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Article

Effects of Supplemental Potassium on the Growth, Photosynthetic Characteristics, and Ion Content of Zoysia matrella under Salt Stress

by
Ling Zhang
1,2,
Qiaofeng Jiang
3,
Junqin Zong
1,2,
Hailin Guo
1,2,
Jianxiu Liu
1,2 and
Jingbo Chen
1,2,*
1
The National Forestry and Grassland Administration Engineering Research Center for Germplasm Innovation and Utilization of Warm-Season Turfgrasses, Institute of Botany, Jiangsu Province and Chinese Academy of Sciences, Nanjing Botanical Garden, Mem. Sun Yat-sen, Nanjing 210014, China
2
Jiangsu Key Laboratory for the Research and Utilization of Plant Resources, Institute of Botany, Jiangsu Province and Chinese Academy of Sciences, Nanjing Botanical Garden, Mem. Sun Yat-sen, Nanjing 210014, China
3
Changzhou Supervision Center of Ecology and Environment, Changzhou 213000, China
*
Author to whom correspondence should be addressed.
Horticulturae 2024, 10(1), 31; https://doi.org/10.3390/horticulturae10010031
Submission received: 15 November 2023 / Revised: 20 December 2023 / Accepted: 26 December 2023 / Published: 28 December 2023
(This article belongs to the Special Issue Advanced Studies in Abiotic Stress Response Mechanism of Horticultural Plants)
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Abstract

:
Potassium is crucial in plant metabolism processes, and sufficient potassium can improve plant tolerance to abiotic stress. We studied the effects of different KCl concentration treatments (0, 1, 5, 20 mM) on the biomass, photosynthetic characteristics, and ion content of Zoysia matrella under salt stress (NaCl 300 mM). The results showed that the plant dry weight, stomatal conductance, transpiration rate, photosynthesis rate, K+ content in plants, and K+/Na+ ratio in leaves of Zoysia matrella under NaCl stress were significantly lower than those under no NaCl conditions. The addition of K can promote an increase in plant dry weight and significantly improve the stomatal conductance, transpiration rate, and photosynthesis rate of plants. In addition, under salt stress, the addition of 20 mM KCl can significantly reduce the accumulation of Na+ in plants and promote the secretion of Na+ in leaves, thus improving the salt tolerance of Zoysia matrella.

