agronomy Article Effect of a Newly-Developed Nutrient Solution and Electrical Conductivity on Growth and Bioactive Compounds in Perilla frutescens var. crispa Thi Kim Loan Nguyen 1,2, Moon-Sun Yeom 1,2 and Myung-Min Oh 1,2,* 1 Division of Animal, Horticultural and Food Sciences, Chungbuk National University, Cheongju 28644, Korea; [email protected] (T.K.L.N.); [email protected] (M.-S.Y.) 2 Brain Korea 21 Center for Bio-Health Industry, Chungbuk National University, Cheongju 28644, Korea * Correspondence: [email protected]; Tel.: +82-43-261-2530 Abstract: We evaluated the effect of a newly-developed nutrient solution of red perilla (NSP) with various electrical conductivity (EC) levels on plant growth, mineral content, and bioactive compounds. Four-week-old seedlings were grown in greenhouse nutrient solution as control (CT) (EC 1–3 dS m−1) or NSP (EC 1–6 dS m−1). NSP 1 dS m−1 induced better growth characteristics, whereas higher EC levels inhibited plant growth. Most of the macro-elements contents significantly decreased under NSP 6 dS m−1, whereas the micro-elements contents fluctuated according to EC levels. Total phenolic concentration in NSP was lower than that in CT, and total phenolic content was highest −1 under NSP 1 dS m . Total anthocyanin and antioxidant concentrations and contents increased at lower EC levels. Rosmarinic and caffeic acids concentrations increased at higher EC levels, Citation: Nguyen, T.K.L.; Yeom, whereas there were no significant differences in these compound contents among the EC levels. No M.-S.; Oh, M.-M. Effect of a difference in perillaldehyde concentration was observed, whereas the content was higher at lower Newly-Developed Nutrient Solution EC levels. Overall, these results suggest that NSP 1 dS m−1 is suitable for cultivating red perilla in and Electrical Conductivity on plant factories. Growth and Bioactive Compounds in Perilla frutescens var. crispa. Agronomy 2021, 11, 932. https://doi.org/ Keywords: bioactive compounds; electrical conductivity; mineral content; newly-developed nutrient 10.3390/agronomy11050932 solution; red perilla Academic Editors: William Carl Fonteno III, Jean-Charles Michel and Brian Eugene Jackson 1. Introduction Perilla (Perilla frutescens) belongs to the Labiatae family. It is consumed as a functional Received: 19 April 2021 food and widely cultivated as an annual herb in China, Korea, Japan, Vietnam, and Thai- Accepted: 5 May 2021 land [1]. Perilla leaves have traditionally been used as a crucial medicine for treating cough, Published: 9 May 2021 anxiety, depression, tumors, and some intestinal disorders because it possesses antiallergic, anti-inflammatory, antioxidative, anticancer, and antibacterial bioactivities [2–5]. The high Publisher’s Note: MDPI stays neutral content of bioactive compounds found in perilla, such as rosmarinic acid, caffeic acid, with regard to jurisdictional claims in and anthocyanins, contributes to perilla’s health-promoting effects [6]. Perillaldehyde is published maps and institutional affil- the main component, comprising up to 50%, of the essential oils extracted from perilla iations. plants. It is responsible for the unique smell and taste of perilla, which is important not only for the flavor of food but also for medical treatment, such as depressive effects on the central nervous system and antibacterial activity [7]. Perilla is conventionally grown in fields and greenhouses, where environmental conditions are variable; this results in an Copyright: © 2021 by the authors. unstable harvest and variable bioactive compounds [8]. Therefore, a new approach that Licensee MDPI, Basel, Switzerland. cultivates perilla in a closed controlled environment system has been utilized to improve This article is an open access article the productivity and quality of perilla plants for market and consumer needs. distributed under the terms and Plant factories, advanced types of protected horticultural systems developed in recent conditions of the Creative Commons years, provide larger cultivation space with multilayer shelves. The well-controlled light- Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ ing, temperature, humidity, and CO2 allow year-round crop production while increasing 4.0/). productivity and quality [9]. Hydroponic culture is an essential technique in plant factories Agronomy 2021, 11, 932. https://doi.org/10.3390/agronomy11050932 https://www.mdpi.com/journal/agronomy Agronomy 2021, 11, 932 2 of 16 because of its advantages, including efficient management of nutrient components, lack of soil contamination, rapid plant growth, and reduced crop cycles. Nutrient film technique (NFT) and deep flow technique (DFT) are the most popular hydroponic systems used to grow leafy vegetables. In the NFT, the plant roots are suspended in the flow of nutrient solution, and the nutrient solution is recirculated continuously by pumping water from a nutrient solution reservoir. Meanwhile, the deep flow technique (DFT) is a modified