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Article

Modulation of Antioxidant Defense Mechanisms and Morpho-Physiological Attributes of Wheat through Exogenous Application of Silicon and Melatonin under Water Deficit Conditions

by
Abdul Sattar
1,2,*,
Ahmad Sher
1,2,
Muhammad Ijaz
1,2,
Sami Ul-Allah
1,3,
Sajjad Hussain
4,
Umair Rasheed
1,
Jamshad Hussain
5,
Salem Mesfir Al-Qahtani
6,
Nadi Awad Al-Harbi
6,
Samy F. Mahmoud
7 and
Mohamed F. M. Ibrahim
8
1
College of Agriculture, University of Layyah, Layyah 31200, Pakistan
2
Department of Agronomy, Bahauddin Zakariya University, Multan 60800, Pakistan
3
Department of Plant Breeding and Genetics, Bahauddin Zakariya University, Multan 60800, Pakistan
4
Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, China
5
Adaptive Research Farm, Lahore 54000, Pakistan
6
Biology Department, University College of Tayma, University of Tabuk, Tabuk P.O. Box 741, Saudi Arabia
7
Department of Biotechnology, College of Science, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia
8
Department of Agricultural Botany, Faculty of Agriculture, Ain Shams University, Cairo 11566, Egypt
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(9), 7426; https://doi.org/10.3390/su15097426
Submission received: 10 March 2023 / Revised: 25 April 2023 / Accepted: 26 April 2023 / Published: 30 April 2023
(This article belongs to the Special Issue Adaptive Response and Mechanism of Crops to Abiotic Stresses)
Browse Figures
Versions Notes

Abstract

:
Although the individual influences of silicon (Si) and melatonin (MT) have been widely studied under various abiotic stresses, little is known about their interaction under drought stress. In this study, an experiment in pots was carried out to investigate the potential of an individual or combined foliar application of silicon (Si) and melatonin (ML) (control (ck), water spray, 4.0 mM Si, 200 µM ML, and 4.0 mM Si + 200 µM ML) on wheat grown at two different water-holding capacity levels (80% well-water condition and 40% drought stress) in order to check of grain yield and some important physiological characteristics. Under drought stress conditions, grain yield and yield attributes, water content and photosynthetic efficiency of wheat crops were significantly decreased. Application of Si+ ML significantly improved leaf pigments (Chl a, Chl b and Chll a + b), leaf relative water content (RWC), proline, total soluble sugars, and total soluble protein. As well as, the activities of important antioxidant enzymes, including catalase (CAT), superoxide dismutase (SOD), peroxidase (POD) and ascorbate peroxidase (APX) were effectively boosted through the combined application of Si + ML. This improvement was correlated with an obvious decrease in the levels of MDA, H2O2, and electrolyte leakage and increased water use efficiency. Conclusively, the combination of Si + ML significantly enhanced the 20.21% yield and various morpho-physiological attributes of drought-stressed wheat plants and can be recommended as a promising treatment to enhance wheat productivity in drought-affected regions. Additionally, the results of this study may open up a whole new area of research opportunities at the transcriptional level to further understand the mechanisms underlying how Si + ML integrates and interacts with plants under drought stress.

