Optimizing lettuce yields and quality by incorporating movable downward lighting with a supplemental adjustable sideward lighting system in a plant factory

Design of plant light source regulation experiment

Overall design of mechanical structures

The primary design and function of the plant light control device are as follows: (1) Adjust the distance of the plant light source and the plant height in a better illumination region. (2) To realize the detection of plant cultivation location. (3) Complete plant growth height detection. (4) Turn on the supplemental sideward plant light source and adjust the angle for a better illumination region. (5) Adjust the lighting area of the light source so that the plant cultivation position is in the optimal light region. (6) Light quality and intensity can be adjusted to meet the plant growth needs at all stages. (7) Human-computer interaction.

The above design requirements, (1), (2), and (5) need to be realized by mechanical structure and control system, (3), (4), (6), and (7) can be done by the control system. Taking the light control device design of the ONE-layer plant as an example, its overall structure is shown in Fig. 1.

System function analysis

In this study, the system’s main functions were divided into four functional modules: a collection of information, decision analysis, instruction execution, and human-computer interaction, as shown in Fig. 2. According to the plant light control device’s design requirements, the control system’s functions were designed as follows: The information collection function module includes information about plants’ growing positions, the height of plant growth, and the working state. The decision analysis function module assesses the various light areas, the calculation of additional light, and the repositioning of the light source. The human-machine interaction functions module, which displays the system’s operating parameters and provides an early warning of the system’s operational status is defined by the system’s operating parameters. This function module executes instructions, including adjusting the installation’s lighting area, lighting ratio, and light source height.

Sideward LEDs control method

The system is controlled by a human-computer interface, which uses infrared sensors to gather distance data from the plant factory and a lighting sensor to measure ambient lighting. Next, it determines the distance between the plate, the LEDs, and the best pulse-width modulation (PWM) for the environmental photosynthetic photon flux density (PPFD) using the target PPFD. In this instance, the PWM signal managed the PPFD. As a result, after passing through the power adjustment module, which converts the PWM signal to DC power, the system generates the optimal level of the PWM signal while taking into account the distance between the center LED and the surrounding PPFD, as shown in Fig. 3.

System overall scheme design

The detection module consists of two parts: the installation position detection module and the installation height detection module. The execution module consists of the sensor position adjustment module, the light source height module, and the light module, as shown in Fig. 4.

The power module adopts a multi-voltage output switching power supply to provide power protection for the remaining modules of the control system. The control module adopts the Arduino Mega 2560 (Shenzhen Ke Zhi You Technology co., Ltd, Shenzhen, China) as the core controller, and the control system achieves the expected function. The plant position detection module uses a micro switch to switch, and the matrix switch is responsible for collecting plant cultivation position information and transmitting it to the control module for processing. The plant height detection module consists of ultrasonic sensor modules, and the adjustment module consists of a driver, a stepping motor, and a screw mechanism. The light source height adjustment module adjusts the distance between the LED panel light source and the plant, and the interactive module is composed of LCD1602 display and setting buttons. The early warning module includes an indicator light and a buzzer to indicate the working status of the system and alarm when the system works abnormally.

Growth conditions

This experiment was conducted in the Key Laboratory of Crop Biotechnology, Fujian Agriculture and Forestry University. The hydroponic seedling cultivation method cultivated roughly 200 romaine lettuce seeds in a plastic cultivation box (110 mm × 235 mm × 40 mm). The seeds were placed on an open sponge block after adding clean water to ensure that the sponge submerged completely. After the lettuce seeds had sprouted, they were grown in an environment-controlled growth chamber at ~17–20 °C (photoperiod/dark period) for 12 h with a PPFD of 50 μmol·m⁻²·s⁻¹ from cool white fluorescent lamps, and the water in the seedling box was changed to fresh nutrient solution. After every 12 h, a new nutrient solution was circulated for 15 min at a pH of 6.1 ± 0.1 and an EC of 1.6 ± 0.1 mS/cm. Following 4 weeks of water cultivation, 36 seedlings that consisted of leaf and of equal growth size were selected for the experiment. Among them 18 were chosen and grown under the C-S (Fig. 5), and the remaining lettuce seedlings were grown under the N-S. The cultivation board was constructed from a polyethylene foam board and measured 110 cm × 50 cm × 10 cm. The deep-flow hydroponics cultivation method was used, and the plant factory controlled several environmental factors during the cultivation period (Causin, Jauregui & Barneix, 2006). The air temperature at night was kept at 20.2 °C and 23 °C during the day. The relative humidity was kept at 60 ± 5%, and the carbon dioxide concentration was maintained at 400 ± 10 ppm. The factories were filled with pure CO₂ injected into the interior to ensure environmental controllability, and the carbon dioxide concentration was the same as the outside atmosphere. The transplanted seedlings were left in an improved fresh nutrient solution, and the pH value (pH = 6.5 ± 0.1 mS/cm) and EC value (EC = 1.7 ± 0.2 mS/cm) of the nutrient solution were regulated accordingly.

