Keywords

Introduction

Global food security is greatly reliant on agricultural crops and their products which need substantial increases for maintenance of the gap between production and consumption. The significance of enhancing crop productivity is even more emergent in recent times because of the growing population of humankind which currently stands at 7.6 billion and is projected to exceed 9.7 billion by the year 2050 (FAOSTAT 2018). Undoubtedly, the increase in population will exert pressure for production of more crops and food resources which seems a challenging task for plant biologists. Concurrently, climate change and several biotic and abiotic stresses challenge the growth and production of agricultural crops. Among abiotic stresses, salinity is considered as one of the leading limiting factors responsible for growth and production decline of agricultural crops throughout the world principally in arid and semiarid regions (Kaashyap et al. 2018). About 20–33% of the cultivated and irrigated land throughout the world is affected by salinity, and the adverse effects are expected to reach to 50% in the year 2050 (Soda et al. 2016; Machado and Serralheiro 2017).

The problem of salinity emerges when ion concentration of different salts (predominantly NaCl) elevates in soils beyond threshold level required for normal germination, growth, and physiological activities of plants in root zones. Standard agricultural soils which promote the growth of cultivated crops have salinity level ~4 dS m−1 which is equivalent to 40 mM NaCl generally determined as the electrical conductivity of the saturated extract (Shrivastava and Kumar 2015). Ionic concentration exceeding 4 dS m−1 in rhizosphere result in stress conditions which challenge the growth of crops in several ways. Leading causes of salinity may be the natural addition of salts from rocks and minerals through weathering process (primary salinity) or extensive human activities like irrigation, agricultural intensification, and deforestation (secondary salinity) (Athar and Ashraf 2009). In addition, high evaporation rate in tropics also seems to increase the level of salts in agricultural land. Salinity has drastic consequences on water availability in the soil for plants, the rate of transpiration and photosynthesis, stomatal opening and closures, and functional activities of plant roots (Khataar et al. 2018). The discrepancies triggered by salinity stress result in impaired growth, physiological functions, and low yield outputs of crops. Annual expenditures associated with crop losses as a result of salinity are documented as 27 Billion US dollars (Singh et al. 2016). Coupled with drought, salinity triggered crop losses range between 20 and 50% in documented studies (Shrivastava and Kumar 2015) while losses may even be much higher as a result of salinity stress in areas out of scientific investigations. From physiological aspects, almost all crops show sensitivity to the salinity stress albeit variation in responses to the imposed stress in different crops does exist based on their tolerance level. Major concerns about the adverse effects of growing salinity stress are associated with crops which have a substantial contribution to global food supply. Rice, maize, wheat, barley, sorghum, and potato are considered as key drivers in the fulfillment of food needs throughout the world, while their growth and productivity are considerably affected by salt stress in many parts.

To maintain a sustainable production of agricultural crops and to ensure future food sanctuary, efforts have been made over decades to address the adversities caused by salinity stress on crops. One of the basic approaches to prevent crop losses triggered by salinization is to induce salinity tolerance in salt-sensitive crops. Genomic and molecular approaches to understand the possibilities of salt resistance induction in crops have proven effective in further elaboration of making crops adaptable to saline conditions (Luo et al. 2017). In addition to molecular methods, breeding for salt tolerance, proper agronomic practices in irrigation, and search for cost-effective and feasible methods to reduce drastic effects of salinity on principal crops are crucially needed to be employed. This chapter presents an overview of the salinity imposed effects on crops, and employment of agronomic approaches, seed priming and plant growth promoting bacteria (PGPB) to induce tolerance in crops to salinity.

