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The unique advantages of LED make it one of the important light sources for plant tissue culture research. Since the 1980s, some countries in the world have successively begun research on the application of LEDs in plant tissue culture. Some colleges and universities and scientific research institutions in China have also started research in this area and have made some gratifying progress. This article briefly summarizes the work that LED has done in plant facility cultivation from the following aspects, and makes an assessment of its application prospects in this field.
1.1 The effect of light on plants
Light is one of the most important environmental factors in plant growth. It not only provides radiation energy for plant photosynthesis, but also provides signal transduction for plants and regulates their development process. Plants are always in a constantly changing light environment throughout its life cycle. In the long-term evolution, plants not only adapt to changes in the light environment, but also influence each other to change the surrounding light environment.
1.2 Light and pigment
The wavelength of sunlight reaching the ground is approximately from 300 to 2 600 nm, of which the effective wavelength for photosynthesis is between 400 and 700 nm, of which blue light at 425 to 490 nm and red light at 610 to 700 nm contribute to photosynthesis. The maximum, and the light absorption rate of 520~610 nm (green) by plants is very low . It can be seen that not all light contributes to the photosynthesis of plants. Pigments can absorb light energy to produce a series of biochemical reactions, and different pigments absorb different wavelengths. There are many pigments in plants that play different roles. However, two types of pigments, phytochrome and cryptochrome, play a key role in regulating the response of plants to light. There are two tautomers of phytochromes-red phytochrome (Pr) and far-red phytochrome (Pfr). Pr absorbs red light with a wavelength of about 660 nm, and Pfr absorbs far-red light with a wavelength of about 730 nm. Phytochrome regulates the response of many different plants to light, including photoperiod, seed germination, leaf development, hypocotyl elongation and de-yellowing. Cryptochromes absorb light waves in the range of blue and ultraviolet light, and other pigments are related to the development of plants . It can be seen that the blue light around 460 nm and the red light around 660 nm are the light waves most needed by plants, and they play a key role in the growth and development of plants.
2.1 Introduction to LED
LED (light-emitting diodes), that is, light-emitting diodes, is a device that can effectively convert electrical energy into electromagnetic radiation. In 1962, the joint laboratories of GE, Monsanto, and IBM developed the red-emitting semiconductor compound GaAsP. In 1965, the world's first commercially available LED that emits infrared light made of germanium was born. With the continuous advancement of technology, the development of white light LEDs has been quite rapid in recent years. The luminous efficiency of white light LEDs has reached 30lm/W, and laboratory research results can reach 60 lm/W, greatly surpassing incandescent lamps and approaching fluorescent lamps.
2.2 Analysis of LED advantages
LED has the advantages of small size, light weight, solid state, long life, special wavelength, low driving voltage, high light efficiency, low energy consumption, safety, reliability and durability, not easy to color decay, and red light LED photons have greater advantages Luminous flux. In addition, LED has a narrow spectrum, the half-width range of the spectrum is from a few nanometers to tens of nanometers, at about ±20 nm, and the wavelength is just in line with the spectral range of plant photosynthesis and photomorphism.
The application of LED in plant tissue culture is developed based on the development of LED technology and the environmental regulation of plant tissue culture. The first in the world to use LEDs for plant cultivation was Japan's Mitsubishi Corporation. As early as 1982, there was a test report on the use of a red LED light source with a wavelength of 650 nm to supplement light in greenhouse tomatoes . Later, LEDs were also applied to environmental regulation in plant tissue culture, and the role of LEDs in energy saving was discussed . At present, the application of LEDs in plant tissue culture mainly focuses on the effects of light quality and light intensity on the growth of tissue cultured seedlings, and there is little research on photoperiod. As far as the world is concerned, the application research of LED in plant tissue culture is mainly concentrated in Japan and the United States. Japanese research is in a leading position in the world. It has not only developed an LED light-emitting system specifically applied to plant tissue culture, but also combined with other environmental control factors to obtain some important basic data. Some scientific research institutions in China have also started research in this area and independently developed some LED light source systems for plant tissue culture research.
3.1 Light quality selection
At the beginning of the birth of LEDs, people used 660 nm red LEDs as the main light source and fluorescent lamps as auxiliary light sources for research. With the continuous development of LED technology, LEDs of various bands have been used for plant tissue culture, mainly 660 nm Left and right red light, blue light around 460 nm, far red light and white light around 730 nm.
