From Coconut Waste to Circular Plant Factories with Artificial Light: Renewable Substrate-Enhanced Crop Yield and Energy Efficiency

Abstract

Developing environmentally friendly and cost-effective substrates is critical to enhance resource efficiency and productivity in plant factories with artificial lighting (PFALs). This study employed a molded coconut coir substrate (coconut coir composited with polyurethane hydrophilic adhesive, MCCS) in PFALs to cultivate lettuce (Lactuca sativa L.) and pak choi (Brassica rapa ssp. chinensis). During the transplanting stage, the roots exposed outside the MCCS of lettuce and pak choi were 13.40% and 19.92% shorter, respectively, than in the sponge treatment, and more amenable to mechanical transplanting. This compensated for the neglect of operational efficiency in traditional lifecycle assessment (LCA). Furthermore, compared with sponge and rockwool, MCCS significantly enhanced the yield of lettuce and pak choi by up to 27.33% and 67.19%, respectively. Meanwhile, MCCS significantly increased the chlorophyll content of lettuce compared to sponge by 8.56%. Compared with rockwool, MCCS significantly increased the chlorophyll b content (7.36%), antioxidant content, and antioxidant activity (total phenolics by 13.59%, total flavonoid by 18.43%, FRAP by 12.96%, and DPPH by 19.87%) of lettuce. For pak choi, MCCS increased the soluble protein content in the blade and total phenolics content in the petiole by 32.01% and 14.76%, respectively. More importantly, the use of MCCS led to a significant reduction in the energy consumption per unit area yield of lettuce and pak choi, with maximum reductions of 22.98% and 40.91%, respectively. This eco-friendly substrate is suitable for replacing sponge and rockwool in the production of lettuce and pak choi in PFALs.

1. Introduction

Given the limited availability of land in urban areas, the potential contamination of urban soil, and the likely negative effects of climate change on the yield of open-field crops, plant factories with artificial lighting (PFALs), could serve as a viable source of fresh produce. This is attributed to the numerous advantages they offer [1,2]. PFALs as a form of vertical farming production are distinguished by high efficiency, labor savings, and stable cultivation. PFALs exhibit a suite of beneficial attributes, including high resource utilization, higher annual yield per unit area, superior crop quality, year-round production capabilities, and lower site selection requirements [3]. Drawing on these advantages and the enclosed cultivation environment, the production in PFALs remains unaffected by external environmental factors. However, 18–20% of the total production cost is constituted by electricity [4]. Carbon footprint analysis also indicated that the lighting system in plant factories was the primary source of carbon emissions (accounting for over 50%). Conversely, by cultivating high light-efficiency cultivars and optimizing planting density, the carbon footprint per unit yield could be reduced by 62%. Moreover, high-yielding plant factories could reduce the dependence on conventional agricultural land, thereby indirectly decreasing the carbon emissions associated with forest conversion [5]. To enhance the conversion efficiency of light, the utilization of light-emitting diodes (LEDs) has become increasingly prevalent, rendering PFALs more efficient and intelligent [6,7]. Numerous studies have explored ways to enhance the converting efficiency of light energy into crop yield, such as by employing varying light intensities [8], spectral qualities [9,10], and lighting strategies at different growth stages [11,12]. However, there are still large knowledge gaps in research focused on improving PFALs’ operating costs and environmental sustainability [13]. The studies aimed at enhancing the efficiency of PFALs and reducing their operational costs most focused on environmental control in PFALs, with scant investigation into the use of cultivation substrates.

Despite the certain advantages of commonly used substrates in hydroponic cultivation, they also have numerous disadvantages (Table 1). Cost and environmental concerns are prompting growers to seek alternative sustainable and recyclable materials, such as bark, compost, and particularly coir, to replace those substrates that are not conducive to sustainable production [14]. During the production in PFALs, the substrates provide structural support to the plants and facilitate the delivery of essential minerals, nutrients, oxygen, and water to the plant roots [4]. An ideal hydroponic growth medium should possess sufficient porosity to facilitate root penetration and expansion. Additionally, it should exhibit robust structural integrity to ensure mechanical stability and provide physical support to the plant over time, without impeding nutrient and water uptake [15]. To adapt to the nutrient solution system of PFALs, the currently used substrates are characterized by their light weight and excellent physical and chemical properties, such as rockwool and sponge (or polyurethane foam). Rockwool, commonly referring to a material crafted from rock fibers, is extensively utilized as a cultivation substrate in plant factories due to its excellent water-retention and aeration properties, as well as its capacity to maintain the moisture and temperature around plant root systems. Similarly, sponges, which are also considered inert substrates, are often manufactured from materials such as polyester, polyether, or polyethylene, and their porous structure endows them with superior water absorption and aeration capabilities. However, these substrates present issues such as being non-renewable or potentially impacting the environment during their manufacturing and disposal processes; in particular, they are not properly recycled and reused [16,17]. Although conventional horticultural substrates (such as rockwool and sponge) demonstrate cost-effectiveness, their production, transportation, and end-of-life management generate substantial greenhouse gas emissions, resulting in significantly elevated global warming potential [18,19].

