Application in facility horticulture and impact on crop growth of LED Light
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Application in facility horticulture and impact on crop growth of LED Light

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The types of facilities for horticultural facilities mainly include plastic greenhouses, solar greenhouses, multi-span greenhouses and plant factories. Due to the fact that the building construction blocks the natural light source to a certain extent, the indoor light is insufficient, which leads to crop yield reduction and quality degradation. Therefore, the fill light plays an indispensable role in the high quality and high yield of facility crops, but it also becomes 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 facilities and horticulture mainly include high-pressure sodium lamps, fluorescent lamps, metal halide lamps, incandescent lamps, etc. The outstanding disadvantages are high heat production, high energy consumption, and high operating costs. The development of a new generation of Light-Emitting Diodes (LEDs) has made it possible to use low-energy artificial light sources in the field of facility horticulture. LED has the advantages of high photoelectric conversion efficiency, use of direct current, small volume, long life, low energy consumption, fixed wavelength, low heat radiation, environmental protection, etc. Compared with the currently used high-pressure sodium lamps and fluorescent lamps, LEDs not only have light quantity and light quality ( The ratio of light in various bands, etc.) can be precisely adjusted according to the needs of plant growth, and because of its cold light, the plants can be irradiated at close range, thereby increasing the number of cultivation layers and space utilization, and achieving energy saving, environmental protection and space that cannot be replaced by conventional light sources. Efficient use and other functions. Based on these advantages, LEDs have been successfully applied to facilities such as horticultural lighting, controlled environment basic research, plant tissue culture, plant factory seedlings and aerospace ecosystems. In recent years, the performance of LED fill lamps has been continuously improved, prices have gradually declined, and various wavelength-specific products have been gradually developed, and its application in agriculture and biology will be broader.

The effect of LED lights on the growth of horticultural crops

The regulation of light on plant growth and development includes 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, illumination period and spectral distribution. The artificial fill light can be used to adjust its elements without being restricted by weather conditions.

Plants have selective absorption of light, and light signals are perceived by different photo receptors. At present, there are at least three types of photo receptors in plants, photo sensitins (absorbing red and far red light), and cryptochrome (absorbing blue light and Near ultraviolet light) and ultraviolet light receptors (UV-A and UV-B). Using a specific wavelength light source to illuminate the crop can increase the photosynthesis efficiency of the plant and accelerate the formation of the light form, thereby promoting the growth and development of the plant. Plant photosynthesis mainly uses red orange light (610 ~ 720 nm) and blue purple light (400 ~ 510 nm). Using LED technology, it is possible to emit monochromatic light (such as red light with a peak of 660 nm and blue light with a peak of 450 nm) in accordance with the wavelength band of the strongest absorption region of chlorophyll, and the spectral domain width is only ±20 nm. At present, it is believed that red orange light will accelerate the development of plants, promote the accumulation of dry matter, the formation of bulbs, roots, leaf balls and other plant organs, causing plants to flower and firm earlier, and play a leading role in plant color enhancement; Blue and violet can control the leaf light of plants, promote stomatal opening and chloroplast movement, inhibit stem elongation, prevent plant growth, delay plant flowering and promote vegetative growth; red and blue LEDs can make up for both monochrome The lack of light forms a spectral absorption peak that is basically consistent with crop photosynthesis and morphogenesis, and the light energy utilization rate can reach 80% to 90%, and the energy saving effect is remarkable.

The installation of LED fill light in the facility gardening can achieve a very significant increase in production. Studies have shown that 300 μmol/(m2·s) LED strips and LED tubes 12h (8:00~20:00) fill the number of cherry tomatoes, total yield and single fruit weight are significantly improved, of which LED The lamp fill light increased by 42.67%, 66.89% and 16.97%, respectively, and the LED lamp fill light increased by 48.91%, 94.86% and 30.86% respectively. The total growth period of LED light fill light [red and blue light ratio of 3:2, light intensity of 300 μmol / (m2 · s)] treatment can significantly increase the single fruit quality and unit area yield of melon and eggplant, the melon increased by 5 .3%, 15.6%, eggplant increased by 7.6%, 7.8%. Through the whole growth period LED light quality and its intensity and duration of air conditioning, it can shorten the plant growth cycle, improve the commercial yield, nutritional quality and form value of agricultural products, and achieve high efficiency, energy saving and intelligent production of facility horticultural crops.

