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Research article2000Peer reviewedOpen access

Nitrogenase activity in Alnus incana root nodules. Responses to O-2 and short-term N-2 deprivation

Lundquist, PO


O-2 and host-microsymbiont interactions are key factors affecting the physiology of N-2-fixing symbioses. To determine the relationship among nitrogenase activity of Frankia-Alnus incana root nodules, O-2 concentration, and short-term N-2 deprivation, intact nodulated roots were exposed to various O-2 pressures (pO(2)) and Ar:O-2 in a continuous flow-through system. Nitrogenase activity (H-2 production) occurred at a maximal rate at 20% O-2. Exposure to shortterm N, deprivation in Ar:O-2 carried out at either 17%, 21%, or 25% O-2 caused a decline in the nitrogenase activity at 21% and 25% O-2 by 12% and 25%, respectively. At 21% O-2, nitrogenase activity recovered to initial activity within 60 min. The decline rate was correlated with the degree of inhibition of N-2 fixation. Respiration (net CO2 evolution) decreased in response to the N-2 deprivation at all pO(2) values and did not recover during the time in Ar:O-2. Increasing the pO(2) from 21% to 25% and decreasing the pO(2) from 21% to 17% during the decline further decreased rather than stimulated nitrogenase activity, showing that the decline was not due to O-2 limitation. The decline was possibly due to a temporary disturbance in the supply of reductant to nitrogenase with a partial O-2 inhibition of nitrogenase at 25% O-2. These results are consistent with a fixed O-2 diffusion barrier in A. incana root nodules, and show that A. incana nodules differ from legume nodules in the response of the nitrogenase activity to O-2 and N-2 deprivation.

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Plant Physiology
2000, Volume: 122, number: 2, pages: 553-561

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