Influence of Electrical Conductivity on Plant Growth, Nutritional Quality, and Phytochemical Properties of Kale (Brassica napus) and Collard (Brassica oleracea) Grown Using Hydroponics

Abstract

Kale (Brassica napus) and collard (Brassica oleracea) are two leafy greens in the family Brassicaceae. The leaves are rich sources of numerous health-beneficial compounds and are commonly used either fresh or cooked. This study aimed to optimize the nutrient management of kale and collard in hydroponic production for greater yield and crop quality. ‘Red Russian’ kale and ‘Flash F1’ collard were grown for 4 weeks after transplanting in a double polyethylene-plastic-covered greenhouse using a nutrient film technique (NFT) system with 18 channels. Kale and collard were alternately grown in each channel at four different electrical conductivity (EC) levels (1.2, 1.5, 1.8, and 2.1 mS·cm⁻¹). Fresh and dry yields of kale increased linearly with increasing EC levels, while those of collard did not increase when EC was higher than 1.8 mS·cm⁻¹. Kale leaves had significantly higher P, K, Mn, Zn, Cu, and B than the collard at all EC levels. Additionally, mineral nutrients (except N and Zn) in leaf tissue were highest at EC 1.5 and EC 1.8 in both the kale and collard. However, the changing trend of the total N and NO3- of the leaves showed a linear trend; these levels were highest under EC 2.1, followed by EC 1.8 and EC 1.5. EC levels also affected phytochemical accumulation in leaf tissue. In general, the kale leaves had significantly higher total anthocyanin, vitamin C, phenolic compounds, and glucosinolates but lower total chlorophylls and carotenoids than the collard. In addition, although EC levels affected neither the total chlorophyll or carotenoid content in kale nor glucosinolate content in either kale or collard, other important health-beneficial compounds (especially vitamin C, anthocyanin, and phenolic compounds) in kale and collard leaves reduced with the increasing EC levels. In conclusion, the kale leaf had more nutritional and phytochemical compounds than the collard. An EC level of 1.8 mS·cm⁻¹ was the optimum EC level for the collard, while the kale yielded more at 2.1 mS·cm⁻¹. Further investigations are needed to optimize nitrogen nutrition for hydroponically grown kale.

1. Introduction

For millennia, plant foods have been consumed for their nutritional and medicinal qualities. The Brassica genus is one group of plants with high nutritional and medicinal value, due to greater levels of mineral nutrient, vitamin C, glucosinolates, and carotenoids [1,2]. Kale (Brassica napus) and collard (Brassica oleracea) of the Brassica genus have been reported to have the highest lutein and β-carotene concentrations among all vegetables [3,4].

Controlled environment agriculture (CEA) is an advanced and efficient method of producing leafy greens, including kale and collard. Within CEA, hydroponics stands out as a pivotal technology where plants are grown in a nutrient solution [5] or different types of soilless substrates [6]. CEA can optimize environmental factors (such as temperature, humidity, light, and CO₂ levels), minimize water and energy usage as well as crop damage, and support year-round production [7,8,9], leading to enhanced growth rates, higher yields, and improved nutritional profiles than soil-grown crops [10]. The global CEA market is predicted to increase at an 18.7 % annual rate and reach USD 172 billion in 2025 [11].

Despite the economic significance of CEA-grown produce and the imperative to diversify hydroponic leafy green crops, there is a notable absence of scientific studies addressing cultural practices influencing their commercial production. Moreover, many hydroponic growers follow the recommendations for lettuce (Lactuca sativa) in kale and collard production, potentially constraining the yield and quality. Therefore, investigation into cultural practices to enhance the production and quality of these crops has become increasingly important.

Researchers have reported the impact of hydroponic nutrient solutions on kale and collard. However, most of these studies have been centered around single nitrogen concentrations [12] or different nitrogen forms [1,13] but not the comprehensive nutrient profile. Mao et al. (2022) reported that the optimized nitrogen and magnesium concentrations for Chinese kale (Brassica albograbra Bailey) were 105 ppm nitrogen and 16.8 ppm magnesium, within ranges of 35 ppm to 315 ppm for nitrogen and 6 ppm to 50.4 ppm for magnesium [12]. While informative, these nutrient level settings are not sufficiently narrow to be useful as a reference for growers. Martínez-Castillo et al. (2022) compared the effect of electrical conductivity on the performance of kale (Brassica oleracea) cv. dwarf blue curled scotch with EC levels of 0.5, 1.0, 1.5, and 2.0 dS·m⁻¹ in a substrate (perlite)-based hydroponic system, but the results may not be applicable to liquid-based hydroponics systems [14]. Cardarelli et al. (2015) investigated the effect of nitrate level (7, 35, 70, 140, 280 ppm) in a subirrigation system on collard growth and found that >140 ppm nitrate is sufficient. The study did not, however, include the effects on phytonutrient contents [15].

