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Abstract
Light-converting polypropylene spunbond was first used in the study of the key physiological parameters of plants. A comparative study of the functioning of the photosynthetic apparatus and the dynamics of growth in late cabbage plants (Olga variety) and leaf lettuce (Emerald variety) was conducted using the ordinary nonwoven polypropylene fabric (spunbond) (density 30 g·m−2) and the spunbond containing a photoluminophore (PL) (1.6% yttrium oxysulfide doped with europium). The plants were grown in a glass greenhouse without spunbond and under the spunbond containing and not containing the PL that transforms a part of UV-radiation into red light radiation. The use of the spunbond led to a decrease in the rate of photosynthesis, activity of the photosystem 2, and the accumulation of plant biomass and to an increase in the stomatal conductance. By contrast to unmodified spunbond, the application of the spunbond containing the PL led to an increase in the rate of photosynthesis, the water-use efficiency (WUE), and the accumulation of the total biomass of plants by 30–50% but to a decrease in the transpiration rate and the stomatal conductance. It is assumed that the positive effect of the PL is associated with an increase in the fraction of fluorescent red light, which enhances photosynthetic activity and accelerates plant growth.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The intensity and spectral composition of light have a significant impact on the plant growth and productivity. Red and blue light are the key spectral components that induce photosynthesis and plant growth. To increase the productivity of plants, it is possible to correct the spectrum of the incident radiation by increasing the proportion of the red or blue components, depending on plant species and environmental conditions.
Polymer materials have found the most extensive application in agriculture for covering greenhouses. The key characteristics of these materials are durability, good spectral composition (in the UV-, visible, and near infrared (NIR) regions), anti-drip properties, and the ability to maintain the required temperature. The UV- and NIR-blocking, as well as fluorescent and thermic films are widely used in agriculture [2,4,6].
The main two approaches to the regulation of light quality during the growth of vegetable and green cultures are the application of polymer coating materials, including light-transforming, light-selective, and light-correcting coverings [1–7] and artificial light courses [8–11].
Fluorescent films contain a photoluminophore (PL) particles that converts a part of UV- and/or blue/green light into luminescent light in the orange-red region of the visible spectrum. As a result, the additional light exerts a beneficial effect by increasing the photosynthesis and plant productivity [12–15]. A great body of data indicating an increase in the yield of many greenhouse vegetable and green crops has been obtained using these PLs as additives introduced into polymer films [12–14]. The positive effect of these modified films is explained by an increase in the proportion of scattered light and orange-red light as well as a decrease in the fraction of UV-radiation, which leads to an increase in the stress resistance of plants as well as in the content of phytohormones in these plants and/or of microorganisms in soil [13,14].
Polyethylene films are traditional and the least expensive of the available covering materials used in many countries [16]. Film coatings transmit light well, protect plants from the wind, and partially retain heat. Plants under their protection can withstand frosts from −2 to −7°C (this indicator depends on the film density and the volume of the greenhouse). However, polyethylene films have several disadvantages. Since a film is air- and waterproof, the plants under its shelter need regular ventilation and watering. In addition, a condensate accumulating on the surface of the film damages the plants, causing fungal diseases.
Another type of covering materials is agro textiles, which also have wide application in sustainable agriculture, horticulture, agro-engineering, and renewable technologies of crop growing [17].
The most suitable material in the production of agro textiles is nonwoven polypropylene, which has a number of advantages over other polymers such as good processability, sufficient chemical resistance, high water adsorption, and low cost [18,19]. The spunbond as a nonwoven fabric textile has found wide use in agriculture as a covering material for beds and greenhouses. It has a number of useful properties. It has fair strength and durability, provides optimal microclimate and moisture retention and protects plants from cold, wind, hail, extra sun as well as pests and pollutants [18–20]. Another valuable property of spunbond is that it is degradable in the soil after the utilization, which is important for use under field conditions.
