Regulatory Effects of S-Abscisic Acid and Soil Conditioner on the Yield and Quality of Hybrid Rice Under Salt Stress

Abstract

Salt stress significantly reduces rice yield and deteriorates rice quality. The present study was conducted to explore the regulatory effects of sole and combined application of S-abscisic acid (S-ABA) and soil conditioner on rice under high salt stress. The experimental treatments comprised 0.1% S-ABA alone (T1), the application of soil conditioner (T2), the combined application of both S-ABA and halotolerant microorganism soil conditioner (T3), and a control without any regulatory substance (CK). The treatments were arranged in a randomized complete block design in triplicate. To simulate high salinity stress, a 0.6% saltwater solution (by mixing natural seawater with freshwater) was used for irrigation. The results showed that T3 alleviated the phytotoxic effects of high salt stress and substantially improved rice yield. Furthermore, the numbers of effective panicles, grains per panicle, and 1000-grain weight under T3 treatment were 13.3–14.5%, 8.9–14.1%, and 4.9–5.5% higher than CK owing to improvement in dry matter accumulation, SPAD values, leaf area index, antioxidant enzyme activity, and reduced malondialdehyde and sodium ion content in rice. Moreover, the T3 treatment increased the output, output rate, and conversion rate of stem sheath matter after the heading stage; improved the milling yield, starch paste viscosity, starch stickiness, and gelatinization enthalpy; and reduced rice chalkiness. In addition, the T3 treatment also increased the amylose contents and decreased the total protein contents, thereby improving the taste of the rice. Overall, the results indicated that the application of exogenous S-ABA and soil conditioner is an effective strategy to alleviate the severity of salt stress in rice.

1. Introduction

Improvement and utilization of saline–alkali land for crop production have consistently been major focuses in agricultural research globally [1,2]. Saline–alkali soils generally limit crop growth and agricultural productivity owing to their unique physicochemical properties [3]. However, with the increasing global population and the growing scarcity of arable land resources, the rational development and utilization of saline–alkali land are particularly important [4]. As one of most populous countries, China has long regarded food security as a fundamental pillar of its national security [5]. Therefore, improving crop productivity through better utilization of saline–alkali land is of great strategic significance for crop production and ensuring national food security.

Rice (Oryza sativa L.) is cultivated globally; however, more than 90% of rice is produced and consumed only in Asia [6,7]. The sensitivity of rice to soil salinity limits its cultivation on saline–alkali land. High salinity stress not only affects the growth and development of rice but also leads to a substantial reduction in yield and quality traits [8,9]. For example, Yao et al. [2] found that rice quality decreased significantly under high salt stress, mainly due to the significant deterioration of starch-related physicochemical properties in rice. In addition, Du et al. [8] improved the growth of rice by using nano-silicon oxide, thus increasing rice yield under high salt stress. Therefore, maintaining high yield and quality of rice under saline conditions is a pressing challenge that needs to be addressed in current crop production systems.

In recent years, exogenous application of growth regulators has shown great potential in improving salt tolerance in crop plants [8]. Exogenous regulators, i.e., plant growth regulators and soil conditioners, have the potential to improve crop growth under adverse conditions by modulating key physiological mechanisms [10,11]. S-abscisic acid (S-ABA) is an important endogenous phytohormone and has been proven to play a key role in plant stress resistance [11,12]. S-ABA enhances plant tolerance to multiple abiotic stresses including salt stress by regulating plant metabolic processes [13]. For example, Jiang et al. [12] found that the application of S-ABA reduced endogenous abscisic acid content, increased endogenous gibberellins and indole-3-acetic acid, and maintained the homeostasis of hormones in rice under salt stress. Yao et al. [13] found an improvement in endogenous ABA, osmotic substances, and antioxidant enzyme activity with a substantial reduction in malondialdehyde (MDA) content with the foliar application of S-ABA at 4 ppm. Therefore, S-ABA can greatly improve salt stress and crop yield.

Soil conditioners may enhance microbial activity and improve soil properties and its ability to retain water and nutrients, thus creating a favorable environment for crop growth [14,15]. In addition, Shan et al. [16] found that the application of bio-fertilizers could improve the metabolism and enzyme activity of rhizosphere soil bacteria. Moreover, the application of bio-fertilizers can improve the nutrient availability and stress tolerance posed by saline–alkali soil [4]. Jin et al. [10] found that under high salinity stress conditions, the application of microbial fertilizer was beneficial in improving soil fertility, increasing the aboveground rice biomass, and thereby improving the yield and quality of rice. Previous studies have predominantly focused on the individual effects of growth regulators or soil conditioners on salt tolerance in crops.

Our study innovatively combines S-ABA with halotolerant microbial soil conditioners. Although previous studies have reported the positive effects of either growth regulators or soil conditioners (sole application) [10,12], we assume that their combined application could be more advantageous in achieving better rice yields with improved salt tolerance under saline–alkali conditions. Furthermore, we hypothesize that the application of S-ABA and soil conditioners could improve the physiological mechanisms of rice under salt stress, including increasing the activity of antioxidant enzymes, reducing the content of malondialdehyde (MDA), and lowering the sodium ion (Na⁺) content, thereby enhancing the salt tolerance of rice. Therefore, the present study was conducted to assess the effects of co-application of plant growth regular (S-ABA) and soil conditioner on the morpho-physiological, yield and quality of hybrid rice. This combined application not only leverages the respective regulatory advantages of both—where S-ABA enhances plant stress resistance by modulating metabolic processes and soil conditioners improve soil properties to foster a more favorable growth environment—but also likely generates synergistic effects. This synergism can further elevate rice yield and quality under salt stress, providing an optimized regulatory approach for rice cultivation in saline–alkali soils. The findings of our study directly contribute to augmenting food production in saline–alkali areas. This is particularly significant in countries and regions with extensive saline–alkali soils, offering a practical solution to bolster global food security.