1. Introduction

Plants encounter various biological and abiotic stresses throughout their life cycle. In abiotic stress, soil salinization seriously restricts the growth and development of plants [1]. Approximately 20% of agricultural land and 50% of farmland globally are salt-stressed, according to the United Nations Environment Programme [2]. In recent years, soil salinization has been aggravated due to global climate change. According to statistics, soil salinization reduces the area of agricultural land by 1–2% every year [3,4]. Saline–alkali soil has the characteristics of a high pH value, high sodium adsorption rate, high sodium exchange rate, and high electrical conductivity, and the excessive accumulation of sodium ions or the increase in soil salt will reduce the stability of soil, resulting in the dispersion of soil particles [5]. Therefore, soil salinity has become one of the most concerning problems in the 21st century. Salt stress occurs when the concentration of salt in soil or plants exceeds threshold levels. NaCl is the most important type of salt, and a large amount of Na+ and Cl− absorbed by plants can cause damage to the normal physiological functions of plants by changing the metabolic process [6,7,8]. The imbalance of the intracellular ion exchange process is the cause of global salt stress. The inflow of Na+ and the outflow of K+ occur through different ion transporters on the cell membrane. Finally, due to the imbalance of ions, the cell membrane will be destroyed, and the homeostasis of cells will eventually be disrupted, affecting the normal growth of plants [9]. Therefore, improving plant resistance to salt stress is an effective way to solve the problem of large-scale planting in saline–alkali land.
Zoysia belongs to the family Poaceae, subfamily Chloridoideae, tribe Zoysieae, which is native to Asia and the South Pacific and is widely distributed in tropical, subtropical, and temperate climate zones [10]. Zoysiagrass, a stoloniferous warm-season lawn grass, exhibits robust tolerance to abiotic stress, making it a popular choice for family lawns, golf courses, sports fields, and parks [11,12]. Z. matrella is a halophyte with strong salt tolerance. Its leaves contain salt gland tissue, which secretes Na+ to diminish the accumulation of salt ions within the plant, thereby enabling it to survive and thrive in higher salinity environment [13,14]. The salt tolerance of zoysiagrass is positively correlated with the density of salt glands in the leaf epidermis and salt secretion rate [10]. In addition, previous studies had shown that the salt tolerance of zoysiagrass was negatively correlated with the content of Na+ in leaf fluid and positively correlated with the content of K+ [15]. H+-pyrophosphatase in zoysiagrass vacuoles can drive Na+ into vacuoles, reduce the pH value of vacuoles, increase K+ assimilation, and improve the salt tolerance of plants, thus reducing damage to plant cells [16,17,18]. Li et al. found that the activity of polyamine synthetase in Z. japonica Steud increased under salt stress. They also found that augmenting the content of polyamines could mitigate the oxidative stress and hypertonic damage induced by salt stress [19]. Sugiura et al. found that under salt stress, Z. matrella diluted salt by shifting water between stolons, stored salt in brown leaves, and activated the defoliation mechanism of brown leaves to alleviate salt stress [20]. However, with the increase in the degree of soil salinization and the aggravation of saline irrigation problems, the salt damage of zoysiagrass was aggravated. Therefore, it is urgent to improve the salt tolerance of zoysiagrass.
K is an essential element for plant growth, and K deficiency can reduce plant resistance to biotic and abiotic stresses. Cakmak et al. found that plants suffer from K deficiency under salt stress, resulting in decreased photosynthetic capacity, slow plant growth, and even death [21]. Sufficient K can promote the operation of relevant stress-resistant pathways and mechanisms in plants, such as by increasing the accumulation of organic osmotic substances, enhancing enzyme activity, protecting chloroplast stability, maintaining high photosynthetic capacity, and maintaining a high K+/Na+ ratio, thus maintaining plant growth and yield [21,22,23,24]. Research has revealed that the growth and K absorption of Festuca arundinacea can be enhanced through the application of an appropriate amount of K fertilizer [25]. Additionally, during the optimization of bermudagrassintegrated turf performance, it was observed that increasing K fertilizer can stimulate root growth [26]. Merwad et al. found that field application of K fertilizer could increase the stem and root yield, total sugar yield, nitrogen, phosphorus, and potassium contents of Beta vulgaris L. growing on saline soil [27]. Yaldiz et al. found that applying potassium sulphate under salt stress could significantly increase the yield of Foeniculum vulgare and Coriandrum sativum [28]. When Ahmad et al. studied the salt tolerance of cabbage (Brassica oleracea L.), they found that the exogenous K application of 10 mM could significantly promote the accumulation of soluble protein and total free amino acids in plants, improve the antioxidant capacity of plants, improve the ratio of K+ and Na+ under salt–alkali conditions, and improve the salt resistance of plants [29]. Under salt stress, the increased application of K fertilizer can promote the water use efficiency of Chenopodium quinoa, promote the accumulation of soybean carbohydrates, and reduce saline–alkali damage [30,31]. Therefore, the internal K demand of plants under salt stress is greater, and increasing K supply may play an important role in alleviating the negative effects of salt stress [32,33]. Currently, there is a lack of relevant reports on the growth characteristics and salt tolerance mechanism of Z. matrella, a perennial halophyte, under salt stress. It is hypothesized that the application of K fertilizer could mitigate the detrimental effects of salt stress on Z. matrella by sustaining photosynthesis and promoting the secretion of Na+. Therefore, this experiment aimed to investigate the impact of increased K fertilizer under salt stress on the growth status, chlorophyll content, photosynthetic characteristics, stomatal changes, Na+ and K+ content, and ion secretion rate of Z. matrella. The objective of this study was to establish an experimental basis for the application of K fertilizer in Z. matrella turf grown in saline soil with saline-water irrigation.

