Effects of Salt Stress on Grain Quality and Starch Properties of High-Quality Rice Cultivars

Abstract

In recent days, there has been a noticeable surge in demand for high-quality rice. However, the influences of salinity on the quality and starch properties of high-quality rice remain unclear. Three high-quality rice cultivars (Nanjing 9108, Nanjing 5055, and Nanjing 46) were studied to investigate the responses of grain quality to salt stress. There were three treatments, including a control zero salt level (0 g·kg⁻¹, CK), and two salt levels of 0.1 g·kg⁻¹ (0.1% salt stress, T1) and 0.2 g·kg⁻¹ (0.2% salt stress, T2). The study involved the assessment of the appearance, milling, cooking, and eating qualities of rice. We also conducted an analysis of pasting properties, an evaluation of starch thermal properties, and an examination of the fine structure of amylopectin. The findings suggest that as the level of salt stress increases, the yield of rice gradually declines, which is primarily due to a significant reduction in the total spikelet number and the ratio of filled grains. Compared with CK treatment, the appearance and milling quality of rice were significantly improved within the T1 treatment. In addition, the protein concentration and amylose concentration were significantly decreased, the gel consistency was significantly increased, and the cooking and eating qualities were improved. In terms of starch properties, the peak viscosity, breakdown value, infrared ratio (1022/995), and short-chain-length amylopectin ratio increased significantly, while the setback value, pasting temperature, gelatinization enthalpy, relative crystallinity, and infrared ratio (1045/1022) decreased significantly. When comparing T2 with CK, the appearance quality and cooking and eating quality had deteriorated, and the milling quality was improved. The changes in the structural and physicochemical properties of starch were opposite to those in the comparison between the T1 treatment and the CK group. Accordingly, we propose that moderate salt stress has the potential to enhance rice quality, even though there may be a slight decrease in yield. This indicates that it is feasible to cultivate high-quality rice in saline–alkali beach areas.

1. Introduction

With the world population continuing to expand, the increase in the yield of staple foods such as rice is expected to be greater than other crops [1,2]. However, global rice yield is severely constrained by salt stress [3,4]. Globally, more than 800 million ha of soil are impacted by salt, and the extent of soil salinization is expanding by 1–2 million hectares annually [5]. Soil salinization primarily results from the use of poor-quality irrigation water, rising sea levels due to climate change, and drought [4]. Asia accounts for ~30% of the world’s saline soil area, with China having the largest saline soil area within Asia [3,6].

Salt stress primarily affects rice through osmotic stress, ion toxicity, and nutritional imbalance [7,8]. Excessive salt levels in the soil lead to osmotic stress. Elevated salt levels in the soil reduce the water potential, making it challenging to uptake water from soil. Consequently, this hampers the growth and development of rice plants [9]. Excessive Na⁺ accumulation in salt-affected plants not only causes physiological metabolic disorders of plant cells, but also has an antagonistic effect on the uptake of the nutrients nitrogen and potassium (K⁺), which results in ion toxicity and disrupts the nutritional balance [10,11,12]. Rice demonstrates varying degrees of salt tolerance throughout its different growth stages. Generally, the seedling and reproductive stages exhibit higher susceptibility to salinity-induced damage compared to the vegetative stage [4]. Salt stress during the seedling stage noticeably affects root growth and can cause leaf chlorosis [13,14]. In the reproductive growth stage, salt stress has detrimental effects on rice plants, resulting in insufficient photosynthate production, hindered panicle differentiation, and significant reductions in panicle length, primary branch number, grain filling rate, grain size, and panicle number [15]. These combined impacts ultimately lead to decreased rice yield [15,16,17].

Salinity stress can reduce the plant photosynthesis, hinder the transport of photosynthates to seeds, and diminish the deposition of starch in the grains. These effects collectively have a detrimental impact on the quality of rice [16,18]. Rice quality traits encompass a range of characteristics, including appearance, milling qualities, cooking and eating qualities, and various other factors [19]. The quality attributes of rice directly impact the market price of rice and the purchasing choices made by consumers [3], of which cooking and eating quality is regarded as the most important one [20]. The physicochemical qualities of starch are commonly used as a standard for assessing the cooking and eating quality, including amylose content, chain length distribution of amylopectin, and the stability of the starch structure [18,21]. It was discovered that subjecting rice plants to moderate salt stress (5 dS/m electrical conductivity) and high salt stress (12 dS/m) during the reproductive growth stage significant results in poor appearance and milling quality of the rice, as well as a higher protein content [22]. Furthermore, there has been an increase in the percentage of medium and long chains of amylopectin in rice, while the amylose content and proportion of short-chain amylopectin have decreased [22]. Sangwongchai et al. [18] studied the impacts of moderate salt levels (4 dS/m) on the physicochemical properties of starch in four rice cultivars during the reproductive growth period, and found that the responses to salt stress can vary due to genotypic differences. Despite the limited number of studies, the findings are inconsistent. It is worth noting that most studies have mainly focused on conventional rice cultivars, with limited research conducted on high-quality rice cultivars.

The rising living standards have led to an increasing demand for high-quality rice among consumers, resulting in a growing popularity of cultivating high-quality rice cultivars in rice production [23]. Cultivation of rice is also considered as an effective approach to enhance saline–alkali land [24]. Hence, in this study, we selected three high-quality japonica rice cultivars (Nanjing 9108, Nanjing 5055, and Nanjing 46), which are famous for their excellent flavor and palatability, and were widely planted in Jiangsu Province of China [25]. The grain quality and starch properties of the rice were measured under different salt stress levels. The study’s objective was to examine how salt stress impacts rice quality and starch properties, with the goal of offering assistance for cultivating high-quality rice varieties in saline–alkali soil.