Polyethylene films are commonly used for vegetable production but there is a problem of the disposal of utilized films [1,3]. Therefore, materials are required that can degrade in the soil after the growing season. Potentially biodegradable plastic films and fabrics, including spunbond, can also be made from renewable materials and are ecologically more favorable [21]. However, these materials have usually a low transmittance in the visible region of the spectrum compared to conventional polyethylene and other polymer films, which can lead to a deficit of light, especially under conditions of low or moderate solar radiation. The key properties of agricultural covering materials have been described in detail in [4,19].
Microdispersed photoluminescent particles increase the light scattering of incident solar radiation. On the other hand, the spunbond itself, which has a stronger light scattering compared to films, rather strongly reduces the intensity of photosynthetically active radiation (PAR) incident on plants, which can decrease plant productivity. However, the use of textiles containing PL, which emits additional light in the orange-red region, can partly solve the problem of decreasing PAR. Besides, by using PLs with different luminescence spectrum, we can improve the quality of light for plant photosynthesis and growth. So far, there are no data in scientific literature on the application of inorganic PL incorporated into spunbond.
Cabbage and lettuce are important green vegetables grown in greenhouse; therefore, they were used in our experiments. The aim of this research was to study the influence of modified white polypropylene spunbond on the physiological parameters such as growth and photosynthetic activity of these plants at early developmental stages and to supply probative data for farmers willing to implement the advantages of these technologies.
2. Materials and methods
2.1. Plant growing conditions
Fourteen-day-old seedlings of late cabbage (Olga variety) and germinated leaf lettuce plants (Izumrudny variety) were used in experiments. Plants were grown in plastic vessels with a height and diameter of 20 cm, filled with gray forest soil with the addition of 5% (by volume) of a mixture of peat, sand, and vermiculite (1: 1: 1).
For comparison, three plant groups were used, which were grown in a glass greenhouse (glass thickness 4 mm) on a special setup (Fig. 1 left): Group 1, control without coating; Group 2, plants grown under the usual unmodified polypropylene white spunbond; and Group 3, plants grown under the modified spunbond containing 1.6% of PL particles. Plants were planted in trays with wooden frames, which were covered with a spunbond. Initially, each group contained 80–90 cabbage seedlings. Then, among these seedlings, about 50–60 seedlings of similar size were selected and further grown for 28 days. During this time, the biomass was measured four times (immediately after covering and after 14, 21, and 28 days). Every time, 10 cabbage plants were used per each measurement. Germinated lettuce plants (35–40 plants of each treatment) grew for 20 days from the beginning of seed germination.
Fig. 1. A photograph of a part of our setup used for plant growing (left). Modified and non-modified spunbond under UV-A radiation (on right). Red light is emitted with photoluminophore.
The average light intensity in the greenhouse at the noontime was 1150 ± 250 µmol photons m−2s−1, and under the spunbond material it was about 1100 ± 200 µmol photons m−2 s−1. The average light intensity at the noontime under modified and unmodified spunbonds at the level of plants was 730 ± 80 and 710 ± 75 µmol photons m−2s−1, respectively. The cultivation was carried out in June and July at an average day/night temperature (25–27)/(20–22)° C.
2.2. Materials
In the experiments, the polypropylene covering material (white thermally-bonded spunbond) with a density of 30 g m−2 was used. The spunbond was developed by the Polisvetan Company (Moscow, Russia) and produced by the United Chemical Company “Shchekinoazot” (Shchekino, Tula Region, Russia). The PL (yttrium oxysulfide doped with europium (Y2O2SEu, size of particles 3–5 microns) was produced by the Luminophor Company (Fryazino, Moscow Region, Russia). PL is non-toxic and retains photoluminescent properties for several seasons of operation. Its concentration in polymeric light-transforming covering materials is about 0.1–2%, which is economically justified for crop production. The method of preparing it is routine and is described in particular in United States Patent [22].