2. Materials and Methods

2.1. Experimental Details

A field experiment was conducted during January to June 2021 and December 2021 to May 2022 at the National Saline–Alkali Rice Technology Innovation Center test base, Yanzao Village (109°11′ E, 18°37′ N) and Yacheng Town (109°16′ E, 18°36′ N), Yazhou District, Hainan Province China. The hybrid rice variety ‘Jingyou 007’ was provided by the Hunan Hybrid Rice Research Center, China. The seeds were sown on 16 December 2021 and seedlings were manually transplanted on 16 January 2022, with a plant spacing of 15 × 30 cm, and any gaps were promptly filled within three days after transplanting. The experimental soil was sandy loam.

The experimental treatments comprised 0.1% S-ABA alone (T1), the application of halotolerant microorganism soil conditioner (T2), the combined application of both S-abscisic acid (S-ABA) and halotolerant microorganism soil conditioner (T3), and a control without any regulatory substance (CK). Abscisic acid (ABA) is commercially known as S-abscisic acid (S-ABA), The S-ABA soluble concentrate was provided by Jiangxi Xinrui Feng Biochemical Co., Ltd. (Ji’an, China). The patent information is as follows: A method for separating natural abscisic acid crude from abscisic acid fermentation liquid, with the following authorized patent number: ZL201510972958.1.

The soil conditioner was a soluble solid with 20% organic matter, 0.2 CFU g⁻¹ effective live bacteria, 2% nitrogen, 6.5% P₂O₅, and 3.5% K₂O manufactured by Foshan Jin Kui Zi Plant Nutrition Co., Ltd. (Foshan, China). The application number is CN202011177498.0 and the public number is CN112341290A. More details of the soil conditioner are shown in the work of Jin et al. [10].

The net plot size of each experimental unit was 50 m² separated by 40 cm high ridges and wrapped with plastic film to prevent water and fertilizer runoff. The treatments were arranged in a randomized complete block design in triplicate. S-ABA was applied as a foliar spray, with 1200 mL per hectare applied at three critical growth stages: 17 days after transplanting, in the tillering stage, and in the panicle initiation stage. In the experiment (T1 and T3), 6 mL S-ABA was taken according to the pot area and then 2.4 L of pure water was mixed and then evenly sprayed on the leaves (diluted 400 times). Plots requiring compound microbial fertilizer (T2 and T3) had 2250 kg hm⁻² of soil-applied compound microbial fertilizer at the time of field preparation. Specifically, 2250 kg hm⁻² of the soil conditioner was applied to the soil, followed by rotary tillage to thoroughly mix the soil conditioner with the soil. All treatments were applied at 210 kg hm⁻² of N, 168.75 kg hm⁻² of P₂O₅, and 210 kg hm⁻² of K₂O. The specific fertilization is shown in

Table 1. Basic physical and chemical properties of soil and the specific fertilization for all treatments.

Basic Indicators		2021	2022
pH		6.17	5.77
Na+ (g kg−1)		5.28	6.17
EC (dS m−2)		1.14	2.12
Organic matter (g kg−1)		18.89	20.54
Total N content (g kg−1)		1.09	1.07
Total P content (g kg−1)		0.53	0.75
Total K content (g kg−1)		26.14	27.4
Available N content (g kg−1)		0.11	0.12
Available P content (g kg−1)		0.046	0.081
Available K content (g kg−1)		0.22	0.32
Treatment	Basal Fertilizer	Tiller Fertilizer	Panicle fertilizer
CK/T1	585 kg hm−2 of compound fertilizer and 1001.25 kg hm−2 of P2O5	91.5 kg hm−2 of urea and 70.5 kg hm−2 of KCl	225 kg hm−2 of compound fertilizer 49.5 kg hm−2 of urea and 37.5 kg hm−2 of KCl
T2/T3	2250 kg hm−2 of compound microbial fertilizer and 375 kg hm−2 of compound fertilizer	75 kg hm−2 of urea and 69 kg hm−2 of KCl	136.5 kg hm−2 of urea and 37.5 kg hm−2 of KCl

Note: The fertilization for CK and T1 treatments was consistent, and the fertilization for T2 and T3 treatments was consistent. Additionally, all treatments (CK, T1, T2, and T3) were applied with 210 kg N hm⁻², 168.75 kg P₂O₅ hm^−2, and 210 kg K₂O hm⁻². Furthermore, N, P₂O₅ and K₂O were applied as urea (46.2% N), superphosphate (12% P₂O₅) and potassium chloride (60% K₂O), respectively. The compound fertilizer used had a 42% nutrient content with a nitrogen–phosphorus–potassium ratio of 18-6-18. Since T2 and T3 involve the application of soil conditioners, which contain nitrogen, phosphorus, and potassium, it is necessary to correspondingly reduce the amount of nitrogen, phosphorus, and potassium (urea, superphosphate, potassium chloride) applied to ensure that the total amount of nitrogen, phosphorus, and potassium in all treatments is the same.

