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Approximately 1 billion ha of the global land surface is currently salt-affected, representing about 7% of the earth's land surface. Whereas most of it results from natural geochemical processes, an estimated 30% of irrigated lands globally are salt-affected through secondary human-induced salinization. Application of lower quality, alternative irrigation water is further threatening expansion of the areal extent of soil salinity, in addition to climate change causing increases of salt-water intrusion in coastal areas and increasing crop water requirements. The reduced availability of freshwater resources for irrigation, the continued reduction of the world's cultivated agricultural area by land degradation and urbanization, in conjunction with a growing world population further complicates the problem seeking sustainable solutions. This scoping review prioritizes critical knowledge gaps and makes recommendations for 10 priorities in soil salinity research toward a sustainable and productive agricultural system for a food-secure future world. We also include basin-specific case studies that illustrate progress of the world's major irrigated areas in addressing impacts of soil salinization. By identifying research priorities, we seek to accelerate enhanced research funding to bring new knowledge and innovative solutions toward mitigation of soil salinity impacts. We further want to inspire the science community to develop new directions in salinity research.

Approximately 1 billion ha of the global land surface is currently salt-affected, representing about 7% of the earth’s land surface. Whereas most of it results from natural geochemical processes, an estimated 30% of irrigated lands globally are salt-affected through secondary human-induced salinization. Application of lower quality, alternative irrigation water is further threatening expansion of the areal extent of soil salinity, in addition to climate change causing increases of salt-water intrusion in coastal areas and increasing crop water requirements. The reduced availability of freshwater resources for irrigation, the continued reduction of the world’s cultivated agricultural area by land degradation and urbanization, in conjunction with a growing world population further complicates the problemseeking sustainable solutions. This scoping reviewprioritizes critical knowledge gaps and makes recommendations for 10 priorities in soil salinity research toward a sustainable and productive agricultural system for a food-secure future world. We also include basin-specific case studies that illustrate progress of the world’s major irrigated areas in addressing impacts of soil salinization. By identifying research priorities, we seek to accelerate enhanced research funding to bring new knowledge and innovative solutions toward mitigation of soil salinity impacts. We further want to inspire the science community to develop new directions in salinity research.

Agriculture is expanding into regions that are affected by salinity. This review considers the energetic costs of salinity tolerance in crop plants and provides a framework for a quantitative assessment of costs. Different sources of energy, and modifications of root system architecture that would maximize water vs ion uptake are addressed. Energy requirements for transport of salt (NaCl) to leaf vacuoles for osmotic adjustment could be small if there are no substantial leaks back across plasma membrane and tonoplast in root and leaf. The coupling ratio of the H+‐ATPase also is a critical component. One proposed leak, that of Na+ influx across the plasma membrane through certain aquaporin channels, might be coupled to water flow, thus conserving energy. For the tonoplast, control of two types of cation channels is required for energy efficiency. Transporters controlling the Na+ and Cl− concentrations in mitochondria and chloroplasts are largely unknown and could be a major energy cost. The complexity of the system will require a sophisticated modelling approach to identify critical transporters, apoplastic barriers and root structures. This modelling approach will inform experimentation and allow a quantitative assessment of the energy costs of NaCl tolerance to guide breeding and engineering of molecular components.

Our previous studies showed that high salt tolerance in Tibetan wild barley accessions was associated with HvHKT1;1, a member of the high-affinity potassium transporter family. However, molecular mechanisms of HvHKT1;1 for salt tolerance and its roles in K+/Na+ homeostasis remain to be elucidated. Functional characterization of HvHKT1;1 was conducted in the present study. NaCl-induced transcripts of HvHKT1;1 were significantly higher in the roots of Tibetan wild barley XZ16 relative to other genotypes, being closely associated with its higher biomass and lower tissue Na+ content under salt stress. Heterologous expression of HvHKT1;1 in Saccharomyces cerevisiae (yeast) and Xenopus laevis oocytes showed that HvHKT1;1 had higher selectivity for Na+ over K+ and other monovalent cations. HvHKT1;1 was found to be localized at the cell plasma membrane of root stele and epidermis. Knock-down of HvHKT1;1 in barley led to higher Na+ accumulation in both roots and leaves, while overexpression of HvHKT1;1 in salt-sensitive Arabidopsis hkt1-4 and sos1-12 loss-of-function lines resulted in significantly less shoot and root Na+ accumulation. Additionally, microelectrode ion flux measurements and root elongation assay revealed that the transgenic Arabidopsis plants exhibited a remarkable capacity for regulation of Na+, K+, Ca2+ and H+ homeostasis under salt stress. These results indicate that HvHKT1;1 is critical in radial root Na+ transport, which eventually reduces shoot Na+ accumulation. Additionally, HvHKT1;1 may be indirectly involved in retention of K+ and Ca2+ in root cells, which also improves plant salt tolerance.

