Critical Leaf Magnesium Concentrations for Adequate Photosynthate Production of Soilless Cultured Cherry Tomato—Interaction with Potassium

Magnesium (Mg) is essential to many plant physiological and biochemical processes; however, understanding how Mg nutrition quantitatively affects the production, partitioning, and utilization of photoassimilates is still lacking, especially in soilless culture systems. We focused on the roles of Mg in yield formation and interactions with potassium (K) nutrition of cherry tomato. Cherry tomato yield, photosynthetic parameters, dry matter weight, and K, Mg, and calcium (Ca) uptake were investigated in two soilless experiments with seven Mg levels and five K levels. The results showed that low (<1 mM) and high (>4 mM) Mg supply limited cherry tomato yield by decreasing dry matter accumulation by 22.6–78.1% and harvest index by 13.9–40.7%. The critical leaf Mg concentrations required for adequate photosynthate production in the first and second harvest periods were 4.67 and 5.52 g·kg−1, respectively. However, over-supply of Mg reduced leaf K and Ca concentrations and limited plant uptake of K and Ca. Moreover, adjusting K concentrations in solution could influence plant Mg functions in photosynthesis and, therefore, cherry tomato growth. Overall, balanced Mg and K application increased Mg, K, and Ca uptake, as well as Mg concentrations in leaves, which could maintain a sustainable photosynthetic rate and plant dry matter formation.


Introduction
Greenhouse-based vegetable crop production has expanded considerably in recent decades [1,2]. Greenhouse soilless culture facilitates high-yield and high-quality vegetable production by controlling growth environment factors such as temperature, light, and nutrients; among these factors, balanced nutrient supply is particularly important [3][4][5][6]. On the other hand, obvious symptoms of Mg deficiency frequently occur in such a system, especially in rapid-growth stages due to imbalanced fertilization or an unfavorable root environment, which causes negative impacts on vegetable yield and quality [7][8][9]. Therefore, it is imperative to enhance Mg supply for optimal plant physiological functions in this system.
Ensuring sufficient biomass and increasing the harvest index (HI, the ratio of fruit dry matter weight to total plant dry matter weight) of crops at the key growth stages are two effective methods

Experimental Setup
Two experiments were conducted under greenhouse conditions from March to July 2016 and March to August 2017 at China Agricultural University (40 • 02 north (N), 116 • 17 east (E)). Cherry tomato (Solanum lycopresicum L. cv. Qianxi) seeds were procured from Shandong Nongyou Seeds Co., Ltd., Weifang, China. Seeds were sown in 50-cell plug trays filled with a commercial substrate and were germinated and grown in a temperature-controlled chamber (the daylight conditions comprised a 12.5/8 h light/dark cycle with an average temperature of 23.2 • C and 18.8 • C during the day and night, respectively). One month after sowing, the cherry tomato plants with four fully unfolded true leaves were transplanted into a growth medium consist of coconut coir at a planting density of 40,000 plants·ha −1 . The available K, Ca, and Mg contents and the cation exchange capacity of coconut coir were 0.50%, 0.30%, 0.13%, and 683 mmol·kg −1 . Daily temperatures in the greenhouse was recorded every 2 h by an automatic temperature recorder (DS1922L, produced by Wdsen Electronic Technology Co., Ltd., Shanghai, China), as shown in Figure S1 (Supplementary Materials).
In the Mg experiment, cherry tomato plants were fertilized with seven Mg treatments (0, 0.5, 1, 2, 4, 8, and 16 mM) together with 12 mM K supplied in nutrient solution. In the K experiment, taking the results of the Mg experiment into account, we applied five levels of K (7,12,17,22, and 27 mM) and 2 mM Mg in nutrient solution. The Mg or K molar concentration was calculated using the following formula: c = m/MV, where c is the concentration of Mg 2+ or K + in the solution (mM), m is the mass of Mg or K (mg), M is the molar mass of Mg or K (g·mmol −1 ), and V is the volume of the solution (L). The magnesium sulfate and potassium nitrate were dissolved in deionized water. The cherry tomato plants were transplanted on 20 April 2016 and 26 April 2017 in the Mg and K experiments, respectively. In both experiments, the experimental design was a randomized block design with three replications. Two side rows were planted as guard rows in each experiment; thus, we planted a total of nine and seven plant rows for the Mg and K experiments, respectively, and there were 15 plants in each row. In all treatments, we supplied 240 mg·L −1 N and 35 mg·L −1 P during the seedling and anthesis periods in all treatments, while 230 mg L −1 N and 22 mg L −1 P were supplied during the fruit stage. We supplied 90 mg L −1 Ca, 6.4 mg L −1 iron (Fe), 0.8 mg L −1 manganese (Mn), 0.2 mg L −1 zinc (Zn), 0.1 mg L −1 copper (Cu), and 0.5 mg L −1 boron (B) during every irrigation period, and we adjusted the pH to 5.5-7.0 using nitric acid (HNO 3 ) or sodium hydroxide (NaOH) every 2 days. Each treatment had an independent fertigation system controlled by an electromagnetic relay. The fertigation frequency was determined by weather conditions and plant requirements. If needed, the electromagnetic relay worked, and the plants were watered up with solutions containing different amounts of K or Mg. Flow-through water was collected and poured once again to recycle the solution in the closed system. During the whole growth period, some measures (e.g., a hanging yellow sticky trap) were adopted to prevent plant diseases and insect pests.

