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Distribution Characteristics and Application Prospect of UV Light in Plant

In agricultural production, the light conditions are directly related to the open field of agricultural production efficiency and efficiency. Therefore, the recognition, utilization and regulation of solar spectral components has always been an important part of agricultural research. Especially in the modern agricultural production, the development of facility agriculture has become a pillar industry. The objective needs of artificial light environment control become realistic and urgent, and the control target has been expanded. The artificial light has become the inevitable development of facility agriculture. select.

Solar light is a continuous spectrum, the wavelength range from 100 nm to 100 m of X-ray radio waves. 99% of the solar radiation energy concentrated in the 280 ~ 500nm band. The contribution of sunlight to plant photosynthesis was different, only light of 400 ~ 700 nm was plant photosynthetically active radiation, which was involved in the photoreaction of plant carbohydrate synthesis. According to the perception of the human eye, usually the solar spectrum is divided into visible and invisible light, invisible light, including infrared and ultraviolet light, which wavelength is less than 380 nm band called ultraviolet light. According to the physical and biological characteristics of UV, the UV-B is divided into three bands: UV-A with wavelength of 320-380 nm, ultraviolet-B with wavelength of 280-320 nm, 280 nm short-wave ultraviolet (UV-C) [1]. In all solar radiation, about 50% of visible light, infrared about 48% to 49%, and the remaining about 1% to 2% of ultraviolet light. The results showed that the photosynthetically active radiation, UV and far-red light had a regulation function on plant growth and development in the solar spectrum, and had application value in agricultural production.

01.Characteristics of UV Radiation in Open Agricultural System

After the solar radiation through the atmosphere, the light intensity and spectrum have changed. The performance of X-ray and some short-wave radiation reduction, mainly by the ionosphere oxygen, nitrogen and other atmospheric components strong absorption filter. For UV rays, ozone in the stratosphere of the atmosphere absorbs almost all of the UV-C, while UV-A and UV-B reach the ground, UV-A reaches 95% of the UV, 1].Radiations with wavelengths greater than 2500 nm have very little energy reaching the ground due to the strong absorption of CO2 and water vapor in the atmosphere. Only the visible part of the solar radiation more complete to reach the ground, the loss in the atmosphere is smaller. Therefore, in the solar spectrum to reach the ground, UV-A and visible light, UV-B and other wavelengths of light quality less, which determines the exposed plants can receive solar energy and optical signal types. In the case of radiant energy, the sun's rays reach the sea level or the surface of the earth through the atmosphere, and the radiation energy of the leaves of the plant gradually decreases. In the end, the fixed solar energy of plant photosynthesis only accounts for 5% of the solar radiation on the plant leaf surface [2]

In recent years, a large amount of chemical substances such as chlorofluorocarbons (CFCs) and nitrogen oxides have been released, leading to a decrease in ozone levels in the stratosphere [3]. The destruction of the atmospheric ozone layer weakens the absorption of solar UV-B radiation, resulting in an increase in the intensity of UV-B radiation reaching the surface of the plant leaves. Ozone depletion has no effect on the spatial variability of UV-A and UV-C radiation, and studies have shown that UV-C does not reach the ground even if the ozone layer is reduced by 90%. Enhanced UV-B radiation reaching the ground has a serious biological effect on the exposed earth plants. It is estimated that for every 1% reduction in atmospheric ozone, the radiation intensity reaching the surface will increase by 2% to 3% and the grain yield will be reduced by 2% [4]. Atmospheric ozone was reduced by 1 to 5 per cent in 1978-1991 and by 16 per cent by 2060. UV-B radiation enhancement, greenhouse effect, acid rain and known as the world's three major environmental problems.

02.Distribution Characteristics of Ultraviolet Light in Plant

As a production form of facility horticulture, the plant is an efficient facility agricultural system which can realize continuous annual production of crops through high-precision environmental control within the facility, and is not subject to new production methods which are rarely subject to natural conditions [5]. Plant plant is internationally recognized as the most advanced stage of development of agricultural facilities, is a measure of a country's agricultural high-tech one of the important indicators. Compared with the traditional facilities, horticulture and plant factories have many production advantages, high production efficiency, good controllability and wide application prospect. Therefore, the plant plant is the inevitable trend of the development of facilities, gardening, to solve the 21st century population, resources, environmental issues of great significance, but also the future of space engineering, the moon and other planets to achieve food self-sufficiency in the important means [5].

In general, the plant plant can be divided into solar plant plants and artificial light plant plant two categories, two types of plant UV radiation background values, UV light conditions are different. Artificial plant plants need to use artificial light source for plants to provide light, and sunlight plant light energy mainly from the sun, artificial fill light only play a supporting role.

Sunlight plants are usually semi-enclosed glass greenhouse or solar greenhouse for the maintenance of the structure, the sun as the light source, so the distribution of its UV radiation and glass, shade nets and film material is closely related. Greenhouses and greenhouses due to cover materials (glass, plastic film and high-quality anti-aging film, etc.) and the natural light absorption, blocking and filtering effect [6]. In general, the transmittance of the covering material to visible light is about 88%, and the ultraviolet transmittance is only 15.9% to 21.1%. Compared with the open field, the retina and glass can filter out most of the UV-A and UV-B, solar plant UV-A and UV-B significantly lower UV radiation significantly reduced. The UV-B irradiance was about 0.5 W · m-2 in open sunny days and only about 0.075 W · m-2 in glass greenhouse [7]. In addition, UV radiation in solar plants is also affected by fill lamps, since high pressure sodium lamps (HPS) or UV lamps contain UV radiation components. Artificial light plants are usually constructed in a closed, light-shielded building. Artificial light sources, such as fluorescent and light-emitting diodes (LEDs), are all dependent on the ultraviolet component of the artificial light spectrum. How much.

