Exogenous Application of Green Titanium Dioxide Nanoparticles (TiO2 NPs) to Improve the Germination, Physiochemical, and Yield Parameters of Wheat Plants under Salinity Stress.

Agriculture is the backbone of every developing country. Among various crops, wheat (Triticum aestivum L.) belongs to the family Poaceae and is the most important staple food crop of various countries. Different biotic (viruses, bacteria and fungi) and abiotic stresses (water logging, drought and salinity) adversely affect the qualitative and quantitative attributes of wheat. Among these stresses, salinity stress is a very important limiting factor affecting the morphological, physiological, biochemical attributes and grain yield of wheat. This research work was carried out to evaluate the influence of phytosynthesized TiO2 NPs on the germination, physiochemical, and yield attributes of wheat varieties in response to salinity. TiO2 NPs were synthesized using TiO2 salt and a Buddleja asiatica plant extract as a reducing and capping agent. Various concentrations of TiO2 nanoparticles (20, 40, 60 and 80 mg/L) and salt solutions (NaCl) (100 and 150 mM) were used. A total of 20 mg/L and 40 mg/L improve germination attributes, osmotic and water potential, carotenoid, total phenolic, and flavonoid content, soluble sugar and proteins, proline and amino acid content, superoxide dismutase activity, and reduce malondialdhehyde (MDA) content at both levels of salinity. These two concentrations also improved the yield attributes of wheat varieties at both salinity levels. The best results were observed at 40 mg/L of TiO2 NPs at both salinity levels. However, the highest concentrations (60 and 80 mg/L) of TiO2 NPs showed negative effects on germination, physiochemical and yield characteristics and causes stress in both wheat varieties under control irrigation conditions and salinity stress. Therefore, in conclusion, the findings of this research are that the foliar application of TiO2 NPs can help to improve tolerance against salinity stress in plants.

1. Introduction

Providing food to ever-growing global populations in a constantly fluctuating climate is one of the most substantial challenges for crop production worldwide. To meet the need for human food requirements, an estimate of 119% growth is needed by 2050 among the production of the major food crops [1]. Wheat ranked second among the most essential cereal crops and shares 20% of the overall dietary calories and a source of proteins globally [2,3]. Wheat and its derivatives are used worldwide, including in Pakistan, Nepal, India, Sri Lanka, and Bangladesh. As a universally vital sustenance crop, wheat is known to have the capability of overcoming food scarcity issues in several underdeveloped countries around the globe. With such food security issues in mind, wheat is amongst the most important crops for research purposes [4].

Numerous abiotic stresses are threatening wheat productivity in major wheat-growing countries, which include the USA, Russia, Pakistan, China, India and Australia [5]. Among the different abiotic stresses, soil salinity is among the most severe agricultural constraints throughout the world [6]. Salinity is amongst the most crucial elements in climate change. The increasing levels of salinity in the soil and irrigation water in combination lead to a negative effect on crop performance. Saline soils are among the main irritants in arid and semi-arid areas worldwide, having a harsh impact on agriculture production [7]. According to the CSSRI (Central Soil Salinity Research Institute) report of 2014, approximately thirty-three percent of irrigated farming lands and twenty percent of overall cultivated lands are already facing salt stress problems globally and are estimated to increase more rapidly than today by the year 2050 [8]. Soil salinity is a severe issue, especially in arid and semi-arid areas, which adversely affects wheat production and its quality [2]. Soil salinity is a major problem that limits food productivity and crop standards [9]. Salinity stress induces a barrier in stomatal conduction, shoot biomass, length of roots, photosynthesis rate and leaf water potential. Due to the accumulation of salts in the body of plants, the relative water content decreases as well as the yield of crops, the number of grains in the spike and the height of the plant, while the concentration of water-soluble carbohydrates and proline in plants increase [10]. An excessive amount of salt accumulation leads to an increase in ROS (reactive oxygen species) production [3]. Reactive oxygen species contain reduced forms of oxygen present in the atmosphere, which are produced in plants through vital processes, that is, photorespiration, photosynthesis and respiration, and result in impairment of cellular membranes and additional essential macromolecules, such as lipids, proteins, DNA and photosynthetic pigments [11].

Numerous technologies have been used to ameliorate the toxicological consequences of saline soils in wheat crops, but they are very expensive and also cause toxicity in the ecosystem. Therefore, it is necessary to take time to develop economical, eco-friendly, convenient and feasible technologies [12]. Currently, engineered nanoparticles have been extensively utilized in various industries worldwide. Therefore, the substantial effect of metallic nanoparticles on the growth and development of crop plants is of great concern for food [13]. Moreover, biofabricated nanoparticles have attracted the attention of nanotechnologists because of their eco-friendly behavior, the use of inexpensive technologies, excellent biocompatibility, and high rate of penetration in plant cells. Furthermore, biofabricated nanoparticles have surface characteristics and extra small-sized structures, resulting in distinctive physiochemical properties [14].

