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Article

Agrivoltaic Farming Insights: A Case Study on the Cultivation and Quality of Kimchi Cabbage and Garlic

by
Da-Yeong Ko
1,
Seung-Hun Chae
2,
Hyeon-Woo Moon
2,
Hye Joung Kim
2,
Joon Seong
3,
Moon-Sub Lee
4 and
Kang-Mo Ku
1,2,*
1
Department of Plant Biotechnology, Korea University, Seoul 02841, Republic of Korea
2
Department of Horticulture, Chonnam National University, Gwangju 61186, Republic of Korea
3
Division of Biotechnology, Korea University, Seoul 02841, Republic of Korea
4
Carl R. Woese Institute for Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(10), 2625; https://doi.org/10.3390/agronomy13102625
Submission received: 29 August 2023 / Revised: 26 September 2023 / Accepted: 14 October 2023 / Published: 17 October 2023
(This article belongs to the Section Innovative Cropping Systems)
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Abstract

:
Agrivoltaic systems, which combine the cultivation of crops with solar panel installations, offer a novel solution to the dual challenges of energy production and agricultural productivity. This research verifies the impact of agrivoltaic (APV) conditions on the growth and quality of garlic and kimchi cabbage over two consecutive years in Naju-si, Jeollanam Province, Republic of Korea. In the 2019–2020 cultivation season, both kimchi cabbage and garlic grown under APV conditions experienced weight reductions of 18% and 15%, respectively, when compared to those grown in conventional settings. Intriguingly, despite the altered light conditions of APV leading to microenvironmental changes (mainly 41% light reduction), the quality of these crops, particularly in terms of their sulfur compound concentrations, remained consistent. This suggests that there was no discernible difference in the sensory quality of APV-grown kimchi cabbage and garlic compared to their traditionally grown counterparts. These findings highlight the potential of APV systems in promoting sustainable agriculture by balancing both crop yield and quality. Based on these results, the study suggests three innovative cultivation techniques to enhance crop growth in APV environments.

1. Introduction

With global electricity demands surging, over 60% of our current electricity production relies on fossil fuels [1]. The International Energy Agency (IEA) has proposed the ‘Net Zero Scenario’, which aims to solve the world’s environmental issues by reducing carbon emissions to production levels until 2050 [2]. As a proposed solution, the suggestion to triple renewable power capacity by 2030 has been put forth [3]. Solar energy, which generates electricity, is a prime example of renewable energy, as it produces electricity without emitting greenhouse gases or other harmful pollutants.
An agrivoltaic (APV) system entails the installation of solar panels above crop cultivation areas, with crops growing beneath [4] (Figure 1). In crop cultivation under solar panels, minimizing shaded areas allows crops to reach their light saturation point [5]. For that, it is important to adjust panel spacing and placement angles to ensure plants below reach the required light saturation point for growth [6]. In the field of agriculture, cultivating crops while generating additional income through solar power contributes to the stability of farming households. Moreover, this is a powerful strategy to overcome the environmental issues our society is facing, including carbon emissions from electricity production [7,8]. APV systems aim for eco-friendly energy production, thereby fostering sustainable and environmentally conscious agricultural systems [9]. Meanwhile, APV systems impact on various environmental and climatic variables, so empirically validated data for each crop and growing season are needed to cultivate crops properly. Recent research on APV has mainly focused on the crop yield beneath the panels. However, modern consumers are also interested in the taste and quality of crops in order to obtain higher nutritional value [10,11]. Thus, if the quality of agricultural products is greatly reduced, it will negatively affect the consumption chain even though the yield remains at the same level as before applying APV. Therefore, quality research should be conducted alongside studies of yield to determine the impact of APV. This study will confirm the crops that are widely consumed in the Republic of Korea, particularly kimchi cabbage and garlic.
According to production data, Kimchi cabbage (Brassica rapa L. ssp. Pekinenis) holds a prominent position among vegetables in Korea, with an annual production of 2.5 million tons of this specific cabbage variety. In 2022, Statistics Korea also reported that the cultivation area dedicated to kimchi cabbages encompassed 23,853 hectares, surpassing that of fruits (9660 hectares) and root crops (18,408 hectares) [14]. This underscores the substantial importance of Kimchi cabbage in Korean agriculture. Nonetheless, climate change has a substantial impact on kimchi cabbage production, making it a primary driving force behind the imperative need for APV in kimchi cabbage cultivation [15]. According to the Global Warming report, the average annual temperature of the Earth is rising by 0.08 °C every 10 years [16]. This could have a negative impact on kimchi cabbage production [17]. Since kimchi cabbage is a low-temperature crop, in summer it is cultivated only in limited highland locations in Gangwon Province, Republic of Korea [18]. This highland area keeps decreasing, so applying new measures to cope with climate change is necessary [19]. The usage of solar panels which make barrier-like shadows can be proposed as a solution to overcome this situation. Based on previous research, solar panels can create more optimal environments with reduced radiation, properly cooled down soil, and low rates of evaporation [20]. Therefore, kimchi cabbage possessing a 40 Klx low light saturation point can be suggested as a potentially suitable crop for APV that may show good photosynthesis performance under partial shading conditions beneath APV solar panels [21].
Glucosinolates (GLS) refer to sulfur-containing compounds that are characteristic of the Brassicaceae family and have many beneficial effects on the human body, including antioxidant effects [22]. GLS hydrolysis products are broken-down products of GLS, unique functional substances that contribute to the distinct taste and aroma of Brassicaceae crops, and react with the hydrolytic enzyme myrosinase to form isothiocyanates known for their bioactive compounds [23,24]. The GSL concentration in Brassicaceae crops may have indirect effects on the morphological characteristics of the cultivated varieties, which may impact the final production of crops or other traits related to their quality [25,26]. Therefore, GLS and GLS hydrolysis products serve as key indicators of the taste and quality of kimchi cabbage. Thus, it is necessary to analyze GLS as an indicator to decide the quality of kimchi cabbage. Likewise, garlic (Allium sativum L.) is a highly valued spice worldwide. In Korea, the garlic field cultivation area was 22,362 ha in 2022, which was the second largest among condiment vegetables [14]. If solar modules are integrated into the garlic and kimchi cabbage cultivation areas, they might hold the potential to become a sustainable and viable source of renewable energy. Sulfur compounds that produce spicy or pungent tastes are characterized to determine the quality of garlic. When garlic is split or crushed, alliin is broken down by the enzyme allinase and becomes allicin, which then immediately makes various sulfur-containing derivatives [27,28]. The derivatives then can give off a spicy taste and also produce antioxidant effects [29]. To date, there have been no reports on how APV affects sulfur compounds in garlic and alters the essential characteristics of garlic.
This experiment will examine not only crop yield but also quality by evaluating secondary metabolites such as anticancer compounds and sensory-related components. Furthermore, the Japanese Ministry of Agriculture reported that a crop yield below 80% during cultivation in an APV system has an adverse impact on food security. Therefore, it is important to consider methods to enhance crop growth under APV conditions by either improving cultivation techniques or utilizing plant physiology. This study will integrate previous studies to provide valuable insights into crop cultivation and production.

