Abstract
To look into the microevolution of weed populations in copper-polluted rice fields, we have been engaged in observations of weed communities. This paper deals with the results so far obtained in the barnyard grass (Echinochloa oryzicola, 4X and E. crusgalli, 6X). A total of 210 barnyard grass strains (progenies of single plants) collected from copper-polluted (25-310ppm) and control (1-18ppm) fields were tested for copper tolerance and other characters. Copper tolerance was evaluated by the ratio of performance in copper-treated plot to that in control (no copper) plot, mainly in gravel culture and partly in water culture. The 4X species tended to be more tolerant than the 6X species. It was also found that 6X strains resembling the 4X species morphologically had higher tolerance than typical 6X strains. In both species, the populations in polluted fields showed a higher frequency of tolerant plants or a higher mean tolerance than those in control fields. But the populations were polymorphic not only morphologically but also in copper tolerance. There were tolerant plants in normal fields and sensitive plants in polluted fields. The tolerant plants were generally small in the size of various organs and inferior in performance when tested in normal conditions. They were also inferior in competitive ability. The tolerant strains showed a lower copper content in the leaves and stems than normal strains in the copper-treated plot of gravel culture. The data were largely comparable with those reported by British workers in perennial grass species growing on strongly polluted sites, though the level of copper pollution in Japanese rice fields was much lower.
