A Mini-Review of Photoconversion Covers for Greenhouses: Assessment Parameters and Plant Experiment Results

Abstract

Photoconversion covers (PCCs) are specialized materials designed to modify light conditions in greenhouses, thereby enhancing plant growth and development. Recently, extensive research and development efforts have focused on improving and characterizing both components of PCCs: the cover material and the photoconversion agent (phosphor(s)). Given that the true impact of PCCs on plant growth can only be assessed through greenhouse experiments, while surveying recent publications from 2020 to 2024, in this review, we specifically tried to focus on such experiments. A total of 58 studies on PCCs for greenhouse applications were analyzed. Of those, 26 studies introduced novel materials, including phosphors and PCCs, with the potential to enhance plant growth, although greenhouse experiments were not conducted to evaluate their performance. The remaining 32 studies provided experimental data on PCC efficiency in promoting plant growth through plant-based experiments. To summarize and compare the findings from these greenhouse experiments, in this work, we systematically classify plant growth parameters and examine their application across the surveyed studies.

1. Introduction

Photoconversion covers (PCCs) are a class of materials designed to modify light conditions in greenhouses to promote plant growth and development. Solar radiation within the wavelength range of 400 to 700 nm is referred to as the photosynthetically active region (PAR). Light from this region is thought to be utilized by plants for photosynthetic processes. Both the entire PAR spectrum and specific wavelengths within it affect plants in distinct ways, and this also applies to variations in light intensity and duration [1]. If sunlight alone is considered, excluding artificial light sources, it becomes evident that increasing the intensity and duration of lighting is challenging. In the present review, two noteworthy studies address this issue, focusing on the use of PCCs under low-light conditions, such as haze, to enhance the PAR [2] and on the bio-hybrid system incorporating long afterglow particles that maintain emission during shading periods, ensuring smoother light availability [3]. However, in greenhouses that rely on sunlight, the primary means of modifying lighting are spectral adjustments, including shading, selective transmission, and photoconversion. Among these, the latter appears particularly promising for enhancing plant growth.

Thus, the conversion of light from other spectral regions into the PAR range can positively influence plant growth, enhancing development rates, biomass accumulation, and the yield of edible parts. To achieve this, PCCs have been developed and implemented by researchers worldwide.

In general, PCCs consist of two primary components: the covering material and the photo-converting agent. The covering materials, typically either polymeric or glass, serve as greenhouse covers, providing protection and aiding in the formation and control of the internal microclimate. In turn, photo-converting materials, or phosphors, transform sunlight into light that promotes plant growth. Phosphors are defined by their optical properties; they should absorb light from less useful spectral regions and emit it within a range beneficial to plants. There are various conversion schemes, with UV-to-PAR being the most common. Other schemes, such as UV-to-blue, UV-to-red, or green-to-red, etc., are also noted in the reviewed literature.

Another critical characteristic of phosphors is their photostability, which should be carefully evaluated and optimized. Meanwhile, covers should be sufficiently transparent to allow adequate light penetration into the greenhouse. They should also possess suitable mechanical properties, as well as high photo, thermal, and, in some cases, chemical stability. At the same time, the integration of phosphors should not compromise their advantageous properties.

A significant number of reviews on this vital field are published annually. For instance, the optical materials for controlling sunlight in greenhouses (covering aspects such as intensity management, distribution optimization, and color changes) were comprehensively reviewed by Timmermans and colleagues [4]. Liu and co-authors conducted a comparative analysis of various light-converting materials, highlighting the unique advantages associated with each [5]. Shen et al. provided a comprehensive review of spectral conversion materials in agriculture, discussing their working principles, examples, and future perspectives [6]. Yang and colleagues conducted an in-depth review of PCC materials, focusing on strategies for modulating photoluminescence properties [7]. Yu and co-authors classified and analyzed a diverse array of nano-sized photo-converting agents [8]. Wang et al. provided a detailed examination of rare-earth agricultural light conversion films [9].

For a more comprehensive practical assessment of PCC effectiveness, greenhouse experiments involving plant growth are typically required. In such experiments, plants grown under PCCs are analyzed and compared with those grown under non-converting covers (which are generally the preferred control) or without any covering. The photosynthetic response of crops to variations in light quality and intensity was analyzed in detail by Shafiq et al. [10]. More recently, Mustaffa and colleagues systematically reviewed the effects of light changes on plant growth and development [1]. Thus far, PCCs have been found to have different effects on different plant species [1]. Hence, each newly developed PCC should undergo greenhouse experiments involving various crops to confirm its suitability for broader applications. In turn, this raises the question of which assessment parameters should be employed, as researchers often use differing criteria to evaluate the effectiveness of PCCs in promoting plant growth. For this reason, in this review, we attempted to compile and classify these diverse parameters.

The present review aimed to examine recent publications from 2020 to 2024 on newly developed PCCs as well as to evaluate and discuss the methodologies used for their characterization, with an effort to compare the reported results. In total, 58 studies on various PCCs for greenhouse applications were analyzed. Among those, 32 studies provided experimental data on PCC effectiveness in promoting plant growth, while the remaining 26 works focused solely on presenting novel materials (both phosphors and PCCs) with potential applications for enhancing plant growth. Greenhouse experiments to evaluate these materials have yet to be conducted.

2. Photoconversion Cover Materials

PCCs generally consist of two main components: cover material and phosphor(s) (Figure 1). The first plays a role in coating, insulating and protecting the interior of the greenhouse. More commonly, the cover is made of glass [11,12,13] or polymer films [14,15,16,17,18,19]. A phosphor or a group of phosphors with accompanying components provides the photoconversion properties of the PCC by changing the spectral composition of the incident light (from the sun or another source, e.g., LED). In the reviewed articles, the phosphors are presented in two main groups: organic and inorganic substances (Figure 1). Organic dyes have been used for photoconversion for a long time and are still of interest [20,21,22,23,24]. They represent 20% of the non-rare-earth phosphors in Figure 2. The traditional luminescent dyes are well studied and have advantages such as good compatibility with the polymer matrix and tunable emission wavelengths [20]. They can also be modified or combined to achieve better results, as in the work of Sanchez-Lanuza and co-authors [24], who developed PCCs with multiple luminescent dyes. However, organic dyes without additional treatments and modifications have relatively low photostability and degrade quite easily under intense light [5,21]. For this reason, inorganic phosphors have been extensively studied for sunlight photoconversion over the last few decades. The most developed group of such phosphors includes compounds based on or containing rare-earth elements, such as Eu [3,11,14,25], La [12,26,27], Yb [13,28], Y [29], Tb [30], and so on. Among the reviewed articles, about 58% are devoted to such rare-earth-based materials used for PCC production (Figure 2).

The second group of inorganic phosphors described by the authors can be called “non-rare-earth compounds”. These materials can also be divided into several subgroups (Figure 1). The most popular subgroup (48%) is represented by phosphors based on semiconducting materials (more often in the form of quantum dots, QDs), such as sulphides [16,31,32], selenides [33,34,35], etc. In second place are carbon dots (CDs) (24%, Figure 2), which have recently become more popular [15,36,37]. Two other materials, namely corundum [38] and borate glass [39], are both doped with Cr³⁺, which belong to the “Other” group in Figure 2.

Light conversion can generally be performed through two main processes: down-conversion and up-conversion. In the first case, higher-energy light is absorbed and emitted at a lower energy level. Examples include UV-to-PAR, UV-to-red and yellow/green-to-red/orange conversions. In the case of up-conversion, the energy of the emitted radiation is higher than that of the absorbed radiation. The mechanisms of PCC performance have been described in detail in other reviews [5,6,7,8,9]. Therefore, this review will not focus on this topic.

In general, the best PCC materials must meet the following requirements [40]. Firstly, they must maximize the transmission of sunlight in PAR. At the same time, they should have maximum absorption in other parts of the spectrum. Thirdly, they should have maximum quantum yield with minimum luminescence reabsorption losses. Finally, the PCC must be stable, i.e., it must not deteriorate under greenhouse conditions such as intense and prolonged sunlight, temperature (including the difference between indoor and outdoor temperature), moisture and humidity, etc.

2.1. PCC Preparation

In general (Figure 3), the photo-converting component(s) can either be applied as a coating to the covering material (see e.g., [27,41]), where the coating can cover both the glass and the film, or it can be incorporated into the covering material (see e.g., [15,36]), i.e., it can be used as a filler to the polymer film or plate. Both types of PCC materials—filled or coated with the phosphor(s)—have their advantages and disadvantages. To name a few, more frequent filling of the phosphors into the polymer matrix provides better homogeneity of the filler location in the PCC. Coatings make it easier to control the thickness of the phosphor-containing component and, thus, to modify the properties of the PCC. On the other hand, in the long term, coatings can lose their adhesion to the surface of the covering material. However, the incorporation of phosphor(s) into the polymer matrix can lead to a deterioration in its mechanical properties and also a reduction in its stability.

It should be noted that covers are often made using a polymeric binder with which the phosphorus-containing component is mixed [13,25]. This binder provides better adhesion of the cover to the glass surface and increases the stability of the cover, making it useful in a wider range of indoor and outdoor greenhouse conditions.

Another way to increase the stability of coatings is to make a “sandwich” structure (Figure 3c), where the coating is additionally coated with another glass or directly applied between two glass sheets [11,12,26]. However, it may also have a disadvantage in the form of reduced light intensity due to the double glass layer.

Thus, the choice of PCC formation is a question related to the final purpose of the PCC application and the conditions of its use. Both coatings and fillers have their applications.

2.2. PCC Characterization Results

In general, we have divided all the papers analyzed into two groups. The first group included papers that did not include greenhouse experiments. Authors in this group developed new phosphors for PCCs, improved PCC photoperformance, stability, etc., but did not carry out experiments with plants. The second group is represented by the papers in which the authors performed greenhouse experiments and evaluated the influence of PCCs on plant growth. In this section, the papers from the first group will be discussed; the second group will be considered in the following sections.

The details of the PCC materials or components, their preparation, photochemical properties and other results reported by the authors of the first group are summarized in Table 1.

Yan and colleagues [42] obtained photoconversion phosphors in form of core–shell-structures with the composition of CaS:Eu²⁺,Cu⁺@CaZnOS:Cu⁺ via high-temperature solid-state reaction. The obtained phosphor converted UV and green light into blue-red light. The authors optimized the composition of the phosphor, changing copper content in order to achieve an effect of multicolor emission. Another Eu-based rare-earth, also in the form of core–shell particles, was prepared by Wang et al. [43]. They used a two-step solid-state reaction to obtain CaS:Eu²⁺@CaZnOS:Mn²⁺. The phosphor had two excitation maxima (at 295 and 530 nm) with emission at 615 nm. Moreover, this phosphor was all-weather stable, including stability against heat, moisture, and UV radiation. Thus, this material can be potentially used to produce long-life agricultural films. Dong and co-authors [3] suggested a new biohybrid system composed of long afterglow particles (LAP) Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ (commercial LAP modified with amino groups) and natural thylakoid membrane (TM) extracted from chloroplasts obtained from spinach. LAP and TM were co-assembled, resulting in UV-to-blue photo-converting material. Under irradiation, LAP converts light, which matches the absorption of TM. Under the dark condition, charged LAP continues emission, which facilitates photosynthesis of TM and thus smoothing shading periods. The authors concluded that such a system can be added to greenhouse covers for the sake of light management, namely for the increase in the lighting periods’ duration.

Phosphors with the composition of Al/Y-doped Na₄CaSi₃O₉:Bi³⁺,Eu³⁺ was developed by You et al. [44]. Irregular particles with sizes in the range of 5–20 µm were produced by high-temperature solid-phase synthesis. The phosphor exhibited broad absorption bands (at 200–400 nm) and red-blue double emission.

