Fifty grams of dried duckweed was soaked in 500 mL of 70%
(v/v) aqueous methanol for 48 h, and filtered through filter paper
No. 1 (Toyo Ltd., Tokyo, Japan) to remove debris. The residue was
re-extracted with methanol for 24 h and filtered. The two filtrates
were combined and evaporated at 40 ◦C to produce an aqueous
methanol extract which was covered by aluminum foil and kept in
a refrigerator until use. The same extraction procedure was applied
to water lettuce. Both aqueous methanol and methanol were used
as solvents for extraction in this study because phenolic substances
can easily dissolve in aqueous methanol (Zhao and Hall, 2008) and
methanol is a solvent known to dissolve many plant secondary
metabolites (Chung et al., 2005; Rouf et al., 2007).