CAN THE N-15 DILUTION TECHNIQUE BE USED TO STUDY N-2 FIXATION IN TROPICAL TREE SYMBIOSES AS AFFECTED BY WATER-DEFICIT

Three methods were used to study N-2 fixation and effects of water def icit on N-2 fixation: C2H2 reduction assay (ARA), N-15 dilution techni que and accumulated N content. In addition, N-15 dilution was calculat ed both in a traditional way and in a modified way, which takes into c onsideration N and N-15 content for the plants before the experiment s tarted. The three methods were applied on the following Rhizobium-symb ioses: Acacia albida Del (Faidherbia albina (Del) A. Chev.) and Leucae na leucocephala (Lam) de Wit., and the Frankia-symbiosis Casuarina equ isetifolia L. The plants were about 4-months-old when they were harves ted. Nitrogen derived from N-2 fixation in control plants of Acacia al bida was 54.2 mg as measured with ARA, while it was 28.5 mg as measure d with the N-15 dilution technique, compared to 30.7 mg calculated as accumulated N. In comparison, L. leucocephala fixed 41.6 mg N (ARA), 5 3.5 mg N (N-15 dilution technique) and 56.3 mg N (accumulated N). The Frankia-symbiosis had fixed 27.4 mg N as measured by ARA, 8.1 mg N as measured by N-15 dilution technique and 12.3 mg N as accumulated N. Th ere were no differences between the estimates based on traditional and modified ways of calculating N-15 dilution. The immediate effect of w ater deficit treatment on N-2 fixation was continuously measured in al l species with ARA, which started to decrease approximately 10 d after the initiation of the treatment, and declined to less than 5% of the initial level after 21-28 d. The decrease in the amount of N derived f rom N-2 fixation was studied in L. leucocephala during the period of t reatment. There was a 26% decrease in amount of N derived from N-2 fix ation as result of water deficit (as measured with ARA), while the dec rease was 23% when measured with both the N-15 dilution method and as accumulated N.