hydroponic culture method, in which the plant’s roots grow into a static and circulating nutrient solution. For overcoming the limited oxygen exchange between the atmosphere and the nutrient solution in static DFT, the nutrient solution is aerated by an air bubbler connecting with the pump in order to provide adequate root oxygenation [10]. Many studies have reported that medicinal herbs and vegetables grown in hydro- ponic systems possess better qualities than conventional soil-based systems [11]. Nutrient factors (macro- and micro-elements, pH, and electrical conductivity [EC]) are crucial for growing plants in hydroponic systems. The optimal macro- and micro-elements in the nutrient solution can determine plant growth characteristics such as leaf number and area, marketable yield, and quality crops such as mineral and chlorophyll contents [12]. Each plant species has a proper uptake rate of the nutrient solution; excessively high or low levels of nutrient solution have a negative effect on plants [13]. Establishing a nutrient solution that provides a favorable ratio of ions for plant growth and development is an important step in cultivating crops in hydroponic systems in plant factories. EC is an important factor in nutrient solutions and reflects the total content of macro- and micro-elements available to plants [14]. Generally, low EC levels lead to nutrient deficiencies and growth inhibition, whereas high EC levels impede nutrient uptake by increasing osmotic pressure and salt stress. Thus, creating waste nutrients that increase nu- trient solution discharge into the environment, resulting in environmental pollution [15,16]. The optimal EC level range should be from 1.5 to 3.5 dS m−1 for most hydroponic crops, but this value varies between crop species and phenological stages [17]. Many studies have been conducted to investigate the effects of EC levels on nutrient solutions of veg- etables, crops, and medicinal plants [18–21]. This study aimed to establish a new nutrient solution for a suitable absorption rate of red perilla and evaluate the effect of EC levels of newly-developed nutrient solution for perilla on the growth, mineral content, and bioactive compounds accumulation in red perilla grown in plant factories. 2. Materials and Methods 2.1. Plant Material and Growth Conditions Red perilla seeds (Perilla frutescens var. crispa, Johnny’s Selected Seeds, Winslow, ME, USA) were sown in sponges and placed in an environmental control room at air temperature of 25 ◦C, relative humidity of 60%, white LEDs, photosynthetic photon flux density (PPFD) of 150 µmol m−2 s−1, and a light period of 16 h. Distilled water was supplied to the seeds every two days. 2.2. Development of Nutrient Solution After 4 weeks of sowing, seedlings were grown in a deep flow technique (DFT) system that was set by the tank (L × W × H, 25.5 cm × 22 cm × 15.5 cm) with the hole cover (distance of hole is about 11 cm). Three types of nutrient solutions (pH 6.0, EC 1.5 dS m−1), such as Hoagland solution ([22], Otsuka solution [23], and greenhouse multipurpose nutrient solution [24]) was supplied into each tank to grow seedlings. Three tanks were used for each nutrient solution, and five seedlings were grown in each tank. All of the seedlings were grown in an environmental control room under the following conditions: 25 ◦C air temperature, 60 ± 10% relative humidity, white LEDs, 230 µmol m−2 s−1 PPFD, and a 16 h light period. To supply the oxygen for seedling roots, a bubble-generating device (air stone; Mimineaqua, Incheon, Korea) was installed inside the DFT tank, which was connected to the pump by lines to aerate the nutrient solution. Agronomy 2021, 11, 932 3 of 17 stone; Mimineaqua, Incheon, Korea) was installed inside the DFT tank, which was con- nected to the pump by lines to aerate the nutrient solution. For 7 weeks after transplanting, the red perilla plants were harvested and divided into the shoot part (including leaves) and the root part at the point in which the root started to grow. The shoots were freeze-dried for 72 h in a freeze dryer (Alpha 1–4 Agronomy 2021, 11, 932 3 of 16 LSCplus; Martin Christ Co., Osterode am Harz, Germany), and then the dried shoots were ground in a milling tube (maximum volume of 40 mL, MMT40.1, IKA-Werke GmbH & Co. KG, Staufen, Germany) to produce a fine powder for analyzing mineral content. A For 7 weeks after transplanting, the red perilla plants were harvested and divided into wet digestion methodthe shootwas used part (including to process leaves) the andsamples the root to part analyze at the point mineral in which contents the root [25] started. to Approximately 1 g ofgrow. each The dry shoots powder were was freeze-dried digested for with 72 h in 20 a freezemL of dryer 70% (Alphanitric 1–4acid LSCplus; (HNO3 Martin) at 125 °C. The remaiChristning procedure Co., Osterode was am performed Harz, Germany), according and then to thethe dried method shoots described were ground in in a Park et al.