Progress and prospect of rice allelopathy research

An improvement in allelopathic potential in rice will have a great impact on both low-input and high-input management systems. Alleopathy alone is not likely to replace other weed control practices, but it surely functions as a component of integrated weed management technology. Marginally reduced use of herbicide over time will provide significant economical benefits to farmers and will also reduce adverse effects on the environment. There is a great possibility of breeding new allelopathic rice by regulating their capacity to produce allelopathic substances. Allelopathic potential in rice was greatly improved by the classical breeding method of crossing between a non-allelopathic commercial cultivar and an allelopathic cultivar. The first priority research areas are identification of allelochemicals and genes responsible for allelopathic effects and these traits can be easily incorporated into improved cultivars by either classical breeding or biotechnology. Allelopathic activities of rice seem to be derived from the joint action of momilactones with phenolic compounds. Phenolics, however, are unlikely to be the primary allelochemicals since they never reach a phytotoxic level in soil. However, it is believed that phenolics like other multiple compounds present at concentrations below the activity threshold could have pronounced allelopathic effects through their joint action. Momilactone B seems to be the responsible allelochemical because its activities were 30-fold to 300-fold greater than phenolic acids. Insertion of two specific genes such as CA4H for p-coumaric acid and OsDTS2 for momilactone into one commercial rice cultivar by biotechnology will lead to the development of allelopathic rice cultivar. Further studies on genetic control of rice allelopathy, the complete array of the responsible chemicals in rice root exudates and their mode of action will shed more light on developing an allelopathic rice crop.