2. Materials and Methods

2.1. Experiment Setup

Three high-quality rice cultivars, Nanjing 9108 (NJ 9108), Nanjing 5055 (NJ 5055), and Nanjing 46 (NJ 46), were used in this study. The experiments were conducted at Yangzhou, Jiangsu Province, China during May–October in the years 2019 and 2020. In both years, the seeds were sown on 12 May and subsequently transplanted into pots on 12 June. There were three hills per pot and two seedlings per pot. Each pot was filled with 13 kg of sieved soil (sandy loam) and 0 g, 13 g, and 26 g of artificial sea salt (Blue Starfish Salt Product Co., Ltd., Hangzhou, China). Subsequently, 94.5% NaCl, 0.11% K⁺, 0.13% Mg²⁺, 0.06% Ca²⁺, and 3.7% SO₄²⁻) were added based on the weight of the soil. Based on a previous study [26], a control zero salt and two salt levels were set: 0 g·kg⁻¹ (CK, soil electrical conductivity of 227 μS cm⁻¹), 1.0 g·kg⁻¹ (T1, soil electrical conductivity of 2550 μS cm⁻¹), and 2.0 g·kg⁻¹ (T2, soil electrical conductivity of 4870 μS cm⁻¹). For each cultivar and each treatment, there were 36 replicates. For basal fertilizer, 2 g urea and 0.5 g KH₂PO₄ were added per pot before transplanting. At the tillering stage, jointing stage, and panicle initiation stage, 1 g of urea was applied per pot. The soil salinity level was monitored by the soil salinity meter (TR-6D, Shunkeda, Beijing, China) during the growth period of rice. A thin (2–3 cm) layer of water was kept until 7 days before harvest. Weeds and insects were controlled through chemical and manual methods following local high-yield practices.

2.2. Rice Yield and Quality

A total of ten pots of rice plants were sampled for all the cultivars and treatments, and yield and yield components were measured at maturity. For the measurement of rice quality, three replicates were measured.

The brown rice rate, milled rice rate, head milled rice rate, chalky grain rate, chalkiness degree, chalkiness area, ratio of length/width, amylose content, amylopectin content, and gel consistency were evaluated following the standard methods of High-Quality Paddy (GB/T 17891-1999) [27]. The protocol of Wei et al. [28] was used to measure protein concentration.

2.3. Starch Granule Morphology

The extraction and purification of starch referred to Syahariza and Tran et al. [29,30]. The starch was dried at 40 °C for 4 h, and then the starch sample were coated with gold and photographed using scanning electron microscopy (GeminiSEM 300, Carl Zeiss, Oberkochen, Germany). The particle distribution of starch was determined using a diffraction particle size analyzer (Master 2000, Malvern, England).

2.4. Starch Properties

For starch properties, including X-ray diffraction (XRD) and relative crystallinity, Fourier transform infrared (FTIR) analysis, pasting properties, and thermal properties were measured according to Zhou et al. [31].

2.5. The Fine Structure of Amylopectin

To measure the fine structure of amylopectin, the chain length distribution (CLD) of amylopectin was analyzed using the methods of Wu et al. [32]. The PA-800 Plus fluorophore-assisted carbohydrate electrophoresis (FACE) System (Beckman-Coulter, Brea, CA, USA), coupled with a solid-state laser-induced fluorescence (LIF) detector and an argon–ion laser as the excitation source, was used.

2.6. Statistical Analysis

The data presented in the tables represent the average values of ten and three replicates for yield traits and grain quality traits, respectively. A three-way analysis of variance (ANOVA) was conducted to identify the effects of treatment, year, and genotype. The ANOVA and Tukey’s test were performed using the SPSS 16.0 Statistical Software Program. Statistical significance was determined at a threshold of p < 0.05.

3. Results

3.1. Yield and Yield Components

Table 1. Effect of salinity stress on yield and yield components.

Year/Cultivar	Treatment	Number of Panicles per Pot	Number of Spikelets per Panicle	Total Spikelets per Pot	Filled Grains (%)	1000-Grain Weight (g)	Grain Yield (g pot−1)
2019/NJ 9108	CK	21.33 ± 0.58 a	125.09 ± 0.64 a	2668.27 ± 58.64 a	93.28 ± 0.14 a	26.28 ± 0.05 a	65.40 ± 1.24 a
	T1	18.67 ± 0.58 b	119.63 ± 0.60 b	2232.86 ± 58.13 b	90.27 ± 0.06 b	25.91 ± 0.04 b	52.23 ± 1.25 b
	T2	14.67 ± 0.58 c	110.71 ± 0.44 c	1623.58 ± 57.73 c	80.41 ± 0.21 c	24.36 ± 0.21 c	31.80 ± 0.97 c
2019/NJ 5055	CK	20.33 ± 0.58 a	120.40 ± 0.14 a	2448.25 ± 72.32 a	90.62 ± 0.25 a	24.85 ± 0.01 a	55.14 ± 1.48 a
	T1	17.67 ± 0.58 b	114.32 ± 0.04 b	2019.71 ± 65.91 b	85.26 ± 0.10 b	24.13 ± 0.02 b	41.56 ± 1.40 b
	T2	13.33 ± 0.58 c	107.26 ± 1.67 c	1429.49 ± 39.08 c	78.23 ± 0.22 c	23.56 ± 0.01 c	26.34 ± 0.65 c
2019/NJ 46	CK	21.67 ± 0.58 a	134.41 ± 1.76 a	2911.61 ± 39.99 a	94.12 ± 0.10 a	26.12 ± 0.03 a	71.59 ± 0.85 a
	T1	19.67 ± 0.58 b	129.21 ± 1.01 b	2540.76 ± 56.32 b	90.58 ± 0.12 b	25.48 ± 0.01 b	58.64 ± 1.26 b
	T2	15.33 ± 0.58 c	115.46 ± 0.10 c	1770.37 ± 68.06 c	83.31 ± 0.17 c	24.86 ± 0.01 c	36.67 ± 1.47 c
2020/NJ 9108	CK	21.00 ± 0.00 a	126.55 ± 0.37 a	2657.62 ± 7.76 a	92.59 ± 0.06 a	26.22 ± 0.10 a	65.40 ± 1.24 a
	T1	18.33 ± 0.58 b	119.63 ± 0.95 b	2192.92 ± 51.83 b	88.51 ± 0.13 b	25.83 ± 0.05 b	52.23 ± 1.25 b
	T2	14.33 ± 0.58 c	112.68 ± 0.16 c	1615.14 ± 67.22 c	78.94 ± 0.08 c	24.59 ± 0.03 c	31.80 ± 0.97 c
2020/NJ 5055	CK	20.00 ± 0.00 a	122.44 ± 0.21 a	2448.87 ± 4.20 a	89.20 ± 0.04 a	24.75 ± 0.03 a	55.14 ± 1.48 a
	T1	17.67 ± 0.58 b	114.86 ± 0.28 b	2029.18 ± 65.91 b	84.53 ± 0.18 b	24.02 ± 0.01 b	41.56 ± 1.40 b
	T2	12.67 ± 0.58 c	110.67 ± 0.52 c	1401.81 ± 65.37 c	76.99 ± 0.65 c	23.20 ± 0.04 c	26.34 ± 0.65 c
2020/NJ 46	CK	21.33 ± 0.58 a	133.27 ± 1.76 a	2842.34 ± 38.73 a	93.25 ± 0.09 a	25.97 ± 0.07 a	71.59 ± 0.85 a
	T1	19.33 ± 0.58 b	128.50 ± 1.61 b	2483.78 ± 42.68 b	89.42 ± 0.05 b	25.32 ± 0.04 b	58.64 ± 1.26 b
	T2	15.00 ± 0.00 c	116.28 ± 0.34 c	1744.25 ± 5.09 c	82.23 ± 0.19 c	24.72 ± 0.01 c	36.67 ± 1.47 c
Analysis of variance
Year (Y)		*	**	NS	**	**	**
Cultivar (C)		**	**	**	**	**	**
Treatment (T)		**	**	**	**	**	**
Y × C		NS	**	NS	NS	**	NS
Y × T		NS	**	NS	NS	NS	NS
C × T		NS	**	**	**	**	**
Y × C × T		*	**	NS	**	**	**

