Silicon Nanoparticle-Induced Regulation of Carbohydrate Metabolism, Photosynthesis, and ROS Homeostasis in Solanum lycopersicum Subjected to Salinity Stress.

Agricultural crops are facing major restraints with the rapid augmentation of global warming, salt being a major factor affecting productivity. Tomato (Solanum lycopersicum) plant has immense nutritional significance; however, it can be negatively influenced by salinity stress. Nanoparticles (NPs) have excellent properties, due to which these particles are used in agriculture to enhance various growth parameters even in the presence of abiotic stresses. The objective of this study was to investigate the effects of silicon NPs (Si-NPs) through root dipping and foliar spray on tomato in the presence/absence of salt stress. Plant root and leaf were used for the measurements of morphological, physiological, and biochemical parameters treated with Si-NPs under salt stress. At 45 days after sowing, the activity of antioxidant enzymes, photosynthesis, mineral concentration, chlorophyll index, and growth attributes of tomato plants were measured. The developmental processes of tomato plants were severely slowed down by salt stress upto 35.8% (shoot dry mass), 44.3% (root dry mass), 51% (shoot length), and 62% (root length), but this reduction was mitigated by the treatment of Si-NPs. Application of Si-NPs significantly increased the growth attributes (height and dry weight), mineral content [magnesium (Mg), potassium (K), copper (Cu), iron (Fe), manganese (Mn), zinc (Zn)], photosynthesis [net photosynthetic rate (P N), stomatal conductance (gs), transpiration rate (E), internal CO2 concentration (Ci)], and activity of antioxidative enzymes including superoxide dismutase and catalase in salt stress. Foliar application of Si-NPs in tomato plants appears to be more effective over root dipping and alleviates the salt stress by increasing the plant's antioxidant enzyme activity.

Introduction

Salinity included the most dangerous environmental constrains affecting agricultural sector in the whole world.[1] Among various salts influencing soil salinity, NaCl is the most copious and influential one because of its capability to fight vital nutrients resultant to ground nutrient shortage and certain toxicity indications to the plants.[2] Mainly, salinity stress can spoil the ecosystem, biodiversity, human fitness, and natural wealth.[3,4] Presently, this difficulty has been provoked in many areas of the earth because of the negative effects of anthropogenic activities, regular weather disturbance, shortage of pure water, and restriction of arable area.[5] Salt stress is known to influence crop nutritional quality and palatability by changing the amounts of secondary metabolites, protein, and micronutrients.[6,7]

In saline situation, the performance of plants and their communication with the taxing feature have been found to be enormously compound, important to modify at the different stages of plant life cycle.[8] This problem can eventually activate normal amounts of stress adaptation among the salt-sensitive and salt-tolerant plant species.[9] Plants cope by oxidative strain primarily through an endogenous resistance system that includes enzymatic, such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), and non-enzymatic, such as ascorbate (AsA), glutathione (GSH), and proline, antioxidants.[10] Salt stress persuades the development of reactive oxygen species (ROS) in numerous parts of the plant at the cell level. Redox homeostasis is established in plant cells as a result of a balance between the formation of ROS and the performance of antioxidative enzymes, with a proficient protection arrangement in plants maintaining the appropriate balance between the formation and abolition of ROS.[11] ROS are well-known as a type of internal level of stress, causing a number of changes in a cell’s metabolism.[12] Silicon (Si) is the main element in the soil and a well-documented advantageous component in plants.[13] Plants gain favor in the presence of Si and it is noticed that Si can augment the biomass production and boost the tolerance to biotic and abiotic stresses and it assists the plant with stability and protection.