2. Materials and Methods

2.1. Material Culture and Treatment

Zoysia matrella was provided by the Flower and Wood Cooperative of Rugao City, Jiangsu Province, China. The washed turf blocks were planted into a 250 mL plastic cup (diameter 6.5 cm) with quartz sand. The plastic cups hung from foam panels supported by wire at the bottom. The foam board was positioned on the plastic turnover box, and 40 L of the pre-culture hoagland nutrient solution was transferred into the turnover box (Table 1). The nutrient solution was allowed to reach 1 cm from the bottom of the cup with continuous ventilation. Throughout the culture period, the nutrient solution was replaced on a weekly basis, and grass was pruned to a height of 3 cm every week. Following approximately 2 months of cultivation, the cup was fully covered with grass, and salt treatment commenced. The NaCl concentrations in the treatment consisted of 0 (control) and 300 mM (salt treatment), totaling two levels. To reduce the salt shock effect at the beginning of salt treatment, the salt concentration was gradually increased at 50 mM per day and maintained for 1 month after reaching the highest salinity to adapt to the salt environment.
After salt pretreatment, plastic cups containing Z. matrella were suspended from perforated foam boards and placed in a small bucket of 2.5 L nutrient solution. KCl was used to treat K at concentrations of 0, 1, 5, and 20 mM. The nutrient solution formula was shown in Table 1. At the same time, a salt treatment group with 300 mM NaCl added and a control group with no NaCl added were set, and a total of 8 treatments were set. Each treatment had 3 replicates, and the treatment time was 1 month.

2.2. Assay Method

At the end of the experiment, the branch density in each cup was determined. The branches and leaves were trimmed to the original pruning height, and the roots were trimmed at the bottom of the cup. The cut branches, leaves, and roots were washed in distilled water 3 times and then dried at 80 °C, then were used as the dry weights of shoot clippings and root clippings. Residual branches and leaves were removed from the cup, rinsed with tap water to eliminate quartz sand, then washed them with distilled water 3 times, dried at 80 ℃, weighed, and combined with shoot and root clippings, and the total dry weight of the entire plant was determined.
The net photosynthetic rate of leaves was measured using a Li-6400XT portable photosynthesis measurement system (Li-COR, Lincoln, NE, USA), with the light intensity set at 1000 µmol·m−2·s−1 and the CO2 concentration set at 400 µmol·mol−1. The parameters measured included the photosynthetic rate (Pn, μmol CO2·m−2·s−1), stomatal conductance (Gs, mol H2O·m−2·s−1), intercellular CO2 concentration (Ci, μmol CO2·mol−1), and transpiration rate (Tr, mmol H2O·m−2·s−1).
The pigment content was assessed using the technique developed by Lichtenthaler et al. [34]. Approximately 0.03 g of healthy intact leaves was cut, and then 10 mL of 95% ethanol solution was added for 24 h under dark conditions. The light absorption values at 665 nm, 649 nm, and 470 nm were measured using a spectrophotometer (Beckman Coulter DU800, Brea, CA, USA). Chlorophyll a (Ca), chlorophyll b (Cb), and carotenoid (Car) contents were calculated thus: Ca = 13.95A665nm − 6.88A649nm, Cb = 24.96A649nm − 7.32A665nm, and Car = (1000A470nm − 2.05 Ca − 114.8 Cb)/245.
To determine the ion secretion capacity of the leaves, the leaves were washed with tap water 3 times and then with deionized water 3 times. After 7 days, 10 pieces were randomly taken from the top of the third reciprocal leaf (mature and robust functional leaf) and immediately placed in a centrifuge tube for 10 s with 10 mL distilled water according to Marcum’s method [35]. Then, the eluent was poured into another centrifuge tube, and the leaves were removed, dried, and weighed (dry weight, DW). The content of Na+ and K+ in the eluent was determined using a flame spectrophotometer (FR6410, Shanghai Xinyi Instrument Inc., Shanghai, China). The contents of Na+ and K+ in leaves and roots were determined according to Glenn et al. [36].