Values in the same column with different letters are significantly different (p < 0.05) for the same cultivar within the same year. *, significant at p < 0.05 level; **, significant at p < 0.01 level; NS, not statistically significant. The sampling size (n) is ten.

displays the grain yields and yield components. With the increasing levels of salinity, the yields of these three high-quality rice cultivars decreased significantly. When compared with CK, the yields of NJ 9108, NJ 5055, and NJ 46 decreased by 21.2%, 24.2%, and 18.2%, respectively, under a salinity level of 0.1%, and decreased by 51.4%, 53.0%, and 48.6%, respectively, under a salinity level of 0.2%. The salinity stress significantly impacted the number of spikelets per panicle, reducing their count. However, the effects on the ratio of filled grains were relatively minor. There were slight variations in grain weight between the CK and the salinity-stressed ones.

3.2. Appearance and Milling Quality of Rice

It appears that, with increasing levels of salinity stress over two years, the percentages of brown rice, milled rice, and head milled high-quality rice continued to increase, as shown in

Table 2. Effects of salinity stress on milling and appearance qualities of rice.

Year/Cultivar	Treatment	Brown Rice (%)	Milled Rice (%)	Head Milled Rice (%)	Chalky Area (%)	Chalky Kernel (%)	Chalkiness (%)	Kernel Length/Width
2019/NJ 9108	CK	88.17 ± 0.06 c	78.36 ± 0.12 c	71.13 ± 0.08 c	20.42 ± 0.06 b	10.22 ± 0.04 b	3.16 ± 0.05 b	1.55 ± 0.01 a
	T1	88.97 ± 0.01 b	79.66 ± 0.09 b	72.57 ± 0.02 b	19.34 ± 0.09 c	9.51 ± 0.04 c	2.56 ± 0.02 c	1.53 ± 0.01 b
	T2	89.52 ± 0.11 a	80.28 ± 0.11 a	73.71 ± 0.06 a	21.40 ± 0.02 a	10.44 ± 0.07 a	4.25 ± 0.03 a	1.51 ± 0.01 c
2019/NJ 5055	CK	88.39 ± 0.03 c	78.74 ± 0.09 c	71.44 ± 0.02 c	19.40 ± 0.06 b	9.93 ± 0.07 b	0.91 ± 0.03 b	1.50 ± 0.01 a
	T1	89.07 ± 0.05 b	79.87 ± 0.05 b	72.77 ± 0.08 b	18.62 ± 0.07 c	9.31 ± 0.03 c	0.67 ± 0.01 c	1.49 ± 0.01 a
	T2	89.78 ± 0.05 a	80.55 ± 0.09 a	73.90 ± 0.03 a	20.26 ± 0.04 a	10.10 ± 0.04 a	1.54 ± 0.09 a	1.45 ± 0.01 b
2019/NJ 46	CK	86.00 ± 0.13 c	75.70 ± 0.17 c	67.19 ± 0.04 c	15.52 ± 0.09 b	20.34 ± 0.11 b	2.41 ± 0.05 b	1.53 ± 0.01 a
	T1	87.02 ± 0.09 b	76.56 ± 0.02 b	68.29 ± 0.07 b	14.76 ± 0.11 c	19.23 ± 0.07 c	1.91 ± 0.05 c	1.52 ± 0.01 a
	T2	87.94 ± 0.08 a	77.07 ± 0.06 a	69.10 ± 0.09 a	16.37 ± 0.03 a	21.42 ± 0.08 a	3.21 ± 0.11 a	1.47 ± 0.01 b
2020/NJ 9108	CK	88.29 ± 0.06 c	78.49 ± 0.04 c	71.20 ± 0.06 c	20.58 ± 0.06 b	10.33 ± 0.05 b	3.36 ± 0.10 b	1.55 ± 0.00 a
	T1	88.99 ± 0.13 b	79.71 ± 0.11 b	72.66 ± 0.11 b	19.52 ± 0.07 c	9.66 ± 0.03 c	2.66 ± 0.04 c	1.53 ± 0.01 b
	T2	89.58 ± 0.13 a	80.33 ± 0.07 a	73.88 ± 0.05 a	21.48 ± 0.04 a	10.48 ± 0.03 a	4.35 ± 0.03 a	1.50 ± 0.01 c
2020/NJ 5055	CK	88.42 ± 0.03 c	78.79 ± 0.09 c	71.58 ± 0.04 c	19.70 ± 0.03 b	10.15 ± 0.02 b	0.96 ± 0.04 b	1.50 ± 0.01 a
	T1	89.20 ± 0.04 b	79.89 ± 0.06 b	72.90 ± 0.03 b	18.74 ± 0.03 c	9.49 ± 0.03 c	0.71 ± 0.03 c	1.50 ± 0.01 a
	T2	89.81 ± 0.07 a	80.58 ± 0.08 a	74.03 ± 0.08 a	20.41 ± 0.05 a	10.32 ± 0.04 a	1.66 ± 0.02 a	1.45 ± 0.00 b
2020/NJ 46	CK	86.12 ± 0.10 c	75.68 ± 0.20 c	67.27 ± 0.05 c	15.64 ± 0.06 b	20.63 ± 0.04 b	2.46 ± 0.02 b	1.54 ± 0.01 a
	T1	87.09 ± 0.09 b	76.66 ± 0.03 b	68.42 ± 0.03 b	14.90 ± 0.10 c	19.38 ± 0.06 c	2.01 ± 0.04 c	1.52 ± 0.01 b
	T2	87.98 ± 0.09 a	77.17 ± 0.05 a	69.38 ± 0.05 a	16.83 ± 0.05 a	21.79 ± 0.12 a	3.55 ± 0.04 a	1.48 ± 0.01 c
Analysis of variance
Year (Y)		**	*	**	**	**	**	NS
Cultivar (C)		**	**	**	**	**	**	**
Treatment (T)		**	**	**	**	**	**	**
Y × C		NS	NS	NS	NS	**	*	NS
Y × T		NS	NS	*	NS	NS	**	NS
C × T		**	**	**	**	**	**	**
Y × C × T		NS	NS	NS	**	*	**	NS

