Potential use of rhizobial exopolysaccharides to protect wheat (Triticum aestivum) germination under salt stress

Abstract

Salinity is a major limiting factor in agricultural productivity. This study evaluates the effect of two exopolysaccharides (EPS), produced by Bradyrhizobium cytisi (EPS_B21) and Rhizobium sullae (EPS_BG15), on the tolerance of wheat (Triticum aestivum) to salt stress. Treatments included a control, NaCl (8 g.L⁻¹), EPS alone (0.5 and 5 mg/mL) and EPS + NaCl combinations. EPS were extracted by ethanol precipitation. R. sullae and B. cytisi yielded 45 g.L⁻¹ and 37 g.L⁻¹ respectively. The extracts showed antioxidant activity, with EPS_BG15 achieving 69.15 % DPPH inhibition at 9 mg/mL, 26.32 % higher than EPS_B21. On the other hand, the FRAP assay revealed a better iron reduction capacity for EPS_B21 (IC₅₀ = 20.74 mg/mL vs. 59.36 mg/mL for EPS_BG15). Under salt stress, the addition of EPS at 0.5 mg/mL improved germination (89.33 %) compared with NaCl alone (70.66 %). However, fresh and dry weights remained lower than those of the control (0.25 g and 0.02 g), particularly for the fresh weight with EPS_B21+NaCl (0.09 g), despite the improved germination rate. EPS_BG15 treatment maintained high levels of chlorophyll a (37.94 μg/g), chlorophyll b (21.52 μg/g) and carotenoids (9.34 μg/g), while reducing NaCl-induced proline accumulation (182.28 %). Antioxidant enzyme activity varied according to the treatments: APX was activated by EPS_B21(5 mg/mL), with activity reaching around 1.035 U.mg⁻¹ of protein, compared with NaCl treatment, while CAT and POD showed contrasting responses depending on the doses and combinations applied. These observations reinforce the interest of EPS, particularly those from R. sullae, as natural biofertilizers capable of mitigating salt stress effects.