Another Al-containing inorganic phosphor doped with Mn was suggested by Lv and colleagues [45]. They prepared SrZnAl₁₀O₁₇:Mn⁴⁺ also via high-temperature solid phase. It possessed conversion of blue and green-yellow light into a far-red light, having emission in the range of 600–800 nm with the peak at 692 nm. The authors concluded that the phosphor had potential as a light converter for plant cultivation.

A luminophore with the composition of Sr₂In_0.5Sb_0.5O₄:Mn⁴⁺ was obtained by Shan et al. [46], and the optimal doping content of Mn was found. The phosphor emits at 695 nm under excitation at 330 and 520 nm. Wang and colleagues used doping by Mn as well [29]. They inserted Mn into Ba₃Y₂WO₉ during the high-temperature solid-state reaction. The luminophore also has emission at 692 nm under excitation at both 367 and 535 nm.

Another inorganic non-rare-earth phosphor was suggested by Babkina et al. [39], chromium-doped borate–glass ceramics with green light conversion to 650–800 nm wavelength. The authors varied annealing temperature during synthesis and found out that its increase led to QY increasing, luminescence shifting to red region, increasing hardness of the material and its chemical stability.

Several fluorescent organic dyes were studied by Ramanna et al. [22]. They thoroughly investigated the organic dye solutions and optimized the concentrations and solvents used. They showed that using some dyes was promising for enhancing PAR, and can be potentially used for the improvement of micro-algae photosynthesis, growth, and metabolite production.

Among the analyzed works, three examples of luminescent film laminated glasses (LFLGs, or LLGs, or LFLG) were found. All of them developed “sandwich”-like structures consisting of two glass sheets and luminescent film between them. Phosphors, namely, Eu³⁺, Mn⁴⁺ co-doped LaAl_0.7Ga_0.3O₃ [11], Ca_2−2xNa_xLa_xMgWO₆:Mn [26], and Tb_0.7La_0.3ZnAl₁₁O₁₉:Cr³⁺ (TLZA:Cr³⁺) [12], were obtained via traditional high-temperature solid-state reaction and added to UV-photosensitive liquid resin. Then, the space between the two glasses was filled with this modified resin. The obtained PCCs exhibited UV-to-red/far-red [11], 250–600 nm light conversion to 650−800 nm [26], and near UV and green-to-far-red [12] photo-activity. The authors concluded that the application of such glass panels can turn the solar spectrum to the optimal action spectrum for plant growth and they can be used in the construction of outdoor horticultural facilities.

Eu-containing phosphors were most frequently reported for the preparation of PCCs in the form of films. Wang et al. [9] obtained Sr₂Si₅N₈:Eu²⁺ via spray pyrolysis and introduced it into the low-density polyethylene (LDPE) matrix. Blue/violet-to-red/orange light conversion of the obtained PCCs indicated their potential usability for the optimization of light distribution for plant growth. Qiao and colleagues [30] intercalated N-Methyl imidazole and Eu³⁺/Tb³⁺ salts into kaolinite, and embedded the phosphors prepared into polyvinylidene fluoride (PVDF) via the slurry casting method. The films obtained were found to be good candidates for PCCs with ultraviolet-to-red conversion for plant growth. Liu and co-authors [47], Wang and colleagues [48], and Zou et al. [49], prepared Eu-based complexes with different ligands, such as phenanthroline [47,48], benzoic acid (HBen), ancinnamic acid (HCin) [49], etc. Complexes obtained were added to the polymers, such as poly lactic acid (PLA) [47], low-density polyethylene (LLDPE) [48], and polymethyl methacrylate (PMMA) [49]. The authors revealed the enhancement of the polymer stability under UV [47], wettability [48], and photothermal stability [49]. Also, all the PCCs exhibited UV-to-red conversion, and can be potentially applied to the greenhouse covering.

Among non-rare-earth phosphors, Hu and co-authors [18] suggested BaSO₄@SiDs (silicon nanodots) for obtaining a dual-functional film. The addition of phosphor to PE led to forming PCCs (UV-to-blue), in which also BaSO4@SiDs prevented influence of UV radiation on the polymer, making it more photo-stable. Kwamman et al. [50] and Wu and colleagues [36] suggested carbon dot (CD)-based PCCs. Kwamman and colleagues [50] obtained CDs from water hyacinth stalks (biomass-derived CDs) and put them into poly(vinyl)alcohol (PVA). Obtained film converted UV-to-visible and blue-to-red light. Wu et al. [36] prepared nitrogen-doped carbon dots (RH-CDs) via the solvothermal technique and inserted them into PMMA film. Green-to-red spectral conversion performance for such PCCs had 23% QY. Barman and colleagues [51] also used N-doped carbon dots (N-CDs) for transparent polymer film preparation. CDs were embedded in a PVA matrix via solution mixing. The resulting PCC exhibited UV-A shielding and blue light emission. At excitation under 360–370 nm, QY was 91%, and visible light transparency was 90%.

Organic phosphors were reported to prepare PCC films as well. Hu and colleagues [20] and Shi and co-authors [23] suggested perylene imide derivatives (PDI or PDIE) embedded into PVA [23], polyvinyl chloride (PVC), polybutylene adipate-polybutylece terephthalate copolymer (PBAT), and ethylene-vinyl alcohol co-polymers (EVA) [20] as PCCs. In the work by Shi [23], light conversion from 380–600 nm to 600–780 nm was achieved by the film, along with high quantum yield, excellent thermal stability, excellent photostability, and high transmittance. Hu and colleagues [20] found that 1% PBAT film with PDI exhibited a 65% increase in photosynthetic photon flux density (PFD) at 600–700 nm. Also, they showed that films based on EVA exhibited enhanced photostability under intensified UV.

A film with shielding and photo-converting properties on the base of organics was developed by Liu and colleagues [21]. The obtained microspheres of 4,4′-bis[(4-anilino-6-hydroxyethylamino-1,3,5-triazin-2-yl) amino] stilbene-2,2′-disulphonate (VBL) onto poly (maleic anhydride-co-α-methyl styrene) (PMAS) with a spherical shape and average dry diameter of 0.954 ± 0.090 µm and incorporated them into polyvinyl alcohol (PVA) films. The presence of PMAS-VBL in films improved UV-shielding and exhibited excellent UV-to-blue light conversion at high optical transparency and photo-stability in the whole UV region.

Luminescent coatings were reported by Cho et al. [25], and Yalcin and colleagues [52]. Cho et al. [25] suggested cheap SiO₂ particles doped with Eu²⁺ and Al³⁺. They prepared SiO₂:Al_0.04Eu_0.01, SiO₂:Al_0.12Eu_0.03, and SiO₂:Al_0.20Eu_0.05 via the base-catalyzed sonochemical sol–gel method. The resulting coating of 100 μm contained 8w.% of phosphor, dispersing agent, defoamer, acrylic resin, and thickeners. It exhibited UV-to-PAR conversion efficiency of 26.2%, and photoluminescent quantum yield of 71.5%. Yalcin and colleagues [52] reported a coating made of K₂SiF₆:Mn⁴⁺ doped BK7 glass with the conversion of blue-green light to red (300–520 nm → 600–650 nm). Actually, they did not conduct experiments; instead, they carried out a greenhouse experiment modeling. They concluded that the introduction of such fluorescent reflectors could result in an increase in crop production of over 35%.

Thus, the works studied were mainly directed towards the production of new phosphors, most of which are based on rare-earth metals. The appearance and development of carbon dots seems to be a promising field in the area of new phosphors for PCCs. New phosphors themselves and new PCCs based on polymer films were most frequently reported by the authors. However, both coatings and “sandwich”-like glass constructions have also been developed and proposed.

To summarize the photoconversion properties of the materials for PCCs (to obtain a more complete picture, we considered Table 1 and the data from the Section 4), we analyzed the reported data and revealed the following points. Rare-earth-based phosphors and their corresponding PCCs emit mainly in the red and far-red region (83% of the reported data), and 17% showed emission in the PAR. The majority of developed rare-earth phosphors (67%) have excitation maxima in the UV region.

Non-rare-earth phosphors and their PCCs also emit more red light (83%), but their excitation can be by UV (50%) as well as by other light (45%, blue, green, yellow, see Table 1). CDs mostly shield UV light and convert light in the visible range (blue to red [50], green to red [36]). However, they can also absorb in the UV range (Table 3). Eight organic phosphors developed in the reviewed works have different photoconversion properties. They can both enhance PAR (50%) and emit red light with excitation in the visible (38%). UV to PAR conversion is also possible [21].

Table 1. Details and results of the researchers obtained PCCs and their components without carrying out the greenhouse experiments.