EC has been used as a standard indicator of hydroponic nutrient solution concentration [16,17,18] due to the ability of soluble salts in the solution to conduct electricity. This conductivity is directly influenced by the concentration of ions (such as calcium, potassium, and nitrate) present in the solution [6]. Monitoring and managing EC helps ensure that plants receive the optimal amounts of nutrients needed for healthy growth [16,17,18]. We have previously demonstrated the optimization of electrical conductivity to improve nutrient and phytonutrient contents in hydroponic-grown Arugula (Eruca vesicaria) [19]. Thus, the objective of the current research was to determine the optimum electrical conductivity (EC) levels for high-yield, high-quality hydroponic-grown kale and collard.

2. Materials and Methods

2.1. Plant Growing Conditions

The experiments were conducted in February and March 2022. A nutrient film technique (NFT) system (CropKing, Lodi, OH, USA) was used in a greenhouse covered with double polyethylene plastic at the Ohio State University, Wooster Campus, Ohio (40.78° N, 81.93° W). Four different nutrient solutions with distinct EC levels were prepared in duplicate and stored separately in a total of eight UV-stabilized plastic tanks. Each tank was randomly assigned to supply two of the sixteen NFT channels, ensuring replication for each EC treatment. The NFT channels, each measuring 4 m in length, were spaced 0.2 m apart and could accommodate 18 plants per channel, also spaced 0.2 m apart. The two outermost channels were connected to a separate tank and were not included in the experimental analysis to minimize edge effects (Figure 1).

Air temperature and humidity in the greenhouse were continuously monitored with a humidity and temperature probe (INTERCAP^® HMP50; Vaisala, Helsinki, Finland), recording data at 10-s intervals. Photosynthetic photon flux density (PPFD) was supplied by a combination of sunlight and 400-watt high-pressure sodium (HPS) lamps (Energy Technics Horticulture Lighting, York, PA, USA), operating for 16 h daily. PPFD levels were measured every 10 s using a sun-calibrated quantum sensor (SQ-110-SS; Apogee Instruments, Logan, UT, USA). All environmental parameters, including air temperature, humidity, and average light intensity, were captured at 10-s intervals and logged by a datalogger (CR3000; Campbell Scientific, Logan, UT, USA). The average (±standard error) daytime and nighttime air temperatures were 20.7 ± 0.1 °C and 13.6 ± 0.1 °C, respectively (Figure 2). Air humidity averaged 49.7 ± 2.3 % and the daily light integral of PPFD was 28.7 ± 1.7 mol·m⁻²·d⁻¹ (Figure 2).

The experiment was conducted using kale (Brassica napus ‘Red Russian’) and collard (Brassica oleracea ‘Flash F1’) (Johnny’s Selected Seeds, Albion, ME, USA). A two-part fertilizer program was applied in which water-soluble fertilizer (Hydro Grow Leafy Green Fertilizer; 4.3% N, 9.3% P, 35% K; Crop King, Lodi, OH, USA) at 100 mL·L⁻¹ and calcium nitrate (CropKing) at 78 mL·L⁻¹ were used as fertilizer stock solutions to prepare nutrient solutions for four different EC treatments. Seed propagation was carried out in 162-cell foam trays (Horticubes^®; Smithers Oasis, Canton, OH, USA) using a hydroponic system with nutrient circulation. A diluted solution with an EC of 1–1.2 dS·m⁻¹ and a pH of 5.8 was applied when the first true leaves emerged, as recommended. Seedlings of kale and collard were transplanted three weeks after germination, with an average leaf number of 2.2 ± 0.1 and a relative chlorophyll content (SPAD value) of 33.39 ± 0.97. SPAD readings, which serve as an index of chlorophyll content per unit leaf area, were taken from each fully expanded leaf using a chlorophyll meter (SPAD-502, Minolta Corporation, Ltd., Osaka, Japan). Five readings were taken per leaf at the central point between the midrib and leaf margin, and the averaged value was recorded. The duration from transplanting to harvest was four weeks.

2.2. Treatments and Experimental Design

The experiment included four EC treatments: 1.2, 1.5, 1.8, and 2.1 dS·m⁻¹ (

Table 1. Electrical conductivity (EC) and pH of nutrient solutions used in the hydroponic production of kale and collard taken before (pre) and after (post) daily adjustment of the nutrient solution. Data represent the mean and standard error of 28 measurements taken daily.

Treatment	Pre-EC	Pre-pH	Post-EC	Post-pH
EC 1.2	1.09 ± 0.03 d	6.46 ± 0.06 a	1.19 ± 0.01 d	5.83 ± 0.01 a
EC 1.5	1.38 ± 0.04 c	6.46 ± 0.06 a	1.50 ± 0.01 c	5.82 ± 0.01 a
EC 1.8	1.68 ± 0.04 b	6.34 ± 0.07 a	1.79 ± 0.01 b	5.83 ± 0.01 a
EC 2.1	1.94 ± 0.06 a	6.41 ± 0.09 a	2.10 ± 0.01 a	5.85 ± 0.01 a

Means with different letters are significantly different according to Tukey’s test (p = 0.05).

). The nutrient compositions of the solution for EC 1.8 dS·m⁻¹ and pH 5.8 are shown in

Table 2. Macronutrient compositions and concentrations used in the nutrient solution with EC of 1.8 mS·cm⁻¹ and pH of 5.8.