Biochemical contents

Statistical analysis

Data were analyzed using “Statistix 8.1” (Gomez & Gomez, 1984; Sánchez-Rodríguez et al., 2010). We performed t-test to find significant differences among treatments. The significant differences between treatments were compared by the least significant difference LSD (p ≤ 0.05) (Halberstadt et al., 2008).

Plant growth

After cultivating the plants under the N-S and the C-S in the plant factory, we observed distinct variations among the various parameters assessed. For instance, the C-S treatment had a significant impact on the morphology of lettuce plants, including the length of the shoots (Fig. 6A), the diameter of the stems (Fig. 6B), and the number of leaves (Fig. 6D). When the supplement adjustable sideward lighting LEDs were switched on and used in conjunction with the movable downward lighting LED system, root fresh weight between N-S and C-S revealed no significant difference (Fig. 6G). However, the leaf area, shoot fresh weight, shoot dry weight, root dry weight content under C-S were significantly higher than N-S at T2, as shown in Figs. 6C, 6E, 6F, 6H–6J, respectively.

The plants treated under the C-S and the N-S had very different morphologies (Fig. 7). For example, plants subjected to the C-S appeared dense, with narrow, twisted leaves, and no excessive elongation was observed. On the other hand, plants under the N-S were short with hypertrophic and thick leaves and a yellowing color, despite it been subjected to light condition for 40 days, and exhibited a sparsest plant architecture with a noticeable elongation of the stem. This implies that the optimal wavelength for plants should be adjusted following plant growth and that the C-S exhibited superior performance.

Photosynthetic pigments content and biochemical contents

The supplementary sideward light source significantly impacted the levels of chlorophyll a and b, soluble sugar, and soluble proteins. The lettuce grown under C-S at T2 had the higher chlorophyll a and b concentrations (Figs. 8A and 8B). It was observed that chlorophyll a concentration under N-S at T1 was the lowest compared with C-S at T2. At T2, lettuce’s carotenoid concentration dropped under C-S largely due to the additional sidelight compared to plants cultivated under N-S. This indicates that time directly affects the transformation of carotenoids into pigment in plants, as shown in Fig. 8C. The supplemental lights helped to increase the amount of soluble sugar in the lettuce leaves, with C-S at T2 yielding the higher level compared with N-S (Fig. 8F). The N-S-treated lettuce had the lowest soluble protein levels at T1, but these levels somewhat increased at T2 relative to those under C-S. The highest soluble protein levels were found at T2 under C-S compared with N-S (Fig. 8E). However, nitrate content exhibited no discernible variations between the two treatments (Fig. 8D).

Ascorbic acid content and total nitrogen utilization efficiency

The ascorbic acid concentrations in plants under the C-S treatment varied significantly compared with those under the N-S treatment. The outer leaf revealed a generally increase in ascorbic acid level than the inner and total leaf (Fig. 9A). Total nitrogen utilization efficiency in plants under the C-S was higher than that under N-S at both T1 and T2 (Fig. 9B).

Photosynthetic productivity

It was noticed that the C-S treatment influenced the photosynthetic rate, stomatal conductance, evaporation rate, and CO₂ concentration in the newest fully developed leaves with the additional adjustable sideward lighting at a PPFD of 50 µmol m⁻² s⁻¹ compared with the N-S treatment. The lowest rate of evaporation was observed in the control group, while stomatal conductance showed similar patterns. Under the C-S, which was about twice that of the control, the stomatal conductance peaked significantly. The leaves showed positive net photosynthetic rates (Table 2). These findings showed that leaves’ ability to increase photosynthesis could be regulated by additional, adjustable side lighting.

Light and energy use efficiency

The segmented energy consumption of the C-S and N-C is shown in Table 3. C-S exhibited an upward trend on day 20, while N-S performed in terms of energy consumption until the harvesting period. The power consumption per unit area, the output per unit area, and the power utilization efficiency (LUEs) of the C-S and the N-S were measured and calculated separately (Table 4). The comparison showed that the power consumption of the C-S was (35.02%) lower than that of the N-S. We pointed out that the power utilization rate of LUEs was a practical value for comparing the power utilization rate between the systems. Therefore, it was observed that at relatively low output and the power utilization rate of the C-S were (135%) higher than that of the N-S. The analysis also indicated that C-S can allow the lettuce to receive the ideal light under the conditions of large-scale plantations in plant factories, obtain relatively more biomass, and consume the least energy value.