Values in the same column with different letters are significantly different (p < 0.05) for the same cultivar within the same year. *, significant at p < 0.05 level; **, significant at p < 0.01 level; NS, not statistically significant. The sampling size (n) is three.

.

Under the salinity level of 0.1%, the appearance qualities were significantly improved compared to CK. However, these qualities significantly deteriorated under the salinity level of 0.2%. Furthermore, the kernel length/width decreased significantly with increasing salt stress. The data were consistent over the two-year period.

3.3. Cooking and Eating Quality of Rice

The three high-quality cultivars showed improved cooking and eating quality levels under mild salt stress (0.1% salinity), but deteriorated under severe salt stress (0.2% salinity). This was evident in the changes in protein percentage, amylose percentage, amylopectin percentage, starch percentage, and gel consistency compared to the CK (

Table 3. Effects of salinity stress on cooking and eating quality of rice.

Year/Cultivar	Treatment	Protein Content (%)	Amylose Content (%)	Amylopectin Content (%)	Starch Content (%)	Ratio of Amylose/Amylopectin	Gel Consistency (mm)
2019/NJ 9108	CK	8.29 ± 0.04 b	14.21 ± 0.05 b	64.36 ± 0.08 b	78.56 ± 0.12 b	0.22 ± 0.00 b	84.40 ± 0.04 b
	T1	7.86 ± 0.03 c	13.60 ± 0.05 c	66.31 ± 0.14 a	79.91 ± 0.19 a	0.21 ± 0.00 c	85.48 ± 0.19 a
	T2	8.47 ± 0.02 a	14.60 ± 0.02 a	63.30 ± 0.14 c	77.89 ± 0.15 c	0.23 ± 0.00 a	83.28 ± 0.11 c
2019/NJ 5055	CK	8.87 ± 0.05 b	12.36 ± 0.09 b	68.27 ± 0.08 b	80.64 ± 0.10 b	0.18 ± 0.00 b	82.33 ± 0.15 b
	T1	8.21 ± 0.08 c	11.54 ± 0.08 c	69.81 ± 0.15 a	81.36 ± 0.08 a	0.17 ± 0.00 c	83.65 ± 0.19 a
	T2	9.34 ± 0.02 a	13.06 ± 0.05 a	66.83 ± 0.17 c	79.89 ± 0.19 c	0.20 ± 0.00 a	80.45 ± 0.05 c
2019/NJ 46	CK	7.64 ± 0.05 b	14.46 ± 0.09 b	65.35 ± 0.12 b	79.81 ± 0.19 b	0.22 ± 0.00 b	81.49 ± 0.15 b
	T1	7.14 ± 0.02 c	13.60 ± 0.05 c	66.90 ± 0.07 a	80.50 ± 0.11 a	0.20 ± 0.00 c	83.14 ± 0.08 a
	T2	8.04 ± 0.02 a	15.22 ± 0.09 a	63.99 ± 0.14 c	79.21 ± 0.08 c	0.24 ± 0.00 a	79.23 ± 0.09 c
2020/NJ 9108	CK	8.36 ± 0.03 b	14.19 ± 0.12 b	64.53 ± 0.13 b	78.72 ± 0.10 b	0.22 ± 0.00 b	84.12 ± 0.03 b
	T1	7.90 ± 0.01 c	13.55 ± 0.07 c	66.80 ± 0.08 a	80.35 ± 0.14 a	0.20 ± 0.00 c	85.58 ± 0.08 a
	T2	8.54 ± 0.02 a	14.66 ± 0.03 a	63.53 ± 0.12 c	78.19 ± 0.10 c	0.23 ± 0.00 a	83.15 ± 0.08 c
2020/NJ 5055	CK	8.91 ± 0.02 b	12.44 ± 0.05 b	68.18 ± 0.04 b	80.63 ± 0.09 b	0.18 ± 0.00 b	82.20 ± 0.14 b
	T1	8.31 ± 0.07 c	11.52 ± 0.15 c	69.42 ± 0.11 a	80.94 ± 0.20 a	0.17 ± 0.00 c	83.33 ± 0.10 a
	T2	9.48 ± 0.02 a	13.10 ± 0.02 a	66.50 ± 0.17 c	79.61 ± 0.17 c	0.20 ± 0.00 a	80.64 ± 0.26 c
2020/NJ 46	CK	7.71 ± 0.02 b	14.53 ± 0.15 b	65.29 ± 0.16 b	79.82 ± 0.10 b	0.22 ± 0.00 b	81.51 ± 0.20 b
	T1	7.24 ± 0.02 c	13.53 ± 0.13 c	67.07 ± 0.10 a	80.59 ± 0.19 a	0.20 ± 0.00 c	83.29 ± 0.15 a
	T2	8.14 ± 0.01 a	15.47 ± 0.24 a	63.91 ± 0.12 c	79.38 ± 0.31 c	0.24 ± 0.00 a	79.37 ± 0.27 c
Analysis of variance
Year (Y)		**	NS	NS	NS	NS	NS
Cultivar (C)		**	**	**	**	**	**
Treatment (T)		**	**	**	**	**	**
Y × C		NS	NS	**	**	NS	NS
Y × T		NS	NS	NS	NS	NS	NS
C × T		**	**	**	**	**	**
Y × C × T		NS	NS	*	NS	NS	*

Values in the same column with different letters are significantly different (p < 0.05) for the same cultivar within the same year. *, significant at p < 0.05 level; **, significant at p < 0.01 level; NS, not statistically significant. The sampling size (n) is three.

).

