VARIATION IN ACID-AL TOLERANCE OF BRADYRHIZOBIUM-JAPONICUM STRAINS FROM AFRICAN SOILS

Seventy-six strains of Bradyrhizohium japonicum (54 African and 22 exotic) were examined for their tolerance to acidity (pH 4.5), low P (5 μ M) and high Al (50 and 100 μ M) levels by using an agar plate method. Forty-four strains were tolerant to acidity regardless of P levels (1,000 or 5 μ M) of the medium and 22 were sensitive. The remaining 10 strains differed in their tolerance to acidity depending on the level of P of the medium; nine required a low P (5 μ M) level for being tolerant but one required a high P level (1,000 μ M). All of the 21 strains which could grow under the most severe stress conditions used, consisting of low P and high Al (100 μ M) levels at pH 4.5, showed tolerance to acidity at both levels of P. Isolates from a highly acidic soil (Ultisol, pH(H2O) 4.3) of Onne, south-eastern Nigeria, showed different levels of acid-Al tolerance but tended to be more tolerant than those from a slightly acidic soil (Alfisol, pH (H2O) 6.4) of Ibadan, south-western Nigeria. However, 20% of the Onne isolates and 30% of the Ibadan isolates were sensitive to acidity, respectively. Thus, the tolerance of the strains was not necessarily determined by the acidity of the bulk soils in which they occurred. Extracellular polysaccharide production by the strains which was observed at low P level did not show any distinct relation to acid-Al tolerance. The great variation in acid-Al tolerance observed among the strains of B. japonicum suggests that it may be possible to select strains for use as inoculants in acid soils. © 1990 Taylor & Francis Group, LLC.