Synthesis and 15N-labelling of glutamine and glutamate in maize roots during early stages of 15N-ammonium assimilation
1994
Lee, R.B. | Lewis, M.J.
The concentration 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.
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