3.4. Starch Granule Morphology and Size Distribution

Figure 1 shows that the starch granules in all treatments of these three high-quality rice cultivars exhibited irregular polygons. Specifically, under the salinity level of 0.1%, the starch was tightly arranged in the endosperm, forming larger starch granules with minimal differences in particle diameter. Additionally, there were fewer gaps observed between starch granules in comparison to CK. However, under high-level salinity stress, there was a higher ratio of small- and medium-sized starch granules in the high-quality cultivars, as depicted in Figure 2.

The diameter distribution of the granules displayed a bimodal pattern, with a peak at 5–6 μm, and the performance of each cultivar remained consistent across treatments (Figure 2). The highest values were observed under 0.1% salinity, followed by the CK treatment and 0.2% salinity. In Figure 2D, the average diameter is depicted. Compared with CK, the diameters of NJ 9108, NJ 5055, and NJ 46 increased by 6.7%, 5.4%, and 9.8%, respectively, under 0.1% salinity, and decreased by 5.0%, 5.0%, and 4.3%, respectively, under 0.2% salinity.

Under mild salt stress, the sizes of starch granules in the high-quality cultivars significantly increased. Conversely, when exposed to severe salt stress, there was a reduction in the starch granule size, as evidenced by Figure 1.

3.5. Chain Length Distribution of Amylopectin

Figure 3 illustrates the chain length distribution of amylopectin in three rice cultivars. The peak type of the chain length distribution did not change under salt stress, and all cultivars displayed a bimodal distribution, with the highest peak value observed at a degree of polymerization (DP) of 12. The salinity level of 0.1% exhibited the highest peak value, followed by CK and the salt stress level of 0.2% (Figure 3).

Amylopectin chains can be grouped into four types: ‘A’, ‘B1’, ‘B2’, and ‘B3’ chains. These types are grouped according to the values of DP: 6–12, 13–24, 25–36, and greater than 37, respectively. Compared to CK, the contents of A-chain and B1-chain in each cultivar increased significantly, and the ratio of the B3-chain and the average chain length of amylopectin decreased significantly under the mild salinity level (

Table 4. The ratios of different chain lengths of amylopectin in the year 2019.

Year/Cultivar	Treatment	A-Chain Content (%)	B1-Chain Content (%)	B2-Chain Content (%)	B3-Chain Content (%)	Average Chain Length (DP)	Degree of Branching (%)
2019/NJ 9108	CK	30.98 ± 0.02 b	49.36 ± 0.03 c	8.61 ± 0.02 c	11.05 ± 0.02 b	19.60 ± 0.02 b	5.11 ± 0.01 b
	T1	31.36 ± 0.03 a	51.23 ± 0.03 a	8.81 ± 0.02 b	8.60 ± 0.04 c	18.64 ± 0.02 c	5.38 ± 0.01 a
	T2	27.46 ± 0.02 c	50.32 ± 0.02 b	9.92± 0.02 a	12.31 ± 0.03 a	20.41 ± 0.01 a	4.91 ± 0.01 c
2019/NJ 5055	CK	29.85 ± 0.02 b	48.14 ± 0.02 b	9.43 ± 0.02 b	12.58 ± 0.01 b	19.76 ± 0.01 b	5.07 ± 0.01 b
	T1	31.98 ± 0.02 a	48.66 ± 0.05 a	9.34 ± 0.01 c	10.02 ± 0.03 c	19.18 ± 0.05 c	5.21 ± 0.00 a
	T2	28.83 ± 0.01 c	48.13 ± 0.03 b	9.94 ± 0.03 a	13.10 ± 0.01 a	20.72 ± 0.03 a	4.83 ± 0.00 c
2019/NJ 46	CK	25.55 ± 0.01 c	52.32 ± 0.02 b	9.43 ± 0.03 c	12.69 ± 0.05 b	20.93 ± 0.03 b	4.78 ± 0.01 b
	T1	27.10 ± 0.04 a	54.49 ± 0.03 a	9.51 ± 0.02 b	8.90 ± 0.08 c	19.42 ± 0.06 c	5.15 ± 0.01 a
	T2	26.26 ± 0.03 b	47.86 ± 0.02 c	11.09 ± 0.05 a	14.79 ± 0.06 a	21.76 ± 0.02 a	4.61 ± 0.01 c

Values in the same column with different letters are significantly different (p < 0.05) for the same cultivar within the same year. The sampling size (n) is three.

). For severe salinity levels, the opposite results were observed. There were genotypic differences in the ratios of different chain lengths of amylopectin in the responses to salt stress. For the content of A-chain, the value was lowest in the T2 treatment for cultivars NJ 9108 and NJ 5055, but was lowest in the CK treatment for cultivar NJ 46. For the content of B1-chain, the value was lowest in the CK treatment for cultivar NJ 9108, but lowest in the T2 treatment for cultivars NJ 5055 and NJ 46. For the content of B2-chain, the lowest value was observed in the CK treatment for cultivars NJ 9108 and NJ 46, but in the T1 treatment for cultivar NJ 5055.

3.6. X-ray Diffraction (XRD) and Relative Crystallinity

Natural starch can be grouped into three types: A, B, and C. The XRD patterns of rice starch of each cultivar had two continuous strong diffraction peaks at 17° and 18°, and two weak diffraction peaks at 15° and 23°, respectively, which indicated typical A-type starch crystals (Figure 4). For all the cultivars, the relative crystallinity of the rice starch initially decreased and then increased as the salt level increased (Figure 4D).

3.7. Fourier Transform Infrared (FTIR) Analysis

Figure 5 depicts the FTIR spectra of three rice cultivars, which exhibited increases with higher salt stress levels. The band intensity ratio at 1045/1022 cm⁻¹ and 1022/995 cm⁻¹ reflects the content of ordered and unordered molecular structures in the outer region of starch granules, respectively. Under a mild salinity level (0.1%), the band intensity ratio of 1045/1022 cm⁻¹ initially decreased, and then increased at a higher salinity level (0.2%). Conversely, the trends of 1022/995 cm⁻¹ were the opposite. These findings suggest that mild salt stress decreases starch stability, while severe salt stress enhances starch stability.

3.8. Pasting Properties and Thermal Properties of Rice Starch

The Rapid Visco Analyzer (RVA) can quickly evaluate the eating and cooking quality.

Table 5. Effects of salinity level on starch pasting properties of rice.

