Combined effects of salinity and soil drying on growth, water, status, and photosynthesis in Medicago ciliaris and Medicago polymorpha

Drought stress along with soil salinity are the most common and frequently co-occurring abiotic stresses and threaten plant productivity, especially in arid and semi-arid regions of the world. The aims of this study were to investigate the interaction between soil drying (D) and salinity (S) in two forage species Medicago ciliaris TNC1.11 line and M. polymorpha TNP1.11 line. Plants were cultivated for 1 month in silty-sandy soil under two irrigation modes: 80% and 33% of field capacity (FC). The amount of the evapotranspirated water was replaced by a nutrient solution containing either 0 or 50 mM NaCl. Morphological parameters, photosynthesis, whole plant growth, water, and nutrient status (Na, K, Ca, Zn) were investigated. Under control or stressed conditions, M. ciliaris produced more biomass when compared to M. polymorpha. The whole-plant biomass production represented 237%, 230%, 175%, and 172%, respectively, in treatment C, S, D, and D + S in M ciliaris when compared to M polymorpha. The higher productivity correlates under all studied treatments in M. ciliaris with (i) a higher photosynthetic activity and water-use efficiency, (ii) more leaves per plant, higher number of ramifications and a longer stem, (iii) a high root water content under C, S, D + S treatments, and (iv) a high Zn accumulation in leaves. Salinity and drought applied individually negatively affect growth, photosynthesis, water, and potassium leaf content in both studied species. The effects of both constraints were not significantly additive. Salinity affects growth of the two studied species by the same manner (reduction by 64% as compared to control). The major advantage of M. ciliaris under dry or saline conditions seems to be the general high water-use efficiency and the corresponding positive impact on the ROS risk. Besides, M ciliaris showed a lower increase of Na+ and less decrease of K+ at salinity leading to an overall lower Na+/K+ ratio as M. polymorpha. This shows that the selectivity of M. ciliaris is higher as the one of M. polymorpha. When compared to M. polymorpha, TNC1.11 line of M. ciliaris produced more biomass under drought, salinity, and combined stresses; therefore, we recommend its multiplication and use by farmers as a forage candidate for the rehabilitation of soils in arid and semi-arid salinized regions.