2.3. Statistical Analysis

Two salinity schemes and four potassium levels were used in the experiment, which was repeated three times in a completely randomized design. SPSS 17.0 software (SPSS Institute, Cary, NC, USA) was used for variance analysis and mean comparison of all data. The difference was statistically significant at the 5% level (p < 0.05), and Duncan’s Multiple Range Test was used for analysis. Data visualization was performed using GraphPad Prism 8.0 (GraphPad Software, La Jolla, CA, USA).

3. Results

3.1. Effects of Different Potassium Levels on the Biomass of Zoysia matrella under Salt Stress

When Zoysia matrella was grown in a K-deficient environment, the plants exhibit slow growth (Figure 1 and Figure A1). Under salt treatment (300 mM NaCl), when the K concentration was 0 mM, the shoot clipping dry weight was the lowest and increased with increasing K concentration (Figure 1A). At 20 mM, the maximum shoot clipping dry weight was 19.66%, 9.9%, and 2.6% higher than those at 0, 1, and 5 mM, respectively. Compared with the control, salt treatment significantly reduced the shoot clipping dry weight. Under the same K concentration, the K concentrations of 1, 5, and 20 mM under the control were 25.39%, 48.16%, and 31.1% higher than those under the salt treatment, respectively. The root pruning dry weight increased with increasing K concentration in both the no-salt treatment and salt treatment, reaching a maximum at 5 mM and slightly decreasing at 20 mM. Under salt treatment, the root pruning dry weight was significantly higher than those under 0 mM K at 5 mM and 20 mM, which were 3.53 and 3.44 times that under 0 mM K, respectively. When the K concentration was 1 mM, there was no significant difference in root pruning dry weight compared with the 0 mM K concentration.
The change trend of whole-plant dry weight was basically consistent with those of branch and leaf dry weight and root dry weight, which increased with increasing K concentration and reached the maximum value when the K concentration was 5 mM but decreased at 20 mM. The dry weight of the whole plant under salt treatment was significantly lower than that under the control treatment. Under salt treatment, the whole plant dry weight under a K concentration of 5 mM was 27.06%, 8.61%, and 7.80% higher than those under 0, 1, and 20 mM, respectively (Figure 1C).
Under the control treatment, the lawn density increased with increasing K concentration and reached a peak value at 5 mM, which was 34.42%, 18.87%, and 8.53% higher than those at 0, 1, and 20 mM, respectively, which was basically consistent with the change trend of the pruning dry weight of branches and leaves. Under salt treatment, the density had the highest value at 5 mM and decreased at 20 mM. The density of K concentration at 5 mM increased by 54.01%, 13.57%, and 11.62% compared with 0, 1, and 20 mM, respectively (Figure 1D).

3.2. Effects of Different Potassium Levels on the Photosynthetic Pigment Content of Zoysia matrella under Salt Stress

Chlorophyll is an important component of plant photosynthesis. Under both the control treatment and salt treatment, the content of photosynthetic pigments showed an increasing trend with increasing K concentration. Under salt treatment, when the K concentration was 20 mM, the content of Ca was the highest, being 5.48%, 2.93%, and 0.55% higher than those at 0, 1, and 5 mM, respectively (Figure 2A). The change trend of Cb, Car and total chlorophyll content was consistent with that of Ca, and all showed a tendency to rise first with the increase in K under NaCl treatment, but there was no significant difference among different K concentrations (Figure 2B–D).

3.3. Effects of Different Potassium Levels on Photosynthesis in Zoysia matrella under Salt Stress

Under 300 mM NaCl treatment, compared with the control treatment, the K concentration at 5 mM could significantly promote the increase in Pn (Figure 3A). With increasing K concentration, the Pn also showed an upwards trend. Under salt treatment, Gs was significantly lower than that under the control treatment. With increasing K concentration, Gs first increased and then decreased and reached a maximum value at 5 mM, which was significantly higher than those at 0 and 1 mM K concentrations (Figure 3B). Ci increased with increasing K concentration and reached its peak at 5 mM. Meanwhile, the Ci of plants was lower than that of control plants with the same K concentration (Figure 3C). The Tr under salt treatment was significantly lower than that under the control treatment. Under 300 mM NaCl treatment, Tr initially increased and then decreased with increasing K concentration, and at 5 mM, Tr was significantly higher than those at 0 and 1 mM. However, at 20 mM, there was a downwards trend (Figure 3D).

