Effects of Potassium Supply in Nutrient Solution on Water and Nutrient Absorption of Substrate-Grown Tomato Plants

Abstract

Potassium (K⁺) functions as a critical “regulator” and “quality element” in plants, with its physiological roles varying across developmental stages. To clarify the effects of different K⁺ amounts in nutrient solution on water and nutrient absorption characteristics and potassium utilization efficiency in substrate-grown tomato, a controlled experiment was conducted in a climate-regulated solar greenhouse using “Saint Ness” tomato as the plant material. Four K⁺ supply levels (1, 4, 8, and 16 mmol/L, designated as K1, K4, K8, and K16 treatment, respectively) were tested to systematically evaluate the responses of tomato plants at different growth stages in terms of water and nutrient absorption capacity, potassium physiological efficiency (KPE), and potassium utilization efficiency (KUE). The results showed that water absorption capacity did not differ significantly among treatments during the vegetative growth stage. However, during the reproductive stage, the K8 treatment exhibited the highest water absorption capacity (47.05 kg/plant) and water absorption efficiency (84.6%). In addition, K8 significantly promoted the coordinated uptake of K⁺, nitrogen, phosphorus, calcium, and magnesium, with a total potassium absorption capacity of 7.2 g/plant and a potassium absorption efficiency of 79.1%. In contrast, excessive K⁺ supply (16 mmol/L) increased total potassium absorption capacity (5.09 g/plant) but led to a marked decline in physiological efficiency (by 27.9%) and water absorption efficiency (by 10.3%) due to luxury consumption and substrate-induced salt stress. Insufficient K⁺ levels (1–4 mmol/L) also restricted root-mediated water and nutrient flux. The study further revealed a dose-dependent and stage-specific pattern in water and potassium absorption. Therefore, an appropriate K⁺ supply of 8 mmol/L not only improved the plant’s absorption capacity for water and nutrients and potassium utilization efficiency but also maintained ionic balance among essential nutrients. These findings provide a theoretical basis for precision water and fertilizer integration strategies in substrate-cultivated tomato production under greenhouse conditions.

1. Introduction

Tomato (Lycopersicon esculentum Mill.) is a typical potassium-loving (K-loving) fruit vegetable. During the vegetative growth stage, the potassium content in tomato tissues ranges from 0.3% to 5.0% [1,2]. During the fruit enlargement stage, it can reach as high as 6.3% to 8.1%, ranking first among all essential nutrient elements [3]. Proper potassium application not only promotes the growth and development of tomato plants but also significantly enhances fruit yield and improves fruit quality [4]. For example, Ramírez et al. [5] found that potassium can enhance tomato quality by influencing carotenoid biosynthesis in the fruit. Similarly, Zhang et al. [6] reported that increasing the potassium concentration in the nutrient solution by 1.5 times not only improved tomato fruit yield but also significantly elevated the contents of vitamin C and soluble proteins. Specifically, K⁺ activates over 60 types of enzymes within the plant [7,8], enhances osmotic potential and stabilizes membrane potential in mesophyll cells [9,10], regulates stomatal aperture to optimize CO₂ assimilation rates [2,11], facilitates the translocation of photosynthates to physiological sink organs [12], and strengthens the plant’s ability to adapt to adverse environmental conditions [13].

In soil-based cultivation systems, the increasing cropping index and intensive production practices have led to K⁺ depletion and disrupted the balance among essential nutrient elements, severely limiting improvements in crop yield and quality [14,15]. In substrate-grown tomatoes supplied with nutrient solution, K⁺ demand also varies significantly across different growth stages [2]. Ineffective regulation of K⁺ supply can impede fruit yield formation and adversely affect fruit quality at later stages [16]. As the country with the largest protected tomato cultivation area, China has made rapid advancements in greenhouse structure and environmental control technologies. However, the yield per unit area of Chinese greenhouse tomatoes still lags considerably behind leading countries such as the Netherlands, Japan, and Israel, primarily due to insufficient management and regulation of nutrient solutions, particularly K⁺ supply [17,18]. Given that K⁺ is recognized as a “quality element” for tomatoes, optimizing K⁺ management in nutrient solutions has become a critical factor for improving both yield and fruit quality in protected tomato production systems.

