NH4/+ assimilation by roots of young barley plants, changes in pool of free glutamine and asparagine and respiratory oxygen consumption

Barley plants (Hordeum vulgare L.) grown for 10-d in nitrogen-free hydroponic culture with a 16 h photoperiod, absorbed NH4/+, added at a final 2 mM concentration, at a constant rate of 15.1 ± 1.2 μnol h-1 g-1 FW root in the light periods, and at a rate of 13.8 ± 1.6 μmol h-1 g-1 FW root in the dark periods. Two days after NH4/+ was supplied, barley plants absorbed NH4/+ at a rate of 7.5 ± 0.33 and 3.5 ± 0.43 μmol h-1 g-1 FW root in the light and in the darkness, respectively. N-Free grown plants showed low concentrations of free amino acids in both toot and shoot tissues. On the 10th day, NH4/+ supplied 6 h after the light period began, caused a continuous increase in the glutamine levels within root tissues, and 2 h later also in shoot tissues. Eight hours after NH4/+ supply, however, the shoots of illuminated plants had 150% higher glutamine than in roots, whereas in the shoots of plants transferred to the dark glutamine was only 80%. Twenty four hours after NH4/+ addition, with the same photoperiod, glutamine content was 9.3 ± 0.6 μmol g-1 FW in roots and 24.7 ± 3.8 μmol g-1 FW in shoots, but after 48 h, similar concentrations of 28.3 ± 3.1 μmol g-1 FW and 26.8 ± 4.0 μmol g-1 FW were found in roots and shoots, respectively. Significant levels of asparagine were detected in the root and shoot 24 h after adding NH4/+, which, after 48 h, increased further to 12.1 ± 1.3 and 6.8 ± 0.9 μmol g-1 FW in the root and in shoot, respectively. Root tissue accumulated methylammonium at a rate of 8.55 μmol h-1 g-1 FW root, which was 56% that of NH4/+ absorption. Methylammonium was never found in shoot tissues of either illuminated or darkened plants. Addition of NH4/+ to 10-d-old N-free grown plants caused a sudden permanent 165-168% increase in respiratory oxygen consumption by roots of darkened and illuminated plants: methylammonium stimulated respiration by 120%. NH4/+ added 2 h after methylammonium stimulated respiration by 145%. In the presence of NH4/+ N-repleted plants respired at an higher rate after several hours of light, than in darkness or in the hours of light immediately following a dark period.