3.4. Effects of Different Potassium Levels on the Ion Content of Zoysia matrella under Salt Stress

Compared with the control, the concentration of Na+ in the leaves and roots was significantly increased under salt treatment (Figure 4A,B). Under salt treatment, the concentration of Na+ in leaves and roots showed a decreasing trend with increasing K concentration. Notably, at the 5 mM and 20 mM K concentrations, the Na+ content in leaves and roots was significantly lower than that at the 0 mM K concentration. Under the control treatment, there was no significant difference in Na+ content between leaves and roots when the K concentration was increased. These results indicated that the Na+ content in Z. matrella could be significantly decreased by increasing K fertilizer under salt stress. Under salt treatment, the concentration of K+ in the leaves and roots was significantly lower than that in the control parts (Figure 4C,D). Under the control treatment, the K+ content in leaves and roots with added K was significantly higher than that with 0 mM K treatment. Under salt treatment, there was no significant difference in K+ content between leaves and roots. Under the control treatment, the K+/Na+ ratio in leaves was not significantly different in the 0, 1, and 5 mM K treatments, but significantly increased in the 20 mM K treatment. There was no significant difference in subsurface K+/Na+ ratio between the 0 and 1 mM K treatment; the result under the 5 mM K treatment was significantly higher than under the 0 and 1 mM K treatment but significantly lower than under the 20 mM K treatment. Under salt treatment, there was no significant difference in K+/Na+ between the leaves and roots at different K concentrations.

3.5. Effects of Different Potassium Levels on Na+ and K+ Secretions of Zoysia matrella under Salt Stress

Under salt treatment, the amount of Na+ secreted by Z. matrella was significantly higher than that in the control treatment, and the amount of K+ secreted by Z. matrella was significantly higher than that in the control treatment. Under salt treatment, there was no significant difference in the amount of Na+ and K+ secreted by Z. matrella across the 0, 1, and 5 mM K treatments. Under the 20 mM K treatment, the amount of Na+ secreted by Z. matrella was significantly increased, and the amount of K+ secreted by Z. matrella was significantly higher than that secreted under the 0 and 1 mM K treatments, and there was no significant difference between the two groups (Figure 5).

