Effects of some salts and sodicity on the growth of a Rhizobium leguminosarum bv. viceae strain isolated from a salt-affected soil.

The effects of sodium (Na+), calcium (Ca2+), magnesium (Mg2+), and boron (B) concentrations and sodicity, as measured by the sodium adsorption ratio (SAR), on the growth of a Rhizobium leguminosarum bv. viceae strain isolated from a salt-affected soil were studied. The rate of growth was measured in a yeast extract-mannitol broth, amended with salts having electrical conductivity (EC) of 4, 8, and 16 dS x m(-1). Each salinity level was prepared to achieve SAR values of 10, 20, and 30 with or without graded B concentrations of 0.5, 1, 3, and 5 mg x L(-1). We found that salinity levels equal to or more than 8 dS x m(-1) had negative effects on Rhizobium growth during the first days of incubation, but the effects became less pronounced after 1 week. Na+ concentrations of more than 1.1 g x L(-1) retarded growth, especially at high SAR values (i.e., at low Ca2+ concentrations). The retardation of growth increased with increases in EC up to 16 dS x m(-1), at all sodicity levels. Mg2+ added together with Na+ or with Ca2+ + Na+ affected growth more negatively than Ca2+ + Na+ alone. The effect of Mg2+ became more pronounced with increased salinities and sodicities. It was concluded that EC of more than 4 dS x m(-1) retarded growth of Rhizobium, but only at high sodicity levels. The relative specific ion effect on growth was in the order Na+ < Ca2+ < Mg2+. The harmful effect of Mg2+ on this strain was accentuated by adding Ca2+ to the cultural medium. When SAR increased from 10 to 30, Na+ had no clear effect on growth, irrespective of the accompanied cations, i.e, Ca2+, Mg2+, or Ca2+ + Mg2+. Growth was reduced by B concentrations as low as 0.5 mg x L(-1), and the B effect was enhanced by increased salinity.

Comparison of hup trait and intrinsic antibiotic resistance for assessing rhizobial competitiveness axenically and in soil.

The competitiveness of dual-strain inocula of cowpea rhizobia for nodulation of Vigna unguiculata (L.) Walp. was studied axenically between one slow-growing strain (P132, HP147, 401, or 22A1) and one fast-growing strain (176A26 or 176A28) at logarithmic inoculum ratios ranging from 10 to 10. Nodule infectivity was determined by multiple intrinsic antibiotic resistance, since both fast-growing strains were sensitive. Different hydrogen uptake (Hup) efficiencies of dual-strain inocula allowed for the comparison of an indirect rapid method. Infectivity data based on antibiotic resistance and Hup efficiency were fit to linearized fractile plots of log-normal distributions to determine C(AB) (percent infectivity at a 1:1 inoculum density) or I(50) (inoculum ratio at 50% infectivity). The slow growers were always better competitors and had I(50) values which ranged from 7 to 160,000 and C(AB) values which ranged from 62 to 97%. P132 was the best competitor of all those tested. Antibiotic resistance and Hup efficiency methods were in agreement with 401 (Hup) and 176A26 (Hup), but the Hup efficiency method overestimated the I(50) index with 22A1 (Hup) and 176A28 (Hup). The competition of each of the four slow-growing strains with indigenous rhizobia was examined in Cajanus cajan from three tropical soils. Nodule infectivity for all strains ranged from 42 to 96%, and P132 was the best competitor in all the soils. Hup efficiency overestimated infectivity by about 2-fold when Hup inocula (P132 and HP147) were used but underestimated infectivity by more than 100-fold when Hup inocula (401 and 22A1) were used. Although the Hup trait has limited quantitative usage axenically, it is only qualitative in soil competition studies and can only be used with Hup inocula.

Kinetics of denitrifying growth by fast-growing cowpea rhizobia.

Two fast-growing strains of cowpea rhizobia (A26 and A28) were found to grow anaerobically at the expense of NO(3), NO(2), and N(2)O as terminal electron acceptors. The two major differences between aerobic and denitrifying growth were lower yield coefficients (Y) and higher saturation constants (K(s)) with nitrogenous oxides as electron acceptors. When grown aerobically, A26 and A28 adhered to Monod kinetics, respectively, as follows: K(s), 3.4 and 3.8 muM; Y, 16.0 and 14.0 g . cells eq; mu(max), 0.41 and 0.33 h. Yield coefficients for denitrifying growth ranged from 40 to 70% of those for aerobic growth. Only A26 adhered to Monod kinetics with respect to growth on all three nitrogenous oxides. The apparent K(s) values were 41, 270, and 460 muM for nitrous oxide, nitrate, and nitrite, respectively; the K(s) for A28 grown on nitrate was 250 muM. The results are kinetically and thermodynamically consistent in explaining why O(2) is the preferred electron acceptor. Although no definitive conclusions could be drawn regarding preferential utilization of nitrogenous oxides, nitrite was inhibitory to both strains and effected slower growth. However, growth rates were identical (mu(max), 0.41 h) when A26 was grown with either O(2) or NO(3) as an electron acceptor and were only slightly reduced when A28 was grown with NO(3) (0.25 h) as opposed to O(2) (0.33 h).