1. Introduction

Wheat is the main staple food crop that is used all over the world, but due to abiotic stresses like drought, salinity and heat stress, the required amount of its yield has not been produced [1]. All environmental abiotic stresses have a negative impact on the growth of wheat and its production, but among all the stresses, drought stress has proved more lethal to the crops [2]. Due to climate change, globally rising temperatures increased threatening aspects of drought stress. In dry regions, higher temperatures cause the rate of water evaporation to increase and thus enhance the risk of drought or prolong periods of drought. About 80–90% of all documented disasters from natural hazards during the past 10 years have resulted from droughts, tropical cyclones, heat waves, and severe storms. Some 55 million people are globally affected by droughts every year, and they are the most serious hazard to livestock and crops in nearly every part of the world. Water scarcity impacts 40% of the world’s population, and as many as 700 million people are at risk of being displaced as a result of drought by 2030 [3]. In Pakistan, about 62% of the area is under scarce water conditions, in which water availability is critical for crop growth and productivity. All stages of growth are negatively affected by a water scarcity condition, but the reproductive stage and grain filling are particularly affected, resulting in a decrease in the number of grains and reduce the grain size of wheat [4]. The grain filling is also reduced because of the essential enzymes that are responsible for synthesizing carbohydrates such as sucrose and starch [5]. Drought stress severely reduced crop growth and productivity by decreasing the leaf water potential, relative water content, nutrient uptake, and the photosynthetic efficiency of plants. It also causes oxidative damage in plants through higher production of reactive oxygen species (ROS) [6].
Silicon (Si) is the second most abundant element on earth that is not essential for crop growth [7,8], but other elements cannot replace its unique function [9,10]. In the cell walls of leaves and stems, a large amount of amorphous form of silica (silicon dioxide/SiO2) is stored, some of which is also accumulated in the inner root cortex cells [11,12]. This silica ultimately acts as a physical barrier in plants. It helps in improving the rate of photosynthesis and uptake of various nutrients and also enhances the levels of antioxidants and hormones that ultimately strengthen the plant tissues and increase the various biotic and abiotic stresses. It improves plant functioning, which increases crop yield and quality [13,14,15,16,17,18]. In many research studies, it has been observed that the uptake of Si by plants was found to mitigate stress that induces the inhibition of photosynthesis. As Si regulates the gaseous exchange and the nutrients balance, increases the water content in leaves and boosts up the activity of antioxidants, the photosynthetic activity also increases [14,19,20].
The Si also maintains the water potential of the leaf, photosynthetic activity, the conductance of stomata, erectness leaves and the xylem’s structure during the high transpiration rates. Thereby, it proved to be vital for improving drought tolerance in plants [21,22]. The selection of drought-tolerant plants has made extensive use of all of these parameters as physiological indicators of drought stress [23]. Si can promote photosynthetic activity in plants that are grown in water-deficit conditions by reducing the electrolyte leakage from the leaves of rice [24]. In several plant species, due to the modification of osmolytes, foliar application of Si improves the tolerance of plants in drought stress [25,26]. For example, the amount of proline and sugar has increased in wheat, and Solanum tuberosum leaves by the application of silicon [27,28]. It has also been observed that the application of silicon on plants can reduce the oxidative damage to plants by increasing the concentration of GSH, which helps to scavenge ROS, and its application also enhances the activity of essential antioxidant enzymes of plants, including SOD, CAT, POD, GR, and APX [15,29].
Melatonin (N-acetyl-5-methoxytryptamine) is a plant growth regulator which is mainly present in plants, bacteria, fungi, and algae [30,31]. According to many previous studies, the involvement of melatonin (ML) in various biological processes in plants has led to results such as germination of seed [32], growth of plant roots [33], flowering [34], senescence of leaves [35], improved capacity of photosynthesis [36], as well as minimizing oxidative damage [37,38,39]. In addition, various studies have demonstrated that the antioxidant effects of melatonin can significantly increase plants’ tolerance to abiotic stresses, including cold stress [40], heat stress [41], salt stress, UV stress [42,43], and drought stress [44]. Furthermore, melatonin enhances the resistance of plants against drought stress by regulating various processes like the metabolism of carbon and nitrogen. Melatonin has a vital role in the mitigation of drought stress by enhancing the activity of antioxidants, improving cellular redox homeostasis, and promoting photosynthesis [45,46]. According to recent research, the accumulation of metabolites occurs, including organic acids and amino acids, after the melatonin’s application on Bermuda grass, which ultimately increases the resistance to cold stress [47]. According to recent research on cotton, it has been observed that melatonin balances the carbohydrates in anthers during drought stress to increase the fertility of pollen [48].
It has also been demonstrated that the resistance of plants to various stresses can be effectively increased through exogenous supplementation of Si or ML [3,49,50]. However, information about the interactive effect of osmoprotectants has not yet been explored. It is hypothesized that the combined application of Si and ML will be more effective in alleviating drought stress. Therefore, the major objective of the study was to explore the Si and ML-driven physio-biochemical mechanism to alleviate the drought stress tolerance in wheat.

