PLANT PHYSIOLOGY , Vol 114, Issue 4 1405-1412, Copyright © 1997 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Nitrogenase Activity Is Affected by Reduced Partial Pressures of N2 and NO3- 1
J. M. Blumenthal, M. P. Russelle and C. P. Vance
Department of Soil, Water, and Climate, 439 Borlaug Hall, University of Minnesota, St. Paul, Minnesota 55108-6028 (J.M.B)
Optimal use of legumes in cropping systems requires a thorough
understanding of the interaction between inorganic N nutrition and
symbiotic N2 fixation. Our objective was to test the hypothesis that
increased NO3- uptake by alfalfa (Medicago sativa L.) would compensate for
lower N2 fixation caused by low partial pressure of N2. Root systems of
hydroponically grown alfalfa at 2 mg L-1 NO3--N were exposed to (a) 80% N2,
(b) 7% N2, (c) 2% N2, or (d) 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 hours after
treatments began, the TNA of plants exposed to 7 and 2% N2 was
substantially higher than pretreatment activities, whereas the TNA of
plants exposed either to 0 or 80% N2 did not differ from pretreatment
values. The decline in TNA due to NO3- exposure over 4 d was not affected
by reduced partial pressure of N2. During the 1st 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 for the rest of the experiment. Although the hypothesis that
alfalfa compensated with increased NO3- uptake for lower N2 fixation was
not validated by our results, we unexpectedly 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.