. The irrigation water was prepared by mixing natural seawater with freshwater in a mixing pool to achieve a 0.6% saltwater solution. After preparing the solution, the salinity was measured using a portable conductivity meter (2266FS, Spectrum, Stevens Point, WI, USA). The meter was calibrated according to the manufacturer’s instructions to ensure accurate readings. Salinity measurements were taken regularly, and the irrigation water was drained every three days and re-applied, with additional drainage after rainy days. A water layer of about 5 cm was maintained throughout the growth period, and water was cut off 7 days before harvest. Pest, disease, and weed control was carried out in accordance with unified rice cultivation measures.

2.2. Observations and Measurements

2.2.1. Determination of Dry Biomass, Leaf Area Index and Leaf SPAD Values

At the heading stage (HS) and maturity stage (MS), six uniform plants were selected from each replicate, cut into leaves, sheaths, and panicles (after heading), oven-dried at 105 °C for 30 min, and then dried at 80 °C till constant weight.

At the HS, three uniform plants were selected from each replicate, all leaves were removed, 15 leaves were randomly selected, the maximum length and widest width of each leaf were quickly measured, and the leaf area was calculated using the method “length × width × 0.75”; then, they were oven dried till constant weight at 80 °C and the leaf area index was calculated [8].

At the HS, 10 days after heading, and 20 days after heading, the leaf SPAD value was recorded using a chlorophyll meter (SPAD-502 PLUS, Konica Minolta, Tokyo, Japan), at a position of 1/2–1/3 from the leaf tip (the upper middle part of the leaf). The chlorophyll meter was clamped to measure five points on both sides of the measurement position along the leaf vein (avoiding the vein position), and the average value was recorded. Six sword leaves were measured in each plot, and the average value was taken as the leaf SPAD value [8].

2.2.2. Determination of Antioxidant Enzyme Activity and Malondialdehyde Content

At the HS, flag leaves were taken and immediately placed in liquid nitrogen and stored at −80 °C for biochemical analyses. The activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and malondialdehyde (MDA) contents were determined using kits according to Jin et al. [10].

2.2.3. Determination of Sodium Ion Content

Dried samples of stems, leaves, and panicles at the HS and MS were ground into powder and sieved. Dry samples (0.5 g) were digested with an H₂SO₄-H₂O₂ system, and then the Na⁺ content of each organ was determined by flame photometry [10].

2.2.4. Yield and Yield Components

At the MS, 20 effective panicles were surveyed in sequence. And according to the average number of effective panicles in that plot, three uniform plants were selected to measure the total number of grains per plant, the number of filled grains, the 1000-grain weight and the grain filling. In each plot, a 10 m² area was harvested. Each plot was harvested and threshed separately, impurities were removed, moisture was determined, and the yield was converted to a standard moisture content of 13.5% [17].

2.2.5. Determination of Processing and Appearance Quality

After MS, the harvested rice was sun-dried and stored for 3 months before rice quality determination. The determination of the brown rice rate, milled rice rate, head rice rate, chalky grain rate, chalkiness degree, grain length, grain width, and length–width ratio were according to Li et al. [18]. Chalky grain rate = Number of Chalky Grains/Total Number of Grains. The chalkiness percentage (%) = (Projected area of the chalky part of the rice grain/Projected area of the rice grain) × 100%. Generally, the average value of 30 rice grains was taken.

2.2.6. Determination of Grain Quality

Total starch content was measured using a starch total content detection kit (Megazyme, K-TSTA, Wicklow, Ireland). The determination of amylose content and viscosity was carried out according to the Jin et al. [10]. PV (cP) = Maximum viscosity reached during the heating phase; TV (cP) = Minimum viscosity during the holding phase; BD (cP) = PV − TV; FV (cP) = Viscosity at the end of the cooling phase; SB (cP) = FV − TV.

2.2.7. Determination of Starch Thermodynamics and Viscosity Characteristics

A differential scanning calorimeter (DSC, Q2000, TA Instruments, New Castle, DE, USA) was used to test the thermodynamic properties of the samples, whereas a rapid viscosity analyzer ‘RVA4800’ manufactured by PerkinElmer Instruments was used to determine the viscosity characteristics.

2.3. Calculation and Statistical Methods

Panicle number per unit area (×10⁸ hm⁻²) = Number of effective panicles per unit area (×10⁴ hm⁻²) × Total number of grains per panicle

Sink capacity (×10³ kg hm⁻²) = Panicle number per unit area × Grain weight

Post-heading stem sheath matter output (×10³ kg hm⁻²) = Dry weight of stem sheath at HS (×10³ kg hm⁻²) − Dry weight of stem sheath at MS (×10³ kg hm⁻²)

Post-heading stem sheath apparent output rate (%) = Post-heading stem sheath matter output/Dry weight of stem sheath at HS × 100

Post-heading stem sheath conversion rate (%) = Post-heading stem sheath matter output/Grain dry weight × 100

Dry matter stage accumulation (×10³ kg hm⁻²) = Dry weight of aboveground part at MS − Dry weight of aboveground part at HS

Community growth rate (×10³ kg hm⁻² d⁻¹) = (W₂ − W₁)/(t₂ − t₁), where W₁ and W₂ are the dry matter weights (×10³ kg hm⁻²) measured at the heading and mature stages, and t₁ and t₂ are the time intervals (d) between the first and second measurements.