2. Materials and Methods

2.1. Experimental Detail

A pot experiment was performed in greenhouse conditions at the College of Agriculture, University of Layyah, Pakistan, situated at 30°57′40.6″ north latitude to 70°56′20.5″ east longitude and 151 feet high from sea level. The seeds of wheat cultivar Akbar-20 used in the present study were obtained from the wheat section of the Ayub Agricultural Research Institute (AARI), Faisalabad, Pakistan. Experimental treatments comprised of two factors: (i) two different water holding capacity (WHC) levels comprising a well-watered condition (80% WHC) and a drought-stressed condition (40% WHC) and (ii) five foliar treatments of Si and melatonin (ML) (Ck; control, water spray, 4.0 mM Si, 200 µM ML, 4.0 mM Si + 200 µM ML). The experiment was carried out by using a completely randomized design with two-factor factorial arrangements. Drought stress was imposed from the reproductive stage (BBCH growth stage 49) to the maturity stage (BBCH growth stage 83) phase of wheat. The foliar application of Si and ML was completed after seven days of imposing the drought stress. The experiment was comprised of ten treatments, and each treatment had five pots and three replications.
The 45 cm tall and 14.5 cm wide earthen pot was filled with 15 kg of well-ground air-dried sieved soil. The soil was collected from the Agriculture Research Farm, Hafizabad, at the University of Layyah. The soil was sandy loam in texture, having a pH of 8.5, organic matter of 0.76 percent, and electrical conductivity (EC) of 2.56 dSm−1. The soil also contains total nitrogen, available phosphorus, and available potassium at the rate of 0.58 g kg−1, 9.53 mg kg−1, and 62.34 mg kg−1, respectively. The bulk density of the soil was 1.71 g cm−3, and water content of the soil at field capacity (FC) was 20.87 percent. During pot filling with soil, fertilizers which contain potassium, phosphorus, and nitrogen were applied at rates of 100, 90, and 60 mg kg−1, respectively. A different source of fertilizer was used: nitrogen from urea, potassium from potassium sulfate, and phosphorus from di-ammonium phosphate. On 10 November 2021, ten seeds of the same size were manually sown in each pot at a uniform depth of 3 cm. After one week of germination, seedlings were thinned out, and three plants were kept in each pot for further studies. All pots were irrigated with tap water up to 80% WHC till the start of drought stress treatments.

2.2. Imposition of Water Holding Capacity

A water holding capacity (WHC, measured on a gravimetric basis) was maintained to impose drought stress during the reproductive phase. Three soil samples (200 g each) were taken from the soil used to fill the earthen pots. The soil samples were kept at 105 °C for 24 h. The average weight of these samples was measured to calculate the humidity level before the seed sowing. Three samples (100 g each) from these oven-dried soil samples were taken and saturated with distilled water. By calculating the water used for suturing the paste, field capacity was determined, as suggested by Nachabe [26].
At the reproductive stage, two levels of soil water-holding capacity (WHC) were maintained: in a set of experimental units, 80% WHC (well-watered) and in a second set, 40% WHC (drought stress) was maintained.

2.3. Photosynthetic Pigments and Relative Water Contents

According to Arnon’s method [51], a 0.5 g fresh sample of fully expanded flag leaves was collected for the determination of photosynthetic pigments. These samples were dipped in 5 mL acetone (80%) by keeping them at 0–4 °C overnight, and the extract was centrifuged (10,000 d 4 of photosynthesis). The supernatant was used to measure the absorbance of chlorophyll at 645 nm and chlorophyll b at 663 nm by using the spectrophotometer (U2001, Hitachi, Tokyo, Japan). According to the elaboration of Hall et al. [52], only flag leaves with full expansion were used to measure relative water contents (RWC). A 24-h soak in distilled water was used to rehydrate some of the leaves. Weighing and oven drying of fully turgid leaves at 80 °C for 48 h yielded:
RWC (%) = [(FW − DW)/(FTW − DW)] × 100.
where FW was fresh weight, DW was dry weight, and FTW was turgid weight.

2.4. Determination of Oxidative Stress Indicators

Wheat leaf samples were prepared for the determination of oxidative stress indicators. Hydrogen peroxide (H2O2) concentration was determined by taking the absorbance at 390 nm, followed by the procedure of Velikova et al. [53]. The amount of MDA contents was determined by using the TBA method [54]. For analysis, fresh leaves (0.15 g) of wheat were ground with 5.0 mL of 5% (w/v) TCA in mortar placed on an ice bath and centrifuged, and the MDA content was measured at 532 and 600 nm by using spectrophotometrically. For the determination of malondialdehyde (MDA), absorbance was recorded at 532 and 600 nm. The electrolyte leakage was determined by following the procedure of Agarie et al. [55].

