Characteristics of Iron Tolerant Rice Lines Developed at IRRI under Field Conditions

In the 2003 wet season, IR64 (check variety) and four lines of rice (Oryza sativa L.) developed at IRRI were cultivated in a field with iron (Fe) toxicity at Iloilo City, Philippines, and also under normal soil conditions at IRRI farm. Two of the lines used in the experiments were the near isogenic lines (NILs) of IR64, selected as Fe-tolerant lines in solution Culture in the greenhouse. The other two lines were elite breeding lines that were selected as Fe-tolerant lines in the Field trial at Iloilo. The tolerances of NILs were not expressed at the Iloilo field site because the yield reductions due to Fe-toxicity were similar to or larger than those for IR64. The yield reduction of elite breeding lines was smaller than that of IR64, indicating that the tolerance of elite breeding lines was expressed at the Iloilo field. At Iloilo, the shoot dry weights of IR64 and its NILs hardly increased after 73 days Of Cultivation. On the contrary, those of elite breeding lines kept oil increasing. While the root dry weight of IR64 and its NILs decreased abruptly after 50 days of growth, those of elite breeding lines remained constant or decreased gradually. This finding suggests that one Of the factors that Suppressed the growth of IR64 and its NILs during the late stage was early root senescence. Since the tolerance of elite breeding lines was associated with the maintenance of root activity during, the late stage of growth, the long Maturity period of elite breeding lines presumably helped to alleviate the toxicity.