Growth, denitrification and nitrate ammonification of the rhizobial strain TNAU 14 in presence and absence of C2H4 and C2H2

Abstract

Soil-N (NO3 −) initiates as far as a threshold concentration is surpassed manifold physiological reactions on N2-fixation. Organic N and ammonium oxidised to NO3 − means oxygen depletion. Plants suffering under O2 or infection stress start to excrete ethylene (C2H4). C2H4 widens the root intercellulars that O2-respiration will continue. Now microbes may more easily enter the plant interior by transforming the reached methionine into C2H4. 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 C2H4 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) N2−C2H2-atmosphere in the laboratory. C2H2 was included into the experiments because it is frequently used to quantify N2-fixing potentials (acetylene reduction array, ARA). Gene-16S rDNA-sequencing and physiological tests revealed a high affiliation of strain TNAU 14 toRhizobium radiobacter andRhizobium tumefaciens. Strain TNAU 14 released N2O into the bottle headspace in all treatments, surprisingly significantly less in presence of C2H2. 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.