Causes of Salinity and General Effects

Root sphere is the zone of soil which provides water and minerals to plants in addition to harboring diverse beneficial microbial communities and plays a determining role in successful germination and the consequent establishment of growth and reproductive phases of plants. Imbalance in water contents and minerals in the rhizospheric soil can lead to deteriorated effects on plant growth and productive outputs. Soil salinity alters water and mineral contents and microbiota and results in poor germinability and growth of sensitive crops. Salinity is a physiological state of soil where the concentration of active ions of salts principally NaCl increases the threshold levels. It is established that salt concentration up to 4 dS m−1 (~40 mM) of NaCl in soils represent a suitable environment for most of the plant species; however, some plants known as halophytes can withstand salinity stress up to 100 mM (examples include Vesicaria, Atriplex, Chenopodium spp.) while most of the cultivated crops including cereals (known as glycophytes) show differential sensitivity to salinity stress and prefer to grow at ≤40 mM salt concentration (Rasool et al. 2013; Ismail and Horie 2017; Yang et al. 2018). Causes of salinity stress in the natural ecosystem and managed agriculture are both natural and anthropogenic. Naturally, the problem of salinity occurs when minerals and salty rocks are weathered and ion concentration accumulates in soil (Athar and Ashraf 2009). Transportation of salt contents to root growing zones from parent rocks (rich in salts) as a result of several types of weathering and from groundwater may result in an increased level of salt stress and could contribute to primary salinity (Daliakopoulos et al. 2016). Agricultural lands located near coastal regions are exposed to high salinity stress because of the presence of high concentrations of salts in seawater. In tropical regions, high temperature can stimulate evaporation rate and hence more salt ions in soils accumulate giving rise to salinity stress. Moreover, flooding and wind erosion may be regarded to some extent as natural causes of salinity due to the imbalanced flow of minerals and salts from one region to another. Shallow groundwater may also serve as a source of salinization when upward migration of salts occurs (Shrivastava and Kumar 2015). Land cover and climatic conditions are also thought to contribute to salinity (Fan et al. 2012). Anthropogenic salinity, representing a significant proportion of the overall global salinity, is caused by massive agricultural activities, poor irrigation, use of imbalanced fertilizers, soil degradation, and poor drainages (Shrivastava and Kumar 2015; Sandhu et al. 2017). Mining activities and the use of wastewater and industrial effluents enriched in diverse salt contents in addition to other hazardous materials may lead to agricultural salinity (Daliakopoulos et al. 2016). Deforestation is an imminent threat to changes in rainfall patterns and soil erosion which would lead to consequent salinity problems. Rhizospheric soils whether salinized by natural processes or human activities are generally classified on the basis of electrical conductivity (EC) as: (1) non-salinized (EC = 0–2 dS m−1), (2) marginally salinized (EC = 2–4 dS m−1), (3) moderately salinized (EC = 4–8 dS m−1), (4) strongly salinized (EC = 8–16 dS m−1), (5) rigorously salinized (EC =16–32 dS m−1), and (6) exceptionally salinized (EC >32 dS m−1) (Rasool et al. 2013). According to Munns and Gilliham (2015), the economic costs associated with salinity are variable in different regions which may range between 300 and 600 US dollars ton−1. Daliakopoulos et al. (2016) stated that soil salinity may impart an economic burden of up to 600 million euros in European countries due to reduced crop yields and agricultural degradation. According to FAO and ITPS, African continent is the most salt-affected region where salinized area exceeds 122 million hectares followed by north and central Asia which represent almost 91.5 Mha of salt-affected land (Table 1).

Table 1 Salt- and sodic-affected soils in different continents of the world
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Effects of Salinity on Crops

