Author: Yamin Li and Houcheng Liu, etc, from College of Horticulture, South China Agriculture University

Article Source: Greenhouse Horticulture

The types of facility horticulture facilities mainly include plastic greenhouses, solar greenhouses, multi-span greenhouses, and plant factories. Because facility buildings block natural light sources to a certain extent, there is insufficient indoor light, which in turn reduces crop yields and quality. Therefore, the supplementary light plays an indispensable role in the high-quality and high-yield crops of the facility, but it has also become a major factor in the increase of energy consumption and operating costs in the facility.

For a long time, artificial light sources used in the field of facility horticulture mainly include high pressure sodium lamp, fluorescent lamp, metal halogen lamp, incandescent lamp, etc. the prominent disadvantages are high heat production, high energy consumption and high operating cost. The development of the new generation light emitting diode (LED) makes it possible to use low energy artificial light source in the field of facility horticulture. LED has the advantages of high photoelectric conversion efficiency, DC power, small volume, long life, low energy consumption, fixed wavelength, low thermal radiation and environmental protection. Compared with the high-pressure sodium lamp and fluorescent lamp commonly used at present, LED can not only adjust the light quantity and quality (the proportion of various band light) according to the needs of plant growth, and can irradiate plants at close distance due to its cold light, Thus, the number of cultivation layers and space utilization rate can be improved, and the functions of energy saving, environmental protection and space efficient utilization which can not be replaced by traditional light source can be realized.

Based on these advantages, LED has been successfully used in facility horticultural lighting, basic research of controllable environment, plant tissue culture, plant factory seedling and aerospace ecosystem. In recent years, the performance of LED grow lighting is improving, the price is decreasing, and all kinds of products with specific wavelengths are being developed gradually, so its application in the field of agriculture and biology will be broader.

This article summarizes the research status of LED in the field of facility horticulture, focuses on the application of LED supplemental light in the light biology foundation, LED grow lights on plant light forming, nutritional quality and the effect of delaying aging, the construction and application of light formula, and analyzes and prospects of the current problems and prospects of LED supplemental light technology.

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 in the facility include light intensity, light cycle and spectral distribution. The elements can be adjusted by artificial light supplement without the limitation of weather conditions.

At present, there are at least three types of photoreceptors in plants: phytochrome (absorbing red light and far red light), cryptochrome (absorbing blue light and near ultraviolet light) and UV-A and UV-B. The use of specific wavelength light source to irradiate crops can improve the photosynthetic efficiency of plants, accelerate the light morphogenesis, and promote the growth and development of plants. Red orange light (610 ~ 720 nm) and blue violet light (400 ~ 510 nm) were used in plant photosynthesis. Using LED technology, monochromatic light (such as red light with 660nm peak, blue light with 450nm peak, etc.) can be radiated in line with the strongest absorption band of chlorophyll, and the spectral domain width is only ± 20 nm.

It is currently believed that the red-orange light will significantly accelerate the development of plants, promote the accumulation of dry matter, the formation of bulbs, tubers, leaf bulbs and other plant organs, cause plants to bloom and bear fruit earlier, and play a leading role in plant color enhancement; Blue and violet light can control the phototropism of plant leaves, promote stomata opening and chloroplast movement, inhibit stem elongation, prevent plant lengthening, delay plant flowering, and promote the growth of vegetative organs; the combination of red and blue LEDs can compensate for the insufficient light of single color of the two and form a spectral absorption peak that is basically consistent with crop photosynthesis and morphology. The light energy utilization rate can reach 80% to 90%, and the energy saving effect is significant.

Equipped with LED supplementary lights in facility horticulture can achieve a very significant increase in production. Studies have shown that the number of fruits, the total output and the weight of each cherry tomato under the supplementary light of 300 μmol/(m²·s) LED strips and LED tubes for 12h (8:00-20:00) are significantly increased. The supplementary light of the LED strip has increased by 42.67%, 66.89% and 16.97% respectively, and the supplementary light of the LED tube has increased by 48.91%, 94.86% and 30.86% respectively. The LED supplement light of LED grow lighting fixture during the whole growth period [the ratio of red and blue light is 3:2, and the light intensity is 300 μmol/(m²·s)] can significantly increase the single fruit quality and yield per unit area of chiehwa and eggplant. Chikuquan increased by 5.3% and 15.6%, and eggplant increased by 7.6% and 7.8%. Through the LED light quality and its intensity and duration of the whole growth period, the plant growth cycle can be shortened, the commercial yield, nutritional quality and morphological value of agricultural products can be improved, and the high-efficiency, energy-saving and intelligent production of facility horticultural crops can be realized.

