Contributions and limitations to symbiotic nitrogen fixation in common bean (Phaseolus vulgaris L.) in Romania

Common beans usually achieve grain yields less than the genotypic potential of the cultivar under Romanian field conditions. To understand better the contribution of nitrogen fixation to the yield formation, I made a long-term evaluation (1977–1994) of inoculation effects of 19 Rhizobium leguminosarum bv. phaseoli strains on common bean cultivated in several locations. Grain yields were significantly influenced by all selected strains, years and locations in all experimental cycles, and only partially by the interactions between strains and years or strains and locations. The average yield increases induced by these strains during the four cycles ranged from 6 to 20%. Four bacterial strains proved to be more stable in their field performances, taking into account the yield increases greater than 10% over controls observed in all individual trials. Mean yields and variation limits recorded during the long-term evaluation of strain efficacy in locations with different soil pH values showed similar patterns of yield increases from soils with acidic to neutral pH values. Linear regression between mean grain yields and average temperatures demonstrated the limiting effect of temperature on yield. The interaction between bacterial strain and nitrogen fertiliser rate demonstrated the ability of dinitrogen fixation to satisfy the crop requirements for this element. An evaluation of the amounts of nitrogen fixed in three common bean cultivars inoculated with two bacterial strains showed different N2-fixing capacities among plant genotypes.