Regulation of Agronomic Traits, Nutrient Uptake, Osmolytes and Antioxidants of Maize as Influenced by Exogenous Potassium Silicate under Deficit Irrigation and Semiarid Conditions

Understanding the link between the protective role of potassium silicate (K2SiO3) against water shortage and the eventual grain yield of maize plants is still limited under semiarid conditions. Therefore, in this study, we provide insights into the underlying metabolic responses, mineral nutrients uptake and some nonenzymatic and enzymatic antioxidants that may differ in maize plants as influenced by the foliar application of K2SiO3 (0, 1 and 2 mM) under three drip irrigation regimes (100, 75 and 50% of water requirements). Our results indicated that, generally, plants were affected by both moderate and severe deficit irrigation levels. Deficit irrigation decreased shoot dry weight, root dry weight, leaf area index (LAI), relative water content (RWC), N, P, K, Ca, Fe, Zn, carotenoids, grain yield and its parameters, while root/shoot ratio, malondialdehyde (MDA), proline, soluble sugars, ascorbic acid, soluble phenols, peroxidase (POD), catalase (CAT), polyphenol oxidase (PPO), and ascorbate peroxidase (APX) were improved. The foliar applications of K2SiO3 relatively alleviated water stress-induced damage. In this respect, the treatment of 2 mM K2SiO3 was more effective than others and could be recommended to mitigate the effect of deficit irrigation on maize plants. Moreover, correlation analysis revealed a close link between yield and the most studied traits.

Maize seeds of white single cross hybrid (Hytech 2030) produced by Misr Hytech Seed Int., Egypt was sown on 17th of May 2018 and 2019, respectively. The experiment was arranged in a split plot design with three replicates. A surface drip irrigation system with three levels (100, 75, and 50% of water requirements) was implemented in the main plots, and the foliar applications of K 2 SiO 3 treatments (0, 1, and 2 mM) were randomly distributed in the subplots. The experimental unit area was 60 m 2 (15 m length × 4 m width) consisting of 5 rows with 0.8 m distance between rows. The plant distance was 30 cm apart on one side. Maize plants were irrigated using drippers of 4 L h −1 capacity and 0.3 m distance between drippers. A flow meter was installed for each irrigation level treatment, and three rows were left without irrigation as a border between different irrigation levels.

Calculations of Water Regimes
Data of class A pan (Epan) for the experimental site expressed in mm/day were obtained from an agrometeorological station located close to the site. Water requirements (Table 3) for different irrigation levels were calculated for 105 days, and then irrigation was stopped for 11 days before the harvesting date (117 days after sowing). The calculation was made according to Doorenbos [28].

Foliar Application and Sampling
Maize plants were subjected to the foliar application of distilled water as a control and K 2 SiO 3 (1 or 2 mM) four times: first at 24 days after sowing (DAS) then the subsequent applications were applied every 15 days. Tween 20 at 0.05 mL L −1 was used as a wetting agent for all foliar treatments (K 2 SiO 3 -treated and control plants). To determine plant growth and physiological and biochemical changes in response to applications, plants samples were collected twice, first after 10 days of the last foliar application. Four plants were randomly collected from the inner rows to determine the vegetative growth (shoots and roots) in each experimental unit. Biochemical analyses were conducted using the 4th fully expanded leaf from the top, which was randomly collected from 3 plants of each experimental plot. In addition, two plants were randomly selected to collect the 4th fully expanded leaf from the top to determine mineral nutrients after drying in an oven at 105 • C. At the end of the experiment (117 DAS), grain yield per plant and its related traits were estimated, while the grain yield per hectare was determined from one inner row that was left for this purpose (12 m 2 /experimental unit).

Vegetative Growth
Shoot and root dry weights were determined by drying four plants from each experimental unit in an air-forced ventilated oven at 105 • C. The dry weight ratio of root/shoot ratio was calculated. Leaf area index (LAI) was calculated as described by Iqbal and Hidayat [29].

