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Review

Recent Trends in Urban Agriculture to Improve Bioactive Content of Plant Foods

by
Jin-Hee Ju
1,
Yong-Han Yoon
1,
So-Hui Shin
2,3,
Se-Young Ju
2,3 and
Kyung-Jin Yeum
2,3,*
1
Department of Green Convergence Technology, College of Science and Technology, Konkuk University, Chungju-si 27478, Korea
2
Department of Integrated Biosciences, College of Biomedical Science, Konkuk University, Chungju-si 27478, Korea
3
Research Institute for Biomedical & Health Science, Konkuk University, Chungju-si 27478, Korea
*
Author to whom correspondence should be addressed.
Horticulturae 2022, 8(9), 767; https://doi.org/10.3390/horticulturae8090767
Submission received: 13 July 2022 / Revised: 17 August 2022 / Accepted: 22 August 2022 / Published: 26 August 2022
(This article belongs to the Special Issue Nutrition, Phytochemistry, Bioactivity of Fresh-Consumed Vegetables)
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Abstract

:
Urban agriculture is an important part of the food and nutrition security of many cities and is growing in importance, especially with social disruptions such as climate change and pandemics. Plant foods, in particular, provide bioactive compounds and other essential nutrients. Therefore, it is important to provide timely and useful research resources to horticultural sector and food-related industries that want to produce high-quality plant foods at low cost to meet the market demands of urban agriculture. This study focuses on up-to-date information on urban agriculture, the mechanisms of production of bioactive compounds in plant foods, and the main factors influencing the levels of bioactive compounds in plant foods. As a strategy to improve the bioactive compounds of plant foods in urban agriculture, the recent trends in urban agriculture were investigated according to four categories: ground-based uncontrolled or controlled agriculture and building-integrated uncontrolled or controlled agriculture. In ground-based urban agriculture, the application of short-term abiotic or biotic stresses, such as agronomic management practices, can significantly affect the bioactive compound levels in fruits and vegetables. On the other hand, in building-integrated urban agriculture, horticultural scientists have been interested in artificial lighting, cultivation medium, and water use efficiency to improve the level and composition of functional components of plants. The future trend of urban agriculture is expected to change from ground-based to building integration considering the sustainability of agriculture. Therefore, ongoing research on the growth and bioactive content improvement of plant foods using building-integrated agriculture is an important aspect for urban agriculture.

1. Introduction

The Food and Agricultural Organization (FAO) defines urban agriculture as any production produced by households or parcels of urban areas. Urban agriculture, which can occur at the individual household or community level, creates green spaces and provides fresh vegetables, fruits and flowers to the community [1]. In recent decades, urban agriculture has gained ground in various forms such as allotted gardens, community gardens, harvest farms, commercial farms, as well as open spaces, rooftops, greenhouses, and indoor farms [2].
It is estimated that over 800 million people participate in urban agriculture, producing more than 15% of the world’s food [3]. As climate and social changes progress, the food and nutritional security essential to maintaining health is increasingly dependent on urban agriculture. Plant foods, in particular, are important for providing essential nutrients as well as bioactive phytochemicals [4,5].
Bioactive phytochemicals can be defined as secondary plant metabolites that have biological or pharmacological effects in humans and animals. In particular, phytochemicals with antioxidant activity reduce the harmful effects of reactive oxygen species produced during normal plant metabolism and are stimulated when plants come under environmental stress [6]. In addition, various phytochemicals have been found to have different types of important functions in living plants, such as protection, attraction, and signaling [7]. These bioactive phytochemicals commonly found in fruits and vegetables provide an important evaluation criterion for determining the quality of plant foods. Bioactive phytochemicals such as phenolics [8,9], flavonoids [10,11], and carotenoids [12,13] in plant foods have been reported to be associated with a reduced risk of chronic disease in humans. However, there is still limited information on how urban farming practices relate to the improvement in bioactive substances in plant foods. Therefore, recent trends in urban agriculture and efforts over the past decade to improve the bioactive phytochemicals of fruits and vegetables were investigated using the Web of Science, PubMed, LISTA, and Library of Congress databases in this study.

