Vegetable oils are an important part of the human diet. Nowadays, cultivation of vegetable-oil producing crops covers about 270 million hectares and produces 170 million tons, which accounts for approximately 75% of world production ( FAO, 2014 ). Egypt's self-sufficiency of vegetable oil declined from 95 % in early 1960's to less than 30 % in 2004, resulting in an increase in the importation of processed vegetable oil ( Hassan & Shafique, 2011 ). In 2011, Egypt spent about US$ 1.875 billion on the importation of vegetable oils ( FAOSTAT, 2011 ). The United States Department of Agriculture, Division of Foreign Agriculture Service, reported in 2013-2014 that Egypt's total oilseeds planted area only covers 3-5% of Egypt's total edible oil consumption. Expanding into edible-vegetable oil production in Egypt is limited due to the limitation of agricultural land and water supplies. Therefore, expanding cultivation into newly reclaimed areas, while developing new high-yielding varieties, as well as minimizing agricultural inputs, might be the only formula for Egypt to fulfill its vegetable oil need. Canola ( Brassica napus L.) is considered a specialty crop in Canada and USA. In Egypt, canola can contribute to reducing oil deficiency gaps. Additionally, it grows vigorously within the northern part of Egypt, newly reclaimed soil.

Salinity is a global issue threatening land productivity and food production. Globally, about 900 million ha are affected by salinity and salinization is predicted to impact 50% of all arable lands by 2050 decreasing the major crop yield by over 50% ( FAO, 2014 ). Under salinity stress, reduction in crop productivity, as well as changes in anatomy and ultra-structure, are sometimes interlinked with concomitant changes in a variety of biochemical, physiological, and molecular responses ( Ahmad et al. , 2018 ). Salinity also acts as pro-oxidant and induces oxidative burst through hyper-accumulation of reactive oxygen species (ROS) that cause injuries to cellular ultra-structure, organic compounds and impaired a variety of metabolic reactions ( Gupta et al. , 2017 ; Ahmad et al. , 2018 ; Kaya et al. , 2018 ; Kholghi et al. , 2018 ; Rahmani et al. , 2018 ).

Currently, there are many strategies for improving plant productivity under salinity, like genetic modifications, and/or using some inducers ( Gupta et al. , 2017 ; Akram et al. , 2018 ). Sodium nitroprusside (SNP, nitric oxide donors) is a small diffusible bioactive signaling molecule, plays multifunctional roles in plant growth, productivity under normal or stressed conditions ( Akladious & Mohamed, 2017 ; Ahmad et al. , 2018 ; Hanafy et al. , 2018 ). SNP can alleviate salt stress by directly scavenging ROS throughout the formation of peroxynitrite (ONOO-) ( Beligni & Lamattina, 2002 ), that is less toxic than peroxides and conjointly by enhancing antioxidant enzyme activities and metabolites ( Sheokand et al. , 2010 ). Moreover, SNP application stimulates activities of proton-pump within the tonoplast and increasing the K ⁺ /Na ⁺ ratio ( Beligni & Lamattina, 2002 ), additionally regulates proteins at the post-translational level needed for cellular division ( Sheokand et al. , 2010 ).

Due to very few reports on inducing salt tolerance of canola plants by SNP, the present study is of special interest for improving canola production. The hypothesis of work is that SNP would improve canola salt tolerance by activating the processes would lead to attenuation of the cellular damage caused by salinity.

The aim of the current investigation was to assess the protective role of SNP for the alleviation of NaCl stress on different parameters of canola plant growth, yield as well as its physiological and anatomical characteristics.

Salinity considerably hampered crop growth that is in harmony with earlier reports ( Gupta et al. , 2017 ; Ahmad et al. , 2018 ; Hanafy et al. , 2018 ; Kaya et al. , 2018 ). These could be attributed to a multifaceted action of salinity on vital processes like water absorption, ion uptake, photosynthesis, etc.

This reduction may be due to: (1) accumulation of salts within the leaves that causes an irreparable damage to the chloroplasts as observed in the current investigation, resulting in metabolic limitations of photosynthesis ( Gong et al. , 2018 ); (2) the osmotic effects resulting from salt stress, that causes increases in abscisic acid and decline in indole acetic acid and gibberellic acid, and consequently inhibition of growth ( Gupta et al. , 2017 ; Hanafy et al. , 2018 ); (3) decreasing in cell elongation ensuing from the repressing impact of water shortage on growth promoting hormones ( Banon et al. , 2006 ). SNP supplementation counteracts the injurious impact of salinity on canola plant development that is in agreement with previous findings in different crops ( Ghadakchiasl et al. , 2017 ; Gupta et al. , 2017 ; Ahmad et al. , 2018 ; Akram et al. , 2018 ; Hanafy et al. , 2018 ; Rai et al. , 2018 ). SNP plays a very important role in inducing plant growth of many plants exposed to environmental stresses by modulating a variety of metabolic alterations affecting plant's tolerance to salt stress, including: (1) scavenging ROS through induction of antioxidant system ( Ahmad et al. , 2018 ; Akram et al. , 2018 ; Rai et al. , 2018 ); (2) SNP as signal molecule induces changes in the expression of stress-responsive genes, involved in enhancing crop growth ( Gill et al. , 2013 ); (3) inhibition of ethylene biosynthesis ( Zhu et al. ,2008 ).

