**3. Absorption of air pollutants in plants**

#### **3.1 Stomata**

The primary entry site for air pollutants in the plants is likely to be stomatal pores, either through exchange of gases or penetration of a liquid layers into the sub-stomatal cavity. By altering the aperture between the guard cells, plants regulates absorption of air pollutants with a diverse range of molecular masses. The movement of guard cells is regulated by the K+ concentration in the cell sap. However, the mechanism of stomatal response to external environmental stimuli involves cellular sodium potassium pump and calcium homeostasis of guard cells as well as K+ ion and its counter-ions, malate, chloride, and nitrate [49, 58]. Several environmental factors (temperature, light, and relative humidity) and cell internal factors (partial pressure of CO2, sucrose concentration, turgidity of guard cells and the presence or absence growth inhibitors) also affect stomatal opening. Absorption of oxides of sulfur (SOx) and nitrogen (NOx) is regulated by light intensity and heavy metals with airborne particulates will directly enter through the stomatal pores, while lipophilic substance enters through cuticular wax [59].

### **3.2 Cuticle**

When stomata are closed, the cuticle acts as the entry point for the pollutants. Lipophilic substances enter through cuticles of the plants, whereas hydrophilic compounds such as gaseous and liquid contaminants can be absorbed by cuticle to some extent. The morphology and composition of cuticles vary with species age, and location of the cuticle in plant as well as some climatic factors such as temperature and relative humidity. Organic compounds such as PAHs, PCBs, dioxins, and charged particulates may alter chemical constituent of cuticle that increase permeability of the cuticle. Once these pollutants enter through cuticle, they infiltrate slowly by diffusion process or get deposited on cell wall or the vacuoles [60].
