Current status of lettuce production in soilless culture: environmental conditions
- Hossein Sheikhi1
- Mojtaba Delshad2
- Cosimo M. Profico1
- Silvana Nicola1*corresponding author
- 1Department of Agricultural, Forest and Food Sciences (DISAFA), Horticultural Sciences - Inhortosanitas Lab, University of Turin, Grugliasco, TO, Italy
- 2Department of Horticultural Science, Faculty of Agricultural Science and Engineering, University of Tehran, Karaj, Iran
Abstract
Lettuce is widely cultivated as a model crop in controlled-environment agriculture and has consequently been the subject of extensive research worldwide. Recent studies have predominantly focused on the effects of light-emitting diodes (LEDs), particularly light quality, intensity, photoperiod, and direction. In response to the development of artificial lighting technologies and the growing body of related research, this review synthesizes and evaluates studies published between 2012 and 2025 on the application of LED lighting in lettuce cultivation. In addition, research published between 2017 and 2025 addressing the effects of other key environmental factors, including temperature, relative humidity, carbon dioxide concentration, and air velocity, on lettuce growth, yield, and quality is critically reviewed. By integrating findings across these environmental variables, this review provides practical insights and reference ranges for optimizing lettuce production under controlled conditions and serves as a valuable resource for researchers and producers seeking to advance controlled-environment lettuce cultivation.
Keywords
1 Introduction
Lettuce (Lactuca sativa L.) is a crucial leafy vegetable (Sapkota et al., 2019) that is extensively consumed worldwide because of its pleasant aroma, crisp texture, and high vitamin and phenolic compound content (Ahmed et al., 2021). Lettuce is cultivated in open fields, greenhouses, and, more recently, vertical farming systems. The comparatively high light-use efficiency (LUE), defined as the shoot dry weight per incident photosynthetic photon flux density (PPFD) (g mol−1), observed in vertical farming has motivated widespread research on this production system. For instance, Jin et al. (2022) reported that the average LUE in lettuce was higher in vertical farms (0.55 g mol−1) than in greenhouses (0.39 g mol−1) and open-field conditions (0.23 g mol−1). Consequently, enhancing yield and quality of lettuce under controlled conditions has become a key aim of recent research (Ouyang et al., 2020).
In recent decades, a considerable proportion of this research has focused on the application of light-emitting diode (LED) technology to increase lettuce growth and quality. At the same time, deeper consideration has been given to optimizing other environmental factors that interact with light to modulate plant performance. Solid-state lighting technologies, particularly LEDs, have become extensively adopted over many industrial sectors, including agriculture (Loconsole et al., 2019). Advances in LED technology have resulted in critical improvements in efficiency, durability, and spectral controllability, making LEDs superior to traditional light sources for cultivation of plants (Nguyen et al., 2021). LEDs offer many advantages, including low weight, low energy consumption, compact size, long operational lifetime and high photosynthetically active radiation (PAR) efficiency (Pennisi et al., 2020b), rendering them excellent for regulating plant growth and quality in controlled conditions.
Comparative research has indicated the superiority of LEDs over conventional lighting systems. Dannehl et al. (2021) reported that lettuce grown under LED lighting consumed 15% less water while producing greater biomass, phenolic compounds, and chlorophyll content than lettuce grown under high-pressure sodium (HPS) lamps, without compromising yield. Likewise, Camejo et al. (2020) reported improved lettuce growth under LEDs compared with fluorescent (FL) lighting. As the cost of LED technology has decreased considerably in recent years, research has increasingly focused on defining optimal lighting strategies, including light intensity, light quality, photoperiod, daily light integral (DLI), and light direction, to maximize lettuce yield and quality (Nguyen et al., 2021). While many studies have investigated the effects of light quality and intensity, photoperiod and light direction have received comparatively less attention, despite their demonstrated roles in regulating lettuce yield and nutritional characteristics.
Besides lighting, other environmental factors, including ambient temperature, carbon dioxide (CO2), relative humidity, and air velocity, highly affect the growth, yield, and organoleptic and nutritional values of greenhouse crops (Kumazaki, 2022; Rouphael et al., 2018). Temperature is a determining factor of lettuce growth rate and the occurrence of physiological disorders, such as tipburn (Kumazaki, 2022). Ambient and root-zone temperatures (RZT) affect lettuce growth and quality and must be optimized for commercial production. Deviations toward excessively low or high temperatures disrupt physiological processes, such as photosynthesis, respiration, and nutrient uptake, eventually reducing growth and yield (Zhao et al., 2025). RZT is particularly crucial because it directly influences water and nutrient absorption and overall root function (Levine et al., 2023).
Relative humidity (RH), often expressed more correctly through vapor pressure deficit (VPD), also plays an important role in lettuce physiology and growth. High RH (low VPD) diminishes evapotranspiration and sap flow, impairing ion transport, resulting in nutrient deficiencies and physiological disorders (Ampim et al., 2022). Therefore, appropriate RH control in indoor cultivation systems is critical for maintaining product quality while also influencing phytochemical composition and antioxidant capacity (Gruda et al., 2025). Furthermore, CO2 enrichment is a well-developed strategy for improving the photosynthetic rate and biomass accumulation under controlled conditions, particularly during periods of minimal natural ventilation in greenhouses (Singh et al., 2020). Higher CO2 concentrations have been shown to enhance growth, increase the accumulation of carbohydrates and antioxidant compounds, and decrease the nitrate content in lettuce plants (Ampim et al., 2022). Air velocity is another important but mostly underestimated factor, as it influences boundary layer resistance, transpiration, gas exchange, and calcium (Ca) transport, thereby playing a crucial role in preventing tipburn and increasing lettuce quality (Persons et al., 2025).
Lettuce serves as an ideal model plant for controlled environment agriculture (CEA) research due to its rapid growth cycle, short life cycle, diverse varieties, well-characterized genome, small and compact size, substantial economic and nutritional value worldwide, exceptional adaptability to indoor and vertical farming systems, and high responsiveness to environmental changes. These attributes have enabled extensive investigation into lettuce-environment interactions, yielding the wealth of data synthesized in this review. Although many studies have examined individual environmental factors affecting lettuce growth, particularly light, recent reviews have shown that lettuce performance in CEA depends on multiple interacting variables rather than isolated inputs. Broad syntheses of plant factory and urban horticulture systems frequently include lettuce as a representative crop but are intentionally framed at a conceptual and system level, with a primary focus on technological innovation, economic viability, and environmental sustainability rather than on crop-specific physiological responses or detailed environmental interactions (Yuan et al., 2025), while recent systematic reviews address environmental factors for lettuce largely on a factor-by-factor basis, with limited integration of interactions among light, temperature, CO2, humidity, and air velocity (Nitu et al., 2025). In parallel, recent work has shown that dynamic, physiology-driven environmental control can improve energy efficiency in vertical farming systems, although these concepts have not yet been systematically evaluated for lettuce in soilless cultivation (Kaiser et al., 2024). In this review, soilless cultivation of lettuce refers to production systems in which plants are grown without soil, using a nutrient solution with or without solid substrates, while the physical and chemical conditions of the root zone are precisely controlled. This cultivation method is widely applied in greenhouse vegetable production as well as in CEA such as plant factories, indoor farms, and vertical farming facilities. Therefore, this review synthesizes recent literature on lettuce grown in soilless cultivation, focusing on LED lighting studies published between 2012 and 2025 and on other key environmental factors reported between 2017 and 2025, with the aim of integrating recommended operating ranges, environmental interactions, quality responses, and remaining research gaps to support more efficient and physiologically informed lettuce production.
Relevant studies were identified through systematic searches in Scopus, Web of Science, and Google Scholar using combinations of keywords related to lettuce or Lactuca sativa and major environmental factors, including artificial lighting, LEDs, indoor farming systems, plant factory, ambient temperature, carbon dioxide or CO2, relative humidity or RH, vapor pressure deficit or VPD, and air velocity or air speed. Priority was given to peer-reviewed articles published in high-impact journals recognized for their scientific rigor. Studies reporting duplicated findings or providing limited additional insight were excluded. The remaining literature was further evaluated based on study design, sample size, statistical robustness, and reproducibility. Despite these efforts, given the breadth and rapid expansion of research in this field, it is possible that some relevant studies were not captured or were inaccessible at the time of review.
2 The effect of artificial light on growth, yield, and quality of lettuce in soilless culture and indoor farming
This section reviews the effects of artificial lighting on lettuce growth, yield, and quality in soilless cultures and indoor farming systems, considering multiple physiological and agronomic perspectives, including photosynthesis, biomass accumulation, and nutritional value. Because light quality, intensity, photoperiod, and direction collectively regulate plant growth and quality (Chen et al., 2021b), this section synthesizes the published findings on the influence of artificial lighting on lettuce performance.
2.1 Quality of light
Light is a key player in the regulation of plant morphology, physiology, and phytochemical composition and is the primary source of energy for photosynthesis (He et al., 2021b). Lettuce, a model leafy vegetable, is particularly sensitive to light quality, which is defined by wavelength or spectral composition (Lin et al., 2013). Recently, most studies on the effect of light quality on the growth and nutritional value of lettuce have been conducted in controlled environments, such as greenhouses, growth chambers, and indoor farms, where LED artificial light fixtures are used to provide lighting.
Red (R) and blue (B) light, as the primary energy sources for CO2 assimilation, have a greater effect on plant growth and biomass production than other light spectra (Shao et al., 2021). R light usually stimulates biomass production, leaf area expansion, and stem elongation, whereas B light is involved in stomatal opening, chlorophyll production, and photosynthesis (Matysiak and Kowalski, 2019). However, Vaštakaitė-Kairienė et al. (2022) reported that end-of-day (EOD) B light elevated shoot elongation in lettuce plants, regardless of their growth stage. According to Karami et al. (2022), R light is responsible for producing the essential substances (primary metabolites and structural components) for plant growth while B light is believed to be responsible for producing secondary metabolites. There is some evidence that suggests that red and green lettuce show distinct physiological adaptive strategies when exposed to high proportions of R or B light, as Cammarisano and Körner (2022) have shown in their study that both lettuce types adapt differently to the higher proportion of B light. Taken together, these studies indicate that R and B light regulate complementary yet distinct physiological processes in lettuce, and that cultivar and timing of exposure critically influence morphological outcomes.
A large body of research has evaluated different R:B ratios across developmental stages and consistently demonstrated that combined R and B lighting is more effective than monochromatic spectra in improving lettuce growth and quality (Chen et al., 2021b; Hooks et al., 2021; Li et al., 2021a; Shao et al., 2021; Zhang et al., 2021; Lee et al., 2023; Sgarbi et al., 2023; Sumi et al., 2025). The highest net photosynthesis in lettuce plants grown under a combination of R and B light compared to monochrome light was obtained by Tarakanov et al. (2022). Continuous RB lighting with low B fractions enhanced yield, carbohydrate accumulation, and metabolic enzyme activity in both ‘Yidali’ and ‘Zishan’ lettuce cultivars (Liu and Liu, 2022). Similarly, high R:B ratios improved the fresh weight and photosystem II (PSII) efficiency in rocket and lamb’s lettuce (Frutos-Totosa et al., 2023). At the physiological level, increased B light intensity stimulates respiration and cyclic electron flow, whereas R light promotes linear electron flow (Yudina et al., 2022). This functional complementarity corresponds to the absorbance spectra of photosynthetic pigments, with chlorophyll a, chlorophyll b, and carotenoids predominantly absorbing in the R and B regions (Wojciechowska et al., 2020). Generally, controlled-environment studies have revealed that optimizing R:B ratios improves photosynthetic efficiency, biomass accumulation, and carbohydrate metabolism, whereas excessive dominance of either wavelength can limit overall performance.