Thus, an increasing number of studies are currently exploring “clean and green” materials as alternatives to these cultivation substrates. Chitin and chitosan are extracted from the shells of crustaceans (such as crabs, shrimp, lobsters, etc.) or the cell walls of fungi. They are considered promising substrate alternatives due to their ecological compatibility and antimicrobial properties [20]. Hydrogel-based soilless cultivation, as a novel and innovative cultivation method, provides a promising practical solution for the application of eco-friendly substrates [21]. However, the production costs of these novel materials are relatively high, and their promotion and application in PFALs still face challenges.

Table 1. Comparison of commonly used substrates in hydroponic cultivation [22].

Substrates	Origin	Advantages	Disadvantages
Rockwool	Melted silicates at 1500–2000 °C	Light volume weight, high total pore space, ease of handling, totally inert, nutrition can be carefully controlled.	Disposal problems, energy consumed during manufacture
Vermiculite	Mg, Al, and Fe silicate sieved and heated to 1000 °C	Light volume weight, high nutrient-holding ability, good water-holding ability, good pH buffering capacity, good aeration due to high pore space.	Compacts when too wet, energy-consuming product, expensive
Perlite	Siliceous volcanic mineral sieved and heated to 1000 °C	Light volume weight, sterile, neutral in pH (6.5–7.5), no decay, sufficient total pore space.	Energy-consuming product, expensive
Coconut coir	By-product of fiber coconut processing	Physical stability, light weight, good air content due to high total pore space and high water-holding capacity, sub-acid–neutral pH (5–6.8)	May contain high salt levels, energy consumption during transport

Table 1. Comparison of commonly used substrates in hydroponic cultivation [22].

Substrates	Origin	Advantages	Disadvantages
Rockwool	Melted silicates at 1500–2000 °C	Light volume weight, high total pore space, ease of handling, totally inert, nutrition can be carefully controlled.	Disposal problems, energy consumed during manufacture
Vermiculite	Mg, Al, and Fe silicate sieved and heated to 1000 °C	Light volume weight, high nutrient-holding ability, good water-holding ability, good pH buffering capacity, good aeration due to high pore space.	Compacts when too wet, energy-consuming product, expensive
Perlite	Siliceous volcanic mineral sieved and heated to 1000 °C	Light volume weight, sterile, neutral in pH (6.5–7.5), no decay, sufficient total pore space.	Energy-consuming product, expensive
Coconut coir	By-product of fiber coconut processing	Physical stability, light weight, good air content due to high total pore space and high water-holding capacity, sub-acid–neutral pH (5–6.8)	May contain high salt levels, energy consumption during transport

Coconut coir, a fibrous material derived from the husk of coconuts, serves as a widely used growing substrate (consisting of 45–50% lignin, 40–45% cellulose, and less than 1% hemicellulose) [23]. Due to its porous structure and water-retention capabilities, coconut coir became a common choice for soilless cultivation. Additionally, the use of coconut coir promotes the recycling of waste from the coconut industry [24]. As a cultivation substrate, coconut coir possesses advantages such as physical stability, light weight, and a favorable air content, attributed to its high total pore space and substantial water-holding capacity, along with a sub-acid to neutral pH range of 5–6.8 [22]. Leveraging the aforementioned benefits, the utilization of coconut coir in conjunction with nutrient solutions can considerably enhance the yield and quality of horticultural crops, such as sweet peppers [25] and microgreen vegetables [26]. The deep flow technique (DFT) and nutrient film technique (NFT) are commonly used in PFALs. However, the loosely packed materials of coconut coir can easily shed fibers into the recirculating water of hydroponic systems, thus causing blockages of water filters and water recirculation systems. This also renders the simple-structured coconut coir substrate challenging compared to rockwool and sponge in PFAL production. However, the low cost of industrial-scale production, high strength to weight ratio, biodegradability, non-corrosive nature, and high abundance of these natural fibers have made them highly appealing for reinforcing composite materials within the biocomposite manufacturing community [27]. Current research has explored methods such as cross-linking reactive coconut coir with polyvinyl alcohol (PVA) to produce coconut coir composites [24], and valorizing coconut fibers (coconut coir) by transforming them into multi-purpose biocomposites using a pectin-based one-pot aqueous process [23], to enhance the suitability of coconut coir in recirculating hydroponic systems. By valorizing coconut coir, a global agricultural by-product exceeding 50 million tons annually [28], this will establish a novel pathway for lignocellulosic waste upcycling.

To develop renewable and biodegradable substrate with good water-retention capacity and air porosity for application in hydroponic systems in PFALs, this study employed a molded coconut coir substrate (MCCS) composed of coconut coir and polyurethane hydrophilic adhesive for cultivating lettuce (Lactuca sativa L.) and pak choi (Brassica rapa ssp. chinensis) in PFALs. These two leafy vegetables are widely consumed and cultivated worldwide and are the main crop species in PFALs. This study compared the effects among vegetable cultivation in MCCS, rockwool, and sponge, thereby achieving energy conservation, environmental protection, and efficient production in PFALs.

