Water deficit is one of the most important environmental stresses that adversely affect crop growth and production; and symbiotic bacteria have an important role in resistance to drought stress. Growth and nodulation of an alfalfa population originating from Ouargla oasis in association with six rhizobial strains (four local strains: E131, E222, O152 and O172, and two introduced strains: Sm1021 and Sm2011) exposed to three levels of water deficit (75%, 50% and 25% of field capacity) were assessed under greenhouse conditions. Phylogenetic analysis of concatenated sequences of glnII and recA showed that tested strains were closely related to reference strain S. meliloti USDA 1002T at similarity level of 97.76 and 98.07%, respectively. Increasing water stress gradually reduced plant growth and nodulation. At irrigation doses D2 and D3, growth was reduced respectively by 30% and 80% of plant height and by 31% and 72% of root length. Likewise, shoot and root dry weights decreased. The negative effect was more pronounced on shoots (39% and 75%) than on roots (no significant difference between control and D2, and between control and D3). Inoculation increased the capability of the plants to counteract the stress compared to the non-inoculated plants. So, the plants inoculated with Sm2011 and O152 strains increased shoot height, root length, shoot and root dry weights and formed the most efficient symbiotic couples compared to the other strains and non-inoculated control in all water stress levels. Inoculation under water stress was found to significantly affect nodule numbers of alfalfa. Under severe stress, a significantly higher nodule number was obtained with the O152 and Sm2011 strains. Under moderate stress, strains Sm2011, Sm1021 had the best nodulation. Our results show that combining alfalfa and rhizobial strains with superior water stress tolerance is an effective strategy to improve alfalfa productivity in drought affected areas. KEYWORDS: Water stress, alfalfa, Sinorhizobium meliloti, resistance, yield.

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