1. Introduction
One of the greatest challenges to the sustainability of irrigated agriculture and food production in arid and semiarid regions of the world is the scarcity of freshwater. This challenge will become more pressing in these regions as the population continues to increase and the impact of climate change intensifies; in particular, current climate changes in these regions are associated with increments in mean and utmost temperatures and low precipitation simultaneously. Presently, global croplands consume 8266 km
3 year
−1 of water: 5406 km
3 from green water and 2860 km
3 from blue water [
1]. The global irrigated croplands cover 23% of the global cropland areas, consume 1083 km
3 year
−1 of blue water resources, and provide approximately 40% of global food production [
1,
2]. Rosa et al. [
1] reported that, with 20% to 50% irrigation deficit scenarios in currently irrigated land, it is possible to irrigate an additional 10% of global croplands and increase food production to feed an additional 800 million people. Therefore, applying deficit irrigation practices instead of full irrigation could be one reasonable solution for handling food crises caused by the freshwater shortage [
3,
4]. However, the exposure of plants to water-deficit stress, especially during critical growth stages, can result in reductions in dry matter production and final grain yields of more than 50% for the majority of cultivated crops [
5,
6,
7,
8]. This is because water-deficit stress causes a broad spectrum of adverse impacts on several physiological, morphological, and biochemical attributes that eventually impair the growth and development of plants, with significant decreases in their production. Exposure of plants to water-deficit stress quite often leads to substantial reductions in cell enlargement rate, biomass accumulation, leaf area, different yield components, photosynthesis rate, stomatal conductance, chlorophyll content (Chl), and relative water content (RWC) [
8,
9,
10,
11,
12]. It also causes an increase in leaf temperature and the generation of reactive oxygen species (ROS) [
13] and leads to the senescence of leaves due to the breakdown of chlorophyll [
13,
14]. In addition, it leads to an imbalance in several plant hormones and growth regulators [
15,
16]. Therefore, applying irrigation water below the full crop water requirements necessitates further complementary approaches to mitigate the harmful outcomes of water deficit on plant growth and production.
Recently, a number of different agronomic and physiological practices have been applied as complementary approaches in order to reduce water-deficit-induced crop losses. Fortunately, plants incubate several complex and well-organized mechanisms to mitigate the deleterious effects of different abiotic stressors. The ability of plants to biosynthesize and accumulate various compatible osmolytes is considered one of the most common of these mechanisms. Plants usually accumulate different compatible osmolytes under stressful conditions in order to maintain cell turgor, maintain continuous water uptake at low soil water potential, protect cellular machinery from stresses, remove excess levels of ROS, enhance the activities of antioxidant enzymes, and protect proteins and biological membranes [
17,
18].
Salicylic acid (SA), as an example of these osmolytes, is present in most plants; however, concentrations differ significantly between species [
19]. For example, although the concentration of SA in tobacco leaves is less than 100 ng g
−1 fresh weight, this concentration can reach 10 mg g
−1 fresh weight in potato leaves [
19]. Additionally, not all plants have the capacity to accumulate SA at a concentration level sufficient to contribute significantly to protecting themselves from the deleterious effects of water-deficit stress. Therefore, previous studies have suggested that external application of SA, either through seed and/or foliar treatments, is one of the most widely applied methods for elevating SA concentration to a sufficient level in plants that are unable to synthesize it under water-deficit stress [
17,
18,
19]. Thus, the exogenous application of SA could be considered an easy and cost-effective approach for alleviating the harmful effects of water-deficit stress on the growth and production of plants.
After this initial characterization, several reports have elucidated the role of SA as a phytohormone and its vital contribution and multifaceted role in plants in enhancing their performance under abiotic stress conditions. There is much evidence that the exogenous application of SA at an appropriate concentration affects multiple aspects related to plant growth, development, and production under stress as well as normal conditions. Exogenous application of SA has also been found to enhance different physiological processes, such as photosynthetic activity, stomatal regulation, nutrient uptake and transport, Chl and protein synthesis, RWC, leaf water potential, and antioxidant capacity [
7,
18,
20,
21,
22,
23]. Plants treated with SA also showed a decrease in leaf senescence [
13,
14,
24]. For example, Ilyas et al. [
12] reported that seeds treated with 10 mM SA under water-deficit stress improved germination percentage by 21% and increased shoot length and leaf water potential by 20% and 47%, respectively, as compared with non-treated seeds. Kareem et al. [
25] investigated the effect of spraying 1.44 and 2.88 mM of SA on wheat under water-deficit stress. They reported that SA enhanced growth, yield, and physiological traits, such as plant height, spike length, number of grains per spike, thousand-grain weight (TGW), Chl content, and RWC, with a concentration of 1.44 mM being shown to have the most positive impact compared with 2.88 mM. Azmat et al. [
7] reported that wheat plants treated with 1.0 mM SA as a foliar application significantly increased Chl a, Chl b, and RWC by 125%, 167%, and 238% under drought stress, respectively, when compared with untreated plants. Hafez et al. [
23] found that synergistic use of biochar and SA significantly improved several physico-agronomic traits, such as Chl content, RWC, photosynthetic rate, stomatal conductance, nutrient uptake (N, P, and K), number of grains spike
−1, TGW, and GY, when compared with control treatments under water-deficit conditions.