Parameter	Concentration (mg·L−1)
Total nitrogen (N)	129.2
P	48.3
K	183.2
S	52.1
Ca	136.5
Mg	32.7

. Each reservoir tank was assigned a specific EC treatment, with the EC adjusted daily to maintain it within ±0.05 using two-part fertilizer stock solutions. Following the EC adjustment, the pH of each reservoir was regulated to 5.82 ± 0.05 by adding either 10% citric acid (to decrease pH) or water (to increase pH). During the plant growth, to sustain the water level, additional water (EC 0.8 dS·m⁻¹, pH 6.4, and alkalinity 192.8 mg·L⁻¹) was added, after which the EC and pH were re-adjusted to meet the treatment requirements. The quantities of 10% citric acid solution or water added were typically less than 1% of the total reservoir volume, resulting in negligible changes in EC (±0.05 mS/cm).

Measurements of EC, pH, redox potential, and alkalinity (as CaCO₃) were taken from the water using a titrator (T7; Mettler Toledo, Columbus, OH, USA) equipped with an autosampler (InMotion Max; Mettler Toledo) and a pH probe (DGi115-SC; Mettler Toledo). The nutrient profile of the nutrient solution was assessed via ion chromatography systems (IC 600; Thermo Fisher Scientific, Waltham, MA, USA). The total nitrogen (TN) and total organic carbon (TOC) levels were measured using a total organic carbon analyzer (TOC-LCSN, Shimadzu, Kyoto, Japan). The EC and pH of each nutrient solution tank were continuously monitored with an EC meter (COM-100, HM Digital Inc., Redondo Beach, CA, USA) and a pH meter (PH-200, HM Digital Inc., Redondo Beach, CA, USA), respectively.

Each NFT channel contained a total of 18 seedlings, with nine kale and nine collard seedlings planted alternately. To avoid edge effects, data were not collected from the two outer channels or from the two outermost plants in each channel. There were four replicate channels for each treatment, with 16 plants per channel. Treatments were assigned to the channels via a completely randomized design.

2.3. Gas Exchange Properties

Gas exchange measurements were conducted using a portable gas exchange system (LI-6400XT; LICOR Biosciences, Lincoln, NE, USA) equipped with a 6 cm² leaf chamber and built-in LEDs (peak wavelengths: 470 nm for blue and 665 nm for red). Lighting was supplied at a photosynthetic photon flux (PPF) of 1000 μmol·m⁻²·s⁻¹ using red and blue LEDs in a 9:1 ratio, and the leaf chamber temperature was maintained at 20 °C under supplemental lighting. The reference CO₂ concentration was set at 400 μmol·mol⁻¹, with a flow rate of 500 μmol·s⁻¹ through the chamber.

Four plants from each channel (eight plants per treatment for both kale and collard) were selected for measurement of their photosynthetic properties during the 1st, 2nd, 3rd, and 4th weeks post-transplant. The photosynthetic rate (Pn) and transpiration rate (Tr) were measured under a PPF of 1000 μmol·m⁻²·s⁻¹ between 9:00 am and 4:00 pm using the third, youngest fully expended leaf of each plant. Data were collected once the coefficient of variation for sample CO₂, sample H₂O, and flow rate stabilized at or below 0.2%, typically within a 10-min period. Intrinsic water use efficiency (WUE) was determined by calculating the ratio of Pn to Tr [20].

2.4. Relative Chlorophyll Content (SPAD) and Chlorophyll Fluorescence

During the 1st, 2nd, 3rd, and 4th week after transplanting, four representative plants were chosen from each NFT channel (resulting in eight plants per nutrient reservoir and 16 plants per treatment) to conduct SPAD readings and chlorophyll fluorescence measurements. Chlorophyll fluorescence was assessed immediately following a 20-min dark adaptation period, using a Plant Efficiency Analyzer (Handy PEA, Hansatech Instruments, King’s Lynn, England). The duration of dark adaptation and the optimal light intensity (3500 µmol·m⁻²·s⁻¹) were set according to the protocol outlined by [21].

2.5. Plant Biomass Measurements

At 28 d after transplanting, six kale and six collard plants per channel were harvested. Plants previously used for photosynthetic measurements were excluded from the final harvest to prevent any potential mechanical damage to the tissues. During harvest, nutrient disorder symptoms were assessed based on qualitative guidelines provided by the University of Arizona [22], primarily focusing on minor chlorosis at the leaf margins of older leaves. The rate of nutrient disorder symptoms was determined by calculating the total number of plants exhibiting these symptoms for each treatment.

Harvested plant samples were separated into roots and shoots, and fresh weight was immediately recorded. Leaf area measurements were then conducted on the shoots, with each leaf scanned using a portable laser leaf area meter (CI-202, CID Bio-Science, Inc., Camas, WA, USA) to calculate the total plant leaf area per plant. Following this, all samples were oven dried at 68 °C until they reached a constant weight, at which point the dry weight was recorded. The dried samples were subsequently ground with a sample mill (Cyclotec™ 1093, FOSS Analytical, Hilleroed, Denmark), passed through a 10-mesh screen, and stored in plastic vials for latter tissue nutrient analysis.