Fluorescence emission and excitation of PL (Fig. 2) were measured on an ALS01 spectrofluorimeter (Institute of Synthetic Polymer Materials, Moscow, Russia). The red fluorescence of the photoluminophore introduced into the spunbond is clearly visible (Fig. 1, on the right). The solar irradiance passing through textiles (Fig. 3) was measured in the wavelength range of 380–780 nm at a clear sky using an MK350N Premium spectroradiometer (United Power Research Technology CORP, Taiwan). Textile samples were placed directly on the photodetector window. It was found that the integral transmittance of solar radiation in this area was somewhat less than 90% and was indistinguishable for ordinary and modified textiles.
Fig. 2. The fluorescence and excitation spectra of the PL introduced into the spunbond material. Excitation of fluorescence at a wavelength of 365 nm. Fluorescence spectrum at a wavelength of 625 nm.
Fig. 3. Changes in solar spectrum after textiles without PL (red curve) and with PL introduced (black points). Black curve - spectrum of solar irradiation before textiles.
In the wavelength range of 380–780 nm, the intensity of solar radiation incident on spunbond at the right angle was 530 W m−2 and that of radiation that passed through the spunbond was 480 W m−2. However, the spectral density of the fluorescent light flux was not identified in a modified spunbond due to a low concentration of PL, 1.6% (Fig. 3).
To evaluate the possible contribution of additional luminescent light from 1.6% PL, polypropylene films with higher concentrations of PL (5% and 10%) (Fig. 4) than in experiments with plants were used. The intensity of the additional luminescent red light from these films, calculated as the areas under luminescence spectral curves was approximately equal to 0.003 (taking the density of materials into account). However, according to our measurements, the intensity of solar light incident on lower tiers of leaves was less than the intensity on the top tier of leaves and ranged, depending on the plant age and place of measurements, from a few µmol photons m−2s−1 to 50–100 µmol photons m−2s−1. The luminescent red light at 710 nm (Fig. 3) is photosynthetically inactive. Changes in the RL/FRL ratio on the surface of top and low tiers of leaves were evaluated using the interference filters of RL (λmax 662 nm and half-width 10 nm) and FRL (λmax -737 nm and half-width 19 nm). The goal of the work was to study both the quality and quantity of PAR that affect plants, but we did not measure changes in the local temperature and humidity.
Fig. 4. Changes in solar spectrum after polypropylene films with a content of 5% and 10% Y2O2SEu. At the top there is a direct recording from a spectroradiometer, at the bottom and in the inset, the luminescence of films with 5 and 10% Y2O2SEu content is shown on an enlarged scale. The intensity of sunlight in region of 380 nm–780 nm was 533.8 W m−2 (film thickness 40 мкм).
2.3. Determination of growth and photosynthetic parameters
The following parameters were examined: accumulation of plant biomass, photosystem 2 (PSII) activity, the rate of photosynthesis, stomatal conductance, transpiration rate, and water using efficiency (WUE). The photosynthesis rate, the stomatal conductance, the transpiration rate, and WUE were determined in a closed system under the day light conditions using an LCPro + portable infrared gas analyzer from ADC BioScientific Ltd. (United Kingdom) connected to a leaf chamber with an area of 6.25 cm−2. The measurements were carried out in the morning with an average light intensity of 300 ± 10 µmol photons m−2 s−1. For analysis, upper fully developed leaves were used.
The activity of the primary light processes of photosynthesis in leaves was assessed using the methods of Chl a variable and delayed fluorescence [23,24]. The parameters of millisecond delayed fluorescence (DF) Im and D (where Im is the maximal amplitude of the DF, and D is the minimum on the curve) were determined on the basis of induction curves obtained using a disk phosphoroscope [24]. DF curves were recorded in the digital form using a computer program developed by us. On the basis of the DF measurements, the (Im-D)/D ratios reflecting PSII photochemical activity were calculated [25].
