The relative importance of root and shoot competition between water-seededrice and Echinochloa phyllopogon

The relationship between root and shoot growth were determined for water-se eded rice and Echinochloa phyllopogon in greenhouse experiments. When grown in monoculture at four nitrogen rates (0, 60, 120, 180 kg N ha(-1)), root dry weight was highly correlated with canopy structure for both species. Ec hinochloa phyllopogon showed a significantly stronger response to the nitro gen rate than rice. When rice and E. phyllopogon were grown in competition, with roots either separated or allowed to mingle, root competition contrib uted more than shoot competition to reductions in the growth of the target species. The results suggest that root competition may be the primary mecha nism determining competitive outcomes between water-seeded rice and E. phyl lopogon. The importance of root competition and the relationship between ro ot and shoot growth demonstrated in this study suggest that researchers sho uld not rely solely on correlations between shoot traits and competitive ab ility as evidence that competition is primarily for light. Our results also suggest the importance of considering the whole plant when assessing rice cultivars for competitive ability.