Innovative Application Strategies of Light-Emitting Diodes in Protected Horticulture

Abstract

Light-emitting diodes (LEDs) in agricultural systems mainly contribute their capacity to create a precise and constant light spectral environment. However, the potential of LED in crop production was underestimated. LEDs serve not only as efficient artificial light sources for plant growth, but are also a good tool for enhancing biomass production with limited energy consumption. This article reviewed innovative applications of LED in facility agriculture, e.g., plant factory, and greenhouse. Compared to conventional application of LED, innovative lighting strategies such as intermittent lighting, night break, continuous lighting, alternate lighting, dynamic lighting, and end-of-day (EOD) far-red provided by LED light can elevate the production efficiency effectively. However, the scientific explanation of the above lighting strategies remains to be clearly revealed, providing theoretical support for the further optimization of conducting parameters. This review summarizes the physiological effects of different lighting strategies on crop cultivation and illustrates their future application in facility agriculture, aiming to provide novel methods for elevating the energy utilization efficiency and lowering the cost in facility agriculture using artificial light.

1. Introduction

Light is a critical environmental factor influencing plant growth and development, and it has a significant regulatory role in the cultivation of horticultural crops grown in facility horticulture [1]. Today, the scale of facility agriculture, including solar greenhouses, plastic greenhouses, multi-story greenhouses, and plant factories in China, has increased gradually and significantly. However, low-light conditions, such as rain, haze, and cloudy days, as well as the shading and filtering of natural light by facility covering materials, resulting in insufficient light inside the facility, often occur. Artificial light supplementation or total artificial light has become essential to guarantee efficient production in modern agriculture. Therefore, artificial light sources are playing an increasingly significant role in the development of modern agriculture.

With the development of LED technology and the drive for energy-efficient LED lighting in facility agriculture, research into optimal light application using LED photoelectric characteristics continues. The content of these applications is also being constantly enriched and expanded. The lighting strategy is to optimize the arrangement of environment-light quality, photoperiod, and light intensity in the time series to achieve the ideal cultivation effect. This review summarizes the relevant research on lighting strategies in recent years, taking light quality, photoperiod, and light intensity as the starting point to study the effects of lighting strategies such as intermittent lighting, night break, continuous lighting, alternate lighting, dynamic lighting, and EOD far-red on vegetable yield, quality, and energy use efficiency in facility agriculture. At the same time, the response mechanism of vegetables under different lighting strategies was explored to provide a new idea for achieving the goal of elevating the energy utilization efficiency and lowering the cost in facility agriculture using artificial light.

2. Intermittent Lighting

Intermittent lighting is a technique in which light is emitted in short cycle periods instead of continuous light intervals. More specifically, intermittent lighting means that different circadian rhythms can be set under artificial light facilities in agriculture, and more than two alternating light/dark (L/D) cycles can be configured within the same circadian rhythm. The key design parameters for intermittent lighting are photoperiod duration, frequency of L/D cycles, and duty ratio. In comparison, the traditional natural light contains only one L/D cycle in a 24 h circadian rhythm. Obviously, artificial intermittent light is much more complex than natural light.

To date, several studies have demonstrated that a specific intermittent lighting strategy can achieve higher output and better quality while maintaining the same energy consumption as traditional lighting [2,3,4]. To be more specific, Chen et al. [3] found that under intermittent RB (R:B = 9:1) with different L/D cycles (L/D = 4; 4 h/2 h, L/D = 6; 3 h/1.5 h and 1 h/0.5 h, L/D = 8; 2 h/1 h) within 24 h, plants showed higher biomass than under continuous RB light, despite equal energy consumption. And the intermittent treatments using two cycles of 8 h/4 h, or three cycles of 6 h/3 h, or 4 h/2 h significantly improved the sweetness and crispness of lettuce (Table 1). Urair et al. [5] obtained the optimal intermittent light conditions which promote efficient cultivation of leaf lettuce for the L/D (light/dark ratio) = 2 and the photoperiod duration within a range of 15 h to 22 h based on equal accumulated time exposed to the light. At the same time, intermittent lighting was also demonstrated as a practical lighting strategy that can reduce total electricity consumption while maintaining a high growth rate and biomass production of the plants (Table 1). For example, Avgoustaki et al. [6] found that basil plants grown under 14 h of intermittent photoperiod produced higher biomass than those under a continuous 16 h photoperiod. Moreover, by arranging intermittent lighting cycles during off-peak electricity demand periods, lower electricity prices can be utilized to reduce operating costs [6] (Table 1).

The biological clock of plants can drive intrinsic rhythmic changes, coordinating their physiological processes to synchronize with the external light/dark rhythm, thereby achieving adaptation to the dynamic changes in daily environmental factors [7,8]. Using LEDs to regulate the plants’ circadian rhythm precisely by regulating the L/D cycle of intermittent light in plant factories can improve cultivation efficiency and ultimately increase yield [9]. Previous reports have suggested that the highest yields and quality are generally achieved by maintaining a single long cycle compared to the more standard single L/D cycle [10]. For example, under the same thermal effectiveness and photosynthetically active radiation, a L/D cycle of 12 h of light alternating with 12 h of darkness can promote lettuce growth, with its growth indicators significantly better than the 6 h/6 h and 3 h/3 h lighting schemes [11] (Table 1). In addition, lettuce cultivated under an 8 h light and 16 h dark photoperiod exhibited significantly better fresh weight, leaf number, and dry weight than the 4 h/8 h light treatment results [12]. The L/D cycles of 12 h/12 h and 10 h/10 h resulted in a heavier dry weight of Doritaenopsis Queen Beer ‘Mantefon’ Orchids than 6 h/6 h and 8 h/8 h; however, the dry weight of the 10 h/10 h treatment was higher than that of 12 h/12 h [13] (Table 1). For cucumber seedlings, higher L/D cycle frequencies significantly reduced fresh and dry biomass accumulation while maintaining stable morphometric parameters, including hypocotyl length, internode count, and stem diameter [14] (Table 1). Furthermore, Intermittent light exposure may disrupt the transmission of circadian clock signals, resulting in premature plant blooming [15]. Hu et al. [16] established intermittent illumination as a viable photomanipulation strategy, inducing concurrent improvements in plant architecture, density adaptation, photosynthetic performance, developmental kinetics, and breeding efficiency.

The beneficial impact of intermittent lighting on plant growth and quality can be partly attributed to its ability to optimize and regulate the plant’s photosynthetic physiological morphology. Intermittent lighting has a significant effect on the effective absorption, transmission, and conversion processes of light energy [17]. Further study revealed that adequate frequencies of L/D cycles could not only enhances key parameters in response curves, such as light compensation point, light saturated net photosynthetic rate (P_n), dark respiration rate, and light saturation point, but also regulates related indicators in CO₂ response curves, including CO₂ saturated photosynthetic rate, initial fixation efficiency, and photorespiration rate [18]. Research by Hang et al. [11] indicated that compared to short-period lighting of 3 h/3 h, extending the lighting cycle to 6 h/6 h can effectively increase leaf stomatal conductance (gs) and P_n. At the same time, intermittent lighting has a significant effect on the regulation of photosynthetic pathways in various Crassulacean Acid Metabolism (CAM) plants due to the controllable L/D cycle [13,19]. In Dendrobium officinale, the photosynthetic pathway could be switched from CAM to C₃ by a shorter L/Dcycle (light:dark = 2 h:2 h or 4 h:4 h), which results in higher daily net CO₂ exchange amounts, biomass accumulation, and soluble polysaccharides yield [20]. However, compared to the light/dark cycle of 12 h/12 h, when the cycle of light and dark periods was adjusted from 4 h/4 h to 12 h/12 h, the net carbon dioxide exchange amount of D. officinale all significantly increased, indicating that this change in light cycle failed to achieve a fully reversible regulatory mechanism [21]. It is well known that full light utilization is achieved through better light interception [22], and intermittent lighting can effectively enhance light energy capture efficiency by regulating the leaf area and leaf shape of plants [11]. A shortened light cycle (3 h/3 h) results in more compact and rounded leaves, which reduces self-shading. This makes the plant more competitive under high-density planting conditions. The longer light cycles, closer to the circadian rhythm, benefit plant growth by promoting leaves to become elongated and open, thereby enhancing light capture efficiency. Moreover, appropriate L/D cycles may enhance plant performance through targeted modulation of sucrose and starch metabolic dynamics [2,3,5]. Carbohydrate metabolism regulation might represent a key mechanism for enhancing production efficiency. Under continuous light, sucrose accumulates within the leaves due to the decreased source capacity [23] and impaired assimilate transport [11], resulting in the accumulation of these assimilated products. Intermittent irradiation offers significant advantages for plant cultivation, and the ideal L/D cycle can simultaneously ensure adequate nocturnal carbon substrate availability while reducing the energetic inefficiencies associated with photoassimilate overaccumulation.

