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Low-concentration NaCl foliar spraying enhances photosynthesis, mineral concentration, and fruit quality of strawberry during greenhouse high-temperature periods

BMC Plant Biology volume 25, Article number: 487 (2025) Cite this article

Abstract

Background

Root-applied low-concentration NaCl (e.g., 40 mM) has been shown to maintain the yield and enhance both nutritional and functional quality in salt-sensitive strawberry cultivars. However, the potential benefits of foliar low-concentration NaCl application on strawberry plants have rarely been investigated to avoid secondary soil salinization through long-term root application, especially the effects on fruit quality during greenhouse high-temperature periods.

Methods

Strawberry (Fragaria × ananassa Duch. cv. Benihoppe) plants were foliar sprayed with 0 (CK), 5, 10, 15, and 20 mM NaCl solutions once a day from fruit setting to ripening under high-temperature period in a solar greenhouse where quality deterioration was observed. The physiological traits of strawberry leaves and the quality of fruits were measured to explore the beneficial effects of low-concentration NaCl solutions.

Results

Compared with the control (CK), foliar spraying with low-concentration NaCl solutions significantly increased the photosynthetic efficiency and mineral element content of strawberry leaves; enhanced the color of strawberry fruits; and increased the weight, size, color, soluble sugar content (e.g., glucose, fructose, and sucrose), and secondary metabolite production (e.g., vitamin C, phenolics, and flavonoids) of strawberry fruits. Additionally, foliar spraying with low-concentration NaCl solutions significantly decreased the organic acid content (e.g., malic and citric acids) in strawberry fruits. According to redundancy analysis, foliar spraying with NaCl induced the accumulation of Na in strawberry leaves and Cl in strawberry fruits, which may have contributed to the increase in physiological activity of leaves and the improvement in fruit quality, respectively.

Conclusion

The foliar spraying of 10–15 mM NaCl, an economical and beneficial method, improves photosynthetic efficiency, thereby promoting formation and accumulation of strawberry fruit nutrients under high-temperature period in greenhouses.

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Introduction

Strawberry (Fragaria × ananassa Duch.) is one of the most popular fruits because of its abundance of health-promoting vitamins, minerals, and secondary antioxidants [1]. Cultivated strawberries gradually become commercially important fruits around the world. It is always cultivated in unprotected and protected fields around the world, including solar greenhouses. Its transplants can be grown in greenhouse cultivation from late August to early September, and its fruits can be harvested throughout the growing season from December to May of the following year. Most strawberry growers encounter challenges of high-temperature exceeding 28 ℃ during greenhouse cultivation in the spring [2]. Under high-temperature period, particularly in the daytime, a decrease in soluble protein [3] and a deactivation of the photosystem II reaction center result in severe photosynthetic photoinhibition of strawberry leaves [4, 5]. Furthermore, increases in temperature can also reduce strawberry fruit size, yield, and quality [6]. In response to high-temperature stress, several approaches, such as the selection of heat-tolerant strawberry varieties [7], moderate shading [8], and foliar spraying of silica fertilizer [9], have been used to alleviate the negative effects of high-temperature stress on strawberry growth and/or quality in greenhouse cultivation. Additionally, the beneficial effect on improvement of strawberry fruit quality was innovatively found in the soil cultivating system under lower level of NaCl (e.g., 40 mM) treatment conditions [10]. Therefore, it is particularly important to enhance the yield and quality of strawberries during greenhouse high-temperature periods to meet the demands of consumers.

Recently, the mild to moderate NaCl treatment (10–50 mM) was considered as an economical and beneficial eustressor which had been proved to enhance the physical quality and functional composition of fruit and vegetables in soil or soilless cropping systems via modulation of electrical conductivity (EC) of nutrient solution composition and/or concentration [11]. Thus, its application could be a useful tool in modern horticulture towards fulfilling the demands of consumers for high quality and functional fruit and vegetable products. However, most horticultural crops are still cultivated in soil, and there is a risk of secondary salinization if low-concentrations of NaCl are applied to soils over a long period of time. This phenomenon may have negative effects on the growth and development of many horticultural crops, especial for some salt-sensitive plants. Therefore, our previous studies indicated that the using method from root application to foliar spraying of low-concentration NaCl solutions significantly alleviated the inhibitory effects of low temperature on growth and development of cucumber seedlings [12], promoting the dry weight accumulation of non-heading cabbage [13], and improving the nutritional value of cherry radishes [14]. These results not only broaden the application of low-concentration NaCl solutions but also further demonstrate the beneficial regulatory effects of foliar spraying low-concentration NaCl solutions, because supplemental foliar spraying of many horticultural plants is also helpful for minimizing soil barriers and maintaining higher nutrient use efficiency to acclimate to diverse sub-optimal stress environments [15].

