Nitrate assimilation: Influence of nitrogen supply

Abstract

We have studied the effect of nitrogen supply on growth as well as nitrate assimilation in Lotus japonicus plants. High nitrate concentrations (higher than 20 mM) were toxic for L japonicus, inhibiting growth, with the optimum nitrate concentration ranging between 4 and 8 mM. The maximum growth and levels of nitrate reductase (NR) and nitrite reductase (NiR) activity as well as nitrate accumulation was obtained in plants grown simultaneously on nitrate plus ammonium. L japonicus could also grow on ammonium as the sole nitrogen source, without a supplement of organic acids like succinate. Low external nitrate concentrations induced a higher growth of roots, while higher external nitrate concentration inhibited the growth of roots and favoured shoots growth. NR and NiR activities were always higher in roots than in shoots of L japonicus plants. An increase in external nitrate concentrations did not alter significantly the plant partitioning of nitrate assimilation among roots and shoots, being predominantly higher in the roots than in stems and leaves. The kinetics of NR activity induction by nitrate showed a different pattern in roots and in leaves, being this induction in roots independent on external nitrate concentration in the short-term. In contrast, the induction kinetics of NiR activity and nitrate accumulation showed the same pattern in roots and leaves, being dependent on the external nitrate concentration. In all cases the levels of NR and NiR activity and nitrate accumulation were higher in roots than in leaves. Significant levels of NR protein but no NR activity were detected in NH 4 + -grown plants.