The artificial light plant depends on the amount of UV radiation emitted by the lamp. The UV radiation component of the fluorescent lamp is very small, and the UV radiation component of the non-UV-LED lamp is almost zero. Therefore, the artificial light plant in the light of UV-A and UV-B radiation is very lacking, or even completely missing.

03.Effects of UV on Plant Growth and Quality.

One of the key features of plant plants is the lack of UV-A and UV-B radiation in sunlight, and the biological effects of UV-B in UV-protected plants are not known; however, UV-B intensities in solar radiation Changes in the UV and its biological effects are currently unclear. On the one hand, the plant plant cover protects the plant from UV radiation, but the complete absence of UV radiation also has a negative production effect. Therefore, it is necessary to regulate the UV radiation level in the plant, but this regulation must be based on production requirements and plant tolerance response laws. At present, the biological effects of greenhouse vegetables exposed to different or dose-varying UV radiation has become a new ecological and biological problem, and become an international research hotspot.

3.1 Biological effects of UV radiation

Suitable UV radiation is very important for high quality and efficient production of vegetable. The lack of UV radiation will have a negative effect on the growth and development of the facility. For example, low UV-B radiation usually causes the plants of plant vegetables to be leggy, low in nutrient quality, dull in color, and not in good taste. It also hinders the synthesis of plant pigments and is not conducive to the production of eggplant vegetables. A reasonable dose of UV-B radiation dwarf plants [8], and promote secondary metabolites such as the accumulation of carotenoids and other effects [9], improve the quality of tomato fruit [10]. However, high-dose UV-B exposure also causes some photosynthetic responses such as CO2 uptake, photosynthetic electron transport chain, dark respiration, stomatal behavior, pigment content, and plant endogenous hormones, which inhibit plant growth and development [11] . UV-A also affects the growth and development of plants, but the biological effects are small.

3.2 Application of UV radiation in plant plants

Facilities Horticultural crop quality is a measure of the value of their products an important indicator of the international horticultural products on the nutritional research in the ascendant. Plant plants in the use of ultraviolet light and regulate the protection of high quality and high yield is very important. Application of artificial control of UV light source in the facility to supplement the UV radiation technology, can reduce the chemical method of vegetables caused by leggy and improve the quality of vegetables is an important guarantee for the production of organic green food. However, little research has been done on the mechanism of mid-long-wave UV light supplementation on the nutrient quality, especially the antioxidant, and the lack of effective UV regulation and control technology and equipment.

Li et al. [12] found that supplementing light with different LED light quality had an effect on the nutritional quality of lettuce, and increased UV-A could improve the content of anthocyanin by 11%. Caldwell et al. [13] studied the effects of supplemental UV-A and UV-A + UV-B treatments on carotenoids and chlorophyll in eight green and red lettuce cultivars, and showed that supplementation with UV-B increased carotenoids and chlorophyll Content, but decreased the content of carotenoid and chlorophyll in leaf lettuce. The difference of UV-A and UV-B radiation in different leaf lettuce may be due to the light-dependent change of plant phenolic level. Red leaf lettuce UV induced relative to the green leaf lettuce significantly higher phenolic substances, may reduce the chloroplast carotenoid photoprotection needs [14]. The results showed that high dose of UV-B could decrease the content of lycopene and vitamin C, and low dose of UV-B could increase the content of lycopene and vitamin C [10]. Tsormpatsidis et al. [15] studied the growth of lettuce and the production of anthocyanins, flavonoids and phenols by different UV radiation through the membrane, including UV completely through the membrane, through 320, 350, 370, 380 nm Film, and a film that does not transmit UV radiation at all. It was found that the dry weight of lettuce biomass was 2.2 times more than that of pure lettuce. On the contrary, the content of anthocyanin in lettuce was about 8 times higher than that under UV completely impermeable membrane.

04.Application and Strategy of UV Artificial Light Regulation in Plant

UV artificial light control in plant plant has a good application value in realizing the high quality and high yield of facility horticulture. However, the application dosage and irradiation time should be selected according to crop type and application target to ensure application safety. In the selection of crop types, the facilities of vegetables, medicinal plants, flowers and fruit trees can be applied; application of the main objectives of the plant to control the leggy and improve the quality of agricultural products, such as increasing antioxidant content of vegetables, medicinal plants to increase secondary The accumulation of metabolites, etc., to speed up the flower leaves and petals in the synthesis and accumulation of color substances. In order to control the effect of good UV control, UV radiation dose must be controlled. Short-term low-dose applications can be used, intermittent repeated exposure, pre-irradiation and other methods to reduce the use of short-term dose. Lamps, the current market sales of the main UV lamp, but the UV-LED light source may be the future of UV light control of the preferred electric light source, precise positioning exposure. From a security point of view, should be based on the facilities of the horticultural crop physiological needs and quality control objectives, to develop a reasonable intelligent light environmental management measures to avoid exposure to UV on the human body, such as the use of night-stop approach during the day, so that UV can play Quality control role, to improve the facilities of the horticultural crop value and nutritional health functions.

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