Pulsed light is an intermittent light with a high frequency, and the mechanism of its effect on plant growth, quality, and photosynthetic properties has become clearer in recent years. Theoretically, the P_n of plant leaves directly affects the amount of material accumulation and material conversion, which in turn affects the yield and quality of plants, and pulsed light affects the yield and quality of plants by affecting the P_n of plant leaves. It has been demonstrated that a pool of photosynthetic intermediates exists within the photosynthetic electron transport chain of plant leaves. The formation of assimilative forces by the photosystem and the photosynthetic electron transport chain stops instantaneously when light is stopped during a cycle of pulsed light. However, carbon assimilation continues for a short period due to the fact that the intermediates formed by the photosystem and the electron transport chain during the period of light exposure are still assimilative forces [24]. The two important parameters of pulsed light are duty cycle (the proportion of light time in a cycle to the whole cycle time) and frequency (0.01 Hz to 10.00 Hz, i.e., from 100 s to 100 ms per L/D cycle). As the frequency or duty cycle increases, the duration of the dark period within each cycle decreases. This reduction shortens the time available for the photosynthetic intermediates accumulated during the light phase to sustain dark-phase carbon assimilation in the electron transport chain. Consequently, the carbon assimilation period becomes briefer, leading to a gradual stabilization of the net photosynthetic rate. Thus, plants exposed to pulsed light with higher duty cycles or frequencies exhibit P_n values that approach, but do not surpass, those achieved under continuous light conditions [25]. Similar conclusions were reached by Son [26] and Jishi [27] (duty cycle of 75%) and others, i.e., at higher intermittent light duty cycles, there was no significant difference between the P_n of lettuce and the control at different frequencies. A lower duty cycle or frequency would produce an effect similar to shade or low light, which is not favorable to the improvement of photosynthesis and biomass accumulation of the crop, and a lower duty cycle or frequency would result in wastage of energy (Table 1).

Intermittent irradiation with customized periods and frequency has been shown to enhance crop yield and quality more than continuous illumination under equivalent energy consumption, which contributes to energy efficiency [6,28]. To maximize the economic benefits derived from achieving both high quality and quantity in plant growth within a closed production system, it appears crucial to optimize the photoperiod, combined with light intensity or light quality, within a plant factory system. Because intermittent irradiation with various light parameters is different. For instance, Kang et al. [29] found that best growth was observed under a high light intensity (290 μmol·m⁻²·s⁻¹) with a shorter L/D cycle (6 h/2 h), and Higashi et al. [9] indicated that circadian resonance (CR) efficiency is higher under LED lighting compared to fluorescent lighting. This improvement was especially evident when exposed to red light. So, further investigation is required into the combined influence of light quality, intensity, and photoperiod under intermittent light conditions. In addition, existing studies have shown that intermittent irradiation can be used not only during general crop cultivation but also for short periods during pre-harvest [8] or post-harvest storage [9]. Moreover, there is a strong need to conduct further research on the application stage of intermittent irradiation.

Table 1. Optimal intermittent lighting parameters for plants.

Plants	The Photoperiod Duration (L + D Periods)	Frequency of L/D Cycles	Duty Ratio	Light Intensity (μmol·m−2·s−1)	Light Quality	Effect	Reference
Lettuce (Lactuca sativa var. crispa ‘Green Oak Leaf’)	8 h +4 h	2	8/12	200	Red (660 nm):Blue (450 nm) = 9:1	Increase shoot biomass, glucose content, sweetness, and crispness	[3]
	6 h + 3 h or 4 h +2 h	3	6/9 or 4/6			Increase glucose content, sweetness, and crispness
	4 h + 2 h	4	4/6			Increase shoot biomass and glucose content
	3 h + 1.5 h or 1 h + 0.5 h	6	3/4.5 or 1/1.5			Increase shoot biomass and glucose content
	2 h + 1 h	8	2/3			Increase shoot biomass and glucose content
Lettuce (Lactuca sativa L. cv. Greenwave)	The light/dark ratio (L/D) of L/D = 2 and the light/dark period of 15–22 h (T15–T22)			200 ± 20 or 245 ± 20	White panel LED	Promotes the efficient cultivation of leaf lettuce and the reduction in running costs	[5]
Long-leaf basil plants (Ocimum basilicum)	4 h light + 4 h dark (receiving radiation every 10 min per h)			228	Red (660 nm) +Blue (450 nm) + near-infrared (NIR) (730 nm)	Increase in the biomass shoot of 47%, and a reduction in electricity consumption	[6]
	The plants received light for 10 min per hour of darkness when the electricity prices were high, and 4 h of light when the electricity prices were low
Butterhead (Lactuca sativa L. var. capitata L. ‘Adriana’)	12 h + 12 h	1	12/24	200	Red/Blue = 83%:17%	Have a growth advantage with slender and open leaves for better light interception.	[11]
	6 h + 6 h and 3 h + 3 h	2 and 4	6/12 and 3/6			Make the leaves compact and rounder, which reduces self-shading
Doritaenopsis (×DoritaenopsisQueen Beer ‘Mantefon’)	10 h + 10 h	/	10/20	160 ± 10	/	Modify the photosynthesis patterns and vegetative growth, resulting in a reduction in the production period	[13]
Tomato (Solanum lycopersicum L. var. Caniles and Cucumber (Cucumis sativus L. var. Litoral)	18 h + 6 h	1	/	/	Red (622, 632, 638 nm)/Blue (454 nm) = 7:2	Increase the concentration of total soluble sugars and proline in the leaf	[14]
	6 h + 2 h	3	6/8			Show the best water content.
Red leaf lettuce (Lactuca sativa L. cv. ‘Sunmang’)	/	1 kHz	75%	173 ± 5	Red (655 nm):white (456 nm + 558 nm)/Blue (456 nm) = 7:2:1	Achieve biomass output equivalent to continuous light and significantly reduce energy consumption	[26]
Cos lettuce (Lactuca sativa)	/	0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, and 12.8 kHz	75%	100	White light-emitting diodes	The CUR at a duty ratio of 75% for each frequency was not significantly different from that in continuous light and reduced energy consumption	[27]

Table 1. Optimal intermittent lighting parameters for plants.

