The relatively closed production system of greenhouses will play an important role in meeting the growing demand for food in the future. In recent years, the lack of greenhouse light has attracted more and more attention. On the one hand, greenhouse light transmittance caused by greenhouse orientation, structure, and covering material characteristics has decreased. On the other hand, due to climate change, greenhouse crops have insufficient light. , such as continuous rainy weather in winter and early spring, frequent haze weather and so on. Insufficient light directly has adverse effects on greenhouse crops, causing serious losses to production. Plant fill light can effectively alleviate or solve these problems.
Incandescent, fluorescent, metal halide, high-pressure sodium, and emerging LED lights have all been used or are being used in the greenhouse for light. Among these types of light sources, high-pressure sodium lamps have higher luminous efficacy, longer service life, and higher overall energy efficiency, occupying a certain market position, but high pressure sodium lamps have poor lighting continuity and low safety (including mercury). Problems such as not being able to reach close range are also prominent. Some scholars take a positive attitude toward LED lamps in the future or can overcome the low performance of high pressure sodium lamps. However, LED’s are expensive, light-filling technology is difficult to support, light-filling theory is not perfect, and LED plant fill light product specifications are confusing, which makes users question the application of LEDs in plant light-filling applications. Therefore, the article systematically summarizes the previous research results and its production and application status, and provides reference for the light source selection and application in the greenhouse.
Differences between high-pressure sodium lamps and LED lighting
♦ Difference in light-emitting principle and external structure
The high pressure sodium lamp consists of mercury, sodium, helium arc tube wicks, glass bulbs, and getters from the inside to the outside. Because of its different core ballasts, it is divided into high-voltage sodium lamp and electronic high-pressure sodium lamp. Different-pressure high-pressure sodium lamps need to use ballasts with corresponding specifications. LED is also called a light emitting diode. The core part is a wafer composed of a P-type semiconductor and an N-type semiconductor. There is a transition layer between the P-type semiconductor and the N-type semiconductor, which is called a P-N junction. When the current flows from the anode of the LED to the cathode, the semiconductor crystal emits light of different colors from purple to red. The intensity of the light is related to the current. According to the luminous intensity and operating current can be divided into ordinary brightness (luminous intensity <10 mcd), high brightness (luminous intensity of 10 ~ 100 mcd) and ultra-high brightness (luminous intensity> 100 mcd) and other types. Its structure is mainly divided into four blocks: the structure of the light distribution system, the structure of the heat dissipation system, the drive circuit, and the mechanical/protective structure.
♦ Irradiation range and spectral range differences
The high-pressure sodium lamp tube has a light emitting angle of 360[deg.], and most of them must be reflected by the reflector before being irradiated to a specified area. The spectral energy distribution is roughly red, orange, yellow, green and blue (only a small part). According to different light distribution designs of LEDs, the effective light emission angles can be roughly classified 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., and can be arbitrarily combined according to different needs.
♦ Differences in applicable conditions and longevity
The high pressure sodium lamp is the third generation of lighting source. It has a wide range of use under the conventional alternating current, high luminous efficiency, and strong penetrability. The maximum life span is 24000h, and the minimum is also maintained at 12000h. The sodium lamp is accompanied by heat generation while it is being illuminated, so the sodium lamp is a heat source. In the process of use, there is also a self-extinguishing problem. LED as the fourth generation of new semiconductor light source, using DC drive, life can reach more than 50000 h, and the attenuation is small, as a cold light source, can be close to the plant irradiation. Compared with LED and high-pressure sodium lamp, it is pointed out that LED has higher safety, does not contain harmful elements, and is more environmentally friendly.
Differences in crop impact between high-pressure sodium lamps and LED fill light
A large number of production practices and scientific research in agricultural production have proven that artificial plant fill light can not only increase crop yield, shorten planting cycle, but also effectively improve crop quality, and is an important guarantee for efficient production of modern agriculture. In the process of nursery and greenhouse crop management, the use of high-pressure sodium lamps and LED to fill the light, can promote the growth of the crop, change the crop yield, morphology, and physiological indicators.
