Current Approaches to Light Conversion for Controlled Environment Agricultural Applications: A Review

Abstract

Modern agriculture cannot be imagined without the introduction of smart and efficient technologies. These, undoubtedly, include technologies for directed regulation of the illumination of agricultural plants. Depending on the climatic conditions of cultivation, farmers shade or additionally illuminate the plants, and also change the spectrum of the light reaching the plants. The aim of this review is to provide an overview of solar light conversion methods and approaches for agricultural applications and discuss their advantages and limitations.

1. Introduction

Light is one of the key factors that determine the growth and development of plants. The quantity and quality of natural lighting determines the economics and feasibility of crop production in various regions of the world. To drive photosynthesis, plants most efficiently use light in the wavelength range from about 400 to 700 nm, called photosynthetically active radiation (PAR). It is known that the quantum efficiency of photosynthesis, calculated on an absorbed photon basis, weakly depends on their wavelength [1,2,3,4]. However, the efficiency of using light of different wavelengths on plants, even within the PAR range, is not the same (Figure 1) due to the different efficiency of photon absorption by photosynthetic antenna complexes, as well as the reflection of light or its absorption by other components of the plant cell [2,4].

It is well known that up to 80% of the energy used in photosynthesis is provided by photons with a wavelength of about 650 nm. Despite the fact that red light (601–700 nm) determines the efficiency of photosynthetic processes and also accelerates the transition of a plant to flowering [6,7], the growth and development of plants depends on the spectral composition of the light. For example, blue (441–500 nm) and violet light (401–440 nm) are involved in the regulation of chlorophyll synthesis, chloroplast development, and photomorphogenesis, induce secondary metabolisms due to cryptochrome and phytochrome, and also regulate stomata opening [8,9,10]. An increase in the proportion of blue light under high light conditions leads to the accumulation of PsbS and LHCSR3 proteins in higher plants and algae, respectively, which are heat dissipation effectors [8]. Green (501–565 nm) light is absorbed by various carotenoids and chlorophylls in the deep layers of the leaf. In nature, yellow (566–600 nm) and green light is important mainly for shade-tolerant and some aquatic plants [11]. Several studies have shown that light in the range from 501 nm to 600 nm has a significant effect on the physiology, morphology, and photosynthetic activity of plants [12,13,14,15,16,17]. Far-red radiation (701–800 nm) accelerates photosynthesis by balancing the stoichiometry of electron transfer in the photosynthetic electron transport chain (PETC), which increases the overall efficiency of photosynthesis, since excess blue and red light can cause PETC overreduction [1,18,19,20,21]. This process may involve specialized chlorophyll, which absorbs light in the far-red range and is associated with photosystem I [20]. It is known that a change in the red/far-red ratio can lead to the activation of the phytochrome system, which, in turn, can intensify photosynthesis, increase stress resistance, and accelerate plant growth [22,23,24]. From one to two percent of the solar energy reaching the Earth’s surface is ultraviolet radiation (UV) (280–400). UV-A (316–400), on the one hand, is one of the factors of photoinhibition [25], and on the other hand, UV-A stimulates an increase in the accumulation of secondary metabolites: phenols, flavonoids, anthocyanin, and ascorbic acid [26]. UV-B (290–315) has traditionally been considered a stressor. It can cause damage to the integrity and function of important macromolecules (DNA, proteins and lipids), oxidative damage, inhibition of photosynthesis and reduced growth of plants [27,28,29]. However, studies have identified regulatory properties of low UV-B levels that induce distinct changes in plant secondary metabolism leading to an accumulation of secondary plant compounds: monoacylated kaempferol di- tri-glucosides, flavonoids such as kaempferol, quercetin, and glucosinolates (GS), especially of 4-methylsulfinylbutyl GS and 4-methoxy-indol-3-ylmethyl GS [30,31,32].

Various methods of sunlight modification are used to increase the yield of a wide range of crops in different climatic zones. For example, in the equatorial and tropical zones, approaches are used to reduce light intensity [33], since not only the deficiency, but also the excess of sunlight has a negative effect on the growth and development of plants [34,35,36]. In temperate and subarctic climatic zones, the deficiency of sunlight is compensated by additional artificial lighting [37,38,39,40,41,42]. In addition to targeted changes in the overall intensity of the incident light, approaches have been developed to improve crop productivity by changing the spectral composition of the light. For this, photoselective and photoconversion covers are used. Photoselective covers completely or partially absorb photons of a certain wavelength [43,44,45,46,47], while photoconversion covers convert photons of one wavelength into photons of another wavelength using luminophores. In agriculture, luminophores, which convert high-energy photons into low-energy photons, are most widely used [48,49,50,51,52]. In addition, up-conversion covers containing luminophores are capable of emitting photons with higher energy than those used for excitation, and are currently being developed [53].

