1. Introduction
Soil salinization is one of the major abiotic stresses encountered in agricultural environments. Its management can be achieved through the leaching and planting of salt-tolerant crops. Crop production in saline soils is inhibited by specific ions, as well as osmotic and toxic effects. Managing saline soil requires sustainable practices that maintain and improve the physicochemical properties of the soil, while attaining optimum production [
1]. The major source of water for crop production in drylands is rainfall, which is often limited and irregular, and results in short-term crop productivity rather than long-term soil resource sustainability [
2]. In semi-arid and arid regions, it is important to continuously monitor the electrical conductivity of the saturated soil extract (EC
se) in the rhizosphere to aid in the management of soil salinization for better crop production. It was previously reported that some drylands had high salt concentrations that rendered them inappropriate for agricultural purposes [
3].
It was speculated that the world’s population may increase from 6 to approximately 10 billion by 2050, and to meet the food demand of this growing populace, a larger increase in food production is inevitable [
4]. The advantages of amending sandy saline soils with organic inputs are generally short-lived due to its rapid decomposition under intense temperature and aeration, thereby causing a frequent yearly application to maintain soil productivity. Furthermore, improving sandy saline soils with inorganic fertilizers could often be unaffordable to peasant farmers [
1,
5]. A more stable compound such as 24-epibrassinolide (BR) could be used as an alternative to mitigate the deleterious effects of salinity on plant growth. Although considered a plant hormone that could regulate an array of physiological and developmental processes in plants under abiotic soil stressors [
6,
7,
8,
9,
10], it was also found to modulate the biosynthesis of plants’ primary metabolites, such as photosynthetic pigments, monosaccharides and protein, which are secreted as exudates through plant roots to the rhizosphere [
11]. Exudates secreted from plant organs, including the roots, are involved in diverse interactions within the rhizospheric soil, such as chelating toxic compounds from the soil and changing the soil pH to help plants cope under stressed soil conditions [
12].
In our previous report [
10], we showed that BR caused significant increases in dry matter accumulation, protein content, photosynthetic activities, nutrient uptake and partitioning, as well as other physiological attributes and seed yield of soybean under salinity stress. However, there is a paucity of information on the influence of BR-treated plants on the rhizospheric electrical conductivity of the saturated soil extract (EC
se). Here, we hypothesized that BR could mediate the composition of root exudates to the advantage of plants growing in such stressed soils. This study aimed at evaluating the effects of the foliar application of BR at different growth stages of soybean, under varying salt concentrations on the rhizosphere EC
se.
4. Discussion
The monitoring of the in situ electrical conductivity of saturated soil extracts (EC
ses) of rhizospheric soils is an important management tool aimed at determining the extent and severity of salinization processes. Overtime, the extent of the salinization of croplands may increase, resulting in accelerated land degradation and desertification, decreased farm output and, consequently, jeopardizing the environment and food security, particularly in drylands [
14]. Such soils lack distinct horizons, an optimal condition for agricultural soils and are generally low in organic matter/carbon, natural fertility and water-holding capacity [
15].
Increases in salinity levels significantly elevated the EC
se in the soil rhizosphere of soybean plants and peaked at the highest concentration. This was possibly a result of excessive accumulations of Na
+ and Cl
− due to high water evaporation; such a concentration of salts could possibly induce ion toxicity [
16] in soils. Excessive Na
+ in the soil would decrease the uptake of NH
4+ and other base cations (K
+, Ca
2+ and Mg
2+) from the soil’s exchange complex. Although, Cl
− is not readily adsorbed, it does move with soil–water and is easily taken up by plants through enzymes in the root cell membrane, thereby reducing the absorption of nitrates from the soil [
17]. It was estimated that by the year 2050, more than 50% of the arable land across the globe would have been salinized [
18]. However, various agronomic practices, including exogenously applied growth regulators, can mitigate the adverse effects of different abiotic stresses such as drought, heavy metals as well as salt stress [
10,
19,
20].
24-epibrassnolide (BR), an active by-product from brassinolide biosynthesis is one of the stress ameliorative approaches adopted to improve and stimulate different plant metabolic processes. It is known to improve the efficiency of transpiration and accelerates metabolic processes that favor the accumulation and release of carbohydrates in the form of gluconic acids, amino acids and organic acids [
21] in plant organs (leaves, shoots or roots) as exudates to their surrounding media. These compounds are involved in numerous interactions within the rhizosphere, and could contribute to the decline in EC
se and salts, including Na
+ and Cl
− [
22].
In this study, the separation of salt extract on the PCA based on BR applications demonstrated its effectiveness in reducing the EC
se of saline soils. It further proved to be sufficient in ameliorating the detrimental effects of salinity on soybean when applied at maximum frequencies of BR
4 and/or BR
5 growth stages. The sampling accuracy indicated by the clustering of similar data points on the PCA further confirmed these findings. The effects could have occurred through changes in salt concentrations in the rhizospheric zones due to exudates from plant roots. Exudates are primary and secondary metabolites (carbohydrates, amino acids, organic acids, flavonoids, glucosinolates, auxins, etc.) secreted by different plant organs, including roots, to their surrounding environment [
23]. These metabolites have been identified and quantified in different plant species, including soybean or the common bean [
24,
25]. The exogenous application of BR was recently reported to boost the yield and dry matter (DM) production in soybean and rice under salinity or high-temperature stress [
10,
26]. The increased DM production could enhance the secretion of exudates from plant roots to complex with salt ions and significantly reduce the EC
se in the rhizospheric soil. Root exudates (flavonoids) obtained from soybean and the common bean whose concentrations increased when plants were under salt stress [
27,
28] could mitigate both osmotic and ion toxicity in the rhizosphere. Similarly, the excised roots of almond trees cultured under in vitro conditions exuded proline in larger quantities when under salt stress [
29].