2. Materials and Methods

2.1. Agrivoltaic Structure and Environmental Data Collection

In this study, an APV system was constructed at a practical training farm belonging to Chonnam National University (34°58′28.4″ N, 126°45′59.3″ E, Naju, Jeollanam Province, Republic of Korea) to investigate crop growth and quality. APV structure was installed according to the regulations of the Ministry of Trade, Industry, and Energy of the Republic of Korea [30]. Briefly, three types of solar panels, namely Bifacial_A (Bif_A, 10.24 kW), Bifacial_B (Bif_B, 9.92 kW), and Monofacial (Mof), were mounted at a height of 3.3 m with a 30° angle. The energy production of solar panels is also economically significant, as it is recommended to use high-efficiency panels, such as bifacial [31]. The average solar power generation efficiency of Bif A and B panels was 14% higher than Mof panel-generated solar power energy (Figure 2). So, in this study, among the three types of solar panel structures, the area covered by Bif_A (162 m2) and Bif_B (162 m2) was used for crop cultivation.
The APV electricity production and microclimatic data were collected using data logger (ZL6, Meter Group Inc., Pullman, WA, USA) in accordance with previous experiments described by Moon et al. [13]. Solar Radiation and photosynthetic photon flux density were measured using PAR sensors (PAR, Meter, Pullman, WA, USA). The Daily Light Integral (DLI) based on PPFD was calculated using Korczynski’s et al. methods [32]. Environmental data such as temperature and precipitation were provided by the RDA (http://weather.rda.go.kr/w/index.do. accessed on 1 January 2023). The GDDs (growing degree days) of each crop were evaluated by previous studies [33,34].

