Rice allelopathy and the possibility for weed management

In attempts to control weeds in rice, much effort has been focused on rice allelopathy research for more than 30 years. Among screening methods that have been developed, some estimate the allelopathic potential of various rice cultivars in a limited time and space, which is less costly and can be conducted year round. Rice allelopathy activity is variety dependent and origin dependent, where Japonica rice shows greater allelopathic activity than Indica and Japonica-Indica hybrids. Allelopathic characteristics in rice are quantitatively inherited and several allelopathy-involved traits have been identified. Numerous phytotoxins such as cytokinins, diterpenoids, fatty acids, flavones, glucopyranosides, indoles, momilactones (A and B), oryzalexins, phenols, phenolic acids, resorcinols and stigmastanols have been identified and determined as growth inhibitors in rice. However, the fate and actual modes of action of these compounds as well as other potent rice phytotoxins in nature are not well understood. The question of which compounds play a major role in rice allelopathy has remained obscure; however, rice allelopathy might be attributable to the interaction of all present allelochemicals. Despite locating genes determining or involving allelopathy in rice having attracted much effort, the introduction of these genes into target rice cultivars has not yet been achieved. Success in breeding new rice cultivars having good weed-suppressing ability would benefit farmers in rice-cultivating countries and play an important role in sustainable agricultural production.