Article	Material/ Device	Phosphor	Synthesis	Photo Conversion Properties	Other Results
Phosphors
[42]	Photoconversion phosphors	Core–shell- structured CaS:Eu2+,Cu+@ CaZnOS:Cu+	High-temperature solid-state reaction synthesis method.	UV/green → blue/red	Content of Cu optimized to enhance luminescence and achieve multicolor emission.
[43]	Phosphor	CaS:Eu2+@CaZnOS:Mn2+ Core-shell spherical agglomerated particles	Two-step solid-state reaction method.	295 nm → 615 nm 530 nm → 615 nm Blue-green → red (650 nm)	Phosphors exhibit all-weather stability, namely, against heat, moisture and UV radiation. UV absorption increased and red emission extended are desirable for long-life agricultural films.
[3]	Biohybrid system	Long afterglow particle (LAP) Sr2MgSi2O7:Eu2+,Dy3+ Natural thylakoid membrane (TM)	Commercial LAP modified by surfacial amino groups. TM extracted from chloroplasts obtained from spinach. Co-assembly of LAP and TM carried out.	UV → blue	Under irradiation LAP light conversion matches the absorption of TM. Under dark, charged LAP emission facilitates photosynthesis of TM thus smoothing shading periods. This system can be added to greenhouse covers for light managing.
[44]	Phosphors	Al/Y-doped Na4CaSi3O9:Bi3+,Eu3+ Irregular particles with the main sizes concentrated in the range of 5–20 µm	High-temperature solid-phase synthesis	Red-blue double emission	Broad absorption bands in the range of 200–400 nm. Under 291 nm excitation increased red emission at 613 nm.
[45]	Phosphor for agricultural cultivation	SrZnAl10O17:Mn4+	Obtained via high-temperature solid-state reaction.	Blue, green-yellow → far-red	Emission in 600–800 nm range, peak at 692 nm. Good match between PL spectra of phosphor and PFR absorption spectra. Phosphor had potential as light converter for plant cultivation.
[46]	Luminophore	Sr2In0.5Sb0.5O4:Mn4+ Irregularly shaped particles with a particle size of 1–2 μm.	Obtained via high-temperature solid-state reaction.	330 nm → 695 nm 520 nm → 695 nm	Optimal doping concentration of Mn4+ was found to be of 0.003.
[29]	Luminophore	Ba3Y2WO9:Mn4+	High-temperature solid-state reaction method.	367 nm → 692 nm 535 nm → 692 nm	Optimal Mn concentration was found to be of 0.3 mol.%.
[39]	Borate glass–ceramics	Chromium-doped borate glass- ceramics	Obtained via bulk crystallization of the borate glass matrix.	Green → 650–800 nm	Annealing temperature during synthesis of ceramics affected its properties. Increasing of temperature resulted in: Luminescence QY increase from 13 to 50%. Luminescence shifted to the red region. Increased hardness and resistance to acids.
[22]	Fluorescent dyes	Diphenylanthracene [DPA], Diphenyloxazole [DPO], Rhodamine 6G [R6G], Rhodamine 8G [R8G], Rhodamine 800 [R800], Fluorescein Isothiocyanate [FITC], Lumogen Yellow [LY], Lumogen Red [LR]	Commercial	PAR enhancement	Thorough study of organic dyes solutions, concentration and solvent optimization. Promise was shown in enhancing of PAR, potential improvement of micro-algae photosynthesis, growth, and metabolite production.
Glass
[11]	Luminescent film laminated glasses (LFLGs)	Eu3+ single-doped LaAl1−yGayO3 solid solution (LAG:Eu3+) Eu3+, Mn4+ codoped LaAl0.7Ga0.3O3 phosphors (LAG:Eu,Mn)	Obtained via traditional high-temperature solid-state reaction from La2O3, Al(OH)3, Ga2O3, Eu2O3, and MnCO3 mixed in mortar and calcined at 1500 °C for 5 h. Phosphor added to UV- photosensitive liquid resin, and inter space between two glass sheets was filled with it.	UV → red/far-red	Optimal LaAl0.7Ga0.3O3:Eu3+ phosphor exhibited excellent luminescent properties having broadband UV-excitation and narrow red emission spectrum. Emission of phosphor in good agreement with absorption of chlorophyll B of phytochrome Pfr.
[26]	Luminous laminated glasses (LLGs)	Ca2−2xNaxLaxMgWO6:Mn	Phosphor obtained via traditional high-temperature solid-phase reaction. Phosphor added to UV- photosensitive liquid resin, and inter space between two glass sheets was filled with it.	250–600 nm → 650−800 nm	Phosphor demonstrated broad excitation band at 250–600 nm (centers at 350, 406, and 490 nm) with emission at 650−800 nm.
[12]	Luminescent film laminated glass (LFLG)	Tb0.7La0.3ZnAl11O19:Cr3+ (TLZA:Cr3+)	Phosphor obtained via high- temperature solid state synthesis. Phosphor added to UV-photosensitive liquid resin, and inter space between two glass sheets was filled with it.	Near UV, green → far-red	Near UV → far-red had internal quantum efficiencies of 74.51%. Green → far-red had internal quantum efficiencies of 76.84%. The authors conclude that application of such LFLGs can turn the solar spectrum to the optimal action spectrum for plant growth and they can be used in the construction of outdoor horticultural facilities.
Films
[53]	Light conversion film	Sr2Si5N8:Eu2+	NPs of phosphor obtained via spray pyrolysis (SP) and solid-state synthesis (SSS). Phosphor combined with low-density polyethylene (LDPE) particles and silicone light diffuser, and processed in a twin-screw extruder. Films fabricated by blowing machine.	Blue-violet → red-orange	SP NPs had luminescence increased by 25% compared SSS NPs. SP NPs exhibited high light conversion capabilities, they can be used for optimization of light distribution for plant growth.
[30]	Composite conversion films	Kaolinite intercalated with N-Methyl imidazole and Eu3+/Tb3+ salts	Obtained via facile stepwise of intercalation strategy. Embedded into polyvinylidene fluoride (PVDF) via slurry casting method.	UV → red	Good candidates for PCCs with ultraviolet-to-red conversion for plant growth.
[47]	Light conversion film	Eu(IAA)2(phen)2 indoleacetic acid (IAA), 1,10-phenanthroline (phen) EuIP	Phosphor synthesized via solution-based chemical method. Phosphor mixed and melt blended with poly lactic acid (PLA).	UV →red	Light conversion properties. Excellent optitcal transparency Absorbing UV by phosphor prevented degradation of PLA. Composite films exhibited antibacterial properties. Such PLA can be used in agricultural films. Phosphor is stable up to 299 °C, so it can be added to melt blend with other polymer materials (PBAT, PE, PP, etc.).
[48]	Fluorescent film	Eu(GI)3Phen Glycerin monostearic acid itaconic acid diester (GI) Phenanthroline (Phen) as co-ligand	Eu complex obtained via chemical synthesis. Films prepared melt mixing with linear l-w-density p-lyethylene (LLDPE) and formed by co-rotating twin-screw extruder.	275 nm → 617 nm	Eu complex introduction led to light converting properties appearance and wettability improvement of PE.
[49]	Photoconversion films	Eu(III) complexes with ligands: 2-pyridine-acrylic acid (H2-PA) 2-pyridine-carboxylic acid (HPic) benzoic acid (HBen) ancinnamic acid (HCin) and binuclear europium-lanthanum complex with HCin	Eu-complexes obtained via chemical synthesis. They mixed with PMMA in solution, and films prepared. Or they were added to PE and films prepared by multilayer co-extrusion film blowing machine.	UV → red	Photothermal stability of the PCC decreased in row La-Eu-Cin > Eu-Cin > Eu-Pic > Eu-Ben >Eu-2-PA.
[18]	Dual-functional film	BaSO4@SiDs (silicon nanodots) Regular granular shape with average diameter about 45 nm	Phosphor prepared by co-precipitation method and added to PE.	UV → blue	UV absorbers BaSO4@SiDs prevented influence of UV radiation onto polymer making it more photo-stable.
[50]	Fluoropolymer films	Carbon dots (CDs)	CDs obtained from water hyacinth stalks (biomass-derived CDs). CDs introduced into poly(vinyl)alcohol (PVA), composite films were obtained via mixing solution and drying.	UV → visible Blue → red	High visible light transmittance (400 nm to 700 nm, 91%), UV-blocking UV region (200 nm to 400 nm, 9%), and increased blue-tored spectral composition by 21.5%.
[36]	Light conversion film	Hydrophobic nitrogen-doped carbon dots (RH-CDs)	CDs obtained via solvothermal method. Mixed with PMMA, films formed at glass.	Green → red	RH-CDs exhibited good compatibility with resins. Green-to-red spectral conversion performance had 23% QY. Emission peak at 594 nm with shoulder at 630 nm (QY 34%).
[51]	Transparent polymer film	N-doped carbon dots (N-CDs)	Phosphor obtained via hydrothermal treatment of citric acid, ethylene diamine, and HCl solution. N-CDs embedded in a PVA matrix via solution mixing.	UV-A shielding and blue light emission	At exitation under 360–370 nm, QY was 91%. UV absorbed with visible light transparency of 90%.
[20]	Light conversion film	Three perylene imide derivatives ((PDI).	PDI were chemically synthesized. Films were prepared on the basis of: Polyvinyl chloride (PVC) Polybutylene adipate-polybutylece terephthalate copolymer (PBAT) 25% and 40% ethylene-vinyl alcohol co-polymers (EVA) Polymer and PDI dissolved in THF (1:100 w. for 1% films, and 1:10 w. for 10% films) and films were obtained.	Yellow-green light conversion properties	Photosynthetic photon flux density (PFD) increased by 65% at 600–700 nm for 1% film with PBAT. Films based on EVA (25 and 40%) exhibited enhanced photostability under intensified UV.
[23]	Light-conversion film	Perylene imide derivative (PDIE)	PDIE obtained via chemical synthesis. Transesterification of PDIE and polyvinyl alcohol (PVA) to prepare PVA-PDIE film.	380–600 nm → 600–780 nm Emission maxima at 620 nm	PVA-PDIE film showed: High quantum yield (0.75). Excellent thermal stability. Excellent photostability. High transmittance. Photosynthetic photon flux density (PFD) for: Red-orange light increased by 25%. Near-IR light increased by 34%. UV light decreased by 39%. Blue-violet light decreased by 27%. Yellow-green light decreased by 24%.
[21]	Shielding and photo-converting films	4,4′ -bis[(4-anilino-6-hydroxyethylamino-1,3,5-triazin-2-yl) amino] stilbene-2,2′-disulphonate (VBL) onto poly (maleic anhydride-co-α-methyl styrene) (PMAS) PMAS-VBL microspheres with spherical shape and average dry diameter of 0.954 ± 0.090 µm	PMAS microspheres prepared via typical precipitation polymerization followed by esterification with VBL. They incorporated into polyvinyl alcohol (PVA) films.	UV-shielding and UV-to-visible light conversion	Presence of PMAS-VBL in films improved UV-shielding, exhibited excellent UV-to-blue light conversion. At the same time the films are characterized with high optical transparency, and photo-stability in the whole UV region. PVA with 7 wt% PMAS-VBL shielded almost 100% UV light having absolute quantum yield of 74.26% and transmittance in visible range of 66.9%.
Coatings
[25]	Luminescent coating	Cheap SiO2 particles doped with Eu2+ and Al3+ SiO2:Al0.04Eu0.01, SiO2:Al0.12Eu0.03, and SiO2:Al0.20Eu0.05	Phosphor synthesized via base-catalyzed sonochemical sol–gel method from aluminum isopropoxide Al(O-i-Pr)3 and Eu(NO3)3 in the presence of NH4OH. Coating (100 μm thickness) contained phosphor (8w.%), dispersing agent, defoamer, acrylic resin, thickeners.	UV-to-PAR	Optimized SiO2:Al0.12Eu0.03 obtained one-tenth of the maximum attainable PAR enhancement Photoluminescent quantum yield of 71.5%. UV-to-PAR conversion efficiency of 26.2%. 0.34% PAR enhancement.
[52]	Fluorescence coatings	K2SiF6:Mn4+ doped BK7 glass	Surface of doped BK7 glass s coated with 100 nm MgF2 antireflection coating for the sake of specular reflection reduction.	300–520 nm → 600–650 nm blue-green light to red	Greenhouse experiment modeling. Concluded that introduction of fluorescent reflectors could result in increase in crop production over 35%

Table 1. Details and results of the researchers obtained PCCs and their components without carrying out the greenhouse experiments.