In recent years, significant advances have been made in elucidating the mechanisms by which K⁺ influences plant water and nutrient uptake [19,20]. K⁺ plays an indispensable role in the process of water absorption and utilization in plants. Under water stress conditions, the accumulation of K⁺ within cells increases the osmotic potential, thereby promoting water influx through osmosis and maintaining cellular turgor pressure [21]. Additionally, K⁺ enhances root water uptake capacity by regulating plasma membrane H⁺-ATPase activity, which facilitates cell elongation and lateral root formation [18,22]. Regarding nutrient absorption and utilization, K⁺ exhibits complex and closely interrelated interactions with other mineral nutrient elements [3,23]. Studies have shown that K⁺ promotes nitrogen uptake and utilization, thereby improving nitrogen use efficiency largely due to its involvement in the enzymatic reactions of nitrogen metabolism within the plant [15]. Moreover, K⁺ influences phosphorus uptake and translocation, optimizing phosphorus distribution within the plant and thereby enhancing overall nutritional status [24]. Notably, K⁺ competes with calcium (Ca²⁺) and magnesium (Mg²⁺) during uptake; high concentrations of K⁺ can inhibit Ca²⁺ influx through non-selective cation channels (NSCCs) but may simultaneously alleviate magnesium deficiency symptoms by upregulating the expression of magnesium transporter genes such as MRS2 [25,26,27].

In substrate-grown tomato cultivation systems, nutrient solution serves as the primary source of water and nutrient elements, and the precise regulation of K⁺ supply levels is of critical importance [19,28]. However, K⁺ is notable for its high mobility both within the plant and in the soil or soilless substrate environment. In the rhizosphere, K⁺ remains predominantly in the substrate solution due to its weak adsorption to substrate particles, making it highly responsive to minor fluctuations in external concentrations. Such sensitivity can markedly influence its uptake kinetics and spatial distribution around the root surface [2,6]. Within the plant, K⁺ is one of the most mobile nutrients in the phloem, allowing for rapid redistribution among tissues to meet dynamic physiological demands. This property is particularly important during reproductive development, when K⁺ is efficiently translocated from mature organs to actively growing sinks such as fruits. However, this high mobility can also result in “luxury uptake” and disrupt internal nutrient balance when potassium is supplied in excess [23,24].

Although numerous studies have focused on K⁺ nutrition in plants, many key questions remain unresolved regarding how varying K⁺ supply levels influence water and nutrient uptake in substrate-grown tomatoes. How do different K⁺ amounts affect the absorption efficiency of water and essential nutrient elements in tomato plants? What are the mechanisms of ion interactions between K⁺ and other elements within the complex substrate environment? Addressing these questions is of substantial theoretical and practical significance for optimizing K⁺ management strategies in substrate-grown tomatoes and enhancing yield and fruit quality. The objective of this study is to systematically investigate the effects of different potassium amounts in nutrient solution on water and nutrient absorption, as well as potassium utilization efficiency in substrate-grown tomato. Furthermore, the study aims to elucidate the physiological mechanisms by which potassium regulates water and nutrient absorption capacity at different developmental stages of tomato.

2. Materials and Methods

2.1. Experimental Materials and Management Practices

The tomato cultivar used in this study was “Oguan” (Seminis, Monsanto Seed Company, St. Louis, MO, USA). This experiment was conducted from August to December at the Key Laboratory of Protected Agriculture Engineering (39°54′ N, 116°14′ E), Ministry of Agriculture and Rural Affairs, Beijing, People’s Republic of China. Seeds were sown in a controlled-environment growth chamber, and seedlings were transplanted into a climate-controlled glasshouse after 28 days. The glasshouse was equipped with evaporative cooling via wet pad–fan systems, internal and external shading screens, humidity control devices, supplemental artificial lighting, and CO₂ enrichment systems.

During cultivation, the maximum photosynthetic photon flux density (PPFD) inside the greenhouse reached 1050 μmol (m²·s). Whenever PPFD fell below 100 μmol (m²·s), an automatic inter-plant supplemental lighting system was activated, supplying approximately 120 μmol (m²·s). Air temperature was maintained between 15 °C and 30 °C, relative humidity between 40% and 70%, and CO₂ concentration was kept above 400 μmol/mol.