In general, plants are broadly categorized as glycophytes (sensitive to salinity) and halophytes (exhibit some degree of tolerance to salinity stress) on the basis of their tolerance in response to salinity stress. Halophytes are plants with typical examples Vesicaria, Atriplex, Chenopodium spp., and several others that can withstand high salt concentration while our major cultivated crops are glycophytes which show differential sensitivity to salinized soils (Borsani et al. 2003; Rasool et al. 2013; Yang et al. 2018). Differential responses of cultivated glycophytes depend on their growth phases, the soil characters, level of salinity, and agricultural practices (Daliakopoulos et al. 2016). Salinity stress primarily affects crops by causing water deficit condition in soils termed as “osmotic stress ” while secondarily due to the accumulation of toxic ions which consequently lead to a poor or delayed germination and post-germination growth abnormalities (Läuchli and Grattan 2007; Munns and Tester 2008). The primary osmotic stress is considered as hastened in its effects to which plants show abrupt responses while the secondary effects of salinity exposures are relatively slower and the adverse effects appear at later stages when sufficiently larger amounts of sodium ions gather in plant tissues and correspond to photosynthetic abnormalities, damage to cells, and many metabolic malfunctions (Munns and Tester 2008; Parihar et al. 2015; Hanin et al. 2016). Tuteja (2007) asserted that when Na ion accumulation in plant tissues increases 100 mM, enzymes’ functions, cell membrane structure, and cell division are severely influenced which result in a reduced growth. In leaves where transpiration proceeds, high level of ions may result in cell injuries and consequent retardation in growth (Parihar et al. 2015). Other drastic effects of salinity exposures on plants are misappropriated opening and closing of stomata, which lead to altered gaseous exchange, photosynthesis, and the rate of transpiration (Munns and Tester 2008; Hanin et al. 2016). Furthermore, it is strongly evident that higher concentration of salts’ ions in soil negatively affects the uptake of other necessary ions which plants require for several metabolic and enzymatic activities (Hanin et al. 2016). The high buildup of toxic ions in leaves outside threshold levels causes eventual death of many cells which are actively involved in photosynthesis with a net result in limited photosynthetic activity (Läuchli and Grattan 2007). The altered ratio of potassium and sodium ions significantly influence the entry of former into plant tissues and intercellular vicinities which lead to drastic consequences on ionic balance inside plants (Tuteja 2007). These abnormalities in physiological and biochemical processes of exposed plants are generally associated with salt stress symptoms which may range from reduced and/or delayed germination, plant growth, reproductive parameters, and yield outputs. In sum, osmotic stress developed due to higher salts concentration, accumulation of Na+ in plants, ion toxicity, altered ratio of Na+ and K+ results collectively in deficit water and nutrient uptake by germinating seeds and seedling roots, cell injuries, changes in vital enzymes and hormones, irregular stomatal opening and closing with disturbed gaseous exchange, rate of transpiration and photosynthesis; thus imposing stress environment for seed to properly germinate and for seedlings to properly grow. The overall result would be a reduction in growth and yields plants exposed to salinity.

In several studies, drastic consequences of salinity on germination, growth, yield, and physiological activities of major cultivated crops have been proven (Table 2). Cha-Um and Kirdmanee (2009) applied 100–400 mM NaCl stress to different genotypes of maize which responded to the imposed stress with reduced synthesis of chlorophylls while increased proline contents. Even lower salinity stress ranging between 3.5 and 8.5 dS m−1 caused a significant retardation in oil content and yield of safflower (Yeilaghi et al. 2012). In our previous study, we recorded a significant decline in germination and seedling length of wheat cultivars which were exposed to 8 dS m−1 NaCl stress (Muhammad et al. 2015). In okra, reduced germination and growth at 75 mM (Habib et al. 2016), while in chickpea arrested germination, growth, and biomass at 40 mM NaCl was observed (Atieno et al. 2017). In response to different level of salinity stress, reduced growth and biomass, leaf area, chlorophyll degradation, altered water status, defective stomatal functions, altered rate of transpiration and respiration, and imbalanced ion ratios in maize (Gul et al. 2017; Konuşkan et al. 2017), rice (Krishnamurthy et al. 2016); Islam et al. 2017; Shahzad et al. 2017; Radanielson et al. 2018), wheat (Gul et al. 2017; Fathi et al. 2017; Bajwa et al. 2018; Khataar et al. 2018), tomato (Rubio et al. 2017; Martinez et al. 2018), faba bean and common bean (Benidire et al. 2017; Hussein et al. 2017; Ahmad et al. 2018; Khataar et al. 2018), barley (Allel et al. 2018), tomato, cotton, and several other crops have been well documented (Meloni et al. 2003; Zhang et al. 2006; Sarabi et al. 2017; Sandhu et al. 2017; Martinez et al. 2018; Ahmadi et al. 2018).

Table 2 Effect of salinity on growth and physicochemical characters of different crops
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Salinity Management Approaches