The reduction in chlorophyll concentration under salinity stress may be due to one or more of the following processes: (1) disruption chloroplasts ultrastructure and activated chlorophyllase enzyme ( Nazar et al. , 2014 ); (2) reduced uptake of some micronutrients like Fe that plays a vital role in chlorophyll biosynthesis ( Santos, 2004 ); (3) oxidation of chlorophylls coupled with the instability of the pigment-protein complex ( Tariq et al. , 2011 ). Increased photosynthetic pigments due to SNP treatment has already been observed in salt-affected plants ( Ghadakchiasl et al. , 2017 ; Gupta et al. , 2017 ; Ahmad et al. , 2018 ; Hanafy et al. , 2018 ). It was noted that SNP impairs the activity of the chlorophyllase and Mg-dechelatase that might have decreased the chlorophyll degradation ( Shi et al. , 2016 ). Moreover, SNP might control Fe uptake in root and increased Fe availability within the cell organelles, by modulating the chlorophyll synthesis gene expression ( Simontacchi et al. , 2012 ).

It is well documented that salinity stress perturbs water status and reduces either WC or LRWC in many plants ( Ahmad et al. , 2018 ; Kholghi et al. , 2018 ), that we also observed in this experiment. This decrease can be attributed to lower water availability; or to lose the plant roots their ability to catch up on the water through a reduction of the absorbing surface ( Yildirim et al. , 2008 ). However, supplementation of SNP has a positive impact on WC and LRWC of canola plants under salt stress, fact supported with previous reports ( Ghadakchiasl et al. , 2017 ; Ahmad et al. , 2018 ). That is still unclear, however SNP is able to maintain LRWC in stressed plants and the rise in water retention induced by exogenous applied SNP in stress-treated plants could also be connected to stomatal closure, a reduced transpiration rate, and/or promoted adventitious root development.

Crop plants accumulate compatible osmolytes that facilitate to regulate turgor and water acquisition under stress conditions ( Ahmad et al. , 2018 ; Kaya et al. , 2018 ; Kholghi et al. , 2018 ). Proline has been related to the relief of cellular osmotic stress, stabilization of proteins and/or membranes and improving the enzyme stability ( Reddy et al. , 2015 ). It is additionally served as a safe guard chloroplast and acts as energy storage under salinity ( Reddy et al. , 2015 ). The regulatory function of SNP in proline biosynthesis has conjointly been verified by inducing of 1-pyrroline -5- carboxylate synthetase activity, the key enzyme of proline biosynthesis; additionally by proline dehydrogenase activity impairment against salinity stress ( Ahmad et al. , 2018 ). Accumulation of total soluble carbohydrates has been widely reported in plants as a response to salt stress ( Rahmani et al. , 2018 ) or SNP application ( Hanafy et al. , 2018 ). Soluble carbohydrates have a vital role in plant metabolism not only by acting as osmoprotection, stabilizing cellular membrane, but also as signal molecules, involved in ROS balance and responses to oxidative stress in plants ( Nasrin et al. , 2010 ).

Salinity causes the impairment of ion equilibrium mechanism by hyper-accumulating toxic ions (Na ⁺ and Cl ^- ) and remarked reduction in useful ions (K ⁺ ) leading to decreasing K ⁺ /Na ⁺ ratio, in numerous plant species ( Gupta et al. , 2017 ; Kaya et al. , 2018 ; Kholghi et al. , 2018 ). Sodium toxicity is related to its competitive nature with K ⁺ for binding essential sites that carry normal cellular function. Additionally, Na ⁺ accumulation contributes to depolarization of the plasma membrane and subsequent opening of K ⁺ outward rectifier channels that triggers an excessive loss of K ⁺ from the interior of the cell ( Bose et al. , 2015 ). Exogenously, applied SNP lowered toxic ions (Na ⁺ and Cl ^- ), yet enhanced K ⁺ in canola plants under saline conditions that are in keeping with many earlier reports ( Gupta et al. , 2017 ; Hanafy et al. , 2018 ). SNP, additionally maintains a high K ⁺ /Na ⁺ ratio within the cytosol (Table 2) by inducing expression of H ⁺ -ATPase, and Na ⁺ /H ⁺ antiporter which facilitate Na ⁺ compartmentation under salinity ( Shi et al. , 2016 ).