Incident light modification, including changing the overall intensity, qualitatively changing the spectral composition, and conversion of part of the light into electrical energy or heat, can significantly increase the yield of most crops, as well as non-chemically control the growth and development of ornamental plants. The various approaches that can significantly improve conditions for the growth of plants are discussed below.

2. First Approach: Change in the Overall Intensity of Natural Light

In many regions, most crops are grown exclusively using natural sunlight. The crops grown are generally well-adapted to the local climate and produce good yields. However, the rapid increase in population requires the intensification of agricultural production (increase in yield, increase in the growth cycles per year, development of new territories that were previously considered unsuitable for use, etc.), which is hindered by various restrictions. Long hot or cold seasons do not allow multiple crops per year. The development of new territories, as a rule, is associated with the development of northern (including the use of greenhouses) or tropical climatic zones, as well as the organization of agricultural enterprises in cities where there is intense heating of the soil and air. This is accompanied by a negative impact on the productivity of agricultural plants and greatly reduces the yield [54]. In subpolar and even temperate latitudes, solar energy is not enough for efficient agriculture due to the short daylight hours and cloudy days. Under such conditions, the morphological and physiological parameters of plants deteriorate [45,55]: flowering and fruiting are delayed, plants have thin stems and shoots, and sparse dark leaves. Shading [33,56,57,58] or additional lighting of plants [37,38,39,40,41,42,45,55] are used to improve the efficiency of agriculture in conditions of excess or deficiency of natural light, respectively. At present, these approaches are most widely used in practice due to their simplicity and high availability for farms.

2.1. Covers That Reduce Overall Light Intensity

In order to reduce the negative effect of excess light and temperature, shading is used. As a rule, covers containing reflective material with light transmission from 10% to 90% are used [33,56,57]. At the same time, when optimizing shading conditions, it is necessary to take into account both the region and the crop planned for cultivation, since excessive or insufficient shading can adversely affect the yield. Several works have shown the high efficiency of shading covers. Kitta et al. showed that shading polyethylene grids with 0.27 mm ∙ 0.27 mm pore size increased the yield of sweet pepper (Capsicum annuum) by up to 100% [33]. It was revealed that the optimal shading for the cultivation of sweet pepper in the eastern part of Greece is about 22%. In another study, used head lettuce (Butterhead lettuce), it was shown that 60% shading with polyethylene film increased the yield by 93% (field experiments were conducted in Kraków from March till May). Thus, a high shading coefficient in Poland is more favorable for the growth and development of lettuce [57]. Authors note that growing crops under shading films leads to a decrease in air temperature by an average of 30% and positively affects soil temperature and air humidity [56,57,58]. Furthermore, to change local conditions under covers, “artificial mist” is used, which is sprayed under a plastic film in the form of small drops [56]. It reduces the soil and air temperature, and increases the humidity under the coating, but practically does not change the light spectrum of light passing through it [56].

2.2. Using Additional Artificial Lighting to Increase Overall Light Intensity

The problem of sunlight deficiency in moderate and subarctic climate zones is solved by installing additional artificial lighting in greenhouses. Until the 1990s, incandescent lamps were practically the single source of additional light. However, at present, these lamps are practically not used for illumination of plants, since they have significant shortcomings: (i) very low efficiency of transformation of electric energy into light energy at 0.15 µmol photons J⁻¹ [59], (ii) a short service life, (iii) a strong increase in temperature in greenhouses, and (iv) a high share of infrared radiation. These factors not only increase expenses on the lighting and cooling of greenhouses, but also adversely affect the growth and development of plants.

The incandescent lamps in agriculture were replaced by sodium lamps. They have higher efficiency of transformation of electrical energy into light 0.67 µmol photons J⁻¹ [59], and longer service life.

Currently, LEDs are gaining more and more popularity. In comparison to incandescent lamps and sodium lamps, they heat the greenhouses little, since their heat generation is much lower, and have almost 20 times higher service life. Moreover, LEDs have a higher efficiency of transformation of electrical energy into light at 0.6–1.5 µmol photons J⁻¹, depending on the type of LED and spectrum of its radiation [60]. Semenova et al. [39] showed that additional LED lighting of lettuce (Lactuca sativa) led to an increase in the accumulation of chlorophyll, leaf area, and dry and fresh weight in comparison to plants grown under sodium lamps [39]. The main disadvantage of LEDs, which hinders their use in farms, is their high price.