♦ Production and quality difference
The high yield and high quality of crops are the ultimate goal of planting and cultivation. The supplement of light by LED can improve the quality of pepper, tomato and eggplant seedlings, and the increase in single fruit quality and yield per plant under light supplementation for 10 hours is significant. The effect of LED light-increasing production is also reflected in the cucumber planting. The LED can improve the quality of the grape fruit, among which the blue light fills the fruit with the fastest development, the single fruit quality is higher, the sugar content is the highest, and the single grain mass is the highest when the UV light-filling treatment fruit matures. Similarly, the 70W high-pressure sodium lamp significantly increased the yield per plant of the strawberry, and the yield increase was 17.9%. High-pressure sodium lamps and LED fill light have a significant effect on plant morphology. The supplemental light treatment on the side of the LED also improved the visual fruit quality of the cucumber. The LED is added on the basis of the sodium lamp. Compared with the sodium lamp, the color of the cucumber is more vivid.
♦ Differences in morphological indicators
Plant morphology index is an important indicator in the process of plant growth, especially in nursery production, it determines whether the plant can grow healthily after transplantation and cultivation. Under normal circumstances, the growth of coniferous plant seedlings under LED growth is better than that of high-pressure sodium lamps. 12h photoperiod, optical density 50μmol/(m2·s), LED red (630~660nm), orange light (590~610nm), blue light (450~460nm), green light (520~540nm), respectively, than natural light [120 μmol/(m2·s)] significantly increased the seedling index of tomato seedlings. After supplementing the light with homemade LEDs, the plant height, stem diameter, and leaf area of the pepper, tomato and eggplant seedlings were also significantly increased, and the inter-LED light supplements significantly increased the mass per unit area of the upper, middle, and lower leaves of the tomato. . The greenhouse tomato variety ‘Maxifort’ used 61±2μmol/(m2·s) high-pressure sodium lamp, natural light and 3 different proportions of red and blue light to make up the light. It was found that tomato leaf area and number of leaves under 95% red+5% blue LED It is higher than high pressure sodium lamp. The effect of LED lights supplementation on the increase of plant height, stem diameter and leaf area of grafted watermelon seedlings was better than that of high pressure sodium lamp treatment. These results all indicate that the LED spectrum ratio is suitable and the plant leaf growth is higher than that of the high pressure sodium lamp. However, there was no significant difference in dry weight and fresh weight between several plant treatments due to the elongation of rose stems and the lower leaf area under LED. This was in line with the study of peppers, tomatoes, geraniums and petunias grown under LED treatment and high-pressure sodium lamp treatment. Seedlings with snapdragon have similar dry matter quality. The tomato seedling height, leaf number, fresh weight, and dry weight under high pressure sodium lamp supplemented with 200μmol/(m2·s) light were greater than that of the red-blue LED lamp combination under the same light density. Moreover, the fresh weight of tomato plants alternately irradiated with LEDs and high-pressure sodium lamps is lower than that of high-pressure sodium lamps alone. The leaves of the leaves under high-pressure sodium lamps have higher transmittance and reflectance, which also allows the light to better enter the crown. After a series of comparisons, the occurrence of different test results was found to be different from the design of the test method. There was a significant relationship between LED light ratio, temperature, and optical density.
