Role of glutamate dehydrogenase and phosphoenolpyruvate carboxylase activity in ammonium nutrition tolerance in roots

Spinach (Spinacea oleracea L. “Correnta F1”) and pea (Pisum sativum L. “Macrocarpon”) plants were grown in a hydroponic culture with nitrate (5 mM), or ammonium (5 mM) as the nitrogen source. Dry matter accumulation declined dramatically in spinach plants fed with ammonium, whereas there was no change in pea plants when compared with nitrate-fed plants. Data obtained from δ15N, the organic nitrogen content, N-assimilation enzyme activity, glutamine synthetase (L-glutamate:ammonia-ligase; EC 6.3.1.2), glutamate dehydrogenase (L-glutamate:NAD+-oxidoreductase; EC 1.4.1.2) and enzymes from the tricarboxylic acid cycle suggest that ammonium incorporation into organic nitrogen is localized in the roots in pea plants and in the shoots in spinach plants. Distribution of incorporated ammonium (in shoots and roots) may determine ammonium tolerance. Our results show that unlike in spinach plants, in pea plants, an ammonium-tolerant species, GDH enzyme plays an important role in ammonium detoxification by its incorporation into amino acids. Furthermore, phosphoenolpyruvate carboxylase (phosphate:oxaloacetate-carboxy-lyase; EC 4.1.1.31) and pyruvate kinase (ATP:pyruvate-2-O-phosphotransferase; EC 2.7.1.40) activities reflect a major flow of carbon for ammonium assimilation through oxalacetate in pea plants and through pyruvate in spinach plants. The differences in the sensitivity to ammonium between the species are discussed in terms of differences in the site of ammonium assimilation as well as in the nitrogen assimilation ways.