In fact, it has been established for many years that low-concentration of NaCl (< 50 mM) exhibits beneficial regulatory effect [16]. On the one hand, Na, as a cheap and safe regulator, can partially replace K to alleviate the reduction in crop yield caused by K deficiency [16]. Meanwhile, an insufficient amount of Na in the soil can inhibit pyruvate transfer, chlorophyll synthesis, and stomatal opening, thereby causing decreases in photosynthetic efficiency and crop yield [17, 18]. Therefore, an appropriate low-concentration of Na can be used as a beneficial yet non-essential element for plant growth and development [17, 19,20,21]. On the other hand, the micronutrient chlorine (Cl), another plant essential nutrient, can accumulate in plant tissues at 0.2–0.4 mg g− 1 dry weight (DW) to meet normal physiological metabolic requirements [22, 23], improving regulation of photosynthetic oxygen-releasing complex activity, as well as asparagine synthetase and amylase activities [24,25,26]. However, Cl can also accumulate to 15–50 mg g− 1 DW, which is similar to the content of the macronutrient K in some plant leaves [24]. Under these conditions, the Cl ion does not produce toxic effects; instead, it maintains cell turgor pressure, promotes cell expansion, and increases water and CO2 utilization [24, 27, 28]. Thus, an appropriate low-concentration of NaCl plays important roles in the production of functional sprouts [29, 30], as well as high-quality tomatoes [31], strawberries [10] and lettuce [32].

Despite this fact, it is unknown whether spraying low-concentration NaCl solutions can regulate the physiology of strawberry leaves and the quality of fruits, during the high-temperature period in solar greenhouses. Therefore, this study was conducted to investigate the effects of foliar spraying of low-concentration NaCl on photosynthetic efficiency, mineral content, and fruit quality of strawberry under high-temperature conditions in a solar greenhouse. Additionally, it aimed to evaluate the potential benefits of an appropriate low-concentration of NaCl on leaf physiology and fruit quality.

Results

Photosynthetic efficiency of strawberry leaves

The photosynthetic parameters of strawberry leaves sprayed with low-concentration NaCl solutions were significantly different (Fig. 1; P < 0.05) from those of the controls. As NaCl concentration increased, Pn, Gs, and Tr first exhibited an upward trend and then a downward trend, reaching their highest values at 10 mM NaCl. Increases of 17.88% in Pn (from 21.51 to 25.36 µmol m− 2 s− 1), 27.71% in Gs (from 226.11 to 288.77 µmol m− 2s− 1), and 34.56% in Tr (from 4.53 to 6.09 µmol m− 2 s− 1) were recorded at 10 mM NaCl relative to the controls. By contrast, the lowest value of Ci was recorded at 10 mM NaCl, although there was no significant difference at 15 mM NaCl compared with the control (P > 0.05). These results indicate that foliar spraying with 10 mM NaCl can improve the photosynthetic efficiency of strawberry under high-temperature conditions in the greenhouse.

Fig. 1
figure 1

Effects of foliar spraying of solutions with different concentrations of NaCl on photosynthetic parameters of strawberry leaves. Pn: photosynthetic rate (A), Ci: intercellular CO2 concentration (B), Gs: stomatal conductance (C), and Tr: transpiration rate (D). Lowercase letters indicate significant differences calculated by Duncan’s multiple range test at P < 0.05

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Foliar nutrient content

Foliar spraying of low-concentration NaCl solutions resulted in significant accumulation of Na and Cl in strawberry leaves compared with the controls (Table 1; P < 0.05). The contents of Na and Cl exceeded those of the controls by 1.50-fold at 15 mM NaCl and 2.19-fold at 20 mM NaCl. Under these conditions, the NaCl-sprayed strawberry leaves also showed increasing trends in N, P, K, Fe, Zn, and Ca contents compared with the controls. The highest contents of Fe, Zn, and Ca were recorded at 15 mM NaCl and were 32.71% (from 209.22 to 277.65 mg kg− 1 DW), 17.96% (from 17.43 to 20.56 mg kg− 1 DW), and 17.97% (from 786.80 to 928.19 mg kg− 1 DW) higher than those of the controls, respectively. However, foliar spraying with 20 mM NaCl resulted in only a 6.14% increase in N (from 30.72 to 32.61 g kg− 1 DW), a 5.88% increase in P (from 5.21 to 5.52 g kg− 1 DW), and a 14.79% increase in K (from 19.50 to 22.38 g kg− 1 DW) compared with the controls. These results reveal that a lower NaCl concentration had a better effect on leaf mineral element accumulation, especially for Fe, Zn, and Ca, than a higher NaCl concentration. In other words, foliar spraying with an appropriate low-concentration of NaCl can significantly increase the contents of Fe, Zn, Ca, and K and alleviate their corresponding deficiencies in strawberry leaves under high-temperature conditions in the greenhouse (P < 0.05).