Leaf Relative Water Content (RWC)
Leaf relative water content was determined according to Ünyayar et al. [30]. Leaf discs (1.8 cm diameter) from 10 fully expanded young leaves (ear leaf) were taken from 6-8 plants at the mid-canopy position before irrigation. Then the discs were weighed (FW) and placed immediately in distilled water for 2 h at 25 • C then their turgid weights (TW) were recorded. The samples were dried in an oven at 110 • C for 24 h (DW). Relative water content (RWC) was calculated using the following formula: RWC = (FW − DW)/(TW − DW) × 100.

Membranes Lipid Peroxidation
Lipid peroxidation was measured by the determination of malondialdehyde (MDA) as described by Heath and Packer [31]. Frozen leaf tissues were homogenized in 0.1% (w/v) trichloroacetic acid (TCA). The absorbance (A) of the supernatant was measured at 535 nm and corrected for nonspecific turbidity at 600 nm using a spectrophotometer (Chrom Tech CT-2200, Taiwan). The MDA concentration (nmol g −1 FW) was calculated using ∆OD (A532-A600) and the extinction coefficient (ε =155 mM −1 cm −1 ).

Proline and Soluble Sugars
Proline levels were determined using the method of acid-ninhydrin reagent as described by Bates et al. [32]. Soluble sugars were determined by anthrone-sulfuric acid reagent as described by Plummer [33].

Determination of Mineral Nutrients
Dry leaves were ground and digested using sulfuric acid and hydrogen peroxide. Leaf mineral concentrations of N, P, K, Ca, Fe and Zn were determined according to Cottenie et al. [34]. Nitrogen (N) was determined by the Kjeldahl method (Velp Scientifica, Europe). The colorimetric method by UV/VIS spectrophotometer was used to determine P; potassium (K) was determined by a Flamephotometer (Jenway, UK). Meanwhile, Ca, Fe and Zn were determined by atomic absorption spectrophotometry (AAS-Hitachi, Tokyo, Japan).

Determination of Carotenoids, Ascorbic Acid and Total Soluble Phenols
Carotenoids were determined using the acetone and petroleum ether method as described by de Carvalho et al. [35]. Ascorbic acid (AsA) was determined using the 2, 6-Dichloroindophenol titrimetric method according to Association of Official Analytical Chemists (A.O.A.C) [36]. Total soluble phenols were determined according to the method of Folin-Denis as described by Skalindi and Naczk [37].

Quantification of Antioxidant Enzymes
Leaf tissue of maize plants (0.5 g) was homogenized in 4 mL 0.1 M K-phosphate buffer (pH 7.0) containing 1% (w/v) polyvinylpyrrolidone (PVP) and 0.1mM Ethylenediaminetetraacetic acid (EDTA). The homogenate was centrifuged at 10,000 rpm for 15 min and the supernatant was used as a crude enzyme extract. All the preparation steps of the enzyme extract were carried out at 0-4 • C. Total soluble protein was determined according to Bradford [38].

Determination of Yield Parameters
Maize ears were harvested at 117 DAS and averages of ear length, ear diameter, number of grains·ear −1 , weight of grains·ear −1 , weight of grains·plant −1 were estimated from 10 random plants per each experimental unit. Eventually, total grain yield (t ha −1 ) was calculated using the average yield of grains/12 m 2 (one inner row was left for this purpose in each experimental unit).

Statistical Analysis
Data of the two seasons were subjected to combined analysis following the two way ANOVA procedure as described by Snedecor and Cochran [43] using MSTAT-C software (Michigan State University, USA). Duncan's test based on a probability of p ≤ 0.05 was used to determine the significant differences between means.

Changes in Plant Growth
The progressive reduction in the irrigation level significantly (p ≤ 0.05) inhibited plant growth in terms of shoot dry weight, root dry weight and LAI. In contrast, root/shoot ratio was not affected compared to the well-irrigated control ( Figure 1). When plants were treated with K 2 SiO 3 (1 or 2 mM), a significant increase was observed in shoot dry weight and LAI either under nonstressed or stressed conditions. Meanwhile, this trend was just obvious in root dry weight under water shortage conditions. Root/shoot ratio revealed a significant decrease in the K 2 SiO 3 -treated plants under well-irrigated conditions. Generally, the highest concentration of the K 2 SiO 3 treatments (2 mM) was more effective in this respect ( Figure 1). Means were presented ± SD. Different letters are significant differences, according to Duncan's multiple range tests (p < 0.05).
On the other hand, no significant differences were detected in Ca between K2SiO3 nontreated and the treated plants under both deficit irrigation levels (75% and 50%). Meanwhile, Zn was significantly decreased by the treatments of K2SiO3 under the moderate level of irrigation. This effect did not occur under the lower level of irrigation (50%).