One-way analysis of variance (ANOVA) was conducted on the collected data using SPSS 19.0 software (SPSS, Inc., Chicago, IL, USA). The differences amongst the treatment means of each experiment were separated using the least significant difference (LSD) test at a significance level of 0.05. In our study, the main effects of different treatments (T1, T2, T3 and CK) on rice yield and quality indexes were analyzed. All graphs were created using Origin 9.0 (OriginLab Corp., Northampton, MA, USA). The analysis of PCA and the plotting of images were conducted using Origin 9.0.

3. Results

3.1. Yield and Yield Components

The application of regulatory substances substantially improved the yield compared to the CK (

Table 2. Effects of S-ABA and soil conditioner on yield and yield components of rice under high salt stress conditions.

Year/ Treatments	Panicles (×104 hm−2)	Spikelet per Panicle	Grain Filling (%)	1000-Grain Weight (g)	Total Spikelets (×108 hm−2)	Sink Capacity (×103 kg hm−2)	Harvested Yield (×103 kg hm−2)
2021
T1	184.42 b	127.55 b	78.26 a	19.68 b	2.38 bc	4.68 bc	3.34 b
T2	188.49 ab	132.66 ab	80.19 a	19.93 ab	2.53 ab	5.04 ab	3.56 ab
T3	201.26 a	136.59 a	78.97 a	20.37 a	2.78 a	5.66 a	3.81 a
CK	175.73 b	125.42 b	77.83 a	19.30 c	2.23 c	4.31 c	2.81 c
2022
T1	197.06 b	152.57 ab	65.80 b	20.03 a	3.00 b	6.05 b	3.47 b
T2	202.09 b	158.33 ab	67.98 ab	20.16 a	3.20 ab	6.45 ab	3.65 ab
T3	208.75 a	163.77 a	69.99 a	20.34 a	3.42 a	7.00 a	3.96 a
CK	184.33 c	143.53 b	63.53 b	19.39 b	2.61 c	5.08 c	2.89 c

Note: Values followed by different small letters in the same column are significantly different among the treatments at the 0.05 probability level. T1, T2, T3 and CK represent S-ABA, soil conditioner, S-ABA+ soil conditioner, and control, respectively.

). Specifically, the yield was increased by 18.86–20.07%, 26.30–26.69%, and 35.59–37.02% in T1, T2, and T3 over CK in both years, respectively. Further, the number of effective panicles and 1000-grain weight compared remained comparatively higher in T1, T2, and T3 than CK in 2022. In addition, no significant differences were noticed in the grains per panicle and seed setting rate between the T1 and T2 treatments. High salinity stress resulted in low panicle number and sink capacity for all treatments, leading to insufficient sinks. Compared to CK, the application of exogenous regulatory substances significantly increased the sink capacity in the following order: T3 > T2 > T1.

3.2. Dry Biomass Accumulation

Compared to CK, the application of regulatory substances increased the total dry matter accumulation at HS and MS (

Table 3. Effects of S-ABA and soil conditioner on aboveground dry matter accumulation under high salt stress conditions.

Treatments	HS (×103 kg hm−2)	MS (×103 kg hm−2)	HS-MS (×103 kg hm−2)	Community Growth Rate (×103 kg hm−2 d−1)
2021
T1	7.14 b	8.95 c	1.80 a	0.056 a
T2	7.77 ab	9.62 b	1.85 a	0.058 a
T3	8.41 a	10.33 a	1.92 a	0.060 a
CK	6.11 c	7.79 d	1.68 b	0.052 b
2022
T1	8.04 b	10.64 b	2.60 b	0.084 a
T2	8.47 ab	11.11 b	2.64 b	0.085 a
T3	9.27 a	12.64 a	3.18 a	0.103 a
CK	6.65 c	8.21 c	1.85 c	0.060 b

Note: Values followed by different small letters in the same column are significantly different among the treatments at the 0.05 probability level. T1, T2, T3 and CK represent S-ABA, soil conditioner, S-ABA+ soil conditioner, and control, respectively. HS, heading stage; MS, maturity stage.

). Compared to CK, the T1, T2, and T3 treatments increased the dry matter accumulation at HS by 16.86–20.90%, 27.17–27.37%, and 37.64–39.40% and at MS by 14.89–29.60%, 23.49–35.32%, and 32.61–53.96%, respectively. The dry matter translocation from HS to MS was the highest in the T3 treatment and also higher than T1 and T2 by 22.3% and 20.5% in 2022, respectively. In addition, no significant differences were noticed in community growth rate among T1, T2, and T3, but all were significantly higher than CK in both years.

Compared to CK, T1, T2, and T3 not only increased the dry weight of the sheath at HS and MS and the dry matter output after HS (

Table 4. Effects of S-ABA and soil conditioner on stem sheath dry matter conversion under high salt stress conditions.

