Nitrogen assimilation in sunflower leaves and upward and downward transport of nitrogen

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Abstract
The assimilation of ammonium and nitrate nitrogen into amino acids of mature sunflower leaves and their transport to the other plant parts were investigated using nitrogen-15 as a tracer. In the leaf, to which 15N-labelled ammonium was vacuum-infiltrated, the 15N content of glutamine was always the highest of the amino acids tested and that of alanine was higher than that of glutamic acid and aspartic acid at 15 min after the infiltration. On the other hand in the leaf to which 15N-labelled nitrate was vacuum-infiltrated, the 15N content of glutamic acid and aspartic acid was superior to that of glutamine. The incorporation of 15N into serine was not active in the case of either 15N-labelled ammonium or nitrate. In the internodes above and below the treated leaf, through which photosynthates were transported into other parts, the 15N content of γ-aminobutyric acid and glutamine was markedly high when both nitrogen sources were supplied. There were no differences in the labelling patterns of amino acids between the upper and lower internodes. From these results it may be concluded that glutamine, glutamic acid, and aspartic acid play an important role in the assimilation of ammonium and nitrate nitrogen in leaves and that nitrogen is transported mainly in the forms of γ-aminobutyric acid and glutamine from the leaves to the other plant parts,