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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.

Aerobic rice is a water-saving technology in which rice grows in non-puddled and non-saturated (aerobic) soil without ponded water. A gradual decline in rice yield was found in field plots at the farm of the International Rice Research Institute, Los Bañ os, Philippines, where rice has been cultivated continuously for 10 cropping seasons under aerobic rice conditions. We investigated whether abiotic soil factors lead to the observed yield decline. An aerobic rice pot experiment was conducted using field soils from flooded rice plots and from the 10-season-long aerobic rice cultivated plots (referred to as 1st-season and 11th-season aerobic rice, respectively). Subtreatments consisted of soil sterilization by oven heating (at 95°C or higher for 24 h) and a control treatment. The above-ground biomass of 1st-season aerobic rice was significantly greater than that of 11th-season aerobic rice in both the oven-heating and control treatments. Oven heating increased soil N availability and above-ground biomass accumulation over the control in both 1st-season and 11th-season aerobic rice, but the above-ground biomass in the oven-heated 11th-season aerobic rice was still significantly lower than that of the oven-heated and even the untreated (control) 1st-season aerobic rice. These results suggest that abiotic factors contribute to the gradual yield decline observed in the field plots.

In a rice cultivation experiment at the International Rice Research Institute (IRRI), water drainage during the fallow period inhibited the growth of paddy rice, especially during the early stage of subsequent cultivation. This symptom became more prominent when rice straw was not applied, and when the amount of fertilizer applied was small. The application of a mixture of nitrogen, phosphorus, and potassium fertilizers reduced this inhibition. The amounts of phosphorus in the growth-inhibited rice and soil were smaller than those without growth inhibition. The findings suggest that one of the factors that inhibited rice growth was phosphorus deficiency. The inhibition was associated with high soil pH and low Fe(II) content in the problem soil. There was a negative correlation between soil pH and Fe(II) content. Water drainage during fallow decreased the amount of Fe(II) in the soil because Fe(II) was oxidized to Fe(III) under oxidative conditions. During the subsequent cultivation season, the amount of Fe(II) in the soil was low and the soil pH was high. The application of rice straw improved rice growth. The phosphorus deficiency of growth-inhibited rice was associated with a small amount of available phosphorus in soil presumably because of the high soil pH and the adsorptopn of phosphorus to Fe(III) oxides in the soil.