Year/Cultivar	Treatment	Peak Viscosity (cP)	Hot Viscosity (cP)	Breakdown (cP)	Final Viscosity (cP)	Setback (cP)	Peaking Time (s)	Pasting Temperature (°C)
2019/NJ 9108	CK	2512.33 ± 18.50 b	1354.67 ± 2.52 b	1157.67 ± 16.01 b	1900.33 ± 5.13 b	−612.00 ± 13.53 b	6.30 ± 0.06 b	71.69 ± 0.07 b
	T1	2658.33 ± 11.72 a	1421.67 ± 9.07 a	1236.67 ± 3.51 a	1932.00 ± 15.39 a	−726.33 ± 4.04 c	6.10 ± 0.03 c	70.56 ± 0.08 c
	T2	2408.33 ± 10.02 c	1295.00 ± 8.19 c	1113.33 ± 2.08 c	1831.00 ± 17.06 c	−577.33 ± 7.09 a	6.49 ± 0.02 a	72.86 ± 0.03 a
2019/NJ 5055	CK	2584.33 ± 9.07 b	1633.33 ± 5.51 b	951.00 ± 3.61 b	2033.33 ± 5.03 b	−551.00 ± 4.36 b	6.43 ± 0.04 b	72.32 ± 0.05 b
	T1	2710.33 ± 10.97 a	1717.67 ± 3.79 a	992.67 ± 8.39 a	2102.33 ± 4.73 a	−608.00 ± 6.24 c	6.26 ± 0.06 c	71.56 ± 0.10 c
	T2	2420.00 ± 13.89 c	1557.33 ± 4.16 c	862.67 ± 15.82 c	1937.67 ± 3.51 c	−482.33 ± 11.37 a	6.83 ± 0.06 a	73.50 ± 0.16 a
2019/NJ 46	CK	2357.00 ± 9.54 b	1318.33 ± 14.29 b	1038.67 ± 4.93 b	1902.00 ± 9.85 b	−455.00 ± 1.73 b	6.37 ± 0.12 b	71.57 ± 0.09 b
	T1	2468.00 ± 10.54 a	1363.00 ± 9.54 a	1105.00 ± 2.00 a	1939.33 ± 6.11 a	−528.67 ± 4.93 c	6.23 ± 0.06 b	70.47 ± 0.17 c
	T2	2216.00 ± 1.00 c	1233.67 ± 5.51 c	982.33 ± 4.51 c	1825.33 ± 9.02 c	−390.67 ± 8.02 a	6.63 ± 0.06 a	72.56 ± 0.10 a
2020/NJ 9108	CK	2487.33 ± 6.03 b	1350.00 ± 5.29 b	1137.33 ± 2.52 b	1889.67 ± 3.06 b	−597.67 ± 3.21 b	6.23 ± 0.09 b	71.59 ± 0.07 b
	T1	2638.67 ± 2.52 a	1404.00 ± 3.61 a	1234.67 ± 1.53 a	1928.00 ± 6.00 a	−710.67 ± 3.51 c	6.14 ± 0.03 b	70.65 ± 0.05 c
	T2	2390.00 ± 4.58 c	1275.33 ± 4.04 c	1114.67 ± 0.58 c	1827.67 ± 7.37 c	−562.33 ± 4.04 a	6.76 ± 0.08 a	72.38 ± 0.06 a
2020/NJ 5055	CK	2575.67 ± 4.16 b	1627.00 ± 4.58 b	948.67 ± 0.58 b	2023.67 ± 11.59 b	−552.00 ± 7.94 b	6.43 ± 0.09 b	72.33 ± 0.16 b
	T1	2685.67 ± 3.06 a	1710.00 ± 4.58 a	975.67 ± 1.53 a	2091.00 ± 3.00 a	−594.67 ± 0.58 c	6.31 ± 0.07 b	71.19 ± 0.16 c
	T2	2422.67 ± 6.51 c	1548.00 ± 7.00 c	874.67 ± 0.58 c	1938.00 ± 13.23 c	−484.67 ± 7.57 a	6.87 ± 0.10 a	73.61 ± 0.06 a
2020/NJ 46	CK	2340.00 ± 6.56 b	1302.33 ± 2.52 b	1037.67 ± 6.35 b	1889.00 ± 4.36 b	−451.00 ± 3.61 b	6.33 ± 0.10 b	71.60 ± 0.07 b
	T1	2437.33 ± 5.13 a	1359.00 ± 2.00 a	1078.33 ± 3.21 a	1930.67 ± 5.77 a	−506.67 ± 4.04 c	6.19 ± 0.06 b	70.38 ± 0.06 c
	T2	2209.33 ± 7.64 c	1238.67 ± 7.57 c	970.67 ± 1.53 c	1806.00 ± 5.29 c	−403.33 ± 4.04 a	6.77 ± 0.10 a	72.50 ± 0.12 a
Analysis of variance
Year (Y)		**	**	**	**	**	*	**
Cultivar (C)		**	**	**	**	**	**	**
Treatment (T)		**	**	**	**	**	**	**
Y × C		NS	**	**	**	*	**	**
Y × T		*	NS	**	NS	**	**	NS
C × T		**	**	**	**	**	NS	**
Y × C × T		NS	*	**	NS	*	NS	**

Values in the same column with different letters are significantly different (p < 0.05) for the same cultivar within the same year. *, significant at p < 0.05 level; **, significant at p < 0.01 level; NS, not statistically significant. The sampling size (n) is three.

displays the pasting properties. As the salt stress levels increased, the peak viscosity, hot viscosity, breakdown value, and final viscosity of the three high-quality rice cultivars significantly increased at a salinity level of 0.1%. However, these values decreased significantly at a salinity level of 0.2%. On the other hand, the setback value, peaking time, and pasting temperature showed the opposite trend, with a significant decrease at a 0.1% salt stress level and a subsequent increase at a 0.2% salinity level.

The thermal properties of starch were significantly different under different salt treatments (

Table 6. Effects of salinity stress on starch thermal properties of rice.

