage to the chloroplasts and membranes represents the site of damage reached by the flow of the contaminant. Faced with either site of damage—impaired water and nutrient uptake due to soil contaminants, or chloroplast and membrane damage caused by airborne pollutants—the plant activates detoxification responses that act upon the flow of the contaminant before it reaches the target site. The plant may sequester the contaminant in compartments where it no longer causes harm, chemically transform it into a less toxic form, or exclude it entirely—either by preventing it from crossing the soil interface or by blocking the entry points provided by stomata at the leaf-atmosphere interface.

Ultimately, the impact of chemical contamination on the plant depends on whether these detoxification responses succeed in halting the flow of the contaminant in time. If they succeed, the balance at the soil interface, the chemical exchange equilibrium, and the balance at the leaf-atmosphere interface are maintained or restored, and the damage never develops. If they fail, the contaminant reaches the target site, and the chemical contamination manifests as impaired water and nutrient uptake, direct oxidative damage to chloroplasts and membranes, or both.

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