Quartz sand was used as the cultivation substrate with EC of 53 μS/cm and pH of 6.8, and the potassium content of the matrix was close to 0 μg/g. The culture troughs were fabricated from 4 mm thick gray PVC panels, measuring 120 cm in length, 90 cm in width, and 30 cm in depth. Eight tomato plants were transplanted per trough, with an in-row spacing of 30 cm, a small inter-row spacing of 60 cm, and a large inter-row spacing of 110 cm, corresponding to a plant density of 3.7 plants/m². Each treatment comprised two culture troughs, totaling 16 plants, and it served as one experimental replicate.

2.2. Experimental Design

Four K⁺ supplies were imposed in the nutrient solution. Using the K concentration in Japanese horticultural experimental nutrient solutions (calcium nitrate tetrahydrate, Ca(NO₃)₂·4H₂O, 944 mg/L; potassium nitrate, KNO₃, 808 mg/L; magnesium sulfate heptahydrate, MgSO₄·7H₂O, 492 mg/L; monoammonium phosphate, NH₄H₂PO₄, 153 mg/L) [29] as a baseline, potassium sulfate (K₂SO₄) and KNO₃ were added to adjust the K concentration in each treatment to 1, 4, 8, and 16 mmol L⁻¹, hereafter referred to as K1, K4, K8, and K16, respectively. To avoid tomato mortality due to severe K deficiency, no zero-K control was included.

Each cultivation trough was equipped with an individual nutrient solution tank and a drainage outlet. Two drip irrigation lines, fitted with pressure regulators spaced at 15 cm intervals, were used to deliver nutrient solution evenly around the tomato root zone. A simple automatic fertigation system, composed of a timer and high-speed solenoid valves, was programmed to irrigate for 5 min every hour between 08:00 and 18:00. While sufficient irrigation was ensured in the morning, additional nutrient solution was supplied in the afternoon to maintain a daily drainage ratio of 10–30% from each trough, thereby preventing salt accumulation within the substrate. The volume of discharged nutrient solution was recorded at each drainage outlet, and samples were collected for subsequent analysis. On clear days and at midday, the amount of supplemental nutrient solution was increased to meet the plants’ additional water demand.

Tomato plants were managed as single-stem specimens. The main shoot was topped (removal of the apical meristem) once the two leaflets above the third fruit cluster had fully expanded. Each cluster was thinned to retain 3–4 fruits. Harvesting began when the first cluster reached full maturity, and the experiment concluded after completion of the third cluster harvest. Additionally, we would like to clarify that limiting the harvest to the first three fruit clusters reflects a typical production practice in Chinese solar greenhouses and also contributes to the consistency and controllability of experimental conditions in this study.

2.3. Measurement Metrics and Methods

2.3.1. Water Absorption Capacity and Water Absorption Efficiency

To maintain a daily leaching rate of 10–30% of the applied nutrient solution, each trough was flushed with deionized water and the total volume of leachate collected was recorded. Together with the known daily irrigation volume, these measurements were used to calculate (1) the water absorption capacity per plant (WAC, kg/plant) and (2) the water absorption efficiency (WAE, %).

WAC = K₁(V − V₀)/n

(1)

WAE = (V − V₀)/V × 100

(2)

where V represents the total irrigation volume (mL); V₀ represents the total leachate volume (mL); n represents the number of plants; and K₁ represents the conversion factor, which is equal here to 0.001 kg/mL.

2.3.2. Continuous Transpiration Measurement

The continuous transpiration of tomato leaves was monitored using a dynamic transpiration meter (YZQ-301A; Yizongqi Technology (Beijing) Co., Ltd., Beijing, China). To minimize the influence of fluctuating environmental conditions, the instrument’s four independent leaf chambers were used to measure leaves from the four treatments simultaneously and continuously. The mean leaf water transpiration (W_t, mol/(m²·d)) over seven days of measurements was then calculated.

$${\mathrm{W}}_{\mathrm{t}}={\displaystyle \frac{1}{n}}{\mathrm{K}}_2\int {\mathrm{T}}_{\mathrm{r}}dt$$

(3)

where T_r represents the leaf transpiration rate (mmol/(m²·s)) and K₂ represents the conversion coefficient, which is equal here to 10⁻³ mol/(mmol·d).

2.3.3. Element Absorption Capacity and Element Absorption Efficiency

Each day, 100 mL aliquots of both the irrigation solution and the leachate were collected from the substrate-grown tomato culture. Concentrations of K⁺, NO₃⁻-N, NH₄⁺-N, PO₄³⁻-P, Ca²⁺, and Mg²⁺ were determined by the ASI method [30]. For each nutrient element, element absorption capacity (EAC, g/plant) and element absorption efficiency (EAE, g/plant) were then calculated.