2. Conceptual Framework

2.1. Recent Classification of Urban Agriculture

Plant growth and yield are closely related to weather conditions and agricultural technological procedures [14]. Urban and suburban agriculture can be found in many forms, such as community gardening in backyards, rooftops, balconies, open spaces and parks, peri-urban agriculture, and livestock grazing in open spaces. Although there are various types of urban agriculture, in this study, the classification of urban agriculture according to O’Sullivan’s location and growth environment control was used [3]. Consequently, four forms were derived according to operational characteristics, capital input, and urban symbiosis potential, as shown in Figure 1.

2.2. Mechanisms of Biological Production of Bioactive Compounds in Plant Foods

Stress induction and stimulation of defense mechanisms are viable strategies to increase the accumulation of secondary metabolites, important plant defense molecules [15]. Because most plant species are sessile organisms, exposure to adverse environmental conditions is unavoidable. When plants are subjected to stressful harsh environmental conditions, the rate of metabolic processes can decrease at the molecular and cellular levels. Although biotic or abiotic stress negatively affect plant growth and development, it can be used as a defense mechanism to maintain homeostasis by activating resistance mechanisms in stressful environments [16]. Activation of enzymatic and non-enzymatic antioxidant defense system, protects plant cells from oxidative damage by quenching reactive oxygen species (ROS). Although ROS is produced under normal conditions, additional ROS can be produced when an organism is exposed to environmental stressors. As a defense mechanism, plants synthesize secondary metabolites, including bioactive compounds, to prevent oxidative damage. These secondary metabolites play important roles as ROS scavenger, UV-absorbing compounds, and anti-infectives [17], as shown in Figure 2.
The antioxidant defense system protects plants from uncontrolled oxidation through enzymatic and non-enzymatic ROS scavenging systems and that protection depends on the strength of ROS production and the plant’s ability to minimize damage [16]. These protection strategies provide a basic understanding of plant tolerance and resistance. However, the same environmental treatment can lead to different types and concentrations of bioactive compounds in different plants [6].
Biotic and abiotic factors, including genetics, climate, water availability, and cultivation, play an important role in maintaining levels of bioactive compounds [18]. These factors induce oxidative stress, which increases the antioxidant defense mechanisms and enhances the production of bioactive compounds, thereby enhancing the antioxidative defense of plants [19]. Lighting conditions have a significant impact on plant growth and development of secondary metabolism. In addition, when growing leafy vegetables, low-temperature treatment can increase the nutritional quality of crops. For example, it has been reported that short-term low temperature activates the biosynthetic pathway of secondary metabolites and induces the buildup of phenolic compounds [20]. Plants have self-resistance mechanisms such as osmotic coordination, photosynthesis, and enhanced antioxidant capacity under drought stress and increased antioxidant capacity to adapt to the environment under these stresses [21]. Therefore, the bioactive compounds of plants may vary according to different cultivation methods [22].

2.3. Major Bioactive Compounds in Plant Foods

More than 25,000 phytochemicals discovered so far, in most cases, are concentrated in colorful parts of the plant, such as fruits, vegetables, nuts, legumes, and whole grains [5]. Phytochemicals vary depending on the type of plant and become abundant during stressful events. They also attract birds and insects that are beneficial in promoting pollination, germination, and seed dispersal [16].
Although there is no evidence that bioactive phytochemicals are deficient, they may exert a wide range of beneficial effects on both the plant itself and human health. Many of the secondary metabolites of plants that make them competitive in their environment are phenolic compounds. In lettuce, for example, about 70% of phenolic compounds have antioxidant properties [23]. Basil and Brassica leafy greens are valuable components of the human diet, with a high diversity of species and varieties and the presence of relatively high levels of bioactive secondary metabolites [24]. Currently, there is considerable interest in the potential health effects of dietary phytochemicals such as carotenoids and phenolic compounds, including phenolic acids and flavonoids. Levels of carotenoids and phenolic compounds vary widely and can be affected by factors such as maturity, genotype, and cultivation [25].
Phenolic compounds widely distributed in plants are generally recognized as major antioxidants because they can provide hydrogen or electrons and form stable radical intermediates [26]. Phenolic compounds can inhibit the oxidative cell damage caused by free radicals in plants and are produced through the phenylpropanoid cycle and can be induced by abiotic stresses [27]. Phenolic compounds, efficient oxygen radical scavengers that protect plants from UV and pathogens, also play an important role in preventing oxidative damage in humans and have anti-carcinogenic and anti-atherosclerotic activity [28]. Flavonoids such as flavonols, flavones, catechins, flavanones, anthocyanidins, and isoflavonoids play significant roles in the human body due to their strong antioxidant activity, as well as biological function. They are also widely used in food industry with various functional roles, many of which are based on antioxidant activity. In addition, they function as important co-enzymes for internal hydroxylation reactions [29]. Therefore, this review focuses on carotenoids and phenolics among the major bioactive compounds contained in plant foods in the field of urban agricultural research (Figure 3).