The maximal quantum yield of PSII (Fv/Fm), which reflects PSII activity, was determined on a JUNIOR-PAM fluorimeter (Walz, Germany) using the modulated pulse techniques. Photoinduced changes in fluorescence were calculated according to the equation: (Fv) = Fm -F0, where F0 is the initial fluorescence level. The light was switched for 10 min (I = 190 µmol (photons) m−2 s−1). The intensity of saturating light was 6000 µmol (photons) m−2 s−1. Saturating pulses were generated every 30 s. The detached developed upper leaves were acclimated to the darkness for 20 min and used for subsequent measurements.
2.4. Statistical analysis
Tables 1–3 and Fig. 5 show the arithmetic means of the values obtained and their standard deviations (± SD). All the photosynthetic experiments were conducted with at least 10 analytical replicates. For this, leaves from 5–6 plants were used. For growth measurements, 28 lettuce plants and 40 cabbage plants for each treatment were used. The significance of differences between two variants was described by the t-test at the 5% significance level. A comparison of the data for three groups of plants was performed by one-factor analysis of variance (ANOVA) and the Tukey multiple comparison test.
Table 1. Photosynthesis and total biomass accumulation by 20-d-old lettuce plants after growing. Control - without any coating. -PL - unmodified spunbond without photoluminophore. +PL - spunbond with the introduced photoluminophore. For biomass, average data from 28 plants are given; for photosynthesis, 10 leaves for each variant were used. Middle light intensity on the leaves of top tiers was 640 µmol photons m−2 s−1 and in the shade of leaves ranged between a few µmol photons m−2 s−1and 100 µmol photons m−2 s−1. The means ± SD are given.
Table 2. The performance of the photosynthetic apparatus 42-d-old cabbage plants grown under the covering material spunbond with the addition of photoluminophore (+PL) and without additive (-PL) and without a coating (Cont.). Water use efficiency (WUE) is equal to the ratio of the rate of photosynthesis (Pn) to the rate of transpiration. The average data for 12 leaves of cabbage plants in each variant are given. The values are means ±SD.
Table 3. Changes in the relative maximal amplitude of the DF induction curves ((Im-D) / D in the case of a spunbond with an introduced photoluminophore (+PL) and without it (-PL). Control - uncoated control. 12 leaves of 42-d-old cabbage plants in each variant and 10 leaves of 20-d-old lettuce plants were taken from 5–6 different plants. The growing conditions are the same as in Tables 1 and 2.
Fig. 5. Dependence of the average total biomass of one cabbage plant on the time of the growing of seedlings under the spunbond and the type of covering. +PL - modified spunbond. Control – control without any covering. -PL - unmodified spunbond. The means ± SD are given, n = 10.
3. Results
3.1. Spectral studies
The fluorescence and excitation spectra of PL introduced into the spunbond material were measured. The excitation spectrum of PL had a wide band in the region of UV (A+B) with a maximum at 310 nm, and the fluorescence spectrum had the main band with a maximum at 625 nm (Fig. 2). The small difference in the region of 600–640 nm in the spectrum of sunlight travelling through the spunbond containing PL and the spunbond without PL was not detected (within the resolution of our device) owing to transmitting high-intensity sunlight (Fig. 3).
Using the spectral data of Figs. 3 and 4, we evaluated the intensity of additional luminescent light in the region of 610–640 nm. According to our estimation, the transmitting solar radiation incident on the top tiers of leaves in the same region was approximately 300 times higher than the intensity of the luminescent light. However, under the low tiers (canopy), the light intensity in the region of 400–750 nm ranged from a few µmol photons m−2 s−1 to 50–100 µmol photons m−2 s−1. Hence, the light intensity in the region of 610–640 nm could be a few tenths of a percent compared to solar radiation incident on top tiers, and therefore could be compared with the intensity of luminescent light, which can reach all areas including lower tiers.