Leaf and Plant Measurements
At anthesis (about 30 days after transplanting, one or two bunches of flowers blooming), first harvest period (about 60-70 days after transplanting, two to four bunches of fruits maturing), and second harvest period (about 85-103 days after transplanting, four to six bunches of fruits maturing), the roots, stems, leaves, and fruits of the cherry tomato plants were harvested. Total fresh fruit yield and marketable fruit yield (single fruit weight >15 g; no damage) were recorded at every harvest. Plant samples were washed with tap water and deionized water, and then dried at 75 • C to constant weight. Dry samples were ground using a stainless-steel grinder for K, Ca, and Mg analyses. A certain amount of samples (0.3 g) were digested with HNO 3 -H 2 O 2 (6 mL of HNO 3 and 2 mL of H 2 O 2 ) in a microwave-accelerated reaction system (CEM, Matthews, NC, USA), and the K, Ca, and Mg concentrations in the digesting solutions were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, OPTIMA 3300 DV, Perkin-Elmer, Waltham, MA, USA).
Standard materials for K, Ca, and Mg analyses (IPE126) were obtained from Wageningen Evaluation Programs for Analytical Laboratories (WEPAL, Naaldwijk, The Netherlands).
In the Mg experiment, we randomly selected one middle leaf from the fourth branch on plant in each repetition in the morning (09:00 a.m.-12:00 p.m.) to measure photosynthetic rate using a portable photosynthesis apparatus (LCpro-SD; ADC BioScientific Ltd., Hoddesdon, UK) under natural sunlight conditions with a light intensity of 500-800 µmol·m −2 ·s −1 and circumambient CO 2 concentration of 340-370 µM. The SPAD readings were taken with a chlorophyll meter (SPAD-502, Minolta, Tokyo, Japan), measurements were conducted in the middle leaf of the third or fourth branch from tip on plant, and the mean of six values was taken as the final result for each repetition. This procedure was followed at anthesis and repeated during the first and second harvests.

Statistical Analysis
SAS v. 8.0 (SAS, Cary, NC, USA) and SPSS v. 20.0 (SPSS, Chicago, IL, USA) software was used for statistical analyses. Means were compared using analysis of variance (ANOVA), followed by Duncan's multiple range test at a significance level of p < 0.05. The responses of SPAD, photosynthetic rate, and plant dry matter to leaf Mg concentration were described using the linear-with-plateau model.

Fruit Yield, Biomass, and HI Were Affected by Mg Treatment Concentration
With increasing Mg concentrations in nutrient solution, the cherry tomato fruit yields increased to 293 and 425 g·plant −1 in the first and second harvest periods, respectively, and then decreased (Table 1). In both harvest periods, the fruit yields in the 1-4 mM Mg treatments were significantly higher than those in the 0, 0.5, and 16 mM Mg treatments (Table 1). In the first harvest period, cherry tomato yields and plant DWs did not differ significantly among the 1-8 mM Mg treatments, but were affected at very low (<1 mM) or high (>8 mM) Mg levels. By contrast, in the second harvest period, cherry tomato yields and plant DWs were more sensitive at higher Mg treatment concentrations (Table 1).