3. Strategies to Improve Bioactive Compounds in Plant Foods by Urban Agriculture

3.1. Ground-Based, Uncontrolled Urban Agriculture

For outdoor ground-based urban agriculture, the focus is on providing ecosystem services and minimizing environmental damage rather than maximizing productivity. Increasing dietary diversity and consumption of a variety of plant foods provide health benefits as well as awareness of the seasonality of foods [30]. As well as commercial farms, home and community gardens and square foot gardens that provide food to many homes and communities and serve charities such as food banks or community kitchens are ground-based, uncontrolled urban agriculture. Abiotic stress can be a major limiting factor in plant cultivation in this type of agriculture. Drought, salinity, cold, and icing stress are some of the most important limiting factors for crop production, and drought has the greatest impact on agriculture in ground-based, uncontrolled urban agriculture [31].
Research studies conducted to improve bioactive compounds in plant foods in ground-based uncontrolled (GBU) urban agriculture are summarized in Table 1.
Croge et al. [18] reported that bioactive compound levels in blackberry fruits harvested in temperate climates had higher concentrations of phenolic compounds, flavonoids, and anthocyanins. This is probably because the concentrations of these metabolites depend on stress level and the period of physiological development. It has been reported that the anthocyanin content of organic raspberries depend on the stage of maturity [20]. In addition, organic fertilization increased the content of ellagic acid and gallic acid at all stages of ripening. Furthermore, higher content of caffeic acid, hydroxybenzoic acid, protocatechuic acid, and vitamin C was observed in overripe organic raspberry [19]. It is interesting to note that total polyphenols and ascorbic acid were higher in the ripe camu-camu (Myrciaria dubia) fruit grown under drought stress conditions [32]. Phenolic compound concentrations in apple (Malus × domestica Borkh.) increased with decreasing crop load, but ascorbic acid levels were not significantly dependent on the crop load [33]. Organic growing systems have been reported to affect tomato quality parameters. Organic tomatoes have been reported to contain significantly higher amounts of total vitamin C and total flavonoids, 3-quercetin rutinoside, and myricetin compared to conventional fruits [25]. There were major differences among the cultivars in the accumulation of the bioactive compounds in fruits during growth and ripening stage. The total phenolic content, total flavonoid content, and ascorbic acid content were found to increase during the growth and ripening of red peppers, and it was found that the highest values were shown in the last stage of ripening [34]. The content of bioactive compounds in a fruit can be depend on the cultivar, harvesting time, and location of the fruit in the crown [41].
Kwon et al. investigated the effect of stem removal and bolting of various lengths on the total quercetin and phenolic contents in onion bulbs [35]. It was found that the total quercetin content varied in response to flower stem removal treatment, and no clear differences were observed compared to the unbolted bulbs. Planting density is an important factor that greatly affects the growth and yield of vegetables and the level of bioactive compounds. Tarragon (Artemisia dracunculus L.) plants growing at high density (40 × 40 cm) have been reported to have higher carotenoid concentrations than plants growing less densely (50 × 50 cm) [14]. The aerial part of blessed thistle (Cnicus benedictus L.) contains high amounts of phytochemicals, and further improvements can be achieved by various agronomic management practices such as nitrogen fertilizer rate and plant density. At high plant densities, a decrease in the nitrogen ratio has been reported to increase the total phenolic and flavonoid contents. These responses can be attributed to the compensatory effects of increased secondary metabolites and antioxidant activity at low nitrogen content and high planting densities [36].
On the other hand, intercropping is considered an interesting option for increasing crop diversity. It is defined as the simultaneous growth of two or more species in the same field for a significant amount of time, but without the need for simultaneous sowing or harvesting due to multiple abiotic and biotic factors affecting yield practices [31]. Fenugreek and buckwheat intercrops were reported to have a higher total phenolic content of fenugreek compared to fenugreek grown alone [37]. In experiments on the fortifying effect of biostimulants in garlic cultivation, the leaves were identified as the most valuable edible organs as the rich source of vitamin C and polyphenols among other organs tested [38]. Interestingly, amaranthus (leafy vegetable) was reported to have significantly higher levels of total carotenoids, vitamin C, beta-carotene, sixteen phenolic acids, and flavonoids depending on the severity of drought stress. Therefore, it can be postulated that semi-arid and dry areas of the world could be suitable areas for the growth of amaranth as a substitute crop [39]. Additional studies related to the effect of abiotic stress on the content of biologically active compounds have been reported. The total phenolic content of broccoli cultivars associated with harvesting and irrigation showed the highest values in spring and under drought stress [40]. As previous studies have shown, short-term abiotic or biotic stress can promote and significantly affect secondary metabolite levels in plants.