♦ Physiological differences
Chlorophyll content directly affects the accumulation of photosynthate in leaves. Studies have shown that the gas exchange law and chlorophyll content of coniferous seedlings under LED growth are higher than that of high pressure sodium lamps. In the high-pressure sodium lamp treatment, the chlorophyll content of the rootstock of the rootstock treated with LED light for 9 to 13 days was significantly higher than that of natural light. LED lights make up for the accumulation of photosynthetic pigments in cabbage. In the eight growth tests conducted by Ptushenko, the average photosynthetic pigment content (per unit leaf area) of five plants grown under LED fill light was higher than that of high pressure sodium lamps. The 200μmol/(m2·s) red-blue LED lamp combination had higher chlorophyll a and chlorophyll b content in tomato seedlings than the high-pressure sodium lamp at the same optical density. Carotenoids are auxiliary pigments for photosynthesis of chloroplasts. Their functions are to consume excess energy in PS II and to protect chlorophyll from glare. Dlugosz research shows that supplementing with high-pressure sodium lamps will increase the concentration of carotenoids and nitrates in lettuce. Under the LED light supplement, the contents of soluble sugar, carotenoids and nitrogen in leaves of pepper, tomato and eggplant seedlings all increased in different degrees, and the transpiration rate was accelerated. When the plants were grown simultaneously and tested with high-pressure sodium lamps and LED (RB, RW) lighting, it was observed that the water use efficiency of tomato and eustoma when supplemented with high-pressure sodium lamps was higher than that of LED treatment, and the transpiration rate was lower than that of the LED treatment, in net CO2 There was no difference between the exchange rate and the final biomass, however, the maximum photosynthetic rate was the same under different treatments. In addition, LED (R:FR=3.09) 500 μmol/(m2·s) can significantly affect the flowering time and flowering rate of lentils. Both LED and high-pressure sodium lamps can increase the photosynthetic pigment content, and the LED is higher in the accumulation of photosynthetic pigment than the high pressure sodium lamp, and the transpiration rate is higher than that of the sodium lamp. The specific spectral ratio of the LED can also be used for the flowering of some plants. The effect has an effect. In addition, it must be pointed out that the chlorophyll content alone can not positively indicate the effect of light on plant photosynthetic ability, because when the plant encounters a low optical density environment, it automatically adapts to low light stress, enriching more in the leaves. Chlorophyll for more light energy.
High pressure sodium lamp and LED production cost difference
Compared to traditional light sources, high-pressure sodium lamps and LED have obvious advantages. With high-pressure sodium lamps and red and blue LED lights, the top of the plant canopy is filled with light, both of which can achieve the same output. LEDs only need to consume 75% of the energy. It has been reported that under the same conditions of energy efficiency, the initial investment cost of LED is 5 to 10 times that of high-pressure sodium lamps. The initial high cost makes the cost per LED of the use of high-pressure sodium lamps 2 to 3 times higher in the use of 5 years. For flowerbed plants, the 150W high pressure sodium lamp and 14W LED can achieve the same effect, compared to 14W LED more economical. In the 550m2 area, the cost of using a high-pressure sodium lamp per kilogram of cucumber alone is $1.3, the cost of a sodium lamp plus a single row of LED lamps is $1.45, the cost of a sodium lamp plus two rows of LEDs is $1.72, and the profit-to-cost ratios are 2.31 and 2.07, respectively. 1.74. The use of LEDs in sheds requires a large number of erections, and the cost of one-time investment is relatively large. For individual vegetable farmers, investment is more difficult. Whether the cost-reduction effect caused by LED energy-saving can fully compensate for its initial investment and subsequent financial costs in its useful life, it needs careful accounting and measurement.
Conclusion and Outlook
The most abundant green plants absorb red orange light with a wavelength of 600-700 nm and blue-violet light with a wavelength of 400-500 nm, and only slight absorption of green light with a wavelength of 500-600 nm. Both high-pressure sodium lamps and LEDs can meet the light requirements of the plants. The initial research purpose of NASA (National Aeronautics and Space Administration) using LEDs was to improve energy efficiency, reduce operating and management costs, and improve the quality of economic crops. In addition, LEDs can be widely used for the production of high-quality medicinal products. There are also scholars who point out that LED technology has great potential for improving plant growth.
The high-pressure sodium lamp is affordable and can be accepted by the majority of farmers. Its short-term efficacy is better than that of LED. Its complementary lighting technology is relatively mature and is still in large-scale use. However, high-pressure sodium lamps need to install ballasts and related electrical equipment, increasing their cost of use. Compared with high-pressure sodium lamps, LEDs have narrow spectral tunability, and have high safety and reliability. LED has the flexibility in the application of plant physiological tests. 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. Specific use should be based on the actual needs of the cultivation needs, application objectives, investment capacity and cost control, reasonable choices.