Author(s):

Gnanachitra M and R Sridar

Abstract: Soil acidity is the major soil environmental stress factor limits <em>Rhizobium</em> survival and thereby reducing nodulation. In addition which results in deficiencies of Calcium, Magnesium, Potassium, Molybdenum and also cause aluminium toxicity which also affects nodulation and inturn the productivity in pulses. Developing a mutant strain of <em>Rhizobium</em> to tolerate acidity from wild type available in the acid soils will survive in soil and enhance the nodulation thereby improve N₂fixation. In the present study the three <em>Rhizobium</em> mutants <em>viz,. </em>VM1, VM2 and VM3 were developed using EMS mutagen for Chloramphenicol resistance from the native <em>Rhizobium</em> strains isolated from blackgram nodules grown under acid soils (pH 4.5 to 5.0) of NPRC, Vamban. Among the three, VM1 alone tolerated 200 ppm Chloramphenicol and others showed meagre growth. In the acid tolerant test VM1 <em>Rhizobium </em>mutant alone grew at both acidic pH levels (5.0 &amp; 4.0) and confirmed in BOX PCR analysis also. The <em>Rhizobium </em>mutant (VM1) increased the ability of wild mutant to grow on acidic pH without causing major alteration in its DNA.

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