Response of the morphology and photosynthetic pigment under different irradiances

Plant’s morphological traits can be influenced by a number of factors, and they have developed mechanisms to adapt to various environments (Kumar et al., 2018; Li et al., 2021). Research has demonstrated that lighting quality, intensity, source, and photoperiod affect plant morphology. However, only a few studies have investigated the orientation of the light source and how it influences the development and growth of plants. Joshi et al. (2017) demonstrated that lettuce grows more efficiently under a combination of downward and upward lighting settings. Moreover, high light levels boost photosynthesis and subsequently increase plant biomass, whereas low light intensities typically result in photoinhibition and impact plant photomorphogenesis (Chen et al., 2016; Kelly et al., 2020). Woltering & Witkowska (2016) showed that higher PPFD increased the dry matter content of lettuce, indicating higher carbohydrate levels and improved post-harvest quality. Here, we observed that C-S significantly outperformed N-S in terms of above-ground and root biomass (fresh weight and dry weight), dry matter content, and leaf weight of lettuce while significantly delaying the senescence of outer leaves in plants, thus leading to a higher photosynthetic activity. These results aligned with recent studies, where in plant shoots’ fresh weight/dry weight was influenced by both plant morphology and photosynthetic capacity (Hernández & Kubota, 2015; Wang et al., 2016). However, tip burn was observed among lettuce plants cultivated indoors under the increased light intensity. Correspondingly, Sago (2016) noted that butterhead lettuce frequently experienced tip burn when exposed to high light intensities (150 and 300 μmol·m⁻²·s⁻¹), which may have been induced by a calcium deficit in the interior leaves (Sago, 2016). The lettuce cultivar used in the current study were grown in C-S and N-S treatments with higher PPFD of 200 + 50 and 280 μmol·m⁻²·s⁻¹, respectively. Neither cultivar displayed the tip burn phenomenon, proving that these PPFD were not excessive for lettuce growth.

Plants rely heavily on chlorophylls (Chl) for photosynthetic capacity and growth (Li et al., 2018). Plant chlorophyll content is also significant for the visual appearance of the vegetable. A consumer will accept or reject a product based on its color and appearance, and these qualities are even more important in a product, such as microgreens, which are highly valued for the colors they include (Barrett, Beaulieu & Shewfelt, 2010). Chlorophyll levels can be influenced by both the intensity of the illumination and the plant’s genotype, as has been described in previous studies (Teng, Liao & Wang, 2021). In lettuce treated with 250 μmol m⁻²s⁻¹, it was evident that higher light intensities were easily accessible to higher chlorophyll a/b levels. In contrast, the concentration of chlorophyll a and b decreased by 300 μmol·m⁻²·s⁻¹. Likewise, the use of 200 μmol m⁻²s⁻¹+ 50 μmol m⁻²s⁻¹ under C-S at T2 was beneficial for the accumulation of chlorophyll a and b (Figs. 8A and 8B), and this enhancement was discovered to be directly associated with the development of the leaf structures.

The senescence of the leaves is a fundamental attribute that decreases the post-harvest performance and the nutritional content of horticultural crops, the yield of agricultural crops, and the build up of biomass. Senescence also restricts the production of agricultural crops (Guo & Gan, 2014). Furthermore, it can be prolonged by a lack of light (Guo & Gan, 2014; Bresson et al., 2017). On the other hand, light regulation remains a major issue in large-scale plantations (high plant density) due to occlusion caused by adjacent plant leaves, making it difficult for the middle and lower leaves to receive adequate light compared to the upper leaves (Chapepa, Mudada & Mapuranga, 2020). Correspondingly, we observed that lettuce grown in the N-S treatment exhibited yellow color at T2. This result could be ascribed to the unbalanced radiation on the cultivar plate, which accelerates the senescence of leaves.

In contrast, lettuce grown in the C-S treatment tent at T2 displayed an appropriate green hue. The senescence of leaves due to low light levels was accompanied by the transfer of nutrients to younger tissues, the loss of chlorophyll, the breakdown of photosynthetic proteins, and a decrease in photosynthetic activity (Thimann & Satler, 1979; Wingler et al., 2006). These findings demonstrated that the sideward and upward combination might delay the middle leaf’s senescence in plant factories. This finding is consistent with the results documented in previous studies, in which it was pointed out that supplemental lighting system delayed leaf’s senescence of lettuce (Zhang et al., 2015; Joshi et al., 2017). This result explains the increment in marketable leaves, as shown in Fig. 10A.

Response of soluble protein, nitrate, and soluble sugar under different irradiances

An accumulation of soluble carbohydrates could result from more intense lighting (Bian, Yang & Liu, 2015). The soluble protein plays a vital role in the osmotic regulation and metabolism of several metabolic enzymes. Plant resistance and metabolism are embodied in soluble protein content. As a result of the synergistic effect of the C-S lighting’s improved stomatal characteristics, photosynthetic pigment concentrations, and use efficiency of light delivered in diverse directions, we observed that the soluble protein levels were increased under C-S lighting at T2 (Fig. 8F).