Plants	The Photoperiod Duration (L + D Periods)	Frequency of L/D Cycles	Duty Ratio	Light Intensity (μmol·m−2·s−1)	Light Quality	Effect	Reference
Lettuce (Lactuca sativa var. crispa ‘Green Oak Leaf’)	8 h +4 h	2	8/12	200	Red (660 nm):Blue (450 nm) = 9:1	Increase shoot biomass, glucose content, sweetness, and crispness	[3]
	6 h + 3 h or 4 h +2 h	3	6/9 or 4/6			Increase glucose content, sweetness, and crispness
	4 h + 2 h	4	4/6			Increase shoot biomass and glucose content
	3 h + 1.5 h or 1 h + 0.5 h	6	3/4.5 or 1/1.5			Increase shoot biomass and glucose content
	2 h + 1 h	8	2/3			Increase shoot biomass and glucose content
Lettuce (Lactuca sativa L. cv. Greenwave)	The light/dark ratio (L/D) of L/D = 2 and the light/dark period of 15–22 h (T15–T22)			200 ± 20 or 245 ± 20	White panel LED	Promotes the efficient cultivation of leaf lettuce and the reduction in running costs	[5]
Long-leaf basil plants (Ocimum basilicum)	4 h light + 4 h dark (receiving radiation every 10 min per h)			228	Red (660 nm) +Blue (450 nm) + near-infrared (NIR) (730 nm)	Increase in the biomass shoot of 47%, and a reduction in electricity consumption	[6]
	The plants received light for 10 min per hour of darkness when the electricity prices were high, and 4 h of light when the electricity prices were low
Butterhead (Lactuca sativa L. var. capitata L. ‘Adriana’)	12 h + 12 h	1	12/24	200	Red/Blue = 83%:17%	Have a growth advantage with slender and open leaves for better light interception.	[11]
	6 h + 6 h and 3 h + 3 h	2 and 4	6/12 and 3/6			Make the leaves compact and rounder, which reduces self-shading
Doritaenopsis (×DoritaenopsisQueen Beer ‘Mantefon’)	10 h + 10 h	/	10/20	160 ± 10	/	Modify the photosynthesis patterns and vegetative growth, resulting in a reduction in the production period	[13]
Tomato (Solanum lycopersicum L. var. Caniles and Cucumber (Cucumis sativus L. var. Litoral)	18 h + 6 h	1	/	/	Red (622, 632, 638 nm)/Blue (454 nm) = 7:2	Increase the concentration of total soluble sugars and proline in the leaf	[14]
	6 h + 2 h	3	6/8			Show the best water content.
Red leaf lettuce (Lactuca sativa L. cv. ‘Sunmang’)	/	1 kHz	75%	173 ± 5	Red (655 nm):white (456 nm + 558 nm)/Blue (456 nm) = 7:2:1	Achieve biomass output equivalent to continuous light and significantly reduce energy consumption	[26]
Cos lettuce (Lactuca sativa)	/	0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, and 12.8 kHz	75%	100	White light-emitting diodes	The CUR at a duty ratio of 75% for each frequency was not significantly different from that in continuous light and reduced energy consumption	[27]

3. Continuous Lighting

Continuous light (CL) is a unique artificial light environment characterized by the absence of dark periods, interruptions, and breaks from the traditional light/dark alternating photoperiod, providing plants with continuous 24 h light conditions [30,31]. Previous studies have shown that CL positively influences plant growth, manifesting in accelerated reproductive cycles, increased biomass, and enhanced quality [30,32,33] (Table 2). However, there are also adverse effects associated with CL, including potentially lethal interveinal chlorosis and necrosis [34], photosynthetic damage, and leaf senescence in tomato [31,34], eggplant [35,36], cucumber [37,38], potato [39,40], and other crops [41,42]. The attribute characteristics of CL include not only the basic elements of light intensity and quality, but also the duration of specific elements and their combination, as well as the conversion rule [30]. It is therefore suggested that these conflicting findings could be attributed to variations in the growth environmental factors of the cultivation conditions. Moreover, plant species and cultivars might be another reason for the contradictory results [32].

The variation diversity of CL is high, which can regulate plant growth and development, as well as yield quality, at multiple plant physiological levels [30]. According to the duration of light, CL can be divided into short-term CL (less than 3 d) and long-term CL (more than 3 d). In general, short-term CL is typically used for the pretreatment of hydroponic leafy vegetables and sprouts to improve the yield of edible parts and the nutritional quality of edible parts [43] (Table 2). Under CL conditions, red and blue spectra are widely applied, and recent studies have shown that the green spectrum can significantly reduce nitrate content by upregulating the expression levels of nitrate reductase (NR) and nitrite reductase (NiR) coding genes [44,45] (Table 2). Many studies have shown that CL of appropriate light intensity and LED red–blue or red–blue–green light for 2 to 3 d before harvesting of hydroponic leafy vegetables can significantly improve the upper yield, vitamin C content, phenolic compound concentrations, and soluble sugar, and decrease nitrate content of hydroponic leafy vegetables [44,46,47] (Table 2). Moreover, the optimal effect of CL and nitrogen limitation on the improvement of crop yield and quality can be realized by coupling the two technologies in the pre-harvest treatment of lettuce. Yang et al. [48] found that a 3 d nitrogen limitation combined with CL significantly enhanced the quality and taste of lettuce while maintaining biomass accumulation (Table 2). Zhang et al. [49] found that applying a continuous 72 h period of 4R:1B light at a nitrogen concentration of 12 mmol·L⁻¹ significantly enhances crop yield and increases AsA content (Table 2). Besides the short-term CL, long-term CL can also regulate plant growth, yield, and quality. The effects of long-term CL on plant physiology depend on the type and variety of the plant. According to the difference in plant adaptability to long-term CL, plants can be divided into CL-sensitive type and CL-adaptable type. Two types of plants require different long-term CL application strategies to obtain maximum application benefits, and inappropriate CL can even cause negative effects on plants, such as leaf chlorosis, leaf blistering, and even reduce the growth and yield [36,50]. The main mechanism of CL to enhance plant growth involves extending the duration of photosynthesis, eliminating respiration consumption in the dark period, and finally increasing photosynthetic product synthesis and accumulation. Previous studies reported that prolonging photoperiod can increase the biomass of plants more effectively than increasing light intensity. Under the same daily light integral (DLI) condition, low light intensity with a long light period is more conducive to the accumulation of biomass of lettuce [51,52,53], radish [54], rose [55], and other plants than high light intensity with a short light period. However, a large number of studies indicate that CL persistently enhances photooxidative processes and disrupts the synchronization between internal circadian biorhythms and the external light/dark cycle (circadian asynchrony) [31,50,56,57,58,59]. The contribution of individual factors to photodamage may vary substantially depending on the biological properties of the object (plant species and variety, plant age, and the stage of development) and environmental conditions. The extent of photodamage observed in plants subjected to CL treatment can vary significantly, and this variation is influenced by a combination of factors, including the plant’s inherent biological characteristics, such as its species, variety, age, and developmental stage, as well as environmental conditions. For example, utilizing CL for microgreen production can improve energy efficiency while maintaining or increasing antioxidants, phenolics, and anthocyanins [60]. Moreover, Demers et al. [61] reported that if CL is provided entirely or partially by solar illumination, the injury is less than that caused by CL from artificial light sources, or it may be entirely absent. In facility agriculture, supplementing with green light has been shown to reduce photo-inhibition by alleviating damage to photosystem II (PSII) and improving the photochemical efficiency in lettuce exposed to CL from red and blue LEDs [44]. Additionally, combining far-red light with red light can also reduce the CL injury caused by red light alone [62]. An alternating spectrum during CL is also a good strategy to effectively alleviate the damage caused by CL [51,63].

The variation diversity of CL is high, which can regulate plant growth, development, and yield quality from multiple plant physiological levels and has broad application prospects. Currently, short-term CL has been widely used in hydroponic leafy vegetable products in artificial light plant factories, and has the advantages of low cost, easy operation, no adverse effects, etc. In comparison, the effect of long-term CL on plant physiology varies depending on the plant species and variety. CL-sensitive plants and CL-adaptable plants should adopt different long-term CL application strategies to obtain the maximum application benefits. In addition, most of the existing research focuses on a few model crops, such as lettuce, while there is a serious lack of research on light management strategies for fruit, vegetables, and specialty vegetables under specific facility types. In the future, it will also be necessary to pay attention to the impact of continuous light supply on various types of vegetables.

Table 2. Optimal continuous lighting parameters for plant.