Among various nanoparticles, titanium dioxide nanoparticles have a wide range of applications as they enhance the growth of crop plants, improve yield and are also used in food and cosmetics. TiO2 NPs have been reported to have various advantageous effects on the biochemical and morphophysiological parameters of various crop plants [15]. TiO2 NPs have also been reported to enhance the photosynthetic rate, chlorophyll formation, the potentiality of antioxidant enzymes and rubisco activity. Furthermore, another study reported the productive influence of TiO2 NPs on broad bean plants under saline conditions [16]. It improves plant growth, proline content and antioxidant defense mechanism and reduces the production of H2O2 and MDA content in broad bean plants. Furthermore, another study reported the improvement in salt stress in the maize crop by the exogenous application of TiO2 NPs through the improvement of chlorophyll contents, phenolics, agronomic parameters and antioxidant capacity and yield [17].

According to the literature, titanium dioxide nanoparticles have both a productive and harmful impact on crop plants. High doses of TiO2 nanoparticles have toxicological effects on plants via ROS production that results in a reduction in chlorophyll content. Lower doses enhance growth attributes and yield of crops. Unfortunately, very few data are available on the beneficial and toxicological influence of biofabricated TiO2 NPs on wheat crops under salinity stress. Green synthesized TiO2 NPs may enhance salinity tolerance by altering germination and physiochemical attributes of wheat plants, ultimately leading to an improved productivity of wheat crops. Hence, this work studies the constructive and detrimental impact of TiO2 NPs on the germination, biochemical and yield characteristics of wheat varieties in response to salinity stress.

2. Results

2.1. Biofabrication and Characterization of TiO2 Nanoparticles

The scanning electron microscopy analysis revealed that nanoparticles are globular in shape and have a size between 30 and 95 nm. Some of them are fused to form tiny aggregates. Energy dispersive X-ray was used to study quantitative and qualitative detail of chemical compositions that are involved in the formation of nanoparticles. In the current study, strong atomic signals of atomic Ti at 4.4 and intensity 44.93 and O at 1.0 and intensity 55.07 indicated the presence of TiO2 NPs. Fourier transform infrared spectroscopy was performed to analyze the possible phytochemical groups accountable for the synthesis and stability of TiO2 nanoparticles. Peaks were obtained at 500–3750 cm−1. The formation of the crest at 3200 indicated the presence of hydroxyl and amino groups; 2962–762, 916 and 2848 indicates the presence of carboxyl groups; and at 2345.62 showed the occurrence of C–O bonds. Furthermore, the crests at 1639, 1437, 1261, 1095 and 1020 showed the presence of alkyl ketones, alkyl amines and carbonyl and alkane groups, respectively. Peak formation between 500 and 600 may be due to a halfway deterioration of the amine and carboxyl groups (Figure 1).

Figure 1

Characterization of TiO2 NPs. (A) UV-visible spectroscopy. (B) EDX. (C) Scanning electron microscopy. (D) Fourier transform infrared spectroscopy.

2.2. Germination Experiment

A germination experiment (Table 1) was performed to investigate the effect of titanium dioxide nanoparticles in the germination characteristics of wheat plants grown under salinity stress and control conditions. Table 1 presents the effect of titanium dioxide NPs on wheat plants grown under salinity conditions. Data were collected and analyzed, showing that salinity stress negatively affects the germination attributes (germination percentage, seedling vigor index, germination index, seedling length and fresh weight) of wheat varieties, while phytosynthesized titanium dioxide nanoparticles improve the germination attributes under both the control and stress conditions. The application of 100 mM NaCl showed a decrease of 58% and 62% in the germination percentage, a decrease of 47.13% and 53.07% in the germination index, a decrease of 69.42% and 73.15% in the seedling vigor index, a decrease of 50.19% and 54.32% in seedling length and a decrease of 42.19% and 47.42% in the fresh weight of both wheat varieties, respectively. The application of 150 mM NaCl showed an 80% and 87% decrease in the germination percentage, a 62.33% and 69.54% decrease in the germination index, an 83.21% and 88.54% decrease in the seedling vigor index, a 64.13% and 69.18% decrease in seedling length and a 60.13% and 66.47% decrease in the fresh weight of both wheat varieties, respectively. The application of 40 mL/l TiO2 showed a 45.33% and 42.15% increase in the germination percentage, a 39.54% and a 35.12% increase in the germination index, a 33.45% and 29.47% increase in the seedling vigor index, a 35.7% and 30.19% increase in seedling length and a 36.53% and 29.42% increase in the fresh weight of both wheat varieties, respectively, at 100 mM NaCl concentration. The treatment of 40 mg/L TiO2 showed a 27.13% and 22.25% increase in the germination percentage, 28.17% and 19.19% increase in the germination index, 23.35% and 19.41% increase in the seedling vigor index, 24.91% and 20.13% increase in the seedling length and 30.23% and 25.23% increase in the fresh weight of both wheat varieties, respectively, at 150 mM NaCl concentration. The application of 40 mg/L TiO2 alone showed a 15.14% and 14% increase in the germination percentage, a 12.5% and 9.2% increase in the germination index, a 16.91% and 10.53% increase in the seedling vigor index, a 9.5% and 7.2% increase in seedling length and 11.52% and 9.76% in the fresh weight concerning the control in both wheat varieties, respectively.

Table 1

Effect of TiO2 NPs on the germination attributes of the wheat varieties.