Table 1 The content of minerals such as nitrogen (N), phosphorus (P), potassium (K), iron (Fe), zinc (Zn), calcium (Ca), Chlorine (Cl), and sodium (Na) in strawberry leaves under foliar spraying of solutions with different concentrations of NaCl
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Fruit nutrient content

Foliar spraying of low-concentration NaCl solutions resulted in a large and significant increase in the accumulation of Cl (1.49- to 2.30-fold) in strawberry fruit relative to the control (P < 0.05), whereas its effect on Na accumulation was much more modest (Table 2). Furthermore, decreasing trends in the contents of K, Ca, Fe, and Zn in strawberry fruit were noted after foliar spraying of low-concentration NaCl solutions compared with the controls (P < 0.05).

Table 2 The content of minerals such as potassium (K), calcium (Ca), iron (Fe), zinc (Zn), Chlorine (Cl), and sodium (Na) in strawberry fruits under foliar spraying of solutions with different concentrations of NaCl
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Fruit weight, size, and color

Foliar spraying of low-concentration NaCl solutions significantly increased strawberry fruit weight and size compared with the control (Table 2; P < 0.05). The highest fruit weight (21.76 g) and diameter (5.54 cm) were observed at 15 mM NaCl concentration, representing increases of 35.66% and 18.84%, respectively, compared to the control values (18.31 g for weight and 4.38 cm for diameter). Moreover, foliar spraying of low-concentration NaCl solutions also had significant effects on strawberry fruit color parameters (Table 3; P < 0.05). The L*-index and a*-index of strawberry leaves increased in response to NaCl spraying: the highest values of the L*-index and a*-index were recorded at 15 mM NaCl and were 6.33% (from 36.16 to 38.45) and 7.99% (from 1.39 to 1.50) higher than those of the control, respectively. However, there was no significant differencein the b*-index between the control and treated strawberry leaves (P > 0.05). These results indicate that foliar spraying with an appropriate low-concentration of NaCl can promote the growth and improve the appearance of strawberry fruit growing under high-temperature conditions in the greenhouse.

Table 3 The weight, size, and color of strawberry fruits under foliar spraying of solutions with different concentrations of NaCl
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Soluble sugars and their components in strawberry fruit

Soluble sugars and their components are major contributors to the nutrition, taste, and flavor of strawberry fruit. In this study, the contents of total soluble sugar, glucose, fructose, and sucrose in strawberry fruit were analyzed after foliar spraying of low-concentration NaCl solutions (Fig. 2). The total soluble sugar content of strawberry fruit increased significantly after foliar spraying of NaCl, and the content of total soluble sugar increased by 15.45% (from 4.63 to 5.35% FW) at 15 mM NaCl compared with the control (Fig. 2A; P < 0.05). When the NaCl concentration exceeded 5 mM, the contents of glucose, fructose, and sucrose first increased rapidly, then decreased slightly for glucose and sucrose only. Significant increases of 72.95% (from 16.79 to 29.04 mg g− 1 FW), 88.48% (from 22.11 to 41.67 mg g− 1 FW), and 77.41% (from 25.76 to 45.7 mg g− 1 FW) were recorded at 10 mM NaCl for glucose, 20 mM NaCl for fructose, and 15 mM NaCl for sucrose compared with the controls, respectively (Fig. 2B–D; P < 0.05). These results reveal that low-concentrations of NaCl can effectively promote the accumulation of total sugars in strawberry fruit, with differences noted for different sugars.

Fig. 2
figure 2

Effects of foliar spraying of solutions with different concentrations of NaCl on the contents of total soluble sugars (A), fructose (B), glucose (C), and sucrose (D) in strawberry fruits. Lowercase letters indicate significant differences calculated by Duncan’s multiple range test at P < 0.05

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Titratable acidity and organic acids in strawberry fruit

Organic acids can affect the taste, flavor and color of high-quality fruit. Thus, we measured titratable acidity and contents of citric acid and malic acid in strawberry fruit after foliar spraying of low-concentration NaCl solutions (Fig. 3). The total organic acid content (0.49% FW) first decreased and then increased with increasing NaCl concentration; decreases of 18.67% in titratable acidity, 24.23% in citric acid content (6.67 mg g− 1 FW), and 7.76% in malic acid content (2.64 mg g− 1 FW) were recorded at 10 mM NaCl compared with the control values (0.6% FW for total organic acid content, 8.8 mg g− 1 FW for citric acid content and 2.86 mg g− 1 FW malic acid content). These results indicate that foliar spraying of 10 mM NaCl can effectively reduce the production of citric acid, thereby improving the taste of strawberry fruit growing under high-temperature conditions in the greenhouse.

