LED grow lights can effectively alleviate or solve these problems. Incandescent lamps, fluorescent lamps, metal halide lamps, high pressure sodium lamps and emerging LED lamps have all been or are being used in greenhouse fill light. Among these light source types, high-pressure sodium lamps have higher luminous efficiency, longer service life, and higher comprehensive energy efficiency, occupying a certain market position, but high-pressure sodium lamps have poor lighting sustainability, low safety (containing mercury), The problem of not being able to irradiate at close range is also prominent. Some scholars have a positive attitude towards LED lamps in the future or can overcome the problem of insufficient performance of high-pressure sodium lamps. However, the price of LED is expensive, the supplementary light technology is difficult to match, the supplementary light theory is not perfect, and the product specifications of LED plant lights are confusing, which makes users question the application of LED supplementary light for plants. Therefore, this paper systematically summarizes the research results of predecessors and the current status of production and application, and provides a reference for the selection and application of light sources in greenhouse supplementary lighting.
Differences between high pressure sodium lamps and LED lighting
1.1 Differences in Lighting Principle and External Structure
The high-pressure sodium lamp is composed of mercury, sodium, xenon arc tube wick, glass bulb, getter lamp holder, etc. from the inside to the outside. Due to the difference in the ballasts of its core accessories, it is divided into inductive high-pressure sodium lamps and electronic high-pressure sodium lamps. High-pressure sodium lamps of different powers need to use ballasts of corresponding specifications. LED is also called light-emitting diode. The core part is a wafer composed of P-type semiconductor and N-type semiconductor. There is a transition layer between P-type semiconductor and N-type semiconductor, which is called P-N junction. When the current flows from the anode to the cathode of the LED, the semiconductor crystal will emit light of different colors from purple to red, and the intensity of the light is related to the current. According to the luminous intensity and working current, it can be divided into ordinary brightness (luminous intensity < 10mcd), high brightness (luminous intensity of 10~100mcd) and ultra-high brightness (luminous intensity > 100mcd) and other types. Its structure is mainly divided into four parts: the structure of the light distribution system, the structure of the heat dissipation system, the driving circuit and the mechanical/protective structure.
1.2 Differences in irradiation range and spectral range
The light-emitting angle of high-pressure sodium lamps is 360°, and most of them must be reflected by the reflector before they can reach the designated area. The spectral energy distribution is roughly red-orange, yellow-green, and blue-violet (only a small part). According to the different light distribution design of LED, its effective luminous angle can be roughly divided into three categories: ≤180°, 180°~300° and ≥300°. The LED light source has wavelength tunability and can emit monochromatic light with narrow light waves, such as infrared, red, orange, yellow, green, blue, etc., which can be combined arbitrarily according to different needs.
1.3 Differences in applicable conditions and lifespan
High-pressure sodium lamp is the third-generation lighting source. It has a wide range of use under conventional alternating current, high luminous efficiency, and strong penetrating ability. The maximum lifespan is 24000h, and the minimum can be maintained at 12000h. When the sodium lamp is used for lighting, it will be accompanied by the generation of heat, so the sodium lamp is a heat source. In the process of use, there is also a self-extinguishing problem. As the fourth-generation new type of semiconductor light source, LED is driven by DC, and its lifespan can reach more than 50,000h, and the attenuation is small. As a cold light source, it can be illuminated close to plants. Liao Ruihui compared LEDs with high-pressure sodium lamps, and pointed out that LEDs are safer, contain no harmful elements, and are more environmentally friendly.
2. Differences in the effects of high-pressure sodium lamps and LED supplementary light on crops
A large number of production practices and scientific research in agricultural production have proved that artificial plant supplementary light can not only increase crop yield, shorten the planting cycle, but also effectively improve crop quality. It is an important guarantee for efficient production in modern agriculture. In the process of seedling raising and greenhouse crop management, high-pressure sodium lamps and LEDs are used to supplement light, which can promote the growth and development of crops and change the yield, morphology and physiological indicators of crops.
