Effects of soil-mutualistic bacterial inoculation on osmolyte production and growth in three generations of alfalfa (Medicago sativa) populations selected for salt tolerance

The halophile Halomonas maura and biological N-fixing Rhizobium may increase salinity tolerance of alfalfa (Medicago sativa L.), especially for alfalfa populations with improved salt tolerance. The objectives of this study were to 1) evaluate whether salt-tolerant bacteria or a 60 kg/ha nitrogen amendment could increase the salinity tolerance of alfalfa; 2) assess whether recurrent selections for salt tolerance in alfalfa populations would differentially interact with soil bacteria.

Three alfalfa generations sequentially selected for improved salt tolerance were inoculated with either highly salt-tolerant (H. maura), moderately-tolerant (Ensifer meliloti) bacteria, or a 60 kg/ha nitrogen amendment in either non- (0 dS/m), moderate-(8 dS/m), or highly (16 dS/m) saline soil in the greenhouse.

Recurrent selection of alfalfa in salinity stress causes increased chlorophyll content, stem count, and plant height; however, yield loss occurred under 120 d of moderate salinity stress. Improved alfalfa generations in combination with nitrogen amendments were able to produce the highest biomass yields in moderate salinity stress. E. meliloti had a positive effect during regrowth, and it increased proline content in both shoot and root tissue. In comparison, H. maura provided no discernible benefits to alfalfa under salt stress.

Recurrent plant selection combined with N-fixing rhizobium could increase alfalfa growth under a multi-harvest system. As alfalfa positively responded to N fertilizer and N fixing rhizobium, improvement of alfalfa traits related to nitrogen utilization and biological N fixation may be vital to alfalfa salt tolerance.