Fig. 3
figure 3

Effects of foliar spraying of solutions with different concentrations of NaCl on the contents of titratable acid (A), citric acid (B), and malic acid (C) of strawberry fruits. Lowercase letters indicate significant differences calculated by Duncan’s multiple range test at P < 0.05

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Vitamin C, total phenolics, and total flavonoids in strawberry fruit

The effects of foliar spraying of low-concentration NaCl solutions on the antioxidant quality of strawberry fruit were evaluated by measuring the contents of vitamin C, total phenolics, and total flavonoids; these compounds are abundant in strawberry fruit and beneficial for human health. Foliar spraying of 5 mM NaCl had no significant effect on the contents of vitamin C, total phenolics, and total flavonoids in strawberry fruit compared with the controls (P > 0.05), but foliar spraying of 10 mM NaCl increased their contents. Significant increases of 35.34% (from 38.67 to 52.34 mg 100 g − 1 FW), 32.27% (from 238.17 to 315.03 mg 100 g − 1 FW), and 36.45% (from 138.26 to 188.66 mg 100 g − 1 FW) were recorded for vitamin C at 10 mM NaCl, total phenolics at 10 mM NaCl, and total flavonoids at 15 mM NaCl compared with controls, respectively (Fig. 4; P < 0.05). However, no significant differences were noted between 15 mM and 20 mM NaCl, except for vitamin C, indicating that foliar spraying with an appropriate low-concentration of NaCl can enhance the antioxidant qualities of strawberry fruit by increasing the contents of vitamin C, total phenolics, and total flavonoids.

Fig. 4
figure 4

Effects of foliar spraying of solutions with different concentrations of NaCl on the contents of vitamin C (A), total flavonoids (B), and total phenolics (C) in strawberry fruits. Lowercase letters indicate significant differences calculated by Duncan’s multiple range test at P < 0.05

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Relationships between the accumulation of Na or Cl and physicochemical parameters

By taking the contents of Na and Cl in strawberry leaves or fruits as independent variables and all measured leaf physiology and fruit quality parameters as response variables, we identified important correlations (Fig. 5). In strawberry leaves, Na accumulation was positively correlated with the contents of K, Zn, Ca, Fe, Pn, Gs, and Tr and negatively correlated with the content of Ci. Accumulation of Cl was positively correlated with the contents of N and P (Fig. 5A) and strongly positively correlated with most quality parameters (Fig. 5B), suggesting that foliar spraying of low-concentration NaCl solutions improves strawberry fruit appearance and other important qualities through fruit Cl accumulation. By contrast, photosynthetic efficiency, macronutrient contents (e.g., K, and Ca), and micronutrient contents (e.g., Fe, and Zn) appeared to be significantly enhanced through Na accumulation rather than Cl accumulation in strawberry leaves after foliar spraying of low-concentration NaCl solutions (P < 0.05).

Fig. 5
figure 5

Biplots of redundancy analysis (RDA) between accumulation of Na and Cl in strawberry leaves (A) and fruits (B) and measurements of multiple physiological and fruit-quality parameters. Red arrows indicate the accumulation of Na or Cl. Blue arrows indicate leaf physiological parameters and fruit-quality parameters. Symbols indicate the concentrations of NaCl applied as foliar sprays: 0, 5, 10, 15, and 20 mM. Abbreviations: Na, sodium; Cl, chlorine; N, nitrogen; P, phosphorus; K, potassium; Fe, iron; Zn, zinc; Ca, calcium; Pn, photosynthetic rate; Ci, intercellular CO2 concentration; Gs, stomatal conductance; Tr, transpiration rate; Fw, fruit weight; Fl, fruit length; Fd, fruit diameter; L*, color value from black to white; a*, color value from green to red; b*, color value from blue to yellow; Tss, total soluble sugars; Fru, fructose; Glu, glucose; Suc, sucrose; Ta, titratable acid; Cac, citric acid; Ma, malic acid; Vc, vitamin C; Tf, total flavonoids; Tp, total phenolics

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Discussion

Effect of foliar spraying of NaCl solutions on the physiological activity of strawberry leaves

Photosynthesis, one of the major metabolic processes in plants, is sensitive to many abiotic factors [33]. The inhibition of photosynthesis by NaCl treatment has been widely reported [34, 35]. However, several studies suggest that soaking broccoli seeds with low-concentration NaCl solution, or spraying them in potted mulberry trees improved the photosynthetic efficiency [36, 37], which is consistent with our findings of this study (Fig. 1). The reasons are mainly reflected in enhanced chloroplast function [36], increased antioxidant enzyme activity [38], and upregulated PSII reaction center activity [39]. Other studies have shown that Cl− provided at beneficial macronutrient levels improves N use efficiency through accumulation and activity of important enzymes involved in NO3− assimilation into amino acids [40], which could increase the mineral N uptake after foliar spraying of low-concentration NaCl during greenhouse high-temperature periods (Table 1). Furthermore, Pn and/or Tr were strongly positively correlated with the contents of Zn, Fe, Ca, and K in strawberry leaves (Fig. 5A), which have been observed to play important roles in plant growth and development under stress conditions such as high temperature [15]. Additionally, enhanced transpiration reduces the leaf temperature through water dissipation and can thus mitigate the negative effects of high temperature on strawberry plants [41].