2.2. Crop Cultivation

Two crops (kimchi cabbage and garlic) grown in two different seasons were investigated (Table 1). To provide nutrients, NovaTec-Supreme (N:P:K; 21:5:10) controlled-release fertilizer (Combo Expert, Münster, Westphalia, Germany) was sprinkled on the growth area. The experiment was designed as a random complete block design (RCBD) with 4 replications. The management of kimchi cabbage followed the standard commercial agricultural practices of Korea. The kimchi cabbage cultivar ‘Hwimori Gold’ seeds (Asia Seed Co., Ltd., Seoul, Republic of Korea) were planted in a 105-plug tray (54 cm × 28 × 4.8 cm, each cell was 3.1 cm × 3.1 cm) and incubated for 30 days after germination in a greenhouse in Cheonnam National University under average 24 °C air temperature. Thirty-day-old seedlings which had 4–5 true leaves were planted and cultivated. Seedlings were then transplanted and cultivated from 9 June 2019 to 4 December 2019 (kimchi cabbage 2019), and from 9 June 2020 to 27 November 2020 (kimchi cabbage 2020). The temperature conditions for cultivation in each year were 5 to 27 °C and 8 to 25 °C, respectively. The average fresh weight (kg) was determined using seven (kimchi cabbage 2019) or five (kimchi cabbage 2020) samples from each replication (n = 4). For further analysis, one-sixth of the kimchi cabbage samples from the kimchi cabbages were pooled and freeze-dried.
On the other hand, the cold-type garlic ‘Uiseong’ was cultivated from 11 October 2019 to 20 June 2020 (Garlic 2019–2020), and again from 26 October 2020 to 5 June 2021 (Garlic 2020–2021), adhering to standard commercial agricultural practices. The temperature conditions for cultivation in each year were −1 to 26 °C and −9 to 24 °C, respectively. For the cultivation, 4–5 g of medium-sized garlic cloves (Daeji Seed Co. Ltd., Gyeonggi-do, Republic of Korea) were selected and immersed to sterilize in 2% benomyl+thiram wettable powder (Samkong Benoram, Samkong Korea, Seoul, Korea) for 30 min. The sterilized garlic cloves were fully dried for one day and planted. The growth was determined by measuring the average fresh weight (g) of twenty garlic bulbs (with skin) from each replication (n = 4). After harvesting, samples were subsequently freeze-dried for allicin and organosulfur analysis.

2.3. Analysis of Kimchi Cabbage Glucosinolates and Hydrolysis Products

Glucosinolates were quantified using the previous desulfoglucosinolates method [35]. Analysis of glucosinolate hydrolysis products was conducted using a method identical to that previously described without incubation by Moon et al. [13]. To put it briefly, 75 mg of freeze-dried kimchi cabbage powder was extracted in 1.5 mL of deionized water. Phenyl isothiocyanate (10 μg·mL−1) was added as an internal standard. The solution was mixed together with dichloromethane and incubated for 16 h at room temperature. After centrifugation, the dichloromethane layer was used. The glucosinolate hydrolysis products in kimchi cabbage were analyzed using GC-MS (GC/MS-QP 2020 NX, Shimadzu, Kyoto, Japan) equipped with the DB-5MS column (Agilent Technologies, Santa Clara, CA, USA; 30 m × 0.25 mm), and these compounds were identified using AMDIS [36].

2.4. Analysis of Garlic Allicin Using Liquid Chromatograph-Mass Spectrometer (LC-MS)

A modified method was employed to extract allicin from garlic powder [37]. Briefly, 50 mg of garlic powder was weighed and added to 2 mL of water. The sample was vortexed for 5 s and placed in an ice bath (0–4 °C) for 10 min. The sample was then centrifuged at 3500 rpm for 5 min and the supernatant was transferred to a 15 mL conical tube. The remaining pellet was re-extracted by adding 2 mL of water, and the two supernatants were combined. Finally, 1 mL of the mixture was filtered through a 0.22 μm PTFE syringe filter and transferred into HPLC vials. An external standard of allicin (0–400 μg·mL−1) was prepared for quantification of allicin in garlic. Allicin analysis in LC-MS was performed using an Xbridge C18 column (50 mm × 4.6 mm, 3.5 µm). The mobile phase was linearly increased from 20% to 80% acetonitrile for 2 min, then decreased to 40% for 2 min. The flow rate was maintained at 0.5 mL·min−1. The UV spectrum was captured from the PDA detector in the range of 190–400 nm, and allicin was detected at 245 nm. The column was purged with 20% acetonitrile for 2 min before elution of allicin at 4.2 min.

2.5. Analysis of Garlic Organosulfur Compounds Using Gas Chromatograph-Mass Spectrometer (GC-MS)

The analysis of volatile organosulfur compounds produced from hydrolyzed garlic was performed using the methods described by [38]. To activate the enzymes for hydrolysis, 60 mL of water and 3 g of freeze-dried garlic powder were shaken for 10 min in a 250 mL triangular flask. A total of 60 mL of petroleum ether was added to extract volatile organosulfur compounds. The mixture was then moved into an incubator and vortexed at 600 g at 37 °C for three hours. Afterward, 10 mL of the petroleum ether layer was transferred to a test tube, and sodium sulfate was added to completely remove moisture. The supernatant was analyzed using GC-MS equipped with the DB-5MS column. The analysis condition was as follows: helium was used as a carrier gas at a flow rate of 1.20 mL·min−1. The injection split ratio was 1:10. The injector temperature was set at 250 °C, and the detector temperature was set at 300 °C. The oven temperature was initially set at 35 °C, held for 1 min, then increased to 220 °C at a rate of 15 °C per minute, and finally increased to 310 °C at a rate of 30 °C per minute.