Several studies further underline trade-offs between yield and nutritional quality depending on the R:B balance. Higher R proportions (e.g., 75R:25B or 90R:10B) are positively correlated with biomass accumulation, photosynthetic pigments, and LUE (Zha et al., 2020; Chen et al., 2021b; Pennisi et al., 2019). Conversely, increasing B proportions enhances antioxidant capacity, ascorbate pools, chlorogenic acid, and other phenolic compounds (Naznin et al., 2019; Shimomura et al., 2020). Similar patterns have been observed in indoor farming systems, where intermediate R:B ratios maximized yield without further gains at higher R dominance (Pennisi et al., 2019). Therefore, R:B optimization involves a physiological trade-off: higher R proportions favor biomass and efficiency, whereas higher B proportions improve phytochemical accumulation and nutritional value.
Beyond R and B light spectrum, recent studies indicate that supplementing additional spectral regions can further regulate lettuce growth and quality (Meng and Runkle, 2019; Viršilė et al., 2020; He et al., 2021b; Hooks et al., 2021; Li et al., 2021a; Yan et al., 2023; Lee et al., 2023). The use of ultraviolet (UV), far red (FR), green (G), amber, yellow, purple, orange, and cyan lights supplemented with R and B light can influence growth and many nutritional values (e.g. content of nitrate and nitrite, content of amino acid, vitamin C, antioxidant capacity, etc.) of lettuce (Chang and Chang, 2014; Bian et al., 2018; Tosti et al., 2018; Li et al., 2021a; Kelly and Runkle, 2023; Hernández-Adasme et al., 2024).
UV (100–400 nm) is classified into three subgroups: UV-C (100–280 nm), UV-B (280–320 nm), and UV-A (320–400 nm). UV-A increases the concentration of photosynthetic pigments, enhances the accumulation of secondary metabolites, and promotes the accumulation of biomass in lettuce under indoor cultivation (Chen et al., 2019; Vaštakaitė-Kairienė et al., 2021; Zhu et al., 2025). However, responses differ remarkably with intensity, photoperiod, cultivar, and timing of exposure. While UV-A supplementation prior to harvest has been shown to enhance protein content, mineral accumulation, and leaf coloration (Lee et al., 2022; Kelly and Runkle, 2023), reductions in fresh weight have also been disclosed under certain conditions (He et al., 2021b). UV-B, particularly short or pre-harvest exposures, enhances flavonoid and anthocyanin accumulation and strengthen disease resistance, whereas UV-C typically exerts negative effects on lettuce quality (McLay et al., 2020; Weiland et al., 2023; Skowron et al., 2024). Briefly, UV light can be an effective tool for improving phytochemical quality, but its application needs accurate control to avoid yield penalties.
FR (750–850 nm) light supplementation has constantly been shown to increase lettuce biomass, canopy expansion, and LUE by enhancing PSII activity and diminishing non-photochemical quenching (NPQ) (Zhen and van Iersel, 2017; Park and Runkle, 2018; Zhang et al., 2019; Legendre and van Iersel, 2021; Van de Velde et al., 2023; Shmarev et al., 2024). Both daytime and end-of-day FR treatments significantly enhance fresh and dry weight (Mickens et al., 2018; Zou et al., 2019; Carotti et al., 2024). Similar results were reported in other studies (Yan et al., 2020; Jin et al., 2021; Li et al., 2021b, 2024; Bi et al., 2024; Dou et al., 2024; Van Brenk et al., 2024; Liu et al., 2025; Yan et al., 2025). To date, no consistent pattern has been identified regarding leaf photosynthetic responses to FR light. Early studies demonstrated that FR light can enhance photosynthetic activity in leaves, a phenomenon known as the Emerson effect, for which Photosystem I (PSI) preferentially absorbs FR light, whereas PSII primarily absorbs R and B wavelengths. Under conditions of insufficient FR light, PSI and PSII are unevenly excited, resulting in reduced efficiency of linear electron transport. Consequently, supplementation with FR light, particularly under low total irradiance, enhances PSI excitation, improves the balance between the two photosystems, increases the efficiency of linear electron transport, and ultimately stimulates photosynthesis (Li et al., 2024). FR light can also reduce nitrate content while improving vitamin C concentration (Meng et al., 2024). Although the mechanisms underlying these effects are not well documented in the literature, it is possible that FR light reduces nitrate accumulation in vegetables by enhancing carbon-derived nitrate uptake and biomass dilution. Simultaneously, it may increase vitamin C content by boosting sugar availability (Zhang et al., 2024) and activating the Smirnoff-Wheeler biosynthetic pathway (Smirnoff and Wheeler, 2000). These results collectively indicate that FR light is an important spectral component for maximizing lettuce biomass and resource-use efficiency (RUE), particularly in compact canopy systems.
G (495–570 nm) light supplementation has been shown to enhance nitrogen (N) metabolism enzyme activity when added to RB lighting (Bian et al., 2018), although monochromatic G light can increase nitrate accumulation and suppress enzyme activity (Zhang et al., 2018b). Contrasting results regarding biomass responses to G light have been reported, ranging from reduced growth to neutral or positive effects, depending on the intensity, developmental stage, and cultivar (Liu et al., 2018; Mickens et al., 2018; Meng and Runkle, 2023). Meta-analytic evidence indicates that G light can be as effective as RB light in enhancing plant biomass (Chen et al., 2024), and recent experimental studies have confirmed its synergistic effects when combined with R and B spectra (Chen et al., 2025). Concerning postharvest perspective, G light has also been shown to improve chlorophyll retention during storage (Salehinia et al., 2024). Overall, G light appears to play a context-dependent modulatory role, with beneficial effects under mixed spectra and at early growth or postharvest stages, but limited value as a sole light source.
Ultimately, studies on cyan, amber, yellow, orange, and purple light remain limited and often incompatible. Cyan (466–532 nm) light has been reported to increase fresh mass and chlorophyll content when added to RB lighting (Chang and Chang, 2014), whereas amber (590–595 nm) and yellow (570–590 nm) light responses differ with PPFD and cultivar (Tosti et al., 2018; Wu et al., 2021; Reddy et al., 2024). Orange (590–620 nm) light generally has minimal effects on lettuce biomass but may increase phenolic accumulation (Samuoliene et al., 2013; Viršilė et al., 2020). Given the variability and limited availability of data, further studies are needed to specify the physiological roles of these minor spectral components and their potential integration into optimized lighting strategies. A summary of the effects of different light spectra on the lettuce growth and quality is shown in Figure 1. The wavelength ranges provided for the various light spectra are based on Duncan and Chang (2012).
2.2 Intensity of light
The photosynthesis rate, biochemistry and many other physiological processes of the crop are highly influenced by the light intensity (Feng et al., 2019). In lettuce, increasing light intensity generally enhances growth and productivity, mostly due to a strong positive relationship between biomass accumulation and total incident light received by plants (Xu et al., 2020; Liu et al., 2020; Palsha et al., 2024). Nevertheless, excessively high light intensity can disrupt cellular redox balance and cause photoinhibition, whereas inadequate light limits photosynthetic capacity and decreases carbohydrate production and yield (Fu et al., 2017; Ghorbanzadeh et al., 2021; Min et al., 2021). Overall, lettuce growth exhibited a classic dose–response relationship with light intensity, in which both light limitation and excessive irradiance negatively affected physiological performance.
A growing body of evidence demonstrates that higher light intensities enhance lettuce biomass, photosynthetic pigments, photosystem efficiency, net photosynthetic rate (Pn), chlorophyll fluorescence parameters, nutrient uptake, water-use efficiency (WUE), nutritional quality, and shelf life. For example, Cammarisano et al. (2021) compared PPFDs of 270 and 570 µmol m−2 s−1 using FL and LED light sources and noted that 570 µmol m−2 s−1 increased rosette area, leaf thickness, and carotenoid content by 13%, 9%, and 15%, respectively. Although biomass did not differ between LEDs and FL lamps at high PPFD, plants grown under low-intensity FL light accumulated less biomass. In a related study, anthocyanin accumulation in lollo rosso lettuce enhanced linearly with irradiation (Cammarisano et al., 2020). Similarly, chlorogenic acid content increased as PPFD rose from 100 to 300 µmol m−2 s−1 (Shimomura et al., 2020). Pennisi et al. (2020b) identified 250 µmol m−2 s−1 as an optimal PPFD, at which lettuce exhibited maximum biomass, stomatal conductance, RUE, antioxidant capacity, and phenolic and flavonoid content compared to both lower and higher intensities. Das et al. (2024) further demonstrated that lettuce seedlings grown under 600 µmol m−2 s−1 accumulated 275% more shoot fresh mass than those grown under 60 µmol m−2 s−1, with substantial gains maintained until harvest. Taken together, controlled-environment studies indicate that moderate-to-high PPFDs significantly enhance lettuce growth and quality, although the existence of optimal intensity thresholds is evident.
Under greenhouse conditions, supplemental lighting also plays a decisive role. He et al. (2021a) noted that lettuce exposed to 300 µmol m−2 s−1 (9R:1B LEDs) combined with natural sunlight showed increased biomass, pigment content, transpiration, and light-saturated CO2 assimilation compared with plants grown under lower supplemental PPFDs or sunlight alone. Likewise, Sutulienė et al. (2022) found that supplementing natural light (80 µmol m−2 s−1) with 250 µmol m−2 s−1 LEDs improved lettuce morphology more effectively than supplementation at 150 µmol m−2 s−1. Ghorbanzadeh et al. (2021) reported that increasing PPFD from 75 to 600 µmol m−2 s−1 (3R:1B LEDs) increased biomass, WUE, PSII efficiency, and NPQ. However, studies by Weaver and van Iersel (2019, 2020) exhibited that although electron transport rate (ETR) increased with PPFD, the quantum yield of PSII (ΦPSII) decreased, indicating reduced photosynthetic efficiency at high irradiance. These results suggest that while high PPFD increases absolute photosynthetic capacity and ETR, lower PPFDs may operate more efficiently on a per-photon basis. At lower PPFDs, most absorbed photons are used for photochemistry, as indicated by higher ΦPSII and a greater proportion of open PSII reaction centers. In contrast, at high PPFD, saturation of the Calvin cycle results in excess excitation energy and increased NPQ, which reduces photochemical and quantum efficiency. Consequently, lower PPFDs lead to greater LUE despite lower absolute carbon assimilation (Mayorga-Gomez et al., 2024b).
Light fluctuation approaches have also been considered as a means of optimizing energy use. Bhuiyan and van Iersel (2021) reported that lettuce tolerated a wide range of alternating PPFD regimes without considerable effects on dry mass, leaf number, leaf area, or chlorophyll content. However, extreme fluctuations (e.g., 400/0 or 360/40 µmol m−2 s−1 applied every 15 min throughout the photoperiod) negatively affected net assimilation and chlorophyll content, resulting in growth reductions. Therefore, moderate light fluctuations may be exploited for energy cost optimization, whereas severe fluctuations compromise photosynthetic stability and growth. In addition, lettuce tolerated multiple consecutive low-DLI days following a high-DLI day without significant growth reductions when the average DLI was maintained at 15 mol m−2 d−1. Only a single high-DLI day followed by one low-DLI day reduced biomass (≈13% lower dry weight), demonstrating a DLI carryover effect that can reduce supplemental lighting requirements and energy use (Mayorga-Gomez et al., 2024a).
Concerning nitrate accumulation, several studies confirm that higher light intensity reduces nitrate content in lettuce by increasing N assimilation. Viršilė et al. (2019) observed the lowest nitrate levels at 300 µmol m−2 s−1 compared with 100 µmol m−2 s−1, although further increases to 400–500 µmol m−2 s−1 did not yield additional reductions. Similarly, Yan et al. (2019a) reported no difference in nitrate content between PPFDs of 200 and 250 µmol m−2 s−1. This reduction in nitrate accumulation under higher light intensities is attributed to increased carbohydrate availability and activation of N-assimilating enzymes (Yi et al., 2021; Chen et al., 2021c). Nitrate content in vegetables is closely linked to light intensity, as nitrate uptake and assimilation depend on photosynthesis to supply carbon skeletons and reducing power for nitrate reduction (Colla et al., 2018). Under low-light conditions, particularly during the autumn–winter season, reduced nitrate reductase activity limits nitrate assimilation, leading to higher nitrate accumulation in plant tissues. Consequently, nitrate levels are generally lower during spring and high-light growing seasons (Colla et al., 2018). Generally, elevated light intensity increases N metabolism and minimizes nitrate accumulation, though diminishing returns occur beyond moderate PPFD levels.
