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Understanding genotypic difference in ion uptake and toxicity is important in breeding plants that are able to adapt to unfavorable soil conditions and to phytoremediate contaminated soils. This investigation was carried out to study kinetic behavior of manganese (Mn) uptake by excised roots of two tobacco (Nicotiana tabacum L.) genotypes with different sensitivities to Mn toxicity. Genotype KY 14 was shown previously to be sensitive to Mn toxicity whereas genotype Tobacco Introduction (T.I.) 1112 was shown to be resistant. Kinetic experiments were conducted by exposing the excised roots of the two genotypes to solution Mn containing 1 mM Mn plus 0.5 mM calcium (Ca). Total Mn uptake and the fraction of Mn that remained in the excised roots after desorption using 2 mM Cu–0.5 mM Ca solution at 5°C were determined to estimate Mn accumulation into the apoplast and symplast compartments. The two tobacco genotypes exhibited a similar biphasic total Mn uptake composed of an initial rapid phase followed by a slow linear phase for the first 180 min. The sensitive tobacco genotype, KY 14, maintained a similar kinetic behavior of total Mn uptake throughout the experiment whereas the tolerant genotype, T.I. 1112, showed a decrease in total Mn uptake after 180 min of exposure. This decrease was mainly due to an exclusion of Mn from the apoplast of T.I. 1112 although its symplastic Mn uptake continued. Comparison of the relative distribution of Mn between the two genotypes indicated a much higher percentage of Mn uptake in the apoplast of the tolerant genotype than that in the sensitive genotype. Overall, total Mn uptake difference by the two genotypes was dominated by the difference in apoplastic absorption characteristics. This apoplastic difference of the two tobacco genotypes appeared to be related to Mn-cell wall binding characteristics. [ABSTRACT FROM AUTHOR]