2.6. Tissue Nutrient Analysis

Nutrient analysis of plant tissue was performed on leaf samples from kale and collard (eight plants per treatment) at the Service, Testing, and Research (STAR) laboratory of Ohio State University (Wooster, OH, USA) to evaluate nutrient uptake across different EC levels. The total concentrations of essential elements (P, K, Ca, Mg, S, Al, B, Cu, Fe, Mn, Mo, Na, and Zn) were determined using a method involving microwave digestion with HNO₃, followed by analysis with inductively coupled plasma optical emission spectrometry (ICP-OES; Agilent 5110, Agilent Technologies, Santa Clara, CA USA), as outlined in [23]. Nitrate nitrogen (NO₃-N) content in the plant tissue samples was measured using a Carbon– Nitrogen Combustion Analyzer (VARIO Max Cube Carbon–Nitrogen Analyzer, Elementar Americas, Mt. Laurel, NJ USA) based on the NO₃-N cadmium reduction method [24]. Total nitrogen content was analyzed using ICP-OES (Agilent 5110) following the Dumas method [25].

2.7. Chlorophyll and Carotenoids

Chlorophyll a, chlorophyll b, and carotenoids were extracted from 25 mg of fresh leaf tissue using 100% methanol as solvent. The samples were stored in a dark environment at 4 °C for 24 h to complete the extraction. Total chlorophyll content was quantified immediately after the extraction process. Absorbance was taken at 661.6 nm and 644.8 nm for chlorophyll pigments and at 470 nm for total carotenoids. The concentrations of chlorophyll and carotenoids were calculated using equations provided by [26]:

$${Chl}_a=11.25A_{661.6}-2.04A_{644.8}$$

(1)

$${Chl}_b=20.13A_{644.8}-4.19A_{661.6}$$

(2)

$${Chl}_{a+b}=7.05A_{661.6}+18.09A_{644.8}$$

(3)

$${Car}_{x+c}={\displaystyle \frac{\left(11.24A_{470}-1.90{Chl}_a-63.14{Chl}_b\right)}{214}}$$

(4)

2.8. Total Anthocyanins

Total anthocyanin content was measured using a modified spectrophotometric method based on [19,27]. The absorbance (A) was calculated using the equation:

$$A={\left(A_{520}-A_{700}\right)}_{pH1.0}-{\left(A_{520}-A_{700}\right)}_{pH4.5}$$

(5)

Anthocyanin content was quantified against a standard calibration curve of methyl orange and expressed as a percentage relative to the dried plant weight.

2.9. Total Content of Phenolic Compounds

Total phenolic content was assessed using the spectrophotometric method outlined by [19,28].The total phenolic content was determined using a standard curve based on gallic acid.

2.10. Vitamin C

Vitamin C content was analyzed using the spectrophotometric method described by [19,29]. The vitamin C content was determined using a standard curve generated from an ascorbic acid solution.

2.11. Total Glucosinolates

Total glucosinolates were estimated using the spectrophotometric method described by [19,30]. Total glucosinolates were calculated using the following formula, where $A_{425}$ represents the absorbance value of each sample:

$$y=1.40+118.86\times A_{425}$$

(6)

2.12. Statistical Analysis

Statistical differences were evaluated through one-way analysis of variance (ANOVA), followed by Tukey’s honestly significant difference (HSD) test at a significance level of p = 0.05 (JMP^® for Windows, Version 14.0 Pro, SAS Institute Inc., Cary, NC, USA). Additionally, multiple regression analysis, encompassing both linear and non-linear models, was performed. Figures with regression plots illustrate the effects of EC on plant response parameters. The significance of the regression coefficient is denoted by *, **, or ***, corresponding to p values of 0.05, 0.01, and 0.001, respectively.

3. Results

3.1. Plant Fresh and Dry Biomass, Leaf Area, and Disorder Symptoms

The shoot fresh and dry weight of kale increased with increasing EC, while those of collard did not further increase when EC was higher than 1.8 mS·cm⁻¹ (Figure 3). The shoot fresh weight and dry weight of kale under EC 2.1 were significantly higher than those under EC 1.2 by 38.9% and 22.7%, respectively. For collard, the shoot fresh weight under EC 2.1 was significantly higher than EC 1.2 by 51.8%, while the shoot dry weight was significantly higher under EC 1.8 and 2.1 than EC 1.2 by 34.7% and 19.9%, respectively.

Similar to the biomass results, the leaf area of kale increased with increasing EC, and the leaf area of collard showed a decreasing trend when EC was higher than 1.8 mS·cm⁻¹ (Figure 4). For both kale and collard, nutrient deficiency symptoms decreased with increasing EC at harvest (Figure 4). The leaf area of kale increased linearly with increasing EC, where the leaf area at EC 2.1 was significantly higher than EC 1.2 by 37.7%. The leaf area values of collard were best fit with a quadratic function, where the leaf area at EC 1.8 was significantly higher than EC 1.2 by 44.2%. For both kale and collard, nutrient disorder symptom values were best fit with a linear function. Nutrient deficiency symptoms were 243.5% higher in kale than collard. In addition, the symptoms of kale under EC 1.2 and EC 1.5 had significantly higher values than that under EC 2.1 by 43.8% and 43.8%, respectively, while the symptoms of collard under EC 1.2 and EC 1.5 had significantly higher values than EC 2.1 by 800%, and 600%, respectively. These results aligned with the higher nutrient levels observed with increasing electrical conductivity.