2. Materials and Methods

2.1. Plant Material, Growth Conditions, Treatments and Substrate Physical Properties

This study was conducted in a PFAL at the South China Agricultural University, from 9 October 2024 to 13 November 2024. The seeds of lettuce and pak choi (Beijing Dingfeng Modern Agricultural Development Company, Beijing, China) were sown in three different cultivation substrates: MCCS (coconut coir composited with polyurethane hydrophilic adhesive; Guangdong Zhuoyu Agricultural Technology Co., Ltd., Guangzhou, China); sponge (Jiangmen Shengjie Sponge Products Co., Ltd., Jiangmen, China); and rockwool (Grodan, Rockwool B.V., Roermond, The Netherlands). The shapes and sizes of each substrate are as follows: MCCS—a frustum of pyramid with upper base side length of 3 cm, lower base side length of 1 cm, and height of 4 cm; sponge—cube with side length of 2.5 cm; and rockwool—cylinder with diameter of 2 cm and height of 3 cm. (Figure 1a). The seedlings were cultivated with modified Hoagland nutrient solution and under photosynthetic photon flux density (PPFD) 250 µmol·m⁻²·s⁻¹ white light-emitting diodes (LEDs), with a 10/14 h light/dark period after germination. The air temperature was 22–26 °C with 65–75% relative humidity; seedlings with third-true leaves were transplanted to a hydroponic system at 15 days after sowing (15DAS).

Plants were grown hydroponically using 1/2 strength Hoagland nutrient solution (pH of 6.6–7.0, electrical conductivity of 1.45–1.55 mS·cm⁻¹). Plant density was maintained at 24 plants per plate (95 × 60 × 3 cm³), which was equivalent to 42 plants per m². Adjustable LED panels (Chenghui Equipment Co., Ltd., Guangzhou, China; 150 × 30 cm²) containing white (peaking at 440 nm) and red (660 ± 10 nm; R) LEDs were used as light sources. The plants were cultivated under PPFD 150 µmol·m⁻²·s⁻¹ white LEDs with PPFD 100 µmol·m⁻²·s⁻¹ red LEDs, with a 10/14 h light/dark period after germination.

There are similar shapes and size of MCCS, rockwool, and sponge (Figure 1a). The physical properties of the substrate, including total pore space (TPS), air volume (AV), water-holding capacity (WHC), and bulk density (BD), were determined through the following procedures [29]. The mass of the substrates after drying in the oven at 105 °C was denoted as W1 and the samples were subsequently submerged in water for 30 min to achieve saturation, after which their masses were recorded as W2. Following removal from water, the substrates were suspended to facilitate drainage until no additional water seeped out, and then their masses were measured again as W3.

$$Total pore space (\%)={\displaystyle \frac{W2-W1}{V}}\times 100\%$$

$$Air volume (\%)={\displaystyle \frac{W2-W3}{V}}\times 100\%$$

$$Water holding capacity (\%)={\displaystyle \frac{W3-W1}{V}}\times 100\%$$

$$Bulk density (\mathrm{g}·{\mathrm{cm}}^{-3})={\displaystyle \frac{W1}{V}}$$

$V$ is the volume of the substrate after drying.

For the assessment of electrical conductivity (EC) and pH of the substrates [23], 10 g of substrates after drying was introduced into a container filled with 10 mL of deionized water. The mixture was agitated for 30 min to ensure homogeneity. Subsequently, the suspension was filtered, and the filtrate was collected for analysis. A double-junction pH electrode was utilized to determine the pH of the filtrate. Meanwhile, a standard platinum EC probe was employed to measure the EC of the filtrate.

In addition, the MCCS maintained its original structure after 35 days of cultivation, without exhibiting disintegration or decay, thus not affecting the nutrient solution circulation system (Figure 1b).

2.2. Measurement of Plant Morphology and Growth Characteristics

From each treatment group, eight uniform plants were randomly sampled for biometrics analysis. The fresh weight of the shoots and roots was weighed separately by electronic balance. Morphological traits were measured by ImageJ 1.52 V (National Institutes of Health, Bethesda, MD, USA), including root length, leaf area, and canopy width (the diameter of the circumscribed circle in a top-view image of plant). Dry weights were determined using an electronic balance following a two-stage dehydration protocol: tissue drying at 75 °C for 48 h, preceded by sample deactivation at 105 °C. Before biochemical analysis, the shoots of lettuce and pak choi were frozen in liquid N₂ and stored at −80 °C. In the biochemical analysis, to gain a more comprehensive understanding of the physiological differences between different organs of pak choi, the leaf blade and petiole of pak choi were separated and measured. Each biochemical index was performed with four analytical replicates.

2.3. Measurement of Pigment Content

Tissues (0.2 g fresh weight) of lettuce and pak choi underwent 24 h pigment extraction in 8 mL acetone-alcohol (1:1, v/v) at 25 °C under dark conditions, followed by mechanical homogenization. Chlorophyll and carotenoid content were quantified spectrophotometrically (Shimadzu UV-1780, Corporation, Kyoto, Japan) via absorbance measurements at 440, 645, and 663 nm, with concentrations calculated as follows [30]:

$$Chlorophyll a (\mathrm{mg}·{\mathrm{g}}^{-1})={\displaystyle \frac{(12.7\times OD663-2.69\times OD645)\times V}{W}}\times 1000$$

$$Chlorophyll b (\mathrm{mg}·{\mathrm{g}}^{-1})={\displaystyle \frac{(22.9\times OD645-4.86\times OD663)\times V}{W}}\times 1000$$

$$Total Chlorophyll (\mathrm{mg}·{\mathrm{g}}^{-1})={\displaystyle \frac{(8.02\times OD663+20.20\times OD645)\times V}{W}}\times 1000$$

$$Carotenoids (\mathrm{mg}·{\mathrm{g}}^{-1})={\displaystyle \frac{(4.7\times OD440-0.27\times Total Chlorophyll)\times V}{W}}\times 1000$$

$V$ is the volume of the extract; W is the weight of the sample.