3.2. Ground-Based, Controlled Urban Agriculture

This type of urban agriculture includes greenhouses or horticultural farms incorporating poly-tunnels or nets where fertilization can be carried out in a growing environment. Urban greenhouse horticulture is a fairly well-established industry and has been supported by research for many years. Greenhouse production of crops has been extended to 10–12 months per year due to consumer demand for a continuous supply of fresh vegetables. In addition, aquaponics, which raise fish in aquaculture systems and use nutrient-rich wastewaters to fertilize horticulture crops, are often introduced as potentially beneficial urban agriculture. However, there are challenges that need to be addressed to ensure the economic and environmental sustainability of these systems [3]. Table 2 summarized the research studies conducted to improve bioactive compounds in plant foods in ground-based controlled urban agriculture.
Titanium dioxide (TiO2) treatments were reported to increase the yield and hardness of strawberries, whereas a decrease in phenolic compound levels was observed as a result of titanium dioxide (TiO2) treatment in low light conditions [42]. Schmitz-Eiberger [43] reported mixed results with respect to heat stress and the content of bioactive compounds. Sweet cherries (Prunus avium) from five early maturation varieties had improved anthocyanin and phenol levels when grown under shading. Although more detailed studies are needed, semi-greenhouse production of high-quality berries can be assumed in the context of sustainable agriculture. [44]. In addition, studies on the beneficial effects of abiotic stress have been reported. Levels of resveratrol and anthocyanins were increased in grape berries under poor irrigation throughout the growing season both before and after fruit set treatment [45]. These results indicate that insufficient irrigation can promote the production of functional and good quality table grapes in greenhouse. The ripening stage also significantly affected the content of bioactive compounds in tomato fruit. The total phenol content increased as the maturation stage progressed from green to pink, and then decreased. Lycopene accumulation increased with the ripening stage, and a sharp increase was observed in the final ripening stage [46].
Applying short-term low-temperature settings during cultivation in controlled greenhouses can be a potential strategy to increase levels of phenolic antioxidant compounds in leafy plants [20]. In addition, salt stress can also affect the accumulation of bioactive compounds in plant foods. It has been reported that exogenous glycine betaine treatment further increases the total antioxidant and total phenolic content of lettuce under salt stress compared to non-glycine betaine-treated plants [47]. Additionally, Saleh et al. [52] reported the effect of growing substrate on the bioactive compound content in plants. They found that the highest content of phenolics and flavonoids was found in dill and parsley plants grown in soil in a net house in Germany. Arbuscular mycorrhizal fungi (AMF), which favored the accumulation of secondary metabolites in lettuce leaves, can interact with fertilizer content such as selenium [48]. Selenium application has been reported to offset the benefits of AMF on the carotenoids and phenolic contents of green-leaf lettuce. On the other hand, sodium selenite interacted positively with AMF in increasing flavanol levels in red-leaf lettuce [48]. The effectiveness of mycorrhizal technology to enhance levels of bioactive compounds in lettuce can be modified with selenium fertilization, and the interaction may vary depending on the lettuce variety [48]. Inorganic calcium sulfate has also been reported to affect the content of bioactive compounds in plant. The total phenol content of CaSO4-treated broccoli sprouts was lower than that of untreated sprouts during the growth phase but higher than that of untreated sprouts during storage [49]. The exogenous application of 5-aminolevulinic acid (5-ALA) has been reported to have growth-regulating effects and may improve the nutritional quality of purslane (Portulaca oleracea). Total phenolic and ascorbic acid contents of purslane shoot were increased in 5-ALA-treated plants [50]. Interestingly. the highest phenolic acid concentrations in chicory (Cichorium intybus) and lettuce (Lactuca sativa), and the highest caffeic acid content in Swiss chard (Beta vulgaris) were observed in high-density breeding of fish in aquaponics treatments [51].