Application of LED lights in vegetable seedlings

LED light source regulation of plant morphogenesis and growth is an important technology in the field of greenhouse cultivation. Higher plants can induce and receive light signals through photoreceptor systems such as phytochromes, cryptochromes, and photoreceptors, and transmit them through intracellular messengers to regulate morphological changes in plant tissues and organs. Photomorphogenesis is the plant's dependence on light to control the differentiation, structure and function of cells, as well as the establishment of tissues and organs, including the effects on the germination of some seeds, the promotion of apical dominance to inhibit lateral bud growth, stem elongation, causing tropism, etc. .

Vegetable seedlings are an important part of facility agriculture. Continuous rainy weather will make the light in the facility insufficient, and the seedlings will be prone to grow up, 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 may be used to regulate the growth of seedlings, but the unreasonable use of plant growth regulators may cause contamination of vegetables and facilities. Human health is not good. LED fill light has many advantages of supplementing light, and it is a feasible way to apply LED fill light to raise seedlings. In the experiment of LED fill light [25±5 μmol/(m2·s)] under low light [0~35 μmol/(m2·s)], it was found that green light promoted the elongation growth of cucumber seedlings, red light Compared with the inhibition of seedlings by blue light, the seedling index of supplemental red and blue light was increased by 151.26% and 237.98%, respectively, compared with the seedling index of natural weak light, and it was red compared with monochromatic light quality. Under the composite light quality of blue component, the seedling index increased by 304.46%. Red light supplementation of cucumber seedlings can increase the true leaf number, leaf area, plant height, stem diameter, dry quality, seedling index, root activity, SOD activity and soluble protein content of cucumber seedlings, and supplement UV-B can improve cucumber The content of chlorophyll a, chlorophyll b and carotenoid in the leaves of seedlings; compared with natural light, supplementing LED red light and blue light significantly increased the leaf area, dry matter quality and seedling index of tomato seedlings, supplemented with LED red light and green light to make tomato seedlings Plant height and stem diameter increased significantly; LED green light supplementation can significantly increase the biomass of cucumber and tomato seedlings, and the fresh and dry weight of seedlings increased with the intensity of green light supplementation, while the stem diameter and seedling index of tomato seedlings were filled with green light. Increased intensity and increase; LED red and blue combined light can increase stem diameter, leaf area, whole plant dry weight, root-shoot ratio, and seedling index of eggplant; compared with white light, LED red light can improve the growth of cabbage seedlings The amount of the cabbage promoted the elongation growth and leaf expansion of the cabbage seedlings; LED blue light promoted the thick growth, dry matter accumulation and seedling index of the cabbage seedlings, which made the cabbage seedlings dwarf. The above results show that the advantages of vegetable seedlings cultivated by combining light regulation technology are very obvious.

The proteins, sugars, organic acids and vitamins contained in fruits and vegetables are nutrients beneficial to human health. Light quality can affect the content of VC in plants by regulating the activity of VC synthesis and decomposing enzymes, and regulate the protein metabolism and carbohydrate accumulation in horticultural plants. Red light promotes the accumulation of carbohydrates, and blue light treatment is beneficial to protein formation. The combination of red and blue light has a significantly higher effect on the nutritional quality of plants than 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 sugar in lettuce, and supplementing infrared light is beneficial to the accumulation of VC in lettuce. The supplementation of blue light can promote the increase of VC content and soluble protein content in tomato; the combined light treatment of red light and red and blue can promote the sugar and acid content in tomato fruit, and the ratio of sugar to acid is the highest under the combination of red and blue light treatment; Red and blue combined light can promote the increase of VC content in cucumber fruit.

The phenolic substances, flavonoids, anthocyanins and other substances contained in fruits and vegetables not only have an important influence 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 human body. The use of LED blue light fill light can significantly increase the anthocyanin content of eggplant by 73.6%, while using LED red light, red and blue combined light can increase flavonoids and total phenol content; blue light can promote tomato red in tomato fruit The accumulation of flavonoids and anthocyanins, red and blue combined light promotes the formation of anthocyanins to a certain extent, but inhibits the synthesis of flavonoids; compared with white light treatment, red light treatment can significantly improve the flowers in the upper part of lettuce Blue pigment content, but the blue-treated lettuce has the lowest anthocyanin content in the shoots; The total phenolic content of green leaf, purple leaf and red leaf lettuce has larger values under white light, red and blue combined light and blue light treatment, but the lowest value under red light treatment; supplement LED light or orange light can increase lettuce leaves The content of phenolic compounds, while supplementing green light can increase the content of anthocyanins. Therefore, the use of LED fill light is an effective way to regulate the nutritional quality of facilities fruits and vegetables.

