INTRODUCTION

Mongolia relies heavily on imported lettuce, primarily from neighboring countries such as China, with imports amounting to approximately $1.43 million in 2022 according to international trade statistics. Domestic production is limited due to harsh climatic conditions, which make year-round cultivation difficult. Traditionally, the Mongolian main diet has been meat-based however, urbanization and increasing health consciousness have led to a growing demand for vegetables, particularly in urban areas such as Ulaanbaatar. Given the limited domestic production and rising consumption, the development of indoor lettuce cultivation methods has become essential to improve food security and reduce dependence on imports.

The indoor environment of Mongolian households presents additional challenges. A questionnaire-based survey of 100 households in Ulaanbaatar revealed mean indoor living- room air annual average temperatures of 25.4 °C and bedroom air temperatures of 24.0 °C, with corresponding relative humidity levels of 26.3 % and 33.2 % respectively (Batsumber et al., 2023). These levels are substantially lower than the optimal range of 50-70% required for lettuce growth. Low humidity indoor conditions inhibit plant growth, emphasizing the necessity of identifying lettuce cultivars that can thrive under these conditions and to optimize artificial lighting for healthy growth.

Natural light in indoor environments is often insufficient or uneven, particularly during winter or in poorly lit spaces. Artificial lighting, such as T5 LEDs and LED grow lamps, can precisely control light intensity, spectrum, photoperiod, and wavelength distribution to enhance photosynthesis and promote vegetative growth. T5 LEDs are energy-efficient and emit minimal heat, making them suitable for the cultivation of leafy vegetables, while LED grow lamps enable spectral adjustment to optimize plant performance at different growth stages (Kim et al., 2022;Massa et al., 2008).

Therefore, the objectives of this study were to identify lettuce cultivars suitable for Mongolian indoor environments and determine optimal lighting conditions using T5 LEDs and LED grow lamps. Specifically, the study aimed to (1) analyze lettuce growth under Mongolian indoor temperature and humidity conditions, (2) compare the effects of T5 LEDs and LED g row lamps on l ettuce growth and physiology to determine optimal lighting, defined as conditions that maximize growth and quality while minimizing energy use, including adjustments in light intensity, photoperiod, and spectrum, and (3) develop a practical indoor cultivation model for Mongolian households to promote vegetable consumption and sustainable indoor farming.

This study addresses the distinctive indoor conditions of Mongolian homes, characterized by high temperature and low humidity, and evaluates artificial lighting for promoting optimal plant growth. By identifying suitable lettuce cultivars and effective lighting systems, it proposes a home-based indoor cultivation model that can reduce import dependence, promote local vegetable production, and contribute to sustainable urban agriculture in Mongolia.

MATERIALS AND METHODS

1. Introduction of Cultivars Used in the Study

Three lettuce (Lactuca sativa L.) cultivars ‘Jeokchima’ (Ywbio, Republic of Korea), ‘Cheongsangchu’ (Ywbio, Republic of Korea), and ‘Meiguodasusheng’ (Beijingshuoyuanseed, China) were obtained from a greenhouse located in Partizan, Mongolia (Table 1). Two of the cultivars were sourced from a Korean seed company and one from a Chinese seed company.

2. Light Conditions Used in the Study

The T5 LED (daylight color 6500K 8W, Sigma LED Co., Ltd., Seoul, Republic of Korea) and a plant-specific LED (LED Plant Growing Light, AliExpress, Shenzhen, China) were used as lighting sources to compare the effects of widely used LEDs in indoor cultivation were used as lights to compare with LEDs, which are recently widely utilized in indoor cultivation.

3. Experimental Design

The experiment was conducted indoors at the Horticultural Laboratory of the Mongolian University of Life Sciences in Khan-Uul District, Ulaanbaatar. Seeds of three lettuce cultivars were sown on February 8, 2024, in 12-cell seedling trays (five seeds per cell, two trays per cultivar). After 9 days, uniform seedlings were transplanted into 63 × 24.5 × 20 cm pots on February 17, with 10 seedlings per cultivars per pot. A total of 18 pots (nine per light treatment) were arranged to ensure uniform light exposure. Growth observations and harvests were conducted twice, on March 31 and April 22, 2024.