	Germination % Age		Germination Index		Seedling Vigor Index		Seedling Length		Fresh Weight
Treatments	V1	V2	V1	V2	V1	V2	V1	V2	V1	V2
To	80 ± 0.88	78 ± 0.35	2.11 ± 0.38	2.08 ± 0.054	1158.3 ± 0.18	1153.3 ± 0.78	14.5 ± 0.28	13.5 ± 0.88	0.12 ± 0.01	0.1 ± 0.54
T1	70 ± 0.51	68 ± 0.00	1.34 ± 0.77	1.3 ± 0.73	945.3 ± 0.59	941.3 ± 0.20	13.44 ± 0.28	12.4 ± 0.28	0.1 ± 0.11	0.08 ± 0.15
T2	90 ± 0.75	87 ± 0.00	2.35 ± 0.36	2.4 ± 0.54	1499.6 ± 0.05	1490.66 ± 0.05	16.64 ± 0.30	15.67 ± 0.30	0.15 ± 0.18	0.13 ± 0.47
T3	50 ± 0.86	48 ± 0.54	1.21 ± 0.75	1.17 ± 0.03	561 ± 0.36	524.3 ± 0.36	11.2 ± 0.57	10.25 ± 0.25	0.08 ± 0.05	0.06 ± 0.45
T4	20 ± 0.45	18 ± 0.88	0.96 ± 0.57	0.9 ± 0.00	179 ± 0.75	175 ± 0.56	9 ± 0.00	8 ± 0.86	0.06 ± 0.17	0.04 ± 0.32
T5	18 ± 0.06	17 ± 0.06	0.53 ± 0.57	0.52 ± 0.57	165.6 ± 0.21	161.7 ± 0.77	8 ± 0.00	7 ± 0.61	0.07 ± 0.35	0.06 ± 0.57
T6	6 ± 0.35	5.3 ± 0.31	0.4 ± 0.028	0.38 ± 0.028	29 ± 0.13	27 ± 0.00	5 ± 0.00	4.8 ± 0.35	0.06 ± 0.44	0.05 ± 0.19
T7	55 ± 0.52	58 ± 0.42	1.4 ± 0.047	1.41 ± 0.049	636.3 ± 0.33	626.67 ± 0.61	11.62 ± 0.23	11.5 ± 0.67	0.11 ± 0.008	0.093 ± 0.11
T8	40 ± 0.88	38.3 ± 0.88	1.1 ± 0.049	1.08 ± 0.054	413.3 ± 0.44	408.03 ± 0.79	10.5 ± 0.22	10.03 ± 0..03	0.09 ± 0.01	0.07 ± 0.08
T9	65 ± 0.39	63 ± 0.00	1.8 ± 0.77	1.84 ± 0.028	816.6 ± 0.30	811.81 ± 0.29	12.59 ± 0.28	12.05 ± 0.67	0.13 ± 0.77	0.11 ± 0.31
T10	60 ± 0.00	58 ± 0.54	1.6 ± 0.75	1.63 ± 0.039	816.6 ± 0.25	691 ± 0.22	11.24 ± 0.17	11.06 ± 0.88	0.11 ± 0.32	0.09 ± 0.11
T11	30 ± 0.00	28 ± 0.06	1.12 ± 0.05	1.10 ± 0.55	816.6 ± 0.14	285.7 ± 0.04	9.5 ± 0.16	9 ± 0.26	0.07 ± 0.57	0.06 ± 0.01
T12	19.6 ± 0.83	19 ± 0.00	0.75 ± 0.03	0.73 ± 0.65	816.6 ± 0.03	162.66 ± 0.33	8.43 ± 0.28	8.26 ± 0.18	0.04 ± 0.66	0.03 ± 0.66
T13	15 ± 0.86	9.3 ± 0.87	0.39 ± 0.05	0.37 ± 0.82	816.6 ± 0.33	101.53 ± 0.78	7.0 ± 0.66	6.7 ± 0.73	0.03 ± 0.47	0.023 ± 0.21
T14	7.3 ± 0.15	8 ± 0.27	0.24 ± 0.04	0.2 ± 0.032	816.6 ± 0.30	29 ± 0.00	5.5 ± 0.57	5.23 ± 0.00	0.02 ± 0.05	0.013 ± 0.17

2.3. Physiological Attributes

The findings indicate that the application of TiO2 NPs showed a considerable improvement (p < 0.05) in physiological attributes. Plants treated with 40 mg/L of TiO2 NPs significantly increase their osmotic potential by 2.32% and 4.17% in comparison to the control (Figure 2A). Salinity stress decreases the osmotic potential by 58% and 53.53%, while 40 mg/L of TiO2 NPs significantly improves the osmotic potential by 44.68% and 43.74% in both wheat varieties, respectively, at a level of salinity of 150 mΜ.

Figure 2

Effect of TiO2 NPs on the physiological parameters of wheat varieties. (A) Osmotic potential. (B) Water potential. (C) Carotenoid contents.

