Keywords

8.1 Introduction

Plants being sessile face multiple adverse environmental conditions throughout their life known as abiotic stresses, more precisely defined as any environmental factor which exerts negative effect on optimum plant functions. The major abiotic stresses are drought, salinity, and low and high temperature which negatively influence the biomass production, economical yields, and ultimately survival of food crops up to 70% and hence are real threat to global food security. Drought stress affects various morphological and molecular cascades of plant at different growth stages. The general effects of drought stress take place at all growth stages irrespective of the plant species (Nawaz et al. 2012; Majeed et al. 2018). In addition, escalating soil salinity is the main cause of arable land degradation, 7% land area of the world is salt-affected, and worryingly, the extent of salinity of salt-affected soils and continuous spread is at alarming rate in densely populated countries (Vashev et al. 2010).

Plants adopt a wide range of resistance mechanisms to alleviate the adversities of abiotic stresses. The production of reactive oxygen species (ROS) under such environmental stress conditions results in activation of antioxidants such as catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX), glutathione reductase (GR), and superoxide dismutase (Wariach et al. 2011). Despite internal resistance of plants, balanced nutrient supply confers resistance against abiotic stress factors. Increasing evidence depict that reduced mineral nutritional status of plants has imposed deleterious impacts on plant resistance adaptations (Marschner 1995). It is evident that the reduced absorption of mineral nutrients to plants under abiotic stresses is because of disturbed activity of membrane transporters (Akram et al. 2009). Exogenous application of inorganic nutrients have proved an essential approach for improving stress tolerance in plants through cell expansion, osmotic adjustment, stomatal aperture, charge balance, protein synthesis, and homeostasis (Wasti et al. 2017) (Fig. 8.1).

Fig. 8.1
figure 1

Schematic overview of the role of mineral nutrition in improving drought and salinity tolerance in crop plants

Full size image

8.2 Nitrogen (N)

Under water scarce conditions, absorption of N by the plants is essential for their growth because of its active role in structural and metabolic processes (Hassan et al. 2005). Its availability promotes the roots ability for more water and nutrients uptake. In N-deficient soils, the processes of cell division, cell expansion, and transpiration are severely affected through closing of stomata which contributes toward reduced growth (Vos and Biemond 1992; Waraich et al. 2011). Rufty et al. (1988) and Marschner (1995) suggested that enhanced N status of plants improves antioxidative defense system, carbon assimilation, accumulation of soluble sugars, and reduced photo-oxidation of chlorophyll pigments, resulting in higher cell growth and final leaf area. Improved photosynthetic ability of plants with higher light interception owing to expanded leaf surface area has also been supposed by N supply. The reduced activity of RuBisCO enzyme and chloroplast pigments during the process of photosynthesis is mainly due to limited N supply. Consequently, higher cell metabolic activities based on stromal and thylakoid proteins attributed to N addition. Under water-deficient conditions, morpho-physiological changes in plants like reduced leaf dry biomass, excised leaf weight loss, relative dry weight, relative water contents (RWC), and chlorophyll pigments could be alleviated with N fertilization. Drought-induced alterations in RNA contents accredited to increased activity of RNAse, leading toward reduced protein contents. In sunflower and wheat roots, nitrate reductase activity under drought stress conditions is severely affected by nitrate deficiency, but was contrasting in the roots of maize plants (Martin and Dasilva, 1972; Gimenez et al. 1992; Lawlor, 2002).

N maintains the metabolic function of plant at low tissue water potential, thereby playing an important role to alleviate drought stress in cereal crops. Application of N at vegetative stage mitigated the negative effects of drought stress in wheat (Abid et al. 2016). Low N supply (0.16 g/kg soil) reduced the grain filling duration and decreased final yield under water deficit conditions. However, high dose of N (0.24 g/kg soil) induced resistance against severe drought stress and increased grain yield, mainly due to enhanced antioxidative defense system and metabolic activities. Similarly, Shi et al. (2014) recorded a marked increase in nitrogen use efficiency (NUE), water use efficiency (WUE), as well as photosynthesis and biomass accumulation of wheat applied with N under drought stress conditions. Increased grain yield in barley, supplemented with N under drought stress conditions, provided further evidence that N supply directly influences quantum yield and activities of antioxidative enzymes to improve yield in crop plants (Movludi et al. 2014). Mannan et al. (2012) observed that increasing N levels improved growth and yield in rice under water deficit conditions. Recently, a comparative study involving application of ammonium (NH4) and nitrate (NO3) under limited water conditions showed that NH4 was more effective than NO3 to induce drought resistance in rice (Ding et al. 2015). Application of NH4 significantly increased the root hydraulic conductivity, plasma intrinsic protein (PIP), expression of root aquaporins (AQPs), and protoplast water permeability suggesting positive association of NH4 with AQPs.

