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

Abstract

SUMMARYBarley plants {Hordeum vulgare L.) grown for 10 d in nitrogen-free hydroponic culture, after a rapid initial phase ahsorbed supplied NH,' ai a constant rate of 131 +1'2//mol h"^ g"'f. wt in the light, and at a rate of 13-81 ± 1-6/imol h"'g"'f. wt in darkness. Atnmonium-grown plants assimilated NH^' at a raie of 7-5 ±0'33//.mo! h"'g"'f. wt and at a 5O'\, lower rate in darkness. Nitrogen-free grown plants showed low concentrations of free amino acids in both root and shoot tissues. Supplying NH^" caused an imtnediate increase in the concentration of glutamine in the root tissues of both illuminated and darkened plants over a 120 tnm period. The increase in concentration of glutaminc then exhibited a lap period of 120 min, after wbich 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 XH^* supply, was 1'61-foId 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 sboots 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 ^^H^' was supplied, glutamine content in root tissue was lower than that in sboot tissue. However, 48 h later, tbe 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 NH/, 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 \'ariations in glucose-6-P concentration. A permanent decrease in free glucose concentration was also detected. Addition of NH/ caused 2-and ]-43-fold increases in respiratory oxygen consumption by roots of ijluminated and darkened plants, respectively. Both in the light and in the dark, the root tissue accumulated methylammonium up to a concentration of 55-67 //mol h'' g"^ 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 NHj' metabolism are discussed.

Appropriate NH4 +: NO3 − ratio improves low light tolerance of mini Chinese cabbage seedlings

“…NH 4 + : NO 3 − (10:90) application alleviated the negative effect of LL on the activities of these Calvin cycle enzymes. As demonstrated by Rigano et al [44], ammonium assimilation required photosynthates imported through the phloem, causing a transitory decrease in the concentration of ATP, along with noticeable variations in glucose-6 -Pconcentration, a permanent decrease in free glucose concentration, an increase in respiratory oxygen consumption. The consumption of photosynthates drives the Calvin cycling process.…”

“…Thus, the activities of Calvin cycling enzymes were activated. Rigano et al [44] also demonstrated that less ammonium was utilized in plants under dark condition than in illuminated plants. Therefore, the addition of moderate NH 4 + in LL plants is beneficial for activating Calvin cycling enzymes.…”

The role of mitochondria in leaf nitrogen metabolism

“…The effect of different levels of N nutrition on respiratory activity has been measured in various plant species; stimulation of respiratory activity by NH 4 + was found (e.g. in barley, Arabidopsis thaliana, spinach, pea and French bean plants; Rigano et al . 1996; Lasa et al .…”

Futile transmembrane NH 4 + cycling: A cellular hypothesis to explain ammonium toxicity in plants

“…Most of this expenditure appears to be due to plasmamembrane transport activity, hence we conclude that the 41% increase in total root respiration observed in the present study for barley at 10 mM NH 4 ϩ can be attributed directly to the approximately 30-fold increase in NH 4 ϩ efflux from the cytosol under this condition. Work with the NH 4 ϩ analogue methylammonium shows that a respiratory increase is also observed in barley (32), as well as other species (33), on addition of this nonmetabolized compound. In the absence of metabolic sinks, it becomes clear that membrane processes per se can impose a substantial energetic burden on the plant root system.…”