Changes in Nonenzymatic Antioxidants
Nonenzymatic antioxidant capacity of plants was investigated by the determination of carotenoids, ascorbic acid and soluble phenols ( Figure 4). Plants that were not applied by K2SiO3 and exposed to continuous deficit irrigation demonstrated a significant (p ≤ 0.05) increase in ascorbic acid and soluble phenols compared to the well-watered conditions, whereas carotenoids did not show any significant differences in this respect. Applied K2SiO3 (1 or 2 mM) significantly enhanced carotenoids and ascorbic acid, while soluble phenols were not changed under well-irrigated conditions. Similarly, K2SiO3 applications, in particular at the highest concentration (2 mM), exhibited the highest significant increases in carotenoids and soluble phenols under both investigated levels of deficit irrigation (75 and 50%). On the other hand, ascorbic acid revealed an opposite trend by the treatment of 2 mM K2SiO3 under the moderate (75%) and lower (50%) levels of irrigation. Means were presented ± SD. Different letters are significant differences, according to Duncan's multiple range tests (p < 0.05).

Changes in RWC, MDA, Proline and Soluble Sugars
Plants that were exposed to deficit irrigation demonstrated a significant (p ≤ 0.05) increase in MDA, proline and soluble sugars, whereas RWC was diminished compared to the well-irrigated conditions ( Figure 2). The foliar applications of K 2 SiO 3 significantly enhanced RWC under both investigated deficit-irrigation levels, while this tendency was conspicuous in soluble sugars under moderate level of deficit irrigation. Conversely, MDA and proline generally exhibited a significant decrease in K 2 SiO 3 -treated plants compared to the untreated ones under stressed conditions. Overall, the treatment of 2 mM K 2 SiO 3 was more efficient than the other treatments.
conditions. Applied-K2SiO3 significantly enhanced the activity of CAT, PPO and APX under the moderate level of irrigation (75%), whereas POD was not affected. When plants were exposed to severe deficit irrigation (50%), POD exhibited a significant increase by the treatment of 1 mM K2SiO3. At the same time, the highest activity of CAT and PPO were obtained by the treatment of 2 mM K2SiO3. On the contrary, APX did not reveal any significant differences between K2SiO3-treated and nontreated plants under the lower level of irrigation.

Changes in Yield Parameters
Grain yield and its parameters, including ear length, ear diameter, number of grains. ear −1 , weight of grains. ear −1 , weight of grains. plant −1 and total grain yield (t ha −1 ) were estimated in this investigation ( Figure 6). Concerning K2SiO3-untreated plants, reducing irrigation level led to significant (p ≤ 0.05), and gradual decreases in all yield parameters studied in parallel with the severity of deficit irrigation. Generally, except for the number of grains. ear −1 under the lower level of irrigation, applied-K2SiO3, specifically at 2 mM, significantly improved all studied traits regardless of the level of irrigation.

Relationships between Grain Yield and RWC, MDA, Osmolytes, Nutrients and Antioxidants
To elucidate the relationships between the grain yield of maize plants as influenced by the foliar applications of K2SiO3 under different irrigation regimes and RWC, MDA, osmolytes, nutrients and antioxidants, the correlation coefficient was analyzed (Figure 7). We observed that grain yield (t ha −1 ) was significantly and positively correlated with leaf relative water content (RWC), carotenoids, N, P, K, Ca and Fe. Meanwhile, MDA, soluble sugars, soluble phenols, POD and PPO demonstrated a negative correlation. On the other hand, proline, ascorbic acid (AsA), CAT, APX and Zn did not reveal any significant correlation in this respect.

