Plant lighting technology plays an important role in modern agricultural construction.But the traditional artificial lighting is mainly carried out through incandescent lamps,halogen lamps and high-pressure sodium lamps. However, these traditional sources have the disadvantages of single wavelength and high energy consumption. Compared with traditional sources, the advantages of new LED source include:
1) As a monochromatic light source, the LED can regulate the light environment according to plant growth needs to form an optimal environment which is more suitable for plant growth;
- The LED source has small volume, so that the structure of the lighting system can be designed freely to increase the cultivation space density, and applied to multi-layer cultivation three-dimensional combination system;
- The LED is a kind of cold light source with low heat generation, it can irraduate to plants in close distance but without burnt for plants.
The application of LED in modern agriculture has attracted widespread attentionat home and abroad, and it has been used in the field of artificial lighting to achieve the purpose of increasing production, high efficiency and high quality.
1. The influence of LED light characteristics on plant growth and development
(1) Light Intensity The light intensity mainly affects the photosynthesis rate of plants, thus in turn to change the morphology of plants. Plants’ demand on light intensity is usually expressed by the light compensation point and the light saturation point. When the light intensity is smaller than the light compensation point, the respiration rate will be faster than the photosynthetic rate; While the light intensity is no less than the compensation point and no more than the light saturation point, the plants will perform normal photosynthesis; When the light intensity is more than the light saturation point, the transpiration will speed up, at this time the plants will close their stomata to prevent excessive water loss, trigger under oxidative stress and cause greater harm to photosynthesis, which is called “photosynthetic lunch break”
It can be seen from Figure 1 that the daily change of plant net photosynthetic rate is a double-peak curve with a peak on the upper and lower respectively. The peak in the afternoon is usually lower than the peak in the morning, and a low valley at noon is formed between the two peaks, called “lunch break phenomenon”, its occurrence time varies in different species or the same species located in different environments, generally concentrated between 11 to 15 o’clock. Photosynthetic lunch break is a physiological phenomenon of self-protection for plants to adapt to the poor external environment, but the utilization rate of photosynthesis is very low at noon when the light intensity is maximum. Studies have shown that when photosynthetic lunch breaks are severe, it can reduce the productivity of daily photosynthesis by 30% to 50% or even more. Therefore, when we use the artificial light sources to cultivate plants, we should provide the light source close to or equal to the light saturation point for the plants.
The light intensity affects not only the photosynthesis rate, but also the fruit quality. Dim light conditions will have an adverse effect on plants. For example, shading the peppers, it will increase the flowering nodes and reduce the rate of flower bud formation, thus the pollen can not be developed normally, which will adversely affect the pepper fruit formation and yield. However, shading in a continuous high temperature environment can effectively improve the water environment and reduce the damage to plants.
(2) The influence of light quality on plant growth and development
Light quality specifically refers to the corresponding spectral distribution of light which plants received. It also has an important impact on plant photosynthesis and morphogenesis. The plants biological effects under different light qualities are shown in Table 1.
|Spectral Wavelength / nm||Plants Biological Effects|
|1000~7000||Promote plant growth, and the far-red light (wavelength 700~760nm) affects the seed germination, bud formation and stem elongation.|
|600~700 (red light)||Absorbed more by chlorophyll with stronger photosynthesis|
|500~600 (yellow green light)||Has active radiation for photosynthesis, but with low utilization due to less absorbed by chloroplasts|
|400~500 (blue light)||Absorbed more by chlorophyll with stronger photosynthesis|
|315~400 (UV-A)||Weak kill ability, provide light protection, stimulate plants to enhance secondary metabolism, improve plant stress resistance.|
|280~315 (UV-B)||Not good to plant growth and development, such as causing plant phenotype short, leaves thick and small, low photosynthetic rate|
|<315nm||May cause stoma close, bad for photosynthesis, thus prevent most of plants’ growth and development|
It can be seen from the Table 1 that most of the solar light spectrum is in the range of 300~2 600 nm, the spectrum light of visible light is 380~720 nm, while the light with the wavelength 400~700 nm can be absorbed by plants for photosynthesis and become active radiation for plant photosynthesis. For the ultraviolet light with a wavelength less than 400 nm and the infrared light with a wavelength at 700-800 nm, although it cannot directly actuate photosynthesis, it can be used as an environmental signal to affect plant growth and metabolism.
