15N INCORPORATION AND GLUTAMINE SYNTHETASE INHIBITION STUDIES OF NITROGEN ASSIMILATION IN LEAVES OF THE NITROPHILE, DATURA STRAMONIUM L.

SUMMARY The N assimilation pathways in leaves of Datura stramonium L. have been investigated at low and high levels of xylem stream nitrate feeding by an 15N infiltration technique in the presence and absence of the glutamine synthetase inhibitor, methionine sulphoximine (MSO). In the absence of MSO, glutamate showed the highest 15N enrichment of any free amino compound at both feeding levels. At the high N feeding level, glutamine showed the highest accumulation of 15N at all stages of the time course, with secondary accumulation in glutamate. At the low N feeding level the reverse was true. These findings indicate the operation of both the glutamine synthetase‐GOGAT and glutamate dehydrogenase pathways in Datura leaves. In the presence of MSO intracellular pools of all the amino compounds, with the exception of glutamine and threonine, became rapidly drained. There was an almost complete restriction of the supply of newly reduced 15N to amino acid assimilation and it accumulated as ammonia. The large glutamine pool remaining contained no 15N enrichment, and was probably an extrachloroplastic storage pool not immediately available for amino acid synthesis. These experimental results indicate the almost exclusive role of the glutamine synthetase‐GOGAT pathway in the assimilation of newly reduced nitrate into amino acid metabolism in Datura leaves.