Article	Material/ Device	Phosphor	Synthesis	Photo Conversion Properties	Other Results
Phosphors
[42]	Photoconversion phosphors	Core–shell- structured CaS:Eu2+,Cu+@ CaZnOS:Cu+	High-temperature solid-state reaction synthesis method.	UV/green → blue/red	Content of Cu optimized to enhance luminescence and achieve multicolor emission.
[43]	Phosphor	CaS:Eu2+@CaZnOS:Mn2+ Core-shell spherical agglomerated particles	Two-step solid-state reaction method.	295 nm → 615 nm 530 nm → 615 nm Blue-green → red (650 nm)	Phosphors exhibit all-weather stability, namely, against heat, moisture and UV radiation. UV absorption increased and red emission extended are desirable for long-life agricultural films.
[3]	Biohybrid system	Long afterglow particle (LAP) Sr2MgSi2O7:Eu2+,Dy3+ Natural thylakoid membrane (TM)	Commercial LAP modified by surfacial amino groups. TM extracted from chloroplasts obtained from spinach. Co-assembly of LAP and TM carried out.	UV → blue	Under irradiation LAP light conversion matches the absorption of TM. Under dark, charged LAP emission facilitates photosynthesis of TM thus smoothing shading periods. This system can be added to greenhouse covers for light managing.
[44]	Phosphors	Al/Y-doped Na4CaSi3O9:Bi3+,Eu3+ Irregular particles with the main sizes concentrated in the range of 5–20 µm	High-temperature solid-phase synthesis	Red-blue double emission	Broad absorption bands in the range of 200–400 nm. Under 291 nm excitation increased red emission at 613 nm.
[45]	Phosphor for agricultural cultivation	SrZnAl10O17:Mn4+	Obtained via high-temperature solid-state reaction.	Blue, green-yellow → far-red	Emission in 600–800 nm range, peak at 692 nm. Good match between PL spectra of phosphor and PFR absorption spectra. Phosphor had potential as light converter for plant cultivation.
[46]	Luminophore	Sr2In0.5Sb0.5O4:Mn4+ Irregularly shaped particles with a particle size of 1–2 μm.	Obtained via high-temperature solid-state reaction.	330 nm → 695 nm 520 nm → 695 nm	Optimal doping concentration of Mn4+ was found to be of 0.003.
[29]	Luminophore	Ba3Y2WO9:Mn4+	High-temperature solid-state reaction method.	367 nm → 692 nm 535 nm → 692 nm	Optimal Mn concentration was found to be of 0.3 mol.%.
[39]	Borate glass–ceramics	Chromium-doped borate glass- ceramics	Obtained via bulk crystallization of the borate glass matrix.	Green → 650–800 nm	Annealing temperature during synthesis of ceramics affected its properties. Increasing of temperature resulted in: Luminescence QY increase from 13 to 50%. Luminescence shifted to the red region. Increased hardness and resistance to acids.
[22]	Fluorescent dyes	Diphenylanthracene [DPA], Diphenyloxazole [DPO], Rhodamine 6G [R6G], Rhodamine 8G [R8G], Rhodamine 800 [R800], Fluorescein Isothiocyanate [FITC], Lumogen Yellow [LY], Lumogen Red [LR]	Commercial	PAR enhancement	Thorough study of organic dyes solutions, concentration and solvent optimization. Promise was shown in enhancing of PAR, potential improvement of micro-algae photosynthesis, growth, and metabolite production.
Glass
[11]	Luminescent film laminated glasses (LFLGs)	Eu3+ single-doped LaAl1−yGayO3 solid solution (LAG:Eu3+) Eu3+, Mn4+ codoped LaAl0.7Ga0.3O3 phosphors (LAG:Eu,Mn)	Obtained via traditional high-temperature solid-state reaction from La2O3, Al(OH)3, Ga2O3, Eu2O3, and MnCO3 mixed in mortar and calcined at 1500 °C for 5 h. Phosphor added to UV- photosensitive liquid resin, and inter space between two glass sheets was filled with it.	UV → red/far-red	Optimal LaAl0.7Ga0.3O3:Eu3+ phosphor exhibited excellent luminescent properties having broadband UV-excitation and narrow red emission spectrum. Emission of phosphor in good agreement with absorption of chlorophyll B of phytochrome Pfr.
[26]	Luminous laminated glasses (LLGs)	Ca2−2xNaxLaxMgWO6:Mn	Phosphor obtained via traditional high-temperature solid-phase reaction. Phosphor added to UV- photosensitive liquid resin, and inter space between two glass sheets was filled with it.	250–600 nm → 650−800 nm	Phosphor demonstrated broad excitation band at 250–600 nm (centers at 350, 406, and 490 nm) with emission at 650−800 nm.
[12]	Luminescent film laminated glass (LFLG)	Tb0.7La0.3ZnAl11O19:Cr3+ (TLZA:Cr3+)	Phosphor obtained via high- temperature solid state synthesis. Phosphor added to UV-photosensitive liquid resin, and inter space between two glass sheets was filled with it.	Near UV, green → far-red	Near UV → far-red had internal quantum efficiencies of 74.51%. Green → far-red had internal quantum efficiencies of 76.84%. The authors conclude that application of such LFLGs can turn the solar spectrum to the optimal action spectrum for plant growth and they can be used in the construction of outdoor horticultural facilities.
Films
[53]	Light conversion film	Sr2Si5N8:Eu2+	NPs of phosphor obtained via spray pyrolysis (SP) and solid-state synthesis (SSS). Phosphor combined with low-density polyethylene (LDPE) particles and silicone light diffuser, and processed in a twin-screw extruder. Films fabricated by blowing machine.	Blue-violet → red-orange	SP NPs had luminescence increased by 25% compared SSS NPs. SP NPs exhibited high light conversion capabilities, they can be used for optimization of light distribution for plant growth.
[30]	Composite conversion films	Kaolinite intercalated with N-Methyl imidazole and Eu3+/Tb3+ salts	Obtained via facile stepwise of intercalation strategy. Embedded into polyvinylidene fluoride (PVDF) via slurry casting method.	UV → red	Good candidates for PCCs with ultraviolet-to-red conversion for plant growth.
[47]	Light conversion film	Eu(IAA)2(phen)2 indoleacetic acid (IAA), 1,10-phenanthroline (phen) EuIP	Phosphor synthesized via solution-based chemical method. Phosphor mixed and melt blended with poly lactic acid (PLA).	UV →red	Light conversion properties. Excellent optitcal transparency Absorbing UV by phosphor prevented degradation of PLA. Composite films exhibited antibacterial properties. Such PLA can be used in agricultural films. Phosphor is stable up to 299 °C, so it can be added to melt blend with other polymer materials (PBAT, PE, PP, etc.).
[48]	Fluorescent film	Eu(GI)3Phen Glycerin monostearic acid itaconic acid diester (GI) Phenanthroline (Phen) as co-ligand	Eu complex obtained via chemical synthesis. Films prepared melt mixing with linear l-w-density p-lyethylene (LLDPE) and formed by co-rotating twin-screw extruder.	275 nm → 617 nm	Eu complex introduction led to light converting properties appearance and wettability improvement of PE.
[49]	Photoconversion films	Eu(III) complexes with ligands: 2-pyridine-acrylic acid (H2-PA) 2-pyridine-carboxylic acid (HPic) benzoic acid (HBen) ancinnamic acid (HCin) and binuclear europium-lanthanum complex with HCin	Eu-complexes obtained via chemical synthesis. They mixed with PMMA in solution, and films prepared. Or they were added to PE and films prepared by multilayer co-extrusion film blowing machine.	UV → red	Photothermal stability of the PCC decreased in row La-Eu-Cin > Eu-Cin > Eu-Pic > Eu-Ben >Eu-2-PA.
[18]	Dual-functional film	BaSO4@SiDs (silicon nanodots) Regular granular shape with average diameter about 45 nm	Phosphor prepared by co-precipitation method and added to PE.	UV → blue	UV absorbers BaSO4@SiDs prevented influence of UV radiation onto polymer making it more photo-stable.
[50]	Fluoropolymer films	Carbon dots (CDs)	CDs obtained from water hyacinth stalks (biomass-derived CDs). CDs introduced into poly(vinyl)alcohol (PVA), composite films were obtained via mixing solution and drying.	UV → visible Blue → red	High visible light transmittance (400 nm to 700 nm, 91%), UV-blocking UV region (200 nm to 400 nm, 9%), and increased blue-tored spectral composition by 21.5%.
[36]	Light conversion film	Hydrophobic nitrogen-doped carbon dots (RH-CDs)	CDs obtained via solvothermal method. Mixed with PMMA, films formed at glass.	Green → red	RH-CDs exhibited good compatibility with resins. Green-to-red spectral conversion performance had 23% QY. Emission peak at 594 nm with shoulder at 630 nm (QY 34%).
[51]	Transparent polymer film	N-doped carbon dots (N-CDs)	Phosphor obtained via hydrothermal treatment of citric acid, ethylene diamine, and HCl solution. N-CDs embedded in a PVA matrix via solution mixing.	UV-A shielding and blue light emission	At exitation under 360–370 nm, QY was 91%. UV absorbed with visible light transparency of 90%.
[20]	Light conversion film	Three perylene imide derivatives ((PDI).	PDI were chemically synthesized. Films were prepared on the basis of: Polyvinyl chloride (PVC) Polybutylene adipate-polybutylece terephthalate copolymer (PBAT) 25% and 40% ethylene-vinyl alcohol co-polymers (EVA) Polymer and PDI dissolved in THF (1:100 w. for 1% films, and 1:10 w. for 10% films) and films were obtained.	Yellow-green light conversion properties	Photosynthetic photon flux density (PFD) increased by 65% at 600–700 nm for 1% film with PBAT. Films based on EVA (25 and 40%) exhibited enhanced photostability under intensified UV.
[23]	Light-conversion film	Perylene imide derivative (PDIE)	PDIE obtained via chemical synthesis. Transesterification of PDIE and polyvinyl alcohol (PVA) to prepare PVA-PDIE film.	380–600 nm → 600–780 nm Emission maxima at 620 nm	PVA-PDIE film showed: High quantum yield (0.75). Excellent thermal stability. Excellent photostability. High transmittance. Photosynthetic photon flux density (PFD) for: Red-orange light increased by 25%. Near-IR light increased by 34%. UV light decreased by 39%. Blue-violet light decreased by 27%. Yellow-green light decreased by 24%.
[21]	Shielding and photo-converting films	4,4′ -bis[(4-anilino-6-hydroxyethylamino-1,3,5-triazin-2-yl) amino] stilbene-2,2′-disulphonate (VBL) onto poly (maleic anhydride-co-α-methyl styrene) (PMAS) PMAS-VBL microspheres with spherical shape and average dry diameter of 0.954 ± 0.090 µm	PMAS microspheres prepared via typical precipitation polymerization followed by esterification with VBL. They incorporated into polyvinyl alcohol (PVA) films.	UV-shielding and UV-to-visible light conversion	Presence of PMAS-VBL in films improved UV-shielding, exhibited excellent UV-to-blue light conversion. At the same time the films are characterized with high optical transparency, and photo-stability in the whole UV region. PVA with 7 wt% PMAS-VBL shielded almost 100% UV light having absolute quantum yield of 74.26% and transmittance in visible range of 66.9%.
Coatings
[25]	Luminescent coating	Cheap SiO2 particles doped with Eu2+ and Al3+ SiO2:Al0.04Eu0.01, SiO2:Al0.12Eu0.03, and SiO2:Al0.20Eu0.05	Phosphor synthesized via base-catalyzed sonochemical sol–gel method from aluminum isopropoxide Al(O-i-Pr)3 and Eu(NO3)3 in the presence of NH4OH. Coating (100 μm thickness) contained phosphor (8w.%), dispersing agent, defoamer, acrylic resin, thickeners.	UV-to-PAR	Optimized SiO2:Al0.12Eu0.03 obtained one-tenth of the maximum attainable PAR enhancement Photoluminescent quantum yield of 71.5%. UV-to-PAR conversion efficiency of 26.2%. 0.34% PAR enhancement.
[52]	Fluorescence coatings	K2SiF6:Mn4+ doped BK7 glass	Surface of doped BK7 glass s coated with 100 nm MgF2 antireflection coating for the sake of specular reflection reduction.	300–520 nm → 600–650 nm blue-green light to red	Greenhouse experiment modeling. Concluded that introduction of fluorescent reflectors could result in increase in crop production over 35%

3. PCC Effect on Plants Estimation

Understanding how these types of covers and their specific properties generally influence crop production and nutritional quality is important. Various articles address this issue to some extent, including those by Kusuma et al. [54], He et al. [55], and Lauria et al. [56].

Newly developed PCCs can have both excellent photoconversion properties and the best stability under the required conditions, but the real effect on plant growth can only be assessed by greenhouse (or similar) experiments. Therefore, the following sections describe some aspects of the plant growth experiments. First, we observed the choice of experimental plants used by the authors. Then we analyzed and classified the plant growth parameters used by the authors of the reviewed papers. The results of the greenhouse experiments themselves are discussed in Section 4.

3.1. Crops for Greenhouse Experiments

Figure 4 and Table 2 summarize the choice of crops used for greenhouse experiments by the authors of the reviewed papers. About 55% of the greenhouse experiments were conducted with fruit crops, with tomato [13,24,27], pepper [19,57], and cucumber [33,35,58] being the most popular crops in this group. These plants are typical greenhouse crops with high light intensity requirements [16,58], and are the main crops grown in greenhouses with quite high demand [14]. Eggplants were mentioned three times and were selected for experiments because they are also important agricultural crops [33,34,35].

Three grain crops (cereals), namely wheat, barley and millet, were used in the work of Bagiyan and co-authors [41]. The ‘Other’ column in the fruit crops section of Figure 4 consists of pumpkin [35], soybean [37] and berries (blackberry [59] and strawberry [60]).

Vegetable crops were used in 35% of the works reviewed (Figure 4). Leafy vegetables are the most common. These crops are also in high demand (economically important and nutritious [61]) and quite sensitive to greenhouse light conditions such as light quality, intensity and photoperiod [58]. The group of leafy vegetables is represented by lettuce (including green [17,24,61,62], red [31,32], Italian [2]), Chinese cabbage ([2,63]), white cabbage [64] and chard [65]. Also in the group of vegetable crops are herbs (basil [31,60]) and beet [41] and mustard [66] in the “Other” group.

