NODULATION OF STRAIN-SPECIFIC RESISTANT (311D) PEA LINES BY RHIZOBIUM-LEGUMINOSARUM STRAINS IN SPLIT-ROOT SYSTEMS

The legume host determines the extent of nodule formation by autoregul ation, a phenomenon called nodulation suppression. In this study, we u sed a split-root technique to study nodulation suppression by peas. No dule formation by peas following removal of tap root was studied using the three pea lines, Afghanistan III, Hero and Iran II. The pea lines Afghanistan III and Iran II were chosen since they are equally incomp letely resistant to infection with the European Rhizobium leguminosaru m strain 311d but differ in susceptibility to infection with the Turki sh strain Tom, a fact that was exploited in a subsequent experiment. T he European pea cultivar Hero, nodulating with both 311d and Tom, was used as control. The plant lines differed in response to removal of th e tap root. The nodulation of Hero and Iran II by 311d and Tom++, resp ectively, was unaffected but dry matter production of Hero decreased w hen grown in the split-root system in contrast to Iran II, which incre ased its dry matter production compared to undivided roots. The nodula tion of Afghanistan III by Tom++ was stimulated by tap root removal bu t dry matter production was unaffected. Nodule efficiency was unaffect ed in all plant lines. We found that the presence of Tom++ on one side of the rootlet of Afghanistan III and Iran II plants neither affected the degree of resistance to 311d on the other root-half nor affected the presence of 311d on one side of the root-half the nodulation of To m++ on the other. We concluded that 311d is incapable to induce the ho st plant to produce a resistance factor towards infection with Tom++. The results also confirms the concept that the decrease of Tom++ nodul es when co-inoculated with the non-nodulating strain 311d depends on b locking of infection sites by the former strain.