However, with greater consumption of water through enhanced transpiration under conditions of high temperature, the necessary osmotic adjustment, in terms of the accumulation of inorganic ions (e.g., Na and Cl) and organic substances (e.g., proline and sucrose), may be an important mechanism for maintaining high leaf water potential and compensating for water consumption [26, 42]. Furthermore, osmotic adjustment by Na and/or Cl was beneficial for than adjustment by organic solutes because less energy and carbon are needed [40]. In this study, the contents of Na and Cl in strawberry leaves were much higher compared with the controls after foliar spraying of low-concentration NaCl solutions (Table 1). Thus, the significant increase in relative water content of strawberry leaves may be due to osmotic adjustment by Na and/or Cl accumulation (Table S1). Our data indicated that the fold change in Na content was lower than that in Cl content after foliar spraying, but Na concentration itself was consistently higher than Cl concentration in strawberry leaves (Table 1). Using redundancy analysis, we observed that Na accumulation in strawberry leaves may have made a substantial contribution to the increase in physiological activity by enhancing photosynthetic efficiency (Fig. 5A) and maintaining relative water content (Table S1). This would support, at least in part, the contention that an appropriate amount of Na can be considered to be a nutrient, one that promotes the physiological activity of plant leaves by maintaining greater leaf water potential and stimulating cell enlargement in diverse environments [19].

Effect of foliar spraying of NaCl solutions on the quality of strawberry fruit

In general, consumers tend to prefer strawberry fruit with a good appearance, high sugar content, and low organic acid content [43,44,45]. However, increasing soil salinity, which is a major environmental constraint, reduces the yield and quality of strawberry fruit [38, 46]. Therefore, a soilless culture system with a nutrient solution that contains low-concentrations of NaCl was developed to produce high-quality fruit after short-term irrigation of bearing strawberry [47, 48]. Similar findings have also been reported for other plants, including tomato fruit [31, 49], wheat microgreens [50], barley seedlings [30], and broccoli sprouts [29, 36].

In this study, foliar spraying of low-concentration NaCl solutions also improved the overall taste and nutritional value of strawberry under high-temperature period (Table 3; Figs. 2 and 3). These results could be explained by higher transport of total assimilates to growing fruit from strawberry leaves with greater photosynthetic efficiency, which then enhanced the biosynthesis of glucose, fructose, and sucrose in fruit (Fig. 2). Additionally, foliar spraying reduced the contents of organic acids, especially titratable acidity and citric acid, in fruits of strawberry plants treated with 10 mM NaCl compared with the controls (Fig. 3). The decrease in citric acid content of strawberry fruit may be due to inhibition of the tricarboxylic acid cycle in sink organs [51], resulting in improved organoleptic qualities of strawberry fruit.

Vitamin C, phenolics, and flavonoids are important secondary metabolites in plants, and they possess strong antioxidant properties [30, 52]. Treatment with NaCl is believed to promote the accumulation of secondary metabolites in the edible parts of plants and to enhance their antioxidant capacity [29, 30, 50, 52]. In this study, secondary metabolite contents were enhanced in strawberry fruit by foliar spraying of low-concentration NaCl solutions (Fig. 4). The contents of vitamin C and total phenolics in strawberry fruit increased by 35.34% and 32.27% at 10 mM NaCl compared with the control, respectively, whereas the content of total flavonoids increased by 36.45% at 15 mM NaCl. Thus, low-concentrations of NaCl had significant positive regulatory effects on the synthesis of vitamin C, phenolics, and flavonoids in strawberry fruits. This increase may be due to upregulation of mRNA and protein levels of phenylalanine ammonia lyase, cinnamate-4-hydroxylase, and 4-coumarate-CoA ligase after foliar spraying of low-concentration NaCl solutions [10, 30, 53], as these enzymes participate in the synthesis of polyphenols, including phenolics and flavonoids. Therefore, increased synthesis of secondary metabolites can further improve the functional value better than nutritional value of strawberry fruit after foliar spraying of low-concentration NaCl solutions under high-temperature period in a greenhouse.

Few studies have simultaneously analyzed Na and Cl concentrations in fruit, and it is unclear whether the accumulation of Na and Cl is beneficial for the main properties of strawberry fruit. Interestingly, foliar spraying of low-concentration NaCl solutions in our study resulted in a proportional increase in the Cl concentration in strawberry fruit compared with the control, but the accumulation of Na in strawberry fruits was more limited (Table 2). This result suggests that Cl is more prone to accumulate in strawberry fruit compared to Na after foliar spraying of low-concentration NaCl solutions due to the highly active absorption and transport capacity of Cl [24,25,26]. Using redundancy analysis, we found that the NaCl-induced accumulation of Cl in strawberry fruit have strongly contributed to the improvement in fruit quality by increasing fruit weight, soluble sugar content, and secondary metabolite production and decreasing organic acid content (Fig. 5B). Given that Cl is not assimilated throughout anabolic metabolism, its accumulation efficiency into strawberry fruit is 2.3 times higher than that of Na, determining more negative osmotic potential and higher turgor to enhance fruit quality of strawberry by specifically improving the particular function in osmoregulation [24,25,26, 54].