Leaf Mg Concentration Regulated Leaf Chlorophyll, Photosynthetic Rate, and Plant DW
Low concentrations of Mg in nutrient solution significantly reduced Mg concentrations in leaves, especially at the later growth stage (Table 2). Cherry tomato plants showed the typical symptom of Mg deficiency with Mg concentration in solution below 1 mM, including leaf yellowing in the form of interval chlorosis in older leaves in early anthesis, slow growth, and small fruits at the harvest period. Compared with the 1 mM Mg treatment, the 0 mM Mg treatment decreased the leaf Mg concentrations by 17.9%, 26.8%, and 31.7% at anthesis, first harvest, and second harvest, respectively ( Table 2). The leaves of plants supplied with low levels of Mg had a significantly lower SPAD reading; thus, net photosynthetic rates were also lower in the lower Mg treatments (Table 2). A linear-with-plateau model produced the best fit for the relationships between SPAD reading and photosynthesis rate against leaf Mg concentration at the first and second harvest; however, the model did not fit at the anthesis stage (Figure 1a

Oversupply of Mg Reduced Leaf K and Ca Levels, and Limited Plant K and Ca Uptake
Leaf K and Ca levels were affected by Mg supply. In the low-Mg treatment (<1 mM), the K and Ca concentrations in leaves decreased as Mg supply increased (Table 3). Compared with the 1 mM solution Mg treatment, the leaf K and Ca levels decreased from 43.4 to 30.0 g·kg −1 and 17.8 to 11.5 g·kg −1 in the 16 mM treatment (Table 3). However, fruit Ca concentration was disturbed by Mg supply levels to a greater extent than K, and it significantly decreased when Mg concentration in solution exceeded 2 mM (Table 3).
Plant DWs were slightly lower in the 16 mM Mg treatment than in the 4 mM treatment at first and second harvest (Table 1). Plant nutrient uptakes are determined by plant DWs and nutrient concentrations. Therefore, plant Mg, K and Ca uptakes showed different trends. The uptakes of K and Ca first increased before reaching 1 mM with the Mg treatment level and then decreased. In contrast, plant Mg uptake increased significantly as nutrient solution Mg concentration increased (Table 3).

Potassium Supply Affected Leaf K and Mg Levels, in Turn Affecting Plant DW, Mg Uptake, and Fruit Yield
K supply determined leaf K and Mg concentrations (Figure 2a,b). In the K experiment, leaf Mg concentration decreased when solution K concentration increased, and leaf Mg concentration was below 4.67 mg·kg −1 when solution K concentration exceeded 17 mM (Figure 2). However, leaf K concentration showed the opposite trend, with levels below 38.0 mg·kg −1 when solution K concentration was less than 12 mM (Figure 2).
The plant DW of cherry tomato was 92.6 g·plant −1 at 12 mM K supply, which was 17% higher than that of the 7 mM K supply treatment. It first increased with leaf K concentration before reaching 41.9 g·kg −1 and then decreased. In contrast, plant DW increased with increasing leaf Mg   (Figure 2a,b). In the K experiment, leaf Mg concentration decreased when solution K concentration increased, and leaf Mg concentration was below 4.67 mg·kg −1 when solution K concentration exceeded 17 mM (Figure 2). However, leaf K concentration showed the opposite trend, with levels below 38.0 mg·kg −1 when solution K concentration was less than 12 mM (Figure 2). concentrations in nutrient solution ( Table 4).
The total yield of cherry tomato was highest when K concentration in solution was 12 mM. The marketable fruit rate decreased from 92.4% to 84.9% as K supply varied from 7 to 27 mM (Table 4). Thus, K supply levels regulated plant DW via leaf K and Mg concentrations, subsequently influencing cherry tomato yield and Mg, K, and Ca uptake (Table 4; Figure S2, Supplementary Materials).   The plant DW of cherry tomato was 92.6 g·plant −1 at 12 mM K supply, which was 17% higher than that of the 7 mM K supply treatment. It first increased with leaf K concentration before reaching 41.9 g·kg −1 and then decreased. In contrast, plant DW increased with increasing leaf Mg concentrations (Figure 2c,d). Generally, higher plant DWs were obtained at 12-22 mM K concentrations in nutrient solution (Table 4). Cherry tomato fruit † Agronomy 2020, 10, x FOR PEER REVIEW 7 of 14 concentrations (Figure 2c,d). Generally, higher plant DWs were obtained at 12-22 mM K concentrations in nutrient solution (Table 4). The total yield of cherry tomato was highest when K concentration in solution was 12 mM. The marketable fruit rate decreased from 92.4% to 84.9% as K supply varied from 7 to 27 mM (Table 4). Thus, K supply levels regulated plant DW via leaf K and Mg concentrations, subsequently influencing cherry tomato yield and Mg, K, and Ca uptake (Table 4; Figure S2, Supplementary Materials).  The total yield of cherry tomato was highest when K concentration in solution was 12 mM. The marketable fruit rate decreased from 92.4% to 84.9% as K supply varied from 7 to 27 mM (Table 4). Thus, K supply levels regulated plant DW via leaf K and Mg concentrations, subsequently influencing cherry tomato yield and Mg, K, and Ca uptake (Table 4; Figure S2, Supplementary Materials).