2.6. Statistical and Multivariate Analysis

The statistical analyses were performed using the statistical software JMP 12 (SAS Institute, Cary, NC, USA). One-way analysis of variance (ANOVA) was performed to analyze differences among the three treatment groups. Tukey’s Honestly Significant Difference (HSD) test was used for post hoc analysis. Differences with p < 0.05 were considered significant. Multivariate analysis was performed using MetaboAnalyst 5.0 (https://www.metaboanalyst.ca, accessed on 1 September 2023).

3. Results

3.1. Cultivation Environments and Crop Growth

The crop growth period represented the number of days from transplanting to harvesting (DTH). GDD (average growing degree day), solar radiation, and precipitation (mm) were observed by the Rural Development Administration (RDA) during the periods of cultivation. Among the multiple environmental factors that can affect yield and weight loss, one of the significant factors altered by the APV system is light [39,40]. During each crop’s cultivation, the average DLI of APV conditions was 14 mol∙m−2∙d which was 59% of control (OF) conditions (Figure 3).
The growth of the kimchi cabbage and garlic crops was measured in each cultivation season, as shown in Figure 4. Crop weight was measured to determine differences in growth between control (OF) and APV conditions (Table 2 and Table 3). In kimchi cabbage, the average weight of control (OF) and APV in 2019 shows significant differences (p < 0.05), as 21% weight loss occurred in APV. In the subsequent season of 2020, there were no significant differences, as shown by a 17% weight loss. However, in season 2020, the average weight increased in both conditions. As described in the previous study by Chae et al., active irrigation resulted in a weight increase in kimchi cabbage, coinciding with a drought in 2020.
When garlic was cultivated during 2019–2020, there was a significant decrease (p < 0.05) in APV garlic weight as 86.0 ± 10.4 g (21% lower than the control). On the other hand, in 2020–2021 there were no significant losses between control (OF) and APV. The growth days of garlic in 2019–2020 were longer than in the following seasons (Table 1) due to the weather.

3.2. Crop Quality Determination

In this study, we investigated GLS, allicin, and their hydrolysis products in crops grown under both control (OF) and APV treatment conditions, with a specific focus on crop growth and crop quality (Figure 5 and Figure 6). The kimchi cabbage contained GLS, namely progoitrin, glucoraphanin, gluconapin, glucobrassicin, 4-methoxyglucobrassicin, gluconasturtin, and neoglucobrassicin. The total GLS content of the control (OF) kimchi cabbage was measured at 9.2 ± 1.3 µmol·g−1 while APV was 7.4 ± 1.7 µmol·g−1 in 2019. In the following seasons in 2020, control (OF) kimchi cabbage registered 7.6 ± 0.3 µmol·g−1, whereas the APV was 7.7 ± 0.5 µmol·g−1. Notably, no significant differences were observed between the two groups in both years (Figure 5A). The detected GLS hydrolysis products in kimchi cabbage detected included phenethyl isothiocyanate, erucin nitrile, erucin, brassicanapin, 3-phenylpropionitrile, 1-cyano-3,4-epithiobutane, and 1-cyano-4,5-epithiopentane. The total GLS hydrolysis content in the control (OF) kimchi cabbage was 110.0 ± 45.6 µg·g−1 in 2019 and decreased to 39.6 ± 3.4 µg·g−1 in 2020. APV shows 99.2 ± 30.3 µg·g−1 and 37.5 ± 4.1 µg·g−1 in 2019 and 2020, respectively. There was a difference in the GLS hydrolysis products between the seasons (Figure 5B). This could have been caused by changes in enzyme activity, such as myrosinase, which might be influenced by climate and seasonal variations [41,42].
The sulfur compounds, the main functional component of garlic, are broken down by the enzyme allinase into a substance called allicin when garlic is crushed or chopped [43,44]. Allicin is converted into the various sulfur compounds which give garlic its pungent taste and aroma. These compounds are categorized as diallyl sulfide, diallyl disulfide, diallyl trisulfide, and diallyl tetrasulfide [45,46]. In this experiment, we detected 10 volatile sulfur compounds in garlic, including the components mentioned above. In the APV garlic, there was no observable significant reduction in allicin content as 10.16 ± 0.92 mg·g−1 and 9.41 ± 0.83 mg·g−1 in 2019–2020 and 2020–2021, respectively, which compares with the control (OF) allicin content of 10.21 ± 1.27 mg·g−1 and 9.35 ± 2.26 mg·g−1 each year (Figure 6A). Moreover, the content of the precursor of volatile sulfur compounds, allicin, may vary depending on the content and activity of alliinase, and these levels and enzyme activity may vary depending on the variety of garlic, the soil conditions, and the climatic environment. However, the average content of organosulfur compounds were statistically similar over the two years in the control (OF, 1.13 ± 0.21 mg·g−1) and APV (1.15 ± 0.27 mg·g−1) garlic. The results obtained from this study will show there is no difference in quality between garlic grown with APV and the control (OF) garlic.

