Abstract
The CO2-exchange rate required to make full use of available N2-fixation capacity, measured as acetylene reduction, was determined in soybean and alfalfa. Carbohydrates of root systems were depleted during a 40-hour dark treatment; then plants were exposed to a 24-hour light period during which different CO2-exchange rates were maintained with various CO2 concentrations. In three- and four-week-old soybeans and four-week-old alfalfa plants, acetylene-reduction capacity was used fully with CO2-exchange rates as low as 10 milligrams CO2 per plant per hour. In six-week-old alfalfa plants, however, acetylene reduction rates increased linearly, and apparent N2-fixation capacity was not used fully when CO2-exchange rates were higher than 40 milligrams CO2 per plant per hour. Under the conditions established, the energy cost of N2 fixation, measured as Δ(respiration of roots + nodules)/Δacetylene reduction over dark-treatment values, was 0.453 milligrams CO2 per micromole C2H4 for all rates of acetylene reduction and for both ages of soybean and alfalfa plants. Thus, root-plus-nodule respiration was not promoted by higher rates of apparent photosynthesis after C2H2-reduction capacity became saturated, and all available capacity for apparent N2 fixation had the same energy requirement.
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Selected References
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