Growth responses of the perennial legume Sesbania sesban to NH4 and NO3 nutrition and effects on root nodulation

Preference for NH4+ or NO3− nutrition by the perennial legume Sesbania sesban (L.) Merr. was assessed by supplying plants with NH4+ and NO3− alone or mixed at equal concentrations (0.5 mM) in hydroponic culture. In addition, growth responses of S. sesban to NH4+ and NO3− nutrition and the effects on root nodulation and nutrient and mineral composition of the plant tissues were evaluated in a hydroponic setup at a range of external concentration of NH4+ and NO3− (0, 0.1, 0.2, 0.5, 2 and 5 mM). Seedlings of S. sesban grew equally well when supplied with either NH4+ or NO3− alone or mixed and had high relative growth rates (RGRs) ranging between 0.19 and 0.21 d−1. When larger plants of S. sesban were supplied with NH4+ or NO3− alone, the RGRs and shoot elongation rates were not affected by the external concentration of inorganic N. At external N concentrations up to 0.5 mM nodulation occurred and contributed to the N nutrition through fixation of gaseous N2 from the atmosphere. For both NH4+ and NO3−-fed plants the N concentration in the plant tissues, particularly water-extractable NO3−, increased at high supply concentrations, and concentrations of mineral cations generally decreased. It is concluded that S. sesban can grow without an external inorganic N supply by fixing atmospheric N2 gas via root nodules. Also, S. sesban grows well on both NH4+ and NO3− as the external N source and the plant can tolerate relatively high concentrations of NH4+. This wide ecological amplitude concerning N nutrition makes S. sesban very useful as a N2-fixing fallow crop in N deficient areas and also a candidate species for use in constructed wetland systems for the treatment of NH4+ rich waters.