Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/12/1997
Publication Date: N/A
Citation: Interpretive Summary: Excessive nitrogen fertilizer applications to agricultural soils have been implicated in nitrate contamination of surface and ground water. Alfalfa is a deeply rooted perennial forage crop that is able to remove nitrate from the subsoil effectively, and, therefore, might be used to clean nitrate-contaminated sites. Nitrogen acquired by alfalfa originates not only from soil, but also from atmospheric nitrogen gas. This gas is captured, or 'fixed', by soil bacteria living in nodules on alfalfa roots by a process called symbiotic nitrogen fixation. Legumes like alfalfa usually obtain less nitrogen from the atmosphere when soil nitrate supply is large, but how plants regulate the uptake of these two nitrogen sources is unknown. Previous research implied that the amount of certain amino acids, the building blocks of protein, in the root nodule might help regulate symbiotic nitrogen fixation. We used a new approach in this research and limited the amount of symbiotic nitrogen fixation that could occur by surrounding the alfalfa roots with less nitrogen gas than is normally present in the atmosphere. We found no differences in the amount of amino acids present in nodules. Our results provide no evidence that particular amino acids are involved in the regulation of symbiotic nitrogen fixation when nitrate is available. We found that the decline in symbiotic nitrogen fixation caused by nitrate did not depend on how much fixation was occurring originally. This finding is important because it indicates that it may be possible to improve simultaneously nitrate uptake and nitrogen fixation in alfalfa, thus leading to a greater efficiency in nitrogen utilization.
Technical Abstract: Optimal use of legumes in cropping systems requires thorough understanding of the interaction between inorganic N nutrition and symbiotic N2 fixation. Our objectives were to characterize the impact of reduced partial pressures of N2 surrounding the root nodules on symbiotic N2 fixation of alfalfa (Medicago sativa L.) and to evaluate the effect of NO3- on symbiotic N2 fixation of alfalfa plants reduced in N2 fixation. Root systems of hydroponically grown alfalfa at 2 mg L-1 NO3- were exposed to either: 1) 80% N2; 2) 7% N2; 3) 2% N2; or 4) 0% N2. Exposure to reduced partial pressures of N2 reduced total nitrogenase activity (TNA, measured as H2 production in 20% O2 and 80% Ar) by 40% within less than 30 min, followed by a recovery period over the next 30 min to initial activity. Five h after treatments began, TNA of plants exposed to 7% and 2% N2 was substantially higher than pretreatment activities, whereas TNA of plants exposed either to 0% or 80% N2 did not differ from pretreatment values. The decline in TN due to NO3- exposure over four days was not affected by reduced partial pressure of N2. During the first h the proportion of electrons used for the reduction of N2 fell from 0.52 to 0.23 for plants exposed to 7% N2 and to 0.09 for plants exposed to 2% N2 and remained unchanged during rest of the experiment. We demonstrated that the decline in TNA upon exposure to NO3- was independent of the N2 fixing efficiency (i.e. the amount of N2 reduced by nitrogenase) of the symbiosis.