This paper reviews the current achievements on plant wax, and in particular on its interaction with the abiotic factors such as water, temperature, CO₂, solar radiation and pollutants, as well as the biotic factors including phytophagous insects and pathogens. The wax coating occurs at the topmost surface layer of the aerial part in the organs (i.e., leaves, stems, petals and fruits) of terrestrial higher plants. This coating is made up of epicuticular and intracuticular waxes featured by a three-dimensional hydrophobic organic micro-structure. Aliphatic and cyclic compounds are measured to be the dominant organics present in wax coating. These organic compounds are demonstrated to vary with species, organs, or even different parts of an individual organ. The adaxial and abaxial parts of the individual leaves are reported to show different chemical composition in some plants. The leaves taken in different developmental stages are also found to be different in the wax chemical composition in some plants.
The wax plays a vital role on plant growth and development, and is believed to be most sensitive in response to the outside environmental conditions due to its formation in the most outside of the plant organs and its direct contact with the environments. This provides us a window to explore the interactions between plants and environments. In some arid regions, due to the absence of water, some plants are found to biosynthesize more hydrocarbons, aldehydes, and in particular the long-chained alkanes, but others are not. Enhanced solar radiation will result in the presence of thick wax coatings, and also the change in the chemical compositions. Temperature will cause the change not only in crystalline micro-structures but also in chemical compositions. The average chain length of normal alkanes is found to increase in some plants due to the elevated temperature. The wax content will increase in some plants but decrease in others when the atmospheric CO₂ concentration is promoted.
The wax coating will also affect the biotic relationship. The difference in crystalline micro-structure of the wax coating will affect the adhesion and movement of the insects. The variation in the content of fatty acids, alkanes and cyclic compounds (in particular terpenoids and sterols) in the plant wax will affect the symbiosis of insects with the plants. The wax structure and chemical composition are also important factors to prevent the pathogenic bacteria from invasion into the plant organs.
It is thus notable that waxes would change the crystalline structure and chemical composition due to the presence of environmental stresses, including both the abiotic (i.e., water, temperature, CO₂, solar radiation and pollutants) and biotic factors (i.e., phytophagous insects and pathogens) as shown herein. The environmental conditions will lead to a change in the metabolic pathway of the organic compound biosynthesis in wax coatings, which in turn causes the change in wax composition. The changes in both the physical structure and chemical composition constitutes the basis of the physiological or biochemical adaptation or response of the plant wax to the changing environmental conditions, and thus the basis of plant survival to deteriorating environments. On this point, the wax could be served as a potential record, in both modern plants or in the wax-bearing rocks, to decipher the relationship between plants and environments.