Application of LED supplement light in vegetable seedling cultivation

Regulating plant morphology and growth and development by LED light source is an important technology in the field of greenhouse cultivation. Higher plants can sense and receive light signals through photoreceptor systems such as phytochrome, cryptochrome, and photoreceptor, and conduct morphological changes through intracellular messengers to regulate plant tissues and organs. Photomorphogenesis means that plants rely on light to control cell differentiation, structural and functional changes, as well as the formation of tissues and organs, including the influence on the germination of some seeds, promotion of apical dominance, inhibition of lateral bud growth, stem elongation, and tropism.

Vegetable seedling cultivation is an important part of facility agriculture. Continuous rainy weather will cause insufficient light in the facility, and seedlings are prone to lengthening, which will affect the growth of vegetables, flower bud differentiation and fruit development, and ultimately affect their yield and quality. In production, some plant growth regulators, such as gibberellin, auxin, paclobutrazol and chlormequat, are used to regulate the growth of seedlings. However, the unreasonable use of plant growth regulators can easily pollute the environment of vegetables and facilities, human health being unfavorable.

LED supplementary light has many unique advantages of supplementary light, and it is a feasible way to use LED supplementary light to raise seedlings. In the LED supplement light [25±5 μmol/(m²·s)] experiment conducted under the condition of low light [0~35 μmol/(m²·s)], it was found that green light promotes the elongation and growth of cucumber seedlings. Red light and blue light inhibit seedling growth. 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. Compared with monochromatic light quality, the index of strong seedlings that contains red and blue components under the treatment of compound light supplement light increased by 304.46%.

Adding red light to cucumber seedlings can increase the number of true leaves, leaf area, plant height, stem diameter, dry and fresh quality, strong seedling index, root vitality, SOD activity and soluble protein content of cucumber seedlings. Supplementing UV-B can increase the content of chlorophyll a, chlorophyll b and carotenoids in cucumber seedling leaves. Compared with natural light, supplementing the red and blue LED light can significantly increase the leaf area, dry matter quality and strong seedling index of tomato seedlings. Supplementing LED red light and green light significantly increases the height and stem thickness of tomato seedlings. The LED green light supplement light treatment can significantly increase the biomass of cucumber and tomato seedlings, and the fresh and dry weight of the seedlings increases with the increase of the green light supplement light intensity, while the thick stem and strong seedling index of the tomato seedlings all follow the green light supplement light. The increase in strength increases. The combination of LED red and blue light can increase the stem thickness, leaf area, dry weight of the whole plant, root to shoot ratio, and strong seedling index of eggplant. Compared with white light, LED red light can increase the biomass of cabbage seedlings and promote the elongation growth and leaf expansion of cabbage seedlings. LED blue light promotes the thick growth, dry matter accumulation and strong seedling index of the cabbage seedlings, and makes the cabbage seedlings dwarf. The above results show that the advantages of vegetable seedlings cultivated with light regulation technology are very obvious.

Effect of LED supplementary light on nutritional quality of fruits and vegetables

The protein, sugar, organic acid and vitamin contained in fruits and vegetables are the nutrition materials which are beneficial to human health. The light quality can affect the VC content in plants by regulating the activity of VC synthesis and decomposing enzyme, and it can regulate the protein metabolism and carbohydrate accumulation in horticultural plants. Red light promotes carbohydrate accumulation, blue light treatment is beneficial to protein formation, while the combination of red and blue light can improve the nutritional quality of plants significantly higher than that of monochromatic light.

Adding red or blue LED light can reduce the nitrate content in lettuce, adding blue or green LED light can promote the accumulation of soluble sugar in lettuce, and adding infrared LED light is conducive to the accumulation of VC in lettuce. The results showed that the supplement of blue light could improve the VC content and soluble protein content of tomato; red light and red blue combined light could promote the sugar and acid content of tomato fruit, and the ratio of sugar to acid was the highest under red blue combined light; red blue combined light could improve the VC content of cucumber fruit.

The phenols, flavonoids, anthocyanins and other substances in fruits and vegetables not only have important influence on the color, flavor and commodity value of fruits and vegetables, but also have natural antioxidant activity, and can effectively inhibit or remove free radicals in human body.