On the contrary, the responses of vitamin C to light intensity appear to be conflicting. While Fu et al. (2017) reported maximum vitamin C content at moderate PPFD (140 µmol m−2 s−1), other studies found either slight increases or no significant differences across similar intensity ranges (Lin et al., 2018; Yan et al., 2019b). These results imply that vitamin C accumulation is influenced by additional factors beyond light intensity alone. Elevated light intensity has been shown to facilitate increased mineral absorption. Zhou et al. (2019) observed greater N, phosphorus (P), and potassium (K) uptake at PPFDs above 350 µmol m−2 s−1 compared with lower intensities. Additionally, pre-harvest exposure to high light intensity improves postharvest quality. Min et al. (2021) indicated that lettuce exposed to 470 µmol m−2 s−1 during the final week before harvest exhibited improved visual quality and extended shelf life by 3–6 days compared to moderate and low light treatments, mainly due to increased carbohydrate and ascorbic acid levels. In brief, high light intensity enhances nutrient uptake and postharvest performance, strengthening its importance not only for yield and growth but also for lettuce shelf life and marketability.
2.3 Photoperiod
Photoperiod, defined as the duration of light within a 24-h cycle, is a key factor influencing lettuce growth and quality in controlled environments. Weaver and van Iersel (2020) showed that under a constant DLI of 17 mol m−2 d−1, extending the photoperiod from 12 to 21 h increased lettuce biomass. Conversely, Pennisi et al. (2020a) reported that a 16 h photoperiod combined with a DLI of 14.4 mol m−2 d−1 maximized biomass, leaf area, and energy-use efficiency (EUE), and LUE under indoor conditions. Longer photoperiods increase chlorophyll content and electron transport capacity (Zhou et al., 2020), but EUE declines when photoperiod exceeds ~18 h (Silva et al., 2022). Taken together, photoperiod prolongation enhances growth and photosynthetic capacity, but prolonged lighting diminishes EUE.
Light–dark cycles management further regulates lettuce growth and quality (Li et al., 2025). Chen et al. (2022b) reported that different RB irradiation patterns over a fixed growth period produced distinct results, with longer light phases favoring biomass and intermittent cycles improving EUE and nutritional quality. Similarly, Yan et al. (2019a) showed greater seedling growth under a 16 h photoperiod than under 14 h at the same PPFD. However, prolonged photoperiod by substantially lowering light intensity is ineffective due to inadequate photosynthesis (Yan et al., 2021), and excessively long photoperiods can cause chlorosis and yield reduction (Song et al., 2020).
Regarding lettuce nutritional quality, longer photoperiods enhance phytonutrient accumulation, including xanthophylls and phenolic compounds, although responses depend on light intensity (Cho et al., 2020; Samuolienė et al., 2021). Preharvest supplemental lighting, particularly at night, has been shown to further enhance phenolic content (Brazaitytė et al., 2022; Hooks et al., 2022). Maximum nitrate assimilation has been reported under PPFDs of 250–400 µmol m−2 s−1 combined with 16–18 h photoperiods (Zhang et al., 2018c; Viršilė et al., 2019). Therefore, photoperiod manipulation provides an adjustable tool for balancing lettuce yield, quality, and nitrate assimilation.
New reports highlight the importance of temporal and spectral dynamics. Alternating R and B light at the same DLI often outperforms simultaneous irradiation in promoting biomass and carbohydrate accumulation (Ohtake et al., 2018; Chen et al., 2022a), although trade-offs with growth reduction have been reported when stress-related metabolites are increased (Shao et al., 2022). Continuous, pulsed, and dynamically adjusted lighting strategies can accelerate growth or reduce energy costs without compromising quality (Ali et al., 2024; Meng and Severin, 2024; Van Brenk et al., 2025; Mäkinen et al., 2025). The mechanisms underlying these responses remain unclear but may involve photoreceptor interactions. Phytochrome B and cryptochrome 1 act synergistically under short photoperiods but independently under prolonged exposure (Casal, 2000).
Prolonging the photoperiod typically increases the DLI, which can improve yield and shorten the production cycle (Kelly et al., 2020; Yudina et al., 2023). Nevertheless, excessive DLI may negatively influence plant appearance, photosynthetic efficiency, and biomass. Numerous studies have identified optimal DLI ranges for lettuce. Matysiak et al. (2022) reported that elevating DLI up to 17.3 mol m−2 d−1 enhanced biomass, head circumference, and leaf number while reducing nitrate accumulation, but negatively influenced PSII quantum yield (Φ PSII) and visual quality. Similarly, Sheikhi et al. (2024a, b) proposed an optimal DLI of ~17.3 mol m−2 d−1 across growth stages. In contrast, Gavhane et al. (2023) recommended a lower DLI (11.5 mol m−2 d−1) for iceberg lettuce under indoor vertical systems, as higher DLI reduced biomass and leaf area. Overall, these studies indicate that optimal DLI is cultivar- and system-dependent, and excessive DLI can reduce lettuce performance. Beyond daily light scheduling, a recent study indicates that lettuce can acclimate to non-natural day lengths, with growth largely unaffected by day length itself and, in some cultivars, improved under extended photoperiods despite reduced dark periods, even when DLI is held constant. These responses, linked to cultivar-dependent increases in leaf area and circadian adjustment of non-structural carbohydrate rhythms, suggest that cumulative light exposure over longer time scales (e.g., weekly or monthly light integrals) may provide a more meaningful framework for interpreting variation in LUE and EUE under dynamic lighting regimes (Núñez Ocaña et al., 2025).
2.4 Light direction
In addition to light quality, intensity, and photoperiod, the light direction has emerged as an important factor influencing plant morphology, physiology, and nutritional quality. In CEA, artificial lighting enables flexible manipulation of light direction; however, the physiological mechanisms and optimal lighting strategies remain poorly understood. Wang et al. (2021) investigated the effects of three lighting directions (top, side, and bottom) on ‘Caesar Green’ and ‘Polla’ lettuce cultivars under identical PPFD (300 µmol m−2 s−1) and photoperiod (12 h). Bottom lighting increased leaf angle and canopy openness but reduced leaf number and root dry weight. It also resulted in lower stomatal density with larger stomata, which was associated with reduced stomatal conductance and carbohydrate content compared with top and side lighting. Noticeably, proline content and PSII quantum yield were not affected by light direction, indicating that plants remained under non-stressful physiological conditions. In another study, Yang et al. (2024) reported that combined top and side lighting at a higher PPFD (600 µmol m−2 s−1) effectively reduced shoot elongation while increasing crown width, leaf size, leaf number and thickness. This lighting arrangement also enhanced chlorophyll fluorescence parameters, photosynthetic capacity, and mineral accumulation prior to harvest. In the same way, Chen et al. (2023b) indicated that side lighting, irrespective of spectral type, significantly improved lettuce yield, reduced size heterogeneity caused by edge effects under top lighting, and altered nutritional quality. Taken together, these studies suggest that side or combined top–side lighting improves canopy architecture, photosynthetic performance, and yield uniformity, whereas bottom lighting primarily induces morphological changes with limited benefits for biomass accumulation and carbohydrate content. One possible explanation is that side lighting enhances plant growth by upregulating key photosynthetic genes. Increased expression of PsaA and PsbA, which encode core components of PSI and PSII, may improve photosynthetic capacity and regulatory signaling, thereby contributing to the observed increase in shoot biomass compared with top or bottom lighting (Wang et al., 2021). Despite these advances, the role of light direction in regulating lettuce nutritional composition and its interaction with light spectrum and intensity remain poorly explored, highlighting the need for further systematic research. A comprehensive summary of these studies is presented in Table 1, providing an integrated overview of how different artificial lighting strategies affect lettuce growth and quality under controlled cultivation conditions.
Table 1. Other published research on the use of LED artificial light on the growth and quality of lettuce under soilless culture.
| Cultivar(s) | Treatments | Cultivation conditions | Outstanding findings | Reference |
|---|---|---|---|---|
| Lactuca sativa L. var. capitata | Four light intensities (50, 100, 150, and 200 µmol m–2 s–1) were applied during 48 h pre-harvest under hydroponic system. | Under PPFD of 100-150 µmol m–2 s–1, nitrate decreased and vitamin C and carbohydrates increased. | (Zhou et al., 2012) | |
| Lactuca sativa L. var. capitata | Four light treatments [HPS, LED, regular greenhouse HPS level (regular), and no supplemental artificial light (control)] were applied under indoor farming; total light irradiance was set at 71.3 mol m–2 for HPS and 35.8 mol m–2 for LED; | photoperiod was set at 18 h. | The LED light treatment produced similar shoot biomass compared with HPS light treatments. LED and HPS provide an energy savings of at least 33.8% and 77.8% over the HPS treatment, respectively. | (Martineau et al., 2012) |
| Lactuca sativa L. ‘Seonhong Jeokchukmyeon’ | Four light spectrum (cool white FL (FL), white (W) LED and an 8:1:1 mixture of R, B and W (RBW) LEDs. | 21 ± 2 °C, 60 ± 10% RH, 140 ± 20 µmol m–2 s–1 PPFD. | The highest biomass was obtained under RBW LED treatment. The highest total anthocyanin was obtained from cool W FLs. | (Park et al., 2012) |
| Lactuca sativa L. ‘Multired 4’, ‘Multigreen 3’ and ‘Multiblond 2’ | Five light treatments including HPS as control with a PPFD of 200 µmol m–2 s–1, HPS + 455 nm, HPS + 470 nm, HPS + 505 nm, HPS + 535 nm (HPS lamps with 16-h; PPFD-170 µmol m–2 s–1 + B 455/470 nm and G 505/530 nm with 16-h; PPFD-30 µmol m–2 s–1) were applied as supplementary lighting under greenhouse conditions. | The highest vitamin C and tocopherol contents was obtained in order: 535 > 505 > 455 > 470 nm; the highest total phenol content: 505 > 535 = 470 > 455 nm; the highest DPPH free-radical scavenging capacity: 535 = 470 > 505 > 455 nm; the highest total anthocyanins: 505 > 455 > 470 > 535 nm. | (Samuoliene et al., 2012a) | |
