An investigation of the pathway of nitrogen incorporation into the leaf and root metabolism of Helianthus annuus L
A survey of the recent literature concerning the assimilation of nitrogen into plant metabolism has been presented. Xylem sap from hydroponically grown and vermiculite- . -3 -3 grown Helianthus annuus fed at 50 µg N cm and 300 ~g N cm feeding level was analysed for nitrate, ammonia, and amino compounds. Nitrate accounted for 771to 94% of the total nitrogen transported via the xylem stream. The pathway of nitrate-N assimilation into amino compounds by the leaves and roots of Helianthus annuus at different feeding levels has been investigated using 15 N - nitrate feeding>enzyme inhibitor studies ~nd enzymological assays. Excised leaves received feeding solutions at two · -3 · -3 concentrational levels: 300 ~g N cm and 50 µg N cm via the xylem stream prior to experimentation. Nitrate - 15 N xylem stream and infiltration feeding experiments on Helianthus leaves indicated an apparent- major routing of newly reduced nitrogen to glutamine. Of the other major soluble amino compounds, alanine, aspartate, serine, and glycine were found to be important in the primary assimilation of newly reduced nitrogen. After pre-treatment with feeding solutions containing 5mM methionine sulphoximine, and after infiltration with 15 N-nitrate containing 5mM methionine sulphoximine, complete suppression of nitrogen routing into amino compounds was noted with the resultant accumulation of 15 N in a large ammonia pool. Conditions of nitrogen stress were indicated from the concentrational changes of free amino compound pools after methionine sulphoximine treatment. Glutamate dehydrogenase activity (NADH-dependent) was not inhibited by the methionine sulphoximine pretreatment. Assimilation of newly reduced nitrogen into amino compounds in Helianthus leaves via the glutamine synthetase/ glutamate synthase pathway, irrespective of nitrogen availability, was indicated by these results. There was a decrease in the 15 N incorporation at the higher feeding level caused possibly, by a decrease in glutamate synthase activity. The incorporation of 15 N-nitrate into the free amino compounds of Helianthus roots indicated that they are capable of reduced nitrate-N assimilation, although at a much lower rate than in the leaves. Glutamine was the most heavily 15 N-labelled soluble amino compounds at both feeding levels used. The effect of feeding Helianthus roots with 15N-nitrate solutions containing 7mM methionine sulphoximine suggested that in the roots, as in the leaves, the glutamine synthetase/glutamate synthase pathway was the sole route for 2-amino production from newly reduced nitrogen.
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