Changes in Mineral Nutrients
To evaluate the nutritional status of plants under continuous deficit irrigation and K 2 SiO 3 foliar applications, N, P, K, Ca, Fe and Zn were quantified (Figure 3). The general tendency was that deficit irrigation obviously and significantly (p ≤ 0.05) decreased N, K, Ca and Fe in K 2 SiO 3 nontreated plants under both examined deficit levels of irrigation (75% and 50%). In comparison, P and Zn were only affected under the severe level of deficit irrigation (50%). Applied K 2 SiO 3 , specifically at 2 mM, significantly improved the concentration of N, P, K, Ca and Fe under unstressed conditions. In contrast, a significant reduction in Zn was manifested in K 2 SiO 3 -treated plants under well-irrigated conditions. When plants were subjected to continuous deficit irrigation, the treatment of 2 mM K 2 SiO 3 exhibited the highest significant increases in N, K and Fe under both investigated levels of deficit irrigation. A similar trend was only observed in P under a moderate level of irrigation.
On the other hand, no significant differences were detected in Ca between K 2 SiO 3 nontreated and the treated plants under both deficit irrigation levels (75% and 50%). Meanwhile, Zn was significantly decreased by the treatments of K 2 SiO 3 under the moderate level of irrigation. This effect did not occur under the lower level of irrigation (50%).

Discussion
Under semiarid conditions, deficit irrigation is thought to be one of the most limiting factors that can restrict plant growth and productivity. In this study, soil analysis and climatic data showed that maize was exposed to high solar radiation and air temperatures with no precipitation during the cultivation periods in the two seasons. All of these factors exhibited drought stress on the maize plants during this study. It is well documented that drought stress reduces the growth of many plant species due to the restriction of cell division and differentiation [44]. In this study, reducing irrigation level exhibited significant decreases in shoot dry weight, root dry weight and LAI of the water-stressed plants (Figure 1). These reductions could be attributed to the disruption that occurred in the photosynthetic process through the degradation of pigments, limitation of stomatal conductance and decreasing the photochemical quantum yield [8,45]. On the other hand, root/shoot ratio as dry weight was unaffected under deficit irrigation (Figure 1). These results were in agreement with those obtained by Ma et al. [46], and may imply that phloem transport and leaf carbon exportation were less sensitive to water deficit under the circumstances of this study.

Changes in Nonenzymatic Antioxidants
Nonenzymatic antioxidant capacity of plants was investigated by the determination of carotenoids, ascorbic acid and soluble phenols (Figure 4). Plants that were not applied by K 2 SiO 3 and exposed to continuous deficit irrigation demonstrated a significant (p ≤ 0.05) increase in ascorbic acid and soluble phenols compared to the well-watered conditions, whereas carotenoids did not show any significant differences in this respect. Applied K 2 SiO 3 (1 or 2 mM) significantly enhanced carotenoids and ascorbic acid, while soluble phenols were not changed under well-irrigated conditions. Similarly, K 2 SiO 3 applications, in particular at the highest concentration (2 mM), exhibited the highest significant increases in carotenoids and soluble phenols under both investigated levels of deficit irrigation (75 and 50%). On the other hand, ascorbic acid revealed an opposite trend by the treatment of 2 mM K 2 SiO 3 under the moderate (75%) and lower (50%) levels of irrigation.  The positive effect of K2SiO3 on the shoot, root dry weight and LAI in the water-deficit stressed plants could be attributed to the synergistic effect of both K and Si on photosynthesis and production of assimilates [20,47]. In the present study, deficit irrigation negatively affected RWC while the K2SiO3 applications significantly mitigated this effect (Figure 2). Applied K positively affected leaf water content under stress conditions by maintenance of turgor potential and enhancing the integrity of cell membranes [48]. Additionally, Si could improve RWC by decreasing the rate of transpiration [49]. Lipid peroxidation is considered a pervasive biochemical response to stress in plant species due to the uncontrolled release of ROS [8]. Applied K and/or Si can promote the antioxidant capacity of the stressed plants [20,21]. This response may explain the significant decrease of MDA in the treated plants with K2SiO3 under water-stressed conditions ( Figure 2). Proline is considered a compatible osmolyte and one of the most contributing factors that maintain intracellular redox homeostasis under stress conditions [50]. Moreover, under drought stress, proline has a crucial role in protecting the integrity of cell membranes and osmotic adjustments that allow the plant to uptake water [50,51]. Soluble sugars are the second compatible osmolytes that were determined in this investigation. The accumulation of soluble sugars during water deficit irrigation could be due to the up-regulation of genes involved in the starch-sucrose pathway [52,53]. All of the above-mentioned responses may explain the dramatic accumulation of proline and soluble

