Plant ionic relation and whole-plant physiological responses to waterlogging, salinity and their combination in barley

Waterlogging and salinity stresses significantly affect crop growth and global food production, and these stresses are often interrelated because waterlogging can lead to land salinisation by transporting salts to the surface. Although the physiological and molecular mechanisms of plant responses to each of these environmental constraints have been studied in detail, fewer studies have dealt with potential mechanisms underlying plant tolerance to the combined stress. This gap in knowledge is jeopardising the success of breeding programs. In the present work we studied the physiological and agronomical responses of 12 barley varieties contrasting in salinity stress tolerance to waterlogging (WL), salinity (NaCl) and combined (WL/NaCl) stresses. Stress damage symptoms were much greater in plants under combined WL/NaCl stress than those under separate stresses. The shoot biomass, chlorophyll content, maximum photochemical efficiency of PSII and shoot K⁺ concentration were significantly reduced under WL/NaCl conditions, whereas shoot Na⁺ concentration increased. Plants exposed to salinity stress showed lower damage indexes compared with WL. Chlorophyll fluorescence F_v/F_m value showed the highest correlation with the stress damage index under WL/NaCl conditions (r = –0.751) compared with other measured physiological traits, so was nominated as a good parameter to rank the tolerance of varieties. Average FW was reduced to 73 ± 2, 52 ± 1 and 23 ± 2 percent of the control under NaCl, WL and combined WL/NaCl treatments respectively. Generally, the adverse effect of WL/NaCl stress was much greater in salt-sensitive varieties than in more tolerant varieties. Na⁺ concentrations of the shoot under control conditions were 97 ± 10 µmol g^–1 DW, and increased to 1519 ± 123, 179 ± 11 and 2733 ± 248 µmol g^–1 under NaCl, WL and combined WL/NaCl stresses respectively. K⁺ concentrations were 1378 ± 66, 1260 ± 74, 1270 ± 79 and 411 ± 92 µmol g^–1 DW under control, NaCl, WL and combined WL/NaCl stresses respectively. No significant correlation was found between the overall salinity stress tolerance and amount of Na⁺ accumulated in plant shoots after 15 days of exposure to 250 mM NaCl stress. However, plants exposed to combined salinity and WL stress showed a negative correlation between shoot Na⁺ accumulation and extent of salinity damage. Overall, the reported results indicate that K⁺ reduction in the plants under combined WL/NaCl stress, but not stress-induced Na⁺ accumulation in the shoot, was the most critical feature in determining the overall plant performance under combined stress conditions.