2.1 Yield and quality differences
The high yield and high quality of crops are the ultimate goal of planting and cultivation. LED supplementary light can improve the quality of pepper, tomato and eggplant seedlings. Under the condition of supplemental light for 10h, the quality of single fruit and the yield per plant of tomato increase significantly. The increase in yield produced by LED supplementary light is also reflected in the cultivation of cucumbers. LED can improve the quality of grape fruit. Among them, blue light supplemented light treatment has the fastest fruit development, higher single-grain quality and highest sugar content, and UV supplemented light treatment has the highest single-grain quality at maturity. Similarly, the 70W high-pressure sodium lamp significantly increased the yield per plant of strawberries, with an increase of 17.9%. High-pressure sodium lamps and LED supplementary light had a significant effect on the morphology of plants. The visual quality of cucumber is also improved through LED side fill light treatment. LED is added on the basis of sodium lamp, and the color of cucumber is more vivid compared with the treatment of only sodium lamp.
2.2 Differences in morphological indicators
Plant morphology index is an important index in the process of plant growth, especially in the production of seedlings, which determines whether the plant can grow healthily after transplantation and cultivation. In general, coniferous plant seedlings grown under LEDs have better growth conditions than high-pressure sodium lamps. Photoperiod 12h, optical density 50μmol/(m2·s), LED red light (630~660nm), orange light (590~610nm), blue light (450~460nm), green light (520~540nm) treatment, respectively, compared with natural light [120μmol/(m2·s)] significantly increased the seedling strength index of 'Saitian' tomato seedlings. Gong Ting et al. used self-made LEDs for light supplementation, and found that the plant height, stem thickness, and leaf area of pepper, tomato and eggplant seedlings increased significantly, and LED interplant supplemental light increased the unit area quality of the upper, middle, and lower leaves of tomato. were significantly increased. The greenhouse tomato variety 'Maxifort' was supplemented with 61±2μmol/(m2·s) high-pressure sodium lamp, natural light, and three different ratios of red and blue light in the early stage. higher than high pressure sodium lamps. The effect of LED light supplementation on the increase of plant height, stem diameter and leaf area of watermelon grafted seedlings is better than that of high pressure sodium lamp treatment. These results all show that the growth of plant leaves is higher than that of high-pressure sodium lamps under the appropriate spectral ratio of LEDs. However, rose stem elongation and leaf area were lower under LED, and there were no significant differences in dry and fresh weight between several plant treatments, in line with the study of pepper, tomato, geranium, petunia grown under LED treatment and HPS lamp treatment Similar dry matter mass to snapdragon seedlings. The height, number of leaves, fresh weight and dry weight of tomato seedlings under 200μmol/(m2·s) high-pressure sodium lamps were all larger than those under the same optical density under the combination of red and blue LED lamps. Moreover, the fresh weight of tomato plants under alternating LED and high-pressure sodium lamps was lower than that of high-pressure sodium lamps alone, and the leaf transmittance and reflectivity of leaves were higher under high-pressure sodium lamps, which also allowed light to enter the canopy better. After a series of comparisons, it is found that the appearance of different test results is different from the design of the test method, and the LED lighting ratio, temperature, and optical density have a significant relationship.
