The types of facilities in facility horticulture mainly include plastic greenhouses, sunlight greenhouses, multi story greenhouses, and plant factories. Due to the fact that facility buildings block natural light sources to a certain extent, indoor lighting is insufficient, resulting in reduced crop yield and quality. Therefore, fill lights play an indispensable role in the high-quality and high-yield of facility crops, but also become the main factor in increasing energy consumption and operating costs within the facility.

For a long time, artificial light sources used in the field of facility horticulture mainly include high-pressure sodium lamps, fluorescent lamps, metal halide lamps, incandescent lamps, etc. The prominent disadvantages are high heat production, high energy consumption, and high operating costs. The development of the new generation of Light Emitting Diodes (LEDs) has made it possible to apply low energy artificial light sources in the field of facility horticulture.

LED has advantages such as high photoelectric conversion efficiency, use of direct current, small size, long lifespan, low energy consumption, fixed wavelength, low thermal radiation, and environmental protection. Compared with commonly used high-pressure sodium lamps and fluorescent lamps, LED not only allows for precise adjustment of light quantity and quality (such as the proportion of light in various bands) according to the needs of plant growth, but also allows for close illumination of plants due to its cold light properties, thereby increasing the number of cultivation layers and space utilization, achieving energy-saving, environmental protection, and efficient space utilization functions that traditional light sources cannot replace.

Based on these advantages, LED has been successfully applied in facility horticultural lighting, basic research on controllable environments, plant tissue culture, factory seedling cultivation, and aerospace ecosystems. In recent years, the performance of LED grow lights has been continuously improving, their prices have gradually decreased, and various specific wavelength products have been gradually developed. Their application scope in agriculture and biology will be even broader.

This article reviews the current research status of LED in the field of facility horticulture, with a focus on the photobiological basis of LED supplementary lighting applications, the impact of LED supplementary lighting on plant photomorphogenesis, nutritional quality and anti-aging, the construction and application of light formulas, and other aspects. It also analyzes and looks forward to the current problems and prospects of LED supplementary lighting technology.

The effect of LED supplementary light on the growth of horticultural crops

The regulatory effects of light on plant growth and development include seed germination, stem elongation, leaf and root development, phototropism, chlorophyll synthesis and decomposition, and flower induction. The lighting environment elements inside the facility include light intensity, light cycle, and spectral distribution. By manually supplementing light, their elements can be adjusted without being limited by weather conditions.

Plants have the characteristic of selective absorption of light, and different light receptors perceive light signals. Currently, it has been found that there are at least three types of light receptors in plants: photosensitizers (absorbing red and far red light), cryptocyanins (absorbing blue and near ultraviolet light), and ultraviolet light receptors (UV-A and UV-B). Using a specific wavelength light source to illuminate crops can improve the photosynthetic efficiency of plants, accelerate their light morphogenesis, and promote their growth and development.

Plant photosynthesis mainly utilizes red orange light (610-720 nm) and blue purple light (400-510 nm). By utilizing LED technology, monochromatic light (such as red light with a peak of 660nm and blue light with a peak of 450nm) that conforms to the strongest absorption band of chlorophyll can be emitted, with a spectral domain width of only ± 20 nm.

At present, it is believed that red orange light can significantly accelerate plant development, promote the accumulation of dry matter, form bulbs, tubers, leaf bulbs, and other plant organs, cause plants to bloom and bear fruit earlier, and play a dominant role in plant coloration; Blue and purple light can control the phototropism of plant leaves, promote stomatal opening and chloroplast movement, inhibit stem elongation, prevent plant overgrowth, delay plant flowering, and promote the growth of nutrient organs; The combination of red and blue LED can compensate for the deficiency of monochromatic light in both, forming a spectral absorption peak that is basically consistent with crop photosynthesis and morphology. The light energy utilization rate can reach 80%~90%, and the energy-saving effect is significant.

