Levels of ammonia, nitrite, and nitrate in alfalfa root nodules supplied with nitrate

Abstract

Protein content, protease activity, and levels of NH4+, NO2- and NO3- were determined in the plant and bacteroid fractions of alfalfa (Medicago sativa L. cv. Aragon) nodules supplied with NO3- (0—200mM).

Protein of the host cell cytosol represented approximately 70 % of the total nodule protein. The effect of NO3- on plant cytosolic protein closely paralleled that on bacteroid protein. Protein of both nodule fractions remained constant upon exposure to NO3- concentrations below 20 mM and was reduced by 30% above 20 mM NO3-. Patterns of proteolytic activity were inversely correlated to those of the corresponding protein for both bacteroids and cytosol.

The nodule level of NH4+ was basically unaffected by increasing added NO3- whereas NO2- accumulated in both bacteroids and plant cytosol, although to a greater extent in the latter. Likewise, NO3- accumulated in the cytosol with much higher relative values than those of NO2-. Only trace or null amounts of NO3- were found in the corresponding bacteroids. Linear correlations either between bacteroidal and cytosolic NO2-, or between NO2- in both fractions and cytosolic NO3- levels were highly significant.

The overall results indicate that the whole nodule capacity of NO2- reduction is not sufficient to avoid accumulation of NO2-, and support the view that NO2- plays a more important role than NH4+ or NO3- itself as inhibitor of nodule activity.