Synthesis and15N-labelling of glutamine and glutamate in maize roots during early stages of15N-ammonium assimilation

Abstract

The concentrations of glutamine and glutamate, and the abundance of ¹⁵N in these compounds, were measured in roots of intact, nitrate-grown maize plants fed with ¹⁵N-nitrate or ¹⁵N-ammonium for periods of 3–80 min. On supplying ¹⁵N-ammonium there was a rapid and almost linear accumulation of glutamine, its concentration in the roots rising 3-fold over 1 h. Supplying ¹⁵N-nitrate instead of ¹⁵N-ammonium did not increase root glutamine, and the concentration of glutamate was not affected by either treatment. The time-course of amide ¹⁵N-labelling seen in glutamine extracted from roots which had been supplied with ¹⁵N-ammonium could best be explained by a model in which (a) the ‘additional’ glutamine which accumulates rapidly during ¹⁵N-ammonium feeding is heavily amide-labelled from the outset, and (b) of the glutamine already present in the roots, only a small proportion (c. 10–15%) incorporates ¹⁵N during the initial 60–80 min of ¹⁵N-ammonium feeding, the remainder (c. 85–90%) remaining essentially unlabelled over this period. The latter is assumed to be located in the vacuoles. Even though prior N-starvation stimulated ammonium net uptake markedly, the data were not of sufficient quality to show whether the relative sizes of the extra-vacuolar and vacuolar glutamine fractions were altered by this treatment. For that reason it was not possible to determine whether cytosolic glutamine has a role in regulating N-absorption.