3.2. Photosynthetic Properties and Plant Growth

Kale and collard showed similar photosynthetic properties and trends throughout the study (Figure 5). In general, net photosynthetic rate (Pn) increased over time, and transpiration rate (Tr) increased in the first two weeks then reduced, while water use efficiency (WUE) reduced in the first week then increased over time.

There was no significant difference in Tr in kale or collard among treatments throughout the study. However, electrical conductivity had a similar effect on the Pn of kale and collard, where kale under EC 2.1 had higher Pn than that under EC 1.2 in week 2 and week 4 by 22.0% and 9.3%, respectively, and collard under EC 2.1 had higher Pn than that under EC 1.2 in week 4 by 24.6%. On the other hand, the WUE of kale under EC 1.5 and 1.8 had significantly higher values than that under EC 2.1 in week 4 by 20.8% and 29.3%, respectively, although there was no treatment effect on the WUE of collard. For kale, there was no significant difference among EC treatments on Pn in week 1 or week 3, or on WUE in week 2 or week 3. For collard, there was no significant difference among treatments on Pn from week 1 to week 3, or on WUE during the whole study.

Nutrient solution concentration did not affect leaf number, SPAD value, or chlorophyll fluorescence rate in collard (Figure 6B,D,F) or leaf number in kale (Figure 6A). However, for kale, SPAD values under EC 1.8 and EC 2.1 were significantly higher than those under EC 1.2 in week 3 by 14.0% and 12.6%, respectively (Figure 6C). The chlorophyll fluorescence values of kale and collard were not significantly different among treatments (Figure 6E), and these values in kale and collard were all higher than 0.83, which meant there was no stress in leaves under any electrical conductivities investigated in this study [31].

3.3. Macro Nutrient Content of the Nutrient Solution

The macro nutrient content in nutrient solutions with different concentrations varied before and after research (Figure 7). Even with daily adjustments of nutrient solution concentration, total nitrogen depleted (reduced by 95.1%–96.8%) in all treatments towards the end of the research, which could explain nutrient deficiency symptoms. However, all other macro nutrients (P, K, Ca, Mg, and S) were accumulated in the system. By the end of the research, the concentrations of phosphorus, potassium, calcium, magnesium, and sulfur had increased by 7.6–17.8 times, 3.3–6.5 times, 14.4–19.8 times, 31.2–39.2 times, and 27.6–40.5 times, respectively. This observation highlighted a preferential uptake of nitrogen compared to the other nutrient elements.

3.4. Plant Tissue Macro and Micronutrient Contents

Electrical conductivity affected macro nutrients contents in kale and collard shoots (Figure 8). Phosphorus and potassium in kale were significantly higher than collard, but there was no difference in other macro nutrient contents between them. For both kale and collard, nitrogen increased linearly with increasing EC, while phosphorus and potassium increased quadratically with increasing EC. Nitrogen content in kale varied with the order of EC 2.1, EC 1.8 > EC 1.5 > EC 1.2. Nitrogen in kale under EC 2.1 and EC 1.8 were significantly higher than EC 1.5 by 20.5% and 13.2%, respectively, and were significantly higher than EC 1.2 by 39.3% and 30.9%, respectively, while the value under EC 1.5 was significantly higher than EC 1.2 by 15.6%. Meanwhile, nitrogen in collard varied with the order of EC 2.1 > EC 1.8, EC 1.5 > EC 1.2. Nitrogen in collard under EC 2.1 was significantly higher than EC 1.8 and EC 1.5 by 8.7% and 18.0%, respectively, as well as higher than EC 1.2 by 43.8%. And nitrogen in collard under EC 1.8 and EC 1.5 were also significantly higher than EC 1.2 by 32.2% and 21.8%, respectively.

Phosphorus in kale varied with the order of EC 1.5, EC 1.8, EC 2.1 > EC 1.2. Phosphorus in kale under EC 1.5 and EC 1.8 were significantly higher than EC 1.2 by 21.7% and 19.6%, respectively. Meanwhile, phosphorus in collard varied with the order of EC 1.5, EC 1.8 > EC 1.2, EC 2.1. Phosphorus in collard under EC 1.5 was significantly higher than EC 1.2 and EC 2.1 by 9.1% and 15.3%, respectively.

Potassium in kale varied with the order of EC 2.1, EC 1.5, EC 1.8 > EC 1.2. Potassium in kale under EC 2.1, EC 1.8 and EC 1.5 were significantly higher than EC 1.2 by 19.2%, 12.4% and 18.8%, respectively. Meanwhile, potassium in collard varied with the order of EC 1.5 > EC 1.8, EC 2.1 > EC 1.2. Potassium in collard under EC 1.5 was significantly higher than EC 1.8 and EC 2.1 by 9.7% and 8.4%, respectively, as well as higher than EC 1.2 by 19.1%. And potassium in collard under EC 1.8 and EC 2.1 also were significantly higher than EC 1.2 by 8.5% and 9.8%, respectively.

