Growth, denitrification and nitrate ammonification of the rhizobial strain TNAU 14 in presence and absence of C2H4 and C 2H2

Abstract

Soil-N (NO3-) initiates as far as a threshold concentration is surpassed manifold physiological reactions on N 2-fixation. Organic N and ammonium oxidised to NO3- means oxygen depletion. Plants suffering under O2 or infection stress start to excrete ethylene (C2H4). C 2H4 widens the root intercellulars that O 2-respiration will continue. Now microbes may more easily enter the plant interior by transforming the reached methionine into C2H 4. Surplus nitrate and C2H4 inhibit nodulation of leguminous plants. Excess NO3- in the nodulesphere could be diminished by N2-fixing bacteria which in addition can denitrify or ammonify nitrate. Consequently, it was asked whether C 2H4 interferes with the potential of N2-fixing bacteria to reduce nitrate. The groundnut-nodule isolate TNAU 14, from which it was known that it denitrifies and ammonifies nitrate, served as inoculum of a KNO3-mannitol-medium that was incubated under N2-, 1% (v/v) N2-C2H4-, and 1% (v/v) N 2-C2H2-atmosphere in the laboratory. C 2H2 was included into the experiments because it is frequently used to quantify N2-fixing potentials (acetylene reduction array, ARA). Gene-165 rDNA-sequencing and physiological tests revealed a high affiliation of strain TNAU 14 to Rhizobium radiobacter and Rhizobium tumefaciens. Strain TNAU 14 released N2O into the bottle headspace in all treatments, surprisingly significantly less in presence of C 2H2. Nitrate-ammonification was even completely blocked by C2H2. C2H4, in contrast rather stimulated growth, denitrification, and nitrate-ammonification of strain TNAU 14 which consumed the released NH4+ during continuing incubation.