1. Introduction
Salinity is a significant global challenge impacting agricultural productivity, water quality, and soil health [
1,
2]. Approximately 954 million hectares, or 20% of the irrigated land, are affected by salinization worldwide; 99 million hectares of land are affected by salinization in China [
3]. The soil salinization is becoming increasingly severe, leading to a substantial reduction in crop yields and posing a formidable challenge to global food security and agricultural sustainability [
4,
5]. Therefore, exploring effective methods to mitigate high-salinity stress and to enhance crop resilience has become crucial to ensure food security.
High-salt stress generally refers to an environment that inhibits the normal growth of rice and seriously reduces the rice yield [
3,
6]. High-salt stress leads to many adverse effects, including root growth retardation, leaf curling, reduced plant height, reduced number of tillers, reduced number of spikelets per spike, reduced grain filling rate, reduced thousand-grain weight, and ultimately, deterioration of rice quality and yield [
7]. It has been shown that some rice cultivars can maintain their yield with the application of 0.3% saline irrigation water [
8]; nevertheless, when the salt concentration exceeds 0.5%, the yield of even salt-tolerant rice varieties was substantially reduced [
9]. However, the reduction in yield of salt-tolerant rice varieties under conditions of high-salt stress was less pronounced than that observed in salt-sensitive rice varieties. It is therefore of great importance to cultivate salt-tolerant rice varieties in order to expand the production of saline-alkali land. However, Li et al. [
10] observed that under conditions of high-salt stress, the yield reduction of salt-tolerant rice varieties remained considerable. Consequently, the corresponding agronomic cultivation measures should also be implemented concurrently.
Silicon (Si) is recognized as an essential plant nutrient and has gained renowned attention owing to its involvement in regulating plant growth, development, and stress responses through various mechanisms [
11,
12,
13]. Previous studies have indicated that Si fertilizers can promote root development, activity of antioxidant enzymes, and the stability of cell membranes, thereby alleviating the inhibitory effects of salt stress on rice [
14,
15]. However, these studies have primarily focused on the effects of traditional silicon fertilizers on conventional rice varieties, with relatively less research on nano-scale silicon.
Silica nanoparticles (SiO
2 NPs), due to their unique nano-scale effects and surface characteristics, may exhibit higher absorption, translocation, and utilization efficiency within plants compared with traditional Si fertilizers [
14,
15]. Generally, SiO
2 NPs can enter plant tissues more rapidly, tightly bind with biological membranes, and form a more stable protective layer that helps in further enhancing the rice tolerance against salt stress [
14]. Additionally, SiO
2 NPs may also improve the stress resistance and rice yield by regulating hormonal balance and promoting nutrient absorption and utilization in the plants [
16]. However, a thorough exploration of the mechanisms and economic benefits of SiO
2 NPs in alleviating salt stress and improving yield and quality traits over conventional Si fertilizer is still lacking.
The development of salt-tolerant rice with the ability to maintain yield is an important achievement in the scenario of saline agriculture [
5,
17]. Nevertheless, the growth, yield, and grain quality of salt-tolerant rice are also substantially affected under high-salt stress conditions [
10,
18]. Although salt-tolerance mechanisms in rice have been explored, there has been limited research on how SiO
2 NPs improve rice growth and yield in saline environments. Therefore, the present study aimed to assess the effects of the application of SiO
2 NPs on the growth, yield, and grain quality of salt-tolerant rice varieties under higher saline conditions.
4. Discussion
Rice is a principal cereal crop among all food crops globally; however, it exhibits sensitivity to soil salinity [
23]. Therefore, developing strategies to mitigate the impact of salinity on rice growth and productivity is important for global food security [
11,
24]. The advent and use of nanotechnology have opened new prospects for agriculture, with particular emphasis on the roles on nano-fertilizers for crop improvement under normal and stress conditions [
11].
In our study, the increase in grain filling rate and grains per panicle are associated with the increased rice yield with the application of SiO
2 NPs. The synergistic application of SiO
2 NPs potentially alleviates the negative effects of soil salinity, with reduced Na+ absorption and improved photosynthetic rate, grain filling rate, and panicle weight [
12]. In our previous study, the YLY957 and JLY534 varieties achieved yields of 9.9 and 9.6 t hm
−2, respectively, under conditions of freshwater irrigation [
25]. However, when subjected to 0.6% saline irrigation, the yield of these two varieties exhibited a marked decline, with reductions of 75.8% and 68.8%, respectively. While the application of SiO
2 NPs has the potential to enhance rice yield, it remains comparatively low in comparison with that observed under freshwater irrigation. It is therefore imperative that further research be conducted on the application of salt-tolerant rice and SiO