Electrical conductivity did not affect calcium in collard and magnesium in kale. Calcium in kale was significantly higher under EC 1.5 than EC 1.2 by 22.2%. Although magnesium in collard increased quadratically with increasing EC, the difference among different treatments were not huge, where values under EC 1.5 and EC 1.8 were significantly higher than EC 2.1 by 14.3% and 10.6%, respectively. Sulphur in kale also increased quadratically with increasing EC, and value under EC 1.5 was significantly higher than EC 1.2 and EC 2.1 by 13.2% and 20.5%, respectively. Sulphur in collard reduced with increasing EC, where EC 1.2, EC 1.5 and EC 1.8 had significantly higher values than EC 2.1 by 27.7%, 16.9% and 17.4%, respectively.

Electrical conductivity also affected micronutrient contents in kale and collard shoots (Figure 9). Except zinc and copper, all other micronutrients in kale and collard increased quadratically with increasing EC. Electrical conductivity did not affect zinc or copper in collard, but in kale, copper and zinc reduced with increasing EC (Figure 9C,D). The zinc content of kale under EC 1.2 was significantly higher than that under EC 2.1 by 52.2%, while the copper contents of kale under EC 1.2, EC 1.5, and EC 1.8 were significantly higher than that under EC 1.2 by 18.5%, 15.7%, and 16.7%, respectively. The iron content of kale under EC 1.5 was significantly higher than that under EC 2.1 by 21.3%. The manganese contents of kale under EC 1.5 and EC 1.8 were significantly higher than that under EC 1.2 by 35.6% and 32.3%. Similarly, the manganese contents of collard under EC 1.5 and EC 1.8 were significantly higher than that under EC 1.2 by 28.7% and 27.9%. The boron contents of kale under EC 1.5 was significantly higher than that under EC 1.2 by 22.1%, while the boron content of collard under EC 1.5 was significantly higher than that under EC 1.2 and EC 2.1 by 16.8% and 12.2%, respectively. The molybdenum contents of kale under EC 1.5 and EC 1.8 were significantly higher than that under EC 1.2 by 47.9% and 49.2%, respectively, while the molybdenum contents of collard under EC 1.5 and EC 1.8 were significantly higher than that under EC 1.2 by 17.9% and 19.5%, respectively, as well as significantly higher than that under EC 2.1 by 22.4% and 24.1%, respectively.

3.5. Plant Tissue Nitrate and Phytochemicals Contents

Nitrate in both kale and collard increased linearly with increasing EC (Figure 10). Kale and collard were considered as medium-nitrate-content vegetable products [32,33]. The nitrate content of kale was significantly higher than collard under EC 1.2 and EC 1.5 by 303.8% and 97.5%, respectively, but there was no significant difference between them under EC 1.8 or EC 2.1. The nitrate content of kale under EC 2.1 was significantly higher than that under EC 1.5 and EC 1.2 by 72.8% and 195.6%, respectively. The nitrate content of collard under EC 2.1 was significantly higher than that under EC 1.8, EC 1.5, and EC 1.2 by 48.6%, 208.2%, and 977.8%, respectively.

Electrical conductivity also affected some key phytochemicals contents in kale and collard shoots, but there was no effect on total glucosinolates (Figure 11). Total chlorophyll and carotenoids increased quadratically with increasing EC for collard, although there was no effect for kale. Collard grown under EC 1.5 had significantly higher total chlorophyll and carotenoids than those grown under EC 2.1 by 48.9% and 55.8%, respectively. For both kale and collard, total anthocyanin, total phenols, and vitamin C levels reduced linearly with increasing EC. Kale under EC 1.2 had significantly higher total anthocyanin than that under EC 1.5, EC 1.8, and EC 2.1 by 28.3%, 20.3%, and 35.0%, respectively, while collard under EC 1.2, EC 1.5, and EC 1.8 had significantly higher total anthocyanin than that under EC 2.1 by 46.7%, 46.7%, and 46.7%, respectively. Although collard under EC 1.2 had significantly higher total phenols than that under EC 1.5 by 9.1%, there was no significant difference among other treatments. Kale under EC 1.2 had significantly higher vitamin C content than that under EC 1.8 and EC 2.1 by 24.9% and 22.7%, respectively, while collard under EC 1.2 and EC 1.5 had significantly higher vitamin C than that under EC 1.8 and EC 2.1 by 24.9%.

