Evidence for N feedback regulation of N₂fixation inAlnus glutinosaL.

Treatments were applied to vary C and N availability in Alnus glutinosa L. and plant growth, nodule activity (including acetylene reduction) and amino acid composition of the xylem sap were measured. Removing the buds, a sink for N, caused a decrease in nodule activity. Flushing root systems daily with 100% O₂ destroyed nitrogenase activity and substantially decreased the amount of citrulline in the xylem sap. The amino acid composition of xylem saps also altered according to the mode of N nutrition. In plants fed $NH4+$, xylem sap composition was similar to N₂-fixing plants, however, when plants were fed $NO3-$, citrulline content increased. The assimilation and subsequent distribution of nitrate was followed in an experiment in which labelled ¹⁵$NO3-$ was added to the base of plant pots. After 12 h 7% of root N was from applied ¹⁵$NO3-$ and this increased to 75% at 7 d; substantial enrichment of N from ¹⁵$NO3-$ also occurred in stems, buds and leaves. After 7 d, 3.5% of nodule N was from ¹⁵$NO3-$, consistent with some N being supplied by recycling of shoot N. Xylem saps were also collected and after 12 h, glutamate and aspartate were enriched with ¹⁵N to 53% and 37% increasing after 7 d to 80% and 49%, respectively. Citrulline content of the xylem sap increased from 3 to 9 μmol cm⁻³ following addition of ¹⁵$NO3-$ and at 7 d 80% of the N in the citrulline had been derived from ¹⁵N. It is hypothesized that the growth and activity of A. glutinosa root nodules is sensitive to the N status of the plant and that the level of citrulline (or other amino acids) returning to the nodules may feed back to regulate nodule growth and activity.