Aluminum tolerance is independent of rhizosphere pH intriticum aestivmL.

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Abstract
Cultivars of Triticum aestivum (wheat) differ in their ability to grow under conditions of aluminum (Al) stress, a phenomenon aptly called differential tolerance to Al. The physiological basis of differential tolerance to Al may be the result of differences among cultivars in patterns of nitrogen (N) assimilation. If cultivars differ in their relative preference for NH4 + and NO3 ‐ in mixed N substrates, this could affect plant‐induced pH of the growth solution, the solubility and speciation of Al and, hence, the expression of Al tolerance. To further test this hypothesis, an Al‐tolerant cultivar (Atlas‐66) and an Al‐sensitive cultivar (Scout‐66) were grown in solution cultures where the relative supply of NH4 + and NO3 ‐ was varied. Plant‐induced pH changes in the growth solutions varied among N treatments and between cultivars. As the supply of NH4 + was increased, both cultivars induced a more acidic pH in the growth solution. Despite differences in plant‐induced pH among treatments, the relative root weight (RRW) of Atlas‐66 and Scout‐66 were relatively unaffected by N supply. Thus the superior Al tolerance of Atlas‐66 could not be explained solely by its ability to maintain a high, rhizosphere pH in mixed N solutions.