Nitrate inhibition of N2 fixation and its effect on micronutrient accumulation in shoots of soybean (Glycine max L. Merr.), Bambara groundnut (Vigna subterranea L. Vedc) and Kersting\u2019s groundnut (Macrotyloma geocarpum\xa0Harms.)

Glory Chinonye Mbah,Felix Dapare Dakora

doi:10.1007/s13199-017-0531-2

Abstract

Although nitrate is known to inhibit nodulation and N2 fixation in symbiotic legumes, little is known about its effect on the uptake and accumulation of trace elements such as Fe, Zn, Mn and Cu. The aim of this study was to evaluate the effect of 5 mM NO3− supply, either with or without rhizobial inoculation, on nodulation, nodule functioning and micronutrient levels in the shoots of soybean (Glycine max L.Merr.), Bambara groundnut (Vigna subterranea L. Vedc) and Kersting’s groundnut (Macrotyloma geocarpum Harm). The results showed reduction in plant growth, nodule formation and nodule dry matter by the supply of 5 mM NO3− to inoculated seedlings of all three species. Nitrate inhibition respectively caused 1.2, 1.4, and 1.5-fold decrease in nodule number per plant in Bambara groundnut, soybean and Kersting’s bean, which resulted in 2.3, 3.3 and 4.5-fold reduction in nodule dry weight of the test species (in that order). The application of 5 mM NO3− to soybean plants also resulted in 2.5, 4.0 and 5.4-fold decrease in shoot accumulation of Fe, Zn and Mn, respectively, when compared to the purely symbiotic control plants. Furthermore, we observed 1.3, 1.8 and 1.3-fold decreases in the concentration of Zn, Mn and Cu in shoots of inoculated Bambara groundnut with NO3− supply, levels lower than those found in soybean. With Kersting’s groundnut, shoot concentration of Fe, Zn and Cu were higher with the application of 5 mM NO3− to inoculated plants when compared to the purely symbiotic treatment, which was opposite to soybean. But pure NO3−feeding of this species respectively resulted in 2.0, 1.4 and 1.3-fold decreases in Fe, Zn and Cu relative to inoculated NO3−-fed plants. Clearly, NO3− supply to landraces/genotypes of the three legume species did not only inhibit nodule formation and functioning, it also reduced shoot micronutrient levels in soybean and Bambara groundnut, but not Kersting’s bean.

Highlights

IntroductionN2-fixing bacteroids in root nodules (Israel 1987; Udvardi and Poole 2013)
Mineral nutrition is important for growth of plants, especially nodulated legumes due to the high demand for nutrients by Presented at the 17th African Association of Biological Nitrogen Fixation (AABNF) Biennial Conference, Gaborone, Botswana, 17–21 October 2016
N2 fixation is the source of N for meeting the N demand of nodulated legumes

Summary

Introduction

N2-fixing bacteroids in root nodules (Israel 1987; Udvardi and Poole 2013). Several factors can inhibit nodulation and N2 fixation in legumes, which in turn affects mineral accumulation in symbiotic legumes. The enzyme nitrogenase, which reduces N2 to NH3, is O2-labile and denatures when the O2 concentration is high (Dakora and Atkins 1989). Mineral N (nitrate and ammonium) is a potent inhibitor of nodule formation and N2 fixation (Streeter and Wong 1988; Ayisi et al 2000). In particular, can combine with leghaemoglobin to form nitrosylleghaemoglobin, which reduces O2 supply to N2-fixing bacteroids (Appleby 1984). Waterlogging of nodulated legumes can result in reduced N2 fixation due to low O2 (Minchin and Summerfield 1976), though some legumes

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