Abstract
Alfalfa and its rhizobial symbiont are sensitive to salinity. We compared the physiological responses of alfalfa populations inoculated with a salt-tolerant rhizobium strain, exposed to five NaCl concentrations (0, 20, 40, 80, or 160 mM NaCl). Two initial cultivars, Halo (H-TS0) and Bridgeview (B-TS0), and two populations obtained after three cycles of recurrent selection for salt tolerance (H-TS3 and B-TS3) were compared. Biomass, relative water content, carbohydrates, and amino acids concentrations in leaves and nodules were measured. The higher yield of TS3-populations than initial cultivars under salt stress showed the effectiveness of our selection method to improve salinity tolerance. Higher relative root water content in TS3 populations suggests that root osmotic adjustment is one of the mechanisms of salt tolerance. Higher concentrations of sucrose, pinitol, and amino acid in leaves and nodules under salt stress contributed to the osmotic adjustment in alfalfa. Cultivars differed in their response to recurrent selection: under a 160 mM NaCl-stress, aromatic amino acids and branched-chain amino acids (BCAAs) increased in nodules of B-ST3 as compared with B-TS0, while these accumulations were not observed in H-TS3. BCAAs are known to control bacteroid development and their accumulation under severe stress could have contributed to the high nodulation of B-TS3.
Highlights
Salinity is an important abiotic stress and a major threat to crop productivity worldwide
The shoot Dry matter (DM) yield was 33 % higher in both populations obtained after three cycles of recurrent selection for salinity tolerance than in initial cultivars
The indoor method of recurrent selection was efficient to improve salinity tolerance in alfalfa, as shown by a 33% higher productivity obtained after three cycles of recurrent selection as compared with initial populations of two different genetic backgrounds
Summary
Salinity is an important abiotic stress and a major threat to crop productivity worldwide. The soil area affected by salt stress is estimated to be over 1030 million hectares in the world [1]. Soil salinization problems will likely worsen in the context of climate change, owing to the predicted increases of temperature and changes in precipitation patterns. The increasing use of poor-quality water for irrigation is an additional factor that exacerbates the global problem of soil salinity [5]. The adoption of crop rotations with deep-rooted perennial legumes is proposed as a way to decrease soil salinization [6]. Alfalfa (Medicago sativa L.) is one of the most important forage legume crops cultivated in the world owing to its high protein content and dinitrogen (N2 ) fixation ability [7]. In Canada, alfalfa is the third-largest crop cultivated, accounting for 76% of the lands in Canadian Prairies [3,8]
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