3.3. Building-Integrated, Uncontrolled Urban Agriculture

Most building-integrated, unconditioned urban agriculture takes the form of roof top gardens. In addition, balcony gardens and green walls also fall into this class. Rooftop agriculture is a form of building-based urban farming that includes both protected and unprotected farming practices, such as roof gardens and farms [53]. Rooftop agriculture practices are typically representative of outdoor farms and gardens, as growth in the field of rooftop greenhouses is still relatively small. Similarly, commercial cases are rare, and the majority of rooftop agriculture cases are aimed at social educational goals or improvements in city life [54]. Although intensive green roof systems (medium depth > 15 cm) are considered the best conditions for vegetable farming, the greatest potential for reliable and continuous productivity is likely through extensive systems (medium depth < 15 cm) because of weight load limitations for most buildings. Therefore, shallow-rooted vegetables, including salad vegetable crops, are considered best suited for extensive systems since they can be produced in large quantities with minimal input [55]. Research studies for improving the levels of bioactive compounds in plant foods in building-integrated uncontrolled urban agriculture are listed in Table 3.
Nour et al. [58] investigated the levels of bioactive compounds in two different types of hydroponic tomato fruits at different stages of ripening. Total phenolic, ascorbic acid, beta-carotene, and lycopene contents increased as ripening progressed in all the cultivars, while the change in ascorbic acid content was cultivar-dependent. The highest total phenolic and total flavonoid contents in the leaves of different chicory cultivars were observed in the potassium (K)-rich nutrient solution, followed by nitrogen (N) and phosphorus (P)-rich solution [56]. Biochar application may increase flavonoid content in vegetables. It has been reported that biochar could potentially be used to improve the nutritional value of pak choi and could be applied in large quantities to obtain lightweight soil mixtures for urban agriculture [57].