Low winter rainfall and irrigation with brackish water are the primary causes of salinity in soils (Maggio and Cavallaro 2011). Salt stress reduces crop growth and production by adversely affecting metabolic activities and physiological process including assimilation, uptake, and translocation of NO3 and increases sap osmolality from 305 to 530 mOs mol kg−1 in roots (Annunziata et al. 2017). Under salt-stress conditions, nitrate reductase enzyme of nitrate reduction pathway is severely affected by salinity. Increased activity of this enzyme in salt-tolerant plants than salt-sensitive means has vast capacity for inducing tolerance against salinity stress. The evidence from previous studies that glutamic acid, a primary product of N assimilation acts as a donor of amino group for many kinds of essential amino acids and actively involved in development of 5-aminolevulinic acid (5-ALA) and proline. Accumulation of proline in plants of salinity stress helps them to survive under such types of punitive conditions by adopting various biochemical adaptations. Its active role in osmotic adjustment as an osmolyte protects the DNA, membrane, and protein structures. Synthesis of ROS (O2·-, H2O2, 1O2) under salinity stress decreases with proline availability because of its scavenging ability for free radicals. Exogenous aminolevulinic acid (5-ALA) application alleviates the lethal effects of salt stress by dominating the anti-stress properties. Production of chlorophyll pigments, hemes, proteins stabilization, plant growth regulation, and cytokinins accumulation is attributed to 5-ALA synthesis (Kuznetsov and Shevyakova 1999; Watanabe et al. 2006). Application of N significantly improved the growth traits and root system because of increased activity of nitrate reductase. In barley plants, salinity stress has imposed serious impacts on reduced activity of nitrate reductase. Its reduced activity in salinity-induced plants caused accumulation of proline and 5-ALA. The decrease in protein contents owing to salinity stress caused significant decline in chlorophyll, 5-ALA, proline, and heme, based on N nutrition. Enhanced synthesis of these contents promotes the activity of peroxidases enzyme for inducing salt tolerance in ROS infected plants. It is well known that salinity has drastic effects on plants growth and development in terms of reduced respiration and the process of photosynthesis. Plants exposed to salinity stress provide protection to physiological processes during its adaptation through increased synthesis of proline, ALA, heme, and chlorophyll contents. Damage to thylakoid membrane results in inhibition of electron transport chain on account of reduced protein-pigment complexes in cytochromes, reaction centers, and antenna complexes of PS-I and PS-II, which consequently lowers the photosynthetic efficiency of plants. Hence, 5-ALA production in response to N nutrition is accountable to synthesis of heme and carotenoids, and mediating the plant growth through enhanced activities of antioxidants as catalase (CAT), peroxidase (POX), superoxide dismutase (SOD), and ascorbate peroxidase (APX) (Parida et al. 2003; Averina and Yaronskaya 2012; Averina et al. 2014).

In pearl millet, application of N at the rate of 225 kg ha−1 increased grain production by 34% compared to 150 kg ha−1 N supply under salts stress conditions (Heidari and Jamshid 2010). Likewise, Abdelgadir et al. (2010) noted that increasing N level significantly increased the straw and grain yield in salt-stressed wheat plants. A marked effect of N fertigation (110 kg ha−1) was also observed on NaCl-treated wheat seedlings by Fallahi et al. (2012). Application of maize with high N dose of 300 kg ha−1 decreased the sensitivity of plants to salinity stress resulting in high yield (Azizian and Sepaskhah 2014). Esmaili et al. (2008) observed that N induced salinity tolerance in sorghum was associated with increased uptake of nutrients and suggested that application of N fertilizers after seedling emergence could minimize N leaching in plants. Similar results were reported by Rawal and Kuligod (2014) in maize exposed to salinity stress. They recorded maximum nutrient uptake and highest yield in plants supplemented with 200 kg ha−1.

