Identification of plumbagin epoxide as a germination inhibitory compound through a rapid bioassay on TLC

The main aim of this study was to develop a rapid and cheap bioassay to identify germination inhibitory compounds directly from plant crude extracts. Extracts of Plumbago auriculata and Carya illinoinensis as well as their previously identified allelopathic compounds, plumbagin and juglone, respectively, were developed on TLC plates. The plates were covered with a thin layer of agar (2-4 mm) and Lactuca sativa (lettuce) and Arabidopsis thaliana seeds placed onto it. After incubation of the plates for three days complete germination inhibition correlating to specific compounds in the extract was clearly visible. Plumbagin and juglone inhibited the growth of the seeds completely while a second compound in the P. auriculata extract also showed inhibition. This compound was identified through chemical synthesis as plumbagin epoxide. This is the first report of its presence in P. auriculata. By using this bioassay on TLC, plant extracts can be tested cheaply and rapidly and the compounds responsible for germination inhibition be identified.