Mg Application Affected Photosynthate Production and Distribution
Cherry tomato yield and dry matter accumulation were significantly affected by solution Mg concentration, which is consistent with the findings of Nzanza (2006) [28]. Increased yield and dry matter accumulation in response to proper Mg application were also observed by Hao and Papadopoulos (2003), who reported decreased fruit yield in the late growth stage at 0.82 mM solution Mg supply in rockwool blocks [4]. Moreover, in later studies, Hao and Papadopoulos (2004) explained this as being the result of a decrease in biomass and fruit biomass allocation in the low-Mg treatment [17]. We observed that lower plant DW and HI reduced yield in response to low-Mg application (<1 mM). The different water-holding capability and buffer ability of growth media were the main reasons for the difference since the marketable yield was also affected by the soilless culture system [29].
The photosynthetic rates of the cherry tomato plants decreased significantly in the 0 and 0.5 mM treatments. A previous study reported that the middle and bottom leaves of cherry tomato plants grown in a soilless production system showed leaf chlorosis under Mg starvation, losing about 50% of their photosynthetic capacity [4]. Impairment of sugar metabolism, photosynthetic CO 2 fixation, and stomatal conductance were reported by Cakmak et al. (1994) and Fischer et al. (1998) in bean and spinach plants [30,31], and Andersson (2008) demonstrated that the involvement of rubisco in CO 2 fixation was adversely affected by poor Mg supply [32].
The decrease in HI among cherry tomato plants observed under lower Mg supply in this study indicates the suppression of assimilate distribution to fruits. Sugar accumulation in source organs and the decline of its distribution to sink tissues were reported previously. Hermans et al. (2004) found that sucrose accumulated in the most recently expanded sugar beet leaves before any loss of photosynthetic activity under Mg deficiency treatment [14]. Farhat et al. (2016) attributed this to preference of Mg transported to source leaves to prevent severe declines in photosynthetic activity [21]. Mg starvation seems to have a direct detrimental effect on function and/or structure of phloem loading [21,30,33,34].

Relationships among Leaf SPAD Reading, Photosynthetic Rate, Plant DW, and Leaf Mg Concentration
Leaf Mg concentrations increased continuously as solution Mg levels increased in this study. A former study of Sulla carnosa plants also showed increased leaf Mg concentrations, to 2.5-, 7-, and 25-fold that of the control (0 mM Mg treatment) in 0.01, 0.05, and 1.50 mM Mg treatments, respectively [12]. In this study, the linear-with-plateau model illustrated the relationship between SPAD reading and leaf Mg concentration at the first and second harvests, and the critical leaf Mg concentrations for SPAD reading were about 4.67 and 5.52 g·kg −1 in these periods. The SPAD reading is an indicator of leaf chlorophyll concentration, which determines photosynthetic rate to a great extent [21]. Therefore, photosynthesis rates also fitted this model, and the critical leaf Mg concentrations for photosynthesis rates were 4.41 and 5.01 g·kg −1 at the first and second harvests. A previous report indicated that maintenance of normal plant growth required 4.0-6.0 g·kg −1 leaf Mg concentration in tomato plants at anthesis, and the marginal concentration in the first harvest period was 3.0 g·kg −1 [35]. The linear-with-plateau model was also applied to dry matter formation, and the critical leaf Mg concentrations were 4.38 and 4.50 g·kg −1 at the first and second harvests, slightly lower than those for the photosynthesis rate. Similarly, dry matter accumulation in Pinus radiata was shown to be inhibited by Mg deficiency [36]. Hauer-Jákli and Tränkner (2019) confirmed 3.9 g·kg −1 as the critical leaf Mg concentration for tomato dry matter accumulation on the basis of the results of Kasinath et al. (2014) using the scattered plot technique, which was lower than this study [16,37] (Table S1, Supplementary Agronomy 2020, 10, 1863 9 of 14 Materials). The different critical leaf Mg concentrations with respect to SPAD reading, photosynthesis rate, and plant dry matter accumulation indicated that sufficient Mg supply can guarantee the chlorophyll concentration and the production of photosynthates. The result was consistent with a study showing that plant growth reduction appeared as a later response compared with chlorophyll content decrease caused by Mg deficiency [15]. Clear relationships were observed between SPAD reading, photosynthesis rate, plant dry matter accumulation, and leaf Mg concentration. These relationships may be explained by the adequate Mg supply during initial growth stages [16]. The results above clearly demonstrate the importance of Mg supply in maintaining strong photosynthesis to produce cherry tomato dry matter.