The effect of LED lights on plant aging

Chlorophyll degradation, rapid protein loss and RNA hydrolysis during plant senescence are mainly manifested by leaf senescence. Chloroplasts are very sensitive to changes in the external light environment, especially affected by light quality. The combination of red, blue and red-blue light is conducive to the formation of chloroplasts. Blue light is beneficial to the accumulation of starch granules in chloroplasts. Red light and far red light have negative effects on chloroplast development. The combined light of blue light and red and blue can promote the synthesis of chlorophyll in cucumber seedling leaves. The combination of red and blue light can also delay the attenuation of 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 seedlings under LED red-blue combined light treatment was significantly higher than that of fluorescent lamp control and monochromatic red and blue light treatment; LED blue light could significantly increase the chlorophyll a/b value of black mulberry and green garlic seedlings.

The cytokinin (CTK), auxin (IAA), abscisic acid (ABA) content and various enzyme activities changed during leaf senescence. The content of phytohormones is easily affected by the light environment, and different light qualities have different regulatory effects on plant hormones, and the initial steps of the light signaling pathway involve cytokinins. CTK promotes leaf cell expansion, enhances leaf photosynthesis, inhibits the activity of ribonuclease, deoxyribonuclease and protease, and delays the degradation of nucleic acids, proteins and chlorophyll, thus significantly delaying leaf senescence. There is an interaction between light and CTK-mediated developmental regulation, and light energy stimulates an increase in endogenous cytokinin levels. When plant tissues are in senescence, their endogenous cytokinin levels decrease. IAA is mainly concentrated in the growing areas, with little content in aging tissues or organs. Violet light can increase the activity of indoleacetal oxidase, and low IAA levels inhibit plant elongation. ABA is mainly formed in senescent leaf tissues, mature fruits, seeds, stems and roots. The ABA content of cucumber and cabbage under red-blue combined light is lower than that of white light and blue light.

Peroxidase (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) are important and light-related protective enzymes in plants. If plants age, The activity of these enzymes will decrease rapidly. The effects of different light qualities on the activities of antioxidant enzymes in plants were significant. After 9 days of red light treatment, APX activity of rapeseed seedlings increased significantly and POD activity decreased. After 15 days of red and blue light irradiation, the POD activity of tomato was higher than that of white light by 20.9%. 11.7%, the POD activity was the lowest after 20 days of green light treatment, only 55.4% of white light; supplementation of 4h blue light could significantly increase the soluble protein content, POD, SOD, APX and CAT activities of cucumber seedling leaves. In addition, the activity of SOD and APX gradually decreased with the extension of illumination time. The activity of SOD and APX under blue light and red light decreased slowly but was always higher than white light. Red light irradiation significantly reduced the activity of IAA peroxidase in tomato leaves, such as peroxidase, IAA peroxidase and eggplant leaves, but caused the peroxidase activity of eggplant leaves to increase significantly. Therefore, the use of reasonable LED fill light strategy can effectively delay the aging of facilities horticultural crops, improve production and quality.

Construction and application of LED light formula

The growth and development of plants are greatly affected by the light quality and its different composition ratios. The light formula mainly includes several factors such as light quality ratio, light intensity and illumination time. Because the need of light between different plants are different, so the need of light in different grow period will be different, therefore will need to combine the best light quality ratios, light intensity and illumination time for the plants.

Light quality ratios

Compared with white light and single red and blue light, LED red and blue combined light showed a comprehensive advantage for 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 stem, fresh weight and strong seedling index of the plant are significantly improved, and it is beneficial to the formation of chloroplast matrix, granule layer and output assimilation product. Under the treatment of 8:1 red and blue light, the plant height, stem diameter, leaf area, seedling index, aboveground and whole plant fresh weight of cucumber seedlings were the highest, and the seedling leaves had higher POD and APX activities; Under the treatment of 6:3, the root activity, leaf soluble protein, soluble sugar content and net photosynthetic rate of cucumber seedlings were the highest, and the SOD activity was relatively high. The use of red, green and blue light combination for red bean sprouts is conducive to the accumulation of dry matter. The addition of green light promotes the dry matter accumulation of red bean sprouts. The ratio of red, green and blue light is 6:2:1. The ratio of red and blue light was 8:1, and the hypocotyl elongation of red bean sprouts was the best. The ratio of red and blue light was 6:3, and the inhibition of hypocotyl elongation was obvious, but the soluble protein content was the highest. When the red-blue light ratio of the loofah seedlings was 8:1, the loofah seedlings had the highest seedling index and the highest soluble sugar content. When the red-blue light ratio was 6:3, the chlorophyll a of the loofah seedlings was used. The content, chlorophyll a/b ratio and soluble protein content were the highest. For the celery using red and blue light ratio of 3:1, the plant height, petiole length, leaf number, dry matter quality, VC content, soluble protein content and soluble sugar content of celery can be effectively promoted; in tomato cultivation, Increasing the proportion of LED blue light promotes the formation of lycopene, free amino acids and flavonoids, increasing the proportion of red light to promote the formation of titratable acid; when using the red and blue light ratio of lettuce leaves to 8:1, it is beneficial to its class. Carotene accumulation, and effectively reduce its nitrate content and increase VC content.