Plants	Treatment Duration (d)	Light Intensity (μmol·m−2·s−1)	Light Quality	Nitrogen Levels (mmol/L)	Effect	Reference
Lettuce (Lactuca sativa L. cv. ‘Yidali’)	15	200	R (630 nm):B (430 nm) = 3:1	/	Obtained greater shoot biomass and ascorbate pool size without leaf injury	[32]
Purple-leaf lettuce (Lactuca sativa ‘Zixiangye’)	2 (Pre-harvest)	150	white LEDs (R:B = 4:1)	/	Improve the levels of carotenoids, AsA, and anthocyanins, while concurrently reducing the content of nitrate.	[43]
Lettuce (Lactuca sativa L. cv. Butterhead)	2	200	R (630 nm):B (460 nm):G (530 nm) = 4:1:1	/	Decreased nitrate content in lettuce leaves and promoted nutritional values	[44]
Butterhead lettuce (Lactuca sativa var. capitata)	1	200	R (630 nm):B (460 nm):G (530 nm) = 4:1:1	/	Decrease nitrate content and enhance lettuce quality.	[46]
Lettuce (Lactuca sativa var. capitata. L.)	2 (pre-harvest)	100–150	R (630 nm):B (460 nm) = 4:1	/	Decrease nitrate concentration and increase vitamin C content and lettuce quality	[47]
Lettuce (butterhead and red oak)	3 (pre-harvest)	200	R (630 nm):B (460 nm) = 3:1	Without nitrogen	Reduce the content of nitrate and sesquiterpene lactones, and improve the quantities of soluble sugar, soluble protein, anthocyanins, and phenolic compounds without reducing the fresh weight of lettuce	[48]
Lettuce (Lactuca sativa L. cv. ‘Yidali’)	3 (pre-harvest)	150	R (655 nm):B (460 nm) = 4:1	12	Obtain higher yield and AsA content	[49]

Table 2. Optimal continuous lighting parameters for plant.

Plants	Treatment Duration (d)	Light Intensity (μmol·m−2·s−1)	Light Quality	Nitrogen Levels (mmol/L)	Effect	Reference
Lettuce (Lactuca sativa L. cv. ‘Yidali’)	15	200	R (630 nm):B (430 nm) = 3:1	/	Obtained greater shoot biomass and ascorbate pool size without leaf injury	[32]
Purple-leaf lettuce (Lactuca sativa ‘Zixiangye’)	2 (Pre-harvest)	150	white LEDs (R:B = 4:1)	/	Improve the levels of carotenoids, AsA, and anthocyanins, while concurrently reducing the content of nitrate.	[43]
Lettuce (Lactuca sativa L. cv. Butterhead)	2	200	R (630 nm):B (460 nm):G (530 nm) = 4:1:1	/	Decreased nitrate content in lettuce leaves and promoted nutritional values	[44]
Butterhead lettuce (Lactuca sativa var. capitata)	1	200	R (630 nm):B (460 nm):G (530 nm) = 4:1:1	/	Decrease nitrate content and enhance lettuce quality.	[46]
Lettuce (Lactuca sativa var. capitata. L.)	2 (pre-harvest)	100–150	R (630 nm):B (460 nm) = 4:1	/	Decrease nitrate concentration and increase vitamin C content and lettuce quality	[47]
Lettuce (butterhead and red oak)	3 (pre-harvest)	200	R (630 nm):B (460 nm) = 3:1	Without nitrogen	Reduce the content of nitrate and sesquiterpene lactones, and improve the quantities of soluble sugar, soluble protein, anthocyanins, and phenolic compounds without reducing the fresh weight of lettuce	[48]
Lettuce (Lactuca sativa L. cv. ‘Yidali’)	3 (pre-harvest)	150	R (655 nm):B (460 nm) = 4:1	12	Obtain higher yield and AsA content	[49]

4. Night-Break Lighting

Night break (NB) is a photoperiodic regulation strategy that imposes a brief irradiation in the middle of the night period and regulates the circadian rhythm of plants by eliminating the long night effect [64]. In recent years, NB has been used extensively as a tool to investigate the photoperiodic regulation of growth and flowering, owing to the advantage offered by NB, which allows for the manipulation of photoperiods without altering the total daily light received by the plant [65]. The current study showed that flowering period regulation and control of overgrowth were positively affected when exposed to NB [66,67,68]. Two important points in NB are light quality and the arrangement of time and interval. With the optimization of NB parameters, including the spectral composition of the LED and the arrangement of the interval, it is possible to achieve precise utilization of the LED light source in facility seedling production.

Plant height is an important indicator to be regulated during the seedling growth period for vegetable production. Excessively tall plants can cause issues during propagation, transport, and planting. NB is an effective method to regulate greenhouse crop height. Red light (RL) and far-red light (FRL) are the most frequently used spectrums in NB lighting, and it has been reported that NB with certain RL/FRL can significantly reduce the height of the plant. For example, NB with a high RL/FRL ratio can significantly inhibit stem elongation in Eustoma grandiflorum and Cymbidium [67,69] (Table 3). In tomato, Cao et al. [68] showed that NB could achieve shorter plants with RL, and as the RL frequency increases, the plant height gradually decreases (Table 3).

Flowering time in the majority of plant species is governed by photoperiod, the duration of the light phase. Long-day plants (LDPs), such as Hordeum vulgare, initiate flowering more rapidly as day length increases. Conversely, short-day plants (SDPs), including sorghum and chrysanthemum, exhibit accelerated flowering in increasing day length and night length, respectively. While day-neutral plants (DNPs) flower under a wide range of day lengths, showing little dependence on photoperiod for this process [65]. As an effective tool to regulate the flowering period, NB with various light qualities has different effects [70,71,72]. Monochromatic light exposure experiments across diverse plant species demonstrated that, whether it is promoting the flowering of LDPs or inhibiting the flowering of SDPs, using RL to interrupt the dark phase has the greatest effect, while the effect was reversed when treated with FRL in many cases [67,68,73] (Table 3). NB lighting has been widely used in cut chrysanthemum production to regulate the flowering and shoot length [74,75,76,77]. Across chrysanthemum’s four flowering types (summer; summer–autumn; autumn; winter). A combination of RL and FRL NB was shown to be a better and more stable inhibitory effect than RL or FRL NB alone on both the flowering types of chrysanthemum cultivars. Consequently, R + FR spectral combinations represent a practical lighting strategy for suppressing flowering in commercial chrysanthemum production [75,76] (Table 3). Intriguingly, the light quality exposure during the daily photoperiod modulates the photosensitivity of NB in the subsequent flower-inductive dark phase [74]. In addition to the effect on flowering, a recent study showed that NB with RL was found to significantly reduce Botrytis cinerea infection on tomato leaves by modulating defense-related transcription factors (MYB and DOF), phytohormones (JA and ABA), and boosting antioxidant enzymes (SOD and APX) in tomatoes, underscoring its utility in plant disease management [78] (Table 3).

Phytochromes have long been recognized as crucial photoperiodic photoreceptors, which are believed to mediate plant responses to night break (NB) light signals [79]. Phytochromes have two photo-interconvertible isomeric forms, including the red-light-absorbing form (Pr) and the far-red-light-absorbing form (Pfr) [80,81]. During the day, phytochromes mainly exist in the Pfr form, inhibiting elongation-related genes. Whereas Pfr gradually reduces to the Pr form in the dark, promoting elongation gene expression [82]. Therefore, NB treatment with RL during the night can promote the conversion of phytochrome from Pr to Pfr, thereby regulating plant elongation growth [83,84]. In addition to regulating plant height, phytochrome also plays a key role in photoperiodic flowering and photomorphogenesis in most SDPs and LDPs [83,84]. Interrupting the dark period during short days (which have long nights) with RL prevents the reversion of Pfr730 to Pr660 and also converts Pr660 into Pfr730. This prevents the level of Pfr730 from dropping below the critical Pfr730 level and, therefore, inhibits flowering in short-day plants and promotes flowering in long-day plants [85]. So far, the molecular mechanisms involved in NB lighting induction of flowering in many plant species have advanced considerably. The florigen-encoding genes, FLOWERING LOCUS T (FT) and its homologs, are key genes involved in floral transition [86,87], which can interact with FLOWERING LOCUS T (FD) and 14-3-3 proteins to form florigen activation complex (FAC), and then, in turn, induce the meristem identity gene APETALA1 and trigger flowering [88,89]. NB differentially affects the flowering of SDPs and LDPs by regulating the expression of the FT gene [90]. In rice, Hd3a (an FT homolog) primarily mediates night-break (NB) induced flowering delay [91], while soybean NB responses predominantly involve FT2a and FT5a downregulation [92]. Conversely, wheat accelerates heading under short days through NB-triggered FT1 induction [65]. Molecular analyses confirm that phytochromes serve as central mediators of NB-induced FT transcriptional regulation. This conserved signaling pathway exhibits functional divergence across species, evidenced by abolished NB responses in rice phyB [93], soybean phyA [94], and wheat phyB/phyC mutants [65]. Circadian regulators further modulate NB pathways: wheat FT1 activation requires PPD1 (orthologous to rice PRR37) [65], whereas poplar NB induces FT2 by suppressing the clock gene LHY2, thereby releasing apical growth arrest [93]. Collectively, NB orchestrates flowering through multi-tiered regulatory networks.