Relative water potential was also enhanced by the application of TiO2 NPs in both control and salinity stress conditions. Figure 2B indicated that salinity stress leads to negative water potential values as compared to the control. The salinity of 100 mM and 150 mM decreases the water potential to 25.4% and 26.94% and 20.31% and 22.74% in both wheat varieties, respectively, while the application of 40 mg/L TiO2 nanoparticles showed a 25% and 27% increase in the water potential under 100 mM salinity stress and a 12% and 15% increase under 150 mM salinity stress in both wheat varieties, respectively. The 40 mg/L TiO2 nanoparticles alone showed a 3% and 5.4% increase in water potential compared to the control (water irrigation). Figure 2C revealed that salinity stress decreases the carotenoid content in both varieties of wheat. Salinity decreases carotenoid contents to 15.32% and 32.22% in V1 and 34.8% and 43.63% in V2 at both salinity levels, respectively.

The highest carotenoids values were observed by the application of TiO2 NPs as 2.02% (V1) and 4.2% (V2) under control irrigation conditions. The application of TiO2 NPs increases carotenoid content to 10.05% and 8.54% in V1 and 13.54% and 12.09% in V2 under both salinity levels, respectively.

2.4. Biochemical Parameters

2.4.1. Production of Osmolytes

The data indicated that the contents of soluble sugar (Figure 3A) and proline (Figure 3B) increased significantly with salt stress. At 100 mM NaCl concentration, soluble sugar increased to 12.53% and 12.10% and 15.75% and 14.46% under 150 mM salt stress conditions in among both wheat varieties, correspondingly. The treatment of 40 mg/L TiO2 also increased by 16.20 and 15.35% at 100 mM salinity stress and 18.27 and 16.72% at 150 mM salinity level in both wheat varieties, respectively. In the case of the proline content, 40 mg/L of TiO2 application resulted in 17.74% and 19.11% increase at 100 mM and 22.48% and 23.55% increase at the salinity level of 150 mM in each wheat variety, correspondingly.

Figure 3

Effect of TiO2 NPs on the biochemical attributes of wheat the varieties. (A) Soluble sugar. (B) Soluble protein. (C) Proline content. (D) SOD. (E) MDA. (F) Amino acid content. (G) Total flavonoid content. (H) Total phenolic content.

2.4.2. Soluble Proteins and Amino Acid

The results show that salinity stress greatly reduced soluble protein (Figure 3C) and amino acid contents (Figure 3D) in both wheat varieties with increasing salinity levels. Salinity stress decreased soluble sugar by 40.69% and 42.6% (100 mM) and 52.33% and 59% (150 mM) in both wheat varieties, respectively, while 40 mg/L TiO2 showed an enhanced level of soluble sugar of 16% and 17.05% (100 mM) and 19% and 18.05% (150 mM) in both wheat varieties, respectively. Similarly, amino acid content is also reduced by 24.785% and 40.27% (100 mM) and 41% and 62.32% (150 mM), while a 40 mg/L of TiO2 application increased the amino acid content by 24.14% and 22.05% (100 mM) and 19% and 17.25% (150 mM) in both wheat varieties, respectively.

2.4.3. Antioxidant Activity

Titanium dioxide nanoparticles have been shown to increase antioxidant activities. The highest value of superoxide dismutase (SOD) was obtained by applying 40 mg/L of TiO2 NPs at the salinity level of 150 mM for NaCl alone. Salinity stress decreases enzyme activity by 12.3% (V1) and 14.75% (V2) at the 100 mM NaCl level and 17.32% (V1) and 21.02% (V2) at the 150 mM NaCl level (Figure 3E), while salinity stress increases MDA content by 45.34% (V1) and 49.12% (V2) and 52.74% (V1) and 55.12% (V2) at salinity levels of 100 and 150 mM, respectively. The application of 40 mg/L TiO2 NPs decreases the presence of MDA by 32.03% (V1) and 30.12% (V2) at 100 mM and 27.43% (V1) and 25.26% (V2) at 100 mM salinity levels, respectively (Figure 3F).

2.4.4. Total Phenolic and Total Flavonoid Contents

The data revealed that salinity stress significantly decreased both TPC (Figure 3G) and TFC (Figure 3H) with increasing salinity levels. A significant difference was observed in the plants treated with NPs and those without them. TiO2 increased TPC and TFC by 15.17% (V1) and 12.32% (V2) and 10.08% (V1) and 9.53% (V2) at the salinity level of 150 mM, respectively.

2.5. Yield Attributes

Data presented the effect of titanium dioxide nanoparticles on the yield parameters of wheat varieties grown in saline soil. Data were collected and analyzed, showing that salinity stress negatively affects the yield attributes: yield per pot is shown in Figure 4A, spike length in Figure 4B, awn length in Figure 4C, number of grains per spike in Figure 4D and hundred grains weight in Figure 4E of wheat varieties. Phytosynthesized titanium dioxide nanoparticles improve yield attributes under both the control and stress conditions.

Figure 4

Effect of TiO2 NPs on yield attributes of wheat varieties. (A) Yield per pot. (B) Spike length. (C) Awn length. (D) Number of grains per spike. (E) Hundred grains weight.