| Lactuca sativa L. ‘Thumper’ and ‘Multibaby’ | Two light treatments including HPS (300 µmol m–2 s–1) and HPS + 638 nm (90 + 210 µmol m–2 s–1) were used under greenhouse conditions three days before harvesting during three growing seasons. | photoperiod was set at 16 h. | The short-term irradiation with supplemental R-LEDs before harvesting improved total phenolics by 28.5%, tocopherols by 33.5% in ‘Multibaby’, sugars by 52.0%, antioxidant capacity by 14.5% and decreased ascorbic acid by 64.3%. | (Samuoliene et al., 2012b) |
| Lactuca sativa L. cv. ‘Banchu Red Fire’ | Four light intensities (100, 200, and 300 µmol m–2 s–1) and four light sources (W FLs, G LEDs 510 nm, G LEDs 520 nm, and G LEDs 530 nm). | The highest rate of leaf photosynthetic was obtained under G LEDs 510 nm and 200 µmol m–2 s–1 PPFD; high intensity and short-wavelength G LED was more effective to promote plant growth. | (Johkan et al., 2012) | |
| Lactuca sativa L. cv. ‘Lvling’ | Five light intensities (100, 200, 400, 600, and 800 µmol m–2 s–1) provided by HPS lamps were applied under phytotron chamber conditions. | 20/16 °C (day/night), 70% RH, 14/10 h (light/dark) photoperiod, and 400 µmol mol−1 CO2 concentration. | Under 200, 400, and 600 µmol m–2 s–1, the highest values of qP, ΦPSII, and ETR were obtained, indicating the highest yield and LUE. Yield and LUE were higher at 600 and 200 µmol m–2 s–1, respectively. | (Fu et al., 2012) |
| Lactuca sativa L. ‘Hongyeom Jeockchukmyeon’ | Four light intensities (200, 230, 260, and 290 µmol m–2 s–1) provided by LEDs (R:B:W = 8:1:1), and three different photoperiods 18/6 (1 cycle), 9/3 (2 cycles), and 6/2 (3 cycles) (light/dark) were used under plant factory system. | 21 ± 1 °C and 70 ± 10% RH. | Under PPFD of 290 µmol m–2 s–1, photoperiod of 6/2 and under PPFDs of 230 or 260, photoperiod of 18/6 and 9/3 increased growth and photosynthesis capacity. | (Kang et al., 2013) |
| Lactuca sativa L. var. capitata | Three light spectra (RB, RBW, and FL) were used under controlled growth room. | 24/20 °C (day/night), 75% RH, 16/8-h light/dark photoperiod, 210 µmol m–2 s–1 PPFD, and 900 µmol mol−1 CO2 concentration. | RBW LED light and FL showed higher biomass as well as crispness, sweetness, and shape compared to RB LED light. RBW LED light also showed the lowest and highest nitrate and soluble sugar contents. | (Lin et al., 2013) |
| Valerianella locusta Laterr. Em Betce‘Noordhollandse’ | Three light spectra (sunlight as control, W, and R+B with ratio of 1:0.8) were used under greenhouse conditions at two seasons (winter and autumn). | PPFD was 100 µmol m–2 s–1. | The highest content of chlorophyll a, b, and carotenoids as well as highest biomass was observed under R+B LED light. | (Wojciechowska et al., 2013) |
| Lactuca sativa L. ‘Thumper’ | Five light spectra e.g. Basal (B 455 nm + R 638 nm + R 670 nm), Basal + UV, Basal + G, Basal + Yellow, and Basal + Orange were applied under growth chambers. | 20/16 ± 2 °C (day/night), 50-60% RH, and 175 µmol m–2 s–1 PPFD. | Supplemental UV or orange LEDs light increased phenolic compounds, supplemental UV or G LEDs light increased α-carotene, and supplemental G LEDs light increased anthocyanins content. | (Samuoliene et al., 2013) |
| Lactuca sativa L. “Tom Thumb” | Three light spectra (R, B, and FL) were applied under room conditions. | 21.9 °C, 12-h photoperiod, and 58.9 µmol m–2 s–1 PPFD. | Total biomass was observed under FL treatment. | (Kobayashi et al., 2013) |
| Lactuca sativa L. ‘Sunmang’ and ‘Grand Rapid TBR’ | Seven light spectra (FL+HPS as control, 0 B/100 R, 13 B/87 R, 26 B/74 R, 35 B/65 R, 47 B/53 R, and 59 B/41 R) were applied growth chambers. | 20 °C, 12-h photoperiod and 171 ± 7 µmol m–2 s–1 PPFD. | Increasing the ratio of blue LED light negatively affected growth of lettuce. Biomass was highest under 0 B/100 R LED light. The SPAD value, total phenolic, total flavonoid, and antioxidant capacity were higher under high ratios of B LED. | (Son and Oh, 2013) |
| Lactuca sativa L. cv. ‘Cheong Chi Ma’ | Three light sources (W LED 420–680 nm, R LED 635 nm, and B LED 460 nm) were used under growth chambers. | 20 °C, 60-65% RH, 16/8-h light/dark photoperiod and 200 µmol m–2 s–1 PPFD. | The highest biomass, chlorophyll content, and antioxidant capacity was observed under B LED light. Botrytis cinerea was suppressed by B LED light. | (Kook et al., 2013) |
| Lactuca sativa L. cv. ‘Hongyeom’ | RGB (11:4:3 chips) LED supplemented with three forms of UV light (UV-A: 352 nm, 3.7 W; UV-B: 306 nm, 4.2 W; and UV-C: 253.7 nm, 7.5 W) LEDs. | 20 °C, 16 photoperiod, and 185 ± 4.3 µmol m–2 s–1 PPFD. | UV-A increased fresh and dry weights of shoots up to day-6, total phenolic content, antioxidant capacity and activity of PAL at day-3, total anthocyanin content up to day-4, and PAL gene expression after day-4.UV-B decreased Fv/Fm, fresh and dry weights of shoot. Improved total phenolic content, antioxidant capacity at day-2, and activity of PAL at day-3. UV-C decreased Fv/Fm, fresh and dry weights of shoot. | (Lee et al., 2014) |
| Lactuca sativa L. var. crispa'Green Oak Leaf' | FL light + R LED (51.9%FL:49.1%R), FL light + B LED (52.7%FL:47.3%B), R LED, B LED, RB LED (48.2%R:51.8%B), and FL. | 24/20 °C (day/night), 60% RH, 900 µmol mol−1 CO2 concentration, 14 h photoperiod, and 133 ± 5 µmol m–2 s–1 PPFD. | FLR or FLB LED increased biomass and pigments content of lettuce than R, B, FL or RB. RB decreased nitrate content. | (Chen et al., 2014) |
| Lactuca sativa L. var. dasusheng | W FL and RB LED (90%R:10%B). Photoperiod of 12, 16, 20, and 24 h. | 25 ± 1 °C, 65% ± 5 RH, 2000 µmol mol−1 CO2 concentration, and 600 µmol m–2 s–1 PPFD. | RB LED increased soluble sugar and vitamin C concentrations and reduced concentration of nitrate.24 h illumination increased the concentration of soluble sugar, vitamin C, and crude fiber, and reduced the concentration of nitrate. | (Shen et al., 2014) |
| Lactuca sativa L. cv. ‘Waldmann’s Green’ | R (630 nm) + B (455 nm); 95%R:5%B. | 25/20 ± 1 °C (day/night) and 65/80% ± 5 RH (day/night). | The technique of “targeted” LED lighting instead of indiscriminate irradiation, improved lettuce growth and reduced energy consumption (less than 1 kWh g−1 dry matter). | (Poulet et al., 2014) |
| Lactuca sativa L. ‘Hongyeom’ | G LED with PPFDs of 70 and 80 µmol m–2 s–1, 522 nm. R LEDs with PPFDs of 88 and 238 µmol m–2 s–1, 639 nm. B LEDs with PPFDs of 80 and 238 µmol m–2 s–1, 470 nm. | 25/18 °C (day/night), 70 + 10% RH, and 16 h photoperiod. | B LEDs with PPFD 238 µmol m–2 s–1 increased biomass, net photosynthesis, stomatal conductance, Fv/Fm, and transpiration rate than other treatments. | (Muneer et al., 2014) |
| Lactuca sativa L. cv. ‘Waldmann’s Green’ | Seven light sources e.g. R (0.3%), R + B (RB; 12%), R + G + B (RGB; 14%), warm W (11%), neutral W (19%), cool W (28%) and B (92%) were used at both 200 and 500 µmol m–2 s–1. | 24.7 ± 0.6/24.1 ± 0.4 °C (day/night), 30 ± 6% RH, 430 ± 50 µmol mol−1 CO2 concentration, and 16/8 h (day/night) photoperiod. | B Light was important to prevent shade-avoidance responses in lettuce. Increasing B Light above 11% continued to decrease hypocotyl length, stem length, petiole length and leaf area, | (Cope et al., 2014) |
| Lactuca sativa L. ‘White Boston’ | R (600–650 nm), B (450–500 nm), G (490–540 nm), magenta filter (W LED covered with a cellophane film magenta), and W LEDs as control. | 20.48 °C, 31.71% RH, and 12 h photoperiod. | The highest fresh and dry weight in lettuce seedlings was obtained in B LED light with increases of 50 and 41%, respectively, compared to control. | (Pardo, 2014) |
| Lactuca sativa L. cv. ‘Ostinata’ | 0G (RB + 0% G), 20G (RB + 20% G), 40G (RB + 40% G), cool-W LED, cool-W FL, and FL. PAR range was separated into five sections. | 20 °C, 70-80% RH, 1200 µmol mol−1 CO2 concentration, 16 h photoperiod, and 150 µmol m–2 s–1 PPFD. | G light (525–575 nm) at less than 30% of PAR improves lettuce growth more than R (625–700 nm) and B (400–475 nm). | (Kong et al., 2015) |
| Lactuca sativa L.cv. ‘Królowa Majowych’ | FL, LED composed of 12 W (12W), LED composed of 9 R + 3 B (9R + 3B), LED composed of 10 R and 2 B (10R + 2B), LED composed of 11 R and 1 B (11R + 1B), LED composed of 12 (12R). | 22/18 °C (day/night), 80 ± 10% RH, and 200 µmol m–2 s–1 PPFD. | The highest and lowest yield was observed under FL light and 12W LED, respectively. 12R LED decreased chlorophyll fluorescence parameters (Fo, Fm, Fv/Fm). 11R + 1B LED showed the most beneficial for lettuce production. | (Borowski et al., 2015) |
| lamb’s lettuce (V. locusta L.) ‘Nordhollandse’ | HPS as control, 100R, 50R/50B, 70R/30B, 90R/10B, and W LEDs as a supplemental to solar radiation under greenhouse conditions. R (peak at 660 nm) and B (peak at 440 nm). | 17 ± 2/16.3 ± 2 °C (2013/2014), 58 ± 2/54 ± 2% RH (2013/2014), 380–400 µmol mol−1 CO2 concentration, 16 h photoperiod, and 200 µmol m–2 s–1 PPFD. | The highest fresh weight, dry mass, soluble sugars, total phenols content and radical scavenging activity were obtained under 90R/10B LED. HPS and 100% R LED showed the lowest ascorbic acid content. | (Wojciechowska et al., 2015) |
| Lactuca sativa L. ‘Sunmang’ | R/FR at the ratios of 0.7, 1.2, 4.1, and 8.6, R LED, and FL as control. | 20 °C, 60% RH, 12 h photoperiod, and 132 ± 7 µmol m–2 s–1 PPFD. | Biomass and leaf area under all R/FR LEDs were higher than those under the FL at 22 days of treatment. All R/FR treatments reduced SPAD values compared to FL. Due to the absence of B light, the photosynthetic rate in all treatments decreased. | (Lee et al., 2015) |
| Lactuca sativa L. ‘Sunmang’ and ‘Grand Rapid TBR’ | Six LED lighting treatments (R:B = 9:1, 8:2, and 7:3; R:G:B= 9:1:0, 8:1:1, and 7:1:2) were used under growth chamber. | 20 °C, 12 h photoperiod, and 173 ± 3 µmol m–2 s–1 PPFD. | R8G1B1 showed the highest fresh weights of ‘Sunmang’ lettuce shoots by 61% compared to R8B2. R7B3 showed the highest concentration of total phenolic and antioxidant capacity of ‘Sunmang’ lettuce. | (Son and Oh, 2015) |
| Lactuca sativa L.‘Cherokee’, ‘Magenta’, ‘Ruby Sky’, and ‘Vulcan’ | Seven LED lighting treatments e.g. Natural and day-extension light (7:11:33:49 B:G:R:FR, 4.5 µmol m–2 s–1) as control, natural light supplemented with HPS (70 µmol m–2 s–1), 100R:0B (100 µmol m–2 s–1), 50R:50B (100 µmol m–2 s–1), 0R:100B (25 µmol m–2 s–1), 0R:100B (50 µmol m–2 s–1), and 0R:100B (100 µmol m–2 s–1) were used at end-of-production (3, 5, 7, and 14 days). | 20/18 °C (day/night), and 16 h photoperiod. | 100:0, 0:100, or 50:50 R:B light enhanced red pigmentation of ‘Cherokee’, ‘Magenta’, ‘Ruby Sky’, and ‘Vulcan’ 5 days of end-of-production at PPFD of 100 µmol m–2 s–1, when DLI was low (< 10 mol m−2 d−1). | (Garrett Owen and Lopez, 2015) |
| Lactuca Sativa L. var. youmaicai | RB LED (R: B = 4:1), HPS, and cool W FLs. | 23 ± 1 °C, 50% RH, 24 h photoperiod, and 260 µmol m–2 s–1 PPFD. | The lowest and highest nitrate content were related to HPS/cool W FLs and RB LED, respectively. | (Liu et al., 2016) |