Equipping LED supplementary lights in facility horticulture can achieve significant yield increase effects. Studies have shown that 300 μ The number of fruits, total yield, and single fruit weight of cherry tomatoes under 12 hours (8:00-20:00) of supplementary lighting with mol/(m? · s) LED strips and LED tubes were significantly increased. The supplementary lighting with LED strips increased by 42.67%, 66.89%, and 16.97%, respectively, while the supplementary lighting with LED tubes increased by 48.91%, 94.86%, and 30.86%, respectively. Full growth period LED light replenishment [red blue light quality ratio of 3:2, light intensity of 300] μ The treatment of mol/(m? · s) significantly increased the single fruit weight and unit area yield of Jiegua and eggplant, with Jiegua increasing by 5.3% and 15.6%, and eggplant increasing by 7.6% and 7.8%. By adjusting the temporal and spatial distribution of LED light quality, intensity, and duration throughout the entire growth period, it is possible to shorten the plant growth cycle, improve the commercial yield, nutritional quality, and morphological value of agricultural products, and achieve efficient, energy-saving, and intelligent production of horticultural crops in facilities.

Application of LED supplementary light in vegetable seedling cultivation

LED light source regulation of plant morphogenesis and growth and development is an important technology in the field of greenhouse cultivation. Higher plants can sense and receive light signals through photoreceptor systems such as photosensitive pigments, cryptocyanins, and phototropins, and regulate morphological changes in plant tissues and organs through intracellular messenger transduction. Photomorphogenesis is the process in which plants rely on light to control cell differentiation, structural and functional changes, as well as tissue and organ development. This includes effects on partial seed germination, promotion of apical dominance to inhibit lateral bud growth, stem elongation, and induction of meridional movement.

Vegetable seedling cultivation is an important part of facility agriculture. Continuous cloudy and rainy weather can lead to insufficient lighting in the facilities, making seedlings prone to elongation, which in turn affects the growth, flower bud differentiation, and fruit development of vegetables, ultimately affecting their yield and quality. In production, some plant growth regulators such as gibberellin, auxin, paclobutrazol, and chloramphenicol are used to regulate the growth of seedlings. However, the unreasonable use of plant growth regulators can easily pollute vegetables and facility environments, which is detrimental to human health.

LED supplementary lighting has many unique advantages in supplementary lighting, and the application of LED supplementary lighting in seedling cultivation is a feasible approach.

In low light [0-35] μ LED replenishment under the condition of mol/(m? · s) [25 ± 5] μ In the mol/(m? · s) experiment, it was found that green light promoted the elongation and growth of cucumber seedlings, while red and blue light inhibited seedling elongation. Compared with natural weak light, the strong seedling index of seedlings supplemented with red and blue light increased by 151.26% and 237.98%, respectively. Moreover, compared with monochromatic light, the strong seedling index of cucumber seedlings treated with composite light containing red and blue components increased by 304.46%. Supplementing cucumber seedlings with red light can increase their true leaf number, leaf area, plant height, stem thickness, dry and fresh weight, seedling strength index, root vitality, SOD activity, and soluble protein content. Supplementing with UV-B can increase the chlorophyll a, chlorophyll b, and carotenoid content in cucumber seedling leaves; Compared with natural light, supplementing LED red and blue light significantly increased the leaf area, dry matter quality, and seedling strength index of tomato seedlings. Supplementing LED red and green light significantly increased the height and stem thickness of tomato seedlings; LED green light supplementation treatment can significantly increase the biomass of cucumber and tomato seedlings, and the fresh and dry weight of young seedlings show an increasing trend with the increase of green light supplementation intensity. However, the stem diameter and strong seedling index of tomato seedlings increase with the increase of green light supplementation intensity; The combination of LED red and blue light can increase the stem thickness, leaf area, whole plant dry weight, root to shoot ratio, and seedling strength index of eggplants; Compared with white light, LED red light can increase the biomass of cabbage seedlings, promote the elongation growth and leaf expansion of cabbage seedlings; LED blue light promotes the thickening growth, dry matter accumulation, and seedling strength index of cabbage seedlings, leading to dwarfing of cabbage seedlings. The above results indicate that vegetable seedlings cultivated by combining light regulation technology have significant advantages.

