Relationship between C2H2 reduction, H2 evolution and 15N2 fixation in root nodules of pea (Pisum sativum)

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Abstract
The quantitative relationship between C2H2 reduction, H2 evolution and 15N2 fixation was investigated in excised root nodules from pea plants (Pisum sativum L. cv. Bodil) grown under controlled conditions. The C2H2/N2 conversion factor varied from 3.31 to 5.12 between the 32nd and the 67th day after planting. After correction for H2 evolution in air, the factor (C2H2‐H2)/N2 decreased to values near the theoretical value 3, or in one case to a value significantly (P < 0.05) below 3. The proportion of the total electron flow through nitrogenase, which is not wasted in H2 production but used for N2 reduction, is often stated as the relative efficiency (1‐H2/C2H2). This factor varied significantly (P < 0.05) during the growth period. The actual allocation of electrons to H2 and N2, expressed as the H2/N2 ratio, was independent of plant age, however. This discrepancy and the observation that the (C2H2‐H2)/N2 conversion factor tended to be lower than 3, suggests that the C2H2reduction assay underestimates the total electron flow through nitrogenase.