## 4 Discussion  
It is known that the quality and quantity of light affects the growth and development of plants, determining the efficiency of photosynthetic processes, as well as the accumulation of secondary metabolites. Plants are capable of adapting to a wide variety of light conditions; however, improving the quality of lighting always increases the productivity of agricultural plants. Plants are believed to utilize between a third and a half of solar radiation. The remaining photons belong to the ultraviolet, infrared and other light ranges, which are generally unavailable to plants for photosynthesis. This makes the development of photoconversion technologies relevant for the temperate and cold regions of the planet, where usable light is at a premium. Based on the investigations of [PERSON] [45],  
Figure 5: Dependence of Y(II) (**A**), Y(NPQ) (**B**) and ETR (**C**) on light intensity in plants growing under common (red, closed squares) and photoconversion (black, opened circles) films. Measurements were done at room temperature. The data are the means of at least three measurements, with the standard deviation of the mean. [PERSON] [46] and [PERSON] [10], we know that additional red light can more efficiently activate photosynthesis in comparison to other light of similar intensity. This principle was applied in this study. For the intensification of plant development, we used a photoconversion coating containing Sr\({}_{0.955}\)Yb\({}_{0.020}\)Er\({}_{0.025}\)F\({}_{2.045}\) nanoluminophores. The coating absorbs IR light and emits visible light (at bandwidths of 660 nm, 545 nm and 525 nm; Figure 3A). These luminescence bands are usually observed under the illumination of the phosphors containing the Er\({}^{3+}\)/Yb\({}^{3+}\) ion pair [39; 40; 41; 42; 47; 48; 49; 50]. Of course, the phosphor we used is much inferior to conventional phosphors in terms of light conversion efficiency (about 6% vs. more than 20%) [30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61]. However, in comparison with many other upconversion luminophores, this phosphor has a fairly high efficiency of photoconversion. The fact that similar particles can convert ultraviolet radiation into PAR increases their usefulness in greenhouses, as discussed in [39]. The difference spectrum shown in Figure 3B confirmed that photoconversion glasses moderately increase PAR intensity and absorb UVA and far-red. Obviously, this changes the red light:far-red light ratio. The difference spectrum is a broadband radiation with two pronounced peaks at 561 nm and 650 nm, which can be correlated with the erbium luminescence bands. The observed broadband radiation was studied earlier for nanoparticles of various chemical compositions [62; 63; 64; 65; 66; 67]. The mechanism of the appearance of such luminescence is the subject of open discussion. As can be seen in Table 1, plants grow and develop quicker under the PCF in comparison with common film, which is consistent with the previous data [41; 42]. The appearance of leaves, and the increase in their area, are accelerated by 50%. PCF accelerates the growth of the stem, without affecting the length of the internodes, which is indicative of accelerated growth rather than avoidance of shadows. Moreover, photoconversion glasses stimulate the accumulation of chlorophyll in the tissues. According to the data published in [68], the obtained values indicate a two-fold increase. A similar improvement in plant growth was previously obtained using a down-shifting red luminophore [32]. As can be seen, luminophore substitution does not significantly reduce the efficiency of photoconversion coatings. It is important to note that only a negligible part of the light absorbed by the coating can be transformed into luminescence in the PAR region. Possible reasons include low absorption of ultraviolet and/ or infrared radiation and high yield of heat dissipation.