Modifications in Agricultural Practices

Rengasamy (2006) categorized three major types of salinity: (1) salinity associated with groundwater, (2) salinity associated with non-groundwater, and (3) salinity associated with irrigation. All the three types of salinity are manageable if preventive measures, depending on the types of salinity and location, are taken into account. Although it seems difficult to employ a particular method in agro farming systems for salinity management, a combination of several techniques would sufficiently help in reducing the drastic impacts of salinity on crops. As for natural factors responsible for salinity, man has little influence on bringing them under control. Since leading causes of anthropogenic salinity include land degradation, poor drainage, and use of substandard water and excessive irrigation, employment of appropriate practices will satisfactorily manage the salt menace. Foremost, increasing population pressure in urban areas stimulate migrations to rural areas—the main hub of agricultural activities—which consequently results in the utilization of agricultural land for dwelling purposes. In parts of fertile land destruction as a result of building constructions, the household outflow of water may cause disturbances in the quality of irrigated water. Thus, restoration and protection of fertile land would limit the potential secondary salinization. Rise in water table caused by irrigation practices, rainfall, and leaching of water which are perceived to be linked with the salinization process can be maintained by surface drainage. The exclusion of excess of water from rhizosphere is also an effective tool to maintain the permanent water table and avoid its rising (Tiwari and Goel 2017). Drainage can be achieved by making trenches in agricultural soils which are saturated with plenty of water or by installation of a drainage system although the latter one cannot be afforded by resource-poor farmers in many countries. Hanson and May (2004) stated that substrata drip irrigation could maintain water table in shallow water regions and correspond to better salinity management and improvement in crops in such adopted system. Ayars et al. (2006) proposed that subsurface drainage and management of water at specific depth in agricultural lands could significantly improve soil profile and salinization problems. They further suggested that proper design and post-installation maintenance of drainage system would be required for getting better results in arid soils with the saline profile. In a study conducted in Pakistan on the importance of different drainage projects in irrigated areas for salt, management revealed a 14–20% decrease in soil salinity (Azhar 2010). Valipour (2014), in a comprehensive review, presented the advantages of the drainage systems in reducing salinity problems in many countries where the problem prevails. He outlined that in Europe the drainage system was the most effective measure in controlling salinity stress; however, such systems were not recommended for areas where phosphorus deficiency remained a challenging problem. Hou et al. (2016) advocated the use of mulch-drip irrigation in soils enriched with salts to minimize salinity concentration and to promote plants’ growth. Besides artificial drainage systems, cultivation of deep-rooted trees such as Eucalyptus sp. may prevent rise in the water table and consequent salinization. Canal system in different countries, particularly in Pakistan and other developing countries, is extensively used for irrigation purposes. Inappropriate designs of canal systems and lack of protective lining in most of the developing countries due to high costs result in water seepage. The seeped water move through different soil zones and lead to the solubility of minerals and their accumulation in the fertile agricultural land which consequently result in soil salinity and water logging.

Another leading cause of salinization throughout the world is excessive irrigation and the use of substandard water for the stated purpose. In areas with arid and semiarid conditions, irrigation processes facilitate the mobility of salts to non-saline areas where secondary salinization occurs and disturb the growth parameters of cultivated plants (Smedema and Shiati (2002). Rietz and Haynes (2003) debated that irrigation with poor water coupled with unmanaged drainage results in water level rise and accumulation of salts in the soil surface layer where plants are grown. The scarcity of fresh water for irrigation and other purposes in many parts of the world coerce farmers to use low standard water which mainly contains municipal wastes and salts, thereby reducing soil quality and elevating salinity and sodicity levels (Qadir and Oster 2004). It is the prime priorities of agricultural stakeholders to stimulate scientific practices in irrigation. This may include drip irrigation (Hou et al. 2016; Ortega-Reig et al. 2017), installation of tube wells (Memon et al. 2017), sprinkler irrigation (Rudrapur et al. 2017), and proper drainage by making drenches and installing water removal equipment. Although it seems costly particularly in developing countries, comparisons with crops losses and land degradation as a result of salinity and poor irrigation can highlight the potential benefits of sustainable irrigation methods for the long term. To minimize energy consumption costs incurred by tube wells irrigation, deployment of solar panels can be specifically useful. Cultivation of deep-rooted trees such as Eucalyptus, Acacia, and Sesbania species can efficiently manage water table and consequently avoid salinity problems. Deforestation and overgrazing are also some of the important indirect contributing factors towards salinization because of changes in climatic condition, altered rainfall, elevated evaporation, and flooding; thus, maximum vegetation and protection of forests can be helpful in long-term salinity management.