The application of 100 mM NaCl showed a 55% and 67% decrease in yield per pot, a 35.53% and 49.07% decrease in spike length, a 37.42% and 53.15% decrease in awn length, a decrease of 50.15% and 57.22% in the number of grains per spike and a decrease of 49.54% and 59.37% in hundred grains weight in both wheat varieties, respectively. The application of 150 mM NaCl showed an 83.12% and 100% decrease in yield per pot, a 75.42% and 87.39% decrease in spike length, an 82.19% and 89.63% decrease in awn length, an 87.43% and 100% decrease in the number of grains per spike and an 80.27% and 100% decrease in hundred grains weight in both varieties, respectively. The application of 40 mg/L of TiO2 showed a 53.32% and 57.25% increase in yield per pot, a 49.91% and 42.54% increase in the germination index, a 53.57% and 59.27% increase in the seedling vigor index, a 55.17% and 50.32% increase in seedling length and a 46.33% and 49.52% increase in the fresh weight of both wheat varieties, respectively, at 100 mM NaCl concentration. The 40 mg/L treatment of TiO2 showed an increase of 38.71% and 35.17% in yield per pot, an increase of 31.33% and 25.19% in spike length, an increase of 36.17% and 39.59% in awn length, an increase of 39.65% and 40.52% in the number of grains per spike and an increase of 39.88% and 37.43% in hundred grains weight of both wheat varieties, respectively, at 150 mM NaCl concentration. The treatment of 40 mg/L of TiO2 alone showed a 12.19% and 10.72% increase in yield per pot, a 12.16% and 10.97% increase in spike length, a 16.87% and 14.73% increase in awn length, a 12.42% and 12.32% increase in the grain numbers on each spike and a 10.15% and 11.37% increase in hundred grains weight concerning control in both varieties, respectively. Figure 5 represents the overall results of the negative effects of salinity stress and mitigation of salinity stress by the application of TiO2 NPs.

Figure 5

Representation of the negative effects of salinity stress and mitigation of salinity stress by the application of TiO2 NPs.

3. Discussion

Salinity is among the major challenges faced by different countries and agricultural land around the world. Salinity stress severely affects germination and healthy plant growth [18]. In the present study of phytosynthesized TiO2 nanoparticles with respect to salinity stress, the experiment showed that germination attributes are severely affected by salinity stress. Both salt concentrations showed a negative effect on all germination characteristics of both wheat varieties, but 150 mM showed the most severe effects at all levels, which is in agreement with ref. [19]. The authors of ref. [20] reported that the decline in germination attributes is possibly due to salt toxicity in cells, which causes a disturbance of different reactions inside the cells. In addition to this, ref. [21] reported that decreased germination may also be due to a reduced drive of H2O to the emerging embryo and a delay in the breakdown of biomolecules deposited in the endosperm. The exogenous application of TiO2 NPs showed an increase in all germination attributes at both salinity levels. Similar findings were also observed by ref. [16] with the use of green synthesized TiO2 on broad beans.

Ref. [22] reported that salinity reduces photosynthetic pigments due to an excessive accumulation of sodium ions, which causes severe changes in their function and structure. Chlorophylls and carotenoids are important molecules, oxygen scavengers and antioxidants, which contribute to lipid peroxidation, light protection and membrane stability. ref. [23] proposed that, carotenoids are involved in protecting the photosynthetic reaction center to avoid auto-oxidation, especially due to abiotic conditions. Phytosynthesized titanium dioxide nanoparticles enhance various photosynthetic pigments, resulting in increased photosynthetic activity.

The authors of ref. [16] described that TiO2 NPs increases photosynthetic pigments in broad bean plants facing salinity conditions and alleviated the harsh effect of salinity on plants. Salinity induces ROS production, which causes oxidative damage to photosynthetic pigments. TiO2 NPs reduce ROS production and enhance photosynthetic pigments, but mitigate the degradation of photosynthetic pigments [2,24,25]. The authors of ref. [23] reported an increase in photosynthetic pigments in Linum usitatissimum L. (Linaceae). In the present study, TiO2 improved the carotenoid content in wheat varieties under salinity stress, which is in line with ref. [22], who described the analogous findings in marigolds in salt stress conditions.

The results show that the water potential of the leaves in the wheat varieties showed negative values under salinity stress conditions compared to the control, while titanium dioxide nanoparticles showed a remarkable increase in the water potential of wheat plants at both salinity levels. Our findings are in line with those of Ahmad [19], who described similar findings in maize plants affected by salinity by applying TiO2 NPs. Salinity stress showed a decrease in osmotic potential at both salinity levels compared to the control, which is also reported by ref. [26]. This decrease is due to the accumulation of sodium ions in saline soils, which can lead to a reduction in the water content of the soil [27].

Salinity stress commonly leads to the generation of osmotic stress, which restricts the access of plants to water, resulting in cellular dehydration, loss of turgidity and, ultimately, plant death [28]. Therefore, plants adopt a crucial mechanism to cope with osmotic stress by accumulating basic osmolytes, such as proline and soluble sugar, to survive under saline conditions [29,30]. Enhanced concentrations of soluble sugar are an important organic solute that may also help in cell homeostasis [21]. In this study, both levels of salinity caused the production of proline and sugar in wheat plants. The exogenous application of TiO2 NPs significantly increased proline and soluble sugar content in both wheat varieties under salinity stress. Our findings are consistent with those of ref. [31,32], who reported similar results by applying TiO2 NPs on Phaseolus vulgaris L. (Fabaceae) and Dracocephalum moldavica L. (Lamiaceae) crops affected by salinity stress. The treatment of TiO2 accelerates the concentrations of proline and soluble sugars in plants, which can help salinity-exposed wheat plants as osmoprotectants by protecting enzymes from denaturing and also by stabilizing membranes [20]. Furthermore, enhanced soluble sugar production could help wheat plants to overcome disturbed cellular mechanisms and maintain growth under saline conditions.