EAC = K₃(CV − C₀V₀)/n

(4)

EAE = M/(CV) × 100

(5)

where C represents the concentration of the nutrient element in the irrigation solution (mg/L); C₀ represents the concentration of the element in the leachate (mg/L); and K₃ represents the conversion coefficient, which is equal here to 10⁻⁶.

2.3.4. Potassium Utilization Efficiency in Tomato Plants

The potassium content in various tomato plant organs, as well as in the nutrient solution leachate, was determined using flame photometry [2]. Total potassium accumulation (TKA, g/plant), potassium physiological efficiency (KPE, g/g), and potassium utilization efficiency (KUE, g/g) were calculated from the biomass and potassium content of each plant organ according to [31].

TKA = Σ(K_o × M_o)

(6)

KPE = M_t/TKA

(7)

KUE = F_y/TKA

(8)

where K_o represents the K⁺ concentration in an organ (mg/g); M_o represents the biomass of that organ (g/plant); M_t represents the total plant biomass (g/plant); and F_y represents fruit yield (g/plant).

2.4. Data Processing and Statistical Analysis

Data processing and statistical analysis were performed using Microsoft Excel 2019 and IBM SPSS 28.0, while figures and tables were generated with Microsoft Excel 2019 and Origin 2021. An analysis of variance (ANOVA) was conducted at a significance level of 0.05, and multiple comparisons were made using the least significant difference (LSD) test.

3. Results

3.1. Effect of K⁺ Supply in Nutrient Solution on Potassium Utilization Efficiency

The total potassium content in tomato plants increased significantly with the rise in K⁺ supply in the nutrient solution, a result jointly determined by the potassium content within different plant organs and their respective biomass accumulation (

Table 1. Potassium physiological efficiency and utilization efficiency of tomato plants.

Treatment	TKA	KPE	KUE
	g/Plant	g/g	g/g
K1	0.71 ± 0.07 d	95.1 ± 4.7 a	18.2 ± 4.4 a
K4	2.19 ± 0.26 c	47.0 ± 3.3 b	9.8 ± 0.4 b
K8	3.24 ± 0.15 b	35.0 ± 1.3 c	9.9 ± 0.8 b
K16	5.09 ± 0.62 a	27.9 ± 1.4 d	8.7 ± 1.7 b

Note: TKA represents the total potassium accumulation, KPE represents potassium physiological efficiency, and KUE represents potassium utilization efficiency. The results are expressed by mean ± standard deviation (SD, n = 3), and treatments with different letters are significantly different at p ≤ 0.05.

). Among the treatments, the K16 treatment exhibited the highest total potassium content, reaching 5.09 g/plant, which was 616.9% higher than that observed in the K1 treatment. Meanwhile, the potassium physiological efficiency of tomato plants declined markedly as the K⁺ supply in nutrient solution increased. Excluding the K1 treatment, tomatoes under the K4 treatment exhibited the highest potassium physiological efficiency at 47.0%, whereas that of the K16 treatment dropped to only 27.9%. A similar trend was observed in potassium utilization efficiency. However, no significant differences were detected among the K4, K8, and K16 treatments.

3.2. Effect of K⁺ Supply in Nutrient Solution on Potassium Absorption Efficiency

As the cultivation period progressed, the potassium absorption capacity of tomato plants under all treatments increased significantly and showed a gradual rise with higher K⁺ supply in the nutrient solution (Figure 1a). Among the treatments, the K16 treatment exhibited the highest potassium absorption capacity, followed by K8, while K1 maintained a consistently low level throughout the experiment. During the seedling stage (1–25 days after transplanting), potassium absorption capacity remained low across all treatments. During the flowering stage (26–42 days after transplanting), the demand for K⁺ from the nutrient solution increased compared to the seedling stage. However, in the later stages of fruit development—specifically during the fruit set stage (43–75 days after transplanting) and fruit maturation stage (76–125 days after transplanting)—the potassium absorption capacity of tomato plants exhibited considerable fluctuations across treatments. Notably, plants in the K8 treatment showed the highest potassium absorption capacity during the late fruit development stage.

In the early stages of tomato growth, the potassium absorption efficiency of all treatments remained relatively low, whereas a substantial improvement in potassium absorption efficiency was observed during the mid-growth stages (Figure 1b). Overall, the K16 treatment exhibited the lowest potassium absorption efficiency, while the K8 treatment achieved the highest, followed by the K4 treatment.

