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

The relationship between root and shoot growth were determined for water-seeded 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. Echinochloa phyllopogon showed a significantly stronger response to the nitrogen rate than rice. When rice and E. phyllopogon were grown in competition, with roots either separated or allowed to mingle, root competition contributed more than shoot competition to reductions in the growth of the target species. The results suggest that root competition may be the primary mechanism determining competitive outcomes between water-seeded rice and E. phyllopogon . The importance of root competition and the relationship between root and shoot growth demonstrated in this study suggest that researchers should not rely solely on correlations between shoot traits and competitive ability 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.