The physiological and biochemical responses of barley (Hordeum vulgare L.) to salinity stress under hydroponic system

Introduction
Barley (Hordeum vulgare L.) is a salinity tolerant crop species that has high economic value in arid and semi-arid regions of the world. Salinity stress is a multiple stress and it is mainly caused by high concentration of sodium ions. Osmotic stress reduces water absorption by plant roots, and leads to various physiological disturbances, such as reduction of photosynthetic activities and accumulation of reactive oxygen species. There are key biochemical processes at the cellular level that plants use to develop salinity tolerance. Salinity stress has important physiological effects, including loss of turgorescence pressure and osmotic adjustments, and reduction of germination rate, leaf water potential, internal CO₂ concentration, CO₂ stomatal conductance, and net photosynthesis rate. Also at the biochemical levels, salinity stress increases the accumulation of proline, glycine betaine, sugars and antioxidant enzymes, and reduces the Rubisco activities and accumulation of reactive oxygen species or Na⁺ /K⁺ ratio in the plants. In this experiment, the physiological and biochemical characteristics of barley varieties with the aim of identifying salinity tolerant varieties were evaluated under non-stress and NaCl stress conditions in the hydroponic system.
Materials and methods
To investigate the physiological and biochemical response of barley to salinity stress, four barley varieties (Reyhan3, Fajr30, Nosrat and Dasht) were assessed at four levels of salinity stress (0, 100, 200, and 300 mM NaCl) under a hydroponic system. The experiment was conducted as a factorial experiment in completely randomized design with three replications in research greenhouse of Department of Biotechnology, Faculty of Agriculture, Azarbaijan Shahid Madani University, Tabriz, Iran. Two weeks after application of salinity stress, shoot and root of seedlings were separated and measured fresh and dry weight. Also, physiological and biochemical characteristics such as relative water content (RWC), ratio of K⁺/Na⁺ ions, osmotic regulators (soluble sugars, glycine betaine, proline), hydrogen peroxide (H₂O₂) and antioxidant enzymes, as well as non-antioxidants enzymes such as photosynthetic pigments (carotenoids, a and b chlorophylls) were separately measured in shoot and root of barley varieties.
Research findings
The results of analysis of variance showed that there was a significant difference among the barley varieties for all measured traits. Also, the effects of salinity stress levels and variety × salinity stress interaction were significant on all studied traits, except for glycine betaine in the root. The results indicated that the salinity stress reduced RWC, shoot dry weight, photosynthetic pigments and K⁺/Na⁺ ratio, while increased the soluble sugars, glycine betaine, proline, H₂O₂, catalase and peroxidase in the root and shoot of barley varieties. The results of comparison of means and principal components analysis showed that Nosrat variety followed by Fajar30 variety had the higher RWC, shoot dry weight, photosynthetic pigments, K⁺/Na⁺ ratio, and catalase and peroxidase levels in the roots and shoot, and showed more tolerance to salinity stress compared to the other two varieties.
Conclusion
The results of this experiment showed that the effects of salinity stress levels and barley varieties, as well as variety × salinity stress interaction on the measured physiological and biochemical characteristics were significant. Comparison of the studied barley varieties also indicated that two varieties, Nosrat and Fajar30, were more tolerant to salinity stress, so it seems that these varieties are more suitable for salinity prone environments.