The application of LEDs is mainly associated with research aimed at identifying the optimal spectral composition of light for growing different plants. For example, the yield of L. sativa under illumination with red (660 nm), green (530 nm), and blue (460 nm) light in a ratio of 4:1:1 (taking the intensity) was significantly higher than under illumination at a ratio of 1:1:1 (in both cases, the total light intensity was 200 µmol m⁻² s⁻¹). At the same time, the fresh weight of the plants increased by 60%, and the leaf area by 30% [37]. According to other data, the L. sativa had increased leaf area (3-fold) when grown under red, yellow, and blue LEDs without changes in the physiological and biochemical parameters of the plants [45]. For radish (Raphanus sativus) and tomato (Solanum lycopersicum), the optimal ratio was 16:3:1; a threefold increase in the fresh and dry weight was observed [40]. Under low light conditions, additional illumination of plants with light of a certain wavelength can also lead to a positive effect. For example, the growing of potatoes (Solanum tuberosum) with additional red light led to a 50% increase in the length of the axial shoot and total fresh weight, and 20% in the dry weight of the root [55]. Works on the optimization of artificial lighting for different types of plants using LEDs indicates the widespread use of this type of light sources (in more than 40 countries, and in some of them they occupy about 60% of all farms) and the ease of working with it [61,62,63,64].

Long service life, low consumption of power, ease of use and efficiency in growing crops will make LED lighting the most economical and common way for both primary and supplementary lighting of agricultural plants. With the optimal parameters of the supplementary illumination of plants, it is possible to achieve a high yield in a wide variety of crops.

3. Second Approach: Qualitative Transformation of the Sunlight Spectrum Using Special Covers

In addition to methods for changing photosynthetic photon flux density, methods of directed qualitative transformation of the light spectrum for growing plants are widely used. As noted earlier, various wavelengths, even in the PAR region, are not all used equally in photosynthesis. As a result, it becomes possible and/or necessary to spectrally transform sunlight. This can be done in several ways. Firstly, it is possible to reduce the spectral component that is harmful or of little use for plant growth and development. Secondly, light from this spectral range can be converted into light of a different wavelength that is effectively used by the plant. The third approach is to convert a light of a certain part of the spectrum into either electrical [65,66,67,68] or thermal [69] energy, which can be used for the needs of the greenhouse itself. A decrease in the sunlight intensity and conversion of light can be achieved using photoselective and photoconversion covers, respectively [23,43,44,46,48,49,50,51,52,70,71]. Photoselective covers contain dyes that reduce the intensity of a certain part of the solar spectrum. Photoconversion covers change the sunlight spectrum due to various luminophores incorporated into the covers, which are capable of converting light from one spectrum region to another [23,49,50,51,52].

3.1. Transformation of the Sunlight Spectrum Using Photoselective Covers

The principle of operation of photoselective covers is based on the dyes, which selectively absorb the light of certain wavelengths. Most often, dyes added to these covers absorb UV [72,73,74,75,76,77,78,79], red or far-red radiation [47,71,80,81,82,83,84,85,86,87,88,89]; less often the dyes absorb blue light [90,91,92]. As a result of such selective absorption, there is a decrease in the proportion of radiation harmful to the plant, as well as a change in the ratio of the intensity of the spectral bands. For example, a change in the red light/far-red radiation ratio (photoequilibrium exchange) (Figure 2) has a great influence on the growth and development of plants (Figure 3) [93,94,95,96]. Figure 4A shows that photoselective covers mainly reduce far-red radiation (in order to increase the ratio of red to far-red intensity and reduce heating), as well as the intensity of red light (to reduce the negative effect of high light).

In early works, copper sulfate (CuSO₄·5H₂O) was used as a dye for photoselective covers. A reduction of the proportion of far red in the light flux using a CuSO₄-based dye was used to regulate the height, number and size of leaves of ornamental crops [82,83,87]. For example, cultivation of garden chrysanthemum (Dendranthema grandiflorum) under polycarbonate panels with a 6% copper sulfate solution between them resulted in a 30% slowdown in growth rate, making the plant small and compact [46]. Similar results were shown in another study [87]. The growth and development of D. grandiflorum slowed down with increasing copper sulfate concentration, and at 8% concentration, the slowdown was 40% [87]. At the same time, it was shown that an aqueous solution of copper sulfate with a concentration above 8%, in particular 16%, practically did not increase the efficiency of the covers, since red light was absorbed at a concentration of copper sulfate of more than 10%, which caused the usual shading effect [46,87]. Covers containing 4–6% copper sulfate slowed down the growth of rosa (Rosa hybrida) [88] and lilium (Lilium longiflorum) by about 20% [83,88]. Although copper sulfate-based covers are effective in controlling plant growth and development, they are of limited value in agriculture due to high cost, maintenance complexity and phytotoxicity.