4. Discussion

4.1. Impact on Crop Yield and Quality

In Korea, kimchi cabbage and garlic are among the most frequently consumed crops. The effects of APV on both the environment and agriculture are investigated through the analysis of microclimatic conditions and crop growth parameters, including crop yield and crop quality [47]. In APV cultivation, changes in light beneath the panels lead to microenvironmental changes and a consequent reduction in crop weight. Shading effects, in particular, have a crucial influence on the yield [48]. Crops can be classified into those thriving in shade and those unsuited to shaded conditions [49]. Each crop variety exhibits unique light saturation points and light requirements for growth. Therefore, when selecting crops for cultivation under photovoltaic panels, it is essential to choose those with lower light saturation points such as kimchi cabbage.
In this study, a 59% DLI reduction under APV was observed and weight loss occurred, which varied from crop to crop and even within each growing season (Table 2 and Table 3). A similar result was observed in the study of kimchi cabbage: a 51% reduced DLI in APV led to yield losses [50]. The typical weight of cabbages in conventional fall cultivation is reported to be 3 kg [51]. Despite the weight loss observed in APV, an average of 3.6 kg of kimchi cabbage (over 2 years) is still expected to hold value as a marketable product. We believe that due to the low light saturation point of kimchi cabbages (253 μmole∙m−2∙s−1), sufficient light intensity was reached even under the solar panels. Moreover, in the case of garlic, Jo et al. [52] reported that the two-year Daeso eco-type garlic harvest yield decreased by 19% in APV. However, in our research, cold ecotype garlic decreased by an average of 15% (Table 2). This result suggests that when cultivating crops beneath APV, the selection of crop cultivars can help reduce yield losses. Crop quality changes are also important indicators for evaluating APV impact. The light environment can affect crop physiology and metabolism, and have a negative influence on secondary metabolite production [53,54]. Secondary metabolites in kimchi cabbage and garlic are mainly sulfur compounds such as GLS hydrolysis products and organo-sulfur compounds, which influence the flavor and have antioxidant and anticancer effects on the crops [55,56,57]. However, in this experiment, the average GLS concentration and GLS hydrolysis products in APV did not decrease in both seasons. A similar result was reported in a study of cabbage. Hydrolysis products are associated with crop taste and consumer preferences, and there were no statistically significant differences observed in the consumer preferences for cabbage juice cultivated beneath APV conditions [9]. Moreover, garlic organo-sulfur compounds are crucial sensory indicators of garlic quality [58], but there was no degradation of those compounds beneath APV garlic over 2 years. Thus, our results indicate that there are no quality changes in kimchi cabbage and garlic cultivated beneath APV.

4.2. Proposed Farming Techniques

Cultivating crops under APV has the potential to generate higher incomes through electricity production but negatively impacts food security due to reduced yields. From this perspective, prior research has suggested approaches to enhance yields beneath photovoltaic panels by taking into account effective photosynthetically active radiation (PAR) accumulation, particularly in green bean cultivation within greenhouses [59].The research aiming to enhance crop yields under solar panels has not been or empirically investigated. Therefore, we propose methods for increasing crop yields under APV and plan to empirically validate them in our future experiments.

4.3. Suggestion 1: Early Transplanting Strategy

In the Republic of Korea, kimchi cabbage is typically transplanted for the autumn season in late August to early September and harvested between late November and early December. A study on kimchi cabbage weight prediction models based on transplanting dates indicated that kimchi cabbages transplanted in early September had lower weights compared to cabbages transplanted two weeks earlier [60]. When applying the growth model for each transplanting date with a two-year average cultivation duration. it was projected that cabbages transplanted two weeks earlier would have a weight of 6.7 kg (Figure 7A). Furthermore, cabbages transplanted without early transplanting exhibited a weight of 5.5 kg, indicating a 17% weight loss. The predicted 17% weight loss without early transplanting using the model coincides with the two-year average weight loss under APV conditions as reported in this study (Table 2). We anticipate that early transplanting under APV conditions could alleviate the weight loss, providing a potential solution.