2.5. Enzymatic Antioxidants Activities

The fresh leaf sample was mixed with a buffer of 5 mL of phosphate (50 mM with 7.8 pH) and was centrifuged at 15,000× g for 20 min. The basics for estimating superoxide dismutase (SOD) activity at 560 nm is the inhibition of NBT (nitroblue tetrazolium) reduction [56]. The 1 mL of NBT (50 nM), 1 mL of riboflavin (1.3 nM), 50 mL of enzyme extract, 950 mL of phosphate buffer (50 mM), 500 mL of methionine (13 mM), and 500 mL of EDTA (75 mM) were the main reactants of this reaction. The reaction was started by exposing the mixture to 30 W of light from a fluorescent lamp. After 5 min, the lamp was turned off, and the reaction stopped. At 560 nm, the blue formazan that resulted from NBT reduction was observed. A blank reading was taken using the same reactants but lacking an enzyme extract. The activity of catalase (CAT) was determined by using a UV-visible spectrophotometer, and the production of H2O2 as a result of the enzyme reaction was recorded at 240 nm. The reaction mixture consisted of 900 µL H2O2 (5.9 mM) and 2 mL phosphate buffer (50 mM), and 100 mL of enzyme extract was added to initiate the reaction. A further µmol of H2O2 per minute per mg of protein was used to show the activity of catalase [57]. The procedure that was provided by Kar and Mishra [58] was used to estimate the peroxidase (POD) activity. The main reactants that were used to determine the activity of peroxidase consisted of 5 mL of Tris-HCL buffer (0.1 M), 5 mL pyrogallol (10 mM), 5 mM of H2O2 (5 mM), and 100 µL enzyme extract. POD activity was measured as POD IU per minute per mg of protein, taking into account the decrease in absorbance at 425 nm caused by H2O2-dependent pyrogallol oxidation.

2.6. Osmolytes Determination

The estimation of total soluble protein and free proline was carried out with 0.5 g of the fresh green flag leaf. For sample grinding, a pre-chilled mortar and pestle have a buffer of 7.02 pH was used. Before the extraction of protein from the samples, cocktail protease inhibitors of 1 µM were added to saline phosphate buffer containing 2 mM KH2 PO4, 2.7 mM KCl, 10 mM Na2HPO4, and 1.37 mM NaCl dissolved in 1 L de-ionized water. The pH of the buffer was maintained by using HCl and autoclaved. Extracted samples were then centrifuged at 12,000× g for five minutes, and the supernatant was collected and used to measure the amount of soluble proteins. The Bradford [59] protocol was used to measure the amount of total soluble proteins. Standard curve construction was based on the dilutions of 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 µg µL−1 (bovine serum albumin). It was vortexed by adding 400 mL of dye stock and DI water to the incubated tubes. The UV 4000 UV-VIS spectrophotometer was used to measure the absorbance. The determination of proline was done by following Simaei et al. [60]. Sulpho-salicylic acid (3% w/v) (10 mL) was used to homogenize fresh leaf samples. For the purpose of color development, the filtrate was separated and kept in test tubes. Glacial acetic acid and ninhidrine (2.5%) were then used to treat it. After that, the filtrate was kept in a water bath for 60 min at 100 °C. In order to separate the chromophores in test tubes, toluene was added when it was removed from the water bath.

2.7. Yield and Yield-Related Traits

Plant height and spike length were measured using a scale from the stem near the soil surface to the ear tip at the maturity of each plant selected from each pot. The number of mature fertile tillers per plant was counted from each pot. The number of grains per spike, 100-grain weight (g), biological yield, and grain yield per plant (g) were recorded from the selected plants that were manually harvested and threshed.

2.8. Water Use Efficiency

The water use efficiency of each treatment was computed using the following formula for different levels of available moisture-holding capacity [61]:
eu = Y W R
where eu = Water use efficiency (g mm−1); Y = Crop yield (g); WR = Total amount of water used by crop plant.

2.9. Statistical Analysis

All of the experimental data were analyzed using Fisher’s two-way Analysis of Variance (ANOVA), and the least significant difference (LSD) test was used to calculate the average of the treatments [62]. The Pearson linear correlation test was used to determine how various traits were linked together. Microsoft Excel ©365 was used to create the figures. Principal components analyses were carried out using XLSTAT software to draw the biplot among treatments and studied parameters.

3. Results

3.1. Photosynthetic Pigments and Relative Water Contents

Drought stress significantly reduced photosynthetic pigments as well as the relative water contents of wheat leaves. Adversities of drought stress can be mitigated by the sole and combined foliar application of Si and ML. However, the foliar application of Si, ML and its combination not only enhanced the Chl a, Chl b, and Chl a + b but also increased the relative water content of wheat leaves in drought conditions (Figure 1). Sole Si application enhanced Chl a, Chl b, and Chl a + b contents by 5.75, 26.6 and 9.55%, respectively, and relative water contents by 15.7%, while sole ML application also increased Chl a, Chl b, and Chl a + b contents by 6.98, 31.25, and 11.19%, respectively, and 17.15% relative water content. Combined application of Si and ML increased the Chl a by 10.12%, Chl b by 42.10% and Chl a + b contents by 16.32% and relative water content is also enhanced by 19.59% (Figure 1). However, overall, a significant increase in Chl a, Chl b, and Chl a + b and relative water content was observed by the combined foliar application of Si and ML under water stress conditions (Figure 1).