Using LED blue light to supplement light can significantly increase the anthocyanin content of eggplant skin by 73.6%, while using LED red light and a combination of red and blue 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 promotes the production of anthocyanins to a certain extent, but inhibits the synthesis of flavonoids. Compared with white light treatment, red light treatment can significantly increase the anthocyanin content of lettuce shoots, but the blue light treatment has the lowest anthocyanin content. The total phenol content of green leaf, purple leaf and red leaf lettuce was higher under white light, red-blue combined light and blue light treatment, but it was the lowest under red light treatment. Supplementing LED ultraviolet light or orange light can increase the content of phenolic compounds in lettuce leaves, while supplementing green light can increase the content of anthocyanins. Therefore, the use of LED grow light is an effective way to regulate the nutritional quality of fruits and vegetables in facility horticultural cultivation.

The effect of LED supplementary light on the anti-aging of plants

Chlorophyll degradation, rapid protein loss and RNA hydrolysis during plant senescence are mainly manifested as leaf senescence. Chloroplasts are very sensitive to changes in the external light environment, especially affected by light quality. Red light, blue light and red-blue combined light are conducive to chloroplast morphogenesis, blue light is conducive to the accumulation of starch grains in chloroplasts, and, red light and far-red light have a negative effect on chloroplast development. The combination of blue light and red and blue light can promote the synthesis of chlorophyll in cucumber seedling leaves, and the combination of red and blue light can also delay the attenuation of leaf chlorophyll content in the later stage. This effect is more obvious 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 treatments. LED blue light can significantly increase the chlorophyll a/b value of Wutacai and green garlic seedlings.

During senescence, there are cytokinins (CTK), auxin (IAA), abscisic acid content changes (ABA) and a variety of changes in enzyme activity. The content of plant hormones is easily affected by the light environment. Different light qualities have different regulatory effects on plant hormones, and the initial steps of the light signal transduction pathway involve cytokinins.

CTK promotes the expansion of leaf cells, enhances leaf photosynthesis, while inhibiting the activities of ribonuclease, deoxyribonuclease and protease, and delays the degradation of nucleic acids, proteins and chlorophyll, so it can significantly delay leaf senescence. There is an interaction between light and CTK-mediated developmental regulation, and light can stimulate the increase of endogenous cytokinin levels. When plant tissues are in a state of senescence, their endogenous cytokinin content decreases.

IAA is mainly concentrated in parts of vigorous growth, and there is very little content in aging tissues or organs. Violet light can increase the activity of indole acetic acid oxidase, and low IAA levels can inhibit the elongation and growth of plants.

ABA is mainly formed in senescent leaf tissues, mature fruits, seeds, stems, roots and other parts. The ABA content of cucumber and cabbage under the combination of red and blue light is lower than that of white light and blue light.

Peroxidase (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) are more important and light-related protective enzymes in plants. If plants age, the activities of these enzymes will rapidly decrease.

Different light qualities have significant effects on plant antioxidant enzyme activities. After 9 days of red light treatment, the APX activity of rape seedlings increased significantly, and the POD activity decreased. The POD activity of tomato after 15 days of red light and blue light was higher than that of white light by 20.9% and 11.7%, respectively. After 20 days of green light treatment, the POD activity of tomato was the lowest, only 55.4% of white light. Supplementing 4h blue light can significantly increase the soluble protein content, POD, SOD, APX, and CAT enzyme activities in leaves cucumber at seedling stage. In addition, the activities of SOD and APX gradually decrease with the prolongation of light. The activity of SOD and APX under blue light and red light decreases slowly but is always higher than that of white light. Red light irradiation significantly decreased the peroxidase and IAA peroxidase activities of tomato leaves and IAA peroxidase of eggplant leaves, but caused the peroxidase activity of eggplant leaves to increase significantly. Therefore, adopting a reasonable LED supplementary light strategy can effectively delay the senescence of facility horticultural crops and improve yield and quality.

Construction and application of LED light formula

The growth and development of plants are significantly affected by light quality and its different composition ratios. The light formula mainly includes several elements such as light quality ratio, light intensity, and light time. Since different plants have different requirements for light and different growth and development stages, the best combination of light quality, light intensity and light supplement time is required for the cultivated crops.

 ◆Light spectrum ratio

Compared with white light and single red and blue light, the combination of LED red and blue light has a comprehensive advantage on the growth and development of cucumber and cabbage seedlings.

When the ratio of red and blue light is 8:2, the plant stem thickness, plant height, plant dry weight, fresh weight, strong seedling index, etc, are significantly increased, and it is also beneficial to the formation of chloroplast matrix and basal lamella and the output of assimilation matters.

The use of a combination of red, green and blue quality for red bean sprouts is beneficial to its dry matter accumulation, and green light can promote the dry matter accumulation of red bean sprouts. The growth is most obvious when the ratio of red, green and blue light is 6:2:1. The red bean sprout seedling vegetable hypocotyl elongation effect was the best under the red and blue light ratio of 8:1, and the red bean sprout hypocotyl elongation was obviously inhibited under the red and blue light ratio of 6:3, but the soluble protein content was the highest.