About 7% of all crops in the present study were non-food crops (Figure 4 and Table 2). Thus, three papers evaluated the effect of PCCs on model representative flowering plants, namely Athaliana plants [15], petunia [37] and Digitalis mariana [67]. The latter was chosen by the authors because it is a source of bioactive cardenolides, including the promising antiviral glucoevatromonoside (GEV) [67]. Shoji and co-authors have cultivated a non-food crop, the Japanese larch [65], which is the most important forest tree in the northern biosphere. These trees are also light demanding and could grow quite rapidly amongst other conifers.

Three percent of the cultures used for experiments are occupied by algae, such as Nannochloropsis oceanica and Phaeodactylum tricornutum, in the work of Zhang and colleagues [68]. They were chosen for the experiments because they are primary consumers of atmospheric CO₂ and sustainable producers of biodiesel. The cetane number and iodine value were additional parameters studied [68].

Table 2. Details on the crops used by the researchers in greenhouse experiments.

Type	Crop	Exact Plant	Article
Fruit crops	Tomatoes	Solanum lycopersicum annuum L.	[13,16,27,28,33,34,35,69,70]
		Campari cultivar	[24]
		Timoty cultivar	[24]
	Peppers	Capsicum annuum L.	[19,33,34,35]
		Sweet pepper	[14]
		Green pepper	[57]
	Cucumbers	Cucumis sativus L.	[27,33,34,35,58]
	Eggplant	Solanum melongena	[33,34,35]
	Grain crops	Wheat	[41]
		Barley	[41]
		Millet	[41]
	Soybean	–	[37]
	Pumpkins	Cucurbita pepo	[35]
	Strawberry	–	[60]
	Blackberry	R. fruticosus var. Loch Ness	[59]
Vegetable crops	Lettuce	Lactuca sativa L.	[17,38]
		Magenta variety	[24]
		Cultivar “Kucheryavets Odesskiy”	[64]
		–	[62]
	Green Leaf Lettuce	Green butterhead lettuce (Lactuca sativa ‘Rex’)	[31]
		‘Buttercrunch’ lettuce	[61]
	Italian Lettuce	Lactuca sativa L. Var. ramosa Hort.	[2]
	Red Leaf Lettuce	L. sativa ‘Outredgeous’	[31,32]
		Rex variety	[24]
	Chinese cabbage	Brassica rapa var. glabra Regel	[2]
		Chinese flowering cabbage	[63]
	White cabbage	Early variety, cultivar “Parel F1”	[64]
	Swiss chard	Beta vulgaris var. cicla L.	[65]
	Basil	Ocimum basilicum var. genovese	[31]
		–	[60]
	Beet	–	[41]
	Mustard	Brassica juncea L.	[66]
Non-food crops	Flowers	Athaliana	[15]
		Digitalis mariana ssp. heywoodii	[67]
		Petunia “Morning glory”	[37]
	Japanese larch trees	Larix kaempferi (Lamb.) Carr.	[65]
Algae	Microalgae	Nannochloropsis oceanica	[68]
		Phaeodactylum tricornutum	[68]

Table 2. Details on the crops used by the researchers in greenhouse experiments.

Type	Crop	Exact Plant	Article
Fruit crops	Tomatoes	Solanum lycopersicum annuum L.	[13,16,27,28,33,34,35,69,70]
		Campari cultivar	[24]
		Timoty cultivar	[24]
	Peppers	Capsicum annuum L.	[19,33,34,35]
		Sweet pepper	[14]
		Green pepper	[57]
	Cucumbers	Cucumis sativus L.	[27,33,34,35,58]
	Eggplant	Solanum melongena	[33,34,35]
	Grain crops	Wheat	[41]
		Barley	[41]
		Millet	[41]
	Soybean	–	[37]
	Pumpkins	Cucurbita pepo	[35]
	Strawberry	–	[60]
	Blackberry	R. fruticosus var. Loch Ness	[59]
Vegetable crops	Lettuce	Lactuca sativa L.	[17,38]
		Magenta variety	[24]
		Cultivar “Kucheryavets Odesskiy”	[64]
		–	[62]
	Green Leaf Lettuce	Green butterhead lettuce (Lactuca sativa ‘Rex’)	[31]
		‘Buttercrunch’ lettuce	[61]
	Italian Lettuce	Lactuca sativa L. Var. ramosa Hort.	[2]
	Red Leaf Lettuce	L. sativa ‘Outredgeous’	[31,32]
		Rex variety	[24]
	Chinese cabbage	Brassica rapa var. glabra Regel	[2]
		Chinese flowering cabbage	[63]
	White cabbage	Early variety, cultivar “Parel F1”	[64]
	Swiss chard	Beta vulgaris var. cicla L.	[65]
	Basil	Ocimum basilicum var. genovese	[31]
		–	[60]
	Beet	–	[41]
	Mustard	Brassica juncea L.	[66]
Non-food crops	Flowers	Athaliana	[15]
		Digitalis mariana ssp. heywoodii	[67]
		Petunia “Morning glory”	[37]
	Japanese larch trees	Larix kaempferi (Lamb.) Carr.	[65]
Algae	Microalgae	Nannochloropsis oceanica	[68]
		Phaeodactylum tricornutum	[68]

Thus, the choice of model experimental crops mainly depends on the target real crop, with food crops being preferred. In some cases, the authors combined crops from different groups, even food crops with non-food crops (see e.g., [37,65]), in order to assess the influence of PCCs on the photosynthetic process and plant growth in more detail.

3.2. Parameters for the PCC Effect on Plants Estimation

Figure 5 illustrates the number of parameters used by different authors to assess the efficacy of PCCs as plant growth regulators in greenhouse experiments in the literature analyzed. The data show that the number of parameters used in these studies ranged from 2 to 16, depending on the depth of analysis. The most common approach was to use four parameters (seven studies or 22%), followed by two parameters (five studies) and eight parameters (four studies). Thus, about half of the researchers (47%) used two to four parameters and 84% of the papers used two to eight parameters (Figure 5). The most comprehensive analyses were carried out by Khramov et al. [64] and Gao and co-authors [14] (15 parameters each), and Hebert and colleagues [16], who used 16 parameters. However, as Figure 5 suggests, a range of two to eight parameters may be sufficient to draw meaningful conclusions about the influence of PCCs on plant growth and development.

In view of these findings, an important question arises: Which parameters are most commonly used to evaluate PCC efficacy and why are they preferred? These aspects are explored in detail below.

All the parameters most commonly used to assess PCC efficacy can be classified in three ways, as shown schematically in Figure 6. The first way is a classification of parameters according to the plant organ being evaluated, such as leaves, roots, fruits, shoots or other specific structures. The second classification is based on which area of the plant’s vital activity was affected by the presence of PCCs. This approach categorizes parameters based on the aspect of plant biology affected by PCC use, including morphology (structural changes), physiology (functional processes), yield (reproductive output), and biochemical composition (levels of specific chemical compounds). The third classification of parameters is based directly on the trait identified. Here, parameters are distinguished by the type of measurement taken, such as size (length, area, volume), quantity (number of leaves, fruits, etc.), weight (fresh or dry biomass), and other quantitative or qualitative characteristics.

This structured classification allows a systematic evaluation of PCC effects at different biological scales, ranging from tissue-specific responses to whole-plant performance.

These classification systems provide a comprehensive framework for understanding the parameters used in greenhouse experiments to assess PCC efficacy. In practice, these parameters are often interrelated. For example, leaf size serves as both a morphological trait and a measurable quantitative trait. Researchers typically combine parameters from different categories within the same study to obtain a holistic assessment of PCC effects.

To determine which parameters are most commonly used, we analyzed their frequency across the reviewed studies. As shown in Figure 7, the most frequently studied parameters were plant part morphology and physiology (excluding photosynthesis), reported a total of 76 times. Photosynthesis-related parameters were also a major focus, appearing in 42 studies. The third most common category was chemical composition analysis, reported in 41 studies, while yield and biomass metrics ranked fourth and fifth in prevalence, respectively. This distribution highlights that researchers prioritize structural and functional plant responses when evaluating PCC efficacy, while also considering biochemical changes and productivity outcomes.

To give a more detailed picture of the frequency of use of the different parameters, we provide a scheme in Figure 8. For plant parts, the most frequently studied objects are the leaves (27 mentions) and the whole plant, in particular height, weight, width, etc. (18 mentions). This is followed by fruits (16 times), stem or shoot (nine times), root (three times) and flowers (three times). More detailed information on the parameters measured on different parts of the plant is provided in Section 3.3.

The second place of parameters used to evaluate the efficacy of PCCs in greenhouse experiments is occupied by the group of photosynthesis parameters. We have composed this group from parameters of the photosynthetic process (different authors may name them differently, e.g., photosynthetic activity [66], photosynthetic efficiency [32], photosynthetic rates [19]), chlorophyll content (total chlorophyll content [41], relative chlorophyll concentration in leaves [13], chlorophyll a and b separately, etc.) and some other parameters used by the authors to characterize the changes in photosynthesis due to PCC application (Fv/Fm value [15], maximum quantum yield of PS2 [28], kinetics of photoinduced changes [28,70], etc.).

Figure 8 shows that the third most popular group of parameters, i.e., the content of chemical components (excluding chlorophyll, which belongs to the photosynthesis group of parameters), contains seven frequently used groups of metabolites. Among them there are sugars (total sugar content determined by the authors, glucose, fructose, sucrose (Brix), etc.), proteins (total or soluble), vitamin C (ascorbic acid, reduced vitamin C), acids (titratable acids, free amino acids, hydroxy acids), phenols (total phenolic content or polyphenolic content), anthocyanin (or its’ index) and carotenoids. We should mention that the last two compounds are photosynthetic pigments and could also be included in the group of photosynthesis parameters, but here we have counted them as chemical parameters according to their rare use.

Among the “other” chemical constituents, the content of which was measured by the researchers and mentioned in no more than one paper each, we found proline [41], lipids [68], total flavonoids [59], triacylglycerol [68], endogenous hormone (fruit levels were determined) [14], eicosapentaenoic acid [68], glucoevatromonoside (GEV) [67], auxin [66], nitrates [17] and cardenolides [67].

The fourth group in terms of size was called ‘Rarely encountered’. It contains parameters that could not be assigned to other groups and whose use was not frequent (one, two or three mentions). Here, we have placed parameters such as gas exchange [27], stomatal conductance [2,59], and transpiration [2,33]. Some authors also carried out gene expression analysis [15,68] and studied the response of plants to stress conditions [27,33].

The fifth group of parameters, mentioned 17 times by authors, includes yield parameters (Figure 8). Here we can find not only absolute yield, but also specific yield, i.e., yield of a single plant [2], yield per unit area [2], and total yield of a plot [68]. In addition, Hebert and co-authors [16] used cumulative saleable production and total fruiting biomass production (saleable production plus waste) to estimate yield change under PCC conditions.

The last and least populated group is the group of biomass parameters (Figure 8). It includes such parameters as total biomass [64,68], [64], total body biomass [65], as well as productivity and kinetic parameters, namely biomass production [65], biomass production dry weight [67], and biomass accumulation rate [27]. Biomass of berries [34] and fruit biomass of a bush [35] were also found in this category.

3.3. Parameters Measured from Plant Parts

This section provides more detailed information on the parameters measured from different plant parts. The plant part parameters used by different authors are summarized and schematically presented in Figure 9.

3.3.1. Leaves

Leaves are the most commonly used part of the plant to assess PCC efficacy. They can be analyzed by their size, i.e., average length and width, or the length and width of the largest leaf of the plant [63]. The number of leaves is also used as a reference parameter. Both the number and size of leaves can be assessed using leaf area. It can be measured using specialized software (see e.g., [27]), even for leaves with atypical color [31].