| Lactuca sativa L. cv. ‘Cos Lettuce’ | Irradiation starting time: The R LED irradiation starting time was simultaneous to B LED light or delayed 1, 4, or 7 h from the B LED light irradiation starting time. | 25 ± 1 °C, 14 h photoperiod, and 90 µmol m–2 s–1 PPFD. | Delay of 4 and 7 h of R LED irradiation starting time from B LED light irradiation was increased shoot fresh weight compared to simultaneous radiation of R and B. | (Jishi et al., 2016) |
| Lactuca sativa L. cv. ‘Astorga’ and cv. ‘Cervantes’ | Light intensities of 52 and 95 µmol m–2 s–1 provided by LEDs. | 28/18 °C (day/night), 80-85% RH, and 16/8 h photoperiod (day/night) | Growth rate was strongly related to photosynthetic response to intensity of light. LUE by LED light was 26% greater than conventional LEDs. | (Urrestarazu et al., 2016) |
| Lactuca sativa L. ‘Green Skirt’ | 100R:0B:0G, 90R:10B:0G, 80R:20B: 0G, 70R:30B:0G, 90R:0B:10G, 80R:10B:10G, 70R:20B:10G, and 60R:30B:10G. | 22/17 °C (day/night), 60% RH, and 16 h photoperiod, 1000 µmol mol−1 CO2 concentration, and 150 ± 15 µmol m–2 s–1 PPFD. | The highest leaf photosynthetic rate was related to 80R:20B:0G LED light. Adding the G light and the absence of B light significantly decreased leaf photosynthetic rate. | (Kang et al., 2016) |
| Lactuca sativa L. var. capitata | RB LEDs as a control (RB-CK, 12-h photoperiod), RB LEDs with preharvest continuous light (RB-CL), RBG LEDs with preharvest continuous light (RBG-CL), W LEDs as the second control (W-CK, 12-h photoperiod), and W LEDs with preharvest continuous light (W-CL). The ratios of RB and RBG LEDs were 4:1 and 4:1:1, respectively. | 25/20 ± 1 °C (day/night), 70% ± 5 RH, and 12 h photoperiod, 400 µmol mol−1 CO2 concentration, and 200 µmol m–2 s–1 PPFD. | RBG-CL had a greater effect on nitrate content reduction. RB LED light (R:B = 4:1) was more effective than W LED light in facilitating growth at the same PPFD. | (Bian et al., 2016) |
| Lactuca sativa L. cv. ‘Sunmang’ | RWB (7:1:2 ratio), RGB (7:1:2 ratio), RWB (7:2:1 ratio), RWB (8:1:1 ratio), RGB (8:1:1 ratio), RWB (8:2:0 ratio), and FL as control. | 20 °C, 60% RH, 12 h photoperiod, and 173 ± 5 µmol m–2 s–1 PPFD. | Adding G LED light to RB LED increased growth and the highest biomass was obtained under RGB with a ratio of 8:1:1 treatment. | (Son et al., 2016) |
| Lactuca sativa L. ‘Sunmang’ | FL as control, B+R/FR 0.7 (0.7, ratio of R to FR LED light), B+R/FR 1.2, B+R/FR 4.1, B+R/FR 8.6, B+R (8:2, ratio of B to R LED light). | 20 °C, 12 h photoperiod, and 130 ± 4 µmol m–2 s–1 PPFD. | Adding FR to R and B LED light increased shoot and root growth compared to B+R LED light and FL. The highest shoot fresh weight and leaf area on day 24 were obtained under B+R/FR 1.2. B+R/FR 0.7 and 1.2 had higher total phenolic levels, antioxidant activity, chlorogenic acid, and caffeic acid contents than the FL. | (Lee et al., 2016) |
| Lactuca sativa L. crispa ‘Bio Saradana’ | Pulse-cycled irradiations (0.5, 1, 1.3, 2.5, 5, 50, 500 Hz, and 1, 1.3, 2, 4, 10, 20 kHz frequencies at 50% duty ratio) and continuous light at PPFD of 200 µmol m−2 s−1. | 22 °C and 60% RH. | The similar ΦPSII and ETR of PSII were observed in wide range of frequency of pulsed light. Fv/Fm, Fv’/Fm’, and qP showed no significant difference between pulsed light and continuous light except at the 0.2 Hz frequency. Chlorophyll content did not change by flashing irradiation. | (Kanechi et al., 2016) |
| Lactuca sativa L. | Different R/B ratios: R, R/B = 12, R/B = 8, R/B = 4, R/B= 1, and B. | 24/20 °C (light/dark), 60% RH, 16 h photoperiod, 400 µmol mol−1 CO2 concentration, and 200 µmol m–2 s–1 PPFD. | Shoot dry weight increased with increasing R to B ratio. R/B = 12 showed the highest shoot dry weight. Amax and Pn increased with decreasing R/B ratio until 1. The lowest ΦPSII and the efficiency of excitation captured by open PSII center were related to B treatment. | (Wang et al., 2016) |
| Lactuca sativa L. var. crispa ‘Green Oak Leaf’ | W (135 µmol m–2 s–1) as control, W+FR, W+R, W+Y, W+G and W+B. For each two-color LED panel, the basal W PPFD was set at 105 µmol m–2 s–1 and PPFDs of Fr, R, Y, G and B LEDs were all set at 30 µmol m–2 s–1. | 22/18 °C (day/night), 65% RH, 16 h photoperiod, 350 µmol mol−1 CO2 concentration. | W+R and W+B increased fresh weight of shoots by 63.2% and 21.7% Compared with control, respectively. The lowest nitrate content was obtained under W+G and W+B. W+FR increased ascorbic acid and decreased biomass and pigment contents. | (Chen et al., 2016) |
| Lactuca sativa L. cv. ‘Great Lakes’ | B (445 nm), R (660 nm) LED light and W FL as the control light. | 22 °C, 70% RH, 1000 µmol mol–1 CO2 concentration, and 120 µmol m–2 s–1 PPFD. | Less and high curly leaves were observed under B and R, respectively. B and R LED light increased the lettuce growth compared to the W FL. | (Miyagi et al., 2017) |
| Lactuca sativa L. cv. ‘Grizzly’ | 100% R, 100% B, 70% R + 30% B, and 100% W. | 25 ± 2 °C, 65 ± 10% RH, 14 h photoperiod, and 300 µmol m–2 s–1 PPFD. | 100% B and 70% R + 30% B LEDs increased fresh mass per plant. Pigments concentrations and Vitamin C content were higher under 70% R + 30% B and 100% B, respectively. | (Amoozgar et al., 2017) |
| Lactuca sativa L. var. youmaicai | Wide spectrum of LED light and wide spectrum LED light lacking G. | 21 ± 2 °C, 35-50% RH, 16 h photoperiod, 350 ± 20 µmol mol−1 CO2 concentration, and 100 ± 5 µmol m–2 s–1 PPFD. | Wide spectrum LED light lacking G decreased biomass, photosynthetic pigment contents, and CO2 assimilation. | (Liu et al., 2017) |
| Lactuca sativa L. ‘Subyana F1’ | W, B and 5%W + 50%B provided by LED lamps. | 22 °C, 65-70% RH, 12 h photoperiod, and 70 µmol m–2 s–1 PPFD. | The highest net photosynthesis rate was observed under 5%W + 50%B LED light. | (Kleiber et al., 2017) |
| Lactuca sativa L. ‘Iceberg’ and ‘Butterhead’ | R/B LEDs (126 µmol m–2 s–1), R/B LEDs (170 µmol m–2 s–1), R/B/FR + W LEDs (266 µmol m–2 s–1), R/B LEDs + FL (201 µmol m–2 s–1), and R/B/FR LEDs (183 µmol m–2 s–1). | 24 °C and 16 h photoperiod. | R/B/FR + W LEDs (266 µmol m–2 s–1) showed the highest yield compared to other treatments. | (Anuar et al., 2017) |
| Lactuca sativa L. | W, G, R, and B LEDs and darkness as control during storage. | 5 ± 1 °C, 90-95% RH, and 10 µmol m–2 s–1 PPFD. | W LED light reduced chlorophyll degradation, and increased the total soluble solid content.G LED light reduced the decay rate.R LED light reduced weight loss. | (Kasim and Kasim, 2017) |
| Lactuca sativa L. | LED illumination (50%R:50%B) and sunlight as control. | 12 h photoperiod. | LED lighting increased leaf area, leaf/root relation, and overall height of plant compared to control. Number of leaf and root length were the same under two types of light. | (Horacio Marin et al., 2017) |
| Lactuca sativa L. cv. ‘Frill ice’, ‘Lvzhu’, and ‘Ziwei’ | FL (R:B ratio of 1.8) and LED (R:B ratio of 1.2) at two light intensities (250 and 350 µmol m–2 s–1) | 24/20 ± 1 °C (light/dark), 65/70 ± 10% RH (light/dark), 16 h photoperiod, and 800 ± 50 µmol mol−1 CO2 concentration. | The highest fresh weight was observed under FL. High light intensity reduced nitrate content. | (Khan et al., 2018) |
| Lactuca sativa L. cv. ‘Ziwei’ | FL (R:B ratio of 1.8), LED (R:B ratio of 1.2), and LED (R:B ratio of 2.2) at four light intensity (150, 200, 250 and 300 µmol m–2 s–1) and two photoperiods of 12 and 16 h. | 22/18 ± 1 °C (day/night), 65/70 ± 5% RH (day/night), and 800 ± 50 µmol mol−1 CO2 concentration. | LED (R:B ratio of 2.2), PPFD of 250 µmol m–2 s–1, and photoperiod of 16 h showed the best results in terms of quality and yield of lettuce. | (Zhang et al., 2018c) |
| Lactuca sativa L. ‘Cherokee’ and ‘Waldmann’s Green’ | Adding three pre-dawn lighting (B, R, and broadband W LEDs light, each providing 50 ± 2 µmol m−2 s−1) and three end-of-day lighting (B, R, and broadband W LEDs light, each providing 50 ± 2 µmol m−2 s−1) to W LED (R:O:B). W LED at PPFD of 200 ± 2 as control. | 20/21 °C (day/night), 60-80% RH, 16 h photoperiod, and 405 µmol mol−1 CO2 concentration. | Plants exposed to end-of-day-B produced up to 18% more fresh weight of shoot compared to those grown under end-of-day-R, pre-dawn-R or control. 1 h of low intensity end-of-day-blue was more useful for improving lettuce growth than other treatments when DLI from monochromatic-source lighting is provided by broadband W LEDs. | (Chinchilla et al., 2018) |
| Lactuca sativa L. var. crispa ‘Green Oak Leaf’ | L/D (1) providing continuous RB LED (9:1 ratio) with one light/dark cycle (16 h/8 h) as control. L/D (2) with two L/D cycles (8 h/4 h during a 24-h period), L/D (3) with three L/D cycles (6 h/3 h or 4h/2h during a 24-h period), L/D (4) with four L/D cycles (4 h/2 h during a 24-h period), L/D (6) with six L/D cycles (3 h/1.5 h or 1 h/0.5 h during a 24-h period), and L/D (7) with eight L/D cycles (2 h/1 h during a 24-h period). | 23 °C, 70 RH, 16 h photoperiod, and 700 µmol mol−1 CO2 concentration. 200 µmol m–2 s–1 PPFD. | The highest shoot biomass was observed under L/D (2), L/D (4), L/D (6), and L/D (7) treatments. Higher soluble sugar content and lower contents of crude fiber and starch were found in plants exposed to L/D (2) and L/D (3) compared other treatments. | (Chen and Yang, 2018) |
| Lactuca sativa L. ‘Rex’ and ‘Rouxai’ | B60R120, B40G20R120, B20G40R120, G60R120, B40R120FR20, B20R120FR40, R120FR60, B20G20R120FR20, warm W180, and equalized-W180. Subscripts indicate the intensity of light (µmol m–2 s–1). | 20-21.2 °C, 44-58% RH, and 379-402 µmol mol−1 CO2 concentration. | Replacing G or FR radiation for B radiation increased leaf expansion and shoot mass but reduced chlorophyll concentrations. Replacing G or FR radiation for B radiation also triggers shade-avoidance responses. | (Meng et al., 2019) |
| Lactuca sativa L. ‘Red Salad Bowl’ | DLI treatments: 1.6, 2.2, 3.7, and 6.5 mol m–2 d–1 (37, 52, 85, and 150 µmol m–2 s–1 for 12 h d–1 from 0600 to 1800h) provided by 4000LED lamps (UV:B:G:R:FR). | 22.8 ± 1.9/21.6 ± 1 °C (day/night), 70% RH, 16 h photoperiod, 400 ppm CO2 concentration. | With increasing DLI, the biomass, pigment content, and anthocyanin concentration of lettuce increased and nitrate content decreased. DLI of 6.5 to 9.7 mol m–2 d–1 is recommended for lettuce growth under indoor gardening systems. | (Paz et al., 2019) |
| Lactuca sativa L. cv. ‘Yidali’ | Low light (3R:1B, 100 μmol m−2 s−1), medium light (3R:1B, 200 μmol m−2 s−1), and high light (3R:1B, 300 μmol m−2 s−1). | 23 ± 3 °C, 50-60% RH, and 24 h photoperiod. | The highest lettuce biomass was observed under higher light intensity. High light intensity enhanced the reactive oxygen species production and the lipid peroxidation degree. Among all the studied enzymes, the maximal and minimal responses to light intensity were observed in ascorbate peroxidase and dehydroascorbate reductase, respectively. | (Zha et al., 2019) |