There was a substantial increase in EC
se across the weeks after the saline water (WAST) treatment. In saline drylands, where high evaporation rates tend to concentrate the water solution, natural salinization can occur regardless of anthropogenic activities, especially when cations and anions are easily leached down the aquifers, resulting in a relative increase in Na
+ ions, which, consequently, may replace Ca
2+ and Mg
2+ in the soil exchange complex [
30]. Although plants take up many salts in the form of nutrients, when more salt is added to the soil than required, it poses a threat to plants. For the various salt concentrations used in this study, there was a gradual increase in EC
se with the number of weeks after the commencement of the salinity treatment, becoming extremely high at the peak of the duration (week 10). Such a high EC
se (as a result of excessive sodium ions accumulated overtime) may reduce the soil’s ability to conduct water and cause the accumulation of a salt crust on the soil’s surface that could leach down during an unexpected precipitation to cause injury to plant roots [
31].
The interaction between BR and salinity levels showed that an increase in BR application frequency significantly reduced the EC
se across various salinity levels. The exogenous application of BR was reported to reduce Na
+ and Cl
− contents, while increasing N, P, K and Ca to boost the activities of antioxidant enzymes, leading to improved plant yield under salinity stress [
32]. The capacity of BR to play this mitigating role could possibly be attributed to its ability to induce the secretion of metabolites from different parts of the plant root system into the soil environment during growth [
33]. In a recent study, the contents of primary metabolites, such as photosynthetic pigments, monosaccharides and protein, improved the growth rate of duckweed (
Wolffia arrhiza) when sprayed with BR [
34]. Root exudates serve as a material exchange and information transmission between plants and the soil in maintaining the functionality and vitality of the rhizosphere [
35]. The root secretions could alter the rhizospheric soil environment, which is a major adaptative response mechanism for plants to cope with environmental abiotic stresses such as salinity [
36].
The temporal effects of 24-epibrassinolide on the EC
se with the number of weeks after the commencement of irrigation with saline water confirmed the efficacy of BR in accelerating metabolic processes that enhanced the accumulation of soluble proteins, and the stability or synthesis of organic acids and monosaccharides, mainly glucose, during photosynthesis [
37]. This was evident across the weeks of saline water irrigation, where it significantly reduced the EC
se to boost soybean yield across the weeks of measurement. The higher frequency of BR application may have promoted the synthesis of metabolites, thereby contributing to an increased plant biomass and, possibly, root exudates, as beneficial support to soybean growth under prolonged salinity stress.
Soils are complex entities that provide substrates for nature as well as agriculture. Saline soils inhibit the growth of most crops because of the higher concentration of neutral soluble salts in them. This occurs majorly through irrigation water, which is usually a gradual process where the salts accumulate over time before their effects are visible. Most salts are taken up by plants in the form of nutrients, but when the soil receives more salt than is removed, plants are eventually affected by salinity injuries [
38]. The increase in salt concentration in our rhizospheric soil increased the EC
se as the saline irrigation week progressed. The plants showed some growth inhibitions from the SL
2 (6.06 dS/m) at week 7, whereas a greater growth inhibition occurred at week 5 when the salt concentration was highest at SL
3 (8.63 dS/m) as compared with SL
1 (3.24 dS/m) [
10]. This trend could be attributed to the fact that the soil physicochemical properties and ecological balance were altered by excessive salt accumulations overtime, leading to high osmotic stress, nutritional disorders and toxicities in the plants [
39]. Soybean is considered to have a moderately tolerant threshold of approximately 5.0 dS/m of saturated soil extract [
40]. The highest frequency of BR application (BR
5) was able to reduce EC
se to 4.99 dS/m at week 9 and 4.93 dS/m at week 6 with saline irrigation water of 6.06 and 8.63 dS/m, respectively. This indicated that the plant was substantially protected at its vegetative and reproductive growth stages when considering the 5.0 dS/m EC
se tolerance threshold.
There was an improvement in the general morphological and physiological wellbeing of soybean plants when BR was exogenously applied. As reported previously [
41], the continuous application of BR improved the biosynthesis of targeted compounds in the plants and facilitated the secretion of primary metabolites that suppressed the abiotic environment. The successive increase in salinity increased the EC
se across the weeks of saline treatment. The highest EC
se was under SL
3, without BR. The application of BR reduced the EC
se substantially between weeks 5 and 7 at SL
2 or SL
3 as compared with BR
0. However, repeat use of BR (BR
4 or BR
5) significantly reduced the EC
se up to week 9, whether at SL
2 or SL
3, relative to BR
1, BR
2 or BR
3. As previously reported [
10], soybean responds to different salinity levels in terms of both the total biomass yield and yield variables. Generally, a lower yield was observed at higher salinity levels. The BR aided the mobilization of nutrients through the exuded metabolites. Plants add glucose to rhizospheres through exudation to reduce electrolyte concentrations and facilitate their survival in saline drylands [
42].