Another group of parameters assessed from the leaves is the weight/mass parameters. Here, the fresh or dry weight of the individual leaf [60], normalized weight/mass per plant or per unit area [31,60] is used. The rate of leaf development can also be used as a kinetic parameter [24].

As the leaves are the most reliable organ for photosynthetic activity, all parameters related to photosynthesis are measured using them. These include physiological parameters, such as photosynthetic rate and net photosynthetic rate, i.e., true photosynthesis excluding photo respiration and dark respiration. Chlorophyll fluorescence parameters, light use efficiency and other photosynthesis-related physiological parameters are also obtained from leaves.

Among other physiological parameters used to assess the influence of PCCs on plant growth, the group of “gas exchange” metrics, including stomatal conductance, transpiration rate, water use efficiency and intercellular CO₂ concentration, etc., are also measured from leaves. A specific parameter, such as the Rubisco (carboxylase) activity used by the authors, is also obtained from leaves.

A group of compositional parameters (Figure 8) can be found in all parts of the plant except photosynthetic pigments. They are usually measured in leaves. Total chlorophyll, chlorophyll A and chlorophyll B separately, total flavonoid content, anthocyanin content or index and carotenoid content were found in the literature reviewed.

It should be noted that in the case of leafy vegetables, the parameters related to the fruit can also be measured in the leaves.

3.3.2. Shoot/Stem

Changes in shoot and/or stem growth under PCCs are considered in terms of parameters of size (length, diameter) [37], structural characteristics (i.e., internode length [14]), weight (fresh/dry) and kinetics, i.e., growth rate [24]. In addition, some primary and secondary metabolites, hormones and activity parameters can be measured from the shoot.

3.3.3. Root

Root size and weight can also be used to analyze the influence of PCCs on plant growth [37]. The authors of [14] described the use of root activity (a representation of the root’s absorptive capacity) to assess the efficacy of PCCs. In addition, in the case of root vegetables such as beets [41], the root can be used as an edible product and its analysis can be carried out similarly to the analysis of fruits.

3.3.4. Fruits

Fruit can be analyzed in terms of its appearance. Hebert and co-authors assessed whether the fruit was saleable or not [16]. Horri and colleagues [59] calculated the berry shape index (ratio of fruit height to fruit width). Yield parameters, fruit number and weight were also determined. Total [60] or specific number of fruits or yield (per plant [58], per area [2], etc.) and their fresh/dry weight (single/individual [2,57,60], total [16], average [16], fruit biomass from one bush [35]) are used by the authors to evaluate the positive or negative effect of PCCs. Organoleptic and biochemical analyses have also been carried out [59]. The content of metabolites such as sugars [60], soluble proteins [14], ascorbic acid [14] and even endogenous hormones [14,58] in fruits was measured in the reviewed papers. A rather thorough analysis was carried out by Hebert and co-authors [16], who obtained and compared “chemical fingerprints” of fruits. In the work of Zakharchenko et al. [66], the number and weight of seeds were determined. As for kinetic parameters, the work of Hebert [16] evaluated the repining time.

Thus, fruits are also a source of information on the influence of PCCs on plants, but they are less popular than leaves.

3.3.5. Flowers

Flower parameters are rarely used. The authors of [16] counted the number of flowers. Flowering speed [16] and fertility [59] were also used to assess the influence of PCCs.

3.3.6. The Whole Plant

The whole plant can also be used to assess PCC efficacy. Most commonly, plant size is measured, namely height [63,65], width [63], head thickness [16], vine length [16], and so on. However, some authors determine kinetic parameters, such as growth rate [66], and weight, such as total body biomass [65].

4. The Effect of PCCs on Plant Growth

The details and results of the researchers who carried out greenhouse experiments are summarized in

Table 3. Details and results of previous research on greenhouse experiments.