3.4. Building-Integrated, Controlled Urban Agriculture

Global warming and extreme weather are having a huge negative impact on the yield and quality of crops grown in the open field [59]. Indoor farms, also known as plant factories or vertical farms, have been rapidly expanding in urban and suburban areas in recent years because they can produce plants year-round and provide significant opportunities to help address global challenges in agriculture. High-value, fast-growing, and short crops, such as leafy vegetables and herbs, are common crops found on indoor farms [60]. Building-integrated, controlled urban agriculture through hydroponic systems can be a viable opportunity to avoid impacts on the environment and public health. Vegetables such as tomatoes, cucumbers, lettuce, as well as cut flowers, have a short growth cycle, so they are often used in hydroponics to better control and standardize the cultivation process. Hydroponics can support the growth of almost any plant species, and different types of pot beds, ranging from containers, channels, and pipes, can be used for this technique [61]. Among various environmental factors that can be controlled in closed plant production system, such as light, temperature, humidity, and CO2, artificial light irradiation is vital because it is a system that does not utilize natural light [62].
Selecting the optimal light source is essential in these systems, which are completely dependent on artificial light sources, as the wavelength of the irradiated light affects the yield and quality of crop. With the advancement of light-emitting diodes (LED) technology and its customizable nature, the combination of red and blue LED, which is often used in closed plant production systems, shows a relatively high production efficiency compared to conventional artificial light sources such as fluorescent lamps, even at the same illuminance [59]. In addition, the ratio of red, green, and blue LEDs is an important factor in the biosynthesis of secondary metabolites in plants, particularly phenolics, flavonoids, and anthocyanins [24]. Although both red and blue wavelengths are involved in the synthesis of secondary metabolites, in most cases, blue wavelengths enrich phytochemical concentrations compared to red wavelengths. Research studies for improving the level of bioactive compounds in plant foods in building-integrated, controlled urban agriculture are summarized in Table 4.
Individual phenolic compounds, including chlorogenic, caffeic, and ferulic acids and kaempferol, in red-leaf lettuce increased with increasing ratios of blue light [63]. In addition, it was reported that the application of far-infrared LED lighting supplemented with a combination of red and blue LEDs on lettuce significantly increased total phenolic levels, antioxidant activity, chlorogenic acid content, and caffeic acid content compared to lettuce treated with red and blue LEDs only. These studies indicate that supplementation of the visible spectrum enhances the bioactive compound content of plants [56,59]. Recent studies on ultraviolet (UV) light have shown that it not only has a sterilization effect but that it also increases the levels of bioactive substances in several crops. It should be noted that high-energy ultraviolet light, an environmental stress, can be used to stimulate the biosynthesis of bioactive compounds in plants. Short-term UV-A irradiation has been reported to increase phenolic compound levels in lettuce leaves [64]. In addition, the significant beneficial effects of UV-A and UV-B on the growth and phenolic content of dropwort (Oenanthe stolonifera) plants also represent a useful strategy of artificial lighting to improve plant quality [65]. Carotenoid and polyphenol contents were greater in red leaf lettuce seedlings treated with blue-containing LED lights than in those treated with fluorescent lamp [66]. Qian et al. [67] reported the highest levels of total phenolics and anthocyanins in kale sprouts and the strongest antioxidant capacity at blue wavelengths. In addition, Pennisi et al. [68] reported that blue wavelength significantly increased total flavonoid concentrations in basil compared to plants grown in a fluorescent lamp. The combined effect of luminous intensity and photoperiod using an external electrode fluorescent lamp showed that the total phenol content continuously increased as the photoperiod increased at 150 μmol m−2 s−1 under the 20 h photoperiod [69]. Different cultivation systems affect and modulate the concentration of bioactive compounds in plants differently according to their class. While parsley had the highest concentrations of cartotenoids in all cultivation systems, the highest content of ascorbic acid was found in parsley in the field, with values 2.6 and 5.4 times higher than those grown indoors and in greenhouses [22].
Both low and high temperatures are forms of abiotic stressors that affect plant growth and metabolic activity. It has been reported that short-term low temperature activates the biosynthetic pathway of secondary metabolites and induces accumulation of phenolic compounds in kale [20]. Additionally, short-term heat-shock treatment has been reported to increase the number of antioxidants and anti-carcinogenic compounds in kale sprouts [70]. Lettuce has also been shown to accumulate antioxidants such as phenolics and flavonoids under water stress [71]. Park at al [72] examined the changes in the concentration of phenolic compounds in three edible sprout species, alfalfa (Medicago sativa L.), broccoli (Brassica oleracea L.), and radish (Raphanus sativus L.), treated with iron-chelates during imbibition in the growth chamber. No significant effect of Fe application on phenolic acids and carotenoid was observed in green lettuce, whereas chlorogenic acid and total phenolic content in red lettuce increased with increasing Fe treatment [73]. Treatment with NaCl significantly maximized beta-carotene, phenolic acid, and flavonoid levels in wheat microgreen extracts [74]. Additionally, salt-stress from NaCl treatment resulted in the highest anthocyanin levels. Therefore, reaching an adequate level of salinity stress could be a useful strategy for the industrial manufacture of new products from wheat microgreen extract.

4. Conclusions and Perspectives

In recent years, due to global population growth and climate change, urbanization, resource competition, and demand for high-quality fresh food, urban agriculture to improve bioactive compound levels in plant foods has been attracting attention. Traditional outdoor ground-based urban agriculture focuses on cultivation practices such as the application of short-term abiotic or biotic environmental stresses to fruit and vegetable production for strategies to increase the levels of health-promoting bioactive compounds in plants. However, the future trend of urban agriculture to improve the bioactive contents in plant foods is expected to shift from ground-based to building-integrated due to the agricultural sustainability in urban areas. These efforts can be found in extensive studies to increase secondary metabolite levels with light spectrum in indoor agriculture. In order to ensure the sustainability of high-quality plant foods in our society, which is undergoing rapidly changing climate and social upheaval, further research is required on various cultivation and application methods to enhance the growth and bioactive content of plant foods. In particular, the importance of urban integrated agriculture research is being emphasized more than ever.