4. Discussion

Salt stress can produce a series of negative effects on plant physiology and biochemistry, including reductions in plant biomass and crop yield [8]. As a halophyte, Z. matrella has a strong tolerance to salt stress, but, under the condition of high salt concentration, it is not conducive to the normal growth of plants [13]. In this study, it was found that 300 mM NaCl treatment significantly reduced the growth of the upper part of Z. matrella compared to the control. The accumulation of salt ions in plants can affect ion homeostasis, destroy biomembranes and subcellular organelles, and alter starch metabolism, leading to abnormal growth [37,38,39]. Plants are subjected to salt stress during their growth and development, and mineral nutrition supplementation can reduce the damage of salt to crops [40,41]. Among various nutrients, the promoting effect of K on plant growth, development, and physiology under salt stress has been fully confirmed [38,40]. According to our current research findings, under salt stress, the exogenous application of K fertilizer can promote the plant growth of Z. matrella to a certain extent, and the growth of deep roots is significantly increased (Figure 1D). Ahmad et al. found that the increased application of K could reduce the osmotic potential of root columns, help maintain turgor pressure, drive xylem solute transport, and maintain the water balance of plants, thus reducing the damage of salt to plants [29]. This is similar to our findings.
Uddin et al. found that high-dose salt treatment promoted a decrease in chlorophyll content in turf grass [42]. The salt-induced chlorophyll reduction may be caused by the disintegration of chlorophyll enzymes induced by NaCl, the deficiency of Mg2+, and structural changes in chloroplasts or related proteins [43,44]. In this study, compared with the control, the total chlorophyll content increased under salt treatment. With an increasing K concentration, the total amount of photosynthetic pigments tended to increase. Kozłowska et al. also found that with a certain amount of salt treatment, all photosynthetic pigment levels in turf grass were increased [45]. K supplementation may protect and promote chlorophyll synthesis and accumulation by reducing the strong NaCl-induced response to improve nutrient absorption [46].
During the growth and development of most crops, photosynthesis is reduced due to salt stress, which is also applicable to halophytes [47,48]. The Pn, Gs, and Tr of Z. matrella were significantly decreased under salt stress. Ashraf et al. and Asrar et al. found that reducing Gs and thus Tr under salt stress was an effective strategy for plant water conservation and reducing Ci and CO2 fixation ability [49,50]. Studies have also shown that the Gs and Tr contents of barley (Hordeum vulgare L. var. Manel) were significantly decreased under salt stress [51]. Under salt stress, the Pn, Gs, and Tr contents of Z. matrella were significantly increased by adding K. Additionally, research has indicated that the foliar spraying of K+ could reduce the damage of salt stress on Thymus vulgaris L. and significantly improve its photosynthetic efficiency [38]. As the main inorganic penetrant, K+ drives stomatal movement and protects against changes in cell expansion. When the K supply is insufficient or the K content in leaves decreases under salt stress, Gs will decrease. Therefore, exogenous K application can improve the K+ concentration in leaves, partially maintain stomatal functional movement, and improve photosynthetic efficiency [29,52]. The balance of Na+ and K+ in plant cells is a key factor in maintaining plant salt tolerance. The salt tolerance of plants is not entirely an adaptation to Na+ toxicity but may be due to water deficiency or impaired absorption of nutrients. K+ absorption is particularly important for plants to resist Na+ stress, and K+ absorption in plants is mainly mediated by K+ channels and K+ transporters [53]. In this study, it was found that the content of K+ in the roots of plants treated with a high concentration of K (≥5 mM) was higher than that in plants treated with a low concentration of K (1 mM) (Figure 4D). Under high K concentrations, plants absorbed and accumulated more K+, which promoted the efflux of Na+ (Figure 5A). Kaddour et al. also found that K supplementation was conducive, in Arabidopsis thaliana, of K+ [54]. This is likely due to the increased application of K, which can improve ion homeostasis and relieve the excessive oxidative stress response under salt stress. Abbasi et al. found that under salt stress, the increased application of K fertilizer in soil was beneficial for enhancing catalase (CAT) activity, thereby protecting the normal growth and photosynthesis of plants and resisting oxidative stress [55]. Higher accumulation of K+ promoted osmotic regulation, stomatal control, and photosynthetic energy dissipation, which was conducive to maintaining normal water content and photosynthesis in plants [21,55,56]. Under salt stress, the content of Na+ in plant cells under high-K treatment (>5 mM) was significantly lower than that under low-K treatment (Figure 4A,B). In the process of plant metabolism, under a sufficient supply of K, especially under a high salt concentration (200 mM NaCl), the absorption of Na+ by plants decreased. Compared with low K treatment, the plants could maintain a lower concentration of Na+, thus alleviating salt damage [29]. In the cell, Na+ will compete for the main binding site of K+ on the enzyme and affect plant metabolism and interfere with plant metabolism processes by inhibiting enzyme activity, thus producing ionic toxicity to plant cells that is unfavorable to plant growth [57]. To promote plant metabolism, the concentration of K+ in the cytoplasm must be maintained at a constant level [57]. The optimal K+/Na+ ratio is very important to activate the enzyme reaction and maintain normal plant growth and development. Under salt stress, the K+/Na+ ratio of plant cells tended to increase with increasing K+ content in the solution (Figure 4E,F). This indicated that the root system absorbed more K+, resulting in a higher K+/Na+ ratio in the root system, and more K+ was transferred from the root system to the aboveground part, thus inhibiting the transfer of Na+ from the root system to the aboveground part [58]. Gul et al. found that increasing the K+ supply (8 mM) under salt stress could reduce the accumulation of Na+ in plant tissues and cells [59]. Under salt stress, foliar-applied potassium increased the levels of the K+/Na+ ratio, which are positively reflected in growth and yield traits [60]. This may be because the high-affinity K transporter (HKT) in plants helps to exclude Na+ and can also lead to K+ inflow [61]. In this study, it was found that the content of Na+ in the leaves and roots of plants under the conditions of high Na+ and low K+ was significantly higher than that under the conditions of high Na+ and high K+ (Figure 4A,B).
As one of the most salt-tolerant turf grasses, zoysiagrass has a special salt gland tissue, which helps to improve its salt tolerance [62]. Studies have found that salt ions absorbed by plant roots are transported to leaves through transpiration, and plants secrete some salt ions (mainly Na+ and Cl−) into the body through salt glands [63,64]. The secretion of salt requires a large amount of energy, and the amount of salt secreted in light conditions is higher than that in dark conditions [63]. Chen et al. found that the expression of H+-ATPase was positively correlated with the amount of salt secretion [65]. Debez et al. found that under increased salinity, H+-ATPase activity and photosynthetic efficiency in the leaves of Cakile maritima increased, and salt resistance was also enhanced [66]. In this experiment, it was found that under salt stress, a certain amount of K fertilizer could significantly improve photosynthetic efficiency to provide more energy for salt secretion in salt glands, significantly increase the secretion of Na+, and improve salt tolerance.