Table 3. Optimal night-break lighting parameters for plant.

Plants	Treatment Duration	Light Intensity (μmol·m−2·s−1)	Light Quality	Effect	Reference
Cymbidium (Red Fire) and Cymbidium (Yokihi)	22:00 pm–02:00 am	3–7	/	Promote flower induction	[67]
		120		Promote flower induction and increase the quality of flowering
Tomato (Solanum lycopersicum)	Lasting for 10 min, with frequencies ranging from every 1, 2, 3, and 4 h	20	R (658 nm)	Decrease stem elongation and obtain more compact and healthier tomato plants	[68]
Eustomab (E. grandiflorum ‘Nail Peach Neo’	/	4.5	R (660 ± 30 nm):FR (730 ± 30 nm) > 5.3	Delay flowering	[69]
			R (660 ± 30 nm):FR (730 ± 30 nm) < 5.3	Promote flowering
		>2.0	R (660 ± 30 nm):FR (730 ± 30 nm) = 0.6	Promote flowering
Chrysanthemum (Dendranthema ×grandiflorum Kitam.)	12:45 am–1:30 am	9.42	Red fluorescent light	Inhibit flowering and promote stem elongation	[73]
Chrysanthemum (‘Jimba,’)	23:00 pm–5:00 am	/	R (630 nm); R (690 nm); and INC lamps	Inhibit flower bud differentiation	[75]
Chrysanthemum (‘Iwa no hakusen.’)			FR (735 nm); R (690 nm); and INC lamps	Delay flower bud differentiation
Chrysanthemum (‘Iwa no hakusen‘) and Chrysanthemum (‘Jimba,’)			R (660 nm) + FR (735 nm)	Inhibit flower bud differentiation
Chrysanthemu	23:00 pm–5:00 am	/	R (630 nm); R (630 nm); R (660 nm) + FR (735 nm)	Inhibited floral differentiation	[76]
Wild-type tomato (Solanum lycopersicum L. cv. ’Ailsa Craig’)	30 min	3.6 W·m−2	R (630 nm)	Reactivate key antioxidant enzymes and defense against Botrytis cinerea infection	[78]

Table 3. Optimal night-break lighting parameters for plant.

Plants	Treatment Duration	Light Intensity (μmol·m−2·s−1)	Light Quality	Effect	Reference
Cymbidium (Red Fire) and Cymbidium (Yokihi)	22:00 pm–02:00 am	3–7	/	Promote flower induction	[67]
		120		Promote flower induction and increase the quality of flowering
Tomato (Solanum lycopersicum)	Lasting for 10 min, with frequencies ranging from every 1, 2, 3, and 4 h	20	R (658 nm)	Decrease stem elongation and obtain more compact and healthier tomato plants	[68]
Eustomab (E. grandiflorum ‘Nail Peach Neo’	/	4.5	R (660 ± 30 nm):FR (730 ± 30 nm) > 5.3	Delay flowering	[69]
			R (660 ± 30 nm):FR (730 ± 30 nm) < 5.3	Promote flowering
		>2.0	R (660 ± 30 nm):FR (730 ± 30 nm) = 0.6	Promote flowering
Chrysanthemum (Dendranthema ×grandiflorum Kitam.)	12:45 am–1:30 am	9.42	Red fluorescent light	Inhibit flowering and promote stem elongation	[73]
Chrysanthemum (‘Jimba,’)	23:00 pm–5:00 am	/	R (630 nm); R (690 nm); and INC lamps	Inhibit flower bud differentiation	[75]
Chrysanthemum (‘Iwa no hakusen.’)			FR (735 nm); R (690 nm); and INC lamps	Delay flower bud differentiation
Chrysanthemum (‘Iwa no hakusen‘) and Chrysanthemum (‘Jimba,’)			R (660 nm) + FR (735 nm)	Inhibit flower bud differentiation
Chrysanthemu	23:00 pm–5:00 am	/	R (630 nm); R (630 nm); R (660 nm) + FR (735 nm)	Inhibited floral differentiation	[76]
Wild-type tomato (Solanum lycopersicum L. cv. ’Ailsa Craig’)	30 min	3.6 W·m−2	R (630 nm)	Reactivate key antioxidant enzymes and defense against Botrytis cinerea infection	[78]

5. Alternating Lighting

Alternating lighting means that plants were alternately irradiated by discriminated light quality, mainly including full-alternated and overlay-alternated lighting modes, that is, the cyclic illumination mode in which two or more spectral compositions appear alternately or superimposed according to certain rules in the time order during the continuous illumination period [95,96,97]. To enhance energy efficiency and crop quality in facility agriculture, customized alternating lighting (in terms of interval, duration, spectrum, and intensity) emerges as a promising strategy. This approach optimizes LED utilization, reduces energy consumption, and simultaneously boosts biomass and nutritional value.

Red and blue light spectra are the primary effective light sources regulating plant photomorphogenesis and growth metabolism [98], which have become the most studied light sources used in alternating irradiation patterns in recent years. The current results have shown that temporally shifting the irradiation times, either as interleaved irradiation (overlay-alternated lighting pattern) or complete spectral alternation (full-alternated lighting pattern), significantly enhances plant growth, nutritional quality, and yield compared to simultaneous irradiation, despite equivalent daily light integrals [96,99,100] (Table 4). However, the effects of alternating light on lettuce growth reported have also been in disagreement due to the differences in cultivation stage [100]. Kuno et al. [101] reported that compared with simultaneous irradiation, alternating irradiation promoted lettuce growth more at 30 DAS (day after sowing) (Table 4), while Jishi et al. [95] found that there is no significant growth difference between the two light treatments at 21 DAS. Considering the influence of alternate irradiation in terms of the yield and energy use efficiency, Ohtake et al. [100] indicated the significant effect of alternating irradiation on plants in the later cultivation period, especially from 22 to 31 DAS (Table 4). Therefore, the influence and development potential of alternating irradiation during the later cultivation stage on shortening the time to harvest and plant yield and quality cannot be underestimated [102] (Table 4). In addition to the cultivation stage, alternating intervals also need to be considered. Chen et al. [96,97] found that based on the same energy consumption, R/B alternating intervals that were 8 h and 1 h during a 16 h photoperiod resulted in higher yield, while R/B alternating intervals that were 4 h and 2 h brought about higher nutritive value compared with the concurrent light RB (Table 4). Takasu et al. [103] indicated that the lettuce grown under the alternating lighting of R21B3 (21 h R + 3 h B), R21B3 (21 h R + 3 h B) and R12B12 (12 h R + 12 h B) exhibited significantly higher photosynthetic capacity and morphological indicators compared to the RB24 (24 h R + B) (Table 4). Chen et al. [104] also reported that different alternating intervals of red and blue light changed the plant morphology of lettuce, which might hold some value as a reference in the targeted cultivation of ornamental plants (Table 4). On the whole, by controlling the interval of alternating red and blue light irradiation, the growth process and quality formation of plants can be effectively regulated without increasing energy consumption. However, the determination of the optimal R/B alternating cycle still requires more systematic experimental research for verification.

Alternate lighting could effectively avoid the adverse effects caused by CL. For lettuce, a CL-adaptable plant, CL with low light intensity can promote plant growth [100], while for CL-sensitive plants, including tomato, potato, and eggplant, CL has been confirmed to inhibit plant growth [23,105]. Nevertheless, alternating red and blue lighting allows for injury-free and high biomass tomato production with continuous 24 h supplemental lighting [63]. For lettuce cultivation in a plant factory, alternate lighting of blue and red light over a 24 h photoperiod resulted in significantly increased biomass while ideal nutritive quality was simultaneously obtained [100] (Table 4).