3.2. Lipid Peroxidation Indicator

Drought stress significantly increased the concentration of H2O2, MDA and the electrolyte leakage in leaves of wheat plants (Figure 2). However, foliar application of Si, ML and Si + ML significantly reduced the concentration of H2O2, MDA and the electrolyte leakage IN wheat leaves under drought stress conditions (Figure 2). In addition, the sole application of Si reduced the level of H2O2, MDA and electrolyte leakage by 26.39%, 24.72%, and 40.28%, respectively. Moreover, sole foliar application of ML also decreased the concentration of H2O2 by 31.16%, MDA by 62.76%, and electrolyte leakage by 64.38%. Furthermore, the application of Si + ML decreased the levels of H2O2, MDA and electrolyte leakage by 49.71%, 79.95%, and 79.87%, respectively (Figure 2).

3.3. Enzymatic Antioxidants Activities

Drought stress significantly affected the activities of antioxidant enzymes such as CAT, SOD, POD, and APX. The activities of these enzymes enhanced by the sole and combined foliar application of Si and ML, whether under drought conditions or not (Figure 3). Separate applications of Si increased the activities of CAT, SOD, POD, and APX by 18.74%, 10.59%, 32.48%, and 23.36%, respectively. Sole foliar application of ML also boosts the activities of CAT, SOD, POD, and APX by 28.85%, 14.32%, 41.69%, and 16.32%, respectively. The combined application of Si and ML enhanced the activity of CAT by 33.89%, SOD by 16.90%, POD by 49.10%, and APX by 30.11%. The combined foliar application of Si and ML showed more significant results in the activities of CAT, SOD, POD, and APX under drought stress conditions (Figure 3).

3.4. Accumulation of Osmolytes

The amount of total soluble protein, free proline and soluble sugar in the wheat plant increased by the foliar application of Si, ML and its combination under water stress conditions. Sole foliar application of Si increases the total soluble protein by 14.64%, free proline by 4.35% and soluble sugar by 14.15% (Figure 4). Separate ML application also increased the amount of total soluble protein, free proline, and soluble sugar by 39.17%, 14.58% and 17.13%, respectively. The effect of the combined foliar application of Si and ML has greater than the separate application of Si and ML. The combined application of Si and ML significantly increases the amount of total soluble protein, free proline, and soluble sugar by 37.48%, 22.47%, and 20.30%, respectively. The effect of the application of Si, ML, and Si + ML on the concentration of total soluble protein, free proline, and soluble sugar is visible both in well-watered and water-deficit conditions, but the effect of Si and ML is more visible in the drought condition (Figure 4).

3.5. Plant Height, Yield and Yield Attributes

Drought stress significantly decreased the plant height and spike length, as well as the number of grains per spike in wheat. The negative impact of water deficit conditions on the plant can be alleviated by the foliar application of Si and ML. Foliar application of Si, ML and its combination not only enhanced the plant height, spike length and number of grains per spike in drought conditions but also in well-watered conditions (Figure 5). Sole application of Si increased the plant height, spike length, and grains per spike by 8.60%, 7.10%, and 26.07%, respectively. Separate application of ML can also increase the plant height, spike length and number of grains per spike by 8.76%, 9.46%, and 21.99%, respectively. The combined application of Si and ML has shown a more significant improvement in plant height, spike length, and grains per spike. The combined foliar applications of Si + ML boost plant height, spike length, and number of grains per spike by 11.44%, 10.33%, and 29.77%, respectively (Figure 5).
Water deficit conditions negatively impact the yield of wheat crops. This impact was alleviated by the foliar application of Si, ML and its combination. Both the sole and combined application of Si and ML have reduced the drought stress and enhanced the biological and grain yield per plant (Figure 6). Sole foliar application of Si increased the 100-grain weight by 12.98%, grains yield per plant by 17.12% and biological yield per plant by 8.01%. Separate application of ML also increased the 100 grain weight, grains yield per plant, and biological yield per plant by 19.99%, 19.65%, and 18.98%, respectively. Foliar application of Si + ML significantly increased the 100-grain weight, grains yield per plant, and biological yield per plant by 26.59%, 20.21%, and 25.51%, respectively (Figure 6). Drought stress influenced the water use efficiency of wheat crop plants. This adverse effect was alleviated by the foliar application of Si, ML and its combination. Both the sole and combined application of Si and ML have reduced the effects of drought stress and enhanced water use efficiency (Figure 4). Foliar application of Si + ML significantly improved 20% of the water use efficiency of wheat plants (Figure 6).

