Magnesium uptake by Al-stressed maize plants with special emphasis on cation interactions at root exchange sites

As aluminium (Al) severely inhibits magnesium (Mg) uptake by many plant species, Mg uptake and Mg-Al interactions in maize (Zea mays L.) were studied in a series of short and long-term experiments. A relationship between Mg uptake and the degree of Mg saturation of exchange or binding sites of the root apoplast (root-CEC) was studied by growing plants in solutions containing: (i) different concentrations of Al, calcium (Ca) and hydrogen (H) ions; and (ii) a number of organic complexes of Al. In short-term experiments, Ca had little effect on the Mg nutrition of maize plants. However, with increasing concentrations of Al and H ions in nutrient solution, there was a decrease in both the degree of Mg saturation of root-CEC and Mg uptake. Effects of pH on cation (H, Al, Mg, Ca) binding at the root apoplasm were pronounced and complicated because of a simultaneous change of H ion concentration, effective root-CEC and Al speciation. The behaviour of Al as organic Al complexes differed from that supplied as aluminium chloride (AlCl₃). In the presence of organo-Al complexes, less Mg was replaced from apoplastic binding sites and Mg uptake was inhibited less severely than with AlCl₃. In a long-term experiment, Al-citrate, in contrast with AlCl₃, was not phytotoxic to maize, expressed by the lack of any inhibition of shoot biomass production.