8.3 Phosphorus (P)

Phosphorus (P) is actively involved in plant growth and development. Its deficiency reduces the uptake and assimilation of nitrates by affecting the nitrate reductase enzyme activity (Pilbeam et al. 1993). A strong relationship between leaf turgor pressure and stomatal conductance has been reported by Radin (1984). Under drought conditions, the deficiency of P is prevalent through impaired root absorption rate and translocation toward shoot. The relative growth rate and photosynthetic ability of plants is negatively affected with P deficiency by reduced cell turgidity. Indeed, P actively involved in energy processes, enzyme regulation, transport of carbohydrates, and building of adenosine triphosphate (ATP), phospholipids, nucleic acids, and phosphorus-proteins. The decline in net photosynthesis is attributed to decreased regeneration capacity of ribulose 1,5 bisphosphate and stomatal conductance. In addition, plants in P-deficient soils are unable to consume photo-assimilates for growth processes (Brooks 1986; Fredeen et al. 1989). The earlier effect of water scarcity in plants is the P deficiency. Its availability improves plant growth even under mild drought stress because of its active role in water status, photosynthesis, stomatal conductance, and cell membrane stability index. Phosphorus induces drought tolerance ability in plants through promoting symbiosis association of roots with mycorrhiza, leading toward enhanced water and nutrients uptake. The more absorption of water and nutrients, in turn, improves nitrate reductase activity for assimilation of nitrates. It is suggested that priming with P nutrient also improves the growth processes under drought stress (Ajouri et al. 2004; Waraich et al. 2011).

A drastic change in morpho-physiological mechanisms through distraction in intercellular osmotic gradient is the main cause of salinity stress. The disturbed functioning as decreased protein synthesis, photosynthesis, enzymatic activity, P, K+, Ca2+ imbalances, water relations, membrane stability index is responsible for decreased stem width, length, diameter, pith, leaf thickness, xylem vessels, and length of leaf vascular bundles (Semida et al. 2014). Higher salinity tolerance with P supply has been suggested in previous reports. It is well documented that only moderate supply of P is essential for salinity tolerance in rice plants while its higher concentration leads toward toxicity in cells, which in turn decreases crop productivity (L’taief et al. 2012). Plants exposed to salt stress conditions causes a significant decline in chlorophyll pigments. The decrease in green pigments reduces photosynthesis, which is a principal source of energy for physiological and biochemical processes. A considerable decline in chlorophyll contents might be caused by disoriented thylakoid membranes and degraded chlorophyll pigments. Indeed, the synthesis of chlorophyllase enzyme, a proteolytic enzyme, is the cause of rapid degradation of chlorophylls, attributed to reduced growth and assimilation rates (Rong-hua et al. 2006).

The production of osmo-protectants like total soluble sugars, proline, and free fatty acid contents are improved in plants of salinity stress, while its concentrations further increase in P-rich plants. These osmo-protectants are involved in salt-tolerant mechanism of osmotic adjustment for plants’ survival under salt-induced conditions (An and Liang 2013).The biosynthesis of osmo-solutes also adopts strategy against salinity stress by acting as protectants for cellular and enzymes structures. Another reason for salinity tolerance by producing osmolytes is that behavior like N storage compounds. The salinity tolerance responses of osmolytes in terms of N fixation and plant development have been reported in Proteus vulgaris, Phaseolus acutifolius, and Medicago sativa (Taie et al. 2013). P’s role in mitigating the salinity-induced adversities in Pistacia vera is because of its contribution to increased sugar contents. P’s active role in osmotic adjustment is because of higher concentrations of produced osmolytes. Addition of P to salt-stressed plants provides support in the pathway of sugar synthesis and its structural formation. The decrease in mineral elements like K+ under prevalent saline conditions could be related to selective absorption and gradient competition between Na+ and K+ resulted in increase in uptake of Na+ in spite of K+. This synergistic effect of P upon Ca+, Mg+, and K+ may be responsible for osmotic adjustment in plants for improved tolerance to salinity stress (Shahriaripour et al. 2011; Bargaz et al. 2016).

