Relationship between symbiotic nitrogen fixation, sucrose synthesis and anti-oxidant activities in source leaves of two Medicago ciliaris lines cultivated under salt stress

The aim of this study was to investigate the effect of NaCl salinity on symbiotic nitrogen fixation (SNF) in a Medicago ciliaris–Sinorhizobium medicae symbiosis system, with special emphasis on two genetically pure lines of M. ciliaris developed from germplasm recovered from saline and non-saline lands. Saline treatment (100 mM NaCl) was applied to these two contrasting lines growing in pots in a greenhouse after the appearance of nodules. At the end of the vegetative period, both lines under salt stress registered a decrease in nitrogenase activity, whole plant and organ biomass production and chlorophyll concentration, but the tolerant line TNC 1.8 was much less affected by salt than the sensitive line TNC 11.9. Salinity also provoked a decrease in sucrose phosphate synthase activity (SPS), mainly in TNC 11.9. The decline in leaf SPS in TNC 11.9 was accompanied by salinity-induced membrane oxidative damage in source leaves of the sensitive line, probably induced by a disturbance in soluble sugar/starch partitioning. We report these physiological and biochemical parameters to promote these useful lines as useful germplasm with potential to reveal detailed transcriptomic and metabolomic information and new genetic tools which could assist breeders in improving salinity tolerance and biomass for pasture legumes.

Research highlights▶ Salinity tolerance and nodulation efficiency in Medicago ciliaris are linked to better nodule growth. ▶ Both parameters depend on the protection of the leaves against oxidative damage and thus on the capacity of leaves to synthesize sucrose. ▶ Sucrose made available to nodules would not only sustain SNF machinery but also provide energy and carbon skeleton for SNF and ammonium assimilation.