Our investigation assessed that salt-stress provokes an over-accumulation of H ₂ O ₂ , which clearly explains salt-mediated oxidative burst. However, the application of SNP effectively eliminated the high levels of H ₂ O ₂ , and thus played an important role in mitigating oxidative stress. These findings also demonstrate the direct ROS quenching role of SNP under salt stress, reported in several plants ( Ghadakchiasl et al. , 2017 ; Gupta et al. , 2017 ; Ahmad et al. , 2018 ). The protective role of SNP may be explained due to (1) reaction with lipid radicals, that stops the generation of lipid peroxidation; (2) SNP might operate as a signal molecule, that activates the cellular antioxidant enzyme-encoding genes leading to higher enzyme activities possible by post-translational modification that renders the plants stress tolerant ( Graß et al. , 2013 ); (3) SNP acts as an antioxidant that reacts with ROS resulting in generation of short lived peroxynitrite (ONOO ^- ), which is less toxic in the cellular environment and thus limits cellular damage.

The hyper-accumulation of ROS in plants is among the most important injuries induced by environmental stresses. The cellular components most degraded by ROS are the membrane lipids. As a consequence, MDA concentrations can increase, which is a symptom of membrane injury in a plant cell ( Kaya et al. , 2018 ; Rahmani et al. , 2018 ). This increment in MDA could also result in the short coming of antioxidants to neutralize ROS ensuing from salt stress. In contrast, exogenous SNP reduced MDA and H ₂ O ₂ concentrations within the plant exposed to salinity. This finding was in accordance with those of other studies that indicated that MDA and H ₂ O ₂ were reduced by SNP treatments throughout abiotic stress as salt ( Gupta et al. , 2017 ; Akram et al. , 2018 ; Hasanuzzaman et al. , 2018 ; Rai et al. , 2018 ).

The cell membrane is one of the most important cellular targets common to different stress conditions. Salt stress induces lipid peroxidation, thus making the membranes leaky as evidenced by increase electrolyte leakage, while SNP alleviates the injuries of NaCl on MDA concentration and membrane permeability percentage. These results are in accordance with those obtained by Ahmad et al. (2018) , Kaya et al. (2018) and Kholghi et al. (2018) . These studies suggest a reduction in electrolyte leakage that may be explained on the basis of the role of SNP facilitated maintenance of membrane functions through induction of antioxidant metabolites, thereby protecting the plants against oxidation damage on membrane permeability ( Akram et al. , 2018 ; Rai et al. , 2018 ).

Under salt stress plant posses a range of non-enzymatic antioxidant molecules ( Shafiq et al. , 2014 ; Ghadakchiasl et al. , 2017 ) to scavenge ROS. Phenolic substances will serve as molecular antioxidants through their ability to destroy free radicals as well as substrates of many antioxidant enzymes ( Blokhina et al. , 2003 ). Accordingly, Franco et al. (2008) stressed that the existence of many hydroxyl groups bonded to associate an aromatic ring provides the molecule with the ability to donate a proton to a radical, thus acting as potential chain-breaking molecules or antioxidants upon secondary oxidation.

Salinity excited the build-up of ascorbic acid (AsA) as compared with that of the non-salinized control. These results harmonize with those reported by Shafiq et al. (2014) . Moreover, the application of SNP markedly induced the accumulation of AsA in canola plants under normal or salinized conditions. Hasanuzzaman et al. (2018) and Rai et al. (2018) assessed that SNP raises the amount of AsA as the results of an increase within the capability of SNP to scavenge ROS and should account partially for the lower concentration of H ₂ O ₂ .

Investigating the plant cell ultra-structural changes under salinity may be a useful tool for unveiling the fundamental strategies involved in conferring salt tolerance ( Atkin & Macherel, 2009 ). Salinity caused the major changes in chloroplasts including swollen of thylakoids, loose shapes of the part of internal lamellae thylakoids, although most of granal thylakoids were destroyed. These results were confirmed by Naeem et al. (2012) . The distortions of grana stacking and swelling of thylakoids caused by salinity within this investigation were probably due to a change within the ionic composition of the stroma.