3.6. Principal Components Analyse Biplot

Principal component analyses (PCA) were run to make nine factors, out of which the first two factors explained 95.03% of the variability and were used to make the biplot (Figure 7). Placements and direction of variables in the biplot confirm the correlation among them and their closeness with two loading factors and this correlation has also been confirmed by the correlation analyses (Figure 8). Biplot grouped the treatments into four, where all the treatment means were away from the origin, showing that they deviated from the overall mean. Si and ML (sole or combined) under drought were grouped into one, Control (CK) and water under drought grouped as one, Sole application of Si and ML along with Ck and water grouped as one, and the combined application of Si and ML remained separate from all the groups and behaved as a separate group.

4. Discussion

Wheat can often continue its growth and development under low water availability but at the cost of yield and biomass accumulation. Drought stress is detrimental, especially at the terminal stage of the process of grain formation, where the accumulation of assimilates in the grain decline and grain size remains small, leading to a lower yield. Since the construction of pigment-protein complexes depends on Chl a, it is well known that Chl a serves as the primary cofactor for photochemical processes in plastids, although Chl b can also function as one of the auxiliary pigments in light-harvesting chlorophyll complexes (LHCs) [63,64]. In the current study, drought stress significantly reduced the chlorophyll contents (both a and b), which was attributed to the destruction of chlorophyll contents by chlorophyllase enzyme whose activity increased under drought stress conditions [65,66]. Chlorophyll contents are directly correlated with photosynthetic efficiency [67]. Therefore, it leads to lower production of assimilates. Although Si is not regarded as a necessary mineral nutrient, numerous lines of research have shown that plants benefit from it, especially when faced with biotic and abiotic challenges [68,69,70]. It can encourage photosynthesis by boosting chlorophyll levels [71] and influencing the activities of RuBisCO and PEP-carboxylase, which are necessary for CO2 fixation [72,73]. In addition, due to its antioxidant properties and ability to prevent the up-regulation of several senescence-associated genes, ML may have a protective effect in allowing chlorophyll to survive under abiotic stressors [74,75]. In this study, the application of Si and ML significantly improved the relative water contents and chlorophyll contents, whereas the combined application performed better. Zahra et al. [76] reported the exogenous application of melatonin regulates chlorophyll catabolism but suppresses the activities of chlorophyll catalyzing. It is well known that plants respond to external stress by producing reactive oxygen species (ROS) to engulf the free radicals. However, under severe stress, the quantity of ROS increases, and it begins to destroy its own plant cells [77]. In the current study, drought stress led to more production of ROS, i.e., H2O2 and MDA. Plant membranes are the favorite sites of the ROS to damage [78,79,80], causing membrane damage and leading to an enhanced electrolyte leakage which is also observed in the current study. Application of Si and ML decreased the production of ROS both under normal and drought stress conditions, and their combined application proved most effective in reducing ROS production. This reduced ROS production under the application of Si and ML is attributed to the ability of both to improve the production of antioxidant enzymes [81,82] and optimization of membranes [83]. This phenomenon is also witnessed in the current study. Both under normal and water stress conditions, maximum antioxidants were produced when both Si and ML were applied in combination.
Moreover, the application of Si and ML also leads to the production of more soluble osmolytes in the cell (i.e., soluble proteins and sugars and free proline), which helps to regulate the cell water contents under drought stress and thus develop resistance against drought stress in the plant [84,85]. Wang et al. [86] reported that enhanced accumulation of osmolytes in the cells of wheat plants sustains a better water status for the plants, which led to relative increases in photosynthesis, decreased oxidative stress damage, and increased grain yield under terminal drought stress.
Plant morphological traits are indicators of physiological health. Drought stress reduced the plant height, spike length, number of grains and grain weight under stress conditions, which is attributed to a disruption in plant physiological and biochemical mechanisms, as described above. Reduction in the development of plant attributes leads to lower biomass accumulation and reduced grain yield. It is reported that drought stress reduced the chlorophyll contents and photosynthesis [87,88,89,90], leading to a reduced accumulation of photosynthates and, thus, lower grain weight and yield which is also evident from there correlation (Figure 8). Application of Si and ML enhanced the antioxidant activities and improved the chlorophyll contents leading to a highly efficient photosynthesis system and increased accumulation of photosynthates, thus leading to improve grain and biomass yield.