Two Side Effects of Mg Application on the Plant K and Ca Content
In the second harvest period, the plant K and Ca uptakes of cherry tomato first increased with Mg concentration in solution lower than 1 mM and then decreased, as indicated by the plant dry matter accumulation and the leaf K and Ca concentration.
The plant dry matter accumulation increased first with Mg concentrations in solution increasing but decreased for Mg treatment concentrations over 8 mM and 4 mM at the first and second harvest periods. The positive effects of Mg nutrient supply on plant growth have been discussed extensively [16,36,38]. The inhibitive effects observed in this study have rarely been reported due to the difficulty in detecting toxicity symptoms, even at high concentrations [39]. The inhibitive effect of high Mg supply on plant dry matter accumulation was caused by slight decreases in the photosynthetic rate at first and second harvest. A similar effect was observed by Rao et al. (1987), who found that net photosynthesis was inhibited to a much greater extent in sunflower plants with a high Mg 2+ content, particularly during dehydration [40]. Moreover, Shaul (2002) and Koch et al. (2019) associated this decrease with K + transport inhibition from the cytosol to the stroma, disequilibrium within the chloroplast, and interference in transport events across the tonoplast [10,20]. Mun et al. (2020) explained the deleterious effects of Mg oversupply on Perilla frutescens growth and yield as the decrease in relative abundance of bioactive phytochemicals, such as triterpenoids, flavonoids, and cinnamic acids [41].
Low leaf K and Ca levels with high Mg supply treatments indicate antagonistic effects among these cations [26]. When solution Mg concentration was higher than 4 mM, the leaf K concentration was lower than 38.0 g·kg −1 , which might induce K deficiency [35,42]. Leaf Ca concentration also decreased in higher Mg supply treatments, but was higher than 10 g·kg −1 [35]. The response of fruit Ca concentration was more sensitive than that of K to Mg concentrations in this study, consistent with the results of Marschner (2012), who reported sevenfold higher K distribution than Ca distribution in pea seeds [26], while Karley and White (2009) also noticed this phenomenon [43].

K Application Influenced Cherry Tomato Growth by Regulating Plant Mg and K
Antagonistic effects of K on Mg, especially under inadequate Mg supply conditions, are a crucial factor influencing Mg-related functions in several crops, including tomato [42,44], green bean [45], potato [46], rice [47], grape [48], and apple [49]. The present study showed that an increasing K concentration in solution adversely affected the leaf Ca and Mg concentrations. Leaf Mg and Ca concentrations were lower than 4.7 and 10.0 mg·kg −1 when solution K concentrations exceeded 17 mM under the 2 mM solution Mg supply, indicating Mg (from the former experiment in this study) and Ca deficiency [35]. However, K deficiency may have occurred when solution K concentration was less than 12 mM because leaf K concentration was lower than 38.0 mg·kg −1 [35,42]. These findings may explain the influence of K supply levels on total yields and plant DWs. This result was in line with Yurtseven et al. (2005) and Sonntag et al. (2019), who reported that significant yield increases with increasing K application [50,51]. However, Nzanza (2006) found that none of the applied K treatments had any significant effect on marketable tomato yield [28]. The difference could be explained by the maximum K supply concentration, 9 mM in the study reported by Nzanza (2006) and 27 mM in this study [28]. Moreover, the different growth media used in the studies may also have been responsible for the difference.
Leaf K, Ca, and Mg concentrations are regulated by the K concentration in solution, as well as plant K, Ca, and Mg uptake. Ali et al. (1991) found that K, Ca, and Mg leaf contents in tomato decreased to 38%, 45%, and 67% of that of control plants under low K, low Ca, and low Mg supply, respectively, and that leaf, stem, and petiole dry matter also decreased significantly [52]. Another study reported that rice shoot DW decreased by 12.9% at high K/Mg ratios in solution, whereas root DW increased by 12.1% as sugar partitioning and root morphological parameters changed [53]. Toumi et al. (2016) also reported that Mg uptake was inhibited by an increase in K/Mg in the nutrient solution in Vitis vinifera, but no significant differences in leaf Ca concentration were detected among treatments [48].