Changes in Antioxidant Enzymes
The activities of antioxidant enzymes (POD, CAT, PPO, and APX) were determined in this study under deficit irrigation conditions and the exogenous application of K 2 SiO 3 ( Figure 5). No significant differences were observed between K 2 SiO 3 -treated, and nontreated plants in the activity of all studied antioxidant enzymes under well-irrigated conditions. Reducing irrigation levels significantly (p ≤ 0.05) increased the activity of these enzymes compared to the well-irrigated conditions. Applied-K 2 SiO 3 significantly enhanced the activity of CAT, PPO and APX under the moderate level of irrigation (75%), whereas POD was not affected. When plants were exposed to severe deficit irrigation (50%), POD exhibited a significant increase by the treatment of 1 mM K 2 SiO 3 . At the same time, the highest activity of CAT and PPO were obtained by the treatment of 2 mM K 2 SiO 3 . On the contrary, APX did not reveal any significant differences between K 2 SiO 3 -treated and nontreated plants under the lower level of irrigation.
Agronomy 2020, 10, x FOR PEER REVIEW 14 of 21 significance of K and/or Si in the protection of cell membranes and maintenance of RWC under deficit irrigation conditions (Figure 2).

Changes in Yield Parameters
Grain yield and its parameters, including ear length, ear diameter, number of grains·ear −1 , weight of grains·ear −1 , weight of grains·plant −1 and total grain yield (t ha −1 ) were estimated in this investigation ( Figure 6). Concerning K 2 SiO 3 -untreated plants, reducing irrigation level led to significant (p ≤ 0.05), and gradual decreases in all yield parameters studied in parallel with the severity of deficit irrigation. Generally, except for the number of grains·ear −1 under the lower level of irrigation, applied-K 2 SiO 3 , specifically at 2 mM, significantly improved all studied traits regardless of the level of irrigation.

Relationships between Grain Yield and RWC, MDA, Osmolytes, Nutrients and Antioxidants
To elucidate the relationships between the grain yield of maize plants as influenced by the foliar applications of K 2 SiO 3 under different irrigation regimes and RWC, MDA, osmolytes, nutrients and antioxidants, the correlation coefficient was analyzed (Figure 7). We observed that grain yield (t ha −1 ) was significantly and positively correlated with leaf relative water content (RWC), carotenoids, N, P, K, Ca and Fe. Meanwhile, MDA, soluble sugars, soluble phenols, POD and PPO demonstrated a negative correlation. On the other hand, proline, ascorbic acid (AsA), CAT, APX and Zn did not reveal any significant correlation in this respect.
leaves of maize plants at 80 DAS as influenced by the foliar application of K2SiO3 (0, 1 and 2 mM) under three irrigation regimes: 100% (white), 75% (green) and 50% (orange) of water requirements. CK: well-watered control, KSi 0: K2SiO3-untreated plants, KSi 1: K2SiO3 (1 mM) and KSi 2: K2SiO3 (2 mM). Data of the two seasons of 2018 and 2019 were subjected to combined analysis. Means were presented ± SD. Different letters are significant differences, according to Duncan's multiple range tests (p < 0.05).   Drought stress strongly affects the uptake of nutrients and it can restrict the translocation of some nutrients acropetally between plant organs [56]. Furthermore, it negatively affects active transport, permeability, and leaf transpiration [25,57]. In our study, plants exposed to moderate or severe stress exhibited a significant decline in N uptake ( Figure 3A). This could be due to decreases