Source	Material/ Device	Phosphor	Cover	Synthesis	Photo- Conversion Properties	Plants	Parameters Measured	Results on Plants
Rare-Earth-Based Phosphors
[27]	Photoconversion cover	Eu2O3 and Eu3+:LaF3 About 230 ± 11 nm and 1210 ± 20 nm in size	Fluoropolymer coating on glass	Laser fragmentation and hydrothermal microwave treatment NPs integrated into fluoropolymer matrix and coated onto glass	395 nm → 591 nm, 615 nm, 622 nm (for Eu3+:LaF3) 395 nm → 612 nm, 625 nm (for Eu2O3)	Tomato Cucumber	Size. Yield. Gas exchange processes. Rate of biomass accumulation. Resistance of plants to stress conditions.	Under PCC with Eu2O3: Plants size increased by 30–40%. Yield rose. Gas exchange activated. Light phase of photosynthesis in the leaves fastened. Sensitivity to heat (+40 °C) and cold (+4 °C) treatments increased. Under PCC with Eu3+:LaF3: Rate of biomass accumulation decreased. Rate of gas exchange activation lowered. Resistance to high and low temperatures increased.
[70]	Photoconversion covers	Graphene oxide (GO) with Eu2O3 Particles of 16 nm ± 5 nm with aglomerates of 200 nm ± 20 nm	Fluoroplast-32L coating on glass	Ultrasonic and laser fragmentation, mixing with fluoroplast-32L and spraying onto glass	UV → blue, red	Tomato	Productivity. Chlorophyll content. Kinetics of photoinduced changes in chlorophyll a.	Under PCC: Productivity increased by 25%. Photosynthesis intensified by 30–35%.
[63]	Light-conversion agricultural film	Sr2Si5N8:Eu2+ Average diameter of 500 nm	Low-density polyethylene (LDPE)	Chemically prepared particles mixed with LDPE, silicone light diffuser added. Films were prepared using film blowing machine.	Blue-violet → red	Chinese flowering cabbage	Plant height. Length of the maximum leaf. Width of the maximum leaf. Breadth of the plants. Soluble protein. Polyphenol content. Soluble sugar content.	Under PCC: Plant height increased by 24.43%. Length of the maximum leaf increased by 15.30%. Width of the maximum leaf increased by 15.60%. Breadth of the plants increased by 19.07%. Soluble protein content increased by 9.09%. Polyphenol content increased by 21.27%. Soluble sugar content increased by 19.15%.
[17]	Photoluminescent panels	CaS:Eu and Sr4Ca4Al22O41:Eu, Dy+3, Nd+3B3 Red and blue components, ratio of 70/30	Poly-methyl methacrylate (PMMA)	PMMA sheets produced by the cell casting method 5% (w/w) of the rare-earth blend was added	Emission peaks at 617, 626, 704, 706 nm (orange-far-red)	Lettuce	Total ascorbic acid content Total phenols content Nitrate content Chlorophyll content Carotenoid content Antioxidant activity Effective quantum yield (ΦPSII) Electron transport rate	Under PCC: Chlorophyll content, carotenoid content, ascorbic acid content, and antioxidant activity without changes. Total phenols content reduced by 22%. Nitrate content increased by 14%.
[57]	Composite coatings for greenhouse films	Complex (Eu (TTA)3phen 2-thenoyltrifluoroacetone (HTTA) and 1,10-Phenanthroline (Phen)	Water borne polyurethane (WPU) PE film	Chemically synthesized complex mixed with WPU and coated onto inner side of PE films by spray coating	UV → red	Pepper	Yield. Mass of single fruits.	Under PCC: Yield increased by 35%. Mass of single fruit increased by 8 g.
[65]	Luminescent cover	[Eu(hfa)3(TPPO)2] Photo-sensitizer (hexafuoroacetylacetonato = hfa), stabilizer (triphenylphosphine oxide = TPPO)	Tris(2,6-dimethoxyphenyl)phosphine oxide (TDMPPO) Polyolefn-type covering film Thickness 0.1 mm	Phosphor/TDMPPO (1:2 molar ratio) dissolved in dichloromethane and painted on polyolefin film	UV → red	Swiss chard Japanese larch tree	Size. Height. Biomass production. Total body biomass.	Under PCC: Size increased. Biomass production rose.
[60]	Luminescent sprayable plastic films incorporated into water-based acrylic varnish that can be spray-coated onto existing greenhouses	Molecular Eu3+-containing polyoxotitanates	Acrylate-based paint for glass	Eu3+-containing polyoxotitanates were chemically synthesized and loaded to the paint of 50 mg/g of the final dry mass of the paint at 30 µm thickness	UV → PAR (quantum yields as high as 68%)	Basil Strawberry plants	Leaf dry weight per plant Individual leaf dry weight Fruit yield Fruit number Fruit weight (g per fruit) Fruit sugars (Brix)	Under PCC: For basil— Leaf dry weight per plant increased by 9%. Individual leaf dry weight increased by 10%. For strawberry— No difference was observed.
[13]	Photoconversion film	Sr0.955Yb0.020Er0.025F2.045 Spherical particles of 75 nm, and agglomerates with 300 nm diameter	Fluoroplate polymer coating on glass	Co-precipitation from nitrate solutions, mixing with fluoroplate polymer (1:100) and sprayed onto glass	976 nm → 660 nm, 545 nm, 525 nm	Tomato	Relative chlorophyll concentration in leaves. Number of leaves. Length of the stem. Leaf area.	Leaves area showed the greatest increase under PCC.
[69]	Photoconversion film	Sr0.46Ba0.50Yb0.02Er0.02F2.04	Fluoroplate polymer coating on glass	Co-precipitation from nitrate solutions, mixing with fluoroplate polymer (1:100) and sprayed onto glass	976 nm → 660 nm, 545 nm, 525 nm	Tomato	Leaf number. Total leaf area. Stem length. Chlorophyll content in the leaves.	Under PCC: Leaf number increased by 12.5% Total leaf area increased by 33%. Stem length increased by 35%. Chlorophyll content in the leaves had two-fold increased.
[28]	Photoconversion cover	Sr0.910Yb0.075Er0.015F2.090 Rounded particles with a 68 and 350 nm mean size	Fluoroplate polymer coating on glass	Co-precipitation from aqueous nitrate solutions, mixed with fluoroplate polymer (7%) and sprayed onto glass	975 nm → 660 nm, 545 nm, 525 nm	Tomato	Leaf area. Chlorophyll content. Kinetics of photo-induced changes. Maximum quantum yield of PS2.	Chlorophyll content increased from 6.2 ± 0.8 to 8.9 ± 0.1 r.u. under PCC.
[2]	Light conversion film	Rare-earth material	Polyvinyl chloride (PVC) Thickness of 0.12 mm	Commercial	Yellow-green (500–600 nm) → red-orange (600–700 nm)	Lettuce Chanese cabbage	Net photosynthetic rate Stomatal conductance Intercellular CO2 concentration Transpiration rate Number of fruits Single fruit weight Single plant yield Yield per unit area Total soluble sugar Soluble protein Reduction-type Vitamin C	Under PCC: Yield increased by 8.97–39.53%. Total soluble sugar increased by 9.22–30.14%. Reduction-type Vitamin C increased by 1.41–21.09%. Net photosynthetic rate, transpiration rate, intercellular CO2 concentration, and stomatal conductance increased.
[58]	Light conversion film	Europium-based	Polyolefin film 0.1-mm thickness	Commercial	Blue, red-orange, far-red ↑ Orange, violet, green ↓	Cucumber	Soluble solid content Fixed soluble protein content Vitamin C content Titratable acid content Free amino acid Soluble sugar content Fruit number per plant Total plot yield Individual fruit weights Length of the cucumber handle Gene expression profiles	Under PCC: Handle length ratio decreased by 24%. Yield increased by 30%. Soluble protein increased by 25% increase. Vitamin C increased by 27%. Free amino acids increased by 28%. Soluble solids increased by 9%. Organic acid content reduced by 35%. Auxin synthesis gene expression and auxin content decreased by 87% and 24%, respectively. Light influences plant development primarily through hormone modulation.
[14]	Rare-earth light conversion film	Rare-earth Eu agent	Polyolefin film 0.1 mm thickness	Commercial	UV, violet, green → blue, red-orange, far-red	Pepper	Plant height Stem diameter Leaf length Leaf width Internode length Root activity Photosynthesis Rubisco activity Net photosynthetic rate Endogenous hormone content in fruit Soluble protein Ascorbic acid Titratable acid Free amino acids Yield	PCC improved growth and yield by advancing photosynthesis, and improved fruit quality through adjusting endogenous hormone content in the low-temperature seasons. Under PCC: Ascorbic acid content increased by 14.29%. Soluble protein content increased by 47.10%. Soluble sugar content increased by 67.69%. Yield increased by 20.34%.
Non-rare-earth inorganic phosphors
[34]	Fluoropolymer films with photoconversion QDs	Cd0.6Zn0.4Se QDs Max size distribution at 7.5 nm	Fluoropolymer films	QDs obtained via chemical synthesis, mixed (7%) with fluoropolymer in a ratio of 1/100 and films were formed	375 nm → 650 nm Quantum yield of 17%	Pepper Eggplant Cucumber Tomatoes	Biomass of berry. Area of leaves.	Under PCC: Biomass of plants increased. Biomass of tomato berry from one bush increased by 20%.
[35]	Photoconversion fluoropolymer films	Cd(1-x)Zn(x)Se QDs Average size of about 7 nm (red QDs), 15 nm (blue QDs)	Fluoropolymer	QDs obtained via chemical synthesis, mixed (7%) with fluoropolymer in a ratio of 1/100 and films were formed	UV/violet → blue and red	Cucumber Pumpkin Pepper Tomato	Biomass growth. Area of leaves. Fruit biomass from one bush.	Under PCC: Leaves area for cucumber increased by 20%. Leaves area for pumpkin increased by 25%. Leaves area for pepper increased by 30%. Leaves area for tomato increased by 55%. Fruit biomass from one bush increased by 15%.
[32]	Luminescent agriculture film	CuInS2/ZnS QDs	Acrylic resin film, polyethylene terephthalate (PET) sheets	QDs were chemically synthesized and added to acrylic resin, then coated between two sheets of PET	UV/blue → red emissions centered at 600 and 660 nm	Lettuce	Edible dry mass. Edible fresh mass. Total leaf area. Photosynthetic efficiency.	Under PCC: Edible dry mass increased by 13%. Edible fresh mass increased by 11%. Total leaf area increased by 13%. Photosynthetic efficiency improved.
[31]	QDs film	CuInS2/ZnS QDs	PE film	Commercial	UV-A, blue → green, red, far-red	Lettuce Basil	Number of leave Total leaf area Shoot fresh weight Shoot dry weight Leaf mass per area Leaf chlorophyll contents Plant height Anthocyanin content index	Under PCC: For red lettuce— shoot fresh weight increased by 10% shoot dry weight increased by 10% total leaf area increased by 8% Leaf expansion and stem elongation promoted for of red and green lettuces and basil. Yield comparable to control group despite 23% decrease in DLI.
[33]	Photoconversion fluoropolymer films	Au NPs Cd(1-x)Zn(x)Se QDs Blue QD of about 12 nm; red QDs of about 7 nm	Fluoropolymer films	Gold nanoparticles were obtained by the method of laser ablation in liquid. QDs obtained via chemical synthesis. 7% QDs solution and 3% Au NPS solution mixed with fluoropolymer in a ratio of 1/100 and films were formed	UV → blue, red	Capsicum Eggplant Cucumber Tomato	Chlorophyll content. Photosynthesis. System of distance stress signals. Length of shoots. Length of roots. Number of leaves. Area of leaves. Transpiration.	Under PCC: Chlorophyll content increased. Intensity of photosynthesis increased. System of distance stress signals suppressed.
[16]	Emission-tunable luminescent film	CuInS2/ZnS quantum dots	Plastic film 350 μm thick	Commercial	Provide diffuse orange light	Tomato	Vine length Head thickness Number of set trusses Number of flowering trusses Number of fruit sets Leaf length Number of leaves Flowering speed Ripening time Cumulative saleable production Average fruit weight Sugar content (Brix) Dry matter content Light use efficiency Waste content Total fruiting biomass production (saleable production plus waste)	Under PCC: Plants grew 2.1 cm/week faster. Tomato production increased. Waste production reduced. Light use efficiency rose.
[68]	Quantum dots	Red, blue, and green quantum dots (QDs)	PE	Commercial	Peak wavelengths of red, blue, and green light at 633, 455, and 513 nm, respectively.	Two species of microalgae	Lipid content. Eicosapentaenoic acid titer. Triacylglycerol content. Cetane number. Reduced iodine value. Biomass accumulation.	Under PCC: For N. oceanica— Growth increased by 11.2%. Lipid content increased by 9.5%. Eicosapentaenoic acid titer increased by 15.5%. Biodiesel production accelerated by 20.2%. Biodiesel improved (increased cetane number and reduced iodine value). For P. tricornutum— Biomass increased by 8.6%. Triacylglycerol content increased by 35.0%. Biodiesel production accelerated by 11.6%. Biodiesel improved (increased cetane number and reduced iodine value).
[15]	Dual light conversion films	Biomass-derived carbon dots (CDs) Monodispers spherical nanoparticles (average diameter of 1.9 nm)	Polyvinyl alcohol matrix	CDs were synthesized from furfural and p -phenylenediamine via hydrothermal method CDs (20 μg/mL and 200 μg/m) integrated into PVA matrix	UV and green → blue and red	Athaliana plants	Fv/Fm value Gene expression analysis	Under PCC: Fv/Fm value increased 12% indicating significant boost in photosynthesis. Gene expression analysis showed upregulating genes involved in light-harvesting and energy conversion resulting in enhance photosynthetic efficiency.
[62]	Photoconversion coating	PVA-coated CDs microcapsules	Polyurethane film	Via “water/oil/water” method	UV → blue	Lettuce	Fresh weight. Dry weight. Chlorophyll a content. Chlorophyll b content. Carotenoids Soluble protein.	Under PCC: Fresh weight increased by 177%. Dry weight increased by 143.5%. Content of chlorophyll a increased by 14.5%. Content of chlorophyll b increased by 188.5%. Content of carotenoids increased by 43.3%. Content of soluble protein increased by 17.9%.
[37]	Light-converting anti-icing superhydrophobic coating	Carbon dots (CDs) on the surface and interlayers of montmorillonite (MMT) (CDs/MMT) Homogeneous spherical particles	Epoxy resin (ER) coating on glass	Chemical synthesis for in situ growth of CDs and hydrolytic polymerization of fluorinated alkyl silane (FAS) on montmorillonite (MMT). Dispersed into ER matrix and covered onto glass	A broad absorption band at 300–600 nm with emission peaks at 483 nm, 485 nm, 517 nm, 596 nm, and 630 nm	Soybean Petunia	Chlorophyll content. Stem length. Stem diameter. Root length.	Under PCC: For soybean— Chlorophyll content increased by 20.0%. Stem length increased by 21.17%. Stem diameter increased by 26.0%. Root length increased by 24.2%. For petunia— Chlorophyll content increased by 9.51%. Stem length by 100.74%. Stem diameter by 23.32%. Root length by 20.56%.
[41]	Smart sunlight window	Silver vanadate nanorods (β-AgVO3)	Glass	Co-precipitation from AgNO3 and NH4VO3, ambient drying. Solution of β-AgVO3 (2 mg/mL) uniformly sprayed on surface.	530 nm → 670 nm Absorb light in the range of 500–600 nm and emit at red region.	Wheat Barley Millet Beet	Total chlorophyll content Photosynthetic rates Proline	Under PCC: Total chlorophyll content ↑ Photosynthetic rates ↑ Proline content ↑
[38]	Photoconversion covers	Chromium-doped alumina (Al2O3:Cr3+) Ruby particles with irregular shape and size 1–10 μm	Fluoroplast-32L coating on glass	Laser ablation and further laser fragmentation, mixing with fluoroplast-32L and spraying onto glass	Two wide bands of excitation 350–450 nm (max at 405 nm) and 500–600 nm (max at 550 nm) with emission at 650–750 nm (max at 695 nm)	Lettuce	Yield. Water use efficiency. Assimilation of carbon dioxide. Chlorophyll content in lettuce leaves. Number of leaves. Leaf area. Chlorophyll a content.	Under PCC: Yield increased by 40%. Water use efficiency increased. During dark respiration increased. Assimilation of carbon dioxide increased.
Organic phosphors
[24]	Photonic thin films	Multiple luminescent dyes combined with photonic crystals (SiO2 and TiO2 layers)	Low-density polyethylene (LDPE)	Mixing of dyes with LDPE and films obtained using corotating twin-screw extrusion line with a pelletizing System. Films were combined with photonic crystals.	Green → red NIR ↓	Tomato Lettuce	Development of leaves. Elongation of stems.	Under PCC: Leaves development fastened. Stems elongated.
[61]	Spectral-shifting microphotonic thin film	Lumogen F Red 305 (LF305)	Poly(methyl methacrylate) (PMMA)	Commercial LF305 dissolved in PMMA, casted and dried	PAR ↑	Lettuce	Photosynthesis. Biomass production.	Under PCC: Photosynthesis increased. Biomass production increased.
[66]	Light-correcting Coatings	Organic photoluminophore (PL)	Polypropylene non-woven spunbond coated with polylactic acid (PLA) films	Polymeric non-woven material obtained via solution method followed by hot matrix pressing Chemically synthesized PL was added (0.25% w/w) to 40 μm thickness containing to the polymeric material	460–560 nm → 660 nm (half-width of 610–730 nm)	Mustard	Auxin content. Analysis of sugars (Glucose, fructose). Hydroxy acids. Growth rates. Seed weights. Seed number. Photosynthetic parameters of Photosystem II (PSII).	Under PCC: Glucose content increased by 28.4 ± 0.3%. Fructose content increased by 60.4 ± 0.3%. Seed weights increased 1.9-fold. Seed number increased 1.6-fold. The authors shown that providing plants with PCC for 4 weeks can lead further plant growth without PCC with higher yields in the future.
[64]	Agrotextile	New organic luminophore (LUM)	Polypropylene (PP) nonwoven spunbond coated with polylactide varnish containing	Polymeric non-woven material obtained via solution method followed by hot matrix pressing Chemically synthesized PL was added (0.25% w/w) to 40 μm thickness containing to the polymeric material	460–560 nm → 660 nm Blue-green → orange-red	White cabbage Lettuce	Photosynthesis rate Photosynthesis rate per unit leaf area Respiration rates Stomatal conductance Transpiration rate Water use efficiency Content of chlorophyl a Content of chlorophyl b Carotenoids content Photosystem II (PS2) maximum Fv/Fm Effective F′v/F′m Biomass accumulation Leaves area Balance of carbon dioxide absorption (difference between photosynthesis and respiration rates)	Under PCC: For lettuce— Biomass increased by 20± 3%. Photosynthesis rate increased by 27 ± 6%. Leaf surface area increased by 12%. For cabbage— Biomass accumulation increased by 42 ± 7%. Photosynthesis rate per unit leaf area increased by 27 ± 6%. Leaf surface area increased by 48 ± 4%.
Non-specified phosphors
[19]	Photoconversion film	Not mentioned	PE film	Commercial	Green → red	Pepper	Dry mass. Petiole length. Photosynthetic rates. Chlorophyll fluorescence parameters.	Under PCC: Dry mass increased. Petiole length decreased. Maximum photosynthetic rates increased. Chlorophyll fluorescence parameters increased.
[67]	Photoconverting nets	Not mentioned	Not mentioned	Commercial	50%-shade black, blue and red photoconverting nets	Digitalis mariana	Biomass production dry weight Photosynthetic pigments Total proteins Cardenolides Glucoevatromonoside (GEV)	Under blue PCC: Production of dry weight increased. Photosynthetic pigments accumulation increased. Accumulation of proteins, total cardenolides, and glucoevatromonoside (GEV) increased.
[59]	Light down-conversion films	Not mentioned	Polyethylene films 150 μm thick	Commercial	red (green → red), pink (UV and green → blue and red) blue (UV → blue light)	Blackberry	Fruit yield Flower Fertility Photosynthetic rate Stomatal conductance Berry shape Fresh weight Total flavonoid Anthocyanin content	Red and blue PCCs boosted photosynthesis and flower fertility, enhanced productivity, and did not affected fruit organoleptic and nutraceutical quality. Under PCCs (blue and red, respectively): Photosynthetic rates increased 23.1 and 14.9%. Stomatal conductance increased 56.0 and 23.6%. Yield increased (49.8% for red PCC). Fresh berry weight increased (36.6% for blue PCC).