As an alternative, photoselective covers containing other dyes allowed to absorb light at specific wavelengths have been developed. As a rule, researchers do not indicate the composition of these dyes in their works. Therefore, in this review, covers are characterized in terms of their ability to absorb light of a certain wavelength. Such covers have a number of advantages over copper sulfate. First, they selectively reduce the intensity of light in a certain part of the spectrum: red, far-red [43,47,71,80,82,84,86,89,91,92,97], and blue [90,91,92]. Secondly, such covers are easy to use and have a relatively simple manufacturing process. Thirdly, their application allows both controlling the growth and development of ornamental flowering plants and increasing the yield or improving the taste of agricultural crops. For example, under covers containing Rabs dye, which reduced light illumination from 550 nm to 700 nm by 50%, the total fresh weight of cucumber (Cucumis sativus) increased by almost 20% [86]. Photoselective covers, which absorbed the light in the wavelength range from 550 nm to 700 nm by 20%, increased the fresh and dry weight of L. sativa by 10–15% [85]. In addition, under covers which block 10% of far-red radiation, the yield of berries of raspberry (Rubus idaeus) increased by 13% [89]. Causin et al. [90] showed that a change in the spectral composition affected the response of plants to oxidative stress and leaf aging. When common wheat (Triticum aestivum) was placed under coating with blue #075 dye, which increased blue light intensity and decreased red light, the leaves showed an increase in catalase activity, as well as a decrease in the amount of degraded proteins and chlorophyll. Under covers with green #089 and red #026 dyes, which reduced the intensity of blue light, on the contrary, a decrease in the catalase activity and an increase in protein degradation were observed. The authors suggest that blue light, which increases the antioxidant activity, plays a decisive role in reducing plant oxidative stress.

As can be seen from the examples above, photoselective covers do not have a very large impact on the productivity of agricultural plants (less than 20%) that limits their scope. Such covers are most in demand in the field of growing ornamental flowering plants to obtain a more favorable presentation. In the examples of pachistachis yellow (Pachystachys lutea), persian shield (Strobilanthes dyrianus) and kidney tea (Orthosiphon stamineus), it was shown that a 70% reduction in the proportion of far-red radiation, using polyethylene covers with a dye that absorbed in the range of 700–800 nm, led to a decrease in stem length, leaf area and plant weight by 15–20%. Under covers that absorbed 50% of red light, the plants were 10% higher due to the increase in internodes, but the size of the leaves and dry and wet weight of the plants did not change. In all cases, flowering began earlier [47]. Similar results were also obtained for D. grandiflorum [80]. Different species of plants react differently to changes in the light spectrum induced by such covers. Species of salvia differ from each other in their response to growing under a coating that absorbs light in the region of 680 nm to 850 nm [47]. The height of salvia blue (Salvia indigo) decreased by almost 50%, and the number of flowers and the dry weight of the stem decreased by three and two times, respectively. The height of sparkling salvia (Salvia splendens) and mexican salvia (Salvia leucantha) decreased much less, by 20%. At the same time, the number of flowers and the dry weight of the stem, on the contrary, increased by 6% and 20%, respectively. Photoselective covers, which reduced the amount of transmitted blue light, have practically no effect [70]. It was shown that a dye which absorbed light with a wavelength of less than 550 nm had no effect on the development of D. grandiflorum. Only a slight decrease (by 5%) in the concentration of chlorophyll and negligible increase in plant height were observed. More information on the effect of photoselective covers on plant development is provided in Table S1.

In addition to the covers described above, photoselective covers that reduce UV intensity are actively used. Photoselective covers that reduce UV intensity are used both to increase the yield [76] and to reduce the activity and amount of insect pests [72,73,78], and to control fungal [79] and viral pathogens of agricultural crops [77].

It was shown that eggplants (Solanum melongena) grown in a greenhouse, the coating of which did not transmit ultraviolet radiation, showed increased stem and leaves (about 20%), as well as fruit weight (17%), in comparison with plants grown in a greenhouse covered with an common polyethylene film, which has a transmittance of ultraviolet radiation of about five percent [76]. Growing S. lycopersicum under the same covers resulted in a decrease in the amount of incoming UV, which induced a significant increase in the leaf area index of plants, as well as a slight increase in the length of internodes, stem length, and dry weight of plants by 5–10%. In plants growing under UV-blocking films, the damage to tomato leaves by late blight reduced fivefold, and as a result, the yield increased by almost 30% [79]. In addition to increasing yields, the number of fungal diseases in plants grown under UV-blocking covers decreases, and the color of fruits, leaves, and flowers can also change. For example, the chroma parameter of the color of eggplant fruit was significantly higher in plants grown under a coating that did not transmit UV radiation [75]. Cases of blackening of rose petals due to excess UV-B are also known. Coatings that absorb UV prevent the blackening of petals and promote the normal development of rose flowers [98,99]. In contrast, the color of petunia flowers did not change [100]. Due to the different needs of plants for UV, it is worth carefully choosing coatings that block UV for a particular plant.