Treatments	Stem Sheath Weight		HS-MS Stem Sheath Dry Matter Conversion
	HS (×103 kg hm−2)	MS (×103 kg hm−2)	Output (×103 kg hm−2)	Output Rate (%)	Conversion Rate (%)
2021
T1	4.56 b	3.87 bc	0.69 b	15.23 a	19.50 ab
T2	4.88 b	4.16 ab	0.72 b	14.67 ab	18.57 ab
T3	5.43 a	4.43 a	1.01 a	18.53 a	24.27 a
CK	3.98 c	3.58 c	0.40 c	9.79 b	13.73 b
2022
T1	5.18 a	4.61 a	0.57 b	10.91 ab	14.72 ab
T2	5.36 a	4.72 a	0.64 ab	11.84 ab	15.63 ab
T3	5.82 a	5.09 a	0.73 a	12.52 a	16.75 a
CK	4.28 b	3.86 b	0.42 c	9.84 b	12.96 b

Note: Values followed by different small letters in the same column are significantly different among the treatments at the 0.05 level. T1, T2, T3 and CK represent S-ABA, soil conditioner, S-ABA+ soil conditioner, and control, respectively. HS, heading stage; MS, maturity stage.

) but, specifically, the T3 treatment significantly increased the dry weight of the sheath at HS and MS and the dry matter output after HS, as well as improving the apparent output rate and conversion rate, all of which were significantly higher than the CK treatment in both years.

3.3. SPAD Values

The exogenous regulatory treatments significantly increased leaf SPAD values compared to the CK (Figure 1). For example, compared to the CK, the T1, T2, and T3 treatments increased the SPAD at HS by 2.91–5.13%, 6.01–6.62%, and 8.61–9.04% in both years, respectively. Furthermore, 20 days after HS, the T1, T2, and T3 treatments increased the SPAD values by 14.11–14.64%, 21.52–21.86%, and 26.51–32.22% in both years, respectively, compared to the CK. The decrease in SPAD values from HS to 20 days after HS was less than that of the CK, with the following order: T1 > T2 > T3. In addition, the T3 treatment increased the leaf area index at HS by 23.1–26.8% and this was significantly higher than CK; however, there were no significant differences in the leaf area index between the T1, T2, and T3 treatments.

3.4. Antioxidant Enzyme Activity and Malondialdehyde Content

The exogenous regulatory treatments significantly increased the activities of antioxidant enzymes, i.e., superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) (Figure 2). Specifically, the activities of SOD, POD, and CAT were the highest in the T3 treatment, i.e., 29.82–35.14%, 45.21–45.34%, and 45.01–48.89% higher than CK in both years, respectively; in addition, there were significant differences in SOD and POD activities among the treatments with the following order: T3 > T2 > T1 > CK. However, there were no significant differences in CAT activity between the T1 and T2 treatments, but both were significantly higher than CK. Furthermore, compared to CK, the T2 and T3 exogenous regulatory treatments significantly reduced the MDA contents in both years.

3.5. Principal Component Analysis (PCA)

The PCA score plots assembled closely within one treatment, and the CK, T0, T1, and T2 treatments were notably separated. The PCA plot explained 68.0% of the variance in PCA1 and 19.7% of the variance in PCA2 (Figure 3). In PCA1, HSDMR, MSDMR, SSHS, and SOD activity had higher loading values and can be used as primary effectors for evaluating yield in rice under brine irrigation.

3.6. Sodium Ion (Na⁺) Content in Different Organs

The exogenous regulatory treatments reduced the Na⁺ content in all parts at HS and MS compared to the CK (Figure 4). For example, the T3 treatment had the lowest Na⁺ content in leaves and stems, which was 19.21% and 24.27% lower than CK, respectively. There were no significant differences regarding Na⁺ content in leaves between the T1, T2 and the CK treatment, but both T1 and T2 reduced the Na⁺ content in stems. At maturity, the Na⁺ content in T1, T2, and T3 treatments was significantly lower than that in the CK. In addition, the reduction in Na⁺ content by T1, T2, and T3 compared to the CK treatment was in the order of T3 > T2 > T1.

3.7. Rice Processing Quality and Nutritional Quality

Compared to CK, the exogenous regulatory treatments significantly increased the milled rice rate and whole milled rice rate (

Table 5. Effects of S-ABA and soil conditioner on the processing and nutritional quality of rice under high salt stress conditions.

Treatment	Brown Rice Rate (%)	Polished Rice Rate (%)	Head Rice Rate (%)	Total Starch Content (%)	Protein Content (%)	Fat (%)
T1	80.27 a	72.53 a	68.97 a	70.95 a	9.99 b	2.70 c
T2	80.60 a	72.87 a	70.50 a	71.78 a	10.19 b	2.67 c
T3	80.97 a	73.77 a	71.30 a	71.58 a	10.22 b	3.13 b
CK	80.40 a	70.33 b	61.70 b	70.89 a	11.48 a	4.37 a

Note: Values followed by different small letters in the same column are significantly different among the treatments at the 0.05 probability level. T1, T2, T3 and CK represent S-ABA, soil conditioner, S-ABA+ soil conditioner, and control, respectively.

). Furthermore, the increase in milled rice rate and head rice rate were ranged from 3.03 to 4.89% and 11.78 to 15.56%, respectively, in exogenous regulatory treatments as compared to CK with the following order: T3 > T2 > T1. However, there were no significant differences in brown rice rate among the treatments.

Compared to CK, the exogenous regulatory treatments enhanced the total starch content to varying degrees (Table 5). There was no significant difference in total starch content among T1, T2, T3, and CK. In addition, compared to CK, the soil conditioner significantly reduced the grain protein and fat content, whereas the reduction in total protein content ranged from 10.96 to 13.07% in the following order: T1 > T2 > T3. Furthermore, the reduction in fat content in the T1, T2, and T3 treatments ranged from 28.38% to 38.90%.