Author Contributions

Conceptualization, J.-H.J. and K.-J.Y.; investigation, J.-H.J., Y.-H.Y., S.-H.S. and S.-Y.J.; writing—original draft preparation, J.-H.J. and Y.-H.Y.; writing—review and editing, K.-J.Y. and S.-Y.J.; visualization, K.-J.Y. and S.-H.S.; supervision, K.-J.Y. and Y.-H.Y.; project administration, S.-H.S.; funding, J.-H.J. and Y.-H.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2021R1F1A1063456).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Horticulturae 08 00767 g001 550
Figure 1. Classification of urban agriculture according to O’Sullivan et al. [3].
Figure 1. Classification of urban agriculture according to O’Sullivan et al. [3].
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Figure 2. Schematic diagram of the mechanism of biological production of bioactive compounds in plant foods.
Figure 2. Schematic diagram of the mechanism of biological production of bioactive compounds in plant foods.
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Figure 3. Major bioactive compounds present in plant foods.
Figure 3. Major bioactive compounds present in plant foods.
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Table
Table 1. Research studies to improve bioactive compound levels in plant foods in ground-based uncontrolled urban agriculture.
Table 1. Research studies to improve bioactive compound levels in plant foods in ground-based uncontrolled urban agriculture.
CropPlant
Species		Cultivation FactorMajor Bioactive CompoundsOutcome EffectReference
FruitRaspberry cultivarOrganic fertilizationPhenolics, anthocyanins, ascorbic acid+ z[19]
Maturity stages+
Camu-camuDrought stressPolyphenols, ascorbic acid+[32]
Blackberry cultivarTemperaturePolyphenols, flavonoids, anthocyanins, ascorbic acid+[18]
Humidity−
Sweet cherryIrrigationPhenolics, tartaric esters, flavanols, anthocyaninsnull[6]
Phosphorusnull
AppleCrop loadsPhenolics, ascorbic acid−[33]
TomatoOrganic cultivationPhenolics, ascorbic acid+[25]
Hot pepperStage of growth and ripeningPhenolics, ascorbic acidRipening +[34]
VegetableOnionBolting and flower stem removalPhenolics, quercetinnull[35]
TarragonPlant densityCarotenoids+[14]
Blessed thistleNitrogen fertilizer ratePhenolics−[36]
Plant density+
BuckwheatIntercropping ratioPhenolics+[37]
GarlicPlant growth biostimulantsPolyphenols,
ascorbic acid		+[38]
AmaranthusDrought stressVitamins phenolics, flavonoids+[39]
BroccoliHarvest seasonPhenolicsSpring[40]
Drought stress+
z Parameters that were increased, decreased, or no effect in the study are denoted by +, −, and null, respectively.
Table
Table 2. Research studies to improve bioactive compound levels in plant foods in ground-based controlled urban agriculture.
Table 2. Research studies to improve bioactive compound levels in plant foods in ground-based controlled urban agriculture.
CropPlant
Species		Cultivation FactorMajor Bioactive CompoundsOutcome EffectReference
FruitStrawberryTitanium dioxide
(TiO2) foliar fertilization		Phenolics− z[42]
Sweet cherryPlastic coverPhenolics−[43]
Anthocyanin+
Red raspberry,
strawberry, blackberry		Photovoltaic coverTotal anthocyanins, phenolic content+[44]
Shading+
GrapeDeficit irrigationResveratrol, anthocyanins+[45]
TomatoStage of growth and ripeningAscorbic acid, lycopene, beta-carotene, total phenolic contentsripening +[46]
VegetableLeafy vegetationShort-term low temperaturePhenolicsnull[20]
LettuceExogenous glycine betaine (GB) under salt stressPhenolics25 mM GB +[47]
Dill
Parsley		Substrate typePhenolics, flavonoidsGermany soil[37]
Green-leaf lettuceSelenium fertilization and arbuscular mycorrhizal fungi (AMF)Carotenoids+[48]