The influence of LED supplementary light on the nutritional quality of fruits and vegetables

The protein, sugars, organic acids, and vitamins contained in fruits and vegetables are beneficial nutrients for human health. Light quality can affect the content of VC in plants by regulating the activity of VC synthesis and decomposition enzymes, and has a regulatory effect on protein metabolism and carbohydrate accumulation in horticultural plants. Red light promotes carbohydrate accumulation, while blue light treatment is beneficial for protein formation. The combination of red and blue light significantly improves the nutritional quality of plants compared to monochromatic light. Supplementing LED red or blue light can reduce the nitrate content in lettuce, supplementing blue or green light can promote the accumulation of soluble sugars in lettuce, and supplementing infrared light is beneficial for the accumulation of VC in lettuce. Supplementing with blue light can promote the increase of VC content and soluble protein content in tomatoes; The combination of red light and red blue light treatment promotes the sugar and acid content in tomato fruits, and the sugar to acid ratio is highest under the combination of red and blue light treatment; The combination of red and blue light can promote the increase of VC content in cucumber fruits.

The phenolic substances, flavonoids, anthocyanins and other substances contained in fruits and vegetables not only have a significant impact on the color, flavor, and commercial value of fruits and vegetables, but also have natural antioxidant activity, which can effectively inhibit or eliminate free radicals in the human body. The use of LED blue light supplementation can significantly increase the content of anthocyanins in eggplant peel by 73.6%, while the use of LED red light and red blue combination light can increase the content of flavonoids and total phenols; Blue light can promote the accumulation of lycopene, flavonoids, and anthocyanins in tomato fruits. The combination of red and blue light can promote the generation of anthocyanins to a certain extent, but inhibit the synthesis of flavonoids; Compared with white light treatment, red light treatment can significantly increase the anthocyanin content in lettuce aboveground parts, but blue light treatment has the lowest anthocyanin content in lettuce aboveground parts; The total phenolic content of green leaf, purple leaf, and red leaf lettuce was higher under white light, red blue combined light, and blue light treatments, but it was the lowest under red light treatment; Supplementing LED ultraviolet or orange light can increase the content of phenolic compounds in lettuce leaves, while supplementing green light can increase the content of anthocyanins. Therefore, using LED supplementary lighting is an effective way to regulate the nutritional quality of fruits and vegetables in facilities.

The effect of LED fill light on delaying plant aging

The degradation of chlorophyll, rapid loss of protein, and RNA hydrolysis during plant aging are mainly manifested as leaf aging. Chloroplasts are highly sensitive to changes in the external light environment, especially significantly influenced by light quality. Red light, blue light, and a combination of red and blue light are beneficial for the morphogenesis of chloroplasts. Blue light is beneficial for the accumulation of starch granules in chloroplasts, while red and far red light have negative effects on chloroplast development. Blue light and the combination of red and blue light can promote the synthesis of chlorophyll in cucumber seedling leaves, while the combination of red and blue light can also delay the decline of chlorophyll content in the later stage. This effect becomes more pronounced with the decrease of red light ratio and the increase of blue light ratio. The chlorophyll content of cucumber seedling leaves under LED red and blue combined light treatment was significantly higher than that under fluorescent light control and monochromatic red and blue light treatment; LED blue light can significantly increase the chlorophyll a/b values of Wutai vegetable and green garlic seedlings.

Changes in content of cytokinin (CTK), auxin (IAA), abscisic acid (ABA), and various enzyme activities occur during leaf senescence. The content of plant hormones is easily influenced by the light environment, and different light qualities have different regulatory effects on plant hormones. The initial steps of the light signal transduction pathway involve cytokinins. CTK promotes leaf cell expansion, enhances leaf photosynthesis, and inhibits the activities of ribonuclease, deoxyribonuclease, and protease, delaying the degradation of nucleic acid, protein, and chlorophyll, thus significantly delaying leaf aging. There is an interaction between light and CTK mediated developmental regulation, where light can stimulate an increase in endogenous cytokinin levels. When plant tissues are in an aging state, their endogenous cytokinin content decreases. IAA is mainly concentrated in areas with vigorous growth, and its content is minimal in aging tissues or organs. Purple light can enhance the activity of indole-3-acetic acid oxidase, while low levels of IAA can inhibit plant elongation and growth. ABA is mainly formed in aging leaf tissue, mature fruits, seeds, stems, roots and other parts. Under red blue combined light, the ABA content in cucumber and cabbage is lower than that under white and blue light.

