| Iron plaque is a special structure for rice on the root surface in order to adapt the flooding environment. The particular electrochemical properties and-OH functional groups of iron plaque affect the bioavailability of cation and anion in the rhizosphere, by the means of adsorption-desorption, oxidation-reduction, organic-inorganic complex ways. In present study, Feiyouduoxi No.1 was selected as an Al-resistant rice cultivar and Hongliangyou 166 was selected as an Al-sensitive cultivar. Solution culture experiments were carried out to characterize the effects of iron plaques amount and the way of iron plaques formation on aluminum(Al) uptake and translocation by rice seedlings, the effects of iron plaque on A1 uptake characteristics and the regulation role of iron plaque and phosphorus on Al uptake and accumulation, with intensive exploration on their mechanisms. The results obtained were summarized as follows:(1) The effect of iron plaques amount on Al uptake and translocation by rice seedlings was studied by supplying 0,10,20,30 and 40 mg/L Fe2+. The result showed that iron plaque amount increased with increasing concentrations of Fe2+in the nutrient solutions. When 40mg/L Fe2+was pretreated, the iron plaque amount of Feiyouduoxi No.1 and Hongliangyou 166 increased 274.4%and 258.6%of the control, respectively. After Al treatment, both rice cultivars reached lowest roots and shoots biomass and highest Al content with 20 mg/L Fe2+. While Al content reached the minimum at 30 and 40mg/L of Fe2+treatment. The Al content and accumulation showed the following rank order:root surface with iron plaques> root> shoot, increased with increasing Fe2+concentrations. So the regulation role of iron plaque on Al uptake by rice seedlings was involved with iron plaques amount. Iron plaques promoted Al uptake when iron plaques amount reached 28-33mg/g at 20mg/L Fe2+in the nutrient solutions, while. prevented roots from absorbing Al and alleviating Al toxicity when iron plaques amount reached 36-42mg/g at 30mg/L and 40 mg/L Fe2+ in the nutrient solutions.(2) The effects of iron plaque and P on Al uptake kinetics was investigated with excised roots of rice seedlings. The results showed that iron plaque removal or not, Al uptake of the two rice genotype roots followed Michaelis-Menten kinetics. The roots Al uptake reached saturation rapidly at lower than 50μmol/L Al when the roots was covered iron plaques, while Al uptake increased with increasing concentrations of A13+in the nutrient solutions with iron plaque removal. At a series of Al concentrations treatment, the Al content and Imax for Hongliangyou 166 roots in the presence of iron plaques significantly decreased, while didn't show the effect for Feiyouduoxi No.1. The Km of both rice cultivars in the presence of iron plaque largely decresed under 0.5mmol/L P pre-treatment. Roots Al uptake reached saturation at 10μmol/L Al when the roots was covered iron plaques. At a series of Al concentrations treatment, the P adsorped on the iron plaque significantly decreased Al content and Imax for Feiyouduoxi No.l, while P adsorped on the iron plaque didn't have a significantly effect on Al uptake for Hongliangyou 166. So iron plaque prevented Al from influxing into rice roots for Al-sensitive cultivar, while the P adsorped on the iron plaque could alleviate Al toxicity of Al-tolerant cultivar.(3) The effect of iron plaque on Al uptake and translocation by rice seedings was studied through low-P induced root surface iron plaque. The results showed that root elongation of the two rice genotypes and the biomass of Hongliangyou 166 decreased under low-P or high-Al treatment, and the effect was more obivious by co-processing of the low-P and high-Al. The root surface iron plaque amount of low-P plants is significantly higher than high-P plants, but the P content of root surface iron plaque, roots and shoots of low-P plants were all largely lower than high-P plants. The low-P plants roots of two genotypes accumulated a large number of Al, and roots Al accumulation reached to 48.8-54.0%, while the root surface iron plaque of high-P plants accumulated a large number of Al, and roots Al accumulation reached to 46.4-52.8%. Furthermore, root rips Fe and Al content of Feiyouduoxi No.l were obviously lower than Hongliangyou 166. It revealed that iron plaque induced by low-P can not prevent the roots Al uptake. The pre-saturation of Fe in Al tolerant genotype root tips played a role in relieving root rips Al toxicity.(4) The effect of iron plaque and P on Al uptake and translocation by rice seedings was investigated with external P and iron plaque induced by low-P. The results showed after Al treatment, root, shoot elongation of Feiyouduoxi No.1 and Hongliangyou 166 decreased 541.9,555.4% and 63.6,186.0% of the control, respectively; roots and shoots Al content increased 886.2,429.5% and 1091.9, 1644.0%, respectively; Fe and P content of different parts of rice plants were not obvious changes. After the formation of iron plaque, the iron plaque amount of both rice cultivars is 11.6 and 19.3 times of roots Fe amount; Compared to Al treatment, Al content on root surface increased 937.7 and 1038.5%, roots and shoots Al content decreased 46.3 and 97.7%. Compared to iron plaque treatment, root and shoot elongation, biomass, Al content on root surface, Fe and P content of different parts of rice plants all increased significantly by co-processing of iron plaque and P. Roots and shoots Al content decreased significantly. The variation trend of Pectin, hemicelluloses 1 and hemicelluloses 2 content in root tips cell wall was consistent with the result of roots and shoots Al content. So iron plaque induced external Fe2+ prevented Al from influxing into rice roots by iron plaque adsorbing Al. After the fomation of iron plaque, the higher the external P, the stronger the double P immobilization effect of iron palque and P, the stronger the alleviating action of roots Al toxicity.
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