2.4. Measurement of Soluble Sugar

The soluble sugar content was determined by anthrone–sulfuric acid colorimetry [31]. Fresh lettuce and pak choi samples (0.5 g) were heated in a boiling water bath with 10 mL distilled water for 30 min; after, the plant tissue was crushed. The supernatant (0.1 mL) was mixed with 1.9 mL distilled water, 0.5 mL anthrone ethyl acetate, and 5 mL vitriol. After shaking to ensure proper mixing, soluble sugars were detected at 630 nm using a UV spectrophotometer.

2.5. Measurement of Soluble Protein

Using the Coomassie brilliant blue G-250 dye technique, the soluble protein content of pak choi and lettuce was ascertained [32]. A quantity of 8 mL of distilled water was used to homogenize about 0.5 g of fresh plant tissue. After that, the resultant homogenate was centrifuged for 10 min at 4 °C at 3000× g. Quantities of 5 mL of Coomassie brilliant blue G-250 (0.1 g·L⁻¹) solution, 0.8 mL of distilled water, and 0.2 mL of the supernatant were combined. A UV spectrophotometer was used to test the mixture’s absorbance at 595 nm following a 5 min incubation period.

2.6. Measurement of Nitrate Content

The nitrate content was determined using UV spectrophotometry [33]. About 1.0 g of fresh lettuce and pak choi tissue were homogenized in 10 mL distilled water and heated for 30 min in a water bath at a rolling boil. The homogenate was moved to a volumetric flask following filtering. A quantity of 9.5 mL of 8% NaOH was added after equal portions (0.4 mL each) of the sample solution and 5% sulfuric and salicylic acids were added. A UV spectrophotometer set to a 410 nm wavelength was then used to measure the amount of nitrate present in this mixture.

2.7. Measurement of Vitamin C

Vitamin C quantification employed molybdenum blue spectrophotometry [34]. Fresh lettuce and pak choi samples (0.5 g) were finely ground into a pulp using 25 mL oxalic acid EDTA solution (w/v). After filtering the resultant combination, 1 mL phosphate–acetic acid, 2 mL 5% vitriol, and 4 mL ammonium molybdate were added to 10 mL of extract solution. A UV spectrophotometer set to 705 nm was used to determine the amount of vitamin C present in this mixture.

2.8. Measurement of Antioxidant Content and Antioxidant Activity

Polyphenol content was quantified via Folin–Ciocalteu assay [35]. Fresh lettuce and pak choi samples (0.5 g) were extracted with 8 mL alcohol. After standing for 30 min, the homogenate was centrifuged at 3000× g for 10 min at 4 °C. A quantity of 1 mL of supernatant was mixed with 7 mL of distilled water, 11.5 mL of 26.7% sodium carbonate, and 0.5 mL of Folin phenol. A UV spectrophotometer was used to detect the absorbance at 510 nm after 2 h.

Total flavonoid content was quantified via the aluminum nitrate method [36]. Following the polyphenol extraction protocol, 11.5 mL of 30% alcohol and 0.7 mL of 5% NaNO₂ were mixed with 1 mL of extract solution. After 5 min, 0.7 mL of 10% Al(NO₃)₃ was added to the reaction solution, and 6 min later, 5 mL of 5% NaOH was added. A UV spectrophotometer was used to measure the absorbance at 760 nm after 10 min.

The 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging rate was quantified via Tadolini et al.’s method [37]. After mixing 2.0 mL of the sample extract with 2.0 mL of DPPH solution (0.0080 g DPPH in 100 mL alcohol), the absorbance of the combination at 517 nm was measured using a UV spectrophotometer.

Ferric-reducing antioxidant power (FRAP) was quantified via Benzie and Strain [38]. Using the same extraction procedure as for polyphenols, a solution containing 0.4 mL of the sample was mixed with 3.6 mL of a solution made up of 0.3 mol·L⁻¹ acetate buffer, 10 mmol·L⁻¹ 2,4,6-tripyridyl-S-triazine (TPTZ), and 20 mmol·L⁻¹ FeCl₃ at a ratio of 10:1:1 (v/v/v). For 10 min, the resultant mixture was incubated at 37 °C. A spectrophotometer was then used to assess the mixture’s FRAP at 593 nm.