5. Conclusions

As a necessary nutrient for plant growth, potassium had a positive effect on the growth of Z. matrella under salt stress. Under salt stress, potassium drove stomatal movement, improved stomatal conductivity, enhanced the photosynthetic rate of Z. matrella, reduced the harm of salt stress, and sustained normal growth state. Moreover, in investigating the mechanism by which potassium enhances the tolerance of Z. matrella to saline and alkaline conditions, we found that under salt stress, the addition of potassium significantly promoted root system growth, thereby improving K+ absorption, reducing the Na+ content in plants, and maintaining cellular homeostasis. Focusing on a halophyte, our study results showed that the addition of potassium under salt stress stimulated plant transpiration, facilitating the transport of more salt ions to the leaves for the secretion by salt glands, thereby enhancing plant salt tolerance. In summary, Z. matrella grown in saline–alkaline land and under saline-water irrigation can experience relief from salt stress, and we can maintain the normal growth state of plants by applying potassium fertilizer. This provides certain guidance for turf cultivation and management under salt stress.

Author Contributions

All authors contributed experimental design oversight. Q.J. and J.C. contributed the plant materials. J.Z. analyzed the data. J.C., H.G. and J.L. designed the experiments. L.Z. wrote the paper. All authors have read and agreed to the published version of the manuscript.

Funding

The work was supported by the Jiangsu Province Social Development Project (BE2023849).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare that they have no competing interests.

Appendix A

Horticulturae 10 00031 g0a1
Figure A1. Effects of different potassium levels on the growth of Zoysia matrella under salt stress.
Figure A1. Effects of different potassium levels on the growth of Zoysia matrella under salt stress.
Horticulturae 10 00031 g0a1