According to the total potassium absorption of tomato plants under each treatment throughout the entire growth period (Figure 2), the K8 treatment exhibited the highest potassium absorption capacity, reaching 7.2 g/plant, with a potassium absorption efficiency of 79.1%. In contrast, the potassium absorption capacity of the K4 and K16 treatments were only 3.19 g/plant and 5.95 g/plant, respectively, with corresponding absorption efficiencies of 60.1% and 38.2%. The K1 treatment showed a potassium absorption capacity of 1.76 g/plant and an absorption efficiency of 59.0%.

3.3. Effects of K⁺ Supply in Nutrient Solution on Water Absorption Efficiency

As shown in Figure 3a, there were no significant differences in water absorption capacity among the tomato plants under different treatments during the early vegetative growth stage. However, as the cultivation period progressed, differences in biomass accumulation among treatments became more pronounced, and corresponding differences in water absorption capacity also gradually emerged. During the flowering stage, water absorption capacity in the K8 and K16 treatments began to exceed that of K1 and K4. In the fruit-setting and enlargement stages, the K8 treatment exhibited the highest water absorption capacity, followed by K16, while K1 maintained the lowest values. During the fruit maturation stage, the general trend observed during fruit enlargement persisted, although the difference between K1 and K4 treatments was not statistically significant. A similar pattern was observed between water absorption capacity and water absorption efficiency among the treatments (Figure 3b), indicating that tomato plants with higher water absorption capacity also demonstrated higher water absorption efficiency, with efficiency generally exceeding 50% across all treatments.

By analyzing the water absorption capacity and water absorption efficiency of tomato plants across the entire growth period under different treatments, it was found that the K8 treatment exhibited both the highest water absorption capacity and the highest absorption efficiency. Throughout the full growth cycle, the K8 treatment achieved a total water absorption of 47.05 kg/plant and a water absorption efficiency of 84.6% (Figure 4a). In contrast, the K1 treatment had the lowest water absorption capacity and absorption efficiency, representing only 75.4% and 76.9% of the values observed in K8, respectively. Water absorption capacity between the K4 and K16 treatments was similar. However, the water absorption capacity and water absorption efficiency values of K16 were 6.3% and 10.3% lower than those of K8, respectively. The continuous monitoring of leaf transpiration rates revealed that the K1 treatment exhibited the highest transpiration rate and cumulative transpiration volume, although the differences from K4 and K8 were not statistically significant, whereas the K16 treatment consistently showed the lowest transpiration rates (Figure 4b).

3.4. Effects of K⁺ Supply in Nutrient Solution on Absorption Capacity and Absorption Efficiency of Other Elements

As shown in Figure 5, the absorption capacity and absorption efficiency of nitrate-nitrogen, ammonium–nitrogen, total nitrogen, available phosphorus, and available calcium by tomato plants exhibited a trend of first increasing and then decreasing with the rise in K⁺ supply in the nutrient solution. For available magnesium, the absorption capacity also followed a similar trend of an initial increase followed by a decline, whereas the absorption efficiency gradually increased. Except for magnesium, tomato plants under the K8 treatment showed higher absorption capacity and absorption efficiency for all measured nutrient elements compared to other treatments, with all nutrient absorption efficiencies exceeding 60%. Tomato plants under the K1 treatment exhibited the lowest absorption capacity and efficiency for nitrate–nitrogen, total nitrogen, available phosphorus, and available magnesium. In contrast, plants under the K4 treatment showed the lowest absorption capacity and efficiency for ammonium–nitrogen and available calcium.

With the increase in K⁺ supply in the nutrient solution, the ratio of K⁺ to other nutrient elements in the solution increases linearly (Figure 6). Based on the absorption of nutrient elements by tomato plants from the nutrient solution, it was found that the ratio of K⁺ to other nutrient elements absorbed by the plants showed a quadratic relationship with the K⁺ supply in the nutrient solution, with correlation coefficients exceeding 0.95. The regression equation of this quadratic relationship suggests that the maximum ratios of K⁺ to nitrogen, phosphorus, calcium, and magnesium correspond to K⁺ supply in the nutrient solution of 12.4, 11.0, 12.7, and 10.5 mmol/L, respectively. The intersection points of the quadratic relationship curves of the nutrient absorption ratios by tomato plants and the nutrient element ratio curves of the nutrient solution, which represent the balance points of K⁺ with nitrogen, calcium, and magnesium, indicate that when the K⁺ amounts in the nutrient solution are 8.1, 12.6, and 11.2 mmol/L, the ratio of nutrient elements absorbed by the tomato plants matches the ratio of nutrient elements supplied in the nutrient solution.