In the past years, some approaches, such as plant genetic transformation and growth stimulating bacteria have been utilized to lessen the salinity stress in plants.[14] Nanoparticles (NPs) are playing an important role in plant tolerance to several abiotic stresses.[15] Introducing NPs, as opposed to fertilizers and pesticides, can improve seed germination, growth, nutrient intake, and stress tolerance.[16] Nanoparticles are a wide range of materials that contain particles ranging in size from 1 to 100 nm in one dimension. Over their bulk sections, silicon nanoparticles (Si-NPs) were able to demonstrate a variety of corporeal and elemental properties.[17] As a result, it is crucial to figure out how Si-NPs interact with their surroundings. Currently, Si-NPs have been shown to be a one-of-a-kind Si resource that might be exploited to increase plant tolerance to abiotic stressors.[13] In Cucumis sativus, silicon dioxide NPs enhanced plant morphological and yield characteristics.[18]

Si-NPs improved the photosynthetic capacity, photochemical efficiency of photosystem II, and photosynthetic pigments in Triticum aestivum under heat stress according to[19] Younis et al. After observing the excellent effects of Si-NPs using it as a fertilizer and stress reliever might be a cost-effective strategy.[20]

This experiment was carried out to demonstrate the effects of Si-NPs on tomato roots and leaves, as well as their putative ameliorative benefits on salt-stressed tomatoes. The morphological, physiological, and metabolic components of these consequences were investigated.

Materials and Methods

Si-NP Preparation

Si-NPs were purchased from Sigma-Aldrich Chemicals Pvt. Ltd, United States of America (USA). A required concentration (100 mg/L) of Si-NPs was ready by diluting the stock solution.

Seed Germination and Plant Growth Conditions

Solanum lycopersicum L. seeds (var. S-22) of equal size were selected and surface sterilized in 70% ethanol for 2 min before being rinsed three to five times with deionized water. Tomato seeds were cultivated in plastic pots (filled with soil) under natural climatic conditions to make the nursery. After 20 days of sowing (DAS), tomato seedlings were transplanted to maintenance pots. Before transplanting, seedling roots were dipped in 100 mg/L of Si-NPs for 30 min. At the time of transplantation, 250 mM of NaCl was added in the soil to ensure the salinity stress. At 35 DAS, the plants were foliar treated with 100 mg/L Si-NPs. The experiment used a totally randomized design, with each treatment being repeated five times with four or five plants in each pot. Six experiments included in this study are categorized as follows: control (T1), salt stress (250 mM NaCl) (T2), roots dipped in 100 mg/L Si-NPs (T3), foliar spray of 100 mg/L Si-NPs (T4), NaCl (250 mM) + root dipping in 100 mg/L Si-NPs (T5), and NaCl (250 mM) + foliar spray of 100 mg/L Si-NP (T6). To analyze the morphological and physiological indicators of tomato plants sampling was done at 45 DAS grown under salinity stress.

Estimation of Photosynthetic Parameters

Gas exchange indicators viz, net photosynthetic rate (PN), stomatal conductance (gs), internal CO2 concentration (Ci), and transpiration rate (E) were calculated using the portable photosynthetic system (LI-COR 6400-40, LI-COR Biosciences; Lincoln, NE, USA).

Calculation of the Chlorophyll Index and Fluorescence of Chlorophyll

A chlorophyll meter from Soil Plant Analysis Development (SPAD) was used to determine the index of chlorophyll (SPAD-502; Konica, Minolta Sensing; Sakai, Osaka, Japan). The leaf chamber fluorometer (Li-COR 6400-40, Li-COR, and Lincoln, NE, USA) was taken to determine fluorescence chlorophyll (Fv/Fm).

Estimation of Antioxidant Enzyme Activity and Proline Content

The CAT, APX, and SOD were determined, as described by Khan et al.[21] However, GSH and AsA activities were determined to follow the procedure of Nakano and Asada.[22] Bates et al.[23] method was implemented to estimate the proline concentrations.

Evaluation of the Mineral Concentration

At 45 DAS, the mineral concentration of tomato plant leaves and roots was measured. Tomato plant leaves and roots were dried at 65 °C until they reached a stable weight, which was then ground into powder using a mill (CIENLAB CE-430) and dissolved in strong nitric acid. According to Hernandez–Hernandez et al.,[24] K, Mg, Fe, Cu, Zn, and Mn concentrations were analyzed using a Plasma Emission Spectrophotometer (ICP, model Thermo Jarrel Ash Irish Advantage 14034000).

Determination of the Activities of Rubisco, Hexokinase, and Fumarase

The Usuda[25] process was used to determine on Rubisco’s investigation. Whittaker[26] et al. and Thimmaiah[27] methods were used to estimate hexokinase and fumarase activity, respectively.

