Applications of Light Wavelength Conversion Materials in Agriculture: Comparison
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Please note this is a comparison between Version 2 by Beatrix Zheng and Version 1 by Jialei Liu.

As new fluorescent materials, light wavelength conversion materials (light conversion agents) have attracted increasing attention from scientific researchers and agricultural materials companies due to their potential advantages in efficiently utilizing solar energy and increasing crop yield. According to the material properties, the light conversion agents can be divided into fluorescent dyes, organic rare-earth complexes, and inorganic rare-earth complexes. The current researches indicates that the fluorescent dyes have relatively high production costs, poor light stability, difficult degradation processes, and easily cause pollution to the ecological environment. The organic rare-earth complexes have short luminescence times, high production costs, and suffer from rapid decreases in luminescence intensity. Compared with fluorescent dyes and organic rare-earth complexes, although rare-earth inorganic complexes have high luminous efficiency, stable chemical properties, and better spectral matching performance, the existing inorganic light conversion agents have relatively poor dispersibility in agricultural films. 

  • light conversion
  • agricultural film
  • luminescent materials
  • agricultural optics
  • optical modulation

1. Introduction

Agricultural films are polymer materials, such as polyethylene (PE), polyvinyl chloride (PVC), Poly-(butylene adipate-co-terephthalate) (PBAT), Ethylene-vinyl acetate copolymer (EVA), etc. Pure polymer materials have many limitations in practical application, so improving the properties of polymer materials has always been the direction of interest for researchers or agricultural materials companies. Three common light wavelength conversion materials (light conversion agents), namely fluorescent dyes, organic rare-earth complexes, and inorganic rare-earth complexes, have potential advantages in potential advantages in efficiently utilizing solar energy and increasing crop yield, so their combination with agricultural film is an inevitable development trend. However, whereas there are still many deficiencies in six aspects: efficiency, cost, compatibility with greenhouse films, light matching, and light transmittance, and this greatly limits the application of multifunctional agricultural film. Therefore, the research on agricultural film with polymer as carrier and its ideal added components is a cause worthy of full-time investment of scientific researchers.
Else, Functionalization has always been an important research direction of polymer materials. The organic combination of light conversion agent and polymer materials realizes the effective regulation of polymer materials on the solar spectrum, which is an important means to realize the optical functionalization of normal polymer materials like PE, PP, PVC, PBAT and so on.
Solar energy is an important energy source and is a necessary condition for plant photosynthesis. The scientific literature shows that the blue-violet (400–480 nm) and red-orange light (600–700 nm) in sunlight can promote the growth and development of crops. The blue-violet light is beneficial in increasing the protein content of crops, while the red-orange light increases the synthesis of crop carbohydrates [1,2,3,4,5,6,7,8,9,10][1][2][3][4][5][6][7][8][9][10]. However, the emission of the ultraviolet light (280–390 nm) is not only unfavorable to the growth of plants, but also increases plant diseases and insect pests. Some sunlight will be reflected when it is emitted to the surface of the earth due to the presence of the atmosphere, which leads to this part of the sunlight being wasted. In recent years, the research in the field of agricultural film materials has mainly focused on the development of agricultural films with high-efficiency light conversion performance in order to make greater use of sunlight. The high-efficiency light conversion performance of agricultural film materials is mainly achieved using light conversion agents. A light conversion agent is a fluorescent material that can convert UV light into blue-violet light or red-orange light. In recent years, it has attracted extensive attention and has been actively developed. An agricultural film added with a light conversion agent is called a light conversion film, which can convert the lower active wavelength band in photosynthesis to the higher active wavelength band (400–700 nm), in turn changing the ultraviolet (280–380 nm) and yellow green (510–580 nm) wavelengths of sunlight into the blue-violet (400–480 nm) and red-orange (600–700 nm) wavelengths required for plant photosynthesis, improving crop yield and quality [11,12,13,14,15,16,17,18,19,20][11][12][13][14][15][16][17][18][19][20].