8.4 Potassium (K)

Potassium (K) is considered the most effective element for inducing drought tolerance in plants. It is involved in various physiological and biochemical mechanisms such as enzymes activation, photosynthesis, turgor pressure maintenance, and photosynthates translocation (Mengel and Kirkby 2001). Increased photosynthesis and plant growth under drought stress conditions is attributable to improved K nutrient status of plants. The possible way for alleviating drought adversities in drought-stressed plants by K supply is the higher water use efficiency. Improved tolerance ability of K-rich plants is due to its crucial role in the maintenance of osmotic potential and turgidity of the cells, which regulates the proper functioning of the stomates (Kant and Kafkafi, 2002). K protects the plants against harmful effects of drought by efficient utilization of soil moisture, maintaining high stroma pH, and reduced photo-oxidative damage to the chloroplast (Cakmak 1997). Plants suffering from abiotic stresses require more K to ameliorate their adverse effects. A high demand for K is relatively linked with its critical role in assimilation of CO2. The decreased leaf water potential due to drought stress conditions causes stomatal closure, which eventually reduces CO2 fixation. Among the harmful effects of environmental stresses like drought stress is the formation of reactive oxygen species (ROS). Its increased production induces oxidative damage to the chloroplast particularly during the process of photosynthesis. The abnormalities in photosynthesis and carbohydrate metabolism are also associated with ROS synthesis. A more decrease in photosynthesis in K-deficient plans under drought stress conditions is due to decrease in K within the chloroplast and further synthesis of ROS. Therefore, to lessen the severity of drought stress, K could be the possible way to avoid the disorders of water relations, stomatal opening, cell oxidative damages, and finally photosynthesis (Mengel and Kirkby 2001; Jiang and Zhang 2002; Waraich et al. 2011).

Exogenous K supply enhances the biomass accumulation and influences grain weight and yield by increasing the translocation of dry matter into grain. Moreover, K plays a critical role in drought tolerance by increasing water use efficiency, maintaining water balance, regulating stomatal conductance, and improving carbohydrate content (Hussain et al. 2016). It decreases the risk of cavitation under low water availability, thus enabling the plants to tolerate drought stress (Trifilo et al. 2008). Application of KCl and K2SO4 was observed to increase drought tolerance in rice by increasing proline content and activities of antioxidative enzymes (Zain and Ismail 2016). In addition, K-induced high transpiration rate increased uptake of nutrients to alleviate the damaging effects of drought stress. Foliar spray of K (using K2SO4 as a source) before silking stage reduced kernel abortion and significantly increased grain yield and K concentration in maize under water deficit conditions (Shahzad et al. 2017). Similarly, soil applied with K (100 mg kg−1 of soil) increased growth and yield of two contrasting maize hybrids, viz., 32F-10 (drought tolerant) and YH-1898 (drought sensitive), under drought stress (Aslam et al. 2013). Hussain et al. (2017) reported that combined application of K with Zn (150 kg ha−1 K + 12 kg ha−1 Zn) was more effective than individual application of these nutrients to mitigate drought stress in maize.

In wheat, K application was observed to increase pigments (chlorophyll and carotenoids) and activity of aminotransferase resulting in improved RWC and yield under water deficit conditions (Jatav et al. 2014). Recent studies by Shah et al. (2017) also showed a marked increase in wheat yield by exogenous K supply. Exogenous K supply improves water relations and prevents the degrardation of pigments and plasma membrane proteins under water stress conditions (Alam et al. 2011; Zareian et al. 2013). Safar-Noori et al. (2018) provided evidence that K application along with salicylic acid (SA) could increase nutritional quality and grain yield of wheat under drought stress. They observed a significant increase in water-soluble pentosan and starch content by SA + K treatment. Likewise, Rohbakhsh (2013) reported a marked increase in forage yield and quality of sorghum by K application under limited water conditions.