Total Soluble Protein Estimation

To determine the total soluble protein amount, the procedure described by Bradford[28] was used.

Estimation the Glucose, Fructose, Sucrose, and Starch Content

The glucose and starch amount in tomato leaves was determined as previously explained by Sadasivam and Manickam.[29] Ashwell[30] procedure was followed for the estimation of the fructose content. The sucrose content was determined by the method of Roe.[31]

Measurement of Growth

The root and shoot lengths were measured on a centimeter scale, while the dry mass was calculated using a digital balance.

Data Analysis

Data were intended by SPSS, 17.0 computer-based statistical tool for the analysis of variance (ANOVA), while LSD was envisioned as a split mean.

Results

Si-NP-Mediated Impacts on Photosynthesis under Salt Stress

Salt stress severely abridged the photosynthesis with concern attributes by 43, 38, 42, and 40% PN, gs, Ci, and E, respectively, over control ones (Figure A–D). However, exogenous application (via leaves and roots) of Si-NPs considerably increased the photosynthetic parameters of the plants and diminished the negative effects caused by salinity. Si-NPs pronounced by foliar spray, showed effectiveness as compared to root dipping and increased the PN (36%), gs (35%), Ci (39%), and E (41%) in contrast to salt-stressed plants (Figure A–D).

Figure 1

Effects of Si-NPs in the existence/non-existence of NaCl on net photosynthetic rate (A), stomatal conductance (B), internal CO2 concentration (C), transpiration rate (D), chlorophyll index (E), and chlorophyll fluorescence (F) of tomato at 45 days stage of growth. T1 = control; T2 = NaCl (250 mM); T3 = Si-NPs (100 mg/L, via root dipping); T4 = Si-NPs (100 mg/L, via foliar spray); T5 = NaCl (250 mM) + Si-NPs (100 mg/L, via root dipping); and T6 = NaCl (250 mM) + Si-NPs (via foliar spray).

Impact of Si-NPs on Chlorophyll Index and Chlorophyll Fluorescence under Salinity Stress

The application (via root and leaves) of Si-NPs to the seedlings of tomato plant increased the chlorophyll index (42 and 48%) and chlorophyll fluorescence (35 and 39%) at 45 DAS (Figure E,F). However, the application of NaCl reduced the chlorophyll index and chlorophyll fluorescence. Seedlings received NaCl decreased the chlorophyll index and chlorophyll fluorescence by 51 and 43%, respectively. However, appliance of Si-NPs partially or completely neutralized the toxicity caused by NaCl (Figure E,F).

Antioxidant Enzyme Activity in Si-NP-Treated Plants

Salt stress significantly improved the GSH, AsA, and proline contents and all antioxidant enzymes (Figure A–F) activities in tomato leaves. Foliar and root dipping treatments of Si-NPs, alone as well as combined with salinity stress, resulted in an augment in the content of above-determined parameters. The highest enhancement in the content of GSH (42%), AsA (51%), proline (28%) SOD (83%), CAT (73%), and APX (62%) was traced after Si-NP application as foliar spray in the presence of salinity stress (Figure ).

Figure 2

Effects of Si-NPs in the existence and non-existence of NaCl on glutathione (A), ascorbate (B), superoxide dismutase (C), catalase (D), ascorbate peroxidase (E), and proline (F) levels of tomato at 45 days stage of growth. T1-control; T2-NaCl (250 mM); T3-Si-NPs (100 mg/L, via root dipping); T4-Si-NPs (100 mg/L, via foliar spray); T5-NaCl (250 mM) + Si-NPs (100 mg/L, via root dipping); and T6-NaCl (250 mM) + Si-NPs (100 mg/L, via foliar spray).

Impact of Si-NPs on Mineral Concentrations

Treatment with Si-NPs increased the mineral contents in the occurrence/nonappearance of salinity in the leaves and roots of tomato (Figures and 4). Foliar spray of Si-NPs showed the best results and increased the K (67%), Mg (41%), Fe (79%), Cu (30%), Zn (26%), and Mn (44%) concentration in the leaves, compared to only NaCl-treated plants (Figures and 4).