Peroxidase (POD), superoxide dismutase (SOD), ascorbic acid peroxidase (APX), and catalase (CAT) are important and light related protective enzymes in plants. If plants age, the activity of these enzymes will rapidly decrease. The effect of different light qualities on plant antioxidant enzyme activity is significant. After 9 days of red light treatment, the APX activity of rapeseed seedlings significantly increases, while the POD activity decreases; After 15 days of red and blue light irradiation, the POD activity of tomatoes was 20.9% and 11.7% higher than that of white light, respectively. After 20 days of green light treatment, the POD activity was the lowest, only 55.4% of that of white light; Supplementing with 4 hours of blue light can significantly increase the soluble protein content, POD, SOD, APX, and CAT enzyme activity in cucumber seedling leaves. In addition, the activities of SOD and APX gradually decrease with the prolongation of light exposure time. The activities of SOD and APX under blue and red light irradiation decreased slowly but remained higher than those under white light. Red light irradiation significantly reduced the peroxidase and IAA peroxidase activities in tomato leaves and eggplant leaves, but caused a significant increase in the peroxidase activity in eggplant leaves. Therefore, adopting a reasonable LED lighting strategy can effectively delay the aging of horticultural crops in facilities, improve yield and quality.

Construction and application of LED light formula

The growth and development of plants are significantly influenced by light quality and its different composition ratios. The light formula mainly includes several elements such as light quality ratio, light intensity, and light duration. Due to the differences in light requirements among different plants and their varying growth and development stages, it is necessary to optimize the combination of light quality, intensity, and replenishment time for the cultivated crops.

Light quality ratio

Compared with white light and single red and blue light, the combination of LED red and blue light shows a comprehensive advantage in the growth and development of cucumber and cabbage seedlings. When the ratio of red and blue light is 8:2, the stem diameter, plant height, plant trunk, fresh weight, and seedling strength index of the plant are significantly improved, while also promoting the formation of chloroplast matrix and basal grain layer and the output of assimilates. Under the red blue light ratio of 8:1, cucumber seedlings had the highest plant height, stem diameter, leaf area, seedling strength index, aboveground and whole plant fresh weight, and the seedling leaves had high POD and APX activities; Under the red blue light ratio of 6:3, the root activity, soluble protein and sugar content, and net photosynthetic rate of cucumber seedlings were the highest, and SOD activity was relatively high. The use of a combination of red, green, and blue light is beneficial for the accumulation of dry matter in red bean sprouts. Adding green light has a promoting effect on the accumulation of dry matter in red bean sprouts, with the most significant increase observed in the red green and blue light ratio of 6:2:1; The red and blue light ratio of 8:1 had the best effect on the elongation of the hypocotyl of red bean sprouts. The red and blue light ratio of 6:3 had a significant inhibitory effect on the elongation of the hypocotyl of red bean sprouts, but the soluble protein content was the highest. When using a red and blue light ratio of 8:1 for the treatment of luffa seedlings, the strongest seedling index and highest soluble sugar content were observed. When using a red and blue light ratio of 6:3, the highest chlorophyll a content, chlorophyll a/b ratio, and soluble protein content were observed in luffa seedlings. When using a red blue light ratio of 3:1 for celery, it can effectively promote the increase of celery plant height, petiole length, number of leaves, dry matter quality, VC content, soluble protein content, and soluble sugar content; In tomato cultivation, increasing the proportion of LED blue light promotes the formation of lycopene, free amino acids, and flavonoids, while increasing the proportion of red light promotes the formation of titratable acids; When using a red blue light ratio of 8:1 on lettuce leaves, it is beneficial for the accumulation of carotenoids, effectively reducing their nitrate content and increasing their VC content.