It has been found that there is a positive correlation between the degree of UV filtration and the level of protection against insects, since insects cannot navigate in a UV-deficient environment. For example, a dramatic decrease in plant disease caused by greenhouse whitefly-borne (Trialeurodes vaporariorum) viruses under UV-absorbing materials may be due to the low activity (movement or feeding) of T. vaporariorum, resulting in slower virus spread [101]. It has been shown that a UV-absorbing coating effectively reduces aphid (Macrosiphum euphorbiae and Acyrthosiphon lactucae) populations and delays the colonization of L. sativa [72]. A decrease in the proportion of plants colonized by aphids and damaged by viral diseases carried by insects was found. These covers are effective in reducing the population density of flower thrips (Frankliniella occidentalis) and the spread of tomato spotted wilt virus, as well as populations of lepidoptera (Autographa gamma), which is a common lettuce pest in Spain. Similar results were obtained with S. lycopersicum in Southeast Spain [78] and with C. sativus in southern Great Britain [73].

However, the use of UV-selective covers has some limitations. On the one hand, a decrease in the population of pollinating insects and disturbance in their color perception, bumblebees in particular, has been observed under such covers [75].

On the other hand, the advantages of ultraviolet radiation are described below. Pre-illumination of L. sativa with ultraviolet (λ = 280–320 nm, up to 12 kJ m⁻² d⁻¹) before infection with a suspension of lettuce downy mildew (Bremia lactucae) led to a significant decrease in pathogen sporulation [102]. In addition, UV radiation (10–20 µmol m⁻² s⁻¹) stimulated an increase in the thickness and increased pigmentation of lettuce leaves, which is an important sign of a high-quality plant for transplanting in the field. According to other data, the addition of UV-A stimulates the accumulation of secondary metabolites [26]. Garcia-Macias et al. [74] demonstrated an increase in the content of beneficial flavonoids in red leaf lettuce growing under a UV-transparent coating.

Photoselective covers may spread in regions with abundant natural light and high daytime temperatures. Figure 4B shows the frequency of use of different plant families in studies that have used photoselective covers. Despite the fact that the greatest research attention was paid to representatives of the nightshade family, which includes important agricultural crops, most of the published works were carried out using ornamental plants (about 70% of cases). Most often, such covers are used as soil-safe growth moderators for agricultural and ornamental plants. The main advantages of such covers are: ease of use, the ability to regulate light intensity in the required spectral range, and a great effect on plants. The disadvantages of such covers are the low efficiency in growing agricultural plants and the inability to be used in temperate latitudes (with the exclusion of UV-blocking covers).

3.2. Transformation of the Sunlight Spectrum Using Photoconversion Covers

The need for artificial lighting is fraught with a significant increase in energy consumption and other costs in comparison to traditional agriculture [103,104,105]. Using photoconversion covers is an alternative way to increase the light intensity in the PAR range [52,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124]. The application of photoconversion covers alone or jointly with artificial lighting can significantly reduce costs and increase yields. Photoconversion covers for greenhouses are produced using various types of luminophores, which convert light that is little-used by plants in photosynthesis, or harmful to plants, into light that is effectively used by plants. As a rule, each photon emitted by luminophores has a lower energy than the one absorbed [125]. A perfect photoconversion cover for greenhouses must have the following features to satisfy the requirements for targeted photoreactions: (1) maximum transmittance of sunlight in critical for the efficiency of photosynthesis spectrum rang; (2) maximum light absorption in other ranges; (3) high quantum yield and minimum reabsorption losses of luminescence; and (4) high chemical stability and photostability. In practice, it is very difficult to produce materials to meet all these requirements; however, photoconversion covers are improving.

Luminophores used in photoconversion covers can be classified into three groups: (i) phosphors based on dyes [48,49,106,108,109,110,111,113,115,116,117,118,119,120,121,124], (ii) luminophores based on metal-containing nanoparticles [52,112,119], from which (iii) nanoparticles based on rare earth metals are distinguished [51,107,120,122]. Currently, dye-based phosphors are the most common (Figure 5), since they are easily integrated into the cover and, in comparison to other types of luminophores, have a low cost. However, covers containing organic fluorophores burn out rapidly and subsequently lose their effectiveness. Rare-earth-metal-based luminophores and their compounds (for example, europium) have increased stability, but a very low quantum yield of luminescence [110,126,127,128,129]. Nanoparticles with plasmonic or exciton emission (cadmium-selenium, zinc-sulfur, etc.) [52,130,131,132] are poorly integrated into polymer matrices and very sensitive to reactive oxygen species formed during the functioning of the luminophore. Figure 6 shows that about 2/3 of the photoconversion covers contain luminophores, which convert absorbed light to red, the use of which is justified in low-light conditions. Plant growth and development are most effectively stimulated by red and, to some extent, blue light [4]. In 15% of the works, blue was the target light, indicating its lower effectiveness in low-light conditions. A relatively large number of studies in which photoconversion covers emitted green light (little used by plants) can be explained by the fact that many phosphors, in addition to the main emission band, have one in the green part of the spectrum. As can be seen in the Figure 6, photoconversion materials used in greenhouses most often absorb the parts of the solar spectrum that are little used by plants in photosynthesis. At the same time, ultraviolet is used in about half of the studies.