Figure 3

Effects of Si-NPs in existence and non-existence of NaCl on leaf potassium (A), root potassium (B), leaf magnesium (C), root magnesium (D), leaf iron (E), and root iron (F) content of tomato at 45 days stage of growth. T1 = control; T2 = NaCl (250 mM); T3 = Si-NPs (100 mg/L, via root dipping); T4 = Si-NPs (100 mg/L, via foliar spray); T5 = NaCl (250 mM) + Si-NPs (100 mg/L, via root dipping); and T6 = NaCl (250 mM) + Si-NPs (100 mg/L, via foliar spray).

Figure 4

Effects of Si-NPs in the existence and non-existence of NaCl on leaf copper (A), root copper (B), leaf zinc (C), root zinc (D), leaf manganese (E), and root manganese (F) content of tomato at 45 days stage of growth. T1-control; T2-NaCl (250 mM); T3-Si-NPs (100 mg/L, via root dipping); T4-Si-NPs (100 mg/L, via foliar spray); T5-NaCl (250 mM) + Si-NPs (100 mg/L, via root dipping); and T6-NaCl (250 mM) + Si-NPs (100 mg/L, via foliar spray).

Si-NPs and NaCl Effects on Rubisco, Hexokinase, and Fumarase

The activities of rubisco, hexokinase, and fumarase are revealed in Figure A–C. These measures were considerably decreased in the presence of salt stress by 52% (rubisco), 62% (hexokinase), and 58% (fumarase) (Figure A–C). However, appliance of Si-NPs appreciably augmented the rubisco, hexokinase, and fumarase activity by 73, 96, and 81% (via root dipping) and 98, 129, and 126% (via foliar spray) in comparison to only NaCl receiving plants (Figure A–C).

Figure 5

Effects of Si-NPs in the existence and non-existence of NaCl on the activity of Rubisco (A), hexokinase (B), fumarase (C), and protein content (D) of tomato at 45 days stage of growth. T1 = control; T2 = NaCl (250 mM); T3 = Si-NPs (100 mg/L, via root dipping); T4 = Si-NPs (100 mg/L, via foliar spray); T5 = NaCl (250 mM) + Si-NPs (100 mg/L, via root dipping); and T6 = NaCl (250 mM) + Si-NPs (100 mg/L, via foliar spray).

Effects of Si-NPs and NaCl on the Protein Content of Tomato Leaves

The protein content significantly increased after the application of Si-NPs of tomato plants (Figure D). Moreover, salt stress reduced the protein amount by 38% over control plants. Treatment of Si-NPs via foliar spray as well as via root dipping diminished the harmful consequences of salinity and increased the protein concentration by 46% (root dipping) and 69% (foliar spray) over salt stress receiving plants (Figure D).

Si-NP Impact on Carbohydrate Metabolism under Salinity

Si-NPs showed positive effects on glucose, fructose, sucrose, and starch content of tomato plants (Figure ). Salt stress reduced the content of aforesaid parameters by 51% (glucose), 42% (fructose), 40% (sucrose), and 39% (starch) over control plants (Figure ). However, the application of Si-NPs diminished the undesirable impacts of salinity and increased the above parameters by 80 and 113% (glucose), 57 and 81% (fructose), 48 and 83% (sucrose), and 55 and 77% (starch), respectively, compared to salt-treated plants (Figure ).

Figure 6

Effects of Si-NPs in the existence and non-existence of NaCl on glucose (A), fructose (B), sucrose (C), and starch (D) content of tomato at 45 days stage of growth. T1 = control; T2 = NaCl (250 mM); T3 = Si-NPs (100 mg/L, via root dipping); T4 = Si-NPs (100 mg/L, via foliar spray); T5 = NaCl (250 mM) + Si-NPs (100 mg/L, via root dipping); and T6 = NaCl (250 mM) + Si-NPs (100 mg/L, via foliar spray).

Si-NP Impact on Plant Growth under Salinity

Under salt stress, tomato plants reduced shoot dry mass (35%), root dry mass (44%), shoot length (51%), and root length (62%) associated to those of controlled (Figure ). However, growth indices had significantly increased with appliance of Si-NPs in the occurrence/nonappearance of salinity (Figure ).

