Plant lighting technology is an important part of modern agricultural construction. Traditional artificial lighting is mainly carried out through incandescent lamps, halogen lamps and high-pressure sodium lamps. However, these traditional light sources have the disadvantages of single emission wavelength and high energy consumption. As a new type of light source, the advantages of LED compared with traditional light sources include:
- As a monochromatic light source, LED can regulate the light environment according to plant growth needs to form an optimal environment suitable for plant growth;
- The LED light source is small in size, and the space cultivation density can be increased by freely designing the structure of the lighting system, which is suitable for multi-layer cultivation three-dimensional combined system;
- LED is a cold light source with a small amount of heat, so that it can achieve close irradiation of plants without being burned.
The application of LED in the field of modern agriculture has attracted widespread attention at home and abroad, and has been used in the field of artificial light supplementation to achieve the purpose of increasing production, high efficiency and high quality.
1.The influence of LED light characteristics on plant growth and development
- The influence of light intensity on plant growth and development
Light intensity mainly affects the photosynthesis rate of plants, and then changes the morphology of plants. Plants’ demand for light intensity is usually expressed by light compensation point and light saturation point. When light intensity <light compensation point, the respiration rate is greater than the photosynthetic rate; when light compensation point <light intensity <light saturation point, plants perform normal photosynthesis ; When the light intensity>light saturation point, transpiration speeds up. Plants close their stomata in order to prevent excessive water loss, triggering oxidative stress and causing greater harm to photosynthesis. This phenomenon is called “photosynthetic lunch break”, as shown in the figure.1 shown.
Figure 1. net photosynthetic rate in a day
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 sides. The peak in the afternoon is usually lower than the peak in the morning. A low valley at noon is formed between the two peaks. Lunch break phenomenon, its occurrence time varies with different species or the same species in different environments, generally concentrated between 11 to 15 o’clock. Photosynthetic lunch break is a physiological phenomenon of self-protection caused by plants poorly adapting to the external environment, but the utilization rate of time cooperation is low at noon when the light intensity is maximum. Studies have shown that when photosynthetic lunch breaks are severe, it can reduce the solar productivity of plants by 30% to 50%, or even more. Therefore, when artificial light sources are used to cultivate plants, the light provided by the plants should be close to or equal to the light saturation point of the plants.
Light intensity not only affects the photosynthesis rate of plants, but also affects the quality of fruits. Low light conditions will have an adverse effect on plants. For example, shading peppers will increase the flowering nodes of peppers, reduce the rate of flower formation, and the pollen will not develop normally, which will adversely affect the fruit formation and yield of peppers. However, under continuous high temperature environment, shading can effectively improve the water environment and reduce the damage to plants.
- The influence of light quality on plant growth and development
Light quality specifically refers to the corresponding spectral distribution of plants receiving light. It also has an important impact on plant photosynthesis and morphogenesis. The biological effects of plants under different light qualities are shown in Table 1.
Table 1. Biological effects of plants under different light quality
It can be seen from Table 1 that most of the solar light spectrum is in the range of 300~2 600 nm, the visible light spectrum range is 380~720 nm, and the light with the wavelength of 400~700 nm can be used by plants for photosynthesis, becoming Plant photosynthetically active radiation. For ultraviolet light with a wavelength of less than 400 nm and infrared light with a wavelength of 700 to 800 nm, although it cannot directly drive photosynthesis, it can be used as an environmental signal to affect plant growth and development and metabolism.
Research found that red light can greatly promote the growth of cotyledons and the elongation of apical hooks, and can increase the growth rate of leaves, but simple red light irradiation will limit the expansion of leaves. By supplementing red light, the plant height of tomato and yellow light seedlings can be significantly increased.
Blue light mainly affects the growth and development of plant roots and stalks. The number of roots of crop seedlings under blue light irradiation, and blue light can increase seedling root vigor and absorption area. Blue light has an inhibitory effect on stem elongation. Compared with white light, blue light obviously reduces plant height, but can increase stem thickness.
Which one has better light quality for plants, LED or HPS?
- 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 themselves by experiencing periodic changes in light time. The growth cycle is related. It has a profound impact on plant seed germination, plant flowering time and plant dormancy. For example, the seeds of begonia must be in the photoperiod of 8-12 h to germinate to the maximum extent; Citi fir needs 16 h to accelerate the seed germination. 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, the photosynthesis time of plants can be increased, and the proportion of carbohydrates will also increase, which is conducive to plant germination.