4. Discussion

4.1. Kale Prefers Higher Electrical Conductivity than Collard in Terms of Photosynthetic Properties and Plant Growth and Yield

Increased plant growth rates over time generally corresponded with changes in the net photosynthetic rate (Pn) of kale and collard. Kale under EC 2.1 showed a higher growth rate (reflected by higher Pn) than kale under a lower electrical conductivity, which could be further verified by its increasing leaf area and shoot fresh and dry weight with the increase in electrical conductivity. These results indicate that an increase in electrical conductivity beyond the highest level (EC 2.1) in the current study may be needed to meet the growth demand of kale. On the other hand, the photosynthetic properties of collard were not greatly affected by electrical conductivity, except for higher Pn under EC 2.1 at the end of the growing season. This phenomenon could be a sign of a slower growth rate under EC 2.1, which was further supported by its highest leaf area value occurring under EC 1.8. These results showed that collard had the best growth performance under the medium-to-high electrical conductivities tested. An EC value of 1.8 aligns with standard EC recommendations for lettuce in hydroponics; hence, collard can be grown in the same system as lettuce.

Martínez-Castillo et al. also found that kale (Brassica oleracea) cv. dwarf blue curled scotch grown using hydroponics showed higher growth rates, yields, SPAD readings, and nitrate concentrations as electrical conductivity increased from 0.5 ds·m⁻¹ to 2.0 ds·m⁻¹, except that the calcium concentration reduced, which was consistent with the current study [14]. Cardarelli et al. also reported that five ornamental cabbage cultivars (Brassica oleracea L. var. acephala D.C.) grown in a closed subirrigation hydroponic system showed a quadratic increase in yield, SPAD, and leaf nitrogen, calcium, and magnesium as nitrogen rate increased from 7 ppm N to 280 ppm N NO₃⁻ and suggested an optimal rate of 140 ppm N [15]. However, our present study found the optimal rate of nitrogen to be higher than that rate. Feng et al. observed that increasing nitrogen level from 232.5 ppm to 930 ppm in hydroponics improved photosynthetic properties (net photosynthetic rate, stomatal conductance, and transpiration rate) in canola plants (Brassica napus L.) [34]. Hu et al. also reported an increased net photosynthetic rate and leaf area in oilseed rape (Brassica napus L. c.v. Huayouza) under synchronous increases in N and K supplementations with a N/K ratio of 3.07 in hydroponics [35]. Mao et al. also suggested the N7.5Mg1.4 (N/Mg = 3.10) treatment for best plant growth (leaf area, fresh weight, and net photosynthetic rate) and quality (vitamin C and chlorophyll) using hydroponics for Chinese kale (Brassica albograbra Bailey var. Cuibao), whose ratio was similar to the setting used in this study (N/Mg = 3.95) [12]. However, Louvieaux et al. did not observe any effects of nitrate with 2.8 ppm N and 70 ppm N on the shoot or root dry biomass of hydroponic grown oilseed rape (Brassica napus L.) [36], which may be a result of cultivar-specific effects. Thus, based on the current study and other reports, the cultivar specificity among brassica species is an important determinant for nutrient recommendations.

4.2. High Electrical Conductivity Did Not Further Improve Tissue Mineral Nutrients or Phytochemicals Contents in Kale or Collard

Mineral nutrient contents in leaf tissue was compared with the sufficiency ranges for kale [37,38] and collard [39]. At all EC levels, tissue nutrient concentrations were within or above sufficiency ranges. However, except nitrogen, the mineral nutrients did not increase due to higher electrical conductivity in the solution. Except zinc and copper in kale (which reduced with higher electrical conductivity), all other mineral nutrient contents were the highest under EC 1.5 or EC 1.8, which were consistent with our previous research regarding total nitrogen, potassium, and calcium in arugula [19]. There were no effects on calcium, zinc and copper in collard or on magnesium in kale. These results were consistent with crop growth performance and yield data, which further confirmed the benefits of medium-to-high electrical conductivity for collard, supporting the findings of [40]. They also suggest that reducing nutrient application for kale, rather than using high-concentration nutrient solutions, may lead to improved production. Kullmann et al. observed an increase in leaf potassium and magnesium as well as a decrease in the leaf calcium of oilseed rape (Brassica napus L.) when nitrogen supply increased from 30 to 100 ppm, although leaf phosphorus was not affected [41].

In terms of phytochemical contents, although electrical conductivity did not affect total glucosinolates in kale or collard, all other key phytochemical contents were affected. Total anthocyanin, vitamin C levels, and total phenols in kale and collard were reduced by higher electrical conductivity, which is consistent with our previous research regarding total anthocyanin in arugula [19], further proving that the high nutritional contents in the current study restrained the secondary metabolism. Collard showed more selectivity regarding the EC setting in this study, while the growth of kale performed without any inhibition at the highest EC treatment, which suggests higher nutrient requirements for better growth. The optimal electrical conductivity for achieving the highest growth rate and phytochemical levels varied, indicating the need for precise nutrient management tailored to specific phytochemicals. Kopsell et al. also found that carotenoid pigments on a fresh weight basis in Brassica oleracea (kale) were not affected by increasing N at a rate of 6, 13, 26, 52, or 105 ppm, and their N settings were lower than those used in the current study (85, 110, 130, and 150 ppm) [1]. Kopsell et al. found that Chinese kale (Brassica oleracea var. alboglabra) had higher total chlorophyll and total carotenoid concentrations at a higher fertility level (half-strength Hoagland’s solution; 105.0 ppm N) than at a lower fertility level (quarter-strength Hoagland’s solution; 52.5 ppm N) [42], and these were consistent with the rates used in the current study.