| Lactuca sativa L. ‘Rouxai’ | Warm W180, R180, B60R120, B20G60R100, B20R100FR60, and B180. Subscripts indicate the intensity of light (µmol m–2 s–1). | 22.5 ± 1 °C, 44 ± 8% RH, 20 h photoperiod and 392 ± 31 µmol mol−1 CO2 concentration. | Increasing B radiation (from 0 to 100%) in static treatments increased foliage redness and reduced fresh and dry weights of shoot. B20R100FR60 increased shoot fresh weight compared to B60R120. Substituting G60 for R radiation decreased shoot weight by 19% on day 25 under the B20 background. | (Meng and Runkle, 2020) |
| Lactuca sativa L. cv. ‘Zishan’ | High light intensity radiation (500 µmol m–2 s–1) e.g. 0.5 h, 1 h, 2 h, and 4 h in the middle of light period (16 h, 150 µmol m–2 s–1, R: B ratio of 4:1) and one control (150 µmol m–2 s–1). | 23 ± 3 °C, 40-50% RH, and 16 h photoperiod. | The highest biomass and secondary metabolite accumulation were found under short-term High light intensity. However, 4 h high light caused photo-oxidative damage to lettuce. | (Shao et al., 2020) |
| Lactuca sativa L. ‘Red Salad Bowl’ | W (cool, neutral, and warm), RB combinations (10, 20, and 30% B), and RBG combinations (10 and 20% B with 20 or 10% G, respectively). Light intensity (200 and 500 µmol m−2 s−1) | 23/20 °C (day/night), 16 h photoperiod, and 450-500 µmol mol−1 CO2 concentration. | G light had no effect on lettuce growth. B LED light at the lowest percentage (10%) showed the highest dry mass of lettuce. | (Kusuma et al., 2021) |
| Lactuca sativa L. ‘Elizium’ | RGB LED lights (80:10:10, 70:10:20, 60:10:30, and 70:18:12) | 20/18 °C (day/night), 65% RH, 16 h photoperiod, and 160 µmol m–2 s–1 PPFD. | The highest leaf biomass and inhibited K and Mg accumulation in leaves were found under RGB ratio of 70:18:12. The most damage of tipburn was observed under RGB ratios of 70:10:20 and 70:18:12. | (Matysiak et al., 2021) |
| Lactuca sativa L. cv. ‘Yidali’ | Plant exposed to normal light (0 µmol m–2 s–1), low light (20 µmol m–2 s–1), and continuous light (200 µmol m–2 s–1) during the dark period (22 pm–6 am) provided by RB LED light (3R:1B, 200 µmol m–2 s–1, 16/8 h photoperiod). | 23 ± 1/20 ± 1 °C (day/night), 50-60% RH, and 400 µmol mol−1 CO2 concentration. | Continuous light (for 9 days) increased the biomass, dry matter content, and specific leaf weight of lettuce compared to the normal light. The highest activities of L-galactono-1,4-lactone dehydrogenase, APX, monode- hydroascorbate reductase, and glutathione reductase were found under continuous light. | (Wen et al., 2021) |
| Lactuca sativa L. cv. ‘Aleppo’ ‘Ferrado’‘Prunai’‘Quintus’ and ‘Skyphos’ | W LED light (170 µmol m−2 s−1), W + R (230 µmol m−2 s−1), and W + B (230 µmol m−2 s−1). | 18/15 °C (day/night), 65–70% RH, and 16/8 h photoperiod (day/night). | The B light increased the growth of the leaf area of plants. R light positively and negatively affected number and length of leaves, respectively, in red and dark-green cultivars. B and R lights enhanced the relative chlorophyll content in the dark-green cultivar. | (Frąszczak and Kula-Maximenko, 2021) |
| Lactuca sativa L. cv. ‘Romana’ ‘Angel’ and ‘Mantecosa’ | W LED light (150 µmol m−2 s−1), longer wavelength of B LED light (141 µmol m−2 s−1), and longer wavelength of R LED light (107 µmol m−2 s−1). | 20 °C, 80–85% RH, and 16/8 h photoperiod (day/night). | Longer wavelength of B and R lights, especially R light, had a positive effect on the quality and growth of lettuce cultivars. | (Ferrón-Carrillo et al., 2021) |
| Lactuca sativa L. ‘Defender’ | Continuous light (15B:10G:75R:10FR) and pulsed lighting (0.5, 1 kHz; 50% duty ratio, V:I:B:G:R:F:R). V and I indicate violet and indigo lights, respectively. | 21 ± 2/18 ± 2 °C (day/night), 50–60% RH, and 16 h photoperiod, 450 µmol mol−1 CO2 concentration, and 250 µmol m−2 s−1 PPFD. | Increase in fresh and dry biomass (~32 and 36%, respectively) and leaf area (~48%) was observed under pulsed irradiation. The higher photosynthetic rate, chlorophyll a and b as well as carotenoid concentration were found under both pulsed light of 0.5 and 1 kHz. | (Miliauskienė et al., 2021) |
| Lactuca sativa L.'Genlina' | Switching frequency for the B LED light (293 and 850 kHz) and FL. Switching frequency for the R LED was similar. | 24 ± 2 °C, 55–70% RH, and 16 h photoperiod, RB ratio of 3, and 215 µmol m−2 s−1 PPFD. | Lettuce morphological parameters were not affected by switching frequency. Compared to switching frequency of 850 kHz, switching frequency of 293 kHz improved the EUE by 40%. | (Carotti et al., 2021) |
| Lactuca sativa L.cv. ‘Tiberius’ | W LED and RB (4R:1B) under light intensities of 150, 200, and 250 μmol m−2 s−1 | 23 ± 2 °C, 60% RH. | W LED significantly decreased the secretion of autotoxins under light intensity of 250 μmol m−2 s−1. Biomass and photosynthetic rate of lettuce increased with increasing light intensity. | (Zhou et al., 2022a) |
| Lactuca sativa cv. ‘Tiberius’ | Three light intensities (100, 200, and 300 μmol m−2 s−1) and three air velocities (0.25, 0.50, and 0.75 m s−1). | 20 °C, 60–70% RH, 16 h photoperiod, 1000 ± 50 µmol mol−1 CO2 concentration. | The photosynthetic rate and fresh weight, and stomatal conductance were highly correlated with light intensity. The boundary layer conductance was strongly correlated with air velocity. The highest occurrence of tipburn was obtained under light intensity of 300 μmol m−2 s−1 and air velocity of 0.25 m s−1. Tipburn decreased significantly with increasing air velocity. | (Ahmed et al., 2022a) |
| Lactuca sativa L.‘Blackhawk’ | 80%R:20%B at three different light intensities (130, 259, and 389 µmol m–2 s–1). | 23/18 °C (day/night), 70/80% RH. | High light intensity (389 µmol m–2 s–1) showed the highest biomass, net photosynthesis, electron transport rate, and content of anthocyanins and carotenoids. | (Modarelli et al., 2022) |
| Lactuca sativa var. capitata ‘Iceberg’ | 14, 16 and 18 h supplementary photoperiod with combination of R and B light (70:30), light intensity of 70 µmol m–2 s–1 | 11.8-25 °C, 66.3–88.3% RH. | Yield, L-ascorbic acid content, firmness, total phenolic content, DPPH radical scavenging activity, and total soluble solids increased significantly with 18 h of supplementary photoperiod. | (Kaur et al., 2022) |
| Lactuca sativa L.‘Coastal Star’, ‘Muir’, ‘Green Butter’, and ‘Rouge d’Hiver’ | LED light compared to FL lamp | 25 °C, 60% RH. | LED light enhanced lettuce quality by increasing photosynthetic activity and reducing stem elongation. | (Nguyen et al., 2022) |
| Lactuca sativa L.‘Azart’ and ‘Lollo Rossa’ | Pulsed, scanning and constant modes of LED irradiation | 25/22 °C (day/night), 75% RH | The impulse illumination mode increased biomass accumulation and anthocyanins synthesis in red leaf lettuce (‘Lollo Rossa’). | (Smirnov et al., 2022) |
| Lactuca sativa L. ‘Expertise’ | PPFD was increased by 16 µmol m−2 s−1 (from 140 to 300 µmol m−2 s−1 from day 0 to 30), decreased (from 300 to 140 µmol m−2 s−1), or kept constant (221 µmol m−2 s−1) | 22.0 ± 0.01 °C, 74.6 ± 0.7% RH, 752 ± 28 µmol mol−1 CO2 | A progressive increase in PPFD positively influenced the accumulation of dry weight and enhanced the dry matter content; however, it did not have a significant impact on shelf life. | (Jin et al., 2023b) |
| Lactuca sativa L. ‘Crunchy’ and ‘Deangelia’ | 120, 180, 240, and 300 µmol m−2 s−1 | 22 ± 2 °C, 60–90% RH, 400 µmol mol−1 CO2, nutrient solution (pH 5.8–6.5 and EC 1.7 mS cm−1) | Light intensities of 240 and 180 µmol m−2 s−1 were found to be optimal for Crunchy and Deangelia, respectively, promoting balanced growth and morphogenesis. | (Miao et al., 2023) |
| Lactuca sativa L.(Enza Zaden Brand) | W LED light (115 µmol m−2 s−1), W + R (102 µmol m−2 s−1), W + B (110 µmol m−2 s−1), and W + G (95 µmol m−2 s−1) | 22-25 °C | There were no significant differences between treatments in terms of biomass and fiber content. W+B-treated lettuce had the highest phenolic content and antioxidant capacity | (Sawatdee et al., 2023) |
| Lactuca sativa L. crispa | 87.5%R:12.5%B and control | Biometric parameters (leaf area, root length, shoot dry weight, and root dry weight) and biochemical parameters (phenolic compounds, flavonoids, chlorophyll a/b ratio, and total chlorophyll) of lettuce seedlings exposed to supplementary light showed an increase compared to the control. | (Rizzon et al., 2024) | |
| Lactuca sativa L. cv. ‘Spanish Green’ | 20%R:80%B, 30%R:70%B, 40%R:60%B, 50%R:50%B, 60% R:40% B, 70%R:30%B, 80%R:20%B, 90%R:10%B | 18 ± 2 °C, 70 ± 5% RH, PPFD of 200 ± 20 µmol m–2 s–1 | The results indicate that growing lettuce with the 80%R:20%B ratio produces the optimal outcomes. | (Luo et al., 2024) |
| Lactuca sativa L. ‘Ideal-205’ | 180, 210, and 240 µmol m−2 s−1 | 25/22 ± 2 °C (light/dark), 85% RH | The levels of total soluble sugar, vitamin C, nitrate, and free amino acids in lettuce increased more significantly under the 210 µmol m−2 s−1 condition. | (Dai et al., 2024) |
| Lactuca sativa L.‘Green Forest’ and ‘Ruby’ | 0, 4, and 8 h of daily supplemental light applied after sunset | 20–25 °C (night/day), 75-80% RH | A light supplementation duration of 4 hours post-sunset is deemed optimal for the ‘Ruby’ lettuce, whereas an 8-hour supplementation period is considered ideal for the ‘Green Forest’ lettuce during the winter growing season. | (Ayankojo et al., 2025) |
| Lactuca sativa L.‘Lollo Bionda’ | (R, peak at 656 nm), R/B (3:1 ratio, peak at 656 nm), B (peak at 450 nm), and W (400–700 nm) | 75 ± 5% RH, 0.3–0.5 m s−1 airflow | The findings highlight the potential of LED technology to enhance germination efficiency and improve seedling quality in controlled environment agriculture. | (Soufi et al., 2025) |
| Lactuca sativa L. cv. ‘Spanish green’ and ‘Butterhead’ | Photoperiod was set at (R/B = 4, 16 h/day) and light intensity ranging from 125 to 300 µmol m−2 s−1. The | 23 ± 2 °C, 70 ± 5% RH, nutrient solution (EC: 2.6 mS/cm) | ‘Butterhead’ lettuce needs 300 µmol m−2 s−1 light for maximum yield. ‘Spanish green’ lettuce thrives at 200 µmol m−2 s−1, with only a 2.11% yield loss, making it more energy-efficient. | (Zou et al., 2025) |
3 Ambient temperature
Temperature is one of the most influential environmental factors regulating plant growth and development and can be precisely controlled in controlled-environment agriculture. In lettuce, temperature strongly affects growth rate, biomass accumulation, and the incidence of physiological disorders such as tipburn. Dynamic temperature management has been shown to be particularly effective. For instance, Kumazaki (2022) reported that a rapid increase in temperature to 23 °C during the first half of the light period followed by a gradual decrease during the second half enhanced lettuce growth while reducing tipburn incidence. Similarly, increasing air temperature from 25 to 30 °C increased lettuce biomass and leaf dry weight by 43% and 40%, respectively (Sublett et al., 2018b), while Kong et al. (2023) observed increases of 30% in fresh weight and 53% in dry weight when temperature was raised from 21 to 30 °C.
