Understanding the physiology of combined salinity and waterlogging tolerance in barley

The world population is expected to reach over 9.3 billion by 2050 prompting the need to increase agricultural food production by 100%. At the same time, agricultural lands globally are suffering from human induced and natural environmental stresses such as salinity and waterlogging. Soil salinization is affecting more than 800 million hectares (about 6 percent) of the land. While some of this land is naturally saline other parts have become saline as a result of secondary salinization caused by irrigation. According to the FAO, 11% of irrigated lands (about 34 million ha) are suffering from secondary (human-induced) salinity. As the growth of most agricultural crops is strongly reduced by high concentrations of salt in the soil, economic penalties are high, and so is the threat to global food security. About 60-80 million hectares of land are affected to some extent by combined waterlogging and salinity stress. Waterlogging reduces the available air in the soil and has a profound effect on plant growth. These two stresses are often interrelated, as waterlogging can lead to salinization by transporting the salts to the surface. In many parts of the world (e.g. Australia, USA, Pakistan, India, Iran, Thailand and Egypt) these two environmental stresses coexist. While the physiological and molecular mechanisms of plant responses to each of these environmental constraints have been studied in detail, the mechanisms underlying plant tolerance to their combined stress have not been well understood. This gap in knowledge is jeopardizing the success of breeding programs and has to be bridged. The current study focuses on plant physiological traits under combined stresses compared to separate stresses and no stress. To address the whole plant physiological mechanisms involved in plant's adaptation to combined waterlogging (WL) and salinity (NaCl) stress, 12 barley varieties contrasting in salinity stress tolerance were grown in potting mixture under controlled light-temperature glasshouse conditions. Two weeks of NaCl, WL and combined WL/NaCl stresses were applied to barley plants. The damage index scoring system was used to evaluate the overall effects of NaCl, WL and combined WL/NaCl on the growth and agronomical characteristics of barley plants, based on the extent of chlorosis and necrosis in the shoot. A 0 to 10 scaling system was used; with grade 0 given to plants that showed no visual symptoms of stress and 10 representing dead plants. Damage symptoms were much stronger in plants under combined WL/NaCl stress compared to separate stresses. The shoot biomass, chlorophyll content, maximum photochemical efficiency of PSII and shoot `K^+` content were significantly reduced under WL/NaCl conditions, while shoot `Na^+` content increased. Plants exposed to salinity stress showed less damage indexes compared to WL. Chlorophyll fluorescence Fv/Fm value showed the highest correlation with the plant damage index under WL/NaCl conditions (R = -0.751) compared to other measured physiological traits and was nominated as a good parameter to rank the tolerance of varieties. The average fresh weight was reduced by 73 ¬¨¬± 2, 52 ¬¨¬± 1 and 23 ¬¨¬± 2 percentage of control upon NaCl, WL and combined WL/NaCl treatments, respectively. Chlorophyll content (SPAD values) of the oldest leaf after 10 days of treatments was also used as a proxy for stress tolerance. Based on these findings, barley varieties were divided into two groups. Genotypes having relative SPAD values less than 10% for combined WL/NaCl treatment were classified as sensitive, and those having more than 60% were classified as tolerant. Amongst these, varieties Yerong, ZUG293 and YYXT were found to be the most tolerant. `Na^+` content under control conditions was 97 ¬¨¬± 10 ˜í¬¿mol/g, and increased to 1519 ¬¨¬± 123, 179 ¬¨¬± 11 and 2733 ¬¨¬± 248 ˜í¬¿mol/g under NaCl, WL and combined WL/NaCl stresses, respectively. `K^+`+ content was 1378 ¬¨¬± 66, 1260 ¬¨¬± 74, 1270 ¬¨¬± 79 and 411 ¬¨¬± 92 ˜í¬¿mol/g under control, NaCl, WL and combined WL/NaCl stresses, respectively. Generally, the adverse effect of WL/NaCl stress was much stronger in salt-sensitive varieties such as ZUG403 and Naso Nijo compared to more tolerant varieties such as ZUG293 and YU6472. In general, coexisting