EVIDENCE FOR N FEEDBACK-REGULATION OF N-2 FIXATION IN ALNUS-GLUTINOSAL

Treatments were applied to vary C and N availability in Alnus glutinos a L. and plant growth, nodule activity (including acetylene reduction) and amino acid composition of the xylem sap were measured. Removing t he buds, a sink for N, caused a decrease in nodule activity. Flushing root systems daily with 100% O-2 destroyed nitrogenase activity and su bstantially decreased the amount of citrulline in the xylem sap. The a mino acid composition of xylem saps also altered according to the mode of N nutrition. In plants fed NH4+, xylem sap composition was similar to N-2-fixing plants, however, when plants were fed NO3-, citrulline content increased. The assimilation and subsequent distribution of nit rate was followed in an experiment in which labelled (NO3-)-N-15 was a dded to the base of plant pots. After 12h 7% of root N was from applie d (NO3-)-N-15 and this increased to 75% at 7 d; substantial enrichment of N from (NO3-)-N-15 also occurred in stems, buds and leaves. After 7 d, 3.5% of nodule N was from (NO3-)-N-15, consistent with some N bei ng supplied by recycling of shoot N. Xylem saps were also collected an d after 12 h, glutamate and aspartate were enriched with N-15 to 53% a nd 37% increasing after 7 d to 80% and 49%, respectively. Citrulline c ontent of the xylem sap increased from 3 to 9 mu mol cm(-3) following addition of (NO3-)-N-15 and at 7 d 80% of the N in the citrulline had been derived from N-15. If is hypothesized that the growth and activit y of A. glutinosa root nodules is sensitive to the N status of the pla nt and that the level of citrulline (or other amino acids) returning t o the nodules may feed back to regulate nodule growth and activity.