3.2.1. UV Conversion to PAR

Obviously, ultraviolet radiation plays an important role in the development of plants [25,54]. However, it is generally accepted that ultraviolet radiation is harmful to plants: UV absorption can result in multiple deleterious effects to plant tissues. The so-called UVA range (315–400 nm) accounts for approximately 95% of ultraviolet radiation reaching the Earth’s surface. Despite the fact that shorter-wavelength ultraviolet can potentially cause severe damage to plants, its effect in natural conditions is minimal due to its low intensity. In greenhouses, it is absorbed by glass and practically does not reach the plants. There are studies on the both negative and positive effects of UVA on plants [133,134,135,136].

Under photoconversion covers, which convert UV to blue or red light, crops grow much faster and yield more in comparison to plants growing under natural light. For example, L. sativa grown under a cover containing phosphors based on yttrium oxosulfide nanoparticles (λex = 270 nm, 330 nm, with full width at half max (FWHM) 130 nm λem = 630 nm with FWHM 50 nm and 710 nm with FWHM 20 nm with a quantum yield of 21–24%) increased plant biomass and photosynthetic efficiency by 40% [23]. At the same time, the overall intensity of light in the PAR region remained practically unchanged. A high positive effect of cover with a quantum yield of about 1% based on yttrium vanadate (λem = 619 nm) on L. sativa productivity was shown [122]. Leaf area, leaf number and fresh weight of plants increased by 50%, 36% and 60%, respectively. Despite 12% shading in the wavelength range from 380 nm to 710 nm, a small increase in red light intensity provided a high effect. Under cellulose acetate cover containing nanoparticles of europium nitrate hexahydrate (λex = 375 nm, λem = 615 nm with FWHM 20 nm), an increase in the number and area of leaves of garden cabbage (Brassica oleracea) by 40% was observed [123]. Biodegradability and the ability to not affect the light intensity in the PAR region are additional advantages of the cellulose acetate cover [137,138]. In tomatoes growing under covers with nanoparticles based on cadmium (λex = 375 nm with FWHM 0.1 eV, λem = 460 nm with FWHM 60 nm) and zinc (λex = 375 nm FWHM, λem = 650 nm with FWHM 80 nm), fruit weight increased by 15–20%. The quantum yield of light conversion by such particles ranges from 17% [112] to 28% [119]. Authors noted that the light intensity in the PAR region under the coating remained unchanged. Moreover, fluoroplate (a base of the coating) prevents the destruction of nanoparticles under the action of reactive oxygen species formed in the light due to its high waterproof effect. In addition to covers with nanoparticles, covers containing dyes had a positive effect on plant growth and development [48,49,106,108,109,110,111,113,115,116,117,118,121,122,124]. For example, yields of dill (Anethum graveolens), S. lycopersicum, L. sativa, and spinach (Spinacia oleracea) grown under KSANTA dye (λex = 340 nm with FWHM 50 nm, λem = 620 nm with FWHM 30 nm) increased by 28%, 39%, 45%, and 53%, respectively [106]. In strawberries (Fragaria ananassa) growing under a cover with blue dye (λex = 365 nm, λem = 450 nm with FWHM 70 nm), the number of fruits increased by 5%, and the weight of fruits increased by 11%. Under the cover with red dye (λex = 365 nm, λem = 620 nm with FWHM 20 nm), the weight of fruits increased by 5% [110]. In both cases, the covers did not affect the light intensity.

