Physiological and biochemical responses to salt stress of alfalfa populations selected for salinity tolerance and grown in symbiosis with salt-tolerant rhizobium
Bertrand, A., Gatzke, C., Bipfubusa, M., Lévesque, V., Chalifour, F.P., Claessens, A., Rocher, S., Tremblay, G.F., Beauchamp, C.J. (2020). Physiological and biochemical responses to salt stress of alfalfa populations selected for salinity tolerance and grown in symbiosis with salt-tolerant rhizobium. Agronomy, [online] 10(4), http://dx.doi.org/10.3390/agronomy10040569
Plain language summary
Salinity is an important abiotic stress and a major threat to crop productivity worldwide. In Canada,
approximately 30% (20 million ha) of agricultural land either openly shows salinization or is at risk of
becoming salinized. In addition, soil salinization problems will likely worsen in the context of
climate change, owing to the predicted increases of temperature and changes in precipitation patterns
worldwide and to the increasing use of poor-quality water for irrigation worlwide. Alfalfa, the most important legume forage crop in Canda, is sensitive to salt stress. In an effort to increase the salinity resistance of alfalfa, we proceeded with the selection of salt-resistant alfalfa populations at the Salt Lab in Swift Current (SK). Here we tested under salinity stress two alfalfa populations obtained after three cycles of selection, in association with a salt resistant rhizobia strain to ascertain the improvement of salinity resistance in response to selection. Our results shows that our selection method is very efficient at improving salt tolerance since the selected alfalfa populations had significantly higher yields than initial populations under salt stress. When looking at the mechanisms of resistance, we found that the osmotic adjustment both in the plant (leaves and roots) and at in the nodules containing rhizobium were key to the increased salt resistance and higher yield.
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.