3.8. Rice Appearance and Cooking Qualities

Compared to CK, the exogenous regulatory treatments significantly reduced the chalky grain rate and chalkiness with a range of 46.60–53.40% and 62.60–68.69%, respectively (

Table 6. Effects of S-ABA and soil conditioner on the appearance of whole rice under high salt stress conditions.

Treatment	Length mm	Width mm	Length Width Ratio	Area mm2	Perimeter mm	Chalky Grain Rate (%)	Chalkiness (%)
T1	5.66 a	1.96 a	2.90 a	9.12 a	13.64 a	2.33 b	0.37 b
T2	5.71 a	1.97 a	2.91 a	9.28 a	13.76 a	2.33 b	0.31 b
T3	5.76 a	2.01 a	2.88 a	9.54 a	13.90 a	2.67 b	0.36 b
CK	5.45 b	1.88 b	2.92 a	8.43 b	13.09 b	5.00 a	0.99 a

Note: Values followed by different small letters in the same column are significantly different among the treatments at the 0.05 probability level. T1, T2, T3 and CK represent S-ABA, soil conditioner, S-ABA+ soil conditioner, and control, respectively.

). However, there were no significant differences that were noted in the chalky grain rate and chalkiness among T1, T2, and T3 treatments. Compared to CK, the T1, T2, and T3 treatments significantly increased the length, width, area, and circumference of rice grain (Table 6). In addition, compared to the CK, the increase in length, width, area, and circumference by the three exogenous regulatory treatments was in the order of T3 > T2 > T1. Furthermore, compared to CK, the exogenous regulatory treatments increased the amylose contents to varying degrees (Figure 5). For instance, the amylose contents increased within a range of 8.76- 11.90% compared to CK, with the following order: T1 > T3 > T2. In addition, the three exogenous regulatory treatments increased the gel consistency within the range of 6.51% to 11.24% compared to CK. The gel consistency of rice grains in T1, T2, and T3 was significantly greater than that in CK; however, T1, T2 and T3 remained statistically similar in this regard.

3.9. Thermodynamic and Viscosity Properties of Grain Starch

Compared to CK, the T1 treatments resulted in a significant increase in the peak temperature and conclusion temperature (

Table 7. Effects of S-ABA and soil conditioner on starch gelatinization properties under high salt stress conditions.

Treatment	Onset Temperature (°C)	Peak Temperature (°C)	Conclusion Temperature (°C)	Temperature Range °C	Enthalpy J g−1
T1	67.23 a	73.94 bc	80.10 b	12.87 ab	4.99 b
T2	68.09 a	75.00 a	81.80 a	13.72 a	5.39 a
T3	68.23 a	74.56 ab	79.95 b	11.72 b	5.08 ab
CK	67.22 a	73.58 c	79.14 b	11.92 b	4.40 c

Note: Values followed by different small letters in the same column are significantly different among the treatments at the 0.05 probability level. T1, T2, T3 and CK represent S-ABA, soil conditioner, S-ABA+ soil conditioner, and control, respectively.

). Except for the T2 treatment, which showed a significant increase in peak temperature and final temperature compared to CK, there were no significant differences among the other treatments. Compared to CK, the three exogenous regulatory treatments significantly increased the gelatinization enthalpy of starch, within a range of 13.41% to 22.50%; however, no significant difference was noted among the three treatments.

The RVA profiles of starch under the three exogenous regulatory treatments showed significant differences compared to CK (

Table 8. Effects of S-ABA and soil conditioner on starch pasting properties under high salt stress conditions.

Treatment	PV	TV	BD	FV	SB
	cP	cP	cP	cP	cP
T1	2640.67 a	1955.33 a	685.33 a	3017.33 a	376.67 a
T2	2616.67 a	1939.67 a	677.00 a	2960.33 a	343.67 a
T3	2696.33 a	2022.67 a	673.67 a	3040.33 a	346.33 a
CK	2172.00 b	1514.33 b	657.67 a	2582.33 b	410.33 a

Note: Values followed by different small letters in the same column are significantly different among the treatments at the 0.05 probability level. T1, T2, T3 and CK represent S-ABA, soil conditioner, S-ABA+ soil conditioner, and control, respectively. BD, breakdown viscosity; FV, final viscosity; PV, peak viscosity; SB, setback viscosity; TV, trough viscosity.

). For instance, compared to CK, all three exogenous regulatory treatments significantly increased the peak viscosity, minimum viscosity, and final viscosity in range of 20.47% to 24.14%, 28.09% to 33.57%, and 14.64% to 17.74%, respectively. Compared to CK, the three exogenous regulatory treatments increased the breakdown value and decreased the setback value to varying degrees, with no significant differences among the treatments.

4. Discussion

Excessive salt accumulation leads to soil compaction and fertility decline, which restrict crop growth and nutrient uptake, ultimately resulting in reduced yield and quality [19,20]. Due to the severe impact of high salinity stress and the lengthy growth cycle of rice, comprehensive regulatory measures should be implemented to manage saline–alkali soil. The aim of the present study is to explore the combined use of exogenous regulators to enhance the salt tolerance of rice, offering a valuable reference for the integrated regulation and cultivation of salt-tolerant rice varieties. Therefore, this study explores the impact of the combined application of salt-tolerant microbial soil conditioner and S-abscisic acid (S-ABA) on the yield and quality of rice grains under salt stress conditions.