Temperature and light act synergistically rather than independently in regulating photosynthesis and growth, and recent studies have increasingly examined their combined effects (Lee et al., 2019; Chen et al., 2021c; Noh and Jeong, 2021). Zhou et al. (2022b) evaluated lettuce photosynthesis under a wide range of day/night temperatures (15/10, 23/18, and 30/23 °C) and light intensities (100–600 µmol m−2 s−1) and found that 23/18 °C consistently produced the highest yield, photosynthetic rate, chlorophyll content, and light saturation point across all light levels. Under this optimal temperature regime, increasing light intensity further enhanced photosynthetic capacity and yield. Comparable trends were reported by Chen et al. (2021c). Likewise, Okazaki and Yamashita (2019) showed that maximum folate biosynthesis and lettuce growth occurred at 25 °C combined with an RGB light intensity of 200 µmol m−2 s−1. However, the optimal temperature–light combination is not universal and depends strongly on cultivar characteristics and production objectives. While moderate to warm temperatures often favor biomass accumulation. Tarr and Lopez (2025) demonstrated that maximizing shoot fresh mass while minimizing energy consumption required cultivar-specific light intensities (150 and 300 µmol m−2 s−1 for ‘Rex’ and ‘Rouxaï RZ’, respectively) under mean daily temperatures of 23–26 °C. In contrast, Lee et al. (2019) reported that high light intensity combined with relatively low temperatures favored proper head formation in crisphead lettuce. Furthermore, Mary Grace et al. (2018) observed no significant difference in productivity between 18 and 25 °C, while Al-Said et al. (2018) found a negative relationship between temperature and shoot growth, noting that higher temperatures (25 °C) promoted bolting and inhibited head formation compared with cooler conditions (13 °C). These conflicting findings emphasize the importance of production objectives, developmental stage, and cultivar type when defining optimal temperature ranges.
Temperature also interacts with photoperiod; however, evidence indicates that air temperature plays a more dominant role in determining the growth rate of lettuce. Noh and Jeong (2021) indicated that a temperature regime of 25/18 °C combined with a 14-h photoperiod accelerated growth, whereas cooler temperatures (20/15 °C) with a longer photoperiod (18 h) delayed growth in two lettuce cultivars. It can be inferred that the effects of photoperiod may be secondary to temperature in regulating growth dynamics.
Another important, though relatively less studied, factor affecting lettuce growth and physiology is RZT. Levine et al. (2023) demonstrated maximum root and shoot dry weights at a RZT of 25 °C, while higher RZTs (35 °C) reduced growth but increased photosynthetic pigment concentrations. Similarly, Moccio et al. (2024) reported that chilled nutrient solution (21.1 °C) did not enhance shoot fresh weight relative to ambient RZTs but enhanced leaf mineral accumulation in several cultivars of lettuce. In greenhouse production systems, effective temperature control strategies are required for keeping productivity and quality. Kumsong et al. (2023) indicated that combining fogging with ventilation fans effectively moderated greenhouse air temperature, resulting in improved growth, yield, and sensory quality of green romaine lettuce. Taking everything into account, synthesis of the available literature suggests that day/night temperatures of approximately 23–25/18–20 °C are commonly optimal for lettuce growth in CEA, particularly when coordinated with suitable light intensity and photoperiod management.
4 Relative air humidity
RH is a critical environmental factor influencing plant growth and physiology in CEA (Vanegas-Ayala et al., 2022). Even though humidity control has long been recognized as important for optimizing crop performance, its precise manipulation to simultaneously enhance both growth and quality remains inadequately understood (Amitrano et al., 2021; Inoue et al., 2021). Hence, VPD is increasingly preferred over RH alone as a more physiologically relevant descriptor, as it directly reflects the evaporative demand between the leaf surface and the surrounding air (Zhang et al., 2018a). VPD is determined by the combined effects of air temperature and relative humidity (Inoue et al., 2021), and therefore changes in VPD should be interpreted as the result of their interaction rather than variations in relative humidity alone. In greenhouses, air temperature frequently exceeds 30 °C during midday on sunny winter days, particularly in Asian regions, requiring roof ventilation to reduce heat accumulation. This ventilation increases air exchange with the cooler and drier outdoor environment, which, together with elevated internal temperatures, leads to higher VPD (Inoue et al., 2021). In controlled environment agriculture systems, high temperatures further enhance VPD and transpiration rates; however, when increased transpiration raises internal humidity, the resulting VPD may help alleviate plant drought stress while reducing the need for energy-intensive dehumidification (Lauwers et al., 2025).
Under conditions of high RH (low VPD), stomata tend to remain open, facilitating greater carbon assimilation and promoting biomass accumulation (Amitrano et al., 2019). In accordance with this, low to moderate VPD levels (approximately 0.2–2 kPa) have been associated with enhanced leaf gas exchange and improved physiological WUE in lettuce plants (Zhang et al., 2018a). However, excessively high RH (> 90%), which corresponds to very low VPD, can limit transpiration and impair the uptake of essential mineral nutrients such as Ca, magnesium (Mg), and K. This reduced mineral transport increases the risk of physiological disorders, particularly tipburn in lettuce (Amitrano et al., 2021). On the contrary, low RH (high VPD) imposes water stress, resulting in partial or complete stomatal closure to decrease transpiration. This reaction restricts CO2 diffusion into the leaf, thus reducing photosynthetic activity and final growth (Ahmed et al., 2020b).
Research findings support this trade-off between growth and quality of lettuce under different VPD conditions. Amitrano et al. (2021) compared lettuce responses under low (0.69 kPa) and high (1.76 kPa) VPD and reported increased biomass production and higher maximum quantum efficiency of PSII (Fv/Fm) ratios under low VPD, whereas higher VPD improved phytochemical characteristics. This implies that moderate atmospheric stress may improve lettuce quality at the expense of maximal biomass accumulation. Likewise, Inoue et al. (2021) demonstrated that abrupt VPD fluctuations significantly reduced stomatal conductance and CO2 assimilation, while moderate VPD fluctuations stimulated leaf expansion, improved CO2 diffusion efficiency, and ultimately enhanced lettuce growth. More recently, Lauwers et al. (2025) reported that very high RH (VPD ≤ 0.5 kPa) physically restricted transpiration, resulting in a decoupling between transpiration and photosynthesis in lettuce plants.
Despite the recognized importance of RH management, recent studies focusing specifically on lettuce remain limited. Evidence from related leafy vegetables suggests that RH levels must be carefully managed to avoid physiological disorders; for example, RH of 40–60% increased tipburn incidence in endive (Bautista et al., 2020), whereas a higher RH of approximately 85% was reported to be optimal for kale growth (Chowdhury et al., 2021). These findings indicate that optimal RH ranges are crop-specific and closely linked to differences in transpiration sensitivity and nutrient transport. Overall, current evidence indicates that moderate RH corresponding to an intermediate VPD range is most suitable for lettuce cultivation in soilless and controlled systems, as it balances biomass production, photosynthetic efficiency, and product quality while minimizing physiological disorders. Figure 2 shows the RH ranges used as experimental levels in nutrient solutions across 100 randomly selected recent articles on soilless lettuce cultivation. The ranges reflect the actual values applied in the experiments, independent of any recommendations for optimal RH management.
5 CO2 concentration
The optimal CO2 concentration for crop growth is generally considered to be approximately three times the ambient atmospheric level (Li et al., 2018). Because current atmospheric CO2 concentrations are insufficient to saturate photosynthesis, enrichment of the growing environment is an effective strategy to stimulate carbon assimilation, particularly in C3 species such as lettuce (Bisbis et al., 2018). Consistent with this, a meta-analysis by Dong et al. (2018) demonstrated that elevated CO2 significantly increased soluble sugar accumulation (27.1%), antioxidant capacity (82.0%), and ascorbic acid content (7.1%) in lettuce, while reducing protein concentration by 5.6%. Their analysis further indicated that CO2 enrichment had no significant effect on phenolic acids, flavonoids, or pigment concentrations. Despite these general trends, responses to CO2 enrichment are not always consistent across studies. Velasquez-Vasconez et al. (2021) observed no significant improvement in lettuce growth or protein content under elevated CO2 (1000 µmol mol−1), whereas positive effects on growth and nutritional quality were reported by Garmendia et al. (2020) and Chen et al. (2023a). Similarly, Song et al. (2023) showed that increasing CO2 concentration from 400 to 800 µmol mol−1 significantly enhanced lettuce growth, vitamin C concentration, chlorophyll content, and photosynthetic rate, while reducing nitrate concentration. Singh et al. (2020) also reported increases of 24.7% and 21.4% in fresh and dry weights, respectively, in lettuce ‘Auvona’ exposed to CO2 enrichment (800 vs. 410 µmol mol−1), although this was accompanied by decreases in chlorophyll and mineral nutrient concentrations (N, P, and Mg), likely due to a dilution effect associated with increased biomass. Increasing CO2 concentration enhances the rate of photosynthesis and carbohydrate production by improving carbon fixation by RuBisCo (ribulose-1,5-bisphosphate carboxylase/oxygenase) and reducing photorespiration in C3 plants (Jin et al., 2023a; Wei et al., 2024). Consequently, biomass accumulation and plant growth increase significantly, provided that water and nutrient availability are not limiting factors. Elevated CO2 also increases ascorbic acid (vitamin C) accumulation by upregulating key genes involved in the Smirnoff–Wheeler biosynthetic pathway. Enhanced expression of enzymes responsible for GDP-D-mannose and GDP-L-galactose formation, culminating in L-galactono-1,4-lactone dehydrogenase activity, promotes ascorbic acid biosynthesis and leads to higher ascorbic acid levels in vegetables (Muthusamy et al., 2019). In addition, elevated CO2 reduces nitrate accumulation in vegetables primarily through a biomass dilution effect associated with enhanced growth, as well as through increased nitrate assimilation driven by greater carbon and energy availability. Partial stomatal closure under elevated CO2 further decreases transpiration-mediated nitrate uptake, contributing to reduced nitrate storage in plant tissues (Dong et al., 2018).
