Varietal Differences in the Kinetics of Iron Uptake by Excised Rice Roots

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Abstract
Iron uptake by excised rice roots can be described in kinetic terms of active absorption and the formation of ion-carrier complexes. Conventional interpretation indicates two carriers, one of which is responsible for iron absorption at low concentrations, with the second primarily functioning at high iron levels. The maximum absorption of both carriers vanes greatly between varieties, the variety Pebifun having much greater absorptive capacity than either Siam-29 or Paldal. Varietal differences in uptake capacity primarily depend on total carrier concentration, 0.93,0.40, and 0.31 μ moles per g roots of Pebifun, Siam-29, and Paldal respectively. Although the kinetic treatment suggests two carriers, the chromatographic evidence indicates the presence of the iron primarily in one major initial complex, with suggestions of accumulation rather than turnover. An alternative to the carrier hypothesis is therefore put forward which regards the accumulation of iron as binding initially on to one or several closely related substances, followed by incorporation into secondary products when a critical level of the initial product is exceeded. Manganese was found to competitively inhibit iron uptake by rice roots. Inhibition was more severe in Siam-29 than in Paldal and can be explained on the basis of carrier concentration and competition for the same carrier site. Copper was found not to be competitive for the iron absorption sites. The effect of copper was not significant with the low levels of iron but only with the 5 × 10−4 M Fe level. The effect varied between varieties, in Pebifun both 5 × 10−5 and 5 × 10−6 M Cu inhibited iron uptake compared with the control. In the case of Siam-29 both copper levels accelerated iron uptake, 5 × 10−5 M Cu giving the greatest uptake. With 5 × 10−5 M Cu Pebifun took up much less iron than Siam-29. The mecharosm of copper inhibition/stimulation is highly speculative.

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