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Abstract This study was applied on two varieties of barely (Hordeum vulgare L.), Giza 132 and Giza 134 with different tolerance to salinity and osmotic stresses in attempt to find out physiological, biochemical and molecular responses. Preliminary experiment was carried out to determine the sublethal dose of NaCl, and plants were treated periodically with three different concentrations of NaCl (100, 150 and 200 mM) till the post-vegetative stage. Results showed inhibitory effects on the growth and yield parameters at 200 mM. Thus, barely seedlings were treated with NaCl (150 mM) as well as isoosmotic polyethylene glycol 6000 (19.5% PEG) and collected after two Salinity and osmotic stresses are main reasons of reduced plant growth and productivity and cause complex set of responses at physiological, cellular, and molecular levels. Since both osmotic and ionic effects are interrelated and co-exist under saline conditions, separating the two components is an important step in understanding the basis of salt tolerance. In the current study, barely seedlings (cultivars: Giza 132 and 134) were treated with NaCl (150 mM) as well as iso-osmotic polyethylene glycol (PEG 6000) (19.5%). Results showed decreasing of almost all growth parameters in seedlings treated with PEG rather than NaCl. The higher levels of different oxidative markers, non-enzymatic antioxidants, in addition to enzymatic antioxidants were recorded mainly under osmotic stress. Furthermore, osmoregulators including proline (Pro) and glycine betaine (GB) increased in the root tissue, in conjunction with enhancement in the antioxidant status of shoots by applying PEG. Results also suggested that different regulatory mechanisms may exist in the regulation of salt tolerance under different situations including types of both cultivar and stress component. Based on molecular analysis using RT-PCR, HvNHX gene (coding for Na+/H+ antiporter) was highly expressed after 48 h from treatment in the roots of salt-stressed seedlings, but its expression level was more pronounced in osmotic-stressed shoots than salt-treated ones, and the opposite was true for HvGORK gene (regulates voltage-gated K+-permeable channels). On the other hand, HvDREB gene (coding for dehydration responsive element binding protein) recorded higher expression in the roots under PEG compared to control. The current study suggested that salttolerant cultivar (Giza 134) performed consistently better under salt stress and poor under osmotic one with regard to salt-sensitive cultivar (Giza 132). Keywords: Barely, PEG, salinity, osmotic stress, oxidative markers, antioxidants, gene expression. |