Under salinity stress, malondialdhehyde content increases in the tissues of plants as a lipid peroxidation product that represents the oxidative damage caused by salinity, leading to injury to the membranes. An increase in the MDA content production leads to membrane injury and then leakage of membrane electrolytes [33,34]. Plants also retain a well-defined detoxification system to regulate ROS levels, which is often by different enzymatic and non-enzymatic antioxidants. These antioxidants are SOD, catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX). SOD works as the first line of defense in the case of oxidative damage. It converts SO radicals to H2O2 according to ref. [1]. Although all antioxidant enzymes remarkably increased under salinity conditions, in the present study, salinity stress caused an increase in the contents of MDA and SOD in both wheat varieties, while TiO2 nanoparticles reduce the production of MDA and SOD contents under salinity stress according to ref. [19].

Salinity stress increases the production of secondary metabolites, such as total phenolic and flavonoid contents. These are compounds that are produced under stress conditions and play an important role in plant growth and development and also have an important nutritional aspect in plants. Furthermore, as they also play an important role in reducing the damage caused by oxidative stress, they are also known as antioxidants according to ref. [35]. Phenolic contents have a specific chemical formula and defend plant cells from oxidative stress by binding to free radicals and chelate metals and help in lower lipid peroxidation. The buildup of phenolic content in plants can be subject to stresses depending on the species of plants and the sensitivity of plants to stresses. Flavonoid content also acts as antioxidants in plants cells according to ref. [36]. Their synthesis and accumulation in plants depend on numerous factors, such as soil type, abiotic stresses, environmental factors and nutritional status according to ref. [37]. In the present study, a reduction in phenolic and flavonoid compounds was observed during salinity stress conditions, while the application of titanium dioxide nanoparticles during stress conditions increases their production. These findings are consistent with those of according to ref. [22].

The yield and biomass of plants are directly related to the uptake of vital nutrients that play numerous important roles in plant metabolism, and their absence or deficiency also results in deterioration effects on plant growth and development according to ref. [6]. In the present study, salinity stress reduced the yield attributes of wheat varieties, as this stress causes a deficiency of nutrients, such as potassium, phosphorus and nitrogen. However, TiO2 NPs increase the yield characteristics by improving the uptake of essential nutrients and blocking the uptake of Na+ ions. It uses the sequestration mechanism of essential elements by nanoparticles and functions as a standard that supplies an adequate quantity of essential elements to roots of plants. NPs also boost the plant’s ability to absorb and use fertilizers and water efficiently from the soil according to ref. [38]. TiO2 NPs were also reported to accelerate the accumulation of essential elements, such as Fe, Ca, Mn, Zn, B and K, in Coriandrum sativum L. (Apiaceae) shoots according to ref. [39].

4. Materials and Methods

4.1. Preparation of the Plant Extract

The ethnomedicinal taxonomically identified plant Buddleja asiatica Lour. (Scrophulariaceae) was collected from Chirala, Bagh Azad Kashmir, located at N 32°26, E 74°64. To remove dust particles and impurities, the leaves were cleaned with distilled water and dried in the shade. The plant decoction was synthesized using 50 g of fresh Buddleja asiatica leaves in 500 mL of distilled water followed by heating at 80 °C for 2 h under stirring conditions. The obtained solution was cooled and filtered two times utilizing Whatman filter paper to obtain the purified extract. The extract of filtered leaves was then preserved at 4 °C.

4.2. Biofabrication of the Targeted TiO2 Nanoparticles

To synthesize TiO2 salt nanoparticles, TiO2 salt (Anatase) was dissolved in distilled water to obtain the salt solution. The solution was then moved to 250 mL conical flasks and shaken for 2 h on an orbital shaker at 100 rpm. A total of 160 mL of solution was placed into a 200 mL flask and 40 mL of leaf extract was poured dropwise during constant stirring conditions. There was no alteration in color when adding the leaf extract. After 24 h of constant stirring, the color changed from milky white to light brown, which shows the formation of titanium dioxide nanoparticles. The centrifugation of the obtained solution was conducted at 12,000 rpm for 20 min. The supernatant was discarded and the pellet was washed with distilled water through centrifugation. The pellet was kept for further characterization [16]. Figure 6 represents the diagrammatic representation of green synthesis of TiO2 NPs.

Figure 6

Diagrammatic representation of the phytosynthesis of TiO2 utilizing the leaf extract of Buddleja asiatica L.

4.3. Physicochemical Characterization of the Biofabricated TiO2 NPs

The biofabricated TiO2 NPs were exposed to numerous characterization techniques, such as UV, SEM, EDX and FTIR. Systronics Halo DB-20, Dynamica Scientific Ltd., was used to confirm the synthesis of TiO2 nanoparticles through UV-visible spectroscopy by using a spectrum between 200 and 650 nm of light wavelength for the reaction sample. An FTIR spectrophotometer was used to perform an FTIR analysis between 400–4500 cm−1 to illustrate the potential biomolecules responsible for the reduction, capping and stabilization of TiO2 nanoparticles. Scanning electron microscopy (SIGMA model, Zeiss, Germany) at 15.5 kV was used to study the size, morphology and image of TiO2 nanoparticles. EDX XL-30 was utilized at 15.5–25.5 keV to study other elements that accompany TiO2 nanoparticles. The values of EDX showed the atomic structure and atomic contents of the biofabricated nanoparticles.

