Separation of Allelopathy from Resource Competition Using Rice/Barnyardgrass Mixed-Cultures

Plant-plant interference is the combined effect of allelopathy, resource competition, and many other factors. Separating allelopathy from resource competition is almost impossible in natural systems but it is important to evaluate the relative contribution of each of the two mechanisms on plant interference. Research on allelopathy in natural and cultivated plant communities has been hindered in the absence of a reliable method that can separate allelopathic effect from resource competition. In this paper, the interactions between allelopathic rice accession PI312777, non-allelopathic rice accession Lemont and barnyardgrass were explored respectively by using a target (rice)-neighbor (barnyardgrass) mixed-culture in hydroponic system. The relative competitive intensity (RCI), the relative neighbor effect (RNE) and the competitive ratio (CR) were used to quantify the intensity of competition between each of the two different potentially allelopathic rice accessions and barnyardgrass. Use of hydroponic culture system enabled us to exclude any uncontrolled factors that might operate in the soil and we were able to separate allelopathy from resource competition between each rice accession and barnyardgrass. The RCI and RNE values showed that the plant-plant interaction was positive (facilitation) for PI312777 but that was negative (competition) for Lemont and barnyardgrass in rice/barnyardgrass mixed-cultures. The CR values showed that one PI312777 plant was more competitive than 2 barnyardgrass plants. The allelopathic effects of PI312777 were much more intense than the resource competition in rice/barnyardgrass mixed cultures. The reverse was true for Lemont. These results demonstrate that the allelopathic effect of PI312777 was predominant in rice/barnyardgrass mixed-cultures. The most significant result of our study is the discovery of an experimental design, target-neighbor mixed-culture in combination with competition indices, can successfully separate allelopathic effects from competition

The chemical cross talk between rice and barnyardgrass

The chemical cross talk between rice and barnyardgrass which is one of the most noxious weeds in rice cultivation was investigated. Allelopathic activity of rice was increased by the presence of barnyardgrass seedlings or barnyardgrass root exudates. Rice allelochemical, momilactone B, concentration in rice seedlings and momilactone B secretion level from rice were also increased by the presence of barnyardgrass seedlings or barnyardgrass root exudates. As momilactone B possesses strong growth inhibitory activity and acts as an allelochemical, barnyardgrass-induced rice allelopathy may be due to the increased momilactone B secretion. These results suggest that rice may respond to the presence of neighboring barnyardgrass by sensing the chemical components in barnyardgrass root exudates and increase allelopathic activity by elevated production and secretion levels of momilactone B. Thus, rice allelopathy may be one of the inducible defense mechanisms by chemical-mediated plant interaction between rice and barnyardgrass and the induced-allelopathy may provide a competitive advantage for rice through suppression of the growth of barnyardgrass