Since it's investigatory and the subject is Biology, it would have to be experimental in approach. Popular (and rather easier than other methods) is the analysis of the use of different plant extracts. Products could be an organic medicine, pesticide or insecticide, disinfectant, etc. using the extracts of the plant/s you have chosen.
You will need the guidance of an adviser to achieve this. Also, you will have to seek advice and opinion from the DOST (Department of Science and Technology) or the DA (Department of Agriculture) personnel, preferably a scientist, to help you determine what components of the plant are essential to your research.
Hope this helps. :)