Allelopathic potential in rice (Oryza sativa L) germplasm

Allelopathy is the direct influence of a chemical released from one living plant on the development and growth of another plant. The use of allelopathy to control weeds of food crops could contribute towards increasing yields in sustainable agricultural systems. Various research teams have identified several hundred rice accessions which show indications of allelopathic potential, 60 of them showing promising allelopathic activity against one or more weeds. These accessions have different origin and stage of improvement. This, combined with the involvement of several allelochemicals and the ability to release these chemicals, indicates that allelopathic potential is a polygenic characteristic weakly correlated with yield or other important agronomic features. Successful allelopathy research, however, must be focused both on the discovery and subsequent validation of allelopathic potential. Experimental factors such as choice of test plants, collection of allelochemicals, concentration, osmotic pressure and bioassay methodology need considerable attention together with validation through assays with known chemicals of known concentrations. Without validation using known chemicals, the contribution of allelopathic research remains obscure. Several putative allelochemicals are found in extracts of rice leaf and straw, decomposing straw, and in rice soil. Evidence that these chemicals are being released from living plants is still lacking, but as it is a prerequisite for allelopathic activity in nature, it must be proved. Methods of collecting test solutions from living plants need further description. Allelopathic activity is believed to be a joint action of several secondary metabolites, and this must be confirmed for rice. Dilution assays with mixtures of putative rice allelochemicals need to be conducted to evaluate possible synergistic effects. In validation, these mixture-dose response curves must be compared with assays using collected test solutions. A joint, coordinated and focused research effort on rice allelopathy is necessary before the full potential of allelopathic cultivars can be realised.