4. Experimental Setup

To minimize light interference, a 2.5 m × 3 m shade was installed on the right side of each test area using agricultural vinyl. T5 LEDs supports were constructed using three 2.5 m stainless steel poles and six tripods, with two T5 LEDs mounted horizontally per support (total of six LEDs). LED lamps were mounted horizontally using screen stands. Light intensity was measured with a light meter (TES-1330A, TES Electrical Electronic Corp, Taiwan) and maintained at 5000 lux for both T5 and LED treatments. The LED lamps consisted of 40 red (620–660 nm), 16 blue (460–470 nm), 8 near-infrared, 8 ultraviolet, and 168 warm white (3500K) LEDs. T5 LEDs emitted light primarily at 570 nm. T5 LEDs were positioned 32.5 cm above the soil surface, and LED lamps at 60 cm. All plants were maintained under a 14-hour photoperiod with 10 hours of darkness using timers (Fig. 1B, 1C, Fig. 2). Pots were filled with 15 L of potting soil (Terravita, Russia), 5 L of humus (Chimt-10, Mongolia), and 10 L of horticultural soil (Number One, Korea) to a depth of 14 cm. Planting distance was 10 cm, with two plants per section and 10 plants per pot (Fig. 1A).

5. Environmental and Growth Management

Indoor temperature was maintained at 24±2 °C during the day and 23±2 °C at night, with relative humidity at 10-11 %. Irrigation was conducted twice per week by applying 2.5 L of water per pot along the pot edges, whenever the soil surface appeared dry.

6. Growth Measurements

For each pot, 10 representative leaves were selected to measure leaf length, petiole length, leaf width, and SPAD values, and leaf number per plant was recorded. Fresh weight was also measured to assess plant biomass　and dry weight was determined after oven-drying the samples at 70°C for 48 h using a precision scale (AJ-220E-D, Japan).

7. Indoor Hydroponic Cultivation Design and Components

A compact three-tier indoor hydroponic system was constructed to simulate vertical farming under limited household space, based on commonly used models in South Korea. The system utilized a mobile drawer unit (manufactured in China, purchased in Mongolia) as the base structure, with a t otal h eight o f approximately 85 c m. L ED grow lights were installed 35 cm above the top drawer and 24 cm above the crops in the first and second tiers to ensure uniform light distribution. To prevent water leakage, perforations in the drawers were sealed with waterproof tape and covered with plastic sheets. Each drawer was f illed with water and aerated using small aquarium pumps. Polystyrene sheets with pre-cut holes were placed on each drawer to serve as transplanting beds for ‘Jeokchima’ lettuce seedlings (Fig. 3). A two-part nutrient solution (Solutions A and B) provided essential macro-and micronutrients for soilless cultivation (Table 2).

8. Statistical Analysis

A three-way analysis of variance (ANOVA) was conducted to evaluate the effects of lighting type (LED plant growth lamps vs. T5 LEDs), lettuce cultivars, and harvest time, including their interactions, on leaf length, petiole length, leaf width, leaf number per plant, fresh weight, and SPAD values.

The ANOVA was used to determine whether significant differences existed among treatment means, and Tukey’s Honest Significant Difference (HSD) test was applied as a post hoc analysis to compare pairwise differences between groups. All statistical analyses were performed using R software, with the significance level set at α = 0.05.

RESULTS AND DISCUSSION

1. Lettuce Growth under Different Light Treatments

Light source had a significant effect on all seven growth parameters (p < 0.05), with LEDs consistently outperforming T5 LEDs across most parameters. Compared with T5 LEDs, LED grow lamps increased leaf length by approximately 20% (17.6 vs. 14.7 cm), leaf width by 40% (9.8 vs. 7.0 cm), chlorophyll proxy (SPAD value) by 24% (10.8 vs. 8.7), fresh biomass by 127% (17.5 vs. 7.7 g), dry biomass by 125% (0.9 vs. 0.4 g), and leaf number by 25% (7.6 vs. 6.1). The only trait favoring T5 LEDs was petiole length, which was about 11% greater under T5 than under LED grow lamps (3.6 vs. 3.2 cm) (Table 3).

2. Growth Comparison of Three Lettuce Cultivars

Significant varietal differences were observed among the three lettuce cultivars for all measured traits, except for leaf width (p<0.05). Among them, ‘Jeokchima’ exhibited the most vigorous growth, showing the highest values in leaf length (19.3 cm), petiole length (3.9 cm), chlorophyll contents (10.3), fresh weight (13.9 g), dry weight (0.7 g), and number of leaves (7.5). In contrast, ‘Meiguodasusheng’ produced the greatest leaf width, whereas ‘Jeokchima’ had the lowest value for this trait, suggesting a trade-off between leaf expansion and other growth characteristics (Table 4). These results suggest that the growth characteristics of lettuce vary depending on the cultivar.