Mg and K Management in Soilless Vegetable Production
Since the functions of Mg in the production, partitioning, and utilization of plant photoassimilates are irreplaceable, adequate Mg supply in the rhizosphere is essential for high-productivity soilless vegetable production systems. According to our results, 1-4 mM Mg in solution was needed to ensure leaf Mg concentrations exceeding 4.67 g·kg −1 at the early harvest and 5.52 g·kg −1 at late harvest, which could also satisfy the requirements for optimized SPAD, photosynthesis rate, and plant dry matter accumulation combined with high fruit yield. These leaf Mg concentrations are slightly higher than that reported in previous studies, which demonstrated that tomato dry matter accumulation responded best at 3.9 g·kg −1 plant Mg concentration [37,54]. However, excessive Mg concentrations (>8 mM) in solution should be avoided due to the risk of adverse effects on photosynthesis. Toxic effects that impair crop growth and development were also shown by Guo et al. (2015) when Mg concentration in soil solution was higher than 8.5 mM [19].
Mg deficiency is a common problem in growth media fertilized only with N, P, and K [7,21]. Consequently, harmonious crop-specific nutrient management requires further attention. Overuse of K fertilizer not only wastes K resources but also disturbs Mg uptake and reduces yield [53,55]. Therefore, K concentrations in soilless culture systems should be managed to supply sufficient leaf K to achieve high yield, while avoiding Mg uptake suppression due to excessive K. Moreover, the plant yield and quality are closely related to plant growth media, which could influence the nutrient supply concentrations in solution [29]. Wang et al. (2009) reported that higher yields were obtained when solution K concentration was at 2.5-5 mM for cherry tomato growing in plastic pots filled with sand [56]. Hao and Papadopoulos (2003) applied 10 mM K in the nutrient solution for greenhouse tomato crop grown on rockwool [4]. Constan-Aguilar et al. (2014) observed that cherry tomato fruit dry matter was higher when K concentrations ranged from 10 to 15 mM using perlite as the growth medium [57]. Comparatively, in this study, we used coconut coir as the growth medium, and the results indicate that 12 mM K in solution is optimal in this environment according to our nutrient uptake and photosynthate production results. Therefore, the optimal nutrient concentration in solution is usually higher when using growth media with a higher cation exchange capacity and greater water-holding capability [58]. We also established relationships of leaf K or Mg concentrations with cherry tomato dry matter in this study, which are crucial for understanding the mechanisms of yield formation in soilless vegetable production systems.

Conclusions
Inadequate Mg supply impairs yield by influencing production and distribution of photosynthesis products. When Mg supply in solution varied from insufficient to adequate, the SPAD reading, photosynthesis rate, and plant dry matter accumulation of cherry tomato increased first with leaf Mg concentration and then plateaued. As indicated by the plateau, critical leaf Mg concentrations were 4.67 and 5.52 g·kg −1 at the first and second harvest periods, illustrating that sufficient Mg supply was crucial for the proper chlorophyll concentration and the production of photosynthates. Moreover, plant dry matter accumulation was inhibited at high Mg treatment levels as a result of a slight decrease in the photosynthetic rate in the first and second harvest periods. As a crucial factor influencing functions of Mg, K concentrations in solution influence cherry tomato growth by regulating plant Mg and K uptake. The leaf Mg concentration decreased when solution K concentration increased. Generally, 1-4 mM Mg in solution was needed for cherry tomato to satisfy the requirements for optimized SPAD, photosynthesis rate, and plant dry matter accumulation combined with high fruit yield. Similarly, a K concentration of 12 mM in solution is recommended on the basis of nutrient uptake and photosynthate production in the substrate cherry tomato cultivation system.

Supplementary Materials:
The following are available online at http://www.mdpi.com/2073-4395/10/12/1863/s1: Figure S1. Mean daily temperatures (recorded every two hours) in the vegetation period during the Mg and K experiments in the greenhouse; Figure S2. Effects of K concentrations in solution on plant potassium (K), calcium (Ca), and magnesium (Mg) uptake by cherry tomato plants; Table S1. Critical leaf Mg concentrations for tomato growth in different substrates; Table S1, Critical leaf Mg concentrations for tomato growth in different substrates.