Discussion
Under semiarid conditions, deficit irrigation is thought to be one of the most limiting factors that can restrict plant growth and productivity. In this study, soil analysis and climatic data showed that maize was exposed to high solar radiation and air temperatures with no precipitation during the cultivation periods in the two seasons. All of these factors exhibited drought stress on the maize plants during this study. It is well documented that drought stress reduces the growth of many plant species due to the restriction of cell division and differentiation [44]. In this study, reducing irrigation level exhibited significant decreases in shoot dry weight, root dry weight and LAI of the water-stressed plants (Figure 1). These reductions could be attributed to the disruption that occurred in the photosynthetic process through the degradation of pigments, limitation of stomatal conductance and decreasing the photochemical quantum yield [8,45]. On the other hand, root/shoot ratio as dry weight was unaffected under deficit irrigation (Figure 1). These results were in agreement with those obtained by Ma et al. [46], and may imply that phloem transport and leaf carbon exportation were less sensitive to water deficit under the circumstances of this study.
The positive effect of K 2 SiO 3 on the shoot, root dry weight and LAI in the water-deficit stressed plants could be attributed to the synergistic effect of both K and Si on photosynthesis and production of assimilates [20,47]. In the present study, deficit irrigation negatively affected RWC while the K 2 SiO 3 applications significantly mitigated this effect ( Figure 2). Applied K positively affected leaf water content under stress conditions by maintenance of turgor potential and enhancing the integrity of cell membranes [48]. Additionally, Si could improve RWC by decreasing the rate of transpiration [49]. Lipid peroxidation is considered a pervasive biochemical response to stress in plant species due to the uncontrolled release of ROS [8]. Applied K and/or Si can promote the antioxidant capacity of the stressed plants [20,21]. This response may explain the significant decrease of MDA in the treated plants with K 2 SiO 3 under water-stressed conditions ( Figure 2). Proline is considered a compatible osmolyte and one of the most contributing factors that maintain intracellular redox homeostasis under stress conditions [50]. Moreover, under drought stress, proline has a crucial role in protecting the integrity of cell membranes and osmotic adjustments that allow the plant to uptake water [50,51]. Soluble sugars are the second compatible osmolytes that were determined in this investigation. The accumulation of soluble sugars during water deficit irrigation could be due to the up-regulation of genes involved in the starch-sucrose pathway [52,53]. All of the above-mentioned responses may explain the dramatic accumulation of proline and soluble sugars in the water-stressed plants under the circumstances of this study ( Figure 2). The exogenous application of K 2 SiO 3 resulted in a notable decrease in proline and a visible increase in soluble sugars. These effects indicate that K and/or Si may enhance the osmotic potential of leaves by stimulating the conversion of starch into soluble sugars, particularly up to the moderate level of irrigation [54,55]. Furthermore, the decrease of proline in the K 2 SiO 3 -treated plants may highlight the significance of K and/or Si in the protection of cell membranes and maintenance of RWC under deficit irrigation conditions (Figure 2).
Drought stress strongly affects the uptake of nutrients and it can restrict the translocation of some nutrients acropetally between plant organs [56]. Furthermore, it negatively affects active transport, permeability, and leaf transpiration [25,57]. In our study, plants exposed to moderate or severe stress exhibited a significant decline in N uptake ( Figure 3A). This could be due to decreases in the activity of the N-uptake proteins (NRT1, NRT2) for inorganic nitrate (NO 3 − ) and (AMT1) or ammonium (NH 4 + ) [58]. Additionally, the availability of N could be reduced under the inadequate water supply [59]. The foliar application of K 2 SiO 3 improved N-uptake under stressed and normal conditions. Applied-K can ameliorate the deleterious effects of drought through the regulation of stomatal movement, increasing root cell elongation, osmotic adjustment and detoxification of ROS [48]. Furthermore, silicon improves photosynthesis, antioxidant activities, and absorption of mineral nutrients of many crops [21,47]. These effects could explain the positive influence of K 2 SiO 3 on N-uptake in our study. Concerning phosphorus (P), it was decreased under severe level of deficit irrigation (50%) ( Figure 3B). This decrease may be attributed to reducing the concentration and/or activity of the P-uptake protein (PHT1) [58]. Moreover, under drought stress, P may be quickly converted into an immobile or insoluble form [60]. On the other hand, the increase in P-uptake prior to K 2 SiO 3 application was significantly under severe and moderate stress. These effects imply that P-uptake in maize is highly dependent on the intensity of drought stress. Similarly, water stress markedly exhibited K deficiency compared to all K 2 SiO 