The antioxidant responses of nodules to water deficit stress were studied in ten symbiotic combinations involving two Moroccan alfalfa populations (Tata and Dem) and five rhizobial strains (RHL2, RHL29, RHL48, RHL68 and RHL80). The experiment was conducted in a greenhouse at 32/22 °C d/n, 50-80% of relative humidity and a photoperiod of 16 h. The seedlings were separately inoculated with the suspensions of five rhizobial strains and grown under two water regime irrigations, 75% of Field capacity (optimal irrigation) and 25% of Field capacity (water deficit), in plastic pots filled with sterile sand and peat at 9/10 and 1/10 ratio, respectively. After 45 days of stress, the nodule biomass and some nodular antioxidant enzyme activities as peroxidase, polyphenol oxidase, catalase and superoxide dismutase were evaluated. The results indicated that the water deficit caused a significant reduction in nodule biomass with the significant differences between the tested symbiotic combinations. The reductions, comparatively to optimal irrigation conditions, varied from 18.63 to 30.14 %. The lowest reductions were marked with the symbiotic combination Tata-RHL80 and Tata-RHL2 (18.63 and 18.98% respectively). However, the highest reductions were observed when the seedlings were inoculated with the rhizobial strain RHL4 (30.14 and 28.54% for Dem-RHL4 and Tata-RHL4, respectively).The activities of the antioxidant enzymes analyzed were found increased in nodules of all combinations under water deficit and the high nodule biomass marked in some combinations was associated with the highest antioxidant activities suggesting that the enhancing of the antioxidant activities of these enzymes could play a critical role in water deficit tolerance.