2.3 Physiological differences
The content of chlorophyll directly affects the accumulation of photosynthetic products in leaves. Studies have shown that the gas exchange rate and chlorophyll content of conifer seedlings grown under LED are higher than those under high pressure sodium lamps. The chlorophyll content of cotyledons of rootstocks treated with high pressure sodium lamp and LED supplemented light for 9-13 days was significantly higher than that of natural light. LED light supplementation is beneficial to the accumulation of photosynthetic pigments in cabbage. Of the 8 growth experiments conducted by Ptushenko, the average photosynthetic pigment content (per unit leaf area) in plants grown under LED supplemental light was higher than that of high-pressure sodium lamps. The contents of chlorophyll a and chlorophyll b in tomato seedlings combined with red and blue LED lamps of 200 μmol/(m2·s) were higher than those of high pressure sodium lamps under the same optical density. Carotenoids are auxiliary pigments for chloroplast photosynthesis, and their function is to consume excess energy in photosystem II (PSII) and protect chlorophyll from damage by strong light. The Dlugosz study showed that supplementing light with high-pressure sodium lamps increased carotenoids and nitrate concentrations in lettuce. The soluble sugar, carotenoid and nitrogen contents in the leaves of pepper, tomato and eggplant seedlings were increased in different degrees under LED supplementary light, and the transpiration rate was accelerated. Jason observed when the plants were grown at the same time and tested with high pressure sodium lamps and LED (RB, RW) lighting, the water use efficiency of tomatoes and eustoma with high pressure sodium lamps was higher than that of LED treatment, and the transpiration rate was lower than that of LED treatment. There was no difference between CO2 exchange rate and final biomass, however, the maximum photosynthetic rate was the same under different treatments. Besides, LED(R:FR=3.09)500μmol/(m2·s) could significantly affect the flowering time and flowering rate of lentils. Both LED and high-pressure sodium lamps can increase the content of photosynthetic pigments, and the accumulation of photosynthetic pigments in LEDs is higher than that of high-pressure sodium lamps, and the transpiration rate is also higher than that of sodium lamps. The special spectral ratio in LEDs can also be used for certain plants. effect has an impact. In addition, it must be pointed out that the index of chlorophyll content alone cannot positively indicate the effect of light on the photosynthetic capacity of plants, because when plants encounter a low light density environment, they will automatically adapt to low light adversity and accumulate more in leaves. chlorophyll for more light energy.
3. Differences in the production cost of high-pressure sodium lamps and LEDs
Compared with traditional light sources, high-pressure sodium lamps and LEDs have obvious advantages. High-pressure sodium lamps and red and blue LED lights are used to fill the top of the plant canopy. Both can achieve the same output, and the LED only needs to consume 75% of the energy. It has been reported that the initial investment cost of LEDs is 5-10 times that of high-pressure sodium lamps under the same energy efficiency. For flower bed plants, 150W high pressure sodium lamps and 14WLEDs can achieve the same effect, and 14WLEDs are more economical in comparison. In an area of 550m2, the cost per kilogram of cucumbers using high-pressure sodium lamps alone is $1.3, the cost of sodium lamps plus a single row of LED lamps is $1.45, and the cost of sodium lamps plus two rows of LEDs is $1.72, and the profit-cost ratios are 2.31, 2.07, 1.74. The use of LEDs in the shed requires a large number of erections, and the cost of one-time investment is relatively large. For individual vegetable farmers, the investment is relatively difficult. Whether the fee reduction effect produced by LED power saving can fully make up for its initial investment and subsequent financial costs during its effective life period needs to be carefully calculated and measured.
4. Conclusion and Outlook
Green plants absorb the most red-orange light with a wavelength of 600-700 nm and blue-violet light with a wavelength of 400-500 nm, and only a small amount of green light with a wavelength of 500-600 nm is absorbed. Both high-pressure sodium lamps and LEDs can meet the lighting needs of plants. The initial research purpose of NASA (National Aeronautics and Space Administration) using LEDs is to improve energy efficiency, reduce operating and management costs, and improve the quality of commercial crops. In addition, LEDs can be widely used in the production of high-quality medicinal crops, and some scholars have pointed out that LED technology has great potential in improving the growth of plants.
The price of high pressure sodium lamp is moderate, which can be accepted by the majority of farmers. Its short-term effect is better than that of LED. Its supporting light supplement technology is relatively mature and is still being used on a large scale. However, high-pressure sodium lamps need to install ballasts and related electrical appliances, which increases the cost of their use. Compared with high-pressure sodium lamps, LEDs have narrower spectral tunability, high safety and reliability. LEDs offer flexibility in plant physiological testing applications. However, in actual production, the cost is high, the light decay is large, and the service life is far from the theoretical value. In terms of crop yield, LED has no obvious advantage over high-pressure sodium lamps. In the specific use, it should be selected reasonably according to the actual situation such as cultivation needs, application goals, investment capacity and cost control.
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