Plants can be divided into 3 categories: long-day plants, short-day plants and day-neutral plants. Many plants have a clear limit of sunshine time for flowering, which is called the critical day. Long-day plants need to bloom longer than the critical day. The critical day of spinach is 13h, and the sunshine is longer than 13h to bloom, and it is shorter than 13h to delay or not bloom; and Short-day plants are required to be shorter than the critical day length. The shorter the sunshine, the earlier the flowering. For example, the 14h of tobacco, artificial shading can induce early flowering; the use of night supplementary light or extended light can delay the flowering period.
The photoperiod is also an important factor in inducing plant dormancy. For example, strawberries grow longer due to short-day sunlight, and woody plants including poplars and mulberries will dormant due to the reduced light period.
What is Photoperiodism？
2.Analysis of design elements of LED artificial light source
Figure 2. LED Grow Light
- Spectral distribution
The peak value of the spectral energy distribution of different light sources is different, and the impact on plants is correspondingly different. The wavelength range required for plant growth and development is 400-700 nm, of which red light (600-700 nm) and blue light (400-500 nm) are the main wavelengths of photosynthesis. Since the quantum efficiency of red light is higher than that of blue light, the proportion of red light is higher than that of blue light in artificial light source applications. Under natural light, blue light energy generally occupies 20%, but the blue light in artificial light sources does not need such a high proportion. For normally developed plants, blue light should not be less than 7%. Because blue light is beneficial to the growth of plant leaves, there can be more blue light components 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 grow too much and easily cause yellowing of the leaves; too much blue light will result in tight plant spacing, which will affect the efficiency of light energy utilization.
Ultraviolet light with a wavelength of less than 400nm and far-red light with a wavelength of 730 nm, although they cannot directly drive photosynthesis, they can be used as environmental signals to affect plant growth, development and metabolic processes.
- Effective fill light position
- Top fill light
Because traditional light sources generate high heat, it is easy to concentrate heat and damage plants, and the light energy utilization rate is low, usually avoid the top fill light mode, that is to install the artificial light source at a certain distance directly above the illuminated plant, the closest distance is 10 cm Left and right LEDs are cold light sources and do not harm plants.
- Fill light between the side of the plant
When the top fill light mode is used, the top leaves of the plant are easily shaded, resulting in insufficient light in the lower canopy. Supplementing light on the side surface between 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 the LED side fill light and the high pressure sodium lamp top fill light are used, there is no significant difference in tomato yield and quality under the two methods, but the LED side fill light can save 36.3%. It can be seen that the LED light source is very suitable for side fill light.
- Photoperiod control
Photoperiod regulation refers to the mode of indirect light supplementation or shading to the plant to regulate its light time, thereby affecting the differentiation of plant flower buds. The photoperiod needs to be determined based on clear plant types (long-day, short-day, 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 if the light length is extended, flowering can be delayed. For most crops, a dark period of at least 4h must be guaranteed every day, otherwise it is easy to accumulate photosynthetic products on the leaves, thereby reducing the efficiency of light energy use.
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, improve product shape, color, etc., and reduce the occurrence of pests. With the development of artificial light source technology, the use of light quality regulation for plant growth and development has become a new type of regulation technology. The study found that under the same light intensity, the leaf area and petiole length of strawberries increased significantly when green film and red film were used for covering, but the leaf area and petiole length decreased significantly under the cover of blue film. Blue light and ultraviolet light reduce the area of single grape leaves, and red light treatment significantly increases the total dry matter accumulation and promotes the thickening 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 relies on the advancement of artificial light source technology on the one hand, and promotes the development of artificial lighting systems on the other hand. Traditional artificial light sources include high-pressure sodium lamps, metal halides, etc.
- The main generation spectrum of the high-pressure sodium lamp is concentrated between 560~640 nm, which is not in good agreement with the spectrum of the photosynthetically active radiation of the plant (400~700 nm). Usually, the high-pressure sodium lamp is used to extend the light time of the plant to improve the plant product. However, the early high-pressure sodium lamps lacked blue light, which played a decisive role in plant growth. After improvement, it will have some negative effects on plants, such as reducing chlorophyll content and dry matter synthesis.
- The intensive spectrum (380~780 nm) of the dysprosium lamp in the metal halide lamp is quite close to the solar spectrum, with high luminous efficiency (> 75 lm/W) and color rendering (Ra>80), but the emission The effective photon beam density of LED is relatively low, which is not conducive to photosynthesis of plants.
- LED lights can not only emit monochromatic light with a narrow light wave, but also can combine light sources arbitrarily according to plant needs, which has obvious advantages in the field of plant lighting.