Seed Priming and Plant Growth Promoting Bacteria (Bio-Priming) for Salinity Management

Pre-germination soaking of seeds with different solutions (or pure water) for the different duration is termed as “seed priming” (Song et al. 2017). Solutions of different compounds, both of natural and synthetic origins, are usually used as pre-soaking materials to prepare seeds for better performance under stressful environment. Seeds may be primed either with water, or salts, hormones, and natural metabolic substances although different priming agents have variable effects on treated seeds in terms of their responses to exposed stress. Soaking duration with priming agents is also a necessary factor which enables seeds to architect stress-combating machinery. In practice, halo priming (treatment of seeds with different concentration of salts) and hydropriming (water treatment) seem economical because of low cost and easy availability of soaking gents (Jisha et al. 2013) although other priming compounds such as polyethylene glycol (PEG), auxins, gibberellins, cytokinins, salicylic acid, jasmonic acid, kinetin, urea, and several natural metabolic substances have been well proven in inducing defense and stress tolerance to subjected seeds (Islam et al. 2015; Salah et al. 2015; Savvides et al. 2016). Primary mechanism underlying the priming-induced stress tolerance in crops is the activation of enzymes, hormones, homeostatic regulation, synthesis of new messenger RNA, improvement in imbibition potential and dormancy, detoxification of reactive oxygen species, enhanced antioxidant systems, and induction of “pre-stress memory ” (Bruce et al. 2007; Varier et al. 2010; Hussain et al. 2016) (Fig. 1). Primed seeds perform better than non-primed ones when sown either under normal conditions or in a stressed environment. Many studies suggest that priming cause better germination, seedling emergence, and vigor (Salah et al. 2015; Hussain et al. 2016). Ajouri et al. (2004) have documented that priming-induced salinity resistance in barley and better germination was achieved. Kaya et al. (2006) observed improved germination and growth of sunflower seedling in saline and drought conditions as a result of hydropriming. Under 100 mM NaCl stress, halopriming, and hydropriming invigorated wheat growth, yield, antioxidant activities, and ions regulation (Islam et al. 2015). Improved germination and reduced mean germination time of Nigella sativa were also recorded under 40 mM NaCl when seeds were treated with 1–2% NaCl and several other priming agents (Gholami et al. 2015). Khaliq et al. (2015) treated seeds of rice with 15–105 μmol L−1 concentrations of selenium and evaluated the primed seeds for germination, growth, and enzymatic activity under salinity stress. They observed the better performance of primed seed than control under stress conditions for all the studied parameters. Osmopriming of bread wheat seeds with CaCl2 resulted in enhanced tolerance to salinity (100 mM) resulting in improved leaf area, Na/K ion ratio, and grain yield (Tabassum et al. 2017). Pretreatment of broccoli sprout with methyl jasmonate and KCl induced salt-stress tolerance in tested plants which resulted in growth and biochemical parameters elevation under salinity stress (Hassini et al. 2017). Recently, Bajwa et al. (2018) effectively induced salt tolerance in wheat by pre-soaking seeds with sorghum extracts and benzyl amino-purine. Germination, growth, and biochemical attributes exhibited improved performance in salinity imposed stress.

Fig. 1
figure 1

Mechanism of stress tolerance induction in seeds by priming (after Varier et al. 2010 and Mahmood et al. 2016)

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In addition to wide applicability of seed priming in agriculture, plant growth promoting bacteria (PGPB) have also a potential role in improving crops’ stress responses. PGPB can manage drastic effects of salinity and other stress conditions on cultivated crops by several mechanisms. They may stimulate the production of specific proteins and osmoprotectants in crops exposed to stressful environment thereby reducing the adverse effects of stresses and protecting crops from stress injuries (Grover et al. 2011). Saleem et al. (2007) argued that one of the leading factors associated with stress is the elevated production of ethylene—a growth regulator produced indigenously by plants when they are challenged with stress conditions—is regulated by several strains of PGPB. It is established that many PGPB produce ACC deaminase and regulate the production and functions of cytokinins, antioxidants, and ethylene which not only help crops to take nutrient and water properly but also induce in them systemic resistance to salinity stress which imparts stimulatory effects on the growth and yield under such stresses (Yang et al. 2009). According to Glick (2012), plant growth suppression in fields is either caused by biotic (pathogenic interaction) or abiotic (salinity, intensive light, drought, temperature, etc.) stresses which are generally correlated with abnormal production of stress hormones, fluctuation in metabolic machinery. He provided evidence that many PGPB can manage the required production of ethylene, indole acetic acid (IAA), trehalose, cytokinin, and several other stress components. Shrivastava and Kumar (2015), in a comprehensive review, highlighted the role of PGPB in conferring salinity resistance to crops. They commented that PGPB induces systemic tolerance in crops besides their active role in the provision of nutrients, soil fertility, and disease suppression. Bharti et al. (2016) documented that Dietzia natronolimnaea, a plant growth promoting bacterium, was involved in regulating transcriptional factors which confer salt resistance to plants and protect them from salinity-induced injuries. Del Cerro et al. (2017) suggested that certain strains of Rhizobium tropici are involved in the production of nodulating factors under high salt concentration which promotes root nodulation in legume crops and helps in avoiding salinity stress. Some species of Streptomyces are recognized for their ability to produce 1-aminocyclopropane-1-carboxylate deaminase and promote the growth, ion uptake, and chlorophyll functions of rice under 150 mM NaCl stress (Jaemsaeng et al. 2018).