2. Application of Light Wavelength Conversion Greenhouse Films

Recently, the application of light conversion films in greenhouse and field planting has gradually increased. Through the conversion of ultraviolet light into blue-violet light or red-orange light, the utilization of sunlight can be significantly improved, thereby providing an optical environment for crops. The increases in red and blue light promote the growth of crops and the accumulation of nutrients and play an important role in increasing crop yield and improving crop quality. In this section, the application effects of light conversion films are discussed in detail.
Many scholars have studied and improved the quality of light conversion films and such kind of light conversion films played a central role in greenhouse and field planting research. Light conversion films can convert ultraviolet light into blue-violet light and red-orange light, which are useful for crops, or can change yellow-green light into red-orange light, which changes the quality of the light passing through the film. This not only increases the leaf area, vine spread, plant height, and petiole length of plants, meaning the leaves have good ductility, but also effectively reduces the ecological pollution at the end of the season. In Gao’s study, a transplanted film used as a modified material in greenhouse lettuce cultivation had a beneficial effect in improving the light energy use efficiency for crop production [45][21]. The research by Yang et al. showed that a translucent film had great application prospects in field crop cultivation, as it promoted plant growth and increased the biological dry weight per plant [46][22]. Gao applied a light conversion film to seedling trays containing tobacco and tomato plants. The film was conducive to the growth of the seedlings, as they effectively increased the temperature in the membrane disk, improved the resistance and quality, and significantly promoted the aboveground biomass and root phenotypic parameters of the seedlings. Additionally, the time of seedling emergence was shortened by 2 d and the period of seedling establishment was 3 d ahead of schedule. The total root length, root surface area, and volume of tobacco seedlings at the seedling establishment stage were significantly increased by 44.88%, 69.87%, and 90.90%, respectively [47][23]. Zhang treated tomatoes with a rare-earth light conversion film, which significantly promoted the growth of green stems and leaves, increased the fruit yield, and improved the fruit quality. In addition, the fruits ripened 7d earlier and yield increased by 6.35% [48][24]. Song covered potato plants with light conversion films, which not only promoted potato emergence and maturity, but also increased the potato yield and output value by 8.91% [49][25]. Wen also covered tomato plants with a light conversion film, which significantly promoted the plant growth and development in the heliostat, with a 9.92% increase in tomato yield per unit area [50][26]. The sugar-to-acid ratio and contents of lycopene, soluble sugar, and VC in tomato fruit samples were also significantly increased, indicating that the addition of suitable light transfer agents to the trellis film could increase the proportions of red-orange and far-red light in the transmitted light. This promoted the growth and development of tomato plants and improved the yield and quality of fruit. In Liu’s studies [51][27], the light conversion film had a promotion effect on the growth of greenhouse strawberry plants. Firstly, the light conversion film caused a marked temperature increase. Compared with the normal film, the accumulated temperature in the greenhouse increased by 75.0 °C and the average daily temperature increased significantly by 0.39 °C. Secondly, the light conversion film promoted the growth and development of strawberry plants. The chlorophyll content and net photosynthetic rate of fruiting strawberry plants in the transplanted film sheds were markedly increased by 14.0% and 14.3%, respectively, and the use of the light conversion film meant strawberries could be picked about 10 d earlier. Thirdly, the light conversion film had improved the quality of the strawberries. The VC contents of strawberries grown under both transplanted films were significantly higher than when grown under normal film. Wen’s research found that the red light conversion film could significantly improve the yield and quality of cucumber fruit compared with the control. The red light conversion film promoted the growth of cucumber plants; prominently improved the plant height, stem diameter, internode length, and single fruit quality; and increased the yield by 16.78%. In addition, the contents of soluble sugar, free amino acids, and VC in cucumber fruit increased significantly [52][28]. Yan applied a broad-spectrum light conversion film to tomato plants, which enhanced the efficiency of the UV-to-red-light conversion and increased the total amount of red light emission. It also facilitated the photosynthesis of tomato plant leaves in the heliostat, promoted the growth of tomato seedlings and the absorption of nutrients and water, increased the yield, and improved fruit quality [53][29]. Sun’s research showed that the white light conversion film treatment significantly promoted the growth of tomato seedlings, increasing the strong seedling index by 67% and stem thickness by 44%, which was suitable for cultivation cover in South China and had good application prospects [54][30]. Li’s study proved that by increasing the ratio of red light to orange light under the red light conversion film, the high-temperature light effect had two advantages. On the one hand, it improved the photosynthetic efficiency of sweet pepper and promoted the absorption of nutrients and the synthesis of amino acids. On the other hand, it accelerated the degradation of chlorophyll and anthocyanin accumulation in fruit during discoloration by promoting carbon metabolism to improve the appearance and nutritional quality of the fruit. At the same time, the addition of suitable light conversion agents could increase the VC content of sweet pepper fruit [55][31].

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