It is difficult to evaluate the changes in the RL/FRL ratio of solar radiation that passes through the canopy. Therefore, we used a model system for evaluating the relative changes in the RL/FRL ratio of sun radiation passing through one or two leaves put on the light receiver window covered with an interference filter (RL or FRL) with known transmission spectra. Leaves of the middle tier of 40-d-old cabbage plants were used. At noon, under spunbond the ratio was approximately 1.0±0.2, whereas under one cabbage leaf and under two leaves (stacked together), it became 0.20 ± 0.04 and 0.011 ± 0.006, respectively. The means ±SE of ten leaves are given. Thus, the R/FRL ratio can be small enough.
3.2. Improvement of photosynthesis and plant growth by the photoluminophore
The use of the spunbond without PL compared with the control (without any covering) led to a decrease in the rate of photosynthesis, the activity of the PSII, and the accumulation of plant biomass, as well as in the WUE. In lettuce plants, the rate of photosynthesis (Pn) decreased by 32%, and the biomass decreased by 23% (Table 1). After 28 days of growing under spunbond without PL, Pn and the biomass in cabbage decreased by 18% and 14%, respectively (Table 2 and Fig. 5). The water-use efficiency (WUE) was reduced by 35%, and stomatal conductance increased by 21%. However, the activity of photosystem 2 did not significantly change (Table 3). The biomass of cabbage plants increased significantly, by 45 g after 14 days and by 70 g (approximately 40%) after 28 days in a test with PL-containing spunbound (modified PL) as compared to the spunbond without PL, and for lettuce it increased by 1.4 g (39%) (Table 1 and Fig. 5). At the same time, an increase in the rate of carbon dioxide absorption in cabbage and lettuce (by 40–50%) and a higher WUE in cabbage plants covered with modified spunbond compared to the plants under the usual spunbond material (Table 2) were found. However, the use of modified spunbond led to a decrease in stomatal conductance and water transpiration rate. The experiments conducted on cabbage showed a slight increase in the relative amplitude of DF (Im-D)/D and the PSII maximal quantum yield (Fv/Fm) in modified spunbond compared to the unmodified one; in the experiments with lettuce, no effect was observed (Table 3).
The data obtained indicate a weak effect of the PL on the efficiency of primary photosynthetic processes, whereas the efficiency of the secondary processes of CO2 fixation by plants increases significantly with the use of modified spunbond, which сompensates for a decrease in photosynthesis under unmodified spunbond.
Presumably, the increase in the photosynthesis rate induced by modified spunbond is one of the main reasons for the increase in plantgrowth.
4. Discussion
From our data, it is clear (Table 1 and Fig. 5) that the application of modified spunbond led to an increase in the value of biomass of plants compared to plants under unmodified spunbond. This suggests that a change in light conditions can play an important role in an increasing the plant productivity. Lowering the photosynthetically active radiation (PAR) under the spunbond is compensated for by changing the spectral conditions under the modified spunbond.
The use of such fluorescent polymer materials as coatings for cultivation structures, as it was shown in our and other studies, significantly increases the plant productivity, which is likely linked to added luminescent radiation [4,12,26]. At the same time, the intensity of luminescent radiation from fluorescent films, depending on the exposure to UV radiation in the light flux, is decisive in changing the productivity of plants [26].
The studies of light-transforming films [13,14,26] assume several mechanisms of positive action of the photoluminophore introduced into spunbond. One such mechanism could be the activation of the phytochrome system by additional red light emitted by the photoluminophore. This activation could enhance photosynthesis, stress-resistance and plant growth [27]. A decrease in UV-component of solar irradiance and an increase in scattered light induced by photoluminophore particles can also play a role in the positive effects of photoluminophore.