In the previous study, salinity tolerance and improved growth of tomato were recorded in response to PGPB (Achromobacter piechaudii) under 43 mM NaCl stress (Mayak et al. 2004). Tank and Saraf (2010) reported 50% increase in tomato growth and biomass under salt stress when plants were inoculated with different strains of Pseudomonas sp. under greenhouse conditions. Wheat seeds inoculated with Arthrobacter sp. exhibited efficient resistance to salt stress and yielded better than non-inoculated plants (Upadhyay and Singh 2015). Wheat inoculated with PGPB Klebsiella sp. (SBP8) and grown in 150–200 mM NaCl stress showed higher growth and chlorophyll activity than non-inoculated control (Singh et al. 2015). Edible peas grown under 70–130 mM NaCl stress after pretreatment with Variovorax paradoxus showed improved growth, photosynthetic performance and ion uptake (Wang et al. 2016). Bharti et al. (2016) reported that wheat inoculation with Dietzia natronolimnaea resulted in a significant elevation of plant height and dry biomass when salinity stress (150 mM NaCl) was imposed on test crop. Under similar salinity stress, Streptomyces sp. (strain GMKU 336) promoted plant growth, water status, chlorophyll and proline contents, and ion ratios of rice (Jaemsaeng et al. 2018). In other similar studies under high salt concentrations, Burkholderia and Enterobacter sp. stimulated the growth and physicochemical attributes of maize and wheat (Akhtar et al. 2015a, b), Bacillus cereus improved mung bean (Islam et al. 2016), and Enterobacter sp. promoted rice growth (Sarkar et al. 2018). These and dozens of other studies employing PGPB in salt stress environment indicate that several PGPB strains have a prospective role in inducing salt-stress tolerance to cultivated crops and enhancing their yield outputs.

Conclusions

Cultivated crops are challenged with several constraints among which salinity is an important one responsible for limited growth and crop production. Salinity stress imposes ion toxicity, water deficit conditions, imbalanced ion uptake, cellular damage, and degradation of chlorophylls, transpiration and respiration activities which definitely influence overall growth attributes of challenged plants. Both natural and man-triggered activities cause salinization in agriculture. Extensive irrigation, poor drainage, low-quality water for irrigation purposes are directly contributing to the problem of salinity. Employment of mechanisms and techniques to diminish the adverse effects of salinity and make crops adapted to withstand salinity would help in crop losses management. Thus, scientific approaches in agriculture such as sprinkler irrigation drench and pumping surface and groundwater, and lining of irrigating canals can significantly lower salinity levels in cultivated land. Seed priming with water, salts, hormones, and other chemicals induces stress adaptability in them and can prove effective tools to divert the adverse effects of salinity on crops. Bio-priming with plant growth promoting bacteria has also been well recognized for their ability to induce salinity tolerance in crops. To make the maximum use of seed priming agents and PGPB for salt management in cultivated crops, commercial availability, economic accessibility, and safety issues concerned with priming chemicals and PGPB strains must be ensured by stakeholders. Since farmers represent a basic component agricultural system, their awareness about the significance of seed priming and PGPB in salinity can contribute to further expansion of these techniques in agricultural areas which are challenged with salinity.