2.9. Measurement of Energy Consumption Indicators

The calculation formulas for relevant energy consumption indicators are as follows:

$$E (\mathrm{J})={\displaystyle \frac{hc}{\lambda }}$$

$$PAR \left(\mathrm{W}·{\mathrm{m}}^{-2}\right)=PPFD\times E\times {10}^{-6}$$

$$DLI \left(\mathrm{mol}·{\mathrm{m}}^{-2}·{\mathrm{d}}^{-1}\right)=PPFD\times LFD (\mathrm{h}·{\mathrm{d}}^{-1})\times {\displaystyle \frac{3.6\times {10}^3}{{10}^6}}$$

$$P_{Total}=PAR\times Area ({\mathrm{m}}^2)$$

$$E_{Total} (\mathrm{Wh})=P_{Total}\times LFD \left(\mathrm{h}·{\mathrm{d}}^{-1}\right)\times Cultivation days (\mathrm{d})$$

$$E_{unit} \left(\mathrm{Wh}·{\mathrm{g}}^{-1}·{\mathrm{m}}^{-2}\right)={\displaystyle \frac{E_{Total}}{Yield \left(\mathrm{g}\right)\times Area ({\mathrm{m}}^2)}}$$

$E$ is the energy of a photon; h is Planck’s constant (6.626 × 10⁻³⁴ J·s); $c$ is the speed of light (3 × 10⁸ m·s⁻¹). $\lambda$ is the wavelength of the light (meters, m); PAR, photosynthetically active radiation; PPFD, photosynthetic photon flux density; DLI, daily light integral; LFD, light fractional duration; $P_{Total}$ is the total power; Area is the cultivation area; $E_{Total}$ is the total energy consumption; Cultivation days represents the number of days for cultivation; $E_{unit}$ is the energy consumption per unit area yield; Yield is the crop yield.

2.10. Statistical Analysis

Data are expressed as mean ± standard error based on three biological replicates per treatment. Each biological replicate consisted of pooled samples from eight individual plants. Analysis of variance (ANOVA) and multivariate analysis of variance (MANOVA) followed by Duncan’s multiple range test were conducted using IBM SPSS Statistics 25 software (SPSS Inc., Chicago, IL, USA).

3. Results

3.1. Plant Morphology and Growth Characteristics

There were significant differences in morphology and biomass of lettuce and pak choi plants cultivated on different substrates (Figure 2 and Figure 3). In the case of lettuce, there were no significant differences in shoot biomass among the three substrate treatments during the early stages of cultivation (15–20DAS). After 25DAS, the shoot biomass showed a trend in which the MCCS and sponge treatments were superior to the rockwool treatment, and the MCCS had a greater increase (fresh weight of shoot by 20.95% than rockwool). The root biomass of lettuce under all three substrate treatments showed a pattern of rapid initial growth followed by a gradual flattening in the later stages, which might be due to the limitation of root growth space in the later stages of cultivation. For pak choi, there was no significant difference in shoot biomass among the three substrate treatments during the early stages of cultivation (15–20DAS). However, at 25DAS, the increase in shoot biomass was most pronounced in the MCCS treatment (fresh weight of shoot 57.65% higher than in rockwool) and at the slowest rate in the sponge treatment. Throughout the entire cultivation period, the accumulation of root biomass in pak choi was greater in MCCS than in the sponge and rockwool.

At 35 days after sowing, compared to sponge, MCCS significantly increased the shoot fresh weight and root dry weight of lettuce by 9.36% and 24.57%, respectively; and compared to rockwool, MCCS significantly enhanced the shoot fresh weight, shoot dry weight, and root dry weight of lettuce by 27.33%, 13.60%, and 19.37%, respectively. Compared to sponge, MCCS significantly increased the shoot fresh weight, root fresh weight, shoot dry weight, and root dry weight of pak choi by 67.19%, 51.10%, 43.07% and 37.25%, respectively; and compared to rockwool, MCCS significantly enhanced the root fresh weight and dry weight of pak choi by 56.21% and 35.11%, respectively.

When cultivated with different substrates, the root length, leaf area, leaf number, and canopy width of lettuce and pak choi were also affected (Figure 4). The trend in root length of lettuce and pak choi under the three different substrate treatments was similar to the root biomass, which might be related to the root zone environment created by the different physical and chemical properties of the substrates. After 20DAS, the differences in the effects of substrates on the leaf area and leaf number of lettuce and pak choi became increasingly obvious, with greater increases in the MCCS. The canopy width of both lettuce and pak choi exhibited an initial phase of rapid growth, followed by a gradual stabilization after 25DAS, which might be due to the plants beginning to shade each other, thus restricting the growth of the plants in terms of canopy width. At 35 days after sowing, leaf area, leaf number, and canopy width of lettuce in MCCS significantly increased by 14.57% and 20.41%, 6.91% and 11%, and 10.61% and 10.70%, compared to those in sponge and rockwool, respectively. Concurrently, the exposed roots outside the MCCS during the transplanting stage of lettuce were shorter than those using the sponge treatment by 13.40%. Additionally, similar trends in root length were observed during the transplanting stage of pak choi, being 19.92% shorter than sponge. The leaf area, leaf number, and canopy width of pak choi in MCCS were significantly enhanced, by 35.18%, 26.26%, and 19.93%, respectively, compared with in the sponge treatment.

3.2. Substrate Physical Properties

Significant differences in the physical properties of various substrates were observed (

Table 2. The physical properties of MCCS, sponge and rockwool.