4.4. Suggestion 2: The Use of Bigger Seedlings at Transplanting to Grow over Short Periods

The 32-cell tray is used for longer seedling periods or larger seedlings with a cell size of approximately 5.7 cm × 5.7 cm per cell (Figure 7B). In a study on the impact of kimchi cabbage seedling size on growth, seedlings grown in 5 cm cell trays were 34% bigger after 4 weeks of growth, and growth weight also increased by 8% [61]. Furthermore, other Brassicaceae studies also show that seedlings planted in larger trays exhibited increased growth in both seedling size and crop weight compared to seedlings grown in smaller trays [62,63].

4.5. Suggestion 3: Cultivation of Low-Light Saturation Cultivar

Figure 7C shows that for crop cultivation beneath APV, one should select a low light saturation point cultivar. The light intensity in the APV setup fluctuates continuously, and the average light intensity is lower compared to open field conditions. Therefore, when cultivating crops in the APV system, it is recommended to utilize cultivars with lower light saturation points. In a previous soybean study, the factor, cultivar was found to be a significant factor affecting yield under shaded conditions receiving 70% of natural light [64]. Furthermore, when different broccoli cultivars were grown under APV conditions, cultivars with higher chlorophyll b content exhibited greater yield reductions under APV conditions [65]. In relation to this, Dash, D. et al. reported that an increase in chlorophyll b under low light stress may result in reduced plant yields by influencing stomatal conductance [66].
Through our research findings and by incorporating insights from previous crop cultivation studies, we propose a strategy to compensate for weight loss when cultivating crops under APV.

5. Direction for Further Research

In this experiment, we wanted to establish a baseline for agricultural productivity and quality under the APV system, as well as to assess land use efficiency for future research focused on agricultural practices within APV systems. The APV system is a relatively novel and emerging concept, with few studies investigating how crop productivity responds to AVP systems. Therefore, a wide range of research efforts will be necessary to optimize crop productivity, develop effective management strategies for the APV system, and fully exploit the potential benefits the APV system offers.

5.1. Research on Crop Breeding and Panel Design to Enhance Crop Photosynthetic Efficiency

Our findings indicate a need to identify high-yield cultivars for optimal APV system productivity. For instance, when a plant reaches its light-saturation point, photosynthesis no longer increases, and the plant cannot utilize additional light sources. In other words, a plant might invest too many resources in light capture rather than improving light use efficiency. Ort et al. [67] suggested that a 50% reduction in chlorophyll could enhance light distribution and increase canopy photosynthesis. Additionally, reduced chlorophyll levels could lead to an increase in nitrogen use efficiency [68,69]. Several studies have shown that low-chlorophyll rice and soybean exhibited similar or even greater photosynthetic efficiency and nitrogen use efficiency when compared to wild types [70,71,72]. Furthermore, Lee et al. [73] observed that changes in light conditions, from high to low or low to high, resulted in a 16% reduction in carbon assimilation for both C3 and C4 crops. Lee et al. [73] suggested that improving photosynthetic efficiency in response to fluctuating light could be a targeted approach to increase crop productivity. Considering the low and changing light conditions in the APV system, improving light use efficiency could be the key trait for compensating for disadvantages caused by the APV system.
Moreover, solar panel technology continues to advance, with the potential for utilizing semitransparent solar cells tailored specifically for APV applications [74]. These panels may improve transparency to specific wavelengths of light, ensuring that crops receive the optimal amount and quality of sunlight required for photosynthesis. In addition, traditional solar panels can result in a reduction of solar radiation ranging from 12% to 40%, which depends on the density and arrangement of solar photovoltaic panels [20,75]. Future experiments will be necessary to investigate the optimal positions for solar photovoltaic panels, including their design, height, and density in APV systems, as suggested by Gomez-Casanovas et al. [76].

5.2. Integration of AI on APV System to Improve Quality and Yield on Crop

In this experiment, our observation was limited solely to two species, kimchi cabbage and garlic. Further studies are essential to assess the physiological responses of various crops within this system. This will help optimize agricultural production and enhance the synergistic effects between agricultural practices and the APV system. For instance, even though shade conditions may result in lower crop productivity, shading in the APV system may prevent photoinhibition from excess lights which can damage the photosynthetic pathway and decrease light use efficiency in photosynthesis [77]. Furthermore, shading in APV systems can reduce evapotranspiration, which ultimately improves water use efficiency and productivity [78,79]. Given the vast array of crops, assessing their suitability and performance becomes a challenge. Incorporating machine learning and AI-based physiology and breeding programs with APV research will be beneficial in the future. Our latest research reveals that APV enhanced the visual appeal of broccoli without causing any significant alterations in its phytochemicals [65]. Additionally, we found that over a span of two years, the APV did not modify any sensorial-important phytochemicals. An earlier study [13] established that juice from winter cabbages grown in APV did not affect consumer preferences. Yet, specific APV conditions might impact the quality and flavor of crops, necessitating extensive research on the dual aspects of yield and quality impacted by APV.