Year/Cultivar	Treatment	To (°C)	Tp (°C)	Tc (°C)	∆Hgel (J/g)	∆Hret (J/g)	R (%)
2019/NJ 9108	CK	61.35 ± 0.04 b	66.52 ± 0.15 b	75.70 ± 0.17 b	9.16 ± 0.05 b	1.85 ± 0.01 b	20.23 ± 0.08 b
	T1	60.30 ± 0.05 c	65.26 ± 0.09 c	74.23 ± 0.13 c	8.84 ± 0.01 c	1.64 ± 0.02 c	18.51 ± 0.16 c
	T2	62.40 ± 0.08 a	67.30 ± 0.06 a	76.36 ± 0.19 a	9.41 ± 0.05 a	2.22 ± 0.01 a	23.59 ± 0.12 a
2019/NJ 5055	CK	61.18 ± 0.14 b	66.16 ± 0.06 b	75.13 ± 0.11 b	9.06 ± 0.01 b	1.78 ± 0.02 b	19.61 ± 0.24 b
	T1	60.20 ± 0.04 c	65.20 ± 0.17 c	74.39 ± 0.08 c	8.76 ± 0.02 c	1.54 ± 0.01 c	17.62 ± 0.07 c
	T2	62.13 ± 0.02 a	67.21 ± 0.03 a	76.55 ± 0.03 a	9.25 ± 0.03 a	2.26 ± 0.03 a	24.43 ± 0.26 a
2019/NJ 46	CK	62.30 ± 0.15 b	67.99 ± 0.13 b	76.59 ± 0.05 b	9.24 ± 0.02 b	1.94 ± 0.02 b	21.02 ± 0.14 b
	T1	61.30 ± 0.12 c	67.16 ± 0.06 c	75.80 ± 0.13 c	9.04 ± 0.02 c	1.74 ± 0.03 c	19.24 ± 0.37 c
	T2	63.46 ± 0.03 a	69.23 ± 0.09 a	77.59 ± 0.04 a	9.46 ± 0.02 a	2.34 ± 0.02 a	24.78 ± 0.20 a
2020/NJ 9108	CK	61.51 ± 0.04 b	66.77 ± 0.02 b	75.72 ± 0.14 b	9.21 ± 0.05 b	1.86 ± 0.01 b	20.20 ± 0.13 b
	T1	60.39 ± 0.08 c	65.34 ± 0.12 c	74.34 ± 0.12 c	8.87 ± 0.02 c	1.66 ± 0.01 c	18.76 ± 0.05 c
	T2	62.80 ± 0.59 a	67.61 ± 0.06 a	76.57 ± 0.11 a	9.37 ± 0.04 a	2.27 ± 0.06 a	24.20 ± 0.50 a
2020/NJ 5055	CK	61.30 ± 0.06 b	66.37 ± 0.04 b	75.28 ± 0.06 b	9.13 ± 0.02 b	1.83 ± 0.01 b	20.01 ± 0.05 b
	T1	60.37 ± 0.04 c	65.30 ± 0.03 c	74.43 ± 0.05 c	8.84 ± 0.02 c	1.57 ± 0.01 c	17.79 ± 0.11 c
	T2	62.30 ± 0.07 a	67.39 ± 0.06 a	76.63 ± 0.06 a	9.26 ± 0.01 a	2.25 ± 0.03 a	24.33 ± 0.32 a
2020/NJ 46	CK	62.40 ± 0.09 b	68.20 ± 0.07 b	76.46 ± 0.10 b	9.29 ± 0.02 b	1.98 ± 0.02 b	21.27 ± 0.17 b
	T1	61.56 ± 0.10 c	67.39 ± 0.08 c	75.73 ± 0.03 c	9.14 ± 0.03 c	1.75 ± 0.02 c	19.19 ± 0.21 c
	T2	63.57 ± 0.04 a	69.67 ± 0.03 a	77.69 ± 0.04 a	9.51 ± 0.04 a	2.42 ± 0.01 a	25.40 ± 0.16 a
Analysis of variance
Year (Y)		**	**	NS	**	**	NS
Cultivar (C)		**	**	**	**	**	**
Treatment (T)		**	**	**	**	**	**
Y × C		NS	NS	NS	*	NS	**
Y × T		NS	*	NS	**	NS	**
C × T		NS	**	**	**	**	**
Y × C × T		NS	NS	NS	NS	*	**

Values in the same column with different letters are significantly different (p < 0.05) for the same cultivar within the same year. *, significant at p < 0.05 level; **, significant at p < 0.01 level; NS, not statistically significant. The sampling size (n) is three.

). When compared to the CK treatments, the values of onset temperature (T_o), peak temperature (T_p), conclusion temperature (T_c), ∆H_gel, ∆H_ret, and R were at their lowest when subjected to a salt stress level of 0.1%. Conversely, these values were highest when exposed to a salinity level of 0.2%.

3.9. Relationships between Starch Properties

Figure 6 illustrates the relationships among rice quality characteristics, starch structure, and physicochemical properties. The results indicate that the A-chain content was positively correlated with gel consistency, peak viscosity, hot viscosity, breakdown value, final viscosity, and the ratio of 1022/995 cm⁻¹. On the other hand, the A-chain content was negatively correlated with setback value, peaking time, pasting temperature, T_o, T_p, T_c, relative crystallinity, the ratio of 1045/1022 cm⁻¹, and ∆H_gel. Conversely, the B3-chain showed opposite correlations compared to the A-chain. This suggests that an increase in the content of short-chain amylopectin (A-chain) leads to a decrease in starch crystal structure stability, but improves the cooking and eating quality of rice.

4. Discussion

4.1. Effects of Salt Stress on Rice Yield and Yield Components

In this study, salt stress significantly decreased grain yield. These findings align with previous research results [17,33,34]. The analysis of yield components showed that, compared to CK, the total spikelets of NJ 9108, NJ 5055, and NJ 46 decreased by 16.9%, 17.3%, and 12.7% on average in two years under moderate salt stress, respectively, and decreased by 39.2%, 42.2%, and 38.9% on average in two years under severe salt stress, respectively.

Salt stress can negatively impact rice yield due to various factors. During the seed germination period, salt stress mainly limits seed physiological water absorption and inhibits seed germination through osmotic effects [35]. At the seedling stage, salinity stress can significantly inhibit the root growth and cause chlorosis or even death of the leaves [13,14]. In addition, salt stress destroys the chloroplast structure, decreases the chlorophyll content, decreases the net photosynthesis rate, inhibits the photosynthesis and assimilate accumulation of rice plants, and reduces the transport of photosynthetic assimilates to grains, resulting in low grain plumpness [36,37]. Salt stress at the booting stage shortens the panicles seriously and significantly affects the effective panicles and 1000-grain weight [38]. These could be the reasons for the small panicle sizes, lower seed weights, and decreased percentages of filled grains for rice plants under salinity stress (Table 1).