Substrates	TPS (%)	AV (%)	WHC (%)	BD (g·cm−3)	EC (μS·cm−1)	pH
MCCS	44.71 ± 0.61 c	35.09 ± 0.89 c	9.62 ± 0.56 b	0.1344 ± 0.0096 a	170.50 ± 1.36 a	6.30 ± 0.03 b
Sponge	79.26 ± 0.91 a	75.38 ± 0.91 a	3.88 ± 0.24 c	0.0229 ± 0.0003 c	8.16 ± 0.05 c	7.92 ± 0.08 a
Rockwool	62.53 ± 0.97 b	46.71 ± 0.85 b	15.82 ± 0.61 a	0.0548 ± 0.0030 b	19.59 ± 0.10 b	7.93 ± 0.03 a

MCCS, molded coconut coir substrate; TPS, total pore space; AV, air volume; WHC, water-holding capacity; BD, bulk density; EC, electrical conductivity. Different letters indicate significant differences among growth stages according to Duncan’s multiple-range test (p ≤ 0.05).

). In comparison with sponge, MCCS, despite having lower TPS and AV, showed significantly higher WHC and BD, with increases of 148.30% and 486.58%, respectively. Similar trends were noted when compared to rockwool; however, rockwool exhibited a higher WHC compared to MCCS. Compared to these two slightly alkaline substrates, MCCS is capable of providing a more acidic environment for root systems, and the EC was significantly increased by 20-fold and 8-fold, respectively.

3.3. Photosynthetic Pigment Content

The different substrates impacted the photosynthetic pigment concentration of lettuce and pak choi leaves, which included chlorophyll a (Chla), chlorophyll b (Chlb), total chlorophyll (TChl), and carotenoid (Caro) (Figure 5). The contents of Chla, Chlb, and TChl in lettuce leaves of MCCS significantly increased by 7.59%, 10.30%, and 8.56%, respectively, compared to those using sponge. However, the content of photosynthetic pigments in pak choi leaves of MCCS was significantly reduced compared to that using sponge and rockwool.

3.4. Contents of Soluble Sugars, Soluble Protein, Nitrates, and Vitamin C

The contents of soluble sugars (SSs), soluble protein (SP), nitrates, and vitamin C (VC) in lettuce and pak choi were affected by different substrates (Figure 6). Compared to rockwool, MCCS significantly enhanced the VC content in lettuce as well as the SP content in pak choi leaf blades by 29.06% and 32.01%, respectively. However, the VC content in pak choi leaf blades cultivated with MCCS was significantly lower than in those cultivated with sponge and rockwool, by 7.85% and 17.15%, respectively, and the VC content in leaf petioles was significantly lower, by 7.48%, than when using sponge.

3.5. Antioxidant Content and Antioxidant Activity

Compared to rockwool, MCCS significantly increased the total phenolics content (TP), total flavonoid content (TF), FRAP, and DPPH in lettuce by 13.59%, 18.43%, 12.96%, and 19.87%, respectively. Although the TP content in pak choi leaf petioles cultivated with MCCS was significantly increased by 14.76% compared to rockwool, the TF content in the leaf blades was significantly reduced by 15.66% compared to rockwool. Moreover, the TF and FRAP in the leaf petioles were significantly decreased by 14.99% and 9.34%, respectively, compared to sponge (Figure 7).

3.6. Energy Consumption Indicators

The energy consumption indicators under different substrate treatments are presented in

Table 3. The energy consumption of MCCS, sponge and rockwool.

Crop Species	Substrates	PAR (W·m−2)	DLI (mol·m−2·d−1)	Eunit (Wh·g−1·m−2)
Lettuce	MCCS	0.058	9.0	0.2665 ± 0.0099 c
	Sponge	0.058	9.0	0.3088 ± 0.0162 b
	Rockwool	0.058	9.0	0.3461 ± 0.0087 a
Pak choi	MCCS	0.058	9.0	0.2312 ± 0.0148 b
	Sponge	0.058	9.0	0.3913 ± 0.0298 a
	Rockwool	0.058	9.0	0.2520 ± 0.0076 b

MCCS, molded coconut coir substrate; PAR, photosynthetically active radiation; DLI, daily light integral; E_unit is the energy consumption per unit area yield. Different letters indicate significant differences among growth stages according to Duncan’s multiple-range test (p ≤ 0.05).

. Compared to sponge, the use of MCCS for lettuce and pak choi cultivation significantly reduced the energy consumption per unit area yield by 13.68% and 40.91%, respectively. Compared to rockwool, the energy consumption per unit area yield of lettuce cultivated with MCCS was significantly reduced by 22.98%, but there was no significant difference in pak choi.

3.7. Multivariate Analysis of Variance

Multivariate analysis of variance (MANOVA) was conducted on the crop species (c), substrates (s), and different organs of pak choi (o) to assess their effects, as shown in

Table 4. The MANOVA of cultivars and substrates on growth characteristics and photosynthetic pigment content.

Source of Variance	SFW	RFW	SDW	RDW	RL	LA	LN	CW	Chla	Chlb	Caro	TChl
Crop species (C)	**	NS	***	*	***	***	***	***	***	***	***	***
Substrates (S)	***	**	NS	**	***	**	**	***	*	*	**	*
C*S	*	NS	NS	NS	NS	NS	**	*	**	***	*	**

SFW, shoot fresh weight; RFW, root fresh weight; SDW, shoot dry weight; RDW, root dry weight; RL, root length of 15 days after sowing; LA, leaf area; LN, leaf number; CW, canopy width; Caro, carotenoid; Chla, chlorophyll a; Chlb, chlorophyll b; TChl, total chlorophyl. NS, non-significant; *, significant at p ≤ 0.05; **, significant at p ≤ 0.01; ***, significant at p ≤ 0.001.

and

Table 5. The MANOVA of cultivars, substrates and organs on phytochemical content.