5. Conclusions

When wheat plants were under water stress, Si and ML therapy improved a number of physio-chemical characteristics. Terminal drought stress disrupts the plant’s physiological and biochemical mechanism leading to poor growth and development and especially a reduction in grain size, which leads to lower grain yield. Application of Si and ML regulated the physiological and biochemical mechanism, where the combined application of Si and ML performed best, leading to better growth and development and high grain and biomass yield. However, before commercial recommendation, more field trials under variable agro-climatic conditions are suggested, along with the determination of the effects of Si and ML on nutritional quality. Future research will also likely shed light on Si + ML’s potential influence on the glutathione-ascorbate cycle in wheat plants under water stress. It is interesting that the application of Si + ML can improve grain quality.

Author Contributions

Conceptualization, A.S. (Abdul Sattar), A.S. (Ahmad Sher), M.I., S.U.-A., and S.H.; formal analysis, S.F.M., U.R.; funding acquisition, J.H.; investigation, S.M.A.-Q.; methodology, N.A.A.-H. and M.F.M.I.; and M.I.; resources, J.H. software, S.U.-A.; supervision, M.I.; validation, A.S. (Ahmad Sher) and S.M.A.-Q.; visualization, S.F.M., N.A.A.-H.; writing—original draft, U.R.; writing—review and editing, S.U.-A. and M.F.M.I. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All concerned data are available in the form of tables and figures.

Acknowledgments

Taif University Researchers Supporting Project number (TURSP-2020/138), Taif University, Taif, Saudi Arabia

