Cassia alata (Caesalpiniaceae), an ornamental shrub, has many biological properties such as antifungal and antibacterial activities. Several bioactive and phytotoxic compounds have already been isolated from C. alata. Phytotoxic substances from plants have drawn attention as an alternative biological approach to control weeds. Thus, we conducted this research to explore other phytotoxic compounds in C. alata leaves. Aqueous methanol extracts of C. alata leaves strongly inhibited the seedling growth of broccoli, cabbage, cress, radish and rapeseed, in which the level of inhibition correlated with concentration. Two active compounds were isolated through chromatographies and identified using spectral data as (S)-4-(3-hydroxybutyl)phenol [(+)-rhododendrol] and (E)-4-((1R,4R)-4-hydroxy-2,6,6-trimethylcyclohex-2-en-1-yl)but-3-en-2-one [3-hydroxy-α-ionone]. These two active compounds inhibited the growth of cress seedlings in a concentration-dependent manner. The required concentrations for 50% growth inhibition (I50 value) of cress seedlings were 192.0–296.1 μM for (+)-rhododendrol and 132.4–195.3 μM for 3-hydroxy-α-ionone. These results indicate that the two phytotoxic compounds play a part in the phytotoxic activity of C. alata leaves.
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