Over two growing seasons, we examined the effects of natural field salinity on grain production by a range of wheat and barley genotypes on rainfed (i.e. non-irrigated) land with a Mediterranean climate. Blocks of wheat and barley were grown at adjacent locations, on saline and non-saline sites. The two growing seasons differed in the amount of rain that fell in late Spring during grain filling, which strongly affected soil moisture, and in the salinity of the soil solution during grain filling. On the non-saline site in 2009 (less rain in Spring) there were similar ranges of grain yield between the 27 barley and 90 wheat genotypes; the 5th–95th percentile ranges of grain yield for the two crops were 0.9–2.1 t ha−1 and 0.9–1.9 t ha−1 respectively. However on the saline site in this year, the average salinity of the soil solution during grain filling was ∼0.4 M, and the earlier flowering barley genotypes had higher grain yields (5th–95th percentile 1.5–3.1 t ha−1) than the later flowering wheat genotypes (0.8–2.3 t ha−1). By contrast, in 2011 (dry early Spring, rain in late Spring), on the saline site the average salinity of the soil solution was less than 0.2 M during grain filling, and the 320 wheat and 14 barley genotypes had similar 5th to 95th percentile ranges in grain yield to each other under saline conditions (barley 1.1–3.1 t ha−1; wheat 1.3–2.9 t ha−1). Comparisons of genotypes common to both sites showed that there were wheat (e.g. Mace, Tammarin Rock, Binnu) and barley cultivars (e.g. Mundah, Parent 19) with consistently higher yields under saline conditions. We conclude that grain yield by cereals on saltland is associated with the severity of salinity, the adaptation of genotypes to local conditions, their salt tolerance, and (in seasons with a dry spring) early flowering.

© 2016 Elsevier. Over two growing seasons, we examined the effects of natural field salinity on grain production by a range of wheat and barley genotypes on rainfed (i.e. non-irrigated) land with a Mediterranean climate. Blocks of wheat and barley were grown at adjacent locations, on saline and non-saline sites. The two growing seasons differed in the amount of rain that fell in late Spring during grain filling, which strongly affected soil moisture, and in the salinity of the soil solution during grain filling. On the non-saline site in 2009 (less rain in Spring) there were similar ranges of grain yield between the 27 barley and 90 wheat genotypes; the 5th–95th percentile ranges of grain yield for the two crops were 0.9–2.1 t ha-1and 0.9–1.9 t ha-1respectively. However on the saline site in this year, the average salinity of the soil solution during grain filling was ~0.4 M, and the earlier flowering barley genotypes had higher grain yields (5th–95th percentile 1.5–3.1 t ha-1) than the later flowering wheat genotypes (0.8–2.3 t ha-1). By contrast, in 2011 (dry early Spring, rain in late Spring), on the saline site the average salinity of the soil solution was less than 0.2 M during grain filling, and the 320 wheat and 14 barley genotypes had similar 5th to 95th percentile ranges in grain yield to each other under saline conditions (barley 1.1–3.1 t ha-1; wheat 1.3–2.9 t ha-1). Comparisons of genotypes common to both sites showed that there were wheat (e.g. Mace, Tammarin Rock, Binnu) and barley cultivars (e.g. Mundah, Parent 19) with consistently higher yields under saline conditions. We conclude that grain yield by cereals on saltland is associated with the severity of salinity, the adaptation of genotypes to local conditions, their salt tolerance, and (in seasons with a dry spring) early flowering.