Although the beneficial effects of SA on stress tolerance are relatively well known, doubts remain about the application method (foliar application or seed soaking) and the appropriate concentration of SA that would provide the best results against water-deficit stress. For instance, Korkmaz et al. [
26] found that both seed soaking and foliar application of SA within the range of 0.1–1 mM provided similar means of protection for muskmelon plants against drought stress. However, seed soaking with up to 0.5 mM SA had a positive effect on all measured traits compared with foliar application with different concentrations or seed soaking with concentrations below 0.5 mM SA. Seed soaking with 100 ppm resulted in significantly higher wheat growth, yield components, and GY in wheat under conserved soil moisture conditions than a foliar application with the same concentration in the study of Mevada et al. [
27]. Otherwise, Farooq et al. [
28] demonstrated that foliar application of SA was more efficient than seed soaking at the same concentrations for enhancing photosynthesis and plant growth in rice against water deficiency. Additionally, they also found that foliar application with 100 mg L
−1 SA was the best treatment to improve the performance of the rice plants under normal and stress conditions compared with 50 and 150 mg L
−1 or the control treatment. Besides the application methods of SA, the concentration can also influence the ability of SA to mitigate the negative impacts of abiotic stresses on the growth and production of plants, as several plant functions can be inhibited or induced with high and low SA concentrations, respectively. Several studies reported that the application of SA at a relatively low concentration was more effective than a higher concentration in enhancing the growth, physiological, and productivity parameters of different field crops under various abiotic stresses. For example, Kang et al. [
29] found that pretreatment seeds of wheat with 0.5 mM SA for 3 days significantly increased the height, fresh weight, and dry wheat of seedlings under drought stress by 10.7%, 15.4%, and 10.4%, respectively, as compared with the control treatment. However, these three traits in the pretreatment seeds with 3.0 mM SA were significantly lower than in the control (by 38.9%, 51.4%, and 36.2%, respectively). In another instance, 1.0 mM SA inhibited the growth of mung bean (
Vigna radiata L.) under salinity stress; however, promoted photosynthesis and growth was evidenced with 0.1 and 0.5 mM SA [
30]. In contrast, Sohag et al. [
24] found that a concentration of 1.0 mmol L
−1 SA equally improved the water-stress tolerance of rice seedlings, as did 0.5 mmol L
−1. Soaking seeds of wheat with 10 mM SA has the potential to enhance the growth of plants under drought stress [
12]. However, in the study of Parveen et al. [
31], it was reported that the resistance of wheat plants to water-deficit stress improved when the plants were sprayed with relatively high concentrations of SA (3 and 6 mM SA). Moreover, 0.6 mM, 0.01–0.05 mM, and 0.1–0.5 mM were found to be appropriate concentrations for enhancing the growth, production, and antioxidant defense mechanisms of soybean, wheat, and bean crops, respectively, under deficit irrigation when their seeds were treated with these concentrations [
32,
33,
34].
Therefore, we hypothesized that the method of application, concentration, level of stress, and crop type are the main factors that determine the role of SA in enhancing the growth and yield characteristics of wheat, particularly under field conditions. Therefore, the principal objective of this study was to identify the proper application methods for SA along with an effective concentration to enhance the vegetative growth and yield characteristics and water use efficiency of wheat under FL and LM irrigation regimes in an arid agro-ecosystem.
3. Discussion
Generally, exposure of wheat plants to water-deficit stress causes drastic reductions in their growth attributes and productivity, which may exceed 50–70%, especially if this water deficit coincides with critical growth stages [
4,
35,
36]. In this study, which was conducted under arid conditions, the LM regime consistently resulted in lower growth and production of wheat compared to the FL regime, while the opposite was true for IWUE (
Table 2 and
Table 5).