Light intensity

Plant growth in weak light is more susceptible to photoinhibition than in strong light. The net photosynthetic rate of tomato seedlings increased first and then decreased with the increase of light intensity [50, 150, 200, 300, 450, 550 μmol/(m2·s)], and at 300 μmol/(m2·s). The plant height, leaf area, water content and VC content of lettuce increased significantly under the light intensity treatment of 150μmol/(m2·s). Under the treatment of light intensity of 200μmol/(m2·s), the fresh weight of lettuce was fresh. The total weight and free amino acid content were significantly increased, while the leaf area, water content, chlorophyll a, chlorophyll a+b and carotenoids of lettuce decreased under the treatment of 300μmol/(m2·s) light intensity; compared with darkness, With the increase of LED fill 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, and the light intensity was 3μmol/(m2·s). The highest content of VC, 9μmol/(m2·s), soluble protein, soluble sugar and sucrose content; under the same temperature condition, with light intensity [(2~2.5) lx×103 lx, (4~4. 5) lx × 103 lx, (6 ~ 6.5) lx × 103 lx] increase, the formation of pepper seedlings The seedling time was shortened, the soluble sugar content was increased, but the chlorophyll a and carotenoid contents were gradually decreased.

Appropriate extension of the illumination time can alleviate the weak light stress caused by insufficient light intensity to a certain extent, and contribute to the accumulation of photosynthetic products in horticultural crops, thereby increasing the yield and improving the quality. The content of VC in sprouts increased gradually with the extension of light time (0, 4, 8, 12, 16, 20h/day), while the free amino acid content, SOD and CAT activities showed a decreasing trend. With the extension of light time (12,15,18h), the fresh weight of Chinese cabbage plants increased obviously. The VC content of leaves and stems of Chinese cabbage was the highest at 15 and 12h, respectively. The soluble protein content of leaves of Chinese cabbage decreased gradually. The stolons were treated at the highest level for 15h; the soluble sugar content of the leaves of Chinese cabbage was gradually increased, while the stolons were the highest at 12h. In the case of a red-blue ratio of 1:2, the 20h light treatment reduced the relative content of total phenolics and flavonoids in green leaf lettuce compared to the 12h light time, but in the case of a red-blue ratio of 2:1, The 20h light treatment significantly increased the relative content of total phenolics and flavonoids in green leaf lettuce.

It can be seen from the above that different light formulas have different effects on photosynthesis, photomorphogenesis and carbon and nitrogen metabolism in different crop types. How to obtain optimal illumination formula, light source configuration and intelligent control strategy requires plant species as the entry point. According to the demand for horticultural crops, production targets, production factors, etc., appropriate adjustments should be made to achieve the goal of intelligent control of light environment and high quality and high yield of horticultural crops under energy-saving conditions.

Existing problems and prospects

The significant advantage of LED fill light is that it can be intelligently adjusted according to the photosynthetic characteristics, morphological integrity, quality and yield of different plants. Different types of crops and different crops have different requirements for light quality, light intensity and photoperiod. This requires further development and improvement of light formula research, forming a huge database of light formulas, combined with the development of professional lamps. The greatest value of LED fill light in agricultural applications, thus saving energy, improving production efficiency and economic benefits. LED fill light has shown strong vitality in the application of facilities and gardening, but the price of LED fill light is relatively high, and the one-time investment is large. The requirements for fill light of various crops under different environmental conditions are not clear, the fill spectrum, Intensity and fill light time are not reasonable, which will inevitably lead to various problems when applying the fill light. However, with the advancement and improvement of technology and the reduction of the production cost of LED fill light, LED fill light will be more widely used in facility gardening. At the same time, the development of LED fill light technology system and the combination of new energy will enable the rapid development of factory agriculture, family agriculture, urban agriculture and space agriculture to meet the demand for horticultural crops in special circumstances.

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