REGULATION OF NH-4+ UPTAKE IN WHEAT PLANTS - EFFECT OF ROOT AMMONIUM CONCENTRATION AND AMINO-ACIDS

When N deficient 10-day-old wheat plants were supplied with 1.5 Mol m-3 NH4+, net NH4+ uptake rapidly decreased during the first 6 h, while root free-NH4+ and free amino acids concentration increased. However, after 24 h the NH4+ uptake rate increased again, as the internal NH4+ concentration decreased. When plants were pretreated during 40 h with different external NH4+ concentrations, net uptake measured on 1.0 mol m-3 NH4+ decreased with the increasing ion concentration during the pretreatment. This decrement coincided with both root free-NH4+ and total free amino acids levels. When N-starved and NH4+ fed plants were treated during 0, 3 or 6h with 1.0 mol m-3 NH4+ in the presence of 1.0 mol m-3 MSX2, net uptake (measured without MSX) decreased with the length of the inhibitor treatment. In both groups, MSX significantly increased root free-NH4+ concentration, while the level of total free amino acids was only increased in N-starved plants. When N-starved plants were externally supplied with 1.0 mol m-3 of different amino acids or amides, net NH4+ uptake was only strongly inhibited in the presence of glutamine or asparagine. It is concluded that rapid changes in the concentration of certain amino acids during NH4+ nutrition might regulate the ion absorption, though at high endogenous levels of free NH4+ net uptake could be suppressed independently of the root concentration of free amino acids.