Plant Growth Regulators (PGRs) are defined as plant hormones that can be synthesized either naturally or in laboratory settings, and are employed to fulfill specific roles within plants. It is important to note that plant hormones that occur naturally and function within the plant system are not classified as plant growth regulators.

PGRs are utilized to regulate and enhance the natural growth processes of plants, thereby aligning with the specific requirements of plant development. It is critical to recognize that plant hormones do not serve as nutrients; rather, they function as chemical agents applied in very low concentrations (ranging from 10−6 to 10−5 mole/L).

In summary, the spectrum of plant hormones is typically categorized into five primary classes, organized based on their structural similarities as well as their physiological effects on plant organisms.