Nitrates are one of the most important molecules in determining the nutritional value of food. Nitrate (NO₃⁻) can accumulate to varying degrees in vegetables and may cause human health issues (Kim, Chiami & Ishii, 2006). Romaine lettuce is a strong provider of nutrients and has low NO₃⁻. Many factors, including light levels, contribute to the final nitrate content in plants (Lillo, 2007). The light intensity and quality affect nitrate uptake, translocation, and decrease in plant organs. Romaine lettuce grown under light-emitting diodes with changing the light conditions during growth had lower levels of nitrate (Nicole et al., 2018). Our results showed that nitrate concentrations were higher under N-S, with a substantial impact from the substrate component. Our finding partly agree with Jannat et al. (2022) study, where in it was established that nitrate levels in coconut fiber dropped dramatically due to exposure to supplemental light (Fig. 8D).

It has been established that soluble sugars are highly sensitive to environmental stresses. They act on the supply of carbohydrates from source organs to sink organs. Besides, regulating lettuce biomass and morphology and light intensity also impacts nutrition quality and anti-oxidative enzyme activities (Fu et al., 2012; Fan et al., 2013). Light intensity increases lettuce’s soluble sugar and ascorbic acid content (Zhou et al., 2012). Furthermore, different lighting conditions that exist throughout the growing cycle have the potential to influence the total amount of carbohydrates present in the plant. These variables can stimulate photosynthetic activity (Cocetta et al., 2017). Vegetables’ sucrose concentration and nutritional profile can be improved with the help of a special LED light, as revealed by Lin et al. (2013). It was discovered that the amount of soluble sugar in strawberries rose in proportion to the direction of the irradiance when the top and side illumination were utilized (Yang, Song & Jeong, 2021, 2022). In a related study, there was an increase in the levels of expression of genes involved in sugar metabolism and signaling in lettuce cultivated with supplemental lighting (Viršilė et al., 2019). Here, we observed that soluble sugar content performed better under the C-S at t2. This could be attributed to usage of the supplemental adjustable sideward lighting and the change in irradiance direction. These results were consistent with prior research, indicating that the principal lighting direction can enhance the concentration of primary metabolites (Yang, Song & Jeong, 2021, 2022).

Response of ascorbic acid under different irradiances

Vitamin C, also known as ascorbic acid, is found in high concentrations in plant foods. It is present in all cell compartments, including the cell wall, and reaches a concentration of more than 20 mM in the chloroplasts where it is found (Smirnoff & Wheeler, 2000; Gallie, 2013). Multiple reports indicate light enhances vitamin C concentration in plants (Li et al., 2010; Massot et al., 2011). The varied light irradiance impacted ascorbic acid in romaine lettuce in the C-S and N-S (Fig. 9). The ascorbic acid content of plants growing under C-S was higher than under N-S. Similar results have been reported for ascorbic acid, alpha-carotene, and phenolic compound levels when treated with UV light (Xie et al., 2015) or red light (Bliznikas et al., 2012) during the latter stages of culture.

Response of stomata under different irradiances

The opening and closing of guard cells control the quantity of stomata, which carry air and water vapor during carbon assimilation, respiration, and transpiration. Light, temperature, and CO₂ are exogenous factors influencing the stomata’s opening and closing. In our study, the supplemental sideward lighting enhanced the stomatal opening compared with the N-S lighting. Moreover, romaine lettuce plants grown under C-S lighting appear to have higher photosynthetic efficiency due to favorable stomatal conditions (Kardel et al., 2009; Moore et al., 2021). We, therefore, believed that this phenomenon led to the highest biomass production rates, plant growth, and development. This result is in accordance with previous findings, where in it was established that side lighting could promote stomatal formation (Yang, Song & Jeong, 2021).

Electric-energy consumption of the lighting systems

Optimizing the plant factories’ performance across their energy spectrum is crucial for ensuring the design’s long-term viability. Lighting fixture position influences cultivation productivity and system lighting efficiency LED panels with different illumination schedules and mounted above butterhead lettuce (Lactuca sativa capitata) seedlings were examined. It was revealed that the highest light efficiencies and lowest electricity consumption were observed in the treatments with irradiation from a shorter distance above the seedlings (Li et al., 2014). In this study, the energy usage fell by 35.02%, and the power utilization rate increased by 135% when the LED panels were mounted adjacent and above the plants, suggesting thaty lighting system built in this study has energy-saving properties and a high energy utilization rate in the seedling stage of plant growth. This result is consistent with Cocetta et al. (2017) research, in which they found that reducing fixture spacing and focusing lights at the canopy could minimize lighting pollution and energy consumption, and lightning-quick efficiency.