2 NPs in order to provide a foundation for the future enhancement of rice yield.
Our results show that the main reasons for the increased yield of SiO
2 NPs are improved root growth and increased dry matter accumulation and leaf characteristics such as chlorophyll content, antioxidant enzyme activity, and K
+ content (
Figure 1,
Figure 2,
Figure 3,
Figure 4 and
Figure 5). The excessive accumulation of Na
+, Cl
−, and sulfate ions in plant roots disturbs osmotic potential, thus restricting water intake and plant growth, and in some cases, leading to plant mortality [
11]. Improvements in root morphological traits may be due to the ability of SiO
2 NPs to accelerate water and nutrient transport within plants, thereby augmenting root morpho-physiological traits [
14]. Furthermore, Yan et al. [
26] observed that Si enhanced the water absorption by increasing the total length and surface area of roots. Maghsoudi et al. [
27] suggested that the mechanisms aiding leaf chlorophyll biosynthesis and root growth may be associated with improved nutrient and water uptake in plants. Alharbi et al. [
14] found that application of SiO
2 NPs showed significant potential in ameliorating salinity stress, not only by enhancing the biosynthesis of photosynthetic pigments but also by improving physiological processes like stomatal conductance and relative water content, while reducing electrolyte leakage and proline content in saline-alkali soils. Yan et al. [
28] also showed that Si mitigates oxidative damage by modulating the activity of antioxidant enzymes such as SOD, CAT, POD, and ascorbate peroxidase (APX). Yan et al. [
26] found that the application of Si significantly increased the activities of SOD, CAT, and APX in rice. Badawy et al. [
11] reported that the application of SiO
2 NPs confers substantial benefits in enhancing ion selectivity by reducing Na
+ absorption and increasing K
+ absorption. Khan et al. [
29] demonstrated that foliar application of SiO
2 NPs enhanced the cellular elongation and ion selectivity, while mitigating the detrimental effects of Na
+ and improving plant growth in saline-alkali soils. Furthermore, deposition of Si in the roots reduces apoplastic bypass flow and provides binding sites for metals ions, resulting in decreased uptake and translocation of Na
+ from the roots to the shoots [
30]. Yan et al. [
31] found that silica reduces the net Na
+ absorption rate, possibly due to the extracellular blocking effect of Si on Na transport, thus lowering the total Na
+ accumulation in plants [
32,
33].
The use of SiO
2 NPs can not only increase the K
+ content in leaves, but it is also an effective method in agronomy to increase the concentration of available P. For example, Akhtar et al. [
34] found that SiO
2 NPs application improves the soil nutrient content, such as N, P, K, and Si, required for normal plant growth. It has been demonstrated in previous studies that the application of Si increases the availability of P, with positive effects on the uptake and utilization of P observed in rice, wheat, and other crops [
35,
36,
37]. The Si application may affect the mechanisms related to P uptake in plants, such as promoting root exudation of organic acids, thereby mobilizing P in the rhizosphere [
38]. Furthermore, silicate anions compete for the same binding sites as phosphate anions, resulting in the release of P into soil solutions and an increase in the P available to plants [
39,
40]. Akca et al. [
41] demonstrated that the application of nano-silica in conjunction with phosphate fertilizer not only enhanced the P content and utilization rate of the fertilizer in crops, but also reduced the quantity of P fertilizer required. The SiO
2 NPs improve the translocation of N, P, K, and Si from leaves to grains to support grain formation [
15,
42]. Therefore, the mechanism of reducing salt stress by increasing P content of nano-silicon oxide should be further studied in the future.
Previous studies demonstrated that the application of Si can enhance the quality and nutritional value of rice grains [
43,
44,
45]. However, research on the impact of SiO
2 NPs on the quality of rice under higher saline conditions is relatively scarce. Our results indicated that the application of SiO
2 NPs improved the processing and appearance quality as well as the cooking and taste quality of rice. Salt stress inhibits the supply of nutrients, especially during the grain filling stage, and limits the transport of photosynthate partitioning into the grains, leading to a loose arrangement of starch granules within the endosperm that results in the formation of cavities and chalkiness in rice grains [
46,
47]. This study also showed that the application of SiO
2 NPs can reduce grain chalkiness, with improved milling and head rice yield. Our results are consistent with Lanning et al. [
48], who reported that the starch granules in the chalky areas of rice grains exhibited a blocky or granular structure with porous and loose arrangement characteristics that caused a reduction in the toughness and milling quality of the rice grains.
The cooking and eating quality attributes of rice are deeply influenced by its starch components, i.e., amylose and amylopectin, as well as the presence of structural and functional proteins [
7]. It is commonly observed that the higher the concentration of amylose and protein, the higher the viscosity of rice, as amylose and protein can enhance the thermal stability of the starch crystalline matrix, thereby limiting gelatinization and solubilization during cooking. Additionally, an increase in protein concentration can hinder the absorption of water by starch granules, which adversely affects the rice flavor [
49]. In our study, the application of SiO
2 NPs significantly increased the levels of total starch and amylose, while reducing the protein content, hence, improving the taste value of the rice. On the other hand, a high gel consistency is conducive to improving the viscosity and hardness of rice, thereby enhancing the taste value. The quality of rice when cooked and consumed is closely related to the RVA profile of the starch. Typically, a superior taste is indicated by higher PV and BD contrasted with a lower SB. Jin et al. [
18] found a significant reduction in PV and BD under salinity compared with CK. Conversely, in our study, the application of SiO
2 NPs treatment led to a marked increase in both PV and BD, which are important for the culinary taste quality.
The complex structure of amylopectin and its chain length distribution play a key role in the formation of rice cooking and taste [
47]. According to Yao et al. [
22], saline conditions can reconfigure the starch composition in rice, especially by changing the distribution pattern of amylose and amylopectin chain lengths. Notably, salt-tolerant rice varieties exhibit an increase in intermediate and extended chains under saline conditions, which typically promotes the formation of a more robust double helix conformation, affecting the crystallinity and gelatinization characteristics of starch. It has been observed that saline stress is also associated with a reduction in the proportion of short chains (designated as A chains and B1 chains) while promoting the prevalence of extended chains (B2 chains and B3 chains) [
18]. Yao et al. [
22] suggested that the increase in gelatinization temperature may be related to the reduction in the number of amylopectin short chains as well as an increase in intermediate and long chains. Additionally, the enthalpy value is positively correlated with gelatinization temperature and crystallinity [
2]. Our study also showed that the application of SiO
2 NPs led to a decrease in gelatinization temperature, and the reduction in crystallinity is mainly due to the decrease in extended chains (B2 + B3) in amylopectin. These shifts are supposed to have repercussions on the crystalline configuration and the overall stability of the starch, which may subsequently alter the sensory attributes and mouthfeel of the rice.