| Ferrous iron (Fe2+) toxicity is a complex and major nutritional disorder thatadversely affects the growth and yield of rice grown on inland swamps and irrigated lowland soils, which are widely distributed in tropical and sub-tropical regions. Many researchers have been exploring varieties of approaches, such as genetic tolerance, plant nutrition, and drainage measures, to eliminate the iron toxicity. However, the economically feasible technique is the elimination of iron toxicity through nutrient management, typically, the application of phosphorus and calcium. In present study, solution culture experiments were carried out to characterize the toxic effect of excessive iron on hybrid rice, the indices estimating Fe2+ tolerance of a large population of rice genotypes, and the ameliorating effect of phosphorus and calcium nutrient on iron toxicity, with intensive exploration on their mechanisms. The results obtained were summarized as follows:1. Toxic effect of excessive Fe2+ on riceThe toxic effect of excessive Fe + on hybrid rice was conducted with five Fe2+ levels. High Fe2+ concentration greatly reduced root and shoot dry weight, and chlorophyll content in lower leaves. Excessive Fe2+ also reduced N, P and Mg absorption, with Cu being enhanced. Fe + increased the absorption of Ca and Mn at 50 and 150 mg kg"1 levels, whereas reduced their uptake at Fe2+ levels higher than 150 mg kg"1. Excessive Fe2"1" stress inhibited translocation of P, K, Ca and Mg from roots to shoots, resulting in nutrient distribution imbalance between shoot and root, and this imbalance, in turn, caused severer iron toxicity. Iron toxicity to rice might be ascribed partially to the disturbance of nutrient distribution between shoots and roots.The activities of POD, CAT and NR were significantly decreased under higherFe stress. Under lower Fe2+ stress, however, the activities of POD, CAT and NR increased, which might be regarded as a mechanism of rice resistance to Fe2+ toxicity.2 Genotypic difference of rice tolerance to excessive Fe2+Genotypic difference of rice tolerance to excessive Fe2+ was examined in solution culture over 96 rice genotypes by iron toxicity scores based on leaf symptom intensity,108growth parameters, and nutrient concentration in plants. Considerable difference in rice tolerance to Fe2+ was observed, which were indicated by the great variation in iron toxicity scores and growth parameters. These results also indicated that this population has been highly segregated under Fe2+ toxicity and has a great potential for screening out iron tolerant genotypes for rice breeding. Principal- component- analysis was performed based on toxicity score, growth parameters and nutrient concentrations, 71.8 percentage of the variance in these data was accounted for by principal 1, 2, and 3, namely growth factor, nutrient factor and element antagonistic factor. Cluster analysis suggested that relative plant height, root length, root dry weight, phosphorus, iron, manganese and zinc content were the best parameters for the evaluation of genotypic difference in iron toxicity tolerance.3 Ameliorative effect of phosphorus and calcium on Fc + toxicityThe amelioration of the negative effect of Fe2+ on rice growth by increasing P level in the cultural solution were investigated on the two genotypes: Azucena, the tolerant genotype; IR64, the sensitive genotype. Phosphorus completely eliminated iron toxicity symptoms of tolerant genotype and alleviated 2/3 of the symptom of sensitive genotype, suggesting that the ameliorating effect of P was more pronounced on tolerant genotype than on IR64.Under excessive Fe2+ stress, ameliorating effect of Ca2"1" at 100-300 mg L"1 level was observed on tolerant genotype, Azucena, which was indicated by the increase of shoot and root biomass. Ca2+ concentration at 300 mg L-1 was more appropriate for IR64 in relation to biomass.4 Amelioration of nutrient disorder by phosphorus(l).The investigation on the relationship between iron plaque quantity and iron concentration in pla... |
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