Studies have found that the red light can promote the growth of cotyledons and the elongation of apical hooks greatly and increase the growth rate of leaves, but irradiating under single red light will limit the expansion of leaves. It can significantly increase the plant heights of tomato and yellow seedlings by supplementing red light.
The blue light mainly affects the growth and development of plant roots and stems, and the number of hair roots of crop seedlings under blue light irradiation, and increase the root activity and absorption area of seedlings. Compared with white light, the blue light has inhibiting effect on stem elongation, significantly reduced plant height but increase stem diameter.
(3) The influence of photoperiod on plant growth and development
Photoperiod is an important signal source for regulating plant growth and development. For example, the flowering time of plants is closely related to the season. Winter jasmine blooms in early spring, while chrysanthemums do not bloom until autumn. This is related to the fact that plants adjust their growth cycle by experiencing periodic change of light time. It has a profound impact on plant seed germination, plant flowering time and plant dormancy, etc.. For example, the seeds of begonia must be in a photoperiod of 8-12hrs to germinate to the maximum extent; Citi fir requires 16hrs to accelerate seed germination. The photoperiod affects the elongation of plant stems and the level of internal growth hormone, and induces the expression of genes related to the promotion of vegetative growth. By appropriately prolonging the photoperiod, it can increase the photosynthesis time and the proportion of carbohydrates, which is conducive to plant germination.
Plants can be divided into three categories: long-day plants, short-day plants and day-neutral plants. Many plants have a clear limit sunshine time for flowering, which is called the critical day length. The flowering time of long-day plants need longer than the critical day length. The spinach’s critical day length is 13hrs, requires the sunshine time longer than 13hrs to bloom, or it will postpone or not bloom if shorter than 13hrs; and the short-day plants require shorter than the critical day length. The shorter the sunshine, the earlier the flowering. For example, the tobacco’s critical day length is 14hrs, it can be induced early flowering by artificial shading; it can be delayed the flowering period by night supplementary light or extended light time.
Photoperiod is also an important factor in inducing plant dormancy. For example, strawberries grow with longer time due to short-day sunlight, and woody plants including poplars and mulberries will dormant due to the reduced light period.
2. Design Element Analysis on LED Artificial Light Source
(1) Spectrum distribution
Different light with different spectral energy distribution and has different impact on plant growth due to their different peak value. The wavelength range required for plant growth and development is 400-700nm, of which the red light (600-700nm) and the blue light (400-500nm) are the main wavelengths of photosynthesis. Since the quantum efficiency of red light is higher than that of blue light, so that the proportion of red light is higher than blue light in artificial light source applications. The blue light energy generally occupies 20% in natural light, but it doesn’t need to be so high in artificial light sources, no less than 7% is enough for normally developed plants. Because the blue light is beneficial to the growth of plant leaves, there can be more blue light spectrum for flower ornamental plants. If the blue light is insufficient and the proportion of red light is too much, it will cause the stems to overgrow and make the leaves yellowing easily; while with too much blue light it will result plants in tight spacing, which will affect the efficiency of light energy utilization.
About the ultraviolet light with a wavelength less than 400nm and the far-red light with a wavelength of 730nm, although cannot directly drive photosynthesis, they can be used as an environmental signal to affect plant growth and metabolism.
(2) Effective position of supplementary light
1) Top Light Supplementation
Due to the traditional light sources generate high heat, it is easy to concentrate the heat and damage the plants, and the light energy utilization rate is low. While the LED is a cold light source, which does not harm the plants. so the mode of top supplementation is usually adopted, that is, the artificial light source is installed at a certain distance directly above the illuminated plant, and the nearest distance is about 10cm.
2) Side Supplementation
When using the top supplementation mode, the leaves of plant on top are easily shaded and resulting in insufficient light for the lower canopy. Supplementing light on the side among plants can promote light distribution in the vertical direction, induce rapid assimilation of the leaves below and improve light utilization efficiency. Studies have found that, when using the LED side supplementation and using the high-pressure sodium lamp top supplementation respectively, there is no significant difference in tomato yield and quality under these two different methods, but the LED side supplementation can save energy by 36.3%. It can be seen that the LED light source is very suitable for side supplementation.