4.4. Experimental Layout and Treatment Application

The seeds of two wheat varieties, Faisalabad-08 and NARC-11, were collected from NARC. A germination experiment was performed under controlled conditions in the laboratory. The seeds were sterilized with 10% sodium hypochlorite and then the seed priming was performed and (10 seeds per plate) placed in Petri plates with two layers (moist) of filter paper. On every second day, 3 mL of distilled water, nanoparticles and salt solution were applied to the respective Petri dish. Seed germination was examined daily up to day 7. After one week, the germination percentage, seedling vigor index, germination index, seedling length and fresh weight were analyzed.

The pot experiment was conducted in the glass house of the Pir Mehr Ali Shah Arid Agriculture University Rawalpindi (Department of Botany). For the pot experiment, the wheat varieties were seeded in specific pots (24 cm wide and 19 cm long), with 4.5 kg of sandy loam soil. After 21 days of germination, two different concentrations (100 and 150 mM) of salt solution were administered to the plants and repeated every third day, followed by the application of TiO2 NPs. Both wheat varieties were arranged into 15 main groups, as shown in Table 2.

Table 2

Different salt and TiO2 NP treatments on the wheat varieties for germination and field experiment.

Treatment Code	Treatments
To	Priming and germination in distilled water
T1	Priming and germination in 20 mg/L−1 of TiO2 NPs
T2	Priming and germination in 40 mg/L−1 of TiO2 NPs
T3	Priming and germination in 60 mg/L−1 of TiO2 NPs
T4	Priming and germination in 80 mg/L−1 of TiO2 NPs
T5	Priming and germination in 100 mM salt solution
T6	Priming and germination in 150 mM salt solution
T7	Priming with 20 mg/L−1 of NPs and germinated in 100 mM salt
T8	Priming with 20 mg/L−1 of NPs and germinated in 150 mM salt
T9	Priming with 40 mg/L−1 of NPs and germinated in 100 mM salt
T10	Priming with 40 mg/L−1 of NPs and germinated in 150 mM salt
T11	Priming with 60 mg/L−1 of NPs and germinated in 100 mM salt
T12	Priming with 60 mg/L−1 of NPs and germinated in 150 mM salt
T13	Priming with 80 mg/L−1 of NPs and germinated in 100 mM salt
T14	Priming with 80 mg/L−1 of NPs and germinated in 150 mM salt

4.5. Germination % Age

Germination % age was calculated using the following formula:

4.6. Seedling Vigor Index (SVI)

The seedling vigor index was measured using the following formula:

4.7. Analysis of Physiological and Biochemical Parameters

After 90–95 days of the experiment, different biochemical parameters were analyzed. Different physiological and biochemical parameters, such as carbohydrate, osmotic potential, water potential, soluble sugar, proline content, carotenoid content, superoxide dismutase and amino acid contents, were analyzed.

4.7.1. Water Potential

The methodology of ref. [40] was followed to determine the water potential of the leaves. To measure the water potential, the fresh leaves were collected and fixed into a pressure chamber. A nitrogen gas cylinder was used to apply pressure and was stopped immediately after the appearance of sap at the end of the petiole. The pressure where the first drop appeared was measured as the leaf water potential.

4.7.2. Osmotic Potential

The method of ref. [5] was used to measure the leaf osmotic pressure. For this, fresh leaves were taken, placed at −20 for 2 weeks and, after that, the cell sap was collected through a syringe from the frozen plant material. The sap obtained was placed on a disc, and the recorded values were converted to MPa from mmol/kg using the following formula:

4.7.3. Carotenoid Contents

The carotenoid contents were measured using the method of ref. [41] and the absorption was taken at 470 nm using a spectrophotometer.

4.7.4. Proline Content

The proline content was analyzed using the method of ref. [42]. To measure the proline content, fresh leaves were collected, purified with double distilled water and then ground in 4 mL of 3% sulfosalicylic acid and filtered with Whatman no. 1 filter paper. A total of 2 mL of the filtrate was taken in a separate test tube and allowed to react with 2 mL of ninhydrin reagent and glacial acetic acid and kept in a water bath for 1 h. After color development, the solution was placed on ice to stop the reaction and 4 mL of toluene was added and properly mixed until a separate layer appeared on top of the solution. This layer was, then, transferred to a separate group of test tubes, and the absorbance was recorded at 520 nm wavelength. The proline content was calculated using the following formula:

4.7.5. Soluble Sugar Content

The method of ref. [43] was used to analyze the soluble sugar content using the phenol sulphuric acid method. A total of 0.5 g of fresh leaves was collected and placed in test tubes containing 80% ethyl alcohol, followed by boiling in a water bath at 80 °C for 1 h. A total of 0.5 mL of sample extract was taken in separate test tubes, followed by the addition of 1 mL of 18% phenol and the mixture was allowed to incubate at room temperature. A total of 2.5 mL of sulfuric acid was added to the mixture and properly shaken, and then the absorbance was recorded at a wavelength of 490 nm using a spectrophotometer. The following formula was used to calculate the soluble sugar content:

4.7.6. Protein Content

The protein content was determined following the method of ref. [34] by grinding 0.5 g of fresh leaves in phosphate buffer. The extract was filtered and, then, 0.5 mL of the extract was taken in a separate test tube followed by the addition of 0.5 mL of distilled water and 3 mL of bio-red dye. The solution was mixed using a vortex mixer and then the absorbance was calculated at a wavelength of 595 nm.

