PNL Volume 13
Feenstra, W. J
E. Jacobsen, and A. D. J. de Visser
University of Groningen, Haren, Netherlands
In root nodules of leguminous plants such as pea, acetylene reduction
(N2~fixation) is inhibited by nitrate. There are significant indications in
the literature suggesting that the origin of the nitrate inhibition has to
be sought in the plant rather than in the bacteroid (4). Nitrate reduction
in the plant seems especially to be involved although the effect of nitrate
in a nitrate reductase (NaR) deficient mutant can provide direct information.
Such mutants have been isolated in the pea (1,2,3). Therefore, we studied
the effect of nitrate on acetylene reduction in our mutant E1. In vitro and
in vivo NaR activity of young leaves of this mutant are about 5Vand 20%. of
the activity in the parent cv 'Rondo1.
Parent and mutant plants were inoculated with Rhizobium leguminosarum
strain PF2 (kindly provided by Dr. Lie, Wageningen). Plant culture, nitrate
treatment, and determination of acetylene reduction of nodulated plants will
be described elsewhere. Nodulation characteristics of the mutant E1 were
not negatively affected by the NaR deficiency.
Without nitrate acetylene, reducing activities of cv Rondo and mutant
E1 were essentially the same. After NO3- treatment parent acetylene reduction
was significantly inhibited (47%) whereas in mutant E1 the inhibition was
only 19% and not significant. This shows that the inhibiting effect of nitrate
is mediated by the plant. Nitrate itself as a causative factor can be elimi-
nated, because uptake in mutant E1 is still intact as the highly increased
nitrate content in leaves of plants cultured on a medium with NH4NO3 as N-source
indicated (1,2).
The observation that in mutant E1 nitrate effect is not completely absent
presumably is due to the fact that in vivo NaR activity is not zero.
The way in which the process of nitrate reduction or reduction products
is involved will be subject of further experiments.
1. Feenstra, W. J. and E. Jacobsen. 1980. Theor. Appl. Genet. 58:39-42.
2. Feenstra, W. J. and E. Jacobsen. 1980. PNL 12:11.
3. Kleinhofs, A., R. L. Warner, F. J. Muehlbauer and R. A. Nilan. 1978.
Mutation Res. 51:29-35.
4. Manhart, J. R. and P. P. Wong. 1980. Plant Physiol. 65:502-505.