No abstract

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 on 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.Discipline: Soil, fertilizers and plant nutrition Additional key words: Fe-tolerance, Fe-toxicity, paddy soil, root JARQ 42 (3), 187 -192 (2008) http://www.jircas.affrc.go.jp This study was supported by the IRRI-Japan collaborative research project phase IV (1999IV ( -2004.

The objective of this study was to characterize the root systems of the new plant type (NPT) of tropical rice (Oryza sative (L)) lines compared with the root system of IR72, one of the elite modern varieties. Two field experiments were conducted to observe plant growth under canopy conditions in the 1997 and 1998 wet seasons. In addition, a pot experiment was conducted in the 1998 dry season to observe plant growth under isolated conditions. The root systems of the NPT lines were characterized by roots heavier than those of IR72. The larger root dry weight (DW) is attributed partly to a genetically determined high root/shoot ratio because, irrespective of growth conditions, the ratio was larger in the NPT lines than in IR72. An increase in root DW was accompanied by an increase in the length of the whole root system, including fine roots. However, despite the differences between the NPT lines and IR72 in root DW and root length, there were no significant differences in the amount of nitrogen (N) absorbed throughout the growth period. Therefore, the specific absorption rate of N per root DW (SAR N , mg N g −1 root DW d −1 ) was lower in the NPT lines than in IR72. We concluded that, although the root systems of the NPT lines grew prolifically, they were not necessarily more efficient in terms of N absorption compared with the root system of IR72.

Under laboratory conditions, submerged paddy soil with the addition of propionate was incubated anaerobically. Propionate oxidation was suppressed by the addition of H2, and the specific inhibitor for methanogenesis (2-bromoethanesulfonate; BES). The inhibition with H2 can be associated to the negative feedback of endergonic propionate oxidation with H2. The BES inhibition on propionate oxidation can be attributed to the uncoupling of propionate oxidation and methanogenesis. When sulfate was added, propionate oxidation was not suppressed by the addition of H2, but it was suppressed by the addition of the specific inhibitor for sulfate-reduction (molybdate). These findings suggest the occurrence of preferential coupling of propionate oxidation with sulfate reduction. During the oxidation of propionate after the addition of sulfate, acetate hardly accumulated in the absence of BES, but accumulated in the presence of BES. Propionate oxidation proceeded irrespective of the addition of BES. These findings indicate that the decomposition of acetate, which was produced from propionate, was associated with methanogenesis even in the presence of sulfate, and that electrons flowed through methanogenesis and sulfate-reduction without competition as long as propionate was present.