The increment in the plastoglobule number assessed in the present study may be a formal indicator of environmental stress effects ( Austin et al. , 2006 ). The physical coupling between the plastoglobules and thylakoid membranes permits the free exchange of lipid molecules among the plastoglobules and thylakoids ( Austin et al. , 2006 ). The increased plastoglobule size and number observed within the salt-treated plants may be one of the adaptive mechanisms which prevent the oxidative damages caused by severe salinity. However, reduction in starch grains will even be attributed to increased conversion of starch into soluble sugars, which are considered as compatible solutes and play a vital role in osmotic adjustment. On the other hand, SNP relieved this structural damage by protecting the photosynthetic membrane system from oxidative stress. Large chloroplasts with no swelling and only minor dilations of the thylakoids in SNP and NaCl treated plants are the concrete indication of less oxidative stress. Another distinction within the chloroplast of the plants grown under salinity and SNP application along was that the dimension of starch grains increased. This means that the plants accumulate starch under favorable growth conditions which they metabolize a large amount of starch under saline to possess an appropriate pool of osmolytes ( Ahmad et al. , 2018 ). Comparatively less number of plastoglobuli in chloroplasts of plants treated with NaCl and SNP along is another indication of lesser oxidative stress ( Zhang et al. , 2010 ).

Under control conditions, the mitochondria had well-organized cristae and an intact structure and were of comparable size and appearance. On the other hand, cells from salt-affected plants had a very small and the lowest number of mitochondria as well as a loss of the integrity of the outer mitochondrial membranes. Similar results have been reported by Zhang et al. (2010) . However, SNP application increased the dimensions and number of mitochondria under control or stressed conditions. The higher mitochondrial number and size meet the increased needs of ATP under unfavorable conditions, when photosynthesis is generally suppressed ( Silva et al. , 2010 ), and additionally these organelles responses to stress by synthesis of various specific mitochondrial stress proteins ( Rizhsky et al. , 2004 ).

Salinization severely affects the agricultural productivity of crop plants. The annual global income losses due to salinization of agricultural land were estimated in US$ 11.4 billion in irrigated land and US$ 1.2 billion in non-irrigated areas ( Zhu et al. , 2008 ). The reduction in seed yield is basically due to a reduction in seed set within the fruit which can be attributed to a decrease in the pollen viability or in the stigmatic surface receptivity, or both ( Khatun & Flowers, 1995 ). A decreasing in the supply of carbon assimilate produced by decreased leaf area per plant (Table 1) is additionally responsible for the reduction of the yield components. Abscission of flowers or young fruit due to ethylene induction by salinity could explain the reduced pod set observed ( Chrominski et al. , 1989 ). SNP-improving stress tolerance and rise yield have been observed in numerous crops ( Kausar & Shahbaz, 2013 ). Improvement salinity tolerance in numerous plants/crops has been shown to be connected with an increased growth and yield as well as multiple metabolic adaptations, including low accumulation of ROS ( Fayez & Bazaid, 2014 ), low uptake of toxic ions and high accumulation of vital osmoprotectants ( Kaya et al. , 2013 ). The present study asserted that exogenous application of SNP significantly increased yield and its components of the canola plant under normal and saline conditions. This increase could also be due to reduced accumulation of Na ⁺ and increased that of K ⁺ enhancing photosynthetic pigments, resulting in increased dry matter accumulation. It has been found that exogenous application of SNP causes stronger stem and roots, improved branching, earlier flowering and a greater number of silique due to increasing water use efficiency. Moreover, SNP-induced high biomass production could also be ascribed to low uptake of Na ⁺ as well as low accumulation of ROS ( Ahmad et al. , 2018 ; Hasanuzzaman et al. , 2018 ; Rai et al. , 2018 ). Finally, application of SNP induced some structural protecting changes within the conductory tissues through increasing the phloem and xylem area, particularly with 50 μM resulting in a fast translocation of photoassimilates from the leaves towards the developing seeds.

In conclusion, exogenous application of SNP proved to be useful in enhancing plant growth and yield characters under normal or salinity conditions. The growth increment under SNP application was found to be associated with increasing photosynthetic pigment concentration, reduced lipid peroxidation-linked mem­brane deterioration, reduced the accumulation of Na ⁺ and Cl ^- and prevented salt-induced K ⁺ leakage thereby maintaining a higher K ⁺ / Na ⁺ ratio. Based on the results of the current work, it can be concluded that SNP sprayed plants can postpone the salinity injuries by peroxide/phenolic/ascorbate system which is involved in scavenging the ROS produced during salt stress and improved biochemical processes and anatomical traits that increased plant growth and yield.

We would like to thank Prof. Dr. A. Keith Cowan (Professor and Director of the Institute for Environmental Biotechnology, Rhodes University, South Africa) for his helpful insights and critical reading of the manuscript.