At present, the growth-promoting effects of alternating red and blue lighting can be attributed to their positive regulation on plant morphogenesis and photosynthetic efficiency [45,100,103]. Alternating lighting with appropriate intervals can promote leaf elongation [95], thereby increasing projected area/leaf area morphological index [103], and subsequently enhance the efficiency of light energy capture. Moreover, plants cultivated under alternating lighting exhibit a particularly high photosynthetic rate per unit leaf area, especially during their early growth phases. This is supported by the significantly higher net assimilation rate (NAR), daily carbon gain estimated, and leaf mass per area (LMA) under alternating red and blue light than under white light [100] (Table 4). Therefore, it can be concluded that the accelerated growth of plants exposed to alternating radiation is due to their enhanced photosynthetic capacity and superior morphology [101,103]. On the other hand, it has been known that red and blue light affect the corresponding photomorphogenesis responses of plants through the specific photoreceptors and signal transduction pathways [106,107]. Moreover, there may be synergistic and antagonistic interactions on the regulation of specific physiological processes between the photoreceptor signaling pathways of R and B [45,96]. For example, phyB and cry1 act synergistically under short photoperiods of simultaneous R and B, whereas they function independently when continuously exposed to the same light combination [108]. Similarly, phot1 and phot2 are independent under low-intensity blue light, but become synergistic under high-intensity blue light [1]. Therefore, alternating R/B light with appropriate intervals may eliminate signal crosstalk or antagonism between the signal transduction pathways of R and B, thus giving full play to the functions of R and B [96]. Kuno et al. [101] concluded that because red light improves photosynthesis and blue light induces lettuce plants to assume the optimum morphogenetic form to receive light, growing lettuce under alternating regimes of red and blue lights represents the most effective course (Table 4).

In the future, if the energy consumption of artificial light sources is not to be increased, it should be possible to optimize the light supply strategy by alternating red and blue light at different frequencies, depending on the target indicators, to increase vegetable yield and improve quality. In addition, the relationship between red and blue, alternating red and blue modes, and the influence of alternating light on the stress resistance of plants should also be an important direction for research into lighting formulations. Besides the alternation of red and blue light, the impact of alternating other light sources (e.g., green light) with red and blue light on energy efficiency and crop quality is also worthy of in-depth exploration.

Table 4. Optimal alternating lighting parameters for plants.

Plants	Lighting Modes	R/B Alternating Intervals	Light Intensity (μmol·m−2·s−1)	Photoperiod (h)	Effect	Reference
Lettuce (Lactuca sativa var. crispa ‘Green Oak Leaf’)	full-alternated	8 h:8 h 1 h:1 h	R (660 nm) = 200 ± 5 B (450 nm) = 100 ± 5	16	Accelerate lettuce growth speed and promote the shoot biomass	[96]
		4 h:4 h 2 h:2 h			Raise the ascorbic acid content and decrease the nitrate content
Lettuce (Lactuca sativa L. var. crispa)	full-alternated	12 h:12 h	R (660 nm) = 100 B (450 nm) = 60	24	Accelerated plant growth	[99]
Lettuce (Lactuca sativa L. ‘Summer Surge’)	full-alternated	18 h:6 h	R (660 nm) = 80 B (450 nm) = 80	24	Enhance plant growth.	[100]
		12 h:12 h	R (660 nm) = 120 B (450 nm) = 40		Enhance plant growth.
Leaf lettuce (Lactuca sativa L. ‘Greenwave’)	full-alternated	12 h:12 h	R (660 nm) = 120 B (450 nm) = 120	24	Increase total and fresh weight	[101]
green and red pak choi (Brassica campestris L. ssp. chinensis var. communis)	full-alternated	1 h:1 h	R (660 nm) = 100 B (460 nm) = 100	16	Increasing the biomass of pak choi	[102]
		4 h:4 h			Enhance the accumulation of health-promoting compounds
Leaf lettuce (Lactuca sativa L. cv. ‘Greenwave’)	full-alternated	21 h:3 h	R (660 nm) = 100 B (450 nm) = 100	24	The shoot fresh weight under this treatment was significantly the highest	[103]
Butterhead lettuce (Lactuca sativa L. ‘Flandria’)	full-alternated	30 min:30 min	R (660 nm) = 180 ± 5 B (450 nm) = 20 ± 5	16	Increase the fresh weight, dry weight, and the content of pigment and soluble sugar, and require the least number of photons and electricity per gram DW produced	[104]
		60 min:60 min			Obtain higher soluble sugar content and the total sweetness index (TSI), as well as lower crude fiber content

Table 4. Optimal alternating lighting parameters for plants.

Plants	Lighting Modes	R/B Alternating Intervals	Light Intensity (μmol·m−2·s−1)	Photoperiod (h)	Effect	Reference
Lettuce (Lactuca sativa var. crispa ‘Green Oak Leaf’)	full-alternated	8 h:8 h 1 h:1 h	R (660 nm) = 200 ± 5 B (450 nm) = 100 ± 5	16	Accelerate lettuce growth speed and promote the shoot biomass	[96]
		4 h:4 h 2 h:2 h			Raise the ascorbic acid content and decrease the nitrate content
Lettuce (Lactuca sativa L. var. crispa)	full-alternated	12 h:12 h	R (660 nm) = 100 B (450 nm) = 60	24	Accelerated plant growth	[99]
Lettuce (Lactuca sativa L. ‘Summer Surge’)	full-alternated	18 h:6 h	R (660 nm) = 80 B (450 nm) = 80	24	Enhance plant growth.	[100]
		12 h:12 h	R (660 nm) = 120 B (450 nm) = 40		Enhance plant growth.
Leaf lettuce (Lactuca sativa L. ‘Greenwave’)	full-alternated	12 h:12 h	R (660 nm) = 120 B (450 nm) = 120	24	Increase total and fresh weight	[101]
green and red pak choi (Brassica campestris L. ssp. chinensis var. communis)	full-alternated	1 h:1 h	R (660 nm) = 100 B (460 nm) = 100	16	Increasing the biomass of pak choi	[102]
		4 h:4 h			Enhance the accumulation of health-promoting compounds
Leaf lettuce (Lactuca sativa L. cv. ‘Greenwave’)	full-alternated	21 h:3 h	R (660 nm) = 100 B (450 nm) = 100	24	The shoot fresh weight under this treatment was significantly the highest	[103]
Butterhead lettuce (Lactuca sativa L. ‘Flandria’)	full-alternated	30 min:30 min	R (660 nm) = 180 ± 5 B (450 nm) = 20 ± 5	16	Increase the fresh weight, dry weight, and the content of pigment and soluble sugar, and require the least number of photons and electricity per gram DW produced	[104]
		60 min:60 min			Obtain higher soluble sugar content and the total sweetness index (TSI), as well as lower crude fiber content

6. Dynamic Lighting

Dynamic lighting refers to breaking the constant lighting mode of plant factories using a dynamic LED light formula to provide dynamic lighting conditions for plants according to the needs of plant growth. The light intensity, light quality, and photoperiod can be regulated with time. On the basis of constant light, the establishment of dynamic light patterns is an important feature of the LED plant lighting system for the future development of lighting strategies, and many dynamic light patterns do not exist under natural conditions. By automating the temporal regulation of light patterns according to plant physiological or production goals, and then integrating them into regulatory strategies for plant production in facilities, it is possible to maximize plant productivity and obtain maximum biomass and carbohydrate content in facilities.

Current studies suggest that the growth of indoor-cultivated plants depends on spectral quality, where optimized light combinations enhance photosynthetic efficiency and biomass accumulation by modulating key physiological parameters [109]. In addition to spectral combinations, light intensity also influences the yield and quality of crops. Previous studies have shown that lettuce exhibits greater dry and fresh biomass when exposed to higher light intensity (170 μmol·m⁻²·s⁻¹) compared to lower levels (85 μmol·m⁻²·s⁻¹) [110]. At the same time, the application of sustained high-intensity (500 μmol·m⁻²·s⁻¹) red light during the later stages of plant development can effectively reduce nitrate levels [111]. Yamada et al. [112] demonstrated that at an identical photosynthetic photon flux, sweet potato plants subjected to stepped illumination exhibited a 1.1-fold increase in dry weight compared to those receiving constant illumination, suggesting that stepped illumination can significantly enhance the efficiency of electrical energy utilization.

