ABSTRACT To search for an alternative to alfalfa under conditions of salinity and drought, a comparative study was carried out to explore the effect of salinity on the symbiosis of alfalfa and local esparcet species (Onobrychis transcaucasica and Onobrychis chorassanica) inoculated with their nodule bacteria. The salinity of up to 30 mM NaCl insignificantly affected the biomass growth of shoots and roots of alfalfa plants, but the increase in the salinity from 30 to as high as 140 mM NaCl led to the biomass decrease. The salinity produced a double effect on the nodulation process in inoculated alfalfa plants as follows: 1) at 30 - 100 mM NaCl the stimulation of nodulation and increased leghemoglobin activity were observed; 2) at salinity concentrations higher than 100 mM NaCl the suppression of both nodule formation and leghemoglobin activity was observed. Alfalfa plants under inoculation with the Sinorhizobium meliloti 10 strain obtained a considerable resistance to salinity (50 - 80 mM NaCl). The efficient symbiosis of O. transcaucasica plants with Rhizobium sp. OT111 and O. chorassanica plants with Rhizobium sp. OC109 enhanced the adaptation of plants to salinity up to 150 mM NaCl. The gradual growth suppression of both Onobrychis plants species started from 200 mM NaCl, and salinity concentration 300 mM NaCl was critical (sublethal) for plants independently of inoculation by nodule bacteria. In field conditions, O. chorassanica was more resistant to salinity than O. transcaucasica, but minimal irrigation for both species of Onobrychis showed a higher effect on their growth and development than the moderate salinity at the concentration 75 mM NaCl. The lower limit (drought threshold) of drought-resistance of Onobrychis plants was 6% - 8% of soil humidity. In shoot and roots of alfalfa, both Onobrychis plant species subject to salt stress, aldehyde oxidase and xanthine dehydrogenase enzymes and different number of their isoforms as well as their electrophoretic mobilities/activities were found.
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