Effect of High Levels of Ammonium or Nitrate on Growth and Nitrogen Metabolism in Roots and Leaves of Sorghum (Sorghum sudangrass) Plants

Sorghum (Sorghum sudangrass) plants were treated with 5, 20 and 50 mM of Nitrogen (Nitrate or Ammonium). Growth parameters (length, dry weight and fresh weight), biochemistry parameters (proteins, amino acids and chlorophyll content) and activities of enzymes involved in nitrogen metabolism such as: Nitrate reductase (NR), Glutamine synthetase (GS) and glutamate dehydrogenase (GDH) were studied in the leaves and roots after 20 days of N exposure. Results showed that sorghum plants exhibited enhanced biomass production under high levels of inorganic nitrogen supply. Biochemistry parameters increased with increasing nitrogen concentration in roots and leaves especially in ammonium-treated plants. Exogenous nitrate but not ammonium induced significant difference in the leaves and roots nitrate reductase activity when compared with control plants (0.5 mM nitrate). While Glutamine synthetase 1 activity and protein accumulated in roots at increasing concentration of either nitrate or ammonium, particularly in ammonium-treated plants, glutamine synthetase 2 protein was more accumulated in leaves of nitrate-treated plants. Glutamate dehydrogenase activity was enhanced in both organs by increasing N source in the culture medium this increase is more pronounced in presence of ammonium. This data showed that sorghum plants exhibited higher adaptive potential under excessive concentration of ammonium by ability to detoxify this ion via nitrogen assimilation in roots.