. All papers have been grouped according to the type of phosphor used for PCC formation. According to Figure 1, rare-earth-based phosphors, non-rare-earth inorganic phosphors and organic phosphors were reported. Table 3 is quite detailed and thoroughly describes the results reported by the researchers, so we will not include their description in the text. In this section, an attempt has been made to analyze the results of the greenhouse experiments, and some interesting or unusual data from the analyzed papers are presented.

In order to compare the results on the influence of PCCs on plant growth, the most frequently used parameters (let us call them “basic parameters”) were grouped and counted.

Table 4. The increase in basic parameters for PCCs with different phosphors.

Parameters Increased Under PCC	Type of Phosphor
	Rare-Earth-Based	Non-Rare-Earth Inorganic	Organic *
Yield and biomass	69%	83%	75%
Photosynthesis and chlorophyll content	38%	67%	75%
Size of plant and leaf area	62%	75%	75%

* Only 4 works on PCCs with organic phosphors were analyzed.

shows three main groups of basic parameters. The group “Yield and Biomass” contains all parameters describing yield, production, biomass accumulation, etc. The group “Photosynthesis and chlorophyll content” includes all parameters characterizing the photosynthetic process (rates, intensity, etc.) and chlorophyll content (total, chlorophyll a, chlorophyll b). The third group, “Plant size and leaf area,” consisted of the parameters of the size of the plant and its parts, and the area of the leaves.

The number of papers reporting an increase in the parameters from these groups was counted and their percentages are shown in Table 4. It should be mentioned that not all authors measured all parameters from these three groups. Therefore, the results of the calculation are quite approximate and are only intended to give a general idea of the reported experimental results.

Another point to note is that only four papers on organic phosphors-based PCCs were analyzed. Therefore, the calculated percentage for this group of materials should not be taken into account.

Thus, when comparing the results for PCCs with rare-earth phosphors and PCCs with non-rare-earth phosphors, it can be seen that non-rare-earth PCCs more often resulted in an increase in the selected basic parameters. Thus, in the majority of works, these PCCs lead to an increase in yield and productivity, photosynthetic activity of the plants, as well as plant size characteristics. This tendency may be related to the emission specificity of the group of non-rare-earth phosphors for PCCs (see some summaries in Section 2.2). Comparing rare-earth and non-rare-earth-based PCCs, it can be seen that the latter emit more in the red than in the far red region. They are also less likely to be excited by UV light than rare-earth phosphors. About 45% of the reported non-rare-earth inorganic phosphors in PCCs have their excitation maxima in the visible range. These factors may have led to higher values of the basic parameters of plants grown under PCCs with non-rare-earth phosphors compared to those with rare-earth-based light converters.

Some selected works from Table 3 are discussed below.

Li and co-authors [2] proposed and studied a rare-earth-based light-conversion film that improved a number of parameters in the greenhouse, including PAR, during frequently hazy conditions. This PCC converted yellow-green light (500–600 nm) into red-orange light (600–700 nm), increasing yield and improving physiological processes and nutrient composition in plants growing under it. The authors concluded that such PCCs can effectively meet the challenge of low light under the frequent haze weather conditions.

According to the results presented in Table 3, most of the compositional parameters (metabolite content) should increase with increasing fruit quality. However, in two articles, for example, the authors presented controversial results. Wu and co-authors [63] and Mola and colleagues [17] developed PCCs with Eu-doped phosphorus. Wu et al. [63] reported a light-converting agricultural film made of low-density polyethylene (LDPE) with Sr₂Si₅N₈:Eu²⁺ with blue-violet-to-red conversion. Under this PCC, Chinese flowering cabbage increased in height (24.43%), width (19.07%), maximum leaf length (15.3%) and width (15.6%). The protein, polyphenol and sugar contents also increased (9.09, 21.27 and 19.15%, respectively). Mola and colleagues [17] proposed PMMA panels with red and blue components, namely CaS:Eu and Sr₄Ca₄Al₂₂O₄₁:Eu, Dy⁺³, Nd⁺³B₃. The orange-red emission influenced the lettuce, resulting in a 22% decrease in total phenolic content and a 14% increase in nitrate content. Thus, the content of phenolic compounds was increased [63] and decreased [17] in two named leafy vegetables. The first possible reason is the slightly different emission of PCCs in these two papers. This may have different effects on the plants. Another possible reason is the point mentioned in the introduction, namely, the different effects of PCCs on different plants.

Three papers also reported the absence of effects of PCCs on some plant parameters. Mola and co-authors [17], mentioned above, showed that chlorophyll content, carotenoid content, ascorbic acid content and antioxidant activity of cabbage were not affected by PCCs. Horri and colleagues [59] investigated the influence of commercial red and blue PCCs on the growth and productivity of blackberries. They found that the PCCs enhanced photosynthesis and flower fertility, increased productivity and did not affect the organoleptic and nutritive quality of the fruit. Muller and colleagues [60] prepared luminescent sprayable plastic films with Eu³⁺-containing polyoxotitanates. UV-to-PAR conversion with a QY of 68% affected basil (increase in leaf dry weight and leaf dry weight per plant) and had no effect on strawberry. This is further evidence that the PCC effect is different for different plants. This is also true for the parameters; different parameters, even in the same plant, are affected differently by the converted light.

Li and co-authors [58] and Gao et al. [14] studied the change in hormone content under PCCs. Li et al. [58] found that under Europium-based polyolefin film, both auxin synthesis gene expression and auxin content decreased in cucumber. They concluded that light affects plant development primarily through hormone modulation. Gao and colleagues [14], studying peppers under Eu-containing polyolefin film, found improved fruit quality and concluded that this was a result of adjusting endogenous hormone levels in the low temperature seasons. However, hormone content determination and gene expression analysis were rarely performed in the papers analyzed, and there is insufficient data for deeper and more general conclusions.

Plant growth and development are thus influenced by PCCs, but in different ways. The controlling influence may be exerted by the type of phosphorus in the PCC composition, by the actual photo-conditions provided by the PCC (shielding, transmittance, photoconversion), or by the plant species growing under the studied PCCs. The use of a greater number of plant growth parameters to assess the efficacy of PCCs can help to provide a more detailed picture of the processes induced by photoconversion agents in plants. However, such experiments are still quite demanding, being labor and resource intensive. A reasonable balance has to be found and maintained in the conduct and analysis of greenhouse experiments for an accurate, competent and realistic assessment of PCCs.

Based on the analysis of the results obtained in the present review, we suggest that at least eight parameters should be used for a minimally reliable assessment of PCC effectiveness. Among these, two should be plant growth characteristics (e.g., size or biomass accumulation, etc.), followed by two parameters of the photosynthetic process. We also propose that at least two parameters of component content should be analyzed (e.g., nutrient content in the edible part of the plant, and so on). Finally, there should be at least two parameters characterizing the yield of the end product. Combining the results of changing the proposed parameters for the plants grown under PCCs will enable one to fully assess the effect of the cover on the growth of a particular crop or group of crops.

Regarding the long-term performance of PCCs in real greenhouse conditions, we note that the articles analyzed in the present review did not provide any information on this topic. The authors either mentioned the possible long-term stability of the developed/proposed phosphors or the PCC itself [51,64,67], or indicated that this would be studied in future work [24]. For example, Kumar Barman et al. [51] hypothesized the long-term stability of carbon dot-modified PVA films based on the stability of their PL spectra after several days. Zhao et al. [62], for instance, reported that PVA-coated CDs microcapsules exhibited a 25.87% decrease in fluorescence intensity on the seventh day compared to the first day, and that the PCC could demonstrate fluorescence performance even after 15 days. Therefore, the long-term stability of PCCs in real greenhouse conditions appears to be an understudied area at present.

5. Conclusions

In the present review, a total of 58 studies on photo-converting covers (PCCs) for greenhouses were analyzed. Among those, 32 studies included data on PCC effectiveness assessed through experiments with plants. The remaining 26 studies focused on new materials (phosphors and PCCs) with potential for improving plant growth, although greenhouse experiments were not conducted.

The reviewed studies were found to primarily target the development of new phosphors, particularly those based on rare-earth elements. Among non-rare-earth-based phosphors, the emergence and development of carbon dots represent a promising avenue for new PCC materials. At the same time, the newly developed PCCs were most commonly presented as polymer films. However, researchers also proposed coatings and “sandwich”-like glass constructions.

An analysis of the photo-converting properties of these materials revealed that both rare-earth-based phosphors and PCCs with non-rare-earth agents predominantly emit light in the red region of the spectrum. Rare-earth-based phosphors frequently emit both red and far-red light simultaneously. Additionally, both categories of phosphors exhibit excitation maxima in the UV region. However, UV excitation is observed less frequently among non-rare-earth-based materials (50%) compared to rare-earth-based materials (67%). Notably, PCCs with inorganic, non-rare-earth photo-converting agents are often excited by light in the visible range (45%). These differences may contribute to the varying performance of PCCs containing rare-earth and non-rare-earth phosphors in greenhouse experiments. In general, covering materials with inorganic phosphors that are free of rare-earth elements showed a greater enhancement in basic plant growth parameters compared to PCCs with rare-earth-based additives. However, this difference could also be attributed to variations in the number of parameters used by researchers to evaluate PCC effectiveness.

In the 32 studies that reported results from plant growth experiments, the authors used between two and 16 parameters to assess changes in plants influenced by PCCs. Among such studies, the most common approach involved using two to four parameters. Overall, the majority of studies (84%) relied on two to eight parameters to determine PCC effectiveness.

The results of greenhouse experiments indicate that PCCs influence plant growth and development, though with varying effects. These variations can be attributed to factors such as the type of phosphor in the PCC composition, the photo-conditions provided by the PCC (e.g., shielding, transmittance, and photoconversion), and the plant species studied. Using a larger number of growth parameters for PCC effectiveness assessment can provide a more detailed understanding of the processes induced by photo-converting agents in plants. However, such experiments require considerable time, effort, and resources, making them very demanding. Therefore, a balance should be found between thorough experimentation and practicality to achieve accurate, reliable, and meaningful evaluations of PCCs. According to the results obtained, we suggest that a minimally reliable assessment of PCC effectiveness requires the use of at least eight parameters: two each for plant growth (e.g., size and biomass), the photosynthesis process, and components content and yield. Combining these parameters allows one to fully assess the effect of the cover on the growth of a particular crop or group of crops when PCCs are used.

When considering future research directions in the field of developing and applying new PCCs, the following points should be mentioned. Firstly, increasing the light conversion efficiency is the main objective. Higher quantum yields with lower re-absorption losses are highly desirable. Secondly, the newly developed technology must be scalable and easily transferable from laboratory conditions to real-world greenhouses. Therefore, the preparation and application methods must be as simple and straightforward as possible. Thirdly, the cost of producing and exploiting such PCCs must be economically viable. Thus, the materials developed should contain cheaper components and be produced in a more accessible way. Additionally, an important challenge is to create and develop up-converting PCCs that convert near-infrared (NIR) radiation into PAR, since NIR is a significant part of solar radiation that is not used by plants.