The current study indicates an EC value of 1.8 to 2.1 mS·cm⁻¹ for the best collard growth performance, which is consistent with other research. Shen et al. found that rapeseed (Brassica napus L.) showed inhibited shoot growth and promoted root growth under N deficiency (1/50 of normal N supply), with more stress-related metabolites accumulated in shoot tissues [43]. Cardarelli et al. did not observe the effect of nitrate rate in nutrient solution on potted collard in terms of chlorophyll and carotenoid contents. This study was similar to the current study, although slightly reducing trends were found, and the concentrations of all nutrients increased [15]. Ogunlela et al. found an increase in chlorophyll a, chlorophyll b, and total chlorophyll contents in oilseed rape (Brassica napus L.) when N supply was increased from 30 ppm to 170 ppm, although they did not find any change in chlorophyll a/b ratios [40]. Similar N rates were used to those in the current study.

4.3. Nutrient Fertilizer Formula Should Be Specifically Designed for Kale and Collard to Optimize Yield and Mineral Nutrient and Phytochemicals Contents

At the end of the study, we observed the depletion of nitrogen and the accumulation of all other macro nutrients in the nutrient solution despite continuous nutrient adjustments. This phenomenon also suggests potential nitrogen deficiency in current EC-based nutrient solution management, as well as potential nutrient toxicity (especially at high electrical conductivities), which needs further investigation to better control the nutrient profile of specific crops in hydroponic production.

Despite an increased growth rate in kale with rising electrical conductivity, there was a decline in zinc, copper, and key phytochemicals (total anthocyanin, total phenols, and vitamin C). These findings challenge the common practice of applying lettuce nutrient guidelines (EC 1.8) to kale hydroponic production. Therefore, specific nutrient fertilizer formula (especially higher nitrogen rate) for kale may enhance their nutritional and phytochemical qualities. These findings were consistent with [16] but did not align with our previous research on arugula [19], which indicated an optimal EC range of 1.5 to 1.8 dS·m⁻¹ to produce the best crop yield and quality. Therefore, nutrient fertilizer formulas should be specifically designed based on the growth characteristics and nutrient requirements of different species.

Nitrate levels in kale and collard in the current study were comparable with previous research [33]. A high intake of dietary nitrate may negatively impact human health through the ‘nitrate–nitrite–nitric oxide (NO) pathway’, which has been linked to gastric and bladder cancers [44,45], and the involvement of nitrite in methemoglobinemia syndrome [46]. According to a report from the European Food Safety Authority (EFSA) [33], the nitrate content of curly kale should be within the range of 19–1846 mg·kg⁻¹ [47] for human consumption; however, only kale under EC 2.1 was close to this value during this study. This observation also suggests the importance of awareness regarding nitrate levels in a nutrient management plan for hydroponically grown kale. Kopsell et al. found that increasing NH₄-N: NO₃-N from 0% to 100% resulted in increasing lutein and β-carotene concentrations in Brassica oleracea (kale) [1], which was similar to research based on Chinese kale [48,49,50] and other Brassica microgreens [51]. Burns also revealed that the growth rate of summer cabbage (Brassica oleracea var. capitata cv. Stonehead) varied linearly with NO₃-N supply when it was withheld [52,53]. Chen et al. reported that phosphorus deficiency restricted the growth of Chinese kale (Brassica alboglabra var. Bailey) but induced higher flavonoid, soluble phenol, and anthocyanin contents in the flower stalks [2]. This may prove that the phosphorus in our study was sufficient as we did not observe any inducement of pigment formation. Inthichack et al. found that increased potassium levels decreased magnesium concentration in cabbage (Brassica oleracea L. cv. green ball) but not calcium concentration, with the highest yields achieved at 482.7 mg·L⁻¹ [54]. van Rooyen and Nicol also found that the optimized phosphate level for hydroponic-grown kale (Brassica oleracea var. Sabellica) could be as low as 0.1 mM (9.5 mg·L⁻¹) [55]. Given the critical role of nutrient ion interactions suggested by this study and previous research, further research is still needed to determine the optimized macro nutrient ratio for kale and collard to refine nutrient management plans based on specific growth characteristics and nutrient requirements.

5. Conclusions

The current study aimed to determine the optimum EC levels for high-yield and high-quality kale and collard grown via hydroponics. It was observed that collard had the best growth performance, with a higher growth rate and higher nutritional and phytochemical contents under EC 1.8, while kale showed an increased growth performance with rising electrical conductivity. Mineral nutrients and phytochemicals did not change in a similar pattern to yield, indicating the compromise between yield and qualitative parameters during EC-based nutrient management. Nitrogen depletion and the accumulation of all other macro nutrients in the solution were found at the end of this study, which led to an imbalanced nutritional composition. Thus, further research will be needed to optimize the nutrient fertilizer formulation with higher nitrogen rate and appropriate macro nutrient ratios along with changes to nutrient replenishment during production cycle.