It is common knowledge that the nutritional and physiological status of fruits and vegetables prior to harvest can positively affect their postharvest quality and self-life [55]. Thus, some studies tried to the application of K and Ca fertilizers during preharvest periods, including foliar spray and root irrigation, which could increase the content of K and Ca in the product organs of fruit and vegetable crops. Their postharvest quality and self-life were significantly enhanced through the coordinating regulation of osmotic balance and signaling pathways by K and Ca on cell wall degradation enzymes, antioxidant enzymes, and organic substances of bell peppers and cherry tomatoes [55, 56]. The findings of the current study suggest that low-concentration NaCl foliar spray slightly decreased the content of K and Ca in strawberry fruits during the harvest (Table 2), which may exert certain impacts on their postharvest quality and shelf life during storage. However, significant increases in secondary metabolites were also found in this study (Fig. 4), which may play an important role in enhancing the postharvest quality and shelf life of strawberries [57]. Moreover, the application of low-concentration NaCl also enhanced pathogen resistance, which reduced postharvest infections and product losses in fresh-cut lettuce [11]. Furthermore, this study presents the first documented evidence on the efficacy of foliar application of low-concentration NaCl in enhancing strawberry fruit quality. While current research provides limited data regarding the direct impact of low-concentration NaCl treatments on postharvest parameters and shelf-life extension, it should be noted that observed alterations in phytochemical compounds (e.g. organic acids, secondary metabolite levels) do not consistently correlate with improved postharvest fruit quality. Given this knowledge gap, subsequent investigations should employ a systematic approach to evaluate the longitudinal effects of foliar spraying low-concentration NaCl (as a high-temperature stress mitigation strategy) on both the postharvest physiological characteristics and commercial shelf stability of strawberry fruits.

Many studies have suggested appropriate concentrations of NaCl for enhancing the edible qualities of plants: 25 mM in cherry tomato [31], 10 mM in barley seedlings [30], 40 mM and 75 mM in strawberry [47], and 40 to 80 mM in broccoli sprouts [36]. Here, we found that 10 to 15 mM NaCl were appropriate concentrations for enhancing photosynthetic efficiency, mineral element content, and fruit quality of strawberry by foliar spraying under high-temperature conditions in a greenhouse. The differences in findings among studies may be due to different application methods, exposure times, experimental systems, sensitivities of plant cultivars, and developmental stages.

Conclusions

Foliar spraying of low-concentration NaCl solutions promotes the photosynthetic efficiency of strawberry leaves and increases the fruit weight, soluble sugar content (e.g., glucose, fructose, and sucrose), and secondary metabolite production (e.g., vitamin C, phenolics, and flavonoids), thus enhancing the quality of strawberry fruit. Redundancy analysis showed that the accumulation of Na in strawberry leaves and Cl in strawberry fruits largely enhanced the physiological activity of leaves and the quality of fruits, respectively. Therefore, we propose that foliar spraying of low-concentration NaCl solutions (10 to 15 mM) is a beneficial and economical method that promotes the growth and development of strawberry plants during the fruit production phase during greenhouse high-temperature period. Future studies of strawberry plants cultivated under high-temperature period in greenhouses will investigate whether foliar application of low-concentration NaCl influences the postharvest self-life and quality of fruit.

Materials and methods

Experimental site and materials

Strawberry seedings (Fragaria × ananassa Duch. ‘Benihoppe’) were bought from Gaodi Agricultural Company, Laiyuan County, Baoding City (115°66′26′′E, 39°02′05′′N), located in Hebei Province, China. Then, they were planted in the Chinese Solar Greenhouse on 2nd September at the Zengxian Agricultural Corporation, Xushui District, Baoding City of Hebei Province, China. The Chinese Solar Greenhouse features a south-facing semi-arch structure with thick insulated back walls, covered by plastic film to maximize sunlight transmission. It only utilizes thermal blankets for nighttime insulation in winter and natural ventilation systems. The contents of organic matter, total nitrogen, available nitrogen, available phosphorus, and available potassium in the cultivated strawberry soil were 1.32%, 0.16%, 112 mg kg− 1, 61.2 mg kg− 1, and 92 mg kg− 1, respectively. Its cation exchange capacity of the soil was 11.6 mg 100 g− 1, with a pH value of 6.61. The cultivated bed was 50 cm in width and 30 cm in height and covered with black polyethylene mulching film. In each cultivated bed, two rows of strawberries were planted with a 20 cm plant spacing and 25 cm row spacing. Drip irrigation tapes with a diameter of 20 mm and 1 mm orifices were placed at the center of each cultivated bed and under the black polyethylene mulching film. The distance between cultivated beds is 50 cm in the facility. In spring, when internal environmental parameters significantly changed in the Chinese Solar Greenhouse, charactered by exceeding of 25 ℃ at day and 8 ℃ at night (Fig. 6), longer than 12 h of photoperiod and approximately 50% relative humidity, the present study was carried out. A total of 15 cultivated beds (each measuring 2 m×2 m, 44 plants) were selected for use in the experimental treatments. The two secondary fruits from a strawberry inflorescence were firstly selected in every area of cultivated bed, and all other flowers and fruits were eliminated.