(3) Photoperiod regulation
Photoperiod regulation refers to the mode of indirect light supplementation or shading to plants to regulate their light time, thereby affecting the differentiation of plant flower buds. The photoperiod is determined by the plant types (long-day, short-day or day-neutral plants). For long-day plants, artificial light supplementation can be used to extend the lighting time to promote flowering in short-day periods; for short-day plants, shading treatment can promote flower bud differentiation, and extending the light length can delay the flowering time. To most of crops, a dark period of4hrs at least must be guaranteed every day, otherwise it is easy to accumulate photosynthetic products on the leaves, thereby reducing the efficiency of light energy ultilization.
3. Current research status of artificial light regulating plant growth and development
When plant seedlings are grown in an artificial environment, artificial supplementary light or full artificial light irradiation can promote plant growth, increase yield, and improve product form and color, etc., and also reduce the occurrence of pests. With the development of artificial light source technology, it has become a new type of regulation technology to use the light quality to regulate plant growth and developments. The study found that under the same light intensity, the leaf area and petiole length of strawberries increased significantly when covered by green film and red film respectively, but the leaf area and petiole length decreased significantly only under the blue film covering. Blue light and ultraviolet light reduce the area of grape simple leaves, and red light treatment significantly increases the total dry matter accumulation and promotes the diameter of new shoots. Supplementing green light on the basis of red and blue light can slow down the degradation of chlorophyll in lettuce leaves and increase the chlorophyll content in tomato seedlings to promote seedling growth.
The development of light control technology, on the one hand, it relies on the advancement of artificial light source technology, and on the other hand, it promotes the development of artificial lighting systems. Traditional artificial light sources include high-pressure sodium lamps, metal halides, etc.
(1) The main generation spectrum of the high-pressure sodium lamp is concentrated between 560~640nm, which is not in good agreement with the spectrum of the photosynthetically active radiation for the plant (400~700nm). Usually, the high-pressure sodium lamp is used to extend the light time to improve the plant product. However, the early high-pressure sodium lamps lacked the blue light, which played a decisive role in plant growth. After improved, it will have some negative effects on plants, such as reducing chlorophyll content and dry matter synthesis.
(2) The spectrum of intensive dysprosium lamp in the metal halide lamp (380~780nm) is quite close to the solar spectrum, with high luminous efficiency (> 75 lm/W) and color rendering (Ra>80), but the emitted effective photon beam density is relatively lower than LED, which is not conducive to photosynthesis of plants.
(3) LED lights can not only emit monochromatic light with a narrow light wavelength, but also can combine light sources arbitrarily according to plant needs, which has obvious advantages in the field of plant lighting.
No matter whether it is from the perspective of promoting the development of modern agriculture or the perspective of achieving energy conservation and environmental protection, LED plant lighting is of great significance, and the status and development needs of agriculture also provide new opportunities for the development of LED plant lighting. Since 2013, the global LED plant lighting market has entered into a period of rapid development, mainly in the United States, Japan, the Netherlands and other regions. In 2010, Japan’s Mitsubishi Chemical used large containers to transform plant factories and used LED light sources for photosynthesis. In 2012, the first set of LED lighting plant factory system was used to cultivate lettuce and young leaf vegetables, thus opening the curtain of LED plant lighting. According to LED inside statistics, the global market scale of LED plant lighting was 100million USD in 2014 and 575million USD in 2016. It is estimated that it will grow up to 1.4billion USD by 2020.
The domestic plant lighting is still in the early stage of industry development. The small scale, small number of manufacturers, lack of core technology and unified standards and specifications are the bottlenecks for domestic plant lighting technology development.
In order to promote the healthy and sustainable development of the LED artificial plant lighting industry, we need to start from the following aspects.
(1) Plant lighting is a professional and comprehensive interdisciplinary technology, so it is necessary to carry out comprehensive and systematic in-depth research on LED light sources and botany.
(2) Standardization is an important measure to promote the industrialization of scientific research achievement and support the industrial standard development. Therefore, it is necessary to speed up the standard formulation and improve the standard testing and certification system.
(3) Strengthen the policy guidance and R&D investment.