These significant reductions in different parameters under the LM regime may be due to the fact that water-deficit stress triggers a broad spectrum of adverse impacts on several physiological, morphological, and biochemical attributes that eventually impair the growth and development of plants, with significant decreases in their production. Water-deficit stress quite often leads to substantial inhibition of several morpho-physiological attributes of plants. It leads to the inhibition of cell division, cell expansion, gas-exchange rates, stomatal conductance, biomass accumulation, and leaf area. It also causes an imbalance in several plant hormones and growth regulators; increases leaf temperature, which reduces RWC and increases transpiration rates; reduces the uptake and translocation of macronutrients; and induces oxidative damage, which may affect leaf water status and leaf pigments [
8,
9,
10,
11,
12,
15,
16,
35,
37,
38]. Consequently, decrease in wheat growth, yield, and yield components is the LM regime’s expected outcome. Therefore, it was difficult to apply the LM regime to wheat without an accompanying reduction in growth and production. Thus, wheat production under the LM regime requires further strategies to decrease the deleterious effects of the shortage of water on wheat growth and production.
Currently, exogenous application of phytohormones such as SA through seed soaking and/or foliar application has been considered an efficient, easy, and economic strategy for ameliorating the harmful effects of water-deficit stress on the growth and production of field crops. This may be because the exogenous application of SA plays an important role in modulating and inducing several biochemical and physiological mechanisms to control different plant responses under stress as well as normal conditions [
19,
31,
39,
40,
41,
42]. In this study, treatments with SA through seed soaking (S
1 and S
2), foliar application (F
1, F
2, and F
3), and a combination of both methods (S
1F
1–S
2F
3) exhibited significant increases in all studied parameters in comparison with the untreated condition (S
0) (
Table 3 and
Table 5). These results indicate that the exogenous application of SA could represent an alternative and technically simple eco-friendly approach to stabilize the growth and production of plants under either stress or normal conditions. These results are in agreement with previous studies showing that the exogenous application of SA plays an important role in enhancing the growth, chlorophyll content, RWC, GY, yield components, and water use efficiency of different field crops under normal and stress conditions [
16,
31,
34,
43,
44,
45]. These findings may be attributed to SA’s being a phenol-based phytohormone that is involved in a wide range of developmental, biochemical, and physiological processes under different growth conditions. It plays a vital role in enhancing cell division, cell elongation, photosynthetic activity, stomatal opening, chlorophyll pigment contents, nutrient uptake, biomass accumulation, and RWC. It also plays an integrating role in delaying the senescence of plant organs and regulating the source-to-sink relationship, as well as in plant growth and development pathways and some physiological responses related to carbon uptake and/or fixation in the chloroplasts and Rubisco concentration and activity [
20,
39,
41,
42,
45]. These abovementioned advantages of SA might explain why the plants treated with SA, in general, exhibited greater enhancements in growth, physiological attributes, yield components, yield, and IWUE. The ability of SA to delay the senescence of plant organs and avoid the premature loss of anthesis and grains leads to increments in yield-related traits in wheat [
31]. The ability of SA to increase the availability of CO
2 for photosynthesis, as well as increase leaf diffusive resistance and decrease transpiration rates by regulating stomatal opening and closing, leads to improved IWUE [
46,
47]. The ability of SA to enhance the flow of metabolites to developing grains leads to improved TGW and thus improved GY [
8]. Additionally, because SA is involved in the formation of antioxidants, stimulating osmotic adjustment, and scavenging ROS, these mechanisms lead to the protection of membrane integrity and photosynthetic pigments, which ultimately lead to enhanced growth characteristics of plants [
20,
48,
49]. These positive effects of SA may provide a rationale for the importance of exogenous application of SA in improving the growth-, physiological-, and yield-related parameters considered in this study, given the positive effects of the SA treatments observed in comparison with the S
0 treatment. Additionally, the enhancement of growth and physiological attributes by the application of SA was always linked to the high production and IWUE of the crop, which was confirmed in this study through significant and positive correlations of growth and physiological parameters with yield parameters and IWUE under the LM regime and largely under the FL regime (
Table 6). Additionally, the strong correlations of RWC and Chlt with the different parameters of growth and yield as well as IWUE under the LM regime (
Table 6) indicated the involvement of SA in the protection of photosynthetic pigments by reducing oxidative stress and improved leaf RWC via the lowering of leaf water potential and the accumulation of compatible solutes, which allows additional water to be taken up from the soil, especially under water-deficit conditions.