For modern controlled environment agriculture, crop production typically separates the seedling stage from subsequent cultivation periods due to the differences in planting densities and optimal environmental conditions required at distinct growth phases [113]. It is very important to optimize the growth of plants, LED quality, and/or light intensity in time and take a growth stage-dependent lighting strategy to produce high-quality plants and achieve low energy consumption [114]. For instance, Chang et al. [115] demonstrated that the provision of appropriate light supplementation during specific plant growth stages (28 d after planting) could effectively enhance the biomass production of leaf lettuce. Additionally, the RB-UV-A light formula can be used during the vegetative stage of lettuce growth to reduce nitrate content. Samuoliene et al. [116] reported that the use of 638 nm LEDs (300 μmol·m⁻²·s⁻¹) in combination with HPS lighting and natural illumination 3 d before harvesting in the greenhouse could reduce the nitrate content in leaf lettuce. A lower R/B ratio was applied by Nicole et al. [114] when lettuce approached its peak growth efficiency (the end of lettuce growth) in order to minimize growth losses caused by spectral changes. Moreover, dynamic lighting is also largely used in the production of the healing and acclimatization stage of grafted seedlings [117,118,119,120]. In the early stage of graft healing, it is recommended to maintain a relative humidity (RH) exceeding 95% and a low light intensity to prevent wilting of the grafted plants [121,122]. Once the graft union has successfully formed, the RH should be gradually reduced over the subsequent 3–4 d, concurrently with an increase in light intensity to facilitate acclimatization to standard conditions [121]. And grafted plants placed in a controlled acclimation chamber almost use dynamic illumination with gradually increasing light intensity [123]. Besides the dynamic lighting of light intensity, Bantis et al. [124] demonstrated that dynamic light spectra significantly influence seedling physiological status, root development, and overall quality during graft healing. Specifically, blue light is critical for the first 3 days post-grafting, while red light becomes essential after vascular reconnection.

Plant growth and development is a complex process, with significant differences in their requirements for external light environment at different growth stages. For instance, applying two distinct multi-spectral LED formulas during the seedling and vegetative stages produced different growth responses in plants [115]. Low light treatment at the seedling stage had no significant effects on the biomass, nutrients, non-edible biomass components, and health indexes of wheat plants. However, low-light treatment during the grain filling stage significantly affected the final yield [125]. Understanding the light requirements of plants and precisely adjusting the light intensity and spectral composition according to each growth stage is an important method to increase yield while reducing production costs. It should be noted that the effective application of dynamic light supply strategies requires data support from the light environment and physiological response data, as well as the precise identification and differentiation of different growth stages of plants.

7. EOD Far-Red

Plants exhibit classic shade-avoidance syndrome (SAS) under low R:FR conditions, which is characterized by stem and petiole elongation, enhanced leaf expansion, and elevated leaf angles [126,127]. These morphological adaptations optimize light capture for photosynthesis. However, SAS responses occur not only during sustained daytime low R:FR exposure but also following brief end-of-day (EOD) far-red illumination, both of which are mediated through phytochrome signaling pathways. Previous studies have found that EOD far-red plays an important role in morphological regulation, yield improvement, and resistance enhancement, and has been widely used in facility vegetable cultivation [128,129,130] (Table 5).

Plant morphology control is essential for greenhouse transplant production [131], and EOD far-red is proven to be an economically feasible non-chemical means allowing transplant propagation, ornamental cut flowers, and the vegetable grafting industry to produce ideal seedlings with longer hypocotyl lengths. In recent years, stem and petiole elongation induced by a higher fraction of FR light during the EOD has been widely studied in various species [132], including poinsettia [133] (Table 5), watermelon (Citrullus lanatus) [134], cowpea (Vigna sinensis) [135] (Table 5), sweet pepper (Capsicum annuum L.) [136] (Table 5), petunia [137] (Table 5), Chrysanthemum morifolium [138,139,140], Antirrhinum majus L. [140], Helianthus annuus L. [140], Matthiola incana L. [140] (Table 5), Euphorbia pulcherrima Willd. ex Klotzsch [128] (Table 5), Lactuca sativa L. [141] (Table 5), Solanum lycopersicum [131] (Table 5), and Cucumis sativus L. [142]. EOD far-red can also induce flowering in long-day plants. However, it can concurrently stimulate excessive internode elongation, compromising transplant architectural quality. Shibuya et al. [143] found that RH can reduce EOD-FR-induced stem elongation as well as inhibit flower development.

In addition to the stimulation of the stem elongation, Zhang et al. [130] indicated that the supplementary short duration of FR light at EOD can also alter leaf morphology. Such plant morphology, with elongated stems and expanded leaf area, under EOD far-red enhances canopy light penetration and homogenizes vertical light distribution (Table 5). This consequently led to higher total plant biomass production and ripe fruit yield. Zou et al. [144] found that adding FR to red and blue light at the end of the day in a fully controlled environment substantially improved biomass production of lettuce (Lactuca sativa L. cv. ‘Tiberius’) and contributed to higher plant radiation use efficiency (RUE). Yang et al. [145] proved that supplementing EOD-FR light to increase hypocotyl length and light interception could promote the growth of squash rootstock (Cucurbita maxima × Cucurbita moschata Tetsukabuto). While substantial initial investment remains a barrier to LED adoption in greenhouse or indoor plant cultivation, strategically applying EOD far-red to enhance production efficiency is vital for the sustainable application of LEDs in agricultural production. In addition, the current research established that EOD far-red irradiation treatment can enhance the stress and disease resistance of plants. Critically, specific tomato (Solanum lycopersicum) cultivars and their rootstocks develop intumescence when cultivated under RB (red and blue) or RGB (red, blue, and green) LED spectra [146]. The application of EOD far-red significantly suppresses intumescence injury. If combined with high-intensity blue light, the root expansion damage could be further reduced, and counteract the undesirable stem elongation caused by far red light [147].

Phytochromes critically mediate plant responses to end-of-day far-red (EOD-FR) irradiation [79]. Under daylight conditions, the predominant Pfr isoform modulates stem elongation and photoassimilate partitioning [148,149]. During nocturnal periods, Pfr undergoes dark reversion to the inactive Pr state, a process that regulates hypocotyl extension and photomorphogenic development [129,149]. Imposing a pulse of FR light at the termination of the light period can effectively remove this residual Pfr from the cell for the duration of the dark period, and results in the conversion of Pfr configuration to Pr configuration in plants [150,151,152], which finally regulates plant growth such as extension of hypocotyls and petioles and leaf hyponasty [153,154,155,156,157,158] through the modulation of auxin [128], abscisic acid [159], erythromycin, ethylene, cytokinin, and brassinolactone [128,160,161]. For example, Dubois et al. [159] indicated that EDO far-red photoconverts PhyB-Pfr to PhyB-Pr, resulting in a derepression of elongation mediated by gibberellin. In addition, ABA levels are reduced following EOD-FR exposure during night breaks in a PHYA-dependent manner, contributing to mesocotyl elongation. In Arabidopsis, EOD-FR treatment acts through PHYB-type PHYs and enhances extension growth of the petioles by regulating expression of GA20-oxidase biosynthetic genes [162]. For cut chrysanthemum ‘Zimba’, Hisamatsu et al. [163] have shown that under photoperiods of between 9 h and 14 h, EOD-FR exposure promotes stem elongation via enhanced GA3 sensitivity, which partially mediates the observed extension growth stimulation.

The molecular mechanism by which FR treatments inhibit leaf development in Arabidopsis has been extensively studied. Existing evidence suggests that EOD far-red, specifically during early leaf development, results in restricted cell proliferation, thereby inhibiting leaf growth. INTERACTING FACTOR 7 (PIF7) acts as a central repressor in repressing cell division by directly inhibiting ANGUSTIFOLIA3 (AN3). Hussain et al. [164] demonstrate that EOD-FR activates PIF7, which competitively displaces AN3 through binding shared cis-regulatory elements (G-box/PBE-box) in cell cycle gene promoters, ultimately restricting the leaf blade expansion. In addition, Mizuno et al. [129] suggested that PIF7 also plays a predominant role in shade avoidance-induced hypocotyl elongation, and proposed the mechanism by which PIF7, together with PIF4 and PIF5, coordinately transcribe a set of downstream genes to promote elongation of hypocotyls under the end-of-day far-red light (EOD-FR) conditions in A. thaliana.

