This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via CrossRef Citing Articles via Web of Science (20) Citing Articles via Google Scholar Google Scholar Articles by Sheehy, J. E. Articles by Phillips, D. A. Search for Related Content PubMed PubMed Citation Articles by Sheehy, J. E. Articles by Phillips, D. A. Agricola Articles by Sheehy, J. E. Articles by Phillips, D. A.

Articles

Carbon Exchange Rates of Shoots Required to Utilize Available Acetylene Reduction Capacity in Soybean and Alfalfa Root Nodules ¹

Department of Agronomy and Range Science, University of California, Davis, California 95616

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 (respiration of roots + nodules)/acetylene 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.

¹ This material is based on research supported by National Science Foundation Grants AER 77-07301 and PCM 78-01146.