Ammonium assimilation by young plants of Hordeum vulgare in light and darkness: effects on respiratory oxygen consumption by roots

Barley plants (Hordeum vulgare L.) grown for 10 d in nitrogen-free hydroponic 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 mol h(-1) g(-1) f. wt and at a 50% lower rate in darkness. Nitrogen-free grown plants showed low concentrations of free amino acids in both root 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 concentration of glutamine then exhibited a lag period of 120 min, after which it resumed, but to a very small extent. Glutamine also accumulated in shoot tissue of illuminated plants at increasing rates, attaining a concentration which, 8 h after NH4+ supply, was 1.61-fold greater than that attained in the roots. In shoots of darkened plants, by contrast, the concentration of glutamine increased slowly and was always smaller than that in the root tissue. Overall formation of glutamine (in shoots and roots) occurred at decreasing rates during the first 4 h, and then at increasing rates. The increase was more pronounced in illuminated plants than in darkened plants. Even 24 h after NH4+ was supplied, glutamine content in root tissue was lower than that in shoot tissue. However, 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 adding NH4+. These increased further during the succeeding period. Ammonium supply caused a transitory drop in the concentration of ATP in root tissue, along with noticeable transitory variations in glucose-6-P concentration. A permanent decrease in free glucose concentration was also detected. Addition of NH4(+) caused 2- and 1.43-fold increases in respiratory oxygen consumption by roots of illuminated and darkened plants, respectively. Both in the light and in the dark, the root tissue accumulated 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+ metabolism are discussed.