Iron toxicity to rainfed lowland rice in Sri Lanka.
1994
Deturck P.
Iron toxicity is a major constraint to rainfed lowland rice in South-west Sri Lanka. The objectives of this work were (a) to identify the causes of iron toxicity; (b) to make recommendations to improve the rice yields on iron toxic soils; and (c) to gain some insight into the underlying principles of iron tolerance of rainfed lowland rice. An extensive field survey revealed that more than 90 per cent of the iron toxic soils of Sri Lanka belong to the category of non-pyritic mineral iron toxic soils. These soils are characterized by a very low intrinsic fertility status as expressed by: light texture, acidic pH, low CEC, and an acute deficiency of exchangeable bases, especially of potassium. They are reasonably well provided with phosphorus and zinc. In South and South-east Asia, an estimated 6.5 million hectares of rainfed lowland rice are grown on soils that have similar chemical properties. Short-term, practical strategies to improve the yield of rice grown on non-pryritic mineral iron toxic soils have been evaluated in 20 pot and field experiments. A farmer using a rice variety tolerant of iron toxicity and a fertilizer combination of about 60 kg N per ha, 30 kg P per ha, 75 kg K per ha and 850 kg dolomite per ha may expect a yield of 3 to 4 t per ha on non-pryritic mineral iron toxic soils. The present average yield of a rainfed rice in South-west Sri Lanka is 2.3 t per ha. It was shown that rice plants responded to iron toxicity by increasing the proportion of assimilates translocated to the root. The reaction was more pronounced in tolerant varieties. The exact nature of this response mechanism, however, could not be elucidated in this work due to the rapid flow-back of part of the radioactive carbon from the below-ground to the above-ground compartment. As it was not possible to distinguish which part of the 14C recovered in the above-ground compartment was contributed by shoot respiration, root respiration, or microbial respiration, the 14C pulse-labelling technique is not directly applicable to calculate carbon fluxes from shoot to root in lowland rice. An immediate challenge for future reseach on stress-response mechanisms of lowland rice is to overcome these methodological difficulties.
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