Light intensity

Plants are more susceptible to light inhibition when growing under weak light than under strong light. The net photosynthetic rate of tomato seedlings varies with light intensity [50, 150, 200, 300, 450, 550] μ The increase in mol/(m? · s) shows a trend of first increasing and then decreasing, and reaches 300 μ Reached maximum at mol/(m? · s); The plant height, leaf area, water content, and VC content of lettuce are within 150 μ Significant increase in mol/(m? · s) light intensity treatment at 200 μ Under the treatment of mol/(m? · s) light intensity, the fresh weight, total weight, and free aromatic acid content of lettuce aboveground parts were significantly increased, while at 300 μ Under the treatment of mol/(m? · s) light intensity, the leaf area, water content, chlorophyll a, chlorophyll a+b, and carotenoids of lettuce all decreased; Compared to darkness, with the increase of LED supplementary light intensity [3, 9, 15 μ The increase of mol/(m? · s) significantly increased the content of chlorophyll a, chlorophyll b, and chlorophyll a+b in black bean sprouts and vegetables, with a light intensity of 3 μ At mol/(m? · s), the VC content is highest at 9 μ The content of soluble protein, soluble sugar, and sucrose is highest at mol/(m? · s); Under the same temperature conditions, with the increase of light intensity [(2-2.5) lx x x 103 lx, (4-4.5) lx x x 103 lx, (6-6.5) lx x 103 lx], the seedling growth time of chili seedlings is shortened, and the soluble sugar content increases, but the chlorophyll a and carotenoid content gradually decreases.

Illumination time

Extending the light exposure time appropriately can alleviate the weak light stress caused by insufficient light intensity to a certain extent, help accumulate photosynthetic products in horticultural crops, and achieve the effect of increasing yield and improving quality. The VC content of sprouted vegetables shows a gradually increasing trend with the extension of light time (0, 4, 8, 12, 16, 20 hours/day), while the content of free amino acids, SOD, and CAT activity all show a decreasing trend; With the extension of lighting time (12, 15, 18 hours), the fresh weight of cabbage plants shows a significant increase trend; The VC content in the leaves and stems of Chinese cabbage was highest at 15 and 12 hours, respectively; The soluble protein content in the leaves of Chinese cabbage gradually decreased, but the highest was observed in the stems after 15 hours of treatment; The soluble sugar content in the leaves of cauliflower gradually increases, while the highest content is observed in the stems after 12 hours. In the case of a red and blue light ratio of 1:2, compared with a 12 hour light time, the 20 hour light treatment reduced the relative content of total phenols and flavonoids in green lettuce. However, in the case of a red and blue light ratio of 2:1, the 20 hour light treatment significantly increased the relative content of total phenols and flavonoids in green lettuce.

From the above, it can be seen that different light formulas have different effects on the photosynthesis, light morphogenesis, and carbon and nitrogen metabolism of different crop types. How to obtain the optimal light formula, light source configuration, and formulate intelligent control strategies needs to take plant species as the starting point, and appropriate adjustments should be made according to the demand for horticultural crops, production goals, production factor conditions, etc., to achieve intelligent control of light environment under energy-saving conditions and the goal of high-quality and high-yield horticultural crops.

Existing problems and prospects

The significant advantage of LED fill lights is their ability to intelligently combine and adjust spectra based on the photosynthetic characteristics, morphological construction, quality, and yield requirements of different plants. Different types of crops and different growth stages of the same crop have different requirements for light quality, light intensity, and light cycle. This requires further development and improvement of light formula research, forming a huge light formula database, and combining with the research and development of professional lighting fixtures, in order to achieve the maximum value of LED fill lights in agricultural applications, thereby better saving energy consumption, improving production efficiency and economic benefits.

The application of LED fill lights in facility horticulture has shown strong vitality, but the price of LED fill lights is relatively high, and the one-time investment is large. The fill light requirements for various crops under different environmental conditions are not clear, and the fill light spectrum, intensity, and fill light time are not reasonable, which inevitably leads to various problems when using fill lights.

However, with the advancement and improvement of technology, the production cost of LED fill lights has decreased, and LED fill lights will be more widely used in facility horticulture. At the same time, the development and progress of LED supplementary lighting technology system combined with new energy will enable the rapid development of factory agriculture, household agriculture, urban agriculture, and space agriculture to meet the needs of people for horticultural crops in special environments.