3 untreated plants ( Figure 3C). This effect could be due to reduction in absorption by the roots and transpiration rate, which consequently reduced water and nutrient transport via xylem [60]. Applied-K 2 SiO 3 significantly increases K content compared to the untreated plants. These results are in agreement with Jiang et al. [61], who found that the application of K can significantly increase its concentration in the different parts of maize plants such as grains and straw. In this study, Ca uptake was also inhibited by reducing water supply ( Figure 3D). This impact was clear under the lower level of stress. Furthermore, the foliar application of K 2 SiO 3 had no significant effect on Ca uptake under both examined treatments. Maize plants could be severely affected by Ca deficiency under drought condition because Ca is relatively an immobile nutrient and its uptake may require sufficient water supply [62]. Deficit irrigation manifestly suppressed the uptake of Fe in K 2 SiO 3 -untreated plants ( Figure 3E). In contrast, applied K 2 SiO 3, specifically at the highest concentration (2 mM), improved Fe-uptake under both treatments. Silicon (Si) can mitigate the symptoms of Fe deficiency in different plant species including soybean, cucumber and rice [63,64]. It could play a crucial role in Fe uptake and its translocation from roots to the aerial parts of the plant [65,66]. This impact could be attributed to the fact that applied-Si can enhance citrate concentration, which acts as an Fe chelator and facilitates its movement through the xylem [67]. The translocation of Zn from roots to leaves may be inhibited by Si application. This effect may be due to the fact that Si precipitates with Zn as zinc silicate around the root epidermis [68], which may reduce Zn translocation via xylem [69].
Under drought stress, plants develop a wide array of complex antioxidant systems that integrated with each other simultaneously to reduce the accumulation of ROS and oxidative damages [70]. The foliar application of K 2 SiO 3 induced dramatic improvement in the concentration of carotenoids under different investigated levels of irrigation. The increase of carotenoids under water stress due to K or Si supplementation could foster the antioxidant capacity of plants under deficit irrigation [71][72][73][74]. Under stress conditions, ascorbate (ASA) could be increased through the overexpression of its synthesis related-genes such as GMP, GME, GalUR, DHAR, and MDHAR [75]. In this study, AsA was substantially increased by reducing the irrigation level ( Figure 4B). This response could help in scavenging ROS and inducing the ascorbate-glutathione cycle [8,76]. Phenolic compounds could also be involved in plant tolerance to drought stress and play a significant role as a sink for carbon under stress conditions [77,78]. These effects could explain the improvement in total soluble phenols by reducing the irrigation level in this study ( Figure 4C). The increase in total soluble phenols by the treatments of K 2 SiO 3 could be due to the effect of Si, which may induce several changes in the phenolic compounds under abiotic and biotic stresses [79,80].
In the present study, our results showed that deficit irrigation increased the activities of POD, CAT, PPO and APX in the leaves of maize plants ( Figure 5). These findings could reflect the integrated regulation between these enzymes in the tolerance of maize plants to water stress. Exogenous applications of K 2 SiO 3 induced a synergistic effect leading to an increase in the activities of all studied antioxidant enzymes under deficit irrigation levels. Previous reports showed that K and/or Si could enhance the antioxidant capacity of plants under stress conditions [20,21]. In this study, these effects were confirmed by the enhancement of RWC and reduction of MDA.
It is well documented that water stress has several deleterious influences on the productivity of maize plants [17,81]. It can affect different metabolic pathways, photosynthesis and translocation of many metabolites required for grain filling [81]. Furthermore, water stress can increase the potential for unsuccessful pollination and poor kernel setting of maize by affecting the anthesis and silking stages [82]. In this study, reducing irrigation levels reduced the yield of grains ( Figure 6). Applications of K 2 SiO 3 not only relatively reversed these adverse effects but also increased the ultimate yield of grains under water stress. These findings could be correlated with corresponding changes in several biochemical and physiological aspects that were found during this work (Figure 7).

Conclusions
In this study, it was found that applied K 2 SiO 3, particularly at 2 mM as a foliar spray, may have several benefits on maize crops under limited irrigation supply. These effects were associated with several changes at physiological and biochemical levels, including adjustment of RWC and osmolytes, alleviation of oxidative damage and reduction of cell membrane dysfunction, as well as enhancement of nutrient uptake of and regulation of several nonenzymatic and enzymatic antioxidant systems. These results could provide a link between the protective role of K 2 SiO 3 against drought stress and the eventual yield of grains, especially under semiarid conditions.