EFFECT OF HIGH NITRATE CONCENTRATIONS ON DICARBOXYLATE TRANSPORT ACROSS THE PERIBACTEROID MEMBRANE OF SOYBEAN ROOT-NODULES

Uptake studies were carried out with intact peribacteroid units isolat ed from nodules of soybean plants (Glycine max L. cv. Hodgson) inocula ted with Bradyrhizobium japonicum strain 273 and treated for 48 h with high concentrations of nitrate. Malate penetrated the peribacteroid m embrane most rapidly when the plants were affected by 0.390 mM nitrate . The 2 mM nitrate treatment had a slightly negative effect on the mal ate uptake, resulting in a 12 % decrease in the dicarboxylate transpor t across the peribacteroid membrane, as compared with the controls. Ma late transport across the peribacteroid membrane decreased by 55 % aft er 7.5 mM nitrate treatment and by 85 % after 15 mM nitrate treatment. At higher nitrate levels poly-beta-hydroxybutyrate synthesis was incr eased. These data support the suggestion that in these conditions mala te is funneled to poly-beta-hydroxybutyrate synthesis, which together with the reduced malate uptake by peribacteroid units could explain th e inhibited nitrogen fixation.