When the ratio of red and blue light is 8:1 for loofah seedlings, the strong seedling index and soluble sugar content of loofah seedlings are the highest. When using a light quality with a ratio of red and blue light of 6:3, the chlorophyll a content, the chlorophyll a/b ratio, and the soluble protein content of the loofah seedlings were the highest.

When using a 3:1 ratio of red and blue light to celery, it can effectively promote the increase of celery plant height, petiole length, leaf number, 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, and increasing the proportion of red light promotes the formation of titratable acids. When the light with the ratio of red and blue light to lettuce leaves is 8:1, it is beneficial to the accumulation of carotenoids, and effectively reduces the content of nitrate and increases the content of VC.

 ◆Light intensity

Plants growing under weak light are more susceptible to photoinhibition than under strong light. The net photosynthetic rate of tomato seedlings increases with the increase of light intensity [50, 150, 200, 300, 450, 550μmol/(m²·s)], showing a trend of first increasing and then decreasing, and at 300μmol/(m²·s) to reach maximum. The plant height, leaf area, water content and VC content of lettuce increased significantly under 150μmol/(m²·s) light intensity treatment. Under 200μmol/(m²·s) light intensity treatment, the fresh weight, total weight and the content of free amino acid was significantly increased, and under the treatment of 300μmol/(m²·s) light intensity, the leaf area, water content, chlorophyll a, chlorophyll a+b and carotenoids of lettuce were all decreased. Compared with darkness, with the increase of LED grow light intensity [3, 9, 15 μmol/(m²·s)], the content of chlorophyll a, chlorophyll b, and chlorophyll a+b of black bean sprouts increased significantly. The VC content is the highest at 3μmol/(m²·s), and the soluble protein, soluble sugar and sucrose content are the highest at 9μmol/(m²·s). Under the same temperature conditions, with the increase of light intensity [(2~2.5)lx×103 lx, (4~4.5)lx×103 lx, (6~6.5)lx×103 lx], the seedling time of pepper seedlings is shortened, the content of soluble sugar increased, but the content of chlorophyll a and carotenoids gradually decreased.

 ◆Light time

Properly prolonging the light time can alleviate the low light stress caused by insufficient light intensity to a certain extent, help the accumulation of photosynthetic products of horticultural crops, and achieve the effect of increasing yield and improving quality. The VC content of sprouts showed a gradually increasing trend with the prolongation of light time (0, 4, 8, 12, 16, 20h/day), while the free amino acid content, SOD and CAT activities all showed a decreasing trend. With the prolongation of the light time (12, 15, 18h), the fresh weight of Chinese cabbage plants increased significantly. The content of VC in the leaves and stalks of Chinese cabbage was the highest at 15 and 12h, respectively. The soluble protein content of the leaves of Chinese cabbage decreased gradually, but the stalks were the highest after 15h. The soluble sugar content of Chinese cabbage leaves gradually increased, while the stalks were the highest at 12h. When the ratio of red and blue light is 1:2, compared with 12h light time, 20h light treatment reduces the relative content of total phenols and flavonoids in green leaf lettuce, but when the ratio of red and blue light is 2:1, the 20h light treatment significantly increased the relative content of total phenols and flavonoids in green leaf lettuce.

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

Existing problems and prospects

The significant advantage of LED grow light is that it can make intelligent combination adjustments according to the demand spectrum of photosynthetic characteristics, morphology, quality and yield of different plants. Different types of crops and different growth periods of the same crop all have different requirements for light quality, light intensity and photoperiod. This requires further development and improvement of light formula research to form a huge light formula database. Combined with the research and development of professional lamps, the maximum value of LED supplementary lights in agricultural applications can be realized, so as to better save energy, improve production efficiency and economic benefits. The application of LED grow light in facility horticulture has shown vigorous vitality, but the price of LED lighting equipments or devices is relatively high, and the one-time investment is large. The supplement light requirements of various crops under different environmental conditions are not clear, the supplement light spectrum, The unreasonable intensity and time of grow light will inevitably cause various problems in the application of grow lighting industry.

However, with the advancement and improvement of technology and the reduction of the production cost of LED grow light, LED supplementary lighting will be more widely used in facility horticulture. At the same time, the development and progress of the LED supplementary light technology system and the combination of new energy will enable the rapid development of facility agriculture, family agriculture, urban agriculture and space agriculture to meet people’s demand for horticultural crops in special environments.