Compared with plants receiving supplemental artificial lighting, the characteristics that are usually considered unfavorable for seedling quality were common across EOD-FR treatments [137]. However, far-red light can induce shade avoidance syndrome (SAS), which provides a basic survival strategy for relatively dense plant populations. The effect of the “afternoon shade event” seen here on stem elongation compared with shorter EOD-FR periods may be of use in rootstock production for crops such as tomato and squash, and in cut flower production. However, the minimum dose (intensity × duration) requirements of EOD-FR and the kinetic response of EOD-FR (the relationship between plant growth and EOD-FR dose) need to be considered for effective and efficient applications of EOD-FR treatment. In the future, we will also need to verify the optimal light intensity and duration of far-red light treatment before the dark period through a large number of experiments, to regulate plant height and flowering period according to specific requirements.

Table 5. Optimal EOD-fr parameters for plants.

Plants	Light Intensity (μmol·m−2·s−1)	Light Quality	Treatment Duration	Effect	Reference
Poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch)	10	R (630 nm)	30 min	Modulate the contents of the growth-controlling hormones to control shoot elongation and counteract the effect of the increased FR proportion at the EOD	[128]
Tomato (Solanum lycopersicum ‘Ruifen 882’)	43	FR (735)	30 min	Stimulated stem elongation, altered leaf morphology toward a higher leaf length/width ratio and larger leaf area, and stimulated tomato growth and production	[130]
Tomato rootstock [Solanum lycopersicum ‘Aloha’ and Solanum × lycopersicum (S. lycopersicum × S. habrochaites) ‘Maxifort’]	/	Incandescent lamps	24 min	Elongation was achieved without compromising growth and development	[131]
Poinsettia (Euphorbia pulcherrima’ White Glitter’ and ‘Marble Star’)	2–3	4 h R + FR (451 nm, 669 nm, 745 nm) + 2 h FR (745 nm)		Increase poinsettia extension growth and delay flowering	[133]
Cowpea (Vigna unguiculata)	total fluence of 222 μmol·m−2	500 W reflector flood lamp	10 min	Increased both epicotyl elongation and GA1 content in the responsive epicotyl	[135]
Sweet pepper (Capsicum annuum L. cv. ‘Frazier’)	30	/	30 min	Led to more stem elongation, more upright branches, and more dry mass partitioning to stems	[136]
Petunia (Petunia × hybrida)	20	R (600–700 nm):FR (700–780 nm) = 0.15	240 min	Greater elongation was observed when the R:FR decreased from 0.8 to 0.15, and when treatment duration increased from 30 min to 240 min	[137]
Chrysanthemum (Chrysanthemum morifolium ‘Dekmona’, ‘Sei-elza’, and ‘Tourmalin’)	/	FR fluorescent tubes	15 min	Promote stem elongation	[140]
Sunflower (Helianthus annuus L.’Sunrich Orange’ and ‘Summer Sunrich Pine 45’); Snapdragon (Antirrhinum majus L. ‘Calyon White’ and ‘Reisen’); Stock (Matthiola incana L. ‘Pink Iron’) and Bupleurum (Bupleurum spp. ‘Green Gold’); Carnation (Dianthus caryophyllus L. ‘Barbara’)			30 min	promote extension growth
Two cultivars each of Helianthus annuus L. and Antirrhinum majus, and Matthiola incana, Bupleurum spp., Dianthus caryophyllus			30 min	Promote flowering
Celosia argentea L., var. cristata (L.) Kuntze (‘Delhi Pearl’)			30 min	Promote stem elongation but delay flowering
Lettuce (Lactuca sativa L. cv. ‘Red butter’ and ‘Green butter’)	White light as the basil light of EOD lighting, and supplementary far-red light		30 min	Stimulate the plant and shoot biomass	[141]

Table 5. Optimal EOD-fr parameters for plants.

Plants	Light Intensity (μmol·m−2·s−1)	Light Quality	Treatment Duration	Effect	Reference
Poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch)	10	R (630 nm)	30 min	Modulate the contents of the growth-controlling hormones to control shoot elongation and counteract the effect of the increased FR proportion at the EOD	[128]
Tomato (Solanum lycopersicum ‘Ruifen 882’)	43	FR (735)	30 min	Stimulated stem elongation, altered leaf morphology toward a higher leaf length/width ratio and larger leaf area, and stimulated tomato growth and production	[130]
Tomato rootstock [Solanum lycopersicum ‘Aloha’ and Solanum × lycopersicum (S. lycopersicum × S. habrochaites) ‘Maxifort’]	/	Incandescent lamps	24 min	Elongation was achieved without compromising growth and development	[131]
Poinsettia (Euphorbia pulcherrima’ White Glitter’ and ‘Marble Star’)	2–3	4 h R + FR (451 nm, 669 nm, 745 nm) + 2 h FR (745 nm)		Increase poinsettia extension growth and delay flowering	[133]
Cowpea (Vigna unguiculata)	total fluence of 222 μmol·m−2	500 W reflector flood lamp	10 min	Increased both epicotyl elongation and GA1 content in the responsive epicotyl	[135]
Sweet pepper (Capsicum annuum L. cv. ‘Frazier’)	30	/	30 min	Led to more stem elongation, more upright branches, and more dry mass partitioning to stems	[136]
Petunia (Petunia × hybrida)	20	R (600–700 nm):FR (700–780 nm) = 0.15	240 min	Greater elongation was observed when the R:FR decreased from 0.8 to 0.15, and when treatment duration increased from 30 min to 240 min	[137]
Chrysanthemum (Chrysanthemum morifolium ‘Dekmona’, ‘Sei-elza’, and ‘Tourmalin’)	/	FR fluorescent tubes	15 min	Promote stem elongation	[140]
Sunflower (Helianthus annuus L.’Sunrich Orange’ and ‘Summer Sunrich Pine 45’); Snapdragon (Antirrhinum majus L. ‘Calyon White’ and ‘Reisen’); Stock (Matthiola incana L. ‘Pink Iron’) and Bupleurum (Bupleurum spp. ‘Green Gold’); Carnation (Dianthus caryophyllus L. ‘Barbara’)			30 min	promote extension growth
Two cultivars each of Helianthus annuus L. and Antirrhinum majus, and Matthiola incana, Bupleurum spp., Dianthus caryophyllus			30 min	Promote flowering
Celosia argentea L., var. cristata (L.) Kuntze (‘Delhi Pearl’)			30 min	Promote stem elongation but delay flowering
Lettuce (Lactuca sativa L. cv. ‘Red butter’ and ‘Green butter’)	White light as the basil light of EOD lighting, and supplementary far-red light		30 min	Stimulate the plant and shoot biomass	[141]

8. Conclusions

Light quality, light intensity, and photoperiod are the three major elements of light environment. The light strategy refers to the arrangement and combination of light quality, intensity, and photoperiod in different time dimensions at different growth stages of crops or within a certain development period in order to achieve a more ideal cultivation effect. This paper not only reviews the effects of lighting strategies such as night-break and dynamic lighting on the yield, quality, and energy utilization efficiency of vegetables in protected horticulture, but also explores the physiological responses of vegetables to various lighting strategies, providing valuable insights for optimizing high-yield and sustainable conservation horticultural production. Intermittent and alternate lighting serve as effective strategies for enhancing crop yield and quality without additional energy consumption; night-break and EOD far-red can precisely control flowering and architecture via phytochrome photoconversion; through dynamic lighting strategies, light environment parameters can be provided as needed at different growth stages of plants; and pre-harvest short-term continuous lighting has emerged as a cost-effective and operationally efficient approach for enhancing the nutritional quality of hydroponic leafy vegetables. With the in-depth research on plant photobiology and the development of technology, in the future, we will be able to adjust the light strategy in a timely manner according to the real-time status of plants, making plants become efficient energy conversion machines and achieving rapid biomass accumulation and target product production in a short period of time.