4. Discussion

4.1. Potassium Utilization Efficiency

Previous studies have demonstrated that potassium physiological efficiency during the reproductive stage of tomato plants is significantly positively correlated with individual plant yield. This is primarily due to the dynamic redistribution of K⁺ between source and sink organs to maximize the use of limited K⁺ resources [32]. In the present study, it was found that although the K16 treatment resulted in the highest potassium content in tomato plants, potassium physiological efficiency significantly declined with increasing K⁺ supply, mainly due to the phenomenon of “luxury consumption” of K⁺ in the plants [33], which is consistent with the results of Zhao et al. [32]. Therefore, when regulating K⁺ in nutrient solutions to promote fruit yield formation, it is essential to simultaneously consider improving potassium physiological efficiency. Notably, there were no significant differences in potassium utilization efficiency among the K4, K8, and K16 treatments, suggesting that moderately reducing K⁺ supply may contribute to more rational potassium fertilizer use [7]. It is also worth mentioning that the potassium physiological efficiency and potassium utilization efficiency observed in this study were relatively lower compared to the results reported by Hou et al. [34], which may be attributed to the cultivation strategy used here, namely the removal of the shoot apex at the fruit-setting stage and the initiation of harvest after the first fruit cluster matured, while the second and third clusters had not yet fully developed.

4.2. Potassium Absorption Efficiency

In the present study, it was observed that the potassium absorption efficiency during the seedling stage was generally low, particularly under low K⁺ supply conditions. This phenomenon may be closely associated with the physiological strategy of prioritizing the allocation of photosynthetic products toward nitrogen assimilation and leaf area expansion during early growth [35]. Across all treatments, the potassium absorption capacity of tomato plants gradually increased with higher K⁺ supply in the nutrient solution, which is consistent with findings reported by Çolpan et al. [36]. Zörb et al. [11] found that the K⁺ absorption rate during the flowering stage could reach 4.5–6.8 mg/(g DW·d)), representing a 5–7 times increase compared to the seedling stage.

In this study, a significant increase in K⁺ absorption during the flowering stage was observed, accompanied by evidence of “luxury consumption,” likely reflecting an adaptive response to the increased K⁺ demand associated with reproductive growth [10,37]. During the late stages of fruit development, K⁺ absorbed by tomato plants exhibited considerable fluctuations across treatments. This variability was mainly attributed to environmental factors affecting water absorption and the transpiration rate, subsequently influencing K⁺ absorption. These findings are consistent with the observations of Kanai et al. [38], who also reported that the variability in K⁺ absorption during fruit maturation is primarily an integrated response of the plant to environmental stresses rather than a simple manifestation of a metabolic disorder.

In this study, it was found that both the potassium absorption efficiency and the total potassium absorption of substrate-grown tomatoes exhibited a clear dose–response relationship throughout the entire growth period. Among the treatments, K16 treatment showed the lowest potassium absorption efficiency, while K8 treatment achieved the highest, indicating that potassium absorption efficiency initially increased and then decreased as K⁺ supply rose. This result differs somewhat from findings reported by Savita et al. [39], possibly due to differences in plant species leading to distinct K⁺ utilization characteristics.

Furthermore, the potassium absorption capacity was highest in K8 treatment plants, while the values for K4 and K16 were relatively lower. The reduced K⁺ absorption observed in K16 treatment may be attributed to the high external K⁺ concentrations facilitating ammonium (NH₄⁺) absorption, which leads to plasma membrane hyperpolarization and subsequently inhibits K⁺ influx [40]. Although this study included the long-term monitoring of selected experimental plots, the results consistently indicate that only an appropriate K⁺ supply in nutrient solution can significantly enhance both potassium absorption capacity and absorption efficiency in tomato plants, which was reported by Song et al. [2].