Source of Variance	SSs	SP	Nitrates	VC	TP	TF	FRAP	DPPH
Crop species(C)	***	***	***	***	***	NS	***	***
Substrates (S)	NS	***	NS	***	NS	**	NS	NS
Organs (O)	*	***	***	***	***	***	***	***
C*S	***	**	NS	***	NS	***	*	***
S*O	*	***	*	***	NS	***	NS	NS

SSs, soluble sugars; SP, soluble protein; VC, vitamin C; TP, total phenolics; TF, total flavonoid; FRAP, ferric-reducing antioxidant power; DPPH, 2,2-diphenyl-1-picrylhydrazyl radical scavenging rate. NS, non-significant; *, significant at p ≤ 0.05; **, significant at p ≤ 0.01; ***, significant at p ≤ 0.001.

. The interaction between cultivars and substrate exhibited significant differences in SFW, LN, CW, photosynthetic pigment content, SSs, SP, VC, TF, FRAP, and DPPH. Moreover, the interaction between substrate and organs showed significant differences in the content of SSs, SP, nitrates, VC, and TF in pak choi. These differences might be attributed to the varying adaptability of different crop species to substrate characteristics, as well as differences in the interaction between root zone environment and nutrient dynamics.

4. Discussion

During vegetable cultivation in PFALs, the root zone environment of plants is not only influenced by the nutrient solution but also by the physical properties of the substrate. The physical and biological properties of substrate provide a biologically stable and non-toxic growth environment for plant root systems [39]. While sponge and rockwool exhibit superior pore space for root aeration, which make them suitable for soilless cultivation systems in PFALs for leafy vegetables such as lettuce [12], pak choi [40], and even for fruiting vegetables such as hot peppers [41] and cucumbers [42], coconut coir provides distinct advantages through its chemical composition. The higher chitin and lower cellulose content in coir enhances root zone buffering capacity [43], and its greater bulk density improves water and nutrient retention [44]. In this study, the MCCS exhibited lower TPS compared to sponge and rockwool, but higher EC and acidic pH levels, due to the major component of coconut coir. Cultivating lettuce and cabbage under slightly acidic conditions (pH 6.2–6.8) was found to be more conducive to enhancing their root biomass compared to cultivation at pH between 4.5 and 6 [45]. Coconut coir, when employed as a seedling substrate, increased the nutrient concentration in the plant roots, stimulated the induction of the primary root system, and contributed to enhanced root biomass development and root hair system development. The root biomass of cucumber and watermelon seedlings improved in coconut coir substrate [46]. Moreover, under cultivation conditions based on coconut coir, the additional application of organic compost further enhanced the root biomass of crops [47]. This study also found that the use of molded coconut coir substrate was more conducive to enhancing the root biomass of lettuce and pak choi seedlings and plants (Figure 2 and Figure 3). These results might be attributed to the physical and chemical properties of the MCCS and the high organic content of coconut coir therein, which might stimulate root development. Compared to pak choi, lettuce has shallower root systems, which might have resulted in a more pronounced spatial limitation on root biomass after 30DAS (Figure 2c). More importantly, compared to sponge and rockwool, lettuce and pak choi cultivated with MCCS had shorter exposed roots outside the substrate exterior during the transplanting stage (Figure 4). This effectively reduced root damage during the transplanting process and enhanced the efficiency of transplantation, improved the adaptability of the cultivation substrate MCCS to mechanical transplanting (reducing root exposure), and facilitated the mechanized transplanting of crops in PFALs. This compensated for the neglect of operational efficiency in traditional lifecycle assessment (LCA).

The relationship between root systems and the shoots of crops is a close one. Different substrates affect the root zone environment, thereby influencing the morphological development and yield of the shoot. Compared with rockwool, cultivation using coconut coir significantly enhanced both the leaf area and yield of cucumber [42]. Coconut coir as a cultivation substrate markedly increased the yield, leaf number, and canopy width of strawberries [48]. In this study, MCCS considerably enhanced the shoot biomass, leaf number, leaf area, and canopy width of lettuce and pak choi (Figure 4). The interrelationship between the root and shoot, as depicted in Figure 3, Figure 4 and Figure 5, is also evident. In lettuce, there was a rapid accumulation of root dry matter at 30DAS (Figure 2e), and the fresh weight of the shoot also increased significantly during the same period (Figure 2b). Concurrently, there was a substantial increase in leaf area, leaf number, and canopy width in lettuce (Figure 4a), which in turn increased the leaf area for light capture and might have enhanced the rate of root dry matter accumulation. In pak choi, the interrelationship between root and shoot was even more pronounced. Moreover, significant interactions between crop species and substrates were observed in shoot fresh weight, leaf number, and canopy width (Table 4), providing insights for optimizing species selection and substrates configuration in PFAL production.

