This study examined phytotoxic activities of several Vietnam localised OM rice (Oryza
sativa L. OM) cultivars on junglerice (Echinochloa colona (L.) Link), an invasive weed
threatening rice production, and identified allelochemicals and their activities for OM
4498, the most allelopathic cultivar. The aqueous MeOH extracts of all rice cultivars
inhibited root and shoot growth of junglerice at a concentration as low as
0.01 g ml 1. Inhibitory effects were greatest in OM 380, OM 2395 and OM 4498,
with the average effective doses (EDs) of extracts required for 90% inhibition (ED90)
on junglerice as low as 0.981, 0.632 and 0.702 g ml 1, respectively. The phytotoxic
substances salicylic acid, vanillic acid, p-coumaric acid, 2,4-dimethoxybenzoic acid,
benzoic acid, p-hydrobenzoic acid, trans-cinnamic acid and coumarin were identified
in OM 4498 at concentrations ranging from 0.066 to 11.412 mg in 100 g fresh
weight of rice tissue. Two of these substances, 2,4-dimethoxybenzoic acid and coumarin,
have never previously been identified in rice. Compared to other compounds,
p-coumaric acid may have the greatest weed-suppressant potential on junglerice,
with the average ED50 being 0.28 mM. The total activities of salicylic acid, vanillic
acid, p-coumaric acid, trans-cinnamic acid and 2,4-dimethoxybenzoic acid in an OM
4498 rice plant on junglerice seedlings were 59.60, 0.78, 3.47, 2.52 and 0.61 mM/
mM, respectively. The results clearly suggest that OM 4498 rice extract is an excellent
source for allelochemicals against junglerice, and this rice cultivar should be considered
in rice production areas invaded by junglerice.