Biological nitrogen fixation: A key to world protein

Autor: R. D. Holsten, R. R. Hebert, E. K. Jackson, Ralph W. F. Hardy, R. C. Burns
Rok vydání: 1971
Předmět:
Zdroj: Plant and Soil. 35:561-590
ISSN: 1573-5036
0032-079X
DOI: 10.1007/bf02661879
Popis: 1. Characteristics and methodology of the C2H2-C2H4 assay forin situ measurement of N2 fixation are outlined. 2. Electron micrographic analysis of the developmental morphology of the natural soybean symbiosis and C2H2-C2H4 analysis indicate that increasing N2-fixing activity from 12–35 days of age is accompanied by an increase in bacteroid number per cell, bacteroid number per vesicle and inclusions per bacteroid. The mole ratio of leghemoglobin to nitrogenase also increases from 50 to a relatively constant plateau of 500 to 1500 during this period. The quantitative validity of the C2H2-C2H4 assay as a measure of N2 fixation during a complete growth cycle of soybeans on nitrogen-free medium is demonstrated by Σ (C2H2→C2H4)×28/3 values which are 75–95% of the values determined for N2 fixed by Kjeldahl analyses. 3. A technique for the establishment of the first callus N2-fixing symbiosis in mixed cultures ofRhizobium legume provides a defined experimental system for exploration of legume symbiosis. N2-fixing activity is about 1% of the natural system and is influenced by exogenous auxins and cytokinins. Morphology, including infection threads and vesicle enclosed bacteroids, is similar to the nodule system. 4. N2-fixing activity of field-grown soybeans, including varieties which differed in flowering characteristics and maturity dates, and of peanuts was determined biweekly with the C2H2-C2H4 assay. Activity extended from nodule initiation to senescence and correlated with the nitrogen demands of the plant and in most cases >90% of the N2 fixed during the 60–70 day period of fruit formation and maturation. A logarithmic relationship between N2-fixing activity and age, and N2 fixed and age was demonstrated as a fundamental characteristic of these annual symbionts,i.e. log N2 fixed =k(t−t 0), wheret 0 is age at activity initiation. The resultant parameters: 1) age at activity initiation, 2) calculated rate of daily increase (7–9% for soybeans and 7–10% for peanuts), 3) age at end of logarithmic phase (about 80 days for soybeans), and 4) total N2 fixed (about 250 mg per soybean plant) are useful bases for evaluation of environmental, bacterial and host effects on N2 fixation. Various N fertilizers applied at planting and flowering inhibited N2 fixation of soybeans by decreasing the rate of daily increase. 5. Physical and chemical characteristics of nitrogenase, including those of crystalline Mo-Fe protein, reactions of nitrogenase, and model studies are consistent with a proposed mechanism. 6. Potential utilities of N2 fixation research include increased food protein production via initially enhanced N2 fixation of legumes such as soybeans and eventually extension of N2-fixing symbioses to non-legumes and new chemistry of N2, including the direct incorporation of aerial N2 into important organic compounds.
Databáze: OpenAIRE
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