3. Growth of Three Lettuce Cultivars under Different Light Conditions

The growth of three lettuce cultivars exhibited distinct responses to different light sources. Under LED lighting, ‘Jeokchima’ showed the greatest leaf length (21 cm), signif i cantly e xceeding t hose o f the o ther cultivars a nd treatments. Although its leaf length slightly decreased under T5, it remained the longest among all cultivars. The longest petiole (4.2 cm) was also observed in ‘Jeokchima’ under T5, surpassing both ‘Meiguodasusheng’ (3.2 cm) and ‘Cheongsangchu’ (3.2 cm). For leaf width, ‘Meiguodasusheng’ produced the widest leaves (10.1 cm) under LED, while no significant differences were observed between Jeokchima (9.4 cm) and ‘Cheongsangchu’ (9.8 cm). However, leaf width decreased significantly in all cultivars under T5. Chlorophyll content was consistently higher under LED, with ‘Jeokchima’ exhibiting the highest SPAD value (12.2). Similarly, fresh weight (19.1 g) and dry weight (1.0 g) reached their maximum values under LED, both significantly greater than those under T5. The highest number of leaves was observed in ‘Cheongsangchu’ (8.3) under LED, followed closely by ‘Jeokchima’ (8.1) (Table 5). These results are consistent with Shin et al. (2012), who reported that red light promotes leaf elongation, while combined red –blue light enhances chlorophyll accumulation and leaf production. In this study, the LED grow lamps had pronounced spectral peaks in the red (620–660 nm) and blue (460–470 nm) regions, with supplemental white, near-infrared, and ultraviolet light, which likely enhanced photosynthetic efficiency and biomass accumulation. In contrast, the T5 lamps emitted a relatively narrow spectrum around 570 nm, providing limited stimulation for vegetative growth. Therefore, the observed increases in leaf length, SPAD values, and fresh and dry weight under LED treatments can be attributed to these differences in light quality.

4. Lettuce Growth at Different Harvest Stages under Light Treatments

Fresh weight, dry weight, and leaf number significantly increased between the two harvest stages under both light conditions (p<0.05), whereas morphological traits - including leaf length, petiole length, and leaf width - and chlorophyll content remained unchanged. In particular, LED lighting promoted markedly greater biomass accumulation compared with T5 lighting. Fresh weight increased from 13.2 g to 21.8 g under LED, nearly doubling, while only a modest increase was observed under T5 (6.1 g → 9.3 g). Dry weight also showed a clear advantage under LED (0.6 g → 1.1 g) relative to T5 (0.3 g → 0.4 g). The number of leaves nearly doubled under LED (5.3 → 9.9), while T5 showed a smaller increase (5.3 → 7.1) (Table 6) These findings suggest that LED lighting accelerates biomass accumulation and promotes continuous leaf initiation, consistent with previous reports indicating that red and blue LED combinations enhance photosynthetic efficiency and lettuce growth (Wang et al., 2025; Park et al., 2024). As leaf expansion stabilizes at later developmental stages, these results further emphasize that optimizing harvest timing in relation to light quality is crucial for maximizing yield potential in indoor lettuce production.

5. Growth Comparison of Three Lettuce cultivars by Harvest Stage

Growth performance differed significantly among the three lettuce cultivars across harvest stages, except for leaf number (Table 7). ‘Meiguodashusheng’ showed a marked increase in biomass between the first and second harvest, suggesting that its growth rate accelerates under prolonged LED cultivation. However, when key parameters such as leaf area, fresh weight, and overall growth rate were considered, ‘Jeokchima’ emerged as the superior cultivar under LED conditions. ‘Meiguodashusheng’ showed relatively slow initial growth, indicating that it may be less suitable for short-cycle cultivation systems. In contrast, ‘Jeokchima’ consistently demonstrated vigorous performance: it excelled in leaf length, petiole length, leaf width, and chlorophyll content during the first harvest, and maintained high values in leaf length, petiole length, fresh weight, dry weight, and leaf number during the second harvest. Taken together, these results indicate that ‘Jeokchima’ possesses the most stable and vigorous growth potential across successive harvest stages, making it the most suitable cultivar for efficient indoor hydroponic lettuce production under LED lighting conditions (Fig. 4).

6. Evaluation of a Prototype Household Hydroponic System

A prototype household hydroponic system was tested using ‘Jeokchima’, which was identified as the most suitable cultivar under LED-based cultivation conditions. The seedlings showed stable rooting and early-stage growth, indicating that the system can support transplant establishment. Since this study was limited to the early growth stage, long-term cultivation trials covering the entire growth cycle are required to fully assess system performance (Fig. 5).

The household hydroponic system presents several potential benefits. First, the system has the potential to produce an estimated yield of approximately 9 kg per harvest, corresponding to up to 54 kg annually when operated with 60 growing units. Second, year-round indoor cultivation may increase vegetable consumption and contribute to household-level food security in Mongolia. Third, the integration of LED lighting and hydroponic cultivation supports resource-efficient and sustainable vegetable production. Nevertheless, current system remains at a prototype evaluation stage. Therefore, further research is required to validate its full growth-cycle performance, long-term stability, and economic feasibility which are essential for practical implementation and commercialization.

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