waterlogging and salinity provoked a combination of the effects of each stress applied autonomously, even though WL had a greater contribution in limiting factors compared to salinity. To study and compare the plant shoot and root response to combined WL/NaCl stress, hydroponic experiments were designed. Eight barley varieties contrasting in salinity stress tolerance were grown in half-strength Hoagland solutions for six days. The seedlings were assigned to separate and combined NaCl and WL treatments after 8 (first sampling) and 16 days (second sampling) of stress. Average shoot fresh weight on the first sampling was reduced by 59 ¬¨¬± 5, 55 ¬¨¬± 2 and 41 ¬¨¬± 2 percentage of control upon NaCl, WL and combined WL/NaCl treatments, respectively, while it was only 36 ¬¨¬± 5, 24 ¬¨¬± 2 and 12 ¬¨¬± 1 percentage for the same treatments on the second sampling. The shoot fresh weight changes of the WL/NaCl treated plants correlated more with effect of WL alone and it is suggested that biomass is more limited by hypoxia than salinity in shoots. Average root fresh weight on the first sampling was reduced by 73 ¬¨¬± 6, 46 ¬¨¬± 4 and 30 ¬¨¬± 2 percentage of control upon NaCl, WL and combined WL/NaCl treatments, respectively, while it was 58 ¬¨¬± 6, 39 ¬¨¬± 3 and 11 ¬¨¬± 1 percentage for the same treatments on the second sampling. Chlorophyll content SPAD value on the second sampling was the lowest under WL/NaCl followed by that in WL-treated plants and then by NaCl-treated plants. Chlorophyll content was used as a proxy for tolerance, therefore varieties YYXT and TX9425 that showed the highest chlorophyll content at both the first and second samplings under combined WL/NaCl conditions were assigned as the most tolerant varieties under WL/NaCl conditions. Shoot `Na^+` content was 95 ¬¨¬± 45, 1736 ¬¨¬± 257, 426 ¬¨¬± 91 and 4401 ¬¨¬± 97 ˜í¬¿mol/g under control, NaCl, WL and WL/NaCl treatments, respectively, and shoot `K^+` content was 2572 ¬¨¬± 127, 1327 ¬¨¬± 96, 1649 ¬¨¬± 117 and 1321 ¬¨¬± 98 ˜í¬¿mol/g under the same treatments. Root `Na^+` content was 540 ¬¨¬± 188, 1374 ¬¨¬± 211, 348 ¬¨¬± 39 and 782 ¬¨¬± 210 ˜í¬¿mol/g under control, NaCl, WL and WL/NaCl treatments, respectively, and root `K^+` content was 2150 ¬¨¬± 328, 397 ¬¨¬± 68, 977 ¬¨¬± 27 and 105 ¬¨¬± 18 ˜í¬¿mol/g for the same treatments. In comparing the effects of stresses on root and shoot ionic relations it was concluded that the major limiting factor in root performance was its ability to retain `K^+`, while shoot performance was more limited by `Na^+` increase. Shoot osmolality under WL/NaCl significantly correlated with WL while root osmolality under WL/NaCl highly correlated with NaCl treatment. To provide some further insights into underlying physiological mechanisms, non-invasive ion-selective microelectrode measurements (the MIFE technique) were used. Selected barley varieties from glasshouse experiments representing low (ZUG403), medium (Gebeina) and high (YU6472) tolerance to combined WL/NaCl stress were used for `K^+` and `H^+` flux measurements. The tolerant variety YU6472 showed `K^+` uptake under all conditions while ZUG403 and Gebeina showed `K^+` efflux in response to all three stress conditions, with the biggest efflux detected from ZUG403 roots. These findings strongly suggest that the root's ability to retain `K^+` under combined stress conditions is a critical determinant of a plant's adaptive potential to saline flooded soils. Comparing root ion flux profiles between intact plants and plants with excised coleoptiles has revealed that oxygen transport from the shoot to the root plays an important role in root `K^+` retention. Oxygen transport from the shoot to the root provides sufficient oxygen to fuel `H^+`-ATPase and maintain membrane potentials negative enough to prevent `K^+` efflux via depolarization-activated outward-rectifying GORK channels. It is concluded that WL/NaCl stress is more severe than either salinity or WL stress alone, and the combined effect of WL and NaCl is synergistic but not additive. It is also shown that tolerance to combined WL/NaCl stress is determined mostly by sensitivity to WL. Taken together, combined morphological, physiological and electrophysiological data clearly indicate that plant `K^+` ionic relations are more critical than `Na^+` in explaining the tolerance to combined WL/NaCl stress in barley.