Figure 7

Effects of Si-NPs in the existence and non-existence of NaCl on shoot dry mass (A), root dry mass (B), shoot length (C), and root length (D) of tomato at 45 days stage of growth. T1 = control; T2 = NaCl (250 mM); T3 = Si-NPs (100 mg/L, via root dipping); T4 = Si-NPs (100 mg/L, via foliar spray); T5 = NaCl (250 mM) + Si-NPs (100 mg/L, via root dipping); and T6 = NaCl (250 mM) + Si-NPs (100 mg/L, via foliar spray).

Discussion

Salt stress is major detrimental environmental stress, which results in the toxicities of ion, osmotic and oxidative.[32] It is predictable that around 20% of ground will be pretentious by salt stress.[33] Salinity stress causes oxidative stress, ion toxicity, and osmotic stress in plants.[34] Plants are susceptible to salt stress at all stages of growth, resulting in lower yields.[35] Soil salinity is an environmentally harmful condition that reduces tomato and other plant yields all around the world.[36] Silicon is the widespread essential element in the earth’s crust, although it usually stays in a form that plants cannot absorb.[37] Transporter proteins such as Nod26-like intrinsic proteins and Lsi2 homologues actively transport Si in plants.[38]

Photosynthesis is a critical mechanism in plants that is directly responsible for biomass production and developmental processes.[39] Photosynthesis is also a stress-susceptible physiological process in which a variety of stress factors have a detrimental impact on photosynthesis and associated properties.[40] The fluorescence of chlorophyll (Fv/Fm) can be used to track a plant’s photosynthetic activity. The Fv/Fm, photosynthesis, and associated properties of tomato plants were significantly reduced by salt stress in the current study (Figure A–D), but they were significantly increased in the presence of additional Si-NPs, indicating improved photochemical efficacy. Similar findings were found in Capsicum annuum.[41] In salt-stressed plants, Si has been demonstrated to modify the assimilation ability of roots and stomatal orifices, affecting water absorption and water consumption efficiency. As a result, Si safeguards the crop against salt stress.[42]

Furthermore, some researchers believe that Si-conciliated plant growth in salt stress is caused by a decrease in sodium (Na+) incorporation and allocation in the crop plants.[43] Our findings are sustained by Sánchez-Navarro et al.[44] on Lilium and Osman et al.[45] on Glycine max. Under salinity circumstances, the application of Si enhanced photosynthesis and related features, as well as the cytochrome b6/f and ATP-synthase molecule according to Muneer et al.[46]

Salt-stressed plants had a significant drop in chlorophyll index (Figure E), indicating that increased Na+ levels are toxic to a variety of biomolecules. Furthermore, chlorophyll biosynthesis was obnoxious when it came to breakdown.[47] Many plants, such as Oryza sativus and Gossypium, have been found to have chlorophyll levels decreased due to salt.[47,48] Salt stress decreases chlorophyll synthesis and turn on chlorophyllase enzymes that have been damaged.[49] Exogenous Si-NP appliances induce a significant increase in chlorophyll quantity in tomato in the presence or absence of salt stress in the current investigation (Figure E). El-Serafy et al.[50] and Badawy et al.[51] provided support for this work.

Because of the extreme and brutal climatic change, salt stress is now common.[52] Silicon increases antioxidant activity, reducing ROS injure grounds by salt stress.[53] According to the findings (Figure ), the activity of SOD, CAT, APX, AsA, GSH, and proline amount were boosted by supplementing NaCl with Si-NPs thereafter. Liang et al.[54] found that Si in plants lowers the permeability of leaf cell plasma membranes and improves the organization of chloroplasts that have been badly damaged by salt stress.[54] Si boosted SOD activity and lowered lipid peroxidation produced by NaCl-stress, as well as encouraged root H+-ATPase in the membranes, signifying that Si can modify the construction, integrity, and meanings of plasma membranes by modulating stress-induced lipid peroxidation.[54] According to Zhu et al.,[55] increased antioxidant enzyme activity can improve salt tolerance by reducing oxidative damage in membranes. Exogenous administration of Si-NPs increased the enzyme activity of APX and CAT in Cucurbita pepo L. under salt stress.[56] According to Gohari et al.,[57] TiO2 NPs reduced the harmful effects of NaCl in Dracocephalum moldavica L. by increasing the activities of SOD, GPX, and APX, resulting in greater ROX detoxification. Shekari et al.[58] found that Si treatment with NaCl significantly increased SOD, CAT, and APX activities in Anethum graveolens. The findings of Osman et al.[45] and Sánchez-Navarro et al.[44] bolster our conclusion.