Salt-induced effects on crop growth and productivity could be alleviated by K application because of its direct involvement in photosynthesis, synthesis of proteins, regulation of stomata, and turgor-pressure-driven solute transport in the xylem (Ashraf 2004). Furthermore, type of cuticle or wax deposition on leaf surface, type, source, type and applied concentration of K, growth conditions and its absorption are the basis for its effective utilization. K deficiency causes a significant decline in growth of plants due to its active role in photosynthesis. The decline in leaf K contents in K-deficient plants is responsible for disturbed stomatal regulation, consequently affecting the rate of photosynthesis. Availability of K in guard cells, epidermal cells, and leaf apoplast is considered to be essential for mediating the proper functioning of stomates. It has been reported that K contributes to optimize the water balance and assimilate partitioning in salt-stressed plants. Its essential role as osmotic adjustment into the vacuole helps the plants to survive under stressful environment by maintaining their water relations (Shabala et al. 2002; Akram et al. 2009). ROS production in salinity-induced plants is the main reason of unbalancing of homeostasis, resulting in ion toxicity, osmotic stress, and lipid peroxidation, eventually reducing permeability of cell membrane for ion leakage (Sairam et al. 2002; Kukreja et al. 2005). Improvement in salinity stress in K-enriched plants has been reported in corn, rice, and wheat by its imperative role in activation of antioxidants. A major decline in ion leakage by K supply has also been reported in salt-induced spinach plants. Availability of K to salt-stressed plants showed positive response in mitigating its drastic effects because of its antagonistic behavior for uptake of Na ions. The enhancement in growth by K application under salinity stress was also reported in rice plants (Lynch and Lauchli 1984; Kaya et al. 2001). A significant decline in chlorophyll contents (a and b) was observed in maize plant when exposed to salt stress conditions. The biochemical alterations which are responsible for reducing chlorophyll pigments in salt-affected plants could be due to degradation of proteins, chlorophyll enzymes, and chloroplast structure. It is suggested that lethal effects are also in terms of reduced K+/Na+ ratio of salt-stressed plants. Addition of K improved the K nutrient concentration in plants, resulting in higher K+/Na+ ratio, which is responsible for salinity tolerance (Hernandez and Alamansa 2002; Abbasi et al. 2012).

8.5 Calcium (Ca)

Calcium (Ca) is considered most effective in enhancing drought tolerance ability in plants by maintaining the integrity of cell wall. It also facilitates the plants to acclimatize and recover from drought injury by its critical role in plant metabolism. To reduce the severe effects of drought, the possible way is the improved Ca nutrition in drought-stressed plants. Calcium protects the plant from drought injury by regulating the enzymatic activity of plasma membrane ATPase. This enzyme is actively involved in pumping back those lost nutrients during cell damage because of Ca deficiency (Palta 2000).

Imbalance nutritional status of salt-stressed plants is due to interaction and competition of Na+ and Cl− with other nutrients, leading to nutrient deficiencies, particularly uptake of Ca and N. Calcium is considered an essential element for ameliorating drastic effects of salinity because of its active role in varying mechanisms such as cell wall stabilization, functional and structural integrity of plant membrane, signaling processes, regulation and selectivity of ion transport, and enzyme activities of cell wall (Hadi and Karimi 2012). Salinity stress causes a decrease in Ca contents in the cytoplasm, attributed to lower Ca signals required for stress tolerance. In various biological systems, Ca acts as messenger for inducing salt tolerance ability in plants under harsh conditions by stimulating the system of signal transduction (Parre et al. 2007).

It has been reported that Ca has protective role in plant growth and development even under high salinity stress. Cell reproduction and volume in cotton roots was stimulated with supplemented Ca nutrition. Lowering of Na+/Ca2+ ratio has significant impacts on cell shape and its production. The root cells become thinner and longer due to narrow Na+/Ca2+ ratio (Leidi and Saiz 1997; Cramer 2002). Transport of water across root cell membranes from roots to leaves is affected when plants are exposed to salinity stress because of reduced hydraulic conductivity of roots with increased Na+/Ca2+ ratio. Exogenous Ca has potential to avoid the inhibition of roots hydraulic conductivity under Na-stress (Azaizeh et al. 1992). Cell wall extension under salinity stress is reduced with increased Na+/Ca2+ ratio as it affects biosynthesis of cell wall. Under high salinity levels, the biosynthesis of cellulose and non-cellulosic polysaccharide in the cell wall is affected in cotton roots with increased contents of uronic acid of cell wall, which eventually lose the integrity of cell wall. Modifications in concentration of cellulose and uronic acid is prevented if plants are supplied with Ca nutrition. With increased evidences, it is suggested that inhibition in enzymes activity and degradation of polysaccharides could be reduced by supplementing Ca. Ca-deficient plants under sodium stress were observed with changed composition of cell wall, specifically that of pectic polysaccharides (Hadi and Karimi 2012).