The root zone environments provided by different cultivation substrates also affect the primary and secondary metabolites in plants. Compared to loamy soil, organic substrates (coconut coir) could enhance the photosynthetic pigment content and photosynthetic rate in tomato [49], but no significant differences were observed in sweet pepper [25]. In this study, MCCS increased the photosynthetic pigment content in lettuce, but showed the opposite trend in pak choi, which might be due to differences in species (Table 4). When cultivating using a mixed organic substrate, the contents of TP, VC, and nitrates in lettuce increased with the increasing proportion of coconut coir, while the SS content showed a declining trend [50]. Although such results were not observed in pak choi in this study (Figure 6b and Figure 7b), similar trends were found in the contents of SSs, VC, and TP in lettuce (Figure 6a and Figure 7a). Significant differences in the phytochemical content were observed between the leaf blade and petiole of pak choi. Moreover, there were interactive effects among different organs of pak choi and various substrate treatments on the contents of SSs, SP, nitrates, VC, and TF (Table 5). The relationships between different organs of pak choi (source–sink relationships) warrant further investigation.

While substrate effects on crop yield and quality remain a primary research focus, increasing attention has been directed toward the environmental sustainability and circularity potential of substrate production and post-use processing. Although closed hydroponic systems demonstrate superior resource efficiency, their energy consumption profiles remain a critical concern [51]. In terms of energy consumption during cultivation, it is noteworthy that the light quality, intensity, and photoperiod for each substrate treatment were consistent, meaning that the PAR and DLI remained unchanged throughout the study. Under consistent light conditions, MCCS implementation significantly reduced the energy consumption per unit area yield of lettuce and pak choi. This energy-saving advantage stemmed directly from the enhanced biomass accumulation and yield parameters achieved through MCCS adoption (Table 3). The negative correlation between crop yield and carbon emissions has been demonstrated in a raspberry soilless cultivation system [52]. This further indicates that yield improvement is a key factor to reducing the carbon footprint (CF). Substrate optimization could complement genetic improvement strategies in emission reduction efforts [5], providing more readily promotable pathways for emission reduction in resource-constrained regions. The substitution of conventional sponge and rockwool substrate with MCCS in PFALs could significantly reduce the production energy consumption, making it a more energy-efficient and environment-friendly form of agricultural production. Currently, optimization strategies focusing on identifying crop-specific DLI for crops in PFALs are also an approach to energy conservation and environmental protection [53]. The synergistic design of substrate and light efficiency might break through the traditional contradiction of high yield and high energy consumption, providing a new pathway for the large-scale emission reduction of PFALs. On the other hand, from the lifecycle perspective, molded coconut coir substrate (MCCS) may exhibit multiple environmental advantages (Figure 8). As a coconut processing by-product, considering the production and subsequent processing, coconut coir substrate aligns with circular economy principles, and is environmentally friendly and renewable [24]. After cultivation, coconut coir can be reused through composting and reusing. In contrast, the production processes of sponge and rockwool consume a considerable quantity of energy [22]. In particular, rockwool, being a non-renewable resource, lacks proper waste processing after the growing cycle [17]. Although an increasing number of novel substrate materials, such as chitin, chitosan [20], miscanthus, and biochar [54], are being explored and applied, the energy-efficient and environmentally friendly coconut coir substrate used in this study is more readily adaptable for promotion and application in the current production of PFALs. The primary focus of the current research was on energy consumption during the cultivation stage. The subsequent treatment and energy consumption of MCCS required further experimental verification. Future research should focus on conducting a detailed LCA and CF analysis of MCCS, including its entire lifecycle, to provide a more complete understanding of its environmental performance and potential for emission reduction. Moreover, research on controlled-environment agriculture currently focuses predominantly on leafy vegetables [13], while the effects of MCCS on other crop types remain under-explored. These studies will enable holistic evaluation of MCCS’s potential in sustainable urban agriculture systems.

5. Conclusions

This study investigated the utilization of molded coconut coir substrate (MCCS) as an environmentally friendly cultivation substrate in PFALs. Comparative analysis revealed distinct physiological impacts on both lettuce and pak choi. When compared with sponge substrates, MCCS significantly increased the shoot fresh weight, root dry weight, leaf area, leaf number, chlorophyll content, and canopy width of lettuce, while decreasing the soluble sugars content. For pak choi cultivation, MCCS increased the biomass, leaf area, leaf number, and canopy width, while reducing the contents of photosynthetic pigments, vitamin C, total phenolic content, and ferric-reducing antioxidant power in the petiole. In comparison with rockwool substrates, MCCS significantly increased the biomass, leaf area, leaf number, canopy width, content of chlorophyll b and antioxidants, and antioxidant activity of lettuce, while decreasing the soluble sugars content. Pak choi grown in MCCS had increased root biomass, leaf number, soluble protein, and total phenolics in the petiole, and reduced contents of photosynthetic pigments, vitamin C, and total flavonoid in the leaf blade. Energy consumption analysis demonstrated that the use of MCCS led to a significant reduction in the energy consumption per unit area yield of lettuce and pak choi compared to conventional substrates. These findings suggest that MCCS was more energy-efficient and environmentally friendly, and offers a renewable alternative to traditional substrates (sponge and rockwool) in PFALs. The observed trade-offs in secondary metabolite accumulation warrant further investigation into optimal cultivation protocols. Further research should integrate spectral optimization strategies to maximize both biomass production and phytochemical content in MCCS-substrate PFALs.