Fig. 6
figure 6

Dynamic changes in daily ambient temperature in a solar greenhouse during the study period

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Experimental design

Five different NaCl solutions, including 0 mM (0.21 dS m− 1, CK), 5 mM (0.52 dS m− 1), 10 mM (0.98 dS m− 1), 15 mM (1.63 dS m− 1) and 20 mM (2.12 dS m− 1), were created by adding different amounts of NaCl to groundwater collected from the experimental area with an electrical conductivity (EC) of 0.21 dS m− 1. To induce the infiltration of NaCl into foliage, 0.2 mL of organosilicone adjuvant was added to each 1 L of the different NaCl solution. Analytical grade NaCl and organosilicone adjuvant were purchased from Wanke Corporation (Baoding, China). Three replicates were established for each treatment, and fifteen replicates were completely randomized arranged to 15 pre-selected cultivated beds. From 9:30 to 10:00 AM each day, 400 mL of different concentration NaCl solution was loaded into a 1 L manual sprayer. Then, 0, 5, 10, 15, and 20 mM NaCl solution was uniformly sprayed to strawberry plants in each experimental area at 20 cm above the shoot top, respectively. Each concentration of NaCl solution was applied a total of 25 times to each replicated experimental area until the maturity of strawberry fruits. The leaves and fruits of strawberries were obtained from each treated plot for evaluation of photosynthetic efficiency, mineral element content and fruit quality.

Leaf photosynthesis

For analysis of photosynthesis, two strawberry plants were randomly selected from each replicate. The total six strawberry plants were collected from each treated plot. A fully expanded, illuminated, and healthy leaf was selected from each strawberry plant for measurement of photosynthetic rate (Pn), intercellular CO2 concentration (Ci), stomatal conductance (Gs), and transpiration rate (Tr), from 9:00 to 11:00 AM each day, using a portable photosynthesis measurement system (model Li-6800; Li-Cor, Lincoln, NE, USA). The measurement conditions were as follows: the Photosynthetically Active Radiation (PAR) was 700 µmol m− 2 s− 1, the CO2 concentration was 380 µmol mol− 1, the chamber temperature was 27 °C, and the relative humidity was 70%.

Mineral element concentration

The third and fourth functional leaves and ripe fruits from ten strawberry plants per replicate were randomly collected using scissors, placed in sampling bag and quickly transported to the laboratory. In the laboratory, strawberry leaves were dried to a constant weight at 65 °C for 72 h. The dried leaves were ground and passed through a 0.5 mm sieve. Subsamples of leaves (0.2–0.3 g) and fresh fruits (10 g) were acid-digested, and the contents of Fe, Cu, Zn, Ca, and Na were analyzed by high-resolution inductively coupled plasma atomic emission spectrometry [58]. The concentrations of nitrogen (N), phosphorus (P), and potassium (K) in strawberry leaves were measured by the Kjeldahl method, vanadate molybdate colorimetric method, and flame photometric method [59]. The content of chloride (Cl) in strawberry leaves and fruits was quantified by the method of silver nitrate (AgNO3) titration according to the manufacturer’s instructions [60].

Fruit weight, size, and color

Four fresh fruits were randomly sampled from four strawberry plant in every replicate. Total twelve fruits in different treatment plot were collected for assessment on the basis of fruit weight, length, and diameter. The strawberry fruit surface color was measured using a colorimeter (model CR-400; Konica Minolta, Tokyo, Japan). The L* (0 [black] to 100 [white]), a* (− greenness to + redness), and b* (− blueness to + yellowness) coordinates were obtained.

Preparation for fruit quality determination

Ten fully mature strawberry fruits were harvested from each area plot, and the total soluble sugar content, sugar components, titratable acidity, organic acid components, total phenolic content, and total flavonoid content were analyzed. Approximately 60 g of strawberry fruit was immediately homogenized, frozen, and stored at − 80 °C until analysis.

Total soluble sugar and individual sugars

Approximately 200 mg of frozen strawberry fruit was ground with 10 mL of distilled water, and the samples were centrifuged at 12,000 × r min− 1 for 10 min at 4 ℃. The supernatants were diluted to 50 mL with distilled water, and 1 mL of extract was combined with 1 mL of 2 g L− 1 anthrone in 706 g L− 1 H2SO4. The mixture was incubated at 100 °C for 15 min and cooled to room temperature in a water bath, and the total soluble sugar content was measured by anthrone colorimetry [61]. The results were expressed as % FW.