8.6 Magnesium (Mg)

Magnesium (Mg) is an important element for plant development because of its direct involvement in physio-biochemical mechanisms. Transport of photosynthates from source to sink is prompted with Mg nutrition supply. Sufficient Mg nutrient increases water and nutrients uptake because of increased root growth, helps in export of carbohydrates, reducing ROS production and photo-oxidative damage to cells under drought stress conditions. An enhanced chlorophyll is directly linked with Mg that is bound in the chloroplast. Interveinal chlorosis in drought-stressed plants is mostly prominent by Mg deficiency. Another reason of its deficiency in plants is presence of competing cations such as Ca, Al, H, NH4, and Na. Reduction in export and carbohydrates accumulation in Mg-deficient plants is the major cause of restricted CO2 fixation. Utilization of electrons is being limited to CO2 fixation due to impaired photosynthetic electron transport chain, thereby leading to ROS generation, which causes damage to membrane lipids and chlorophyll pigments. Tolerance to drought stress in plants is induced, however, by activating various kinds of enzymes such as RuBisCO, protein kinases, and ATPases by Mg supply (Mengel and Kirkby 2001; Mittler 2002; Shaul 2002; Epstein and Bloom 2004).

Harmful effects of salt stress on account of reduced photosynthesis may be due to alterations in stomatal conductance. More negative leaf water potential and osmotic potential under adverse conditions of salinity mediates turgidity of stomatal cells, which ultimately reduces stomatal regulation in plants. Crop productivity may eventually decrease in relation to depressed photo-assimilation during the process of photosynthesis (Xu et al. 1994). It is depicted that under salinity stress, even plants are osmotically adjusted; water absorption could not regain their turgidity. To balance the water loss, Mg has strong impact on maintaining the water status of salt-stressed plants. Higher leaf stomatal conductance in Mg rich plants might be because of decreased leaf water potential and subsequent increase in leaf turgor pressure. Mg is involved in many enzymatic reactions, protein synthesis, osmoregulation, and growth of salt-stressed plants (Furriel et al. 2000).

8.7 Sulfur (S)

Sulfur (S), due to its importance in plant growth, development, and involvement in several defense mechanisms, is now being considered as fourth major macronutrient required by plant. Its importance is evident from the fact that it is a vital constituent of vitamins, pantothenic acid, and prosthetic groups. S-containing compounds like GSH, thiols, and sulfolipids are involved in defense mechanisms and also normal functioning of plants (Brychkova et al. 2007; Münchberg et al. 2007). In addition, it plays key roles in enzyme activation, chlorophyll formation, increasing photosynthesis, and synthesis of nucleic acids (Kaur et al. 2013). Glutathione, an S-containing compound, is involved in improving the assimilation of other nutrients. Furthermore, it stimulates the defense system against oxidative stress (Münchberg et al. 2007).

Sulfur plays an important role in plant growth, tolerance mechanisms, and formation of root nodules in legumes. Assimilation of S begins from the absorption of S from soil by SULTR (sulfate transporters) genes present in the roots. However, in S-deficient soils, plants can also absorb foliar S especially through hydrogen sulfide (H2S) (Koralewska et al. 2008). Plants response to stress results in increased sulfate flux compared to other ions like NO3 or PO4 showing that there is a high demand for S under abiotic stress (Ernst et al. 2010). S nutrition can enhance the efficiency of essential primary macronutrients like N and P (Azza et al. 2011), and plant needs similar amounts of S and P (Ali et al. 2008). In cereals, S nutrition improves the efficiency of N absorption and assimilation because the enzymes involved in metabolism of N have S as their vital constituent (Salvagiotti et al. 2009; De Bona et al. 2011).