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Effect of sole and combined application of silicon and melatonin on chlorophyll a (a), chlorophyll b (b), chlorophyll a + b (c) and relative water contents (d) of wheat genotype Akbar-20 grown under drought stress conditions. For each parameter, means followed by the same letter are not significantly different at p = 0.05; Mean ± SE; WHC, Water holding capacity; Ck, Control; Si, Silicon; ML, Melatonin.
Figure 1. Effect of sole and combined application of silicon and melatonin on chlorophyll a (a), chlorophyll b (b), chlorophyll a + b (c) and relative water contents (d) of wheat genotype Akbar-20 grown under drought stress conditions. For each parameter, means followed by the same letter are not significantly different at p = 0.05; Mean ± SE; WHC, Water holding capacity; Ck, Control; Si, Silicon; ML, Melatonin.
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Figure 2. Effect of sole and combined application of silicon and melatonin on H2O2 (a), MDA (b) and electrolyte leakage (c) of wheat genotype Akbar-20 grown under drought stress conditions. For each parameter, means followed by the same letter are not significantly different at p = 0.05; Mean ± SE; WHC, Water holding capacity; Ck, Control; Si, Silicon; ML, Melatonin.
Figure 2. Effect of sole and combined application of silicon and melatonin on H2O2 (a), MDA (b) and electrolyte leakage (c) of wheat genotype Akbar-20 grown under drought stress conditions. For each parameter, means followed by the same letter are not significantly different at p = 0.05; Mean ± SE; WHC, Water holding capacity; Ck, Control; Si, Silicon; ML, Melatonin.
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Figure 3. Effect of sole and combined application of silicon and melatonin on catalase (a), superoxide dismutase (b), peroxidase (c), and ascorbate peroxidase (d) of wheat genotype Akbar-20 grown under drought stress conditions. For each parameter, means followed by the same letter are not significantly different at p = 0.05; Mean ± SE; WHC, Water holding capacity; Ck, Control; Si, Silicon; ML, Melatonin.
Figure 3. Effect of sole and combined application of silicon and melatonin on catalase (a), superoxide dismutase (b), peroxidase (c), and ascorbate peroxidase (d) of wheat genotype Akbar-20 grown under drought stress conditions. For each parameter, means followed by the same letter are not significantly different at p = 0.05; Mean ± SE; WHC, Water holding capacity; Ck, Control; Si, Silicon; ML, Melatonin.
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Figure 4. Effect of sole and combined application of silicon and melatonin on total soluble protein (a), free proline (b), and soluble sugar (c) of wheat genotype Akbar-20 grown under drought stress conditions. For each parameter, means followed by the same letter are not significantly different at p = 0.05; Mean ± SE; WHC, Water holding capacity; Ck, Control; Si, Silicon; ML, Melatonin.
Figure 4. Effect of sole and combined application of silicon and melatonin on total soluble protein (a), free proline (b), and soluble sugar (c) of wheat genotype Akbar-20 grown under drought stress conditions. For each parameter, means followed by the same letter are not significantly different at p = 0.05; Mean ± SE; WHC, Water holding capacity; Ck, Control; Si, Silicon; ML, Melatonin.
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Figure 5. Effect of sole and combined application of silicon and melatonin on plant height (a), spike length (b), number of grains per spike (c), and 100-grain weight (d) of wheat genotype Akbar-20 grown under drought stress conditions. For each parameter, the means bar followed by the same letter is not significantly different at p = 0.05; Mean ± SE. WHC, Water holding capacity; Ck, Control; Si, Silicon; ML, Melatonin.
Figure 5. Effect of sole and combined application of silicon and melatonin on plant height (a), spike length (b), number of grains per spike (c), and 100-grain weight (d) of wheat genotype Akbar-20 grown under drought stress conditions. For each parameter, the means bar followed by the same letter is not significantly different at p = 0.05; Mean ± SE. WHC, Water holding capacity; Ck, Control; Si, Silicon; ML, Melatonin.
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Figure 6. Effect of sole and combined application of silicon and melatonin on biological yield per plant (a), grain yield per plant (b), and water use efficiency (c) of wheat genotype Akbar-20 grown under drought stress conditions. For each parameter, the means bar followed by the same letter is not significantly different at p = 0.05; Mean ± SE; WHC, Water holding capacity; Ck, Control; Si, Silicon; ML, Melatonin.
Figure 6. Effect of sole and combined application of silicon and melatonin on biological yield per plant (a), grain yield per plant (b), and water use efficiency (c) of wheat genotype Akbar-20 grown under drought stress conditions. For each parameter, the means bar followed by the same letter is not significantly different at p = 0.05; Mean ± SE; WHC, Water holding capacity; Ck, Control; Si, Silicon; ML, Melatonin.
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Figure 7. Principal component analyses biplot showing the active variables as placement of genotypes in different groups. In this figure F1 is on X-axis and F2 is on Y-Axis (active observations on primary Y-axis and Active variables on secondary Y-Axis).
Figure 7. Principal component analyses biplot showing the active variables as placement of genotypes in different groups. In this figure F1 is on X-axis and F2 is on Y-Axis (active observations on primary Y-axis and Active variables on secondary Y-Axis).
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Figure 8. Correlation analysis among the various attributes of wheat subjected to exogenous application of silicon and melatonin under water deficit conditions.
Figure 8. Correlation analysis among the various attributes of wheat subjected to exogenous application of silicon and melatonin under water deficit conditions.
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MDPI and ACS Style

Sattar, A.; Sher, A.; Ijaz, M.; Ul-Allah, S.; Hussain, S.; Rasheed, U.; Hussain, J.; Al-Qahtani, S.M.; Al-Harbi, N.A.; Mahmoud, S.F.; et al. Modulation of Antioxidant Defense Mechanisms and Morpho-Physiological Attributes of Wheat through Exogenous Application of Silicon and Melatonin under Water Deficit Conditions. Sustainability 2023, 15, 7426. https://doi.org/10.3390/su15097426

AMA Style

Sattar A, Sher A, Ijaz M, Ul-Allah S, Hussain S, Rasheed U, Hussain J, Al-Qahtani SM, Al-Harbi NA, Mahmoud SF, et al. Modulation of Antioxidant Defense Mechanisms and Morpho-Physiological Attributes of Wheat through Exogenous Application of Silicon and Melatonin under Water Deficit Conditions. Sustainability. 2023; 15(9):7426. https://doi.org/10.3390/su15097426

Chicago/Turabian Style

Sattar, Abdul, Ahmad Sher, Muhammad Ijaz, Sami Ul-Allah, Sajjad Hussain, Umair Rasheed, Jamshad Hussain, Salem Mesfir Al-Qahtani, Nadi Awad Al-Harbi, Samy F. Mahmoud, and et al. 2023. "Modulation of Antioxidant Defense Mechanisms and Morpho-Physiological Attributes of Wheat through Exogenous Application of Silicon and Melatonin under Water Deficit Conditions" Sustainability 15, no. 9: 7426. https://doi.org/10.3390/su15097426

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