Across developmental stages and cultivars, CO2 concentrations around 800 µmol mol−1 appear particularly effective. Sheibani et al. (2024) reported the highest productivity at this concentration during both small and standard baby lettuce stages, a finding corroborated for ‘Rex’ and ‘Rouxaï RZ’ cultivars by Tarr and Lopez (2025). Nutrient responses to elevated CO2 levels, however, remain complex. Wheeler et al. (2024) reported that CO2 enrichment increased K concentrations in most lettuce cultivars but reduced Mg levels, showing that mineral nutrition responses may be element-specific and cultivar-dependent.
Cultivar type also strongly influences secondary metabolite responses to CO2 enrichment. Studies by Pérez-López et al. (2018) and Sgherri et al. (2017) indicated that red lettuce cultivars generally respond more strongly than green cultivars in terms of accumulation of phenolic compound. Also, Pérez-López et al. (2018) reported that elevated CO2 (700 vs. 400 µmol mol−1) increased total free phenolic acids by 46% in anthocyanic-red lettuce and by 38% in acyanic-green cultivars. From a sustainability viewpoint, most CO2 enrichment systems rely on fossil fuel–derived CO2 (Li et al., 2018); however alternative approaches have recently begun to emerge. Jung and Son (2021) demonstrated that CO2 emitted from mushroom cultivation could be effectively utilized in mixed lettuce–mushroom systems, increasing lettuce shoot dry weight by 18.1% while reducing CO2 emissions by 80.6% compared with mushroom-only cultivation. This integrated approach underscores the potential of CO2 recycling strategies to enhance crop productivity while mitigating environmental impact.
Given that CO2 and light cooperatively regulate photosynthesis, recent studies have increasingly focused on their interaction. Esmaili et al. (2021) showed that lettuce growth parameters enhanced significantly as CO2 concentration increased from 400 to 1200 µmol mol−1 in combination with light intensities from 75 to 300 µmol m−2 s−1. Similarly, Ahmed et al. (2022b) reported that increases in CO2 concentration (500 to 1500 µmol mol−1) and light intensity (100 to 300 µmol m−2 s−1) independently and synergistically enhanced lettuce fresh and dry weights. Improved LUE under elevated CO2 (800 ± 50 and 1600 ± 50 µmol mol−1) was also reported by Chen et al. (2021a). In addition to light intensity, light quality interacts with CO2 enrichment. Shimomura et al. (2020) and Endo et al. (2022) demonstrated that B light combined with high CO2 concentration (1000 vs. 400 µmol mol−1) effectively promoted the accumulation of chlorogenic acid, a key antioxidant compound in lettuce.
Beyond ambient enrichment, root-zone CO2 has also emerged as an effective approach to stimulate lettuce growth. Leibar-Porcel et al. (2020a) reported that elevating root-zone CO2 concentration to 1500 µmol mol−1 increased shoot fresh and dry weights by approximately 22%, although root dry weight was unaffected. In a similar study, the application of bicarbonate (HCO3-, 1–5 mM) in hydroponic systems increased shoot biomass and leaf area by up to 10% (Leibar-Porcel et al., 2020b). Additional approaches, such as the application of carbon dots, have also been shown to improve lettuce photosynthesis by enhancing chlorophyll content, chloroplast activity, PSII performance, and Rubisco activity (Hu et al., 2022).
In general, available evidence indicates that CO2 enrichment substantially enhances lettuce biomass accumulation and antioxidant capacity, although responses of mineral nutrients and pigments can vary depending on cultivar, growth stage, and environmental context. Importantly, CO2 effects cannot be considered in isolation and must be evaluated in conjunction with light intensity, light quality, temperature, and air movement. Based on the reviewed literature, ambient CO2 concentrations of approximately 800–1200 µmol mol−1 and root-zone CO2 concentrations around 1500 µmol mol−1 can be recommended to promote growth and nutritional quality of lettuce in controlled cultivation systems. A schematic overview summarizing the effects of elevated CO2 on lettuce growth and nutritional attributes is presented in Figure 3.
6 Air velocity
Air velocity is an important yet often underestimated environmental factor influencing plant growth, physiology, and product quality in CEA (Kozai et al., 2018). Insufficient air distribution or non-uniform airflow over the crop canopy can destabilize production rates by altering microclimatic conditions surrounding the leaves (Naranjani et al., 2022). From a physiological perspective, air movement plays a dual role: it enhances plant growth by improving gas exchange and transpiration, and it contributes to plant health by facilitating nutrient transport through the transpiration stream (Peiro et al., 2020). In lettuce, insufficient air velocity is closely correlated with the occurrence of tipburn, a physiological disorder that significantly reduces market quality. Tipburn typically arises from the formation of a stagnant boundary layer around inner and new developing leaves, which restricts transpiration and limits Ca transport (Ahmed et al., 2020a). Acceleration of air velocity reduces boundary layer resistance, enhances transpiration, and improves Ca distribution within the plant. Yu et al. (2024) also obtained similar findings, indicating that airflow (300 m3 h−1), by increasing transpiration and Ca ion transport, reduces tipburn in lettuce.
Beyond disorder prevention, moderate air velocity promotes uniform growth and photosynthetic performance. Multiple investigations point to an optimal airflow range of approximately 0.3–0.5 m s−1 for lettuce, which improves canopy uniformity and biomass accumulation (Peiro et al., 2020; Xu et al., 2021). This range promotes photosynthesis by facilitating efficient exchange of CO2 and water vapor between stomatal cavities and the surrounding air (Naranjani et al., 2022). Nevertheless, excessive air velocity can be detrimental; Peiro et al. (2020) reported that air velocities exceeding 0.6 m s−1 suppressed lettuce growth, indicating that airflow must be cautiously regulated.
Recent investigations further highlight that slightly higher air velocities may be beneficial under specific conditions. Zhang et al. (2025) reported that an air velocity of 0.8 m s−1 increased photosynthetic and transpiration rates by 17.8% and 21.7%, respectively, and improved fresh and dry biomass of ‘Tiberius’ lettuce by approximately 30%. Furthermore, Ahmed et al. (2022a) found that under high light intensity (300 µmol m−2 s−1), increasing air velocity from 0.25 to 0.75 m s−1 reduced tipburn incidence by 87.3%, effectively offsetting the negative effects of high irradiance on lettuce quality. In agreement with these results, Ferrarezi et al. (2024) recommended air velocities between 0.4 and 0.7 m s−1 for ‘Rex’ lettuce cultivated in indoor farming systems.
Airflow direction is also important in determining its effectiveness. Vertical airflow has been shown to reduce boundary layer thickness more efficiently than horizontal airflow, thereby enhancing water vapor diffusion from leaf surfaces (Kozai et al., 2018). Conversely, horizontal airflow may be more effective for CO2 distribution within the canopy, resulting in improved photosynthesis and growth (Ahmed et al., 2020a; Naranjani et al., 2022). These contrasting effects suggest that combining airflow directions or optimizing airflow patterns could further improve plant performance. Briefly, current evidence indicates that maintaining air velocity in the range of approximately 0.3–0.7 m s−1 is effective for disrupting the leaf boundary layer, enhancing CO2 and O2 exchange, promoting transpiration-driven nutrient transport, and reducing tipburn incidence in lettuce grown under controlled environments. Future research should focus on refining optimal airflow velocity and direction across different cultivation systems, including indoor farms and greenhouses. The key physiological processes influenced by air velocity in lettuce are summarized in Figure 4.
7 Conclusions and future perspectives
This review synthesized recent literature on the application of LED lighting (2012–2025) and other key environmental factors (2017–2025) in lettuce cultivation under controlled environments. The effects of LED light intensity, spectral quality, photoperiod, and light direction were critically evaluated alongside temperature, relative humidity, CO2 concentration, and air velocity.
Overall, the evidence indicates that combining red (R) and blue (B) light in proportions of approximately 75–80% R and 20–25% B, delivered at a light intensity of 250–300 µmol m−2 s−1 and a photoperiod of 16–20 h (adjusted according to light intensity), effectively enhances lettuce growth, photosynthetic performance, and product quality in controlled systems. Supplementation with white light generally increases biomass accumulation, while far-red light improves both biomass production and light use efficiency. Yellow light has shown potential to enhance biomass and antioxidant properties, whereas green light is particularly beneficial for microgreens and lettuce sprouts due to its pronounced effect on hypocotyl elongation. For several other spectral regions, however, available evidence remains insufficient, highlighting the need for further targeted research before firm recommendations can be made.
Beyond lighting, environmental optimization plays a decisive role in maximizing lettuce productivity. Air temperature regimes of approximately 23–25/18–20 °C (day/night) consistently promote growth and photosynthesis, while relative humidity in the range of 60–70%, corresponding approximately to a moderate VPD of ~0.8–1.3 kPa under typical lettuce-growing temperatures, supports efficient gas exchange and nutrient uptake while minimizing physiological disorders such as tipburn. With respect to carbon management, CO2 enrichment has proven effective both in the aerial environment and the root zone, with recommended concentrations of approximately 800–1000 µmol mol−1 in the growing ambient and around 1500 µmol mol−1 in the root zone under controlled conditions. Appropriate air velocity, typically in the range of 0.3–0.7 m s−1, is also essential for disrupting the leaf boundary layer, enhancing gas exchange and transpiration, and minimizing tipburn, thereby improving photosynthetic efficiency and market quality.
Taken together, these findings demonstrate that precise and coordinated control of environmental factors, particularly maintaining a balanced light–temperature–VPD–CO2–airflow regime, is fundamental to the successful and sustainable production of lettuce in controlled environments. Future research should move beyond single-factor optimization and increasingly focus on multifactor interactions, especially among light intensity and quality, air temperature, VPD, CO2 concentration, and air movement. In addition, cultivar-specific responses, developmental stage–dependent requirements, energy-use efficiency, and environmentally sustainable strategies—such as CO2 recycling and adaptive climate control—should be prioritized. Addressing these gaps will support the development of more resilient, resource-efficient, and high-quality lettuce production systems and contribute to the broader advancement of controlled-environment agriculture.
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Footnotes
Edited by: Xiao Yang, Chinese Academy of Agricultural Sciences, China
Reviewed by: Annalsia Somma, University of Bari Aldo Moro, Italy
Cristian Hernández, Universidad de O’Higgins, Chile
Statements
- Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The author SN declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.
- Funding
The author(s) declared that financial support was received for this work and/or its publication. This work was supported by the University of Turin.
- Author contributions
HS: Writing – review & editing, Validation, Formal analysis, Investigation, Data curation, Methodology, Writing – original draft, Visualization, Conceptualization. MD: Writing – review & editing. CP: Writing – original draft, Writing – review & editing, Data curation. SN: Project administration, Funding acquisition, Writing – review & editing, Supervision, Writing – original draft, Methodology, Validation, Investigation, Conceptualization, Resources.
- Acknowledgements
We regret that, owing to constraints on space, it was not possible to include all pertinent references.
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Citation
Sheikhi, H., et al. (2026). Current status of lettuce production in soilless culture: environmental conditions. Frontiers in Horticulture, Volume 5. https://doi.org/
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- 5 - 2026
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Copyright © 2026 Sheikhi, Delshad, Profico and Nicola.
License: https://creativecommons.org/licenses/by/4.0/
*Correspondence: Silvana Nicola
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