4.3. Water Absorption Efficiency

In this study, it was observed that there were no significant differences in water absorption capacity among treatments during the early vegetative stage; however, during the reproductive stage, water absorption capacity under the K8 treatment was significantly higher than that of other treatments, consistent with the findings of Ullah et al. [41] and Cui et al. [42]. This phenomenon can be attributed to the fact that an increased K⁺ supply enhances water absorption capacity, particularly under water stress conditions [43]. The reduced water absorption capacity in the K16 treatment may result from excessive nutrient solution concentrations, which increase the resistance of the tomato root system to water absorption capacity [16,44]. Throughout the entire growth stage, the K8 treatment achieved the highest total water absorption capacity and absorption efficiency, indicating that the water demand of tomato plants first increased and then decreased with rising K⁺ supply, in agreement with previous studies reported by Qin et al. [45]. However, no significant differences in water absorption efficiency were found between the K1 and K4 treatments.

The continuous monitoring of transpiration rates enables the real-time quantification of dynamic water metabolism, revealing the diurnal rhythms of water use and transient responses to environmental stress [45]. Moreover, transpiration, as the primary driving force for nutrient absorption and transport, plays a crucial role in enhancing plant tolerance to salt stress [46,47]. Notably, this study found that the continuously monitored transpiration rate and cumulative transpiration volume were highest under the K1 treatment and lowest under the K16 treatment, which may be due to salt stress imposed on the roots by high electrical conductivity (EC) of the nutrient solution [13,48]. However, this observation presents a clear discrepancy with total water absorption capacity results. The likely explanation is that the lower biomass of K1 treatment limited the increase in effective transpiring surface area [49]. Therefore, when regulating the K⁺ supply in nutrient solutions, it is essential to ensure sufficient water supply and a relatively stable root zone environment [50], thereby promoting a balanced absorption of water and K⁺ by tomato plants.

4.4. Absorption Capacity and Absorption Efficiency of Other Elements

In the present study, it was found that an appropriate K⁺ supply significantly enhanced tomato plants’ absorption of other nutrient elements, which is consistent with the results of Darko et al. [51]. This improvement was related not only to a balanced ionic concentration in the nutrient solution but also to interactions among different ions [52,53]. Varying K⁺ supply disrupted the original balance between K⁺ and other nutrient elements in the nutrient solution, leading to changes in the absorption efficiency of each element [54]. For instance, in the K16 treatment, the high K⁺ concentration restricted the absorption of available nitrogen and calcium by tomato plants while having little impact on the absorption of available phosphorus and magnesium, which was similar to the results of Chapagain et al. [54].

The intersection between the ratio of K⁺ to other nutrients in the nutrient solution and the corresponding ratio in plant absorption represents the point at which the absorption proportions by tomato plants align with the supplied proportions [55,56]. In this study, it was observed that the nutrient absorption ratios of tomato plants closely matched the nutrient supply ratios, particularly at the intersection points of the quadratic correlation curves, which closely corresponded to the nutrient absorption values recorded in the K8 treatment. This finding is consistent with the highest water and fertilizer absorption capacity and efficiency observed in the K8 treatment and was also similar to the results reported by Song et al. [2]. Therefore, when regulating K⁺ supply in nutrient solutions to promote K⁺ absorption in tomatoes, it is essential to also consider the relationship between K⁺ and other nutrient elements. Ensuring a balanced absorption of all essential nutrients is critical to preventing physiological disorders, particularly calcium absorption deficiency, which can lead to the occurrence of blossom end rot in tomato fruits [57].

5. Conclusions

This study systematically analyzed the effects of different K⁺ amounts in nutrient solutions on water and nutrient absorption characteristics and potassium utilization efficiency in substrate-grown tomato plants. The results demonstrated that K⁺ supply exerts significant regulatory effects on water and fertilizer absorption, as well as on the coordinated utilization of nutrient elements in tomato plants. An appropriate K⁺ supply (8 mmol/L) resulted in the highest water absorption capacity and absorption efficiency during the reproductive growth stage while also promoting the efficient absorption and utilization of potassium and other major nutrient elements (nitrogen, phosphorus, calcium, and magnesium).

Based on the overall results of this study, it can be concluded that water and potassium uptake in tomato plants exhibits a typical dose-dependent response and stage-specific variation. Scientifically regulating the potassium supply in nutrient solutions—particularly maintaining an appropriate concentration at 8 mmol/L—not only enhances water and nutrient absorption capacity and potassium utilization efficiency but also contributes to achieving a balanced uptake of other essential nutrient elements.