Independence of differential aluminum tolerance inlotuson changes in rhizosphere pH or excretion of organic ligands

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Abstract
Detoxification of aluminum (Al) in the rhizosphere has been proposed as a means whereby some plant species are able to tolerate soluble Al. Two mechanisms have been suggested, viz. increase in rhizosphere pH, and excretion of organic ligands into the rhizosphere. A solution culture experiment was conducted to evaluate these mechanisms as possible reasons for the differential tolerance of two Lotus spp., L. pedunculatus cv. Grasslands Maku (Al‐tolerant) and L. corniculatus cv. Maitland (Al‐sensitive). Two solution pH treatments (pH adjusted daily to 4.7 or pH not adjusted, declining to c. 4.0 after 26 days) and two Al treatments (0 or 10 μM monomeric Al) were imposed. The two species showed a marked difference in response to Al. After 26 days’ growth, the total dry matter yield of Grasslands Maku increased by 27% at 10 μM Al, while that of Maitland decreased by 40%. Tissue analysis indicated that Al toxicity was not associated with deficiencies or toxicities of essential nutrients. Higher concentrations of monovalent cations were present in Grasslands Maku tops and roots, while Maitland contained higher concentrations of divalent cations. The two species did not differ in their effect on solution pH, the amount of Al added to maintain 10 μM monomeric Al in solution, or on the quantity of organically‐bound Al in solution at the end of the experiment. This suggested that detoxification of Al in the rhizosphere (either by change in solution pH or by excretion of organic ligands) was not a reason for differential Al tolerance in these two species.