Eco-physiological responses and symbiotic nitrogen fixation capacity of salt-exposed Hedysarum carnosum plants

Abstract

Nitrogen nutrition of Hedysarum carnosum, a pastoral legume common in Tunisian central and southern rangelands, largely depends on atmospheric nitrogen fixation. Yet, this process is greatly affected by environmental factors such as salinity. This study aimed to characterize the tolerance limits and the physiological responses of H. carnosum to salinity under symbiotic nitrogen fixation. Salt treatment was imposed by adding NaCl at different concentrations (0, 50, 100 and 200 mM) to the nutrient solution. Na⁺ content generally increased in the plant organs with increasing salinity in the culture medium. Especially, an excess accumulation of this cation was observed in leaves. Despite the fact that Na⁺ accumulation decreased plant growth, both nodulation and symbiotic nitrogen fixation capacity of H. carnosum appeared to be relatively salt-tolerant, owing to the plant capacity to maintain tissue hydration, control Na⁺ accumulation in shoots, and to conserve nodule efficiency to fix N₂. Taken together, our findings indicate that H. carnosum is a glycophyte that can tolerate moderate salinity (100 mM), suggesting its possible utilization (i) in the improvement of soil fertility and (ii) in saline pastures, where the survival of other fodder species is critical.

Key words: Hedysarum carnosum, nodulation, salinity, symbiotic nitrogen fixation.