4.1. Effects of Different Exogenous Regulators on Rice Growth, Development, and Yield Under High Salt Stress

Salt stress severely affects rice in many ways. For instance, studies have shown that salt stress leads to curling and chlorosis of rice leaves, a decrease in SPAD values and LAI, a decline in photosynthetic rate, and a reduction in the accumulation and translocation of dry matter [20]. With excessive accumulation of Na⁺ in the soil, crops transport Na⁺ into their organs through transport proteins, increasing the Na⁺ content in the aboveground parts and disrupting the ionic homeostasis within cells [21]. Additionally, salt stress causes a large accumulation of reactive oxygen species (ROS) within plants, leading to lipid peroxidation of cell membranes [22]. To quench ROS and enhance the adaptability of crops to adverse conditions, a series of stress responses occur within the plant, such as increasing the activity of antioxidant enzymes (SOD, POD, and CAT) and reducing MDA content [21,22]. After a series of damages including osmotic stress, ionic imbalance, and the destruction of the antioxidant system, crop growth is severely limited, leading to a decrease in yield attributes, i.e., the number of panicles per unit area, the number of grains per panicle, the setting rate, and the 1000-grain weight, ultimately resulting in reduced yields [19].

Our research indicates that the application of S-ABA and soil conditioners alone increased rice yield, mainly due to the improvement in 1000-grain weight and the grains per panicle. The results are consistent with those of Jin et al. [10], who found that the application of soil conditioners as a basal fertilizer can effectively improve the yield by increasing the number of grains per panicle and the seed setting rate. However, the combined application of S-ABA and soil conditioners can further increase the number of effective panicles, thus achieving a greater yield increase than either ABA application alone. Zhu et al. [23] found that the application of biofertilizers improved the activity of antioxidant enzymes, chlorophyll content, photosynthesis, and growth of cotton whilst reducing the MDA contents. In addition, Jiang et al. [12] found that the application of S-ABA under salt stress improved the agronomic traits of rice, enhanced photosynthesis, reduced the Na⁺/K⁺ ratio, improved antioxidant enzyme activity, reduced ROS, and mitigated the impact of salt stress on rice growth. The combined application of S-ABA and soil conditioners likely enhances the expression of genes related to antioxidant enzyme production. S-ABA has been shown to regulate the expression of stress-responsive genes, including those involved in antioxidant defense [12]. Soil conditioners, by improving soil health and nutrient availability, provide the necessary resources for the plant to produce these enzymes more efficiently [10]. Shan et al. [16] found that the application of biofertilizers increased the diversity of microorganisms in the soil, restructured the bacterial community, and enhanced the metabolism and enzyme activity of soil bacteria within the rhizosphere, thus improving the overall nutrient supply as well as rice yield. Jin et al. [10] found that the application of soil conditioner increased the soil cation exchange capacity and enhanced the content of soil organic matter and available potassium content, indicating that soil conditioner improved the soil nutrient status. Microorganisms in soil conditioners improve soil structure and nutrient retention capacity by enhancing the activity of beneficial microorganisms in the soil, thereby creating a favorable environment for crop growth [16,19]. The interaction between ABA and reactive oxygen species (ROS) is complex and multifaceted. As discussed in Li et al. [20], ABA can stimulate the production of ROS through the NADPH oxidase of guard cells. This ROS production is part of the ABA signal transduction pathway and is essential for the plant’s adaptive response to stress. The generated ROS can act as a second messenger, triggering a series of downstream events that lead to stomatal closure and other stress responses. In addition, S-ABA enhances the plant’s tolerance to salt stress by regulating the internal hormonal balance within the plant [20,21]. The application of S-ABA reduces the content of endogenous abscisic acid (ABA) under salt stress conditions but increases the content of endogenous gibberellins (GAs) and indole-3-acetic acid (IAA), maintaining the hormonal homeostasis in rice plants [12]. The combined application of S-ABA and soil conditioners likely enhances the plant’s ability to scavenge reactive oxygen species (ROS) more effectively. By reducing ROS levels, the treatments prevent the initiation of lipid peroxidation, leading to lower MDA content.

Furthermore, the present study showed that the application of S-ABA under salt stress increased the leaf area index, promoted chlorophyll biosynthesis, enhanced dry matter accumulation, and reduced Na⁺ content. Our findings are in agreement with Yao et al. [13], who observed that application of S-ABA alleviated the salt stress by increasing antioxidant enzyme activity and reducing MDA content. Moreover, the application of S-ABA increased yield by improving the number of panicles per unit area, the number of grains per panicle, the seed setting rate, and the 1000-grain weight, thereby enhancing the overall salt tolerance of rice [24].

Chen et al. [25] found that although ABA alone can improve stress tolerance, its application in combination with other substances (sucrose) may be more effective. For example, the possible reason for the increased yield from the combined application of ABA and sucrose is the enhanced accumulation of dry matter and non-structural carbohydrates in the rice panicles. This improvement is primarily due to the significant boost in sucrose transport within the sheath stem resulting from the ABA and sucrose treatment. The present study also confirmed that the application of soil conditioners as a basal fertilizer and the external application of ABA can potentially improve the dry matter accumulation, LAI, leaf SPAD values, antioxidant enzyme activity, the output of matter in the stem sheath after HS, and the output rate and conversion rate, thereby enhancing rice yield under salt stress conditions. In addition, previous research has demonstrated that exogenous hormones, such as gibberellins and cytokines, have the capacity to alleviate salt stress conditions, thereby enhancing the photosynthetic rate of leaves and promoting plant growth [26,27]. Consequently, further trials should be conducted to investigate the combined application of other exogenous hormones and soil amendments, with a view to evaluate their potential synergistic effects on rice growth under salt stress conditions.