4.2. Effects of Salinity on Appearance and Milling Quality of Rice

Chalkiness in rice is a major problem that affects rice quality and price [39]. The formation of numerous gaps between starch granules during maturity, and the resulting alterations in light reflection, are considered to be the main causes of chalkiness [39]. Under mild salt stress conditions, it has been observed that the chalkiness of salt-susceptible rice cultivars tends to increase, while the chalkiness of salt-tolerant rice cultivars tends to decrease [26]. Under mild salt stress, the chalkiness of high-quality rice cultivars, namely, NJ 9108, NJ 5055, and NJ 46, decreased significantly. Conversely, under severe salt stress, there was an observed increase in their chalkiness. These results align with findings from Rao’s research [40]. This consistency across different studies highlights the potential positive impact of mild salinity stress on the appearance quality.

Milling quality is important for rice-processing enterprises. In a study conducted by Zhang et al. [3], it was found that salt stress had a positive impact on the brown rice rate, milled rice rate, and head milled rice rate when the salinity level was below 34.2 mM NaCl. However, when the salinity level exceeded 51.3 mM NaCl, there was a significant decrease in milling quality, indicating that the effect of salt stress on rice appearance quality was dependent on the salinity level in the soil. However, in the present analysis, the milling quality of three high-quality rice cultivars, NJ 5055, NJ 9108, and NJ 46, was significantly improved under salt stress. This phenomenon can potentially be attributed to a decrease in the length/width ratio of the rice grains, which decreased breakage during the milling process and led to a significant increase in the head milled rice rate (Table 2).

4.3. Effects of Salinity on Cooking and Eating Quality and Starch Properties of Rice

Starch and protein, accounting for more than 80% of the dry weight of rice, has an important influence on the cooking and eating quality [31]. Traditionally, rice varieties with high protein and high amylose content tend to be harder, to have lower viscosity, and to be perceived to have a poorer taste compared to sticky rice varieties. Sticky rice, on the other hand, is often associated with better taste quality [41,42]. Higher protein and amylose content increase the stability of the starch crystal structure and heat resistance, which limits the expansion and leaching of starch during rice cooking and results in a harder, less sticky texture of rice [43,44,45]. It is reported that salt stress significantly increases the protein content in rice, but decreases the amylose content and gel consistency [40,46]. In this study, the protein content and amylose content decreased significantly, but the amylopectin content increased significantly under a salinity level of 0.1%. Conversely, under a salinity level of 0.2%, the protein content and amylose content increased significantly, but the amylopectin content decreased significantly. These different findings could possibly be attributed to variations in the level of salinity stress applied in the experiments. In general, the cooking and eating quality became better under low salinity stress and tended to deteriorate under severe salt stress.

The cooking and eating quality is closely linked to the RVA spectrum of its starch. Generally, it is believed that better taste correlates with a higher peak viscosity and breakdown value, as well as a lower setback value [3]. It is reported that salt stress during the reproductive growth stage has been found to slightly enhance the pasting properties of salt-tolerant rice cultivars [22]. In the current analysis, it was observed that mild salt stress treatment (T1) increased breakdown value. Additionally, the pasting properties were decreased under severe salinity stress (T2).

The fine structure of amylopectin in rice starch would influence the crystal structure, pasting properties, and thermal properties. These factors contribute to the overall quality and functionality of rice in cooking and processing [18,47]. The synthesis of amylopectin is regulated by multiple enzymes, such as starch branching enzymes (SBEs); debranching enzymes (DBEs); and three isoenzymes of soluble starch synthase (SSS): SSI, SSII, and SSIII [48]. The important function of SBE is to cut the α-1,4 glycosidic bond and link the cut short chain to the receptor short chain through α-1,6 glycoside to form amylopectin [49]. SSI extends the chain length of amylopectin and synthesizes short chains. SSII and SSIII are responsible for synthesizing medium-length and long chains, respectively [48]. In this study, the T1 treatment resulted in each cultivar having the highest percentage of short chains (A-chain, 6 ≤ DP ≤ 12) and the lowest percentage of long chains (B3-chain, DP ≥ 37). On the other hand, under the T2 treatment, each cultivar had the lowest proportion of short chains and the highest proportion of long chains of amylopectin. When the proportion of short-chain amylopectin is high, the short chains are unable to accumulate effectively into the starch crystal layer, leading to an unstable starch crystal structure and lower pasting temperatures. In contrast, a high proportion of long chains can form a long double helix structure, which strengthens intermolecular forces and stabilizes the crystal structure. This inhibits starch expansion during gelatinization and requires higher temperatures for dissociation, resulting in higher pasting temperatures [31,41,50]. The A-chain content was negatively correlated with relative crystallinity and the ratio of 1045/1022 (Figure 6), and the B3-chain content was positively correlated with relative crystallinity and the ratio of 1045/1022 (Figure 6). This indicates that salt stress changes the stability of starch crystal structures by changing the chain length distribution, thus affecting rice quality.

Hormesis may be responsible for the improvement in rice quality observed under moderate salt stress. Hormesis is a dose–response phenomenon where low doses of stress can stimulate certain processes, while high doses can have inhibitory effects [51,52]. By exposing plants to low levels of biotic or abiotic stress, adaptive responses can be triggered, which activate cellular defense mechanisms and protect the plants. While hormetic effects were observed in all three rice genotypes, significant genetic variations were noted in terms of yields, yield components, and grain quality traits. This suggests that there is potential for enhancing grain quality through the exploration of genetic variations.

5. Conclusions

Under mild salt stress, rice quality significantly improved. The appearance quality, milling quality, and cooking and eating quality were enhanced, along with an increase in starch granule size. The proportion of short-chain-length amylopectin increased, while the relative crystallinity and structural stability of starch decreased. This resulted in improved peak viscosity, hot viscosity, and breakdown value, while setback value, gelatinization temperature, and ∆H_gel decreased. On the other hand, severe salt stress had a negative impact on appearance quality and cooking and eating quality, with opposite changes in starch structure and properties compared to mild salt stress. Mild salt stress can be beneficial for high-quality rice, but may lead to a lower overall yield. Based on these findings, it is indeed feasible to cultivate high-quality rice in saline–alkali beach areas. However, careful management and appropriate strategies should be implemented to ensure that the salt stress levels remain within the acceptable range for optimal yield and quality.