During drought stress, when leaves are only site for ABA synthesis, sulfate in the xylem of plant acts as chemical signal for closing stomata. In addition, it also acts as a chemical signal for ABA-dependent stomatal closure in leaves during early stages of water stress when ABA biosynthesis is restricted to leaves (Ernst et al. 2010). In recent years, important roles of S in alleviating various stresses have been studied in detail (Rausch and Wachter 2005). In wheat, exogenous sodium hydrosulfide (NaHS) supply increased RWC and biomass accumulation at seedling stage compared to control plants under water deficit conditions. Moreover, it also upregulated the ABA catabolism genes, ABA reactivation genes, and expression levels of ABA synthesis in the roots of wheat plants. The results from this research indicated that exogenous NaHS can mitigate drought stress by the participation of ABA. Shan et al. (2011) evaluated the effects of H2S application on ascorbate and glutathione assimilation in wheat leaves under drought stress. Sodium hydrosulfide (NaHS; H2S donor) was used as a source of S. Pretreatment with NaHS reduced the contents of malondialdehyde (MDA) and electrolyte leakage caused by drought stress. Moreover, pretreatment with S enhanced the activities of GR, APX, gamma-glutamylcysteine synthetase (γGCS), and dehydroascorbate reductase (DHAR) compared to control plants. Application of trehalose (Tre) and SA resulted in increased activities of peroxidase (POD) and phenylalanine ammonia lyase (PAL), increased ascorbic acid oxidase (AAO) under stress, and decreased level of lipid peroxidation, preventing membrane leakage (Aldesuquy and Ghanem 2015). They concluded that SA and Tre are very effective in mitigating the negative effects of drought stress in wheat.

Salinity stress severely affects stomatal conductance leading to the restriction of gaseous exchange in plants. Hence, CO2 absorption and availability in plants reduce, resulting in decreased rate of photosynthesis (Flexas et al. 2007). S-containing compounds have the ability to modify the physiological processes of plants to increase tolerance of plants under saline conditions (Khan et al. 2014). Hazardous effects of salinity on rice yield and quality can be alleviated by combined application of gypsum and S (Shaban et al. 2013). They reported that application of gypsum and S can be very helpful in improving vegetative growth, grain yield, and quality of rice grown under saline conditions. Supplementation of wheat with CaSO4 at the rate of 150 kg ha-1 markedly increased tillers, spike length, 1000 grain weight, grains spike−1, and straw yield under salt stress conditions (Arshadullah et al. 2013). Furthermore, increase in Ca, Mg, and S and a decline in Na were also observed in grains compared to control plants, indicating that CaSO4 application can significantly increase essential macronutrients Ca, Mg, and S and avoid uptake of Na. Khan et al. (2006) investigated the importance of S in increasing the yield and yield-related traits of maize under saline conditions and concluded that fertilization with 60 kg ha−1 S markedly improved biomass (41%), 1000-grain weight (5%), and total grain weight (43%) in salt-stressed plants. In rice, application of 600 kg ha−1 S along with 6-day irrigation interval resulted in the highest water productivity rates and significantly improved grain and biological yield under saline conditions (Zayed et al. 2017). Maize seedlings treated with 60 and 80 mM CaSO4 exhibited the highest germination percentage, mean emergence time, germination energy, mean daily emergence, and germination speed under salt stress conditions (Riffat and Ahmad 2016). Ye et al. (2015) reported that pretreatment of wheat grains with H2S during imbibition can increase the germination of wheat seeds by reducing the inhibitory effects of salt stress. In addition, seeds treated with NaHS showed higher activities of esterase and amylase compared to control plants. It also reduced the levels of MDA and the alterations made in solidarity of plasma membrane by NaCl particularly in the tips of radicle.

8.8 Boron (B)

Boron (B) is considered as essential element because of its primary role in the integrity of cell wall. Mineral nutritional supply of B plays a vital role in stimulating plant resistance against drought stress factor. Reduced stunted growth of plants under drought stress is related to the fact that B nutrition helps to strengthen the plants through promoting flower retention, pollen tube formation, sugar transport, and carbohydrates metabolism. A wide range of physiological and biochemical changes at molecular and cellular levels are induced in drought-stressed plants. Alleviation of these drought adversities by B nutrition is because of increased uptake of water from the soil rhizosphere with more root hairs and mycorrhiza production, resulting in higher CO2 assimilation and stomatal conductance (Bartels and Sunkar 2005; Christensen 2005; Gustav et al. 2008). Accumulation of polyamines and chlorogenic acid and reduced biosynthesis of indole acetic acid (IAA) and cytokinins (CK) in drought-stressed plants might be due to B deficiency. Crop productivity increased because B has a role in improving photosynthesis, water use efficiency, and pollen viability and assimilates partitioning under water deficit conditions. Imposition of drought stress caused considerable decline in leaf water potential, while B supplementation changed the water potential more positively. More negative leaf water potential was also perceived in B-deficient legume plants due to reduced transpiration efficiency in response to water scarcity conditions (Wei et al. 2005; Will et al. 2011; Upadhyaya et al. 2012).