3.2.2. Green and Yellow Light Conversion into Red

The spectral region from 500 nm to 600 nm (green-yellow light) is a part of the PAR region. However, a decrease in its intensity does not have a significant effect on plant development. As can be seen from Figure 6 (see also Table S2), photoconversion covers that absorbed green-yellow light and emitted red light had been used in several investigational studies. Nishimura et al. tested photoconversion covers based on a dye (AGS Green) which converts green light to red (λex = 500–560 nm, λem = 580–680 nm with a quantum yield of 8%) [115]. In cucumbers growing under this cover, the yield of fruits and plant wet weight increased by 40%. Covers containing another phosphor (Lumogen*F-Red 300, λex = 450 nm with FWHM 60 nm, 544 nm with FWHM 70 nm and 581 nm with FWHM 70 nm, λem = 616 nm with FWHM 90 nm) stimulated an increase in the mass and number of tomato fruits by 20–25% [116]. A second absorption band in the blue region (450 nm), according to the author’s suggestion, additionally optimized the light spectrum. Photoconversion covers with phosphors based on lanthanum rhodamine dye (λex = 552 nm with FWHM 100 nm, λem = 590 nm with FWHM 100 nm) increased the yields of cotton (Gossypium hirsutum), winter lettuce (Winter lettuce), red cabbage (Brassica purpuraria) and C. annuum by 10%, 30%, 40 and 60%, respectively [113]. In all cases, a slight decrease in light intensity in the range of 450 nm to 600 nm, and an increase in light intensity from 600 nm to 680 nm, were observed. In L. sativa growing under a cover containing the dye Lumogen Red (LR305, λex = 440 nm, 560 nm, λem = 610 nm), the fresh weight of shoots and leaves, as well as their area, increased by almost two times [117]. In addition to the dye, photonic crystals based on SiO₂/TiO₂ were included in this cover. Photonic crystals reflect IR radiation with a wavelength of 1000–1600 nm, which led to a significant decrease in air and soil temperatures.

3.2.3. Up-Conversion Luminophores

All of the luminescent materials described above act as Stokes-shifted emitters, when each emitted photon has a lower energy than the absorbed one. A few luminophores are capable of emitting photons of higher energy than those used for excitation. Such phosphors are known as up-conversion phosphors. It is known that photons from the so-called PAR region make up less than half of all photons reaching the Earth’s surface. At the same time, sunlight contains a large amount of low-energy radiation, which is practically unused by plants. Therefore, the development of up-conversion luminophores, which convert near-infrared radiation (NIR) into visible light, and their practical implementation may provide good perspective. Such luminophores were proposed in the middle of the 20th century [139,140]. To date, a large number of such luminophores have already been developed; for example, the NaYF₄:YbEr luminophore, which converts near-IR (976 nm) to green (534 nm and 549 nm) and red light (654 nm) [141]. Covers containing phosphors based on Yb³⁺ and Tm³⁺ are excited by NIR illumination (980 nm) and luminesce at 474 nm with FWHM 40 nm, 646 nm with FWHM 10 nm and 803 nm with FWHM 50 nm [142]. Such materials can apply to amplify waveguides. Another cover with fluorophore based on rare earth metals (YbErTi₂O₇) also absorbs NIR (980 nm) and emits at 523 nm, 546 nm and 665 nm [143]. The authors assume that their development will be effectively applied in various optical devices. Recently, the applying of coatings with up-conversion phosphors for greenhouse glass has been demonstrated [144,145,146]. Nanoparticles of nominal composition Sr_0.46Ba_0.50Yb_0.02Er_0.02F_2.04 and Sr_0.910Yb_0.075Er_0.015F_2.090 and Sr_0.955Yb_0.020Er_0.025F_2.045, capable of converting NIR radiation to red (λ = 665 nm with FWHM 40 nm) and green (λ = 545 nm with FWHM 30 nm and λ = 525 nm with FWHM 20 nm) light, were used in the research. As a result, in S. lycopersicum grown under low light conditions (70 μmol s⁻¹ m⁻²), the photoconversion cover induced an increase in the number and area of leaves, stem length and chlorophyll content in leaves. It was found that in plants grown under photoconversion covers, disturbances in the functioning of the photosynthetic apparatus (in response to the appearance of an ultraviolet component in growth illumination) were significantly less, and the recovery of normal parameters was much faster (about 20–25%). It was assumed that the positive effect of the photoconversion covers on plant development and photochemical processes occurring in their photosynthetic apparatus can be associated both with an increase in light intensity in the PAR region and with a decrease in the intensity of harmful ultraviolet radiation, as well as with an increase in plant resistance due to light spectrum optimization.

As the analysis of publications showed, photoconversion covers are most often used to increase the yield of agricultural crops (for example, tomato, lettuce, cucumber, cabbage, pepper, etc.) (Figure 7). Less commonly, these covers are used in the cultivation of ornamental plants, in particular, some species of Red Book orchids [108,109]. This is due to the fact that photoconversion covers change the solar spectrum more qualitatively and directionally. The main disadvantage of photoconversion covers is the relative difficulty in the manufacturing of nanoparticles as phosphors that leads to an increase in their cost. At the same time, photoconversion covers can be considered a universal tool for increasing the productivity of crops.

