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Background: Winter climate change including frequent freeze‐thaw episodes and shallow snow cover will have major impacts on the spring regrowth of perennial crops. Non‐bloating perennial forage legume species including sainfoin, birdsfoot trefoil, red clover, and alsike clover have been bred for their adaptation to harsh winter conditions. In parallel, the selection of cold‐tolerant rhizobial strains could allow earlier symbiotic nitrogen (N) fixation to hasten spring regrowth of legumes. Methods: To identify strains forming nodules rapidly and showing high N‐fixing potential, 60 rhizobial strains in association with four temperate legume species were evaluated over 11 weeks under spring soil temperatures for kinetics of nodule formation, nitrogenase activity, and host yield. Results: Strains differed in their capacity to form efficient nodules on legume hosts over time. Strains showing higher nitrogenase activity were arctic strain N10 with sainfoin and strain L2 with birdsfoot trefoil. For clovers, nitrogenase activity was similar for control and inoculated plants, likely due to formation of effective nodules in controls by endophyte rhizobia present in seeds. Conclusions: Selection based on nodulation kinetics at low temperature, nitrogenase activity, and yield was effective to identify performant rhizobial strains for legume crops. The use of cold‐tolerant strains could help mitigate winter climatic changes. [ABSTRACT FROM AUTHOR]

Specific amino acids have protective functions in plants under stress conditions. This study assessed the effects of rhizobial strains on the amino acid composition in alfalfa under salt stress. Two alfalfa cultivars ( Medicago sativa L. cv Apica and salt-tolerant cv Halo) in association with two Sinorhizobium meliloti strains with contrasting growth under salt stress (strain A2 and salt-tolerant strain Rm1521) were exposed to different levels of NaCl (0, 20, 40, 80 or 160 mM NaCl) under controlled conditions. We compared root and shoot biomasses, as well as root:shoot ratio for each association under these conditions as indicators of the salt tolerance of the symbiosis. Amino acid concentrations were analyzed in nodules, leaves and roots. The total concentration of free amino acids in nodules was mostly rhizobial-strain dependent while in leaves and roots it was mostly responsive to salt stress. For both cultivars, total and individual concentrations of amino acids including asparagine, proline, glutamine, aspartate, glutamate, γ-aminobutyric acid (GABA), histidine and ornithine were higher in Rm1521 nodules than in A2 nodules. Conversely, lysine and methionine were more abundant in A2 nodules than in Rm1521 nodules. Proline, glutamine, arginine, GABA and histidine substantially accumulated in salt-stressed nodules, suggesting an enhanced production of amino acids associated with osmoregulation, N storage or energy metabolism to counteract salt stress. Combining the salt-tolerant strain Rm1521 and the salt-tolerant cultivar Halo enhanced the root:shoot ratios and amino acid concentrations involved in plant protection which could be in part responsible for the enhancement of salt tolerance in alfalfa. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Salt stress can affect alfalfa growth directly by adversely affecting metabolism, or indirectly by its effect on Rhizobium capacity for symbiotic N 2 fixation. Growth and carbohydrate metabolism in leaves, roots and nodules of two alfalfa cultivars ( Medicago sativa cv Apica and salt-tolerant cv Halo) in association with two rhizobial strains (A2 and salt-tolerant Rm1521) exposed to different levels of NaCl (0, 20, 40, 80 or 160 mM NaCl) were assessed under controlled conditions. For both cultivars, shoot and root biomasses and shoot to root ratio significantly declined with increasing NaCl concentrations. Under 80 mM NaCl, Halo plants yielded 20% more fresh shoot biomass than Apica while plants inoculated with Rm1521 allocated more biomass to the roots than to the shoots compared to A2. Halo plants maintained a steady shoot water content (about 80%) under the entire range of NaCl concentrations. Shoot water content was more variable in Apica. Apica in association with salt-tolerant strain Rm1521 maintained a better water status than with strain A2, as indicated by the higher shoot water content at 80 mM NaCl. Under salt stress, two major compatible sugars involved in plant osmoregulation, sucrose and pinitol, increased in leaves while a large accumulation of starch was observed in roots. In nodules, pinitol, sucrose and starch increased under salt stress and were much more abundant with strain Rm1521 than with A2. This suggests that there could be an active transport from the shoot to the nodules to help maintain nodule activity under NaCl stress and that strain Rm1521 increases the sink strength toward nodules. Our results show that combining cultivars and rhizobial strains with superior salt tolerance is an effective strategy to improve alfalfa productivity in salinity affected areas. [ABSTRACT FROM AUTHOR]