Salt-induced reduction in growth of plants could be minimized in the presence of B. Its increased supply causes accumulation of B in different plant organs such as shoot, root, style, stigma, and ovary, which results in better pollination, seed setting, and vigorous grain formation. Improved salinity tolerance in rice plants seems possible due to ion exclusion mechanisms (Mehmood et al. 2009: Aftab et al. 2015). Reduced transpiration rate because of salinity stress has been suggested to affect the interaction of B with salinity, its uptake through the roots, and translocation to shoot. The root uptake of B is severely affected in salt stress conditions. Uptake of B is predominantly based on membrane permeability in plants subjected to salt stress. Salt-induced changes in membrane composition, changed aquaporin functionality, or membrane damage, resulted in reduced B translocation to shoots. Salinity-induced changes in transpiration rate of plants might be caused by closed stomata and transpiration-driven water flow (Hu and Brown 1997; Wimmer and Goldbach 2012).

8.9 Zinc (Zn)

The possible way to mitigate the damaging effects of drought stress in plants is the efficient use of Zn nutrition. The decrease in crop productivity under environmental stress conditions is only because of Zn deficiency. Zn supplementation under drought conditions balances the hormonal status of plants and ensures its survival under adverse conditions of drought. It has been suggested that drought stress alter the normal functioning of auxin in plants. Its application under such harsh conditions acts as co-enzyme for the synthesis of tryptophan, a precursor for auxin production, and thereby increases root development for improved water status of plants. In previous studies, stunted growth and chlorotic leaves in maize, reduced photosynthesis because of decreased intercellular CO2 concentration, carbonic anhydrase activity, and stomatal conductance in cauliflower, lowered osmotic potential in cabbage, and declined transpiration rate in pecan plants were observed in Zn-deficient plants. In addition, Zn nutrition protects the plants from oxidative damage of ROS by enhancing the activities of antioxidants and reduced activity of membrane-bound NADPH oxidase under drought stress conditions (Waraich et al. 2011). Improvement in drought tolerance in crop plants by Zn application is also because of protein and carbohydrate metabolism, starch formation, and membrane integrity (Fageria et al. 2002).

Zn-nutritional status of plants is essential for higher crop productivity. Its deficiency is considered a most limiting factor for plant development under salinity stress environmental conditions (Khoshgoftar et al. 2004). It seems in previous studies that improvement in salt tolerance in plants could be possible by Zn additive. Plants exposed to saline conditions have higher concentrations of Na+ and Cl− ions. Generally, reduced drastic effects in salt-stressed plants with supplemented Zn are because of increased membrane stability index. Enriched plants with exogenously applied Zn have critical role in mediating the permeability of membranes by sustaining the membrane lipids that are active structurally and functionally. Instability of cellular membrane due to Zn deficiency in salt-affected plants is attributable to excessive uptake of Na+ and Cl− ions at toxic level, indicating damage to membrane permeability (Kong et al. 2005; Aktas et al. 2006). Tavallali et al. (2009) also suggested a possible role of Zn in improving salt tolerance in plants with reduced uptake of Na+ and Cl− ions. Absorption of Na+ and Cl− ions under salt stress conditions is the major reason of declining leaf RWC that in turn affect the relative growth rate, which is most prominent in Zn-deficient plants. Supplemented Zn nutrition ameliorates the drastic effects of salinity in terms of maintained water status of plants attributable to improved vascular tissues. By maintaining the Zn requirement of plants accumulates the Ca and K ions in the cell that provides protection to salt-stressed plants against osmotic stress and helps the roots for more water absorption (Gadallah 2000; Mehrizi et al. 2011).