Phenolics−
Red-leaf lettuceFlavanols+
BroccoliCalcium sulfate (CaSO4) fertilizationPhenolics−[49]
PurslaneFoliar fertilizationPhenolics, ascorbic acid+[50]
Chicory
Lettuce
Swiss chard		Stocking density of fishPhenolic acid, caffeic acid+[51]
z Parameters that were increased, decreased, or not determined in the study are denoted by +, −, and null, respectively.
Table
Table 3. Research studies to improve bioactive compound levels in plant foods in building-integrated, uncontrolled urban agriculture.
Table 3. Research studies to improve bioactive compound levels in plant foods in building-integrated, uncontrolled urban agriculture.
CropPlant
Species		Cultivation FactorMajor Bioactive CompoundsOutcome EffectReference
FruitTomatoStage of ripeningPhenolics
ascorbic acid,
lycopene, beta-carotene,
total flavonoid content		Ripening + z
VegetableChicory
cultivars		Nutrient (nitrogen, potassium, or phosphorus) solutionsPhenolics, total flavonoidPotassium +[56]
CabbageBiocharFlavonoids,
glucosinolates		+ z[57]
z Parameters that were increased, decreased, or not determined in the study are denoted by +, −, and null, respectively.
Table
Table 4. Research studies to improve bioactive compound levels in plant foods in building-integrated, controlled urban agriculture.
Table 4. Research studies to improve bioactive compound levels in plant foods in building-integrated, controlled urban agriculture.
CropPlant
Species		Cultivation FactorMajor Bioactive CompoundsOutcome EffectReference
VegetableLettuceMonochromatic or combined LED lightPhenolicsBlue LED + z[63]
LettuceCombined light
(ratio of blue + red/far-red LED light)		Phenolics, chlorogenic acid, caffeic acidO.7 and 1.2 LEDs ratio +[59]
LettuceUV-A irradiationPhenolicsnull[64]
DropwortUV lamp or LED irradiationPhenolics+[65]
Red lettuceBlue LEDCartenoid, polyphenol+[22,66]
Chinese kale sproutLight qualityPhenolics anthocyaninsBlue LED+[67]
BasilRed:blue LED ratioFlavonoidR1B3[22,68]
LettuceLight intensityPhenolics150 μ mol m−2 s−1 +[69]
Photoperiod20 h +
ParsleyManagement practices (indoor, greenhouse, field cultivation)Carotenoids,
flavonoids		null[22]
Ascorbic acidField +
BasilAnthocyaninsnull
KaleShort-term low temperaturePhenolics+[20,22]
KaleShort-term
heat shock		Anti-carcinogenic compounds+[70]
LettuceShort-term
water stress		Phenolics,
flavonoids		+[70,71]
Alfalfa, broccooli, radishIron-chelatesPhenolics+[72]
Green lettuceIron (Fe) biofortificationPhenolic acids, carotenoidnull[73]
Red lettuce+
Wheat microgreenSalinity stressBeta-carotene, phenolic acid, flavonoid, vitamin, anthocyanin12.5 and 25 mM +[74]
z Parameters that were increased, decreased, or not determined in the study are denoted by +, −, and null, respectively.
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Ju, J.-H.; Yoon, Y.-H.; Shin, S.-H.; Ju, S.-Y.; Yeum, K.-J. Recent Trends in Urban Agriculture to Improve Bioactive Content of Plant Foods. Horticulturae 2022, 8, 767. https://doi.org/10.3390/horticulturae8090767

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Ju J-H, Yoon Y-H, Shin S-H, Ju S-Y, Yeum K-J. Recent Trends in Urban Agriculture to Improve Bioactive Content of Plant Foods. Horticulturae. 2022; 8(9):767. https://doi.org/10.3390/horticulturae8090767

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Ju, Jin-Hee, Yong-Han Yoon, So-Hui Shin, Se-Young Ju, and Kyung-Jin Yeum. 2022. "Recent Trends in Urban Agriculture to Improve Bioactive Content of Plant Foods" Horticulturae 8, no. 9: 767. https://doi.org/10.3390/horticulturae8090767

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