DISTRIBUTION AND N-2-FIXING ACTIVITY OF FRANKIA STRAINS IN RELATION TO SOIL DEPTH

The qualitative and quantitative distribution of Frankia strains infec tive on Elaeagnus angustifolia L. from different depths in the same so il (10-20 cm; 30-40 cm; 50-60 cm) were compared. The soil samples coll ected from a natural stand devoid of Elaeagnaceae were planted with El aeagnus angustifolia L. seedlings. All plants became nodulated, demons trating that Frankia strains were present at least down to 60 cm in th e soil. The decreased density of Frankia strains was correlated to the decline of soil organic matter content with soil depth. DNA extracted from nodules produced on Elaeagnus angustifolia L. seedlings were pol ymerase chain reaction (PCR) characterized by amplifying the nifD-K in tergenic spacer (IGS), using the polymerase chain reaction (PCR) follo wed by restriction fragment length polymorphism (RFLP) analysis. This showed the presence of seven nif-HaeIII profiles within this Frankia c ommunity. Diversity of Frankia strains was maintained throughout the s oil column, but the relative distribution of strains varied. Some nif- HaeIII profiles were only found in the deeper soil. suggesting differe nt selective advantages to withstand the constraints of soil depth. N- 15(2) experiments indicated that all the strains tested had N-2-fixing activity, However. efficiency was not significantly different among n odules of different nif-HaeIII profiles. Therefore, N-2-fixing activit y does not seem to be the main factor responsible for the different di stribution of Frankia strains at different soil depths.