Nitrogen absorption and assimilation in NaCl-resistant and NaCl-susceptible millet genotypes (Pennisetum americanum)

Nitrogen absorption and assimilation were compared in salt-resistant and salt-susceptible genotypes of millet (Pennisetum americanum L. Leeke) in the absence of NaCl (control) and under saline conditions (300 mM NaCl). At 300 mM NaCl, Na+ and Cl− contents of leaves were lower in the resistant than in the susceptible genotypes and K+ content decreased with respect to the control in both genotypes. In the susceptible genotypes, the uptake of both and was inhibited by NaCl, while in the resistant genotypes only uptake was inhibited. The activities of several enzymes of nitrogen assimilation were also related to salinity tolerance: in vivo nitrate reductase and NADH- and NADPH-dependent glutamate dehydrogenase increased; in vitro nitrate reductase and the rate of [14C]leucine incorporation decreased. Only the glutamine pathway enzymes, glutamine synthetase and glutamate synthase, gave different responses to salinity in relation to the genotype: both enzyme activities were higher in the resistant and lower in the susceptible genotypes. These results suggest that the shift of nitrogen assimilation toward the glutamine route is the main feature of the salt adaptation in millet.