Abstract
Symbiotic interactions between peas and Rhizobium leguminosarum were investigated by inoculating four pea lines, three of which are strain-specific resistant to the European strain 311d, with various combinations of two strains of Rhizobium, 311d and Tom++. The strains were almost equally good to infect the susceptible European cultivar Hero when added singly inoculated. After mixed inoculation (1:1 proportion) strain analysis by ELISA revealed that the nodules were preferentially formed by 311d, although some Tom++ nodules were also found mainly on the upper part of the root. Our conclusion is that Tom++ is less compatible in comparison with 311d. In addition, we found that as the Hero plants emerged, they were becoming more resistant towards infection with not adapted bacteria. The strain-specific resistant lines from Afghanistan belong to two different systems: Afgh. I, completely resistant to 311d and highly nodulating with Tom++, and Afgh. III, incompletely resistant to 311d and poorly nodulating with Tom++. Mixed inoculations resulted in nodule depressions, as compared to single inoculations with Tom++ ranging from 87% to 14%. The ability of 311d to block infection sites on the roots were found to depend on the degree of symbiotic adaptation between Afgh. I and Tom++, respectively Afgh. III and Tom++. Strain analysis after double strain inoculation of Afgh. I plants revealed that some nodules were induced by strain 311d. Thus, the presence of Tom++ in this case influences the degree of host resistance. However, in Afgh. III plants the resistance towards nodulation were unaffected by the presence of Tom++. We suggest that the degree of symbiotic adaptation may change the barrier of resistance towards infection.
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