It appears from previous studies that the efficiency of the exogenous application of SA for enhancement of the growth and production of crops and the alleviation of the negative effects of abiotic stresses depended on several aspects, including plant species, concentrations, application methods, application time, stress duration, and also the physiological state of the plant. Among these aspects, the identification of the proper concentrations and the appropriate application methods for SA are the two factors that should be investigated to avoid unanticipated results from the exogenous application of SA. In this study, in general, foliar application of SA was more effective than seed soaking for improving growth-, physiological-, and yield-related parameters under the FL and LM regime conditions (
Figure 1,
Figure 2 and
Figure 3). Additionally, under both irrigation regimes, the treatments consisting of the combination of foliar application and seed soaking with 0.50 mM (S
1F
1, S
1F
2, and S
1F
3) were more effective than those combinations of foliar application and seed soaking with 1.00 mM (S
2F
1, S
2F
2, and S
2F
3) in enhancing growth-, physiological-, and yield-related parameters (
Figure 1,
Figure 2 and
Figure 3). These results indicate that before treating plants with SA as an alternative strategy to enhance wheat performance under either normal or stress conditions, application methods and concentrations of SA should be taken into account to achieve the maximum results from the exogenous application of SA. Unfortunately, there is still debate in previous studies about the appropriate application methods and concentrations of SA for attaining better results from the exogenous application of this plant hormone. For instance, Farooq et al. [
28] reported that foliar application of SA was more efficient than seed soaking and that the moderate concentration (100 mg L
−1) was also more efficient than the low (50 mg L
−1) and high (150 mg L
−1) concentrations for enhancing the performance of rice under normal and water-deficit stress conditions. Additionally, Aires et al. [
50] confirmed that foliar application of SA is an efficient technique capable of mitigating the negative impacts of water-deficit stress on the production and photosynthetic efficiency of tomato crops. In contrast, the exogenous application of SA was more efficient in improving the growth and production of crops when applied by seed soaking than foliar application at the same concentration [
27,
51,
52]. However, other investigations found that seed soaking and foliar application did not show any significant differences with respect to enhancing plant growth and reducing the negative impacts of water deficit and that low concentrations were more effective than high concentrations [
26,
53]. The findings of these investigations and our results once again confirm that environmental conditions, plant species, and stress levels might be the main reasons why the application methods and concentrations of SA differ from one study to another.
Similarly, the PCA and heatmap, which provide an overview of the responses of plant parameters to different SA treatments, showed that the following treatments: F
1, F
2, F
3, S
1F
1, S
1F
2, and S
1F
3 were grouped together and situated along the quarters with the highest PC1 under both irrigation regimes (
Figure 4). Additionally, these treatments were clustered into one group under both irrigation regimes and displayed the highest values for all parameters (
Figure 5). All of these findings confirm that the foliar application of SA at concentrations of 1–3 mM alone or in combination with seed soaking at a concentration of 0.5 mM seems to be an effective practice for achieving the optimal performance of wheat under arid conditions. These results also confirm that SA application methods are essential factors regarding the effectiveness of SA treatment under control and/or stress conditions. Thus, the foliar application of SA can be suggested as the best method for treating wheat with SA in order to sustain wheat production under similar agro-ecosystem conditions. Similarly, there are some studies that have reported that foliar application of SA is the most effective technique capable of mitigating the negative impacts of environmental stress on the growth and production of field crops [
28,
50,
54]. Since the majority of physiological and biochemical processes and reactions in plants occur in leaves, this may explain why the foliar application of SA seems to be an effective practice for enhancing the growth and production of wheat. Additionally, foliar application of SA may enable adequate accumulations of SA in leaves, helping plants to synthesize/mobilize defense effectors and ensure the quick relief of physiological stress, particularly under stress conditions. The efficiency of the combination of foliar application with seed soaking in this study may indicate that seed soaking, which may induce modification before stress, and foliar application, which may increase the accumulation of SA in leaves during stress, could be required to ensure the growth and production of wheat under stress conditions. Therefore, the application of SA before and during water-deficit stress exposure may also play an important role in mitigating the negative impacts of this kind of stress on the growth and production of wheat.
Interestingly, the heatmap also revealed that the S
0 treatment was clustered into one group under the LM regime (
Figure 5). This indicates that the exogenous application of SA, regardless of the application method and concentration, plays an efficient role in mitigating the negative impacts of water-deficit stress and improving the growth, production, and IWUE of wheat when grown in regions with low water availability.