5.3. Changing Microenvironmental Conditions and Beyond Direct Impact

Recent studies suggest that the shaded conditions and reduced air turbulence under the APV system improved water use efficiency by reducing water loss per carbon fixation during photosynthesis [78,79]. As observed in this experiment, the yield of kimchi cabbage and garlic decreased in the APV system when compared to normal conditions. However, if crops are exposed to continued or severe drought conditions, the yield response in the APV system might prove to be more resilient than that of crops grown in conventional agricultural systems. Similarly, we observed less decrease or no significant difference in yields of kimchi cabbage and garlic during the year 2020, which had less precipitation than the growing season. Beyond these direct impacts on crop yields, APV systems inherently modify soil microclimates, impacting temperature, moisture levels, and UV radiation exposure. Such alterations can reshape soil microbial community dynamics, potentially shifting bacterial–fungal ratios, influencing organic matter decomposition rates, and impacting root microbiomes [80]. The potential for soil compaction and pollutant accumulation from APV infrastructure may further introduce variables that can influence crop health and yields. As APVs gain prominence, understanding their nuanced impact on both crop yield and soil microbiology becomes pivotal for optimizing system design and ensuring ecological integrity. It is imperative for future research to expand comparisons between APV and conventional farming, investigate the water-saving and carbon-fixation merits of APV, evaluate its robustness against varied environmental challenges, and gauge the economic viability of APV for widespread adoption.

6. Conclusions

Our two-year study in Naju-si, Jeollanam Province, Republic of Korea, examined the yield and quality of kimchi cabbage and garlic under both open field and APV conditions. The results revealed an average weight loss of 17% for kimchi cabbage and 15% for garlic in APV conditions. However, sensory-related compounds remained unaffected in both crops. These findings, combined with earlier research, suggest strategies to amplify agricultural efficiency in APVs. This not only offers potential for supplementary income but also advocates for sustainable electricity and crop production without carbon emissions.

Author Contributions

Conceptualization, formal analysis, investigation, D.-Y.K., S.-H.C., H.-W.M. and H.J.K.; writing—original draft preparation, D.-Y.K., S.-H.C., H.-W.M., H.J.K. and K.-M.K.; formal analysis and original draft preparation, J.S.; data interpretation, writing—review and editing, D.-Y.K., M.-S.L. and K.-M.K.; methodology, validation, supervision, project administration, funding acquisition, K.-M.K. All authors have read and agreed to the published version of the manuscript.

Funding

National Research Foundation of Korea (NRF) grant funded by the Korea government. (MSIT) (2023R1C1C1007733).

Data Availability Statement

The datasets generated and/or analyzed during the current study are not publicly available but are available from the corresponding author on reasonable request.

