Altered aluminum inhibition of soybean root elongation in the presence of magnesium

Variations in genotype rankings among screenings for Al tolerance in hydrop onics may be related to differences in the composition of the solutions. In the present study, we investigated the involvement of Mg ions in modifying Al rhizotoxicity in soybeans. Root elongation was strongly inhibited by Al in a simple, 800 muM CaSO4 solution, but elongation increased noticeably w hen the solutions also contained Mg. Amelioration of Al rhizotoxicity was n ot associated with an increase in ionic strength of treatment solutions bec ause Al3+ activities were kept constant. Concentration series experiments i ndicated that the Mg effect occurred in the muM range, while Ca amelioratio n of Al toxicity occurred at mM concentrations. The positive effect of Mg o n root elongation was greatest for Al-sensitive genotypes and minimized gen otypic differences for Al-tolerance. The Mg protection against Al rhizotoxi city apparently does not occur with all species, because it was not observe d in Atlas and Scout 66 wheat varieties. The ability of Mg to ameliorate Al toxicity in soybean at muM levels suggests the involvement of distinct phy siological factors.