The contents of glucose, fructose, and sucrose were determined with an Agilent 1100 high-performance liquid chromatography system (Thermo Separation Products, San Jose, CA, USA) equipped with a Sugar-Pak column (4.6 mm × 250 mm; Supelco Analytical Products, Bellefonte, PA, USA) and refractive index detector according to the manufacturer’s instructions [8]. Briefly, 1 g of frozen strawberry fruit was ground with 10 mL of distilled water, extracted by ultrasonication at 42 kHz for 1 h at 45 ℃, and centrifuged at 12,000 × r min− 1 for 10 min at 4 ℃. The supernatants were filtered through 0.45 μm syringe filters. After diluting the filtrates with distilled water, the separation was conducted at 30 °C with a mobile phase of acetonitrile: water (75:25, v/v) at a flow rate of 1 mL min− 1. The contents of glucose, fructose, and sucrose were determined by comparing the retention times and peak areas of the examined samples with those of the reference solution. The results were expressed as mg g− 1 FW.

Total acidity and individual organic acids

Approximately 5 g of frozen strawberry fruit was ground with 10 mL of distilled water, incubated at 100 °C for 20 min, cooled to room temperature in a water bath, and diluted to 50 mL with distilled water. The titratable acidity content was measured by titrating 5 mL of the fruit suspension with 0.1 mM NaOH to pH 8.2 and presented as the citric acid percentage [57]. The results were expressed as % FW.

The contents of malic and citric acids were determined with an Agilent 1100 high-performance liquid chromatography system equipped with an Acid-Pak column (4.6 mm × 250 mm; Cayman Chemical Company, Ann Arbor, MI, USA) and photodiode array detector according to the manufacturer’s instructions [47]. Briefly, 1 g of frozen strawberry fruit was ground with 8 mL of distilled water and centrifuged at 10,000 × r min− 1 for 10 min at 4 °C. The supernatants were diluted to 10 mL with distilled water and then filtered through 0.45 μm syringe filters. After diluting the filtrates with distilled water, the separation was conducted at 35 °C with a mobile phase of methanol: sodium dihydrogen phosphate (5:95, v/v) at a flow rate of 0.8 mL min− 1. The contents of malic and citric acids were determined by comparing the retention times and peak areas of the examined samples with those of the reference solution. The results were expressed as mg g− 1 FW.

Analysis of antioxidant compounds

Approximately 10 g of fresh strawberry fruit was ground and diluted to 50 mL with oxalic acid. After adding a small amount of activated carbon, the suspension was centrifuged at 12,000 × r min− 1for 10 min at 4 ℃, and 10 mL of extract was quickly used to measure the vitamin C content by the 2,6-dichloroindophenol titrimetric method [62]. The results were expressed as mg 100 g− 1 FW. Briefly, 5 g of strawberry fruit was precisely weighed, ground, and mixed with 30 mL of absolute alcohol, followed by incubation at room temperature for 0.5 h. The supernatants were collected by centrifugation at 12,000 × r min− 1for 15 min at 4 °C, and the contents of total flavonoids and total phenolics were determined by the aluminum chloride colorimetric method [63] and the Folin–Ciocalteu method [64], respectively. The results were expressed as mg 100 g− 1 FW.

Statistical analysis

All data were expressed as the mean ± standard error and evaluated by one-way analysis of variance (ANOVA) followed by Duncan’s multiple range test at P < 0.05. All statistical analyses were performed using the SPSS 23.0 software package (Armonk, NY, USA). All figures were constructed using Origin 9.1 software (San Clemente, CA, USA). Redundancy analysis was carried out with the CANOCO 5.0 software (South Bohemia, České Budějovice, Czech).

Data availability

Data is provided within the manuscript or supplementary information files”. Find some help on our Data availability statements page.

Abbreviations

Tr:

Transpiration rate

EC:

Electrical conductivity

Gs:

Stomatal conductance

Ci:

Intercellular CO2 concentration

PAR:

Photosynthetically Active Radiation

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Acknowledgements

We are grateful for the plant materials provided by Zengxian Agricultural Corporation, Xushui District, Baoding City of Hebei Province, China.

Funding

This work was financially supported by the earmarked fund for China CARS— Specialty Vegetable (CARS-24-G-03).

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Authors and Affiliations

  1. College of Horticulture, Hebei Agricultural University, Baoding, 071001, China

    Wei Lu, Hui Fan, Yuchang Zhang, Bingbing Cai, Xinxin Wang, Zhanjun Xue & Qingyun Li

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Contributions

WL and HF: Writing – original draft, Investigation, Data curation, Visualization, Methodology. YZ: Data curation. BC and XW: Writing – review & editing. ZX: Methodology, Writing – review & editing, Supervision. QL: Investigation, Funding acquisition.

Corresponding authors

Correspondence to Zhanjun Xue or Qingyun Li.

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The plant material of this study protocol comply with relevant institutional, national, and international guidelines and legislation. All the strawberry materials required for the experiment were approved by Zengxian Agricultural Corporation of Baoding. All experimental protocols were approved by the Hebei Agricultural University.

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Lu, W., Fan, H., Zhang, Y. et al. Low-concentration NaCl foliar spraying enhances photosynthesis, mineral concentration, and fruit quality of strawberry during greenhouse high-temperature periods. BMC Plant Biol 25, 487 (2025). https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s12870-025-06518-6

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