Acknowledgments

This research was funded by GS construction. We thank for those who helped field preparation (tillage) work (Jeung Young Guen) and administration work of Institute for Agricultural Practice Education, Chonnam National University in Naju experiment station (team leader Jin Kyoung Kim).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The conceptual diagram of agrivoltaic (APV) systems (A), photograph of field experiment (B) and photograph of panel shading area on crop cultivation (C) photograph of crops under APV [12,13].
Figure 1. The conceptual diagram of agrivoltaic (APV) systems (A), photograph of field experiment (B) and photograph of panel shading area on crop cultivation (C) photograph of crops under APV [12,13].
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Figure 2. Solar generated electricity (kWh) by the Bifacial A, Bifacial_B, and Monofacial.
Figure 2. Solar generated electricity (kWh) by the Bifacial A, Bifacial_B, and Monofacial.
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Figure 3. The daily light integral (DLI) of kimchi cabbage (A) and garlic (B) over growing period.
Figure 3. The daily light integral (DLI) of kimchi cabbage (A) and garlic (B) over growing period.
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Figure 4. Photograph of kimchi cabbage (A) and garlic (B) grown in control (OF) and agrivoltaic systems (Bif _A, Bif _B). Each kimchi cabbage and garlic photograph shows a representation of treatments.
Figure 4. Photograph of kimchi cabbage (A) and garlic (B) grown in control (OF) and agrivoltaic systems (Bif _A, Bif _B). Each kimchi cabbage and garlic photograph shows a representation of treatments.
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Figure 5. Total glucosinolate concentration (A) and glucosinolate hydrolysis products concentration (B) of kimchi cabbage were measured average concentration of treatment (Control n = 4, APV n = 8). APV included Bif_A and Bif_B. ns indicates that there was no significant difference, as determined by the Student’s t-test (p > 0.05), between OF and APV within the same year.
Figure 5. Total glucosinolate concentration (A) and glucosinolate hydrolysis products concentration (B) of kimchi cabbage were measured average concentration of treatment (Control n = 4, APV n = 8). APV included Bif_A and Bif_B. ns indicates that there was no significant difference, as determined by the Student’s t-test (p > 0.05), between OF and APV within the same year.
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Figure 6. Allicin concentration (A) and total sulfur compounds concentration (B) of garlic were measured average concentration of treatment (n = 4, APV n = 8). APV included Bif_A and Bif_B. ns indicates that there was no significant difference, as determined by the Student’s t-test (p > 0.05), between OF and APV within the same year.
Figure 6. Allicin concentration (A) and total sulfur compounds concentration (B) of garlic were measured average concentration of treatment (n = 4, APV n = 8). APV included Bif_A and Bif_B. ns indicates that there was no significant difference, as determined by the Student’s t-test (p > 0.05), between OF and APV within the same year.
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Figure 7. Proposed cultivation methods for agrivoltaic systems. Early transplanting strategy (A), transplanting larger seedlings (B), and selecting cultivars with a lower light saturation point (C).
Figure 7. Proposed cultivation methods for agrivoltaic systems. Early transplanting strategy (A), transplanting larger seedlings (B), and selecting cultivars with a lower light saturation point (C).
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Table 1. Environmental data during crop cultivation.
Table 1. Environmental data during crop cultivation.
CropDTH (day)GDD (°C) zHumidity
(%)		Solar Radiation
(Wh/m2)		Precipitation (mm)
Kimchi cabbage 2019 891004723186429
Kimchi cabbage 202082917703291188
Garlic 2019–20202531552653673510
Garlic 2020–20212221312633483386
DTH and GDD stand for number of days from transplant to harvest and growing degree day, respectively. Humidity represents average humidity during cultivation period. z: GDD was calculated based on previous research, using 5 °C as in previous studies [33,34].
Table 2. Weight (kg) and APV weight loss (% of control) of kimchi cabbages grown under control (OF) and APV conditions.
Table 2. Weight (kg) and APV weight loss (% of control) of kimchi cabbages grown under control (OF) and APV conditions.
CropWeight (kg)APV Weight Loss (%)
Control (OF)APV
Kimchi cabbage 2019 z 3.5 ± 0.42.8 ± 0.6 *21%
Kimchi cabbage 2020 y5.2 ± 0.64.3 ± 0.7 ns17%
Two years average4.4 ± 1.03.6 ± 1.0 ns18%
Kimchi cabbage weight and APV weight loss (% of control) were represented as mean values of each treatment (OF n = 4, APV: n = 8). APV were included Bif_A and Bif_B. Asterisk indicates significant differences between OF and APV by Student t-test (p < 0.05) in same year. z: each replication represents the average weight (kg) of seven kimchi cabbages. y: each replication represents the average weight (kg) of five kimchi cabbages.
Table 3. Weight (g) and APV weight loss (% of control) of garlic grown under control (OF) and APV conditions.
Table 3. Weight (g) and APV weight loss (% of control) of garlic grown under control (OF) and APV conditions.
CropWeight (g)APV Weight Loss (%)
Control (OF)APV
Garlic 2019–2020 z109.2 ± 10.586.0 ± 10.4 *21%
Garlic 2020–2021 z93.4 ± 11.486.5 ± 20.4ns7%
Two years average101.3 ± 13.286.2 ± 15.6 *15%
Garlic weight and APV weight loss (% of control) were represented as mean values of each treatment (OF n = 4, APV: n = 8). APV were included Bif_A and Bif_B. Asterisk indicates significant differences between OF and APV by Student t-test (p < 0.05) in the same year. z: each replication represents the average weight (g) of 20 garlic bulbs.
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Ko, D.-Y.; Chae, S.-H.; Moon, H.-W.; Kim, H.J.; Seong, J.; Lee, M.-S.; Ku, K.-M. Agrivoltaic Farming Insights: A Case Study on the Cultivation and Quality of Kimchi Cabbage and Garlic. Agronomy 2023, 13, 2625. https://doi.org/10.3390/agronomy13102625

AMA Style

Ko D-Y, Chae S-H, Moon H-W, Kim HJ, Seong J, Lee M-S, Ku K-M. Agrivoltaic Farming Insights: A Case Study on the Cultivation and Quality of Kimchi Cabbage and Garlic. Agronomy. 2023; 13(10):2625. https://doi.org/10.3390/agronomy13102625

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Ko, Da-Yeong, Seung-Hun Chae, Hyeon-Woo Moon, Hye Joung Kim, Joon Seong, Moon-Sub Lee, and Kang-Mo Ku. 2023. "Agrivoltaic Farming Insights: A Case Study on the Cultivation and Quality of Kimchi Cabbage and Garlic" Agronomy 13, no. 10: 2625. https://doi.org/10.3390/agronomy13102625

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