NH4-CONSUMPTION( ASSIMILATION BY ROOTS OF YOUNG BARLEY PLANTS, CHANGES IN POOL OF FREE GLUTAMINE AND ASPARAGINE AND RESPIRATORY OXYGEN)

Barley plants (Hordeum vulgare L.) grown for 10-d in nitrogen-free hyd roponic culture with a 16 h photoperiod, absorbed NH4+ added at a fina l 2 mM concentration, at a constant rate of 15.1 +/- 1.2 mu mol h(-1) g(-1) FW root in the light periods, and at a rate of 13.8 +/- 1.6 mu m ol h(-1) g(-1) FW root in the dark periods. Two days after NH4+ was su pplied, barley plants absorbed NH4+ at a rate of 7.5 +/- 0.33 and 3.5 +/- 0.43 mu mol h(-1) g(-1) FW root in the light and in the darkness, respectively. N-free grown plants showed low concentrations of free am ino acids in both root and shoot tissues. On the 10th day, NH4+ suppli ed 6 h after the light period began, caused a continuous increase in t he glutamine levels within root tissues, and 2 h later also in shoot t issues. Eight hours after NH4+ supply, however, the shoots of illumina ted plants had 150% higher glutamine than in roots, whereas in the sho ots of plants transferred to the dark glutamine was only 80%. Twenty f our hours after NH4+ addition, with the same photoperiod, glutamine co ntent was 9.3 +/- 0.6 mu mol g(-1) FW in roots and 24.7 +/- 3.8 mu mol g(-1) FW in shoots, but after 48 h, similar concentrations of 28.3 +/ - 3.1 mu mol g(-1) FW and 26.8 +/- 4.0 mu mol g(-1) FW were found in r oots and shoots, respectively. Significant levels of asparagine were d etected 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 mu mol g(-1) FW i n the root and in shoot, respectively. Root tissue accumulated methyla mmonium at a rate of 8.55 mu mol h(-1) g(-1) FW root, which was 56% th at of NH4+ absorption. Methylammonium was never found in shoot tissues of either illuminated or darkened plants. Addition of NH4+ to 10-d-ol d N-free grown plants caused a sudden permanent 165-168% increase in r espiratory oxygen consumption by roots of darkened and illuminated pla nts; methylammonium stimulated respiration by 120%. NH4+ added 2 h aft er methylammonium stimulated respiration by 145%. In the presence of N H4+ N-repleted plants respired at an higher rate after several hours o f light, than in darkness or in the hours of light immediately followi ng a dark period.