Selection of salt-tolerant Rhizobium isolates of Acacia nilotica.

Among 35 Rhizobium isolates of Acacia nilotica, from different agro-climatic zones, two, ANG4 and ANG5, tolerated up to 850 mM NaCl and one, ANG3, was sensitive to NaCl above 250 mM. Nodulation and nitrogenase activity of the three isolates decreased with increasing concentration of salt up to 150 mM. Nodulation by ANG3 was 15% at 75 mM NaCl and nil at 100 mM. With ANG4 and ANG5, nodulation was only slightly decreased at 150 mM NaCl. Nitrogenase activity associated with plants inoculated with ANG3 was halved at 25 mM NaCl compared with salt-free controls, whereas isolates ANG4 and ANG5 retained 25% and 15% activity, respectively, even at 100 mM NaCl. Salt-tolerant Rhizobium isolates can therefore nodulate and fix N2 in saline soils.