4.2. Effect of Different Exogenous Regulators on Rice Quality Under High Salinity Stress

Currently, there is a lack of research on the effects of individual regulatory substances on rice quality under saline stress conditions, especially the combined application of S-ABA and soil conditioner. Rice quality mainly includes four aspects, i.e., appearance quality, milling quality, cooking and eating quality, and nutritional quality. These qualities collectively determine the market price of rice and the degree of consumer preference [28]. Previous studies have shown that high salinity stress deteriorates the processing quality of rice, reduces the amylose content, increases the total protein content, and changes the physicochemical properties and fine structure of starch, thus leading to a decline in the cooking and eating quality of rice [9,20]. Liu et al. [29] found that the application of exogenous ABA improves the processing and appearance of rice, reduces the prevalence of grain chalkiness, increases the viscosity of starch paste, and improves the overall cooking quality of rice grains. Sun et al. [30] found that increasing the ABA content is beneficial for reducing the impact of high temperatures on the amylose content, gelatinization temperature, and starch viscosity, thereby improving the taste of rice. Chen et al. [25] found that the combined application of exogenous ABA and sucrose is conducive in regulating the source–sink relationship and increasing the grain starch contents, thus improving the rice quality. Jin et al. [10] found that the application of soil conditioner not only improves the milling and appearance of rice but also enhances the taste of rice by increasing the amylose content and reducing the total protein content, which is in line with the current study which indicated that the simultaneous application of ABA and soil conditioner improved the processing of rice. In addition, it was found that rice treated with soil conditioner resulted in excellent gelatinization properties, characterized by a reduction in the proportion of short chains of amylopectin and a lower gelatinization temperature and gelatinization enthalpy, which are conducive to improving the taste of rice [10]. Furthermore, compared with CK, all treatments increased the disintegration value to varying degrees and lowered the reduction value, which indicates that both S-ABA and soil conditioners can increase the viscosity of starch and reduce the rice hardness, thereby improving the eating quality; nevertheless, the combined application is superior to their individual application.

The methods used in this study are feasible for commercial agricultural settings. The application of S-ABA and soil conditioners is a practical and cost-effective approach to improving rice yield and quality under salt stress. These treatments can be easily integrated into existing agricultural practices without requiring significant changes to current farming systems. Many crops exhibit similar physiological responses to salt stress. For example, crops like oilseed rape and eggplant also suffer from reduced growth, increased oxidative damage, and disrupted ion homeostasis under salt stress conditions [31,32,33]. Therefore, it is plausible that the application of S-ABA and halotolerant microorganisms could have similar beneficial effects on these crops.

The use of 0.1% S-ABA and the specific soil conditioner composition ensures that the treatments are both effective and economically viable for large-scale applications. Additionally, the irrigation system used in the study, which involves the use of 0.6% saltwater, can be adapted to commercial settings with appropriate water management practices [7,10]. While the treatments show promising results in improving rice yield and quality under salt stress, it is important to consider the potential trade-offs. The cost of S-ABA and soil conditioners is a significant factor for large-scale application. However, the economic benefits of increased yield and improved quality may offset these costs, especially in regions where salt stress severely limits rice production. Regarding long-term soil health, the organic matter and microbial components in the soil conditioner can improve soil structure and fertility over time, potentially leading to sustained productivity. However, continuous monitoring is recommended to ensure that there are no adverse effects on soil biodiversity or other ecological parameters. Moreover, the irrigation system used in the study, which involves the use of 0.6% saltwater, requires careful water management to ensure that the saltwater concentration is maintained at optimal levels for rice growth. Improper management may lead to increased salinity in the soil, exacerbating salt stress. Therefore, the feasibility of the irrigation system in different regions and its impact on water resources should be considered.

This study focused on a single rice variety, ‘Jingyou 007’, which limits the generalizability of the findings to other varieties. Different rice varieties may have varying responses to S-ABA and soil conditioners due to genetic differences in their stress tolerance mechanisms. Moreover, the study was conducted over two years, which does not fully capture the long-term effects of the treatments on soil health and rice productivity. Future studies should include a broader range of rice varieties and long-term experiments to assess the universal applicability of these treatments. This would provide a more comprehensive understanding of how different genetic backgrounds influence the response to S-ABA and soil conditioners.

5. Conclusions

The application of S-ABA and soil conditioners (sole or in combination) can improve the agronomic traits of rice and increase rice yield; however, in this study, the most significant increase in yield was observed with combined application, owing to improved leaf SPAD values, delayed leaf senescence, and increased total above-ground biomass. In addition, the S-ABA and soil conditioner also improved the antioxidant enzyme activities, reduced the MDA content, and reduced the Na⁺ content in stems and leaves. In addition, the application of S-ABA and soil conditioner also improved the appearance and processing quality and increased the amylose content, viscosity of the starch paste, starch stickiness, and gelatinization enthalpy, thereby improving the taste of rice cultivated under saline conditions. This study provides a theoretical foundation for the cultivation of rice with the application of growth regulators and soil conditioners under high salinity stress conditions. Future research and policy initiatives should conduct long-term field trials to evaluate the cumulative effects of S-ABA and soil conditioners on soil health, microbial diversity, and rice productivity over multiple years.