Plant Physiol, February 2000, Vol. 122, pp. 553-562
Nitrogenase Activity in Alnus incana Root Nodules.
Responses to O2 and Short-Term N2
Deprivation1
Per-Olof
Lundquist*
Department of Plant Biology, Swedish University of Agricultural
Sciences, P.O. Box 7080, S-750 07 Uppsala, Sweden.
O2 and host-microsymbiont
interactions are key factors affecting the physiology of
N2-fixing symbioses. To determine the relationship among
nitrogenase activity of Frankia-Alnus
incana root nodules, O2 concentration, and
short-term N2 deprivation, intact nodulated roots were
exposed to various O2 pressures (pO2) and
Ar:O2 in a continuous flow-through system. Nitrogenase
activity (H2 production) occurred at a maximal rate at 20%
O2. Exposure to short-term N2 deprivation in
Ar:O2 carried out at either 17%, 21%, or 25%
O2 caused a decline in the nitrogenase activity at 21% and
25% O2 by 12% and 25%, respectively. At 21%
O2, nitrogenase activity recovered to initial activity
within 60 min. The decline rate was correlated with the degree of
inhibition of N2 fixation. Respiration (net CO2
evolution) decreased in response to the N2 deprivation at
all pO2 values and did not recover during the time in
Ar:O2. Increasing the pO2 from 21% to 25% and
decreasing the pO2 from 21% to 17% during the decline
further decreased rather than stimulated nitrogenase activity, showing
that the decline was not due to O2 limitation. The decline
was possibly due to a temporary disturbance in the supply of reductant
to nitrogenase with a partial O2 inhibition of nitrogenase
at 25% O2. These results are consistent with a fixed
O2 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
O2 and N2 deprivation.
1
This work was supported by the Swedish Natural
Science Research Council (grant to K.H.-D.).
*
E-mail Per-Olof.Lundquist{at}vbiol.slu.se; fax 46-18-673279.
© 2000 American Society of Plant Physiologists