Japan’s Tanaka et al. earlier used LEDs as the light source for orchid tissue culture seedlings and found that red light can promote the growth of Cymbidium test-tube seedling leaves, but will reduce the chlorophyll content, but this phenomenon can be offset by blue light. The best ratio of red and blue light for the growth of test-tube plantlets is 8:2. The study of Le Van et al. showed that the growth effect of callus is the best when the ratio of red to blue light is 3:1, but 100% red light has the highest induction rate of callus. Anzelika et al.  found in the tissue culture of grapes that the blue light component in the spectrum prevented the elongation of test-tube plantlets, but could promote the formation of leaves and the synthesis of various photosynthetic pigments. The increase or decrease of the far-red light component PPF also has a significant effect on the accumulation of dry and fresh weight and the synthesis of photosynthetic pigments. Red LED helps increase plant height, internode length and rooting rate, while blue light is related to chlorophyll synthesis and stomata development. Although the chlorophyll content is lower under red LED conditions, this effect can be weakened by irradiating blue LEDs or fluorescent lamps. In addition, red light is conducive to the accumulation of soluble sugar and starch, and reduces pigment content. Blue light can reverse this effect and promote the synthesis of pigments and soluble proteins. The soluble sugar and starch content and root activity of the leaves treated with the combination of red light and blue light were higher than those treated with white light. Especially, the tissue cultured seedlings treated with high R/B ratio light grew vigorously and had the highest survival rate after transplantation. Research by Zhang Jie et al. showed that the growth of chrysanthemum tissue cultured seedlings was the best under the condition of the ratio of red to blue light of 7:3. Jiang Yaowei showed that the LED light source with a larger proportion of red light has better influence on Cymbidium and Phalaenopsis test-tube seedlings than the LED light source with a larger proportion of blue light. Yue Lan showed that when the ratio of red to blue light was 3:1, the growth indexes of peony varieties "Wulongpengsheng" and "Luoyang Red" test tube plantlets were better, while the ratio of red to blue light of Huhong test tube plantlets was It grows well at 1:1. However, the test-tube seedling plants are shorter and grow worse in full red light and full blue light. In general, red light is beneficial to the elongation of plant stems and roots and promotes morphogenesis. Plants treated with far red light and blue light are short and have short and thin roots.
3.2 Light intensity selection
For plants, light intensity is the photosynthetic quantum flux (PPF, whose unit is μmol/(m2·s)), which is one of the important parameters that affect plant photosynthesis. Nhut et al. found that when the PPF was 60μmol/(m2·s), the growth of strawberry tissue cultured seedlings was the best; when the white crane taro tissue cultured seedlings were 60~70μmol/(m2·s), the aboveground and underground Some parts have higher fresh weight. Anzelika et al.  used four different wavelength LED lights to cultivate grape tissue culture seedlings, and found that the total PPF value suitable for the growth of grape tissue culture seedlings was in the range of 40~55μmol/(m2·s). Zhang Jie et al.’s research on chrysanthemum proved that the plant height, leaf number, root number, longest root length, dry weight and other main growth indicators of tissue cultured seedlings were significantly higher than the control when the light intensity was 60μmol/(m2·s). .
3.3 photoperiod selection
The research of Chen Yusong et al. showed that the most suitable LED light environment for gentian out of the bottle index is 50% of blue light, PPF of 120μmol/(m2·s), and photoperiod of 16 h. The best light environment for gentian research using fluorescent lamps in Taiwan Sugar Research Institute is PPF 80μmol/(m2·s), and the photoperiod is 12Hr. Research by RCJao et al. showed that when the photoperiod is 16 h, the potato tissue cultured seedlings are better than the alternating red and blue light when the red and blue light is irradiated at the same time. In addition, the long-term irradiation of low PPF is better than the long-term irradiation of high PPF. The effect is good.
3.4 Power supply mode selection
Studies have shown that using direct current to drive LEDs, the driver provides a power supply frequency of 40 Hz, which can save more power than 60 Hz. However, to further reduce the cost, AC power can be directly used for driving, which can avoid the production cost of the AC-DC conversion circuit. Studies have shown that it is feasible to use AC-powered all-red LEDs to produce color alocasia tissue culture seedlings. Jao Ruey-Chi et al. showed in the research on potatoes that if only the growth rate is considered, the plant grows best when the LED is at 720 Hz (1.4 ms), the working ratio is 50%, and the photoperiod is 16 h. Considering the issue of energy consumption, the LED is the most energy-efficient when the working ratio is 50% at 180 Hz (5.5 ms), and the light period is 16 h.
4.1 Restrictive factors and countermeasures
Some shortcomings of the LED itself, such as low brightness, difficulty in PN junction heat dissipation, poor spot brightness and chromaticity uniformity, and high price, restrict the promotion and application of LEDs in plant tissue culture, and are not conducive to the industrialization of LEDs in plant tissue culture. Large-scale application in production. Solving this problem requires the development and improvement of optoelectronic technology, as well as the introduction of relevant policies and regulations. With the development of optoelectronic technology, the technical problems of LED itself will be solved, and the price of LED will also be reduced, which will help Wide application of LED in tissue culture.
4.2 Development Trend
LED is a new type of high-efficiency and energy-saving light source. Using LED as a light source in plant tissue culture can not only reduce the cost of tissue culture, but at the same time, due to the characteristics of LED light quality, adjustable light intensity, and narrow band, it makes the research on plant photophysiology more important. Go deep. In the future, the application of LEDs in plant tissue culture should be strictly controlled from the lighting device, select the appropriate LED, consider the performance and reliability and the special use conditions of plant lighting; combine the characteristics of the LED, use the LED rationally, and consider the rated working conditions of the LED. The design of the drive circuit and the selection of the power supply are combined with other factors of environmental regulation, such as CO2 fertilization, temperature regulation and so on. In addition, it must be combined with the use of plant growth regulators to make the application research of LED in plant tissue culture more in-depth and systematic.
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