AMMONIUM ASSIMILATION BY YOUNG PLANTS OF HORDEUM-VULGARE IN LIGHT ANDDARKNESS - EFFECTS ON RESPIRATORY OXYGEN-CONSUMPTION BY ROOTS

Barley plants (Hordeum vulgare L.) grown for 10 d in nitrogen-free hyd roponic culture, after a rapid initial phase absorbed supplied NH4+ at a constant rate of 15.1 +/- 1.2 mu mol h(-1) g(-1) f. wt in the light , and at a rate of 13.81 +/- 1.6 mu mol h(-1) g(-1) f. wt in darkness. Ammonium-grown plants assimilated NH4+ at a rate of 7.5 +/- 0.33 mu m ol h(-1) g(-1) f. wt and at a 50% lower rate in darkness. Nitrogen-fre e grown plants showed low concentrations of free amino acids in both r oot and shoot tissues. Supplying NH4+ caused an immediate increase in the concentration of glutamine in the root tissues of both illuminated and darkened plants over a 120 min period. The increase in concentrat ion of glutamine then exhibited a lag period of 120 min, after which i t resumed, but to a very small extent. Glutamine also accumulated in s hoot tissue of illuminated plants at increasing rates, attaining a con centration which, 8 h after NH4+ supply, was 1.61-fold greater than th at attained in the roots. In shoots of darkened plants, by contrast, t he concentration of glutamine increased slowly and was always smaller than that in the root tissue. Overall formation of glutamine (in shoot s and roots) occurred at decreasing rates during the first 4 h, and th en at increasing rates. The increase was more pronounced in illuminate d plants than in darkened plants. Even 24 h after NH4+ was supplied, g lutamine content in root tissue was lower than that in shoot tissue. H owever, 48 h later, the concentrations of glutamine in root and shoot were similar, attaining values that were almost 47-fold (in root) and 134-fold (in shoot) greater than initial values. Significant levels of asparagine were detected in the root and in the shoot 24 h after addi ng NH4+. These increased further during the succeeding period. Ammoniu m supply caused a transitory drop in the concentration of ATP in root tissue, along with noticeable transitory variations in glucose-6-P con centration. A permanent decrease in free glucose concentration was als o detected. Addition of NH4(+) caused 2- and 1.43-fold increases in re spiratory oxygen consumption by roots of illuminated and darkened plan ts, respectively. Both in the light and in the dark, the root tissue a ccumulated methylammonium up to a concentration of 55-67 mu mol h(-1) g(-1) f. wt. Methylammonium was never found in shoot tissue of either illuminated or darkened plants. Methylammonium stimulated respiration of root barley plants by a factor of 1.2. Regulatory aspects of NH4+ m etabolism are discussed.