MAGNESIUM UPTAKE BY AL-STRESSED MAIZE PLANTS WITH SPECIAL EMPHASIS ONCATION INTERACTIONS AT ROOT EXCHANGE SITES

As aluminium (Al) severely inhibits magnesium (Mg) uptake by many plan t species, Mg uptake and Mg-Al interactions in maize (Zea mays L.) wer e studied in a series of short and long-term experiments. A relationsh ip between Mg uptake and the degree of Mg saturation of exchange or bi nding sites of the root apoplast (root-CEC) was studied by growing pla nts in solutions containing: (i) different concentrations of Al, calci um (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 nutri tion of maize plants. However, with increasing concentrations of Al an d 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 compli cated because of a simultaneous change of H ion concentration, effecti ve root-CEC and Al speciation. The behaviour of Al as organic Al compl exes 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 AlC l3. In a long-term experiment, Al-citrate, in contrast with AlCl3, was not phytotoxic to maize, expressed by the lack of any inhibition of s hoot biomass production.