It should be noted that the contribution from the luminescent radiation of the photoluminescent phosphor in the region of the commonly used “orange-red” photoluminescent phosphors (610–640 nm) is small compared with the intensity of the solar radiation incident on plants [26]. In our experiments, the luminescent light intensity was also small. However, even the small red light intensity can play a role under certain conditions. In our opinion, it is necessary to take into account that components of solar radiation in the red and blue regions do not penetrate well under the shade of leaves [28]. Luminescence red light from photoluminophore particles spreads in all directions, including lower leaves. The red light intensity in the bottom part of leaves and in some bottom leaves can be much less than the intensity of red light incident on upper leaves (Fig. 6). This is consistent with our data on changes in the RL/FRL ratio during the passage of solar radiation through leaves. Hence, under these conditions the contribution of low-intensity red light from the luminophore can be significant affecting the fraction of phytochrome active form, which depends on the red light/far-red light ratio in the irradiance incident on leaves. Increasing the red light/far-red light ratio enhances the content of the phytochrome active form, which leads to an increase in photosynthesis and plant growth [27,29]. For example, the total dry weight of 20-d-old tomato plants grown at different RL/FRL values used during plant growing could differ by 30% [29].
Fig. 6. Spectral distribution of day solar radiation incident on plants (continuous curve) and in the dense shadow of plants (intermittent curve). Red triangles show additional luminescent light. From [30] with modification.
It should be also noted that the additional red light in light-transforming films can induce an increase (by more than one order) in the concentration of native soil microflora, as well as changes in the content of phytohormones [13,14]. This is consistent with the well-known facts about the regulatory effect of red fluorescent light from photo-converting films on the hormonal balance of plants [13]. It is also known about the high sensitivity of many microorganisms to the action of light, including low-intensity red light [31,32] and the positive influence of soil microflora on plant growth [34], which is due to the light-induced activation of respiration chain [32].
At the same time, the surface layer of the greenhouse soil, due to the increased activity of the aboriginal soil microflora, showed increasing the temperature by 1–2°C (compared with the control). Also, we observed a low intensity of solar RL incident on shaded places. In this case, RL from photoluminophore incident on soil surface can be comparable with solar RL in this place. Apparently, the interaction of microbiota and plants and changes in the hormonal balance can serve as one of key factors in stimulating the plant growth [13,14,33] and increasing their stress-resistance [34]. It is highly likely that, in many cases, when using light-converting coatings, native soil microbiota and a change in the phytohormone balance along with an increase in photosynthesis make a decisive contribution to stimulating the plant development but this requires further research.
5. Conclusions
The use of photoluminescent light-converting materials for the improvement of photosynthetic activity of algae [35] and higher plants [12,36,37] has been described in literature. Examples of successful application of modern fluorescent films for increasing the yield of plants in greenhouses have been reported [11,12,37]. Thus,, the increment in the yield of cabbage and pimento with the use of films containing phosphor (emitting light in blue and red regions) was 23.9% and 21.3%, respectively [36]. However, studies involving simultaneous measurement of photosynthetic parameters and plant productivity using the films with inorganic photoluminophore are scarce [38]. In the present study, we have developed a new light-converting spunbond and tested its application during the growth of cabbage and lettuce, simultaneously measuring the growth and photosynthetic characteristics. We believe that the use of the light-transforming spunbond will make it possible to combine the advantages of agrotextile materials with the advantages of light-converting films modified by photoluminophores, which can improve and optimize the photosynthesis and growth of a variety of plant species.
Funding
Russian Foundation for Fundamental Investigations (18-29-17073); Ministry of Education and Science of the Russian Federation.
Acknowledgments
Authors thank Dr. Andrey B. Gapeev and Svetlana V. Sidorova for help in preparation of the MS and Prof. Sergey. A. Ponomarenko and Dr. Yuriy N. Luponosov for constructive comments on the manuscript. Spectral characterization of all materials was performed with the financial support from the Ministry of Science and Higher Education of the Russian Federation using the equipment of Collaborative Access Center “Сenter for Polymer Research” of ISPM RAS.
Disclosures
No potential conflict of interest was declared by the authors.
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