3.3. Transformation of the Sunlight Spectrum into Electricity Using Photoconversion Covers

The main goal of farms is to obtain a high yield at low cost. To reduce dependence on adverse weather conditions, greenhouses are built. The use of greenhouses is one of the most effective ways to increase crop productivity, crop quality and the number of growth cycles per year [147]. Greenhouses are one of the largest food producers in the modern agro-industrial complex [148]. One of the disadvantages of greenhouses is their high electricity demand [149,150]. There are several approaches to integrate renewable energy sources in greenhouses: geothermal [151], wind [152], and solar [153]. The use of photovoltaic cells is considered one of the safest and most environmentally friendly technologies of renewable and sustainable energy [154]. The principle of operation of photovoltaic cells is based on light harvested from a certain wavelength range and its conversion into electrical energy due to the photovoltaic effect [155]. Several types of photovoltaic cells are used in experimental greenhouses: semitransparent amorphous silicon glass (a-Si), semitransparent organic solar cells (ST-OSCs), and crystalline silicone solar cells (CSC) [65].

a-Si glass with photovolvatic cells is one of the shading methods used [156,157,158]. The use of such panels helps to convert electricity, even in excess [155]. Greenhouses with such elements have stable climatic conditions (CO₂ concentration, air temperature) and shading of the PAR zone of about 30–35%. Under such conditions, lettuce and tomato plants produce slightly less yield (10–15%), but according to the authors, these values are not statistically significant [156,157,158]. In this regard, such systems can be successfully used in regions with increased light intensity, where plants will receive enough energy for photosynthesis, growth and development. Typically, to convert solar energy into electrical energy, greenhouses use green light [67] or near-infrared radiation [65,67]. The high performance of the ST-OSCs FTAZ:IT-M, FTAZ:PC71BM, and PTB7-TH:IEICO-4F was shown [67]. Despite some decrease in photosynthetic activity under each cover, lettuce yield did not decrease. At the same time, the conversion of near-infrared light into electricity additionally reduced the temperature in the greenhouse [65,67].

In greenhouses with glasses coated with CSC-type photovoltaic cells, a slight decrease in the productivity of cultivated plants is possible due to a shading of the plants. For example, cucumbers, green beans and wheat grown under CSC ripen later and have fewer fruits with a low average weight compared to plants grown in common greenhouses [159], although Kadowaki et al. observed a slight decrease in onion mass by 5% [160]. In greenhouses fully- or half-covered with CSC, a slight decrease in lettuce yield (less < 10%) was observed [161]. At the same time, the leaves became thinner, but larger than when grown in common greenhouses [162]. Modern researchers put forward two strategies for using such covers for greenhouses [160,161]. The first is the use of numerous densely installed photocells. This strategy is perfect for regions with high sunlight intensity, because shading occurs under the covers that allows both the light intensity and the temperature inside the greenhouse to be normalized. At the same time, a large amount of energy is generated for the needs of the greenhouse. The second strategy is to use an incompact arrangement of photocells on the glasses. This strategy allows the production of a relatively small amount of electricity and avoids significant reduction in the crop yield due to shading. Thus, such a strategy is suitable for regions with a temperate climate.

In the northern latitudes, due to the lack of heat, greenhouses have to be additionally heated, which reduces the profitability of farms. To solve this problem, it was proposed to include photothermal nanoparticles in greenhouse photoconversion covers. Photothermal nanoparticles can be based on noble metals: Au, Ag, Cu and Pt [163], which have intensive bands of localized surface plasmon resonance—a phenomenon caused by the collective oscillation of surface electrons after exposure to incident light. [164]. Photoexcited particles are heated and warm the environment [165]. Gudkov et al. introduced gold nanoparticles with an absorption peak of 530 nm into the photoconversion cover [69]. As a result, the cover heated the greenhouse air. Furthermore, the gold nanoparticles protected the phosphors against photodegradation.

Equipment for the automatic generation and storage of electricity and heat for greenhouses is still under development and/or optimization. The implementation of such systems will stimulate the development of farms in regions that are currently unsuitable for growing agricultural and ornamental flowering crops.

4. Conclusions

The current and prospective methods of light conversion can significantly increase the yield of most cultivated plants. However, the expediency of choosing, and the application of, specific technologies should be determined by climatic and seasonal conditions, as well as the needs of cultivated crops. For example, shading covers can be used in high-insolation conditions for the growing of almost all crops. LEDs can be used everywhere, as a supplementary or primary source of light, with certain settings for a particular culture. Photoselective coatings are best used for ornamental plants, while photoconversion coatings are suitable for both ornamental plants and crops. At the same time, there are no limiting factors for use in different climatic zones. Coatings with PV elements are mainly for climatic zones with high insolation, since such coatings have a very high percentage of shading (up to 30–40%), which helps to protect plants against photoinhibition.

Approaches for light conversion in agriculture are developing very rapidly. According to the international organization IRENA [153], in the future, the area under greenhouses will rapidly increase, and the area occupied by greenhouses, in which photoconversion technologies will be used, will reach 8 million hectares [154].