For a long time, it has been recognized that the absorption and transfer of suitable amounts of mineral nutrients by plant leaves and roots, as well as their distribution in organs, are critical for optimal plant growth and development.[59] The primary environmental constraint impacting the nutritional rate of the plants has been identified as salinity. In the current study (Figures and 4), NaCl significantly reduced the quantity of K, Mg, Fe, Cu, Zn, and Mn in the leaves and roots (Figures and 4). As proven by Benito et al.,[60] the decrease in these minerals might be linked to the roots’ severe Na+ amalgamation. SiO2 NPs can affect nutrient translocation in plants by a variety of methods, including the formation of organic acids by the roots, which can eventually allow for nutrient absorption.[61] By networking with plant membranes, NPs can change the mobility of transporters.[62] Through inclines, conduits, and transporters, silicon has been demonstrated to turn on H+-ATPases in the membrane and boost K absorption.[63] SiO2 NPs enhanced the amount of Cu and Zn in tomato leaves according to González-Moscoso et al.[41] Si-NPs increased the levels of K and P in Pisum sativum.[64] Alsaeedi et al.[65] found that adding Si-NPs to C. sativus boosted K absorption in a variety of organelles. In salinity stress, our findings correspond with those of Parvez et al.[66] on Chenopodium quinoa and Petropoulos et al.[67] on Cichorium spinosum.

Salinity affects the quantity of glucose in the leaves of plants depending on the intensity of salt stress. In tomato, for example, differences in the partition of assimilates under stress have been accounted for that.[68] According to the findings (Figure ), the application of Si-NPs under NaCl enhanced the concentration of soluble carbohydrates viz, glucose, fructose, sucrose, and starch. These observations are lined with the study of Lu et al.,[69] who found that in NaCl-treated tomato plants, the appearance of a gene encoding a Sucrose Synthase, SUS3, was enhanced while the expression of another gene, SUS2, was lowered. According to Saied et al.,[70] the aforementioned carbohydrate levels in plants exposed to salt stress decreased considerably.

However, the results revealed that saline circumstances reduced plant development substantially. However, the addition of Si-NPs reduced the toxicity induced by salt stress (Figure ). According to research by Janmohammadi and Sabaghnia[71] on Lens culinaris, Si-NPs increased seed germination and growth. In Ocimum basilicum, Si-NPs decreased the toxicity caused by NaCl while also increasing plant biomass.[40] With the fertigation of Si-NPs, salt-stressed genes such as AREB, TAS14, NCED3, and CRK1 become more visible in tomato under salt stress, but RBOH1, APX2, MAPK2, ERF5, MAPK3, and DDF2 genes become less visible.[72] Si increased salt tolerance in Zea mays by increasing soluble protein, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCase) activity, and macronutrient levels.[73] In general, its defensive role is linked to mechanical defense, as well as its capacity to influence gene expression and convince the plant protection course.[37] Silicon is the crucial determinant during salt stress as it acts as signal molecules to mediate stress acclimation response in plants. Further Si-based study should be focus on the specific aspects of omics techniques to understand the genes involved in the regulation of salt stress. This would proffer significant opportunity for increasing food security though the exploitation of Si under a negative environment.

Conclusions

Our findings reveal that under salt stress, tomato plants undergo morphological, physiological, and biochemical changes as a result of ionic disruption. Treatment of Si-NPs (both foliar and root dipping) reduces ionic toxicity by increasing the antioxidant enzymes activity. We propose that, the treatment of Si-NPs can reverse the salinity-induced fatal consequence, which results in a sharp fall in Fv/Fm and photosynthesis in tomato plants. Examination at large-scale field checks could be helpful in building a sustainable crop production system, particularly in salt-infected areas. An integrated move is also required that cumulates physio-morphological and biochemical aspects with molecular techniques domineering for the prosperous establishment of plants in salt-affected regions.