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Figure 1. Effects of different potassium levels on shoot clipping dry weight (A), root clipping dry weight (B), whole plant dry weight (C), and density (D) of Zoysia matrella under salt stress. Analysis of variance was used, and lowercase letters indicate significant differences between different KCl concentrations under the same salt treatment (p < 0.05); * indicates significant differences between different salt treatments (p < 0.05).
Figure 1. Effects of different potassium levels on shoot clipping dry weight (A), root clipping dry weight (B), whole plant dry weight (C), and density (D) of Zoysia matrella under salt stress. Analysis of variance was used, and lowercase letters indicate significant differences between different KCl concentrations under the same salt treatment (p < 0.05); * indicates significant differences between different salt treatments (p < 0.05).
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Figure 2. Effects of different potassium levels on chlorophyll a content (A), chlorophyll b content (B), total chlorophyll content (C), and carotenoid content (D) of Zoysia matrella under salt stress. Analysis of variance was used, with lowercase letters indicating significant differences between different KCl concentrations under the same salt treatment (p < 0.05).
Figure 2. Effects of different potassium levels on chlorophyll a content (A), chlorophyll b content (B), total chlorophyll content (C), and carotenoid content (D) of Zoysia matrella under salt stress. Analysis of variance was used, with lowercase letters indicating significant differences between different KCl concentrations under the same salt treatment (p < 0.05).
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Figure 3. Effects of different potassium levels on photosynthetic rate (A), stomatal conductance (B), intercellular CO2 concentration (C), and transpiration rate (D) of Zoysia matrella under salt stress. Lowercase letters indicate a significant difference between different KCl concentrations under the same salt treatment (p < 0.05); ** indicates a significant difference between different salt treatments (p < 0.01).
Figure 3. Effects of different potassium levels on photosynthetic rate (A), stomatal conductance (B), intercellular CO2 concentration (C), and transpiration rate (D) of Zoysia matrella under salt stress. Lowercase letters indicate a significant difference between different KCl concentrations under the same salt treatment (p < 0.05); ** indicates a significant difference between different salt treatments (p < 0.01).
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Figure 4. Effects of different potassium levels on Na+ content in branches and leaves (A), Na+ content in roots (B), K+ content in branches and leaves (C), K+ content in roots (D), and K+/Na+ ratio in leaves (E) and roots (F) of Zoysia matrella under salt stress. Analysis of variance was used, and lowercase letters indicate significant differences between different KCl concentrations under the same salt treatment (p < 0.05); ** indicates significant differences between different salt treatments (p < 0.01).
Figure 4. Effects of different potassium levels on Na+ content in branches and leaves (A), Na+ content in roots (B), K+ content in branches and leaves (C), K+ content in roots (D), and K+/Na+ ratio in leaves (E) and roots (F) of Zoysia matrella under salt stress. Analysis of variance was used, and lowercase letters indicate significant differences between different KCl concentrations under the same salt treatment (p < 0.05); ** indicates significant differences between different salt treatments (p < 0.01).
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Figure 5. Effects of different potassium levels on Na+ secretion (A) and K+ secretion in leaves (B) of Zoysia matrella under salt stress. Analysis of variance was used, and lowercase letters indicate significant differences between different KCl concentrations under the same salt treatment (p < 0.05); * indicates significant differences between different salt treatments (p < 0.05), ** indicates extremely significant differences between different salt treatments (p < 0.01).
Figure 5. Effects of different potassium levels on Na+ secretion (A) and K+ secretion in leaves (B) of Zoysia matrella under salt stress. Analysis of variance was used, and lowercase letters indicate significant differences between different KCl concentrations under the same salt treatment (p < 0.05); * indicates significant differences between different salt treatments (p < 0.05), ** indicates extremely significant differences between different salt treatments (p < 0.01).
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Table 1. Nutrient solution formula of major element during the pre-culture and potassium treatment.
Table 1. Nutrient solution formula of major element during the pre-culture and potassium treatment.
NutritionPre-Culture Medium (mM)K 0
(mM)		K 1
(mM)		K 5
(mM)		K 20
(mM)
Ca(NO3)22.52.52.52.52.5
KNO32.50000
MgSO411111
KH2PO40.50000
(NH4)2SO402.52.52.52.5
KCI001520
NaH2PO400.50.50.50.5
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Zhang, L.; Jiang, Q.; Zong, J.; Guo, H.; Liu, J.; Chen, J. Effects of Supplemental Potassium on the Growth, Photosynthetic Characteristics, and Ion Content of Zoysia matrella under Salt Stress. Horticulturae 2024, 10, 31. https://doi.org/10.3390/horticulturae10010031

AMA Style

Zhang L, Jiang Q, Zong J, Guo H, Liu J, Chen J. Effects of Supplemental Potassium on the Growth, Photosynthetic Characteristics, and Ion Content of Zoysia matrella under Salt Stress. Horticulturae. 2024; 10(1):31. https://doi.org/10.3390/horticulturae10010031

Chicago/Turabian Style

Zhang, Ling, Qiaofeng Jiang, Junqin Zong, Hailin Guo, Jianxiu Liu, and Jingbo Chen. 2024. "Effects of Supplemental Potassium on the Growth, Photosynthetic Characteristics, and Ion Content of Zoysia matrella under Salt Stress" Horticulturae 10, no. 1: 31. https://doi.org/10.3390/horticulturae10010031

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