Influence of acid soil on nodulation and interstrain competitiveness in relation to tannin concentrations in seeds and roots of Phaseolus vulgaris

The relationship between acid soils and (1) early nodulation in common beans, (2) interstrain competitiveness of Rhizobium leguminosarum bv. phaseoli strains and (3) tannin concentration in roots, were evaluated in two experiments. Ten Phaseolus vulgaris cultivars of different seed tannin content were grown in pots in a greenhouse in soil at pH 4.5 or 5.5, inoculated with equal amounts of strains CIAT 899 and KIM5. At pH 5.5, on all cultivars strain KIM5 occupied a greater proportion of nodules than strain CIAT 899. This was reversed at pH 4.5. Nodule number and nodule weight, 21 days after inoculation, were strongly reduced but root tannin concentrations doubled at pH 4.5. Nodule numbers were negatively correlated with tannin concentration in roots at both pH values. Seed tannin content was negatively correlated with early nodulation, but this was statistically significant only at pH 5.5. Nodulation of cultivars with high seed tannin contents was generally more acid tolerant than of those with low seed tannin content. In a second experiment, six Phaseolus vulgaris cultivars were planted at two field sites with acid soil and one site with neutral soil. In the field, as in the pots, nodule numbers at early stages of plant development were reduced by 30-50% in the acid soils compared to the neutral soil. In contrast, root tannin content in acid soils was twice that in the neutral soil. There were significant cultivar and cultivar by location effects. Strains of rhizobia were tested for tolerance to bean seed tannin extracts, to seed diffusates and for survival on seed surfaces. The Rhizobium strains were sensitive to tannin extracts, with obvious strain differences. Sensitivity of most strains was greater at acidic pH. Seed diffusates, in contrast, did not inhibit growth of most strains.