Effectiveness of native Chilean strains of Rhizobium meliloti in symbiosis with Medicago polymorpha

Alfonso Herrera O.1, Luis Longerl S.1 y Carlos Ovalle M.2

Fifty strains of Rhizobium meliloti were field collected in the Mediterranean zone of central Chile, between the IVtll and IXtll Regions (300 07' - 380 47' S). These strains were then tested for relative nitrogen fixation efficiency with a local ecotype ("Cauquenes") of Medícago polymorpha. Emerging seedlings were inoculated with one of the 50 strains (n =3) and then planted individually in test tubes containing an N-free nutrient solution in agar, and randomly distributed in a heated growth chamber.
Statistically significant differences were found in phytomass production, a clear indication of varying N2-fixation potential. However, the range of differences occurs in large groups of similarly peñorming strains. This makes agricultural selection more difficult.
The range of pH (water) in soils of the collecting sites ranged from 5.2 to 7.5, with 54% of strains coming from soils ranging from 6.0 to 6.4 pH. Moreover, a significant correlation (P S 0.05) occurred between soil pH and nitrogen fixation efficiency of the rhizobial strains collected. The 10 most efficient strains were found in soils with pH over 6.0; 60% of these strains comes from soils with over 6.5 pH. The most efficient strain (H7, Combarbalá) was found in a 7.3 pH soil, and the least efficient one (11 a, Arauco) was found in a highly acidic soil (pH 5.2).
The agricultural implications of the relation between strain fixation efficiency and a high soil pH are briefly discussed.

Keywords: Rhizobium melíloti, rhizobial strains, Medícago polymorpha, annual medic, nitrogen fixation efficiency.
1 Universidad de Concepción, Facultad de Agronomia, Casilla 537, Chlllán, Chile.
2 Centro Regional de Investigación Quilamapu (INIA), Casilla 426, Chlllán, Chile.