Xylem sap analyses of 5-week-old plants of Helianthus annuus L. fed at high and low nitrate concentrations show that between 77 and 94 % of the nitrogen conveyed to the leaf by the xylem is in the form of nitrate, present at up to 300 % of its concentration in the feeding solution, thus confirming nitrate as the major nitrogen source of leaves of this species when its roots are nitrate fed. Organic nitrogen is mainly in the form of glutamine.

The pathways of nitrogen assimilation in the leaves of H. annuus following petiolar nitrate feeding at two concentrational levels have been investigated by an 15N infiltration technique in the presence of the glutamine synthetase inhibitor, methionine sulphoximine (MSO). In the absence of MSO and over a 30 min time-course the main diversion of newly reduced nitrate was to glutamine with a secondary enrichment of glutamate. In the presence of MSO intracellular pools of all the amino compounds, with the exception of threonine, became conspicuously depleted, suggesting that conditions of nitrogen stress had been induced. There was an almost complete restriction of the supply of newly reduced 15N to amino acid assimilation with the resultant accumulation of large 15N ammonium pools. Glutamate dehydrogenase activity was unaffected by the MSO treatment.

These experimental results indicate the almost exclusive role of the glutamine synthetase- glutamate synthase pathway in the assimilation of newly reduced nitrate into amino acid metabolism in the leaves of H. annuus.