Roots of manganese-tolerantTriticum aestivum(cv. Norquay), grown in solution culture with 500ndash;1000ensp;mu;M manganese, showed brown discolouration, reflecting precipitation of manganese on root surfaces. Under similar conditions, roots of manganese-sensitive 'Columbus' lacked a precipitate and remained white. Precipitation of manganese on root surfaces was correlated with a plant-induced rise in pH of culture solutions above 5.5. Increasing ammonium in solution from 0.3 to 0.6ensp;mM prevented the rise in pH, and precipitation of manganese was not observed. Dry weights of roots and leaves of 'Norquay' were higher than those of 'Columbus' over a range of concentrations of manganese (200ndash;1000ensp;mu;M), regardless of amount of ammonium supplied. With 0.3ensp;mM ammonium in solution, roots of 'Norquay' accumulated approximately twofold higher concentrations of manganese than roots of 'Columbus', whereas concentrations of manganese in leaves were similar. With 0.6ensp;mM ammonium in solution, concentrations of manganese associated with roots decreased and concentrations of manganese in leaves increased. Nonetheless, roots of 'Norquay' maintained twofold higher concentrations of manganese than roots of 'Columbus', and differences for leaves remained small. In contrast with previous reports, brown discolouration of roots ofT.aestivumis not necessarily a symptom of manganese toxicity. Precipitation of manganese on root surfaces was related to pH of solutions, could be reduced by the addition of ammonium, and did not affect tolerance of manganese for the cultivars studied.
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