4.7.7. Amino acid Content

The amino acid content was measured by using the method of ref. [44]. To measure the free amino acid content, the fresh leaves of all the treatments were collected and subjected to grinding in 80% ethyl alcohol (cold). The homogenized material was centrifuged for 20 min at 12,000 rpm and the supernatant was collected. HPLC was then used to find the free amino acid content.

4.7.8. Malondialdhehyde Content

The malondialdhehyde content was analyzed using the thiobarbituric acid (TBA) test with slight modifications. A total of 1 g of fresh leaves was ground in 0.1% trichloroacetic acid (TCA) and then centrifuged at 8000 rpm for 30 min. The supernatant was taken and, in a separate test tube, 3 mL of TCA and TBA (1:1) were added to the plant extract and heated in a water bath at 95 °C for 30 min and then placed on ice to stop the reaction. After cooling down, the mixture was again centrifuged at 8000 rpm for 10 min and the supernatant was collected. To analyze the malondealdhehyde content, the absorbance was recorded at 562 and 600 nm wavelength using a spectrophotometer, and then the value obtained at 600 nm was subtracted from 562 nm to obtain the actual value, as described by authors of ref. [45].

4.7.9. Superoxide Dismutase (SOD) Activity

To measure SOD activity, 0.5 g of the fresh leaf sample was homogenized in chilled phosphate buffer with pH 7, followed by centrifugation at 15,000 rpm at 4 °C for 15 min and the supernatant was collected. A total of 0.1 mL of supernatant was taken in a separate test tube and 3 mL of SOD buffer and 0.1 mL of riboflavin were added. The mixture was mixed with a vortex mixer and then kept under a fluorescent lamp (40 watts) for 8 min at 25 °C until the color changed to brown or black. Then, the wavelength at 560 nm was used, ref. [8].

4.7.10. Total Phenolic Content

Total phenolic content was measured using the method of ref. [46] with a few alterations. Freshly taken leaves (0.7 g) were placed in 15 mL of methyl alcohol overnight. The leaves were then filtered, and 100 mL of distilled water was added to be considered as the stock solution. Almost 225 μL of standard solution was taken in a separate test tube containing 1.5 mL of deionized water and 0.10 mL of 50% Folin–Ciocalteu phenol reagent and kept for 3 min. A total of 0.4% sodium carbonate was added to the solution and the subsequent mixture was kept for 2–3 h followed by a slight vortex. The absorbance was measured at 765 nm and gallic acid was used as standard.

4.7.11. Total Flavonoid Content

The method of ref. [47] was used to determine the total flavonoid content. A total of 520 mg of fresh leaves was taken and cut into small discs and ground in 80% methyl alcohol to obtain 8 mL of the final volume. The mixture was then centrifuged at 20,000 rpm for 10 min and the supernatant was collected. Then, 10 mL of deionized water was poured into the supernatant following the addition of 0.75 mL of 5% sodium nitrate and 0.6 mL of 10% aluminum chloride and kept aside for 5 min. Then, 3 mL of 1M sodium hydroxide and 2.5 mL of distilled water were added to the solution and kept for 1 min. Absorbance was recorded at 510 nm.

4.8. Yield Parameters

The yield parameters, such as, yield per pot, spike length, awn length, grain numbers on each spike and hundred-grains weight of seeds, were observed ref. [48].

4.9. Statistical Analysis

Analysis of variance (two-way) was used to measure the differences between the two varieties and various treatments using the SPSS software. Duncan’s new multiple range test was used, with p < 0.05 set as the significant difference.

5. Conclusions

Salinity stress is the predominant environmental factor that limits crop production worldwide. Dealing with salinity is the major challenge of the present time. In the present study, salinity stress considerably altered the germination, physiochemical and yield attributes of the wheat varieties at all levels. However, the exogenous applications of TiO2 alleviated the negative effects of salinity stress. Among the different concentrations of TiO2 NPs, 20 and 40mg/L were found to be the best concentrations to ameliorate salinity stress in plants. However, the best results were obtained by the exogenous application of 40 mg/L of TiO2 NPs, which improved the germination attributes, water potential and osmotic potential, increased the carotenoid, total phenolic and flavonoid, soluble sugar, soluble proteins, total amino acid and proline contents, while it decreased the MDA content in both wheat varieties. It also improved the yield attributes at both salinity levels. However, 60 and 80 mg/L of TiO2 NPs also cause stress in plants. Therefore, it was determined that TiO2 NPs play a considerable role in alleviating salinity stress by improving germination and physiochemical attributes resulting in an increased plant yield.