There are five major groups, based on chemical structure. With the exception of the latter two, each group represents a family of related compounds. In the past, physiologists simply applied the substance to a plant or excised part to see if it caused a response. If there was a response, then we assumed that the substance was a hormone involved in the response. V. Mechanism of hormone action .
Hormones act on target tissues to activate a receptor.
Techniques to study hormones A. Bioassays . A bioassay examines the effect of a test substance on a plant tissue. To perform a hormone bioassay, a test plant is chosen that lacks the hormone for a response. To determine if a sample contains the hormone, the test plant is treated in a similar fashion. If present the hormone can be quantified by comparing its response to the samples of known concentration.