TheParasponia parviflora — Rhizobium symbiosis. Isotopic nitrogen fixation, hydrogen evolution and nitrogen-fixation efficiency, and oxygen relations

Abstract

Isotopic15N2 experiments confirmed nitrogen fixation inParasponia parviflora. The conversion ratio C2H4/N2 was 6.7 under the experimental conditions employed. Measurements of the δ15N in leaves of Parasponia and Trema showed on the basis of these determinations thatParasponia parviflora possesses N2-fixing capacity and can be distinguished in this respect from the non-nitrogen-fixingTrema cannabina tested by the same method. Therefore, δ15N can be used to monitor N2 fixation in natural ecosystems. Hydrogen evolution and the relative efficiency of N2 fixation in this relation have been determined. DetachedParasponia parviflora root nodules grown in soil and tested in an argon/oxygen atmosphere produced appr. 4 μmol H2.h−1.g−1 fresh weight root nodules. The relative efficiency of hydrogen utilization as measured in argon, air, and in the presence of C2H2 10% (v/v) was for both equations\(\left( {1 - \frac{{H_2 (air)}}{{H_2 (Ar)}}} \right) and \left( {1 - \frac{{H_2 (air)}}{{C_2 H_2 }}} \right)\) used for to express this efficiency 0.96 and 0.97, respectively. This indicates that Parasponia like the root nodules of some actinorhizal symbioses (Alnus, Myrica, Elaeagnus) and some tropical legumes (Vigna sinensis) has evolved mechanisms of minimizing net hydrogen production in air, thus increasing the efficiency of electron transfer to nitrogen. The oxygen relation of nitrogen fixation (C2H2) inParasponia parviflora root nodules was determined. The nitrogenase activity of Parasponia root nodules increased at increasing oxygen concentrations up till c. 40% O2. At oxygen levels above 40% O2, the nitrogenase activity of the root nodules was nil or very erratic suggesting that at these oxygen levels the nitrogenase is not longer protected against the harmful effect of oxygen. In this respect Parasponia root nodules differ from actinorhizal root nodules in other nonlegumes, where optimal nitrogenase activity was observed in the range of 12–25% oxygen. Respiration experiments with Parasponia root nodules showed that in the range 10, 20, and 40% oxygen, the respiration rate (CO2 evolution) increased concomitantly with an increase of the acetylene reduction rate. The CO2/C2H4 values obtained varied between 8.1 and 19.2, being therefore 2–3 times higher than similar estimations in some actinorhizal and legume root nodules. The respiratory quotient (RQ) of detachedParasponia parviflora root nodules was in air initially approximately 2.0, but this value dropped to about 1.0 in a 3-hours period.