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Article

Adding Phyto-LED Spectrum to White-LED Light Increases the Productivity of Lettuce Plants

by
Mikhail Vereshchagin
1,*,
Pavel Pashkovskiy
1 and
Ivan Tarakanov
2
1
K.A. Timiryazev Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya Street 35, Moscow 127276, Russia
2
Moscow Timiryazev Agricultural Academy, Russian State Agrarian University, Timiryazevskaya Street 49, Moscow 127550, Russia
*
Author to whom correspondence should be addressed.
Horticulturae 2024, 10(8), 795; https://doi.org/10.3390/horticulturae10080795
Submission received: 3 June 2024 / Revised: 1 July 2024 / Accepted: 25 July 2024 / Published: 27 July 2024
(This article belongs to the Section Plant Nutrition)
Browse Figures
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Abstract

:

Highlights

  • The spectral characteristics of different types of white LEDs differ significantly.
  • The Phyto-LED spectrum increases the productivity of lettuce plants
  • The combination of white LEDs and Phyto-LEDs had the same effect on the productivity of lettuce plants as the Phyto-LEDs alone.
  • The combination of white LEDs and Phyto-LEDs has a significantly greater effect on color rendering than does the use of sole Phyto-LED spectrum
  • Phyto-LEDs provoke flowering of lettuce plants

Abstract

The effect of light of various spectral compositions on the complex morphophysiological parameters of lettuce plants in hydroponic was studied. The light sources had the following light spectra: warm white light—2700 K, cold white light—6500 K, and Phyto-LED light, as well as 2700 K + Phyto-LED and 6500 K + Phyto-LED. The dry and fresh biomass, leaf area, stem length, photosynthetic pigment content, photosynthesis and transpiration rates, chlorophyll fluorescence parameters, and percentage of plants that passed into the generative stage of development were studied. The results showed that partial and complete replacement of white LEDs by Phyto-LEDs with lower green light content and greater amounts of far-red light in the radiation spectrum caused an increase in plant productivity of 37%, average leaf area, and transpiration rate in the treatments but also promoted an earlier transition of plants to flowering under light treatment, Phyto-LEDs, and Phyto-LEDs + white LEDs. The 2700 K + Phyto-LED treatment had one of the highest productivities, as did the Phyto-LED and 6500 K + Phyto-LED treatments, but this lighting treatment provoked less flowering on the 60th day of the growing period.

1. Introduction

The optimization of light in artificial cultivation systems is important for current crop production. Modern narrowband LEDs allow control of the spectral characteristics of light sources created on their basis, which enables the regulation of photosynthesis and plant growth responses. By changing the spectral characteristics of light sources, not only does the regulation of photosynthesis become possible, but also the regulation of plant transition to various ontogenetic stages. The plant determines the spectral composition, intensity, and duration of ambient light by photoreceptors such as cryptochromes, phototropins, and phytochromes [1,2,3,4]. Photoreceptors play a crucial role in regulating circadian rhythms by interacting with a complex of transcription factors, such as PIF, FT, and CO. This interaction enables plants to adapt their transition from the vegetative to the generative stage of development in accordance with the seasonal rhythms of their environment [5]. During the generative phase, the apical meristem of the plant shoot stops forming leaves and lateral shoots and starts producing generative organs. Besides the spectral composition of light, factors such as day and night length, as well as endogenous factors like plant age and water status, can also induce the transition to the generative phase of development [6]. In the cultivation of salad greens, the vegetative period is the most favorable because it is at this time when commercially profitable plant organs are formed. The generative phase of ontogenesis provokes the outflow of assimilates from the leaves to generative organs, reducing the nutritional value of the leaves. The most commercially optimal option would be to increase the vegetative period while increasing the rate of biomass recruitment for the plant to gain useful vegetative mass as quickly and for as long as possible.
Some studies of the spectral effect on photosynthesis have shown that GL (green light) (500–600 nm) stimulates photosynthesis less effectively since it is less strongly absorbed by the chlorophylls and, as a result, has less effect on plant productivity, unlike RL (red light) and BL (blue light) [7,8,9,10]. Based on these results, polychromatic LEDs with special spectral characteristics were developed, namely with a reduced GL content in their spectrum, the so-called full-spectrum plant growth light emitting diode, which is often called Phyto-LEDs [11,12,13] and is supposed to replace white polychromatic LEDs. At the same time, at present, there are no studies devoted to the direct comparison of white-LED spectra and Phyto-LED spectra even though these types of LEDs are actively used in many crop-production farms.
To date, many researchers are working towards optimizing light for the growth of green crops; in this regard, we currently have a large amount of experimental data. The plants of lettuce grown without GL (25% BL, 75% RL) had a 13% larger biomass compared to plants grown under white fluorescent light, which indicates that light with a deficiency of green light in the spectrum stimulates plant biomass [14]. At the same time, L. sativa plants grown in simulated sunlight and under white fluorescent light received the same or greater plant biomass than plants grown under different combinations of BL and RL; moreover, plants grown in modulated sunlight showed signs of transition to flowering, which indicates the role of the light spectrum in regulating the stages of ontogenesis [15]. Additionally, L. sativa plants grown under the lighting option BL + GL + RL (15% BL, 24% GL, 61% RL) had a 47% higher fresh vegetative mass than plants grown under BL + RL (16% BL, 84% RL), which indicates that GL is able to increase the productivity of L. sativa plants [16]
Despite significant progress in the field of artificial lighting technologies, there are still contradictory results regarding the optimal light spectrum when growing green crops such as L. sativa, especially GL. Therefore, many crop-production farms using LED lighting still prefer to use white-LED lamps [17], unlike bi- or polychromatic LED lamps with narrowband LEDs, since white-LED lamps are less expensive, which will allow the costs of enterprise equipment to be covered faster and the added cost of final products to be reduced.
The use of narrowband LEDs allows one to adjust the spectral characteristics much more precisely and create light conditions that are not even close to those found in nature; however, white-LED lights can also differ significantly, which can also be used when selecting the optimal lighting spectrum for growing various crops.
White LEDs have a number of spectral features that allow them to be distinguished from other light sources. Unlike Phyto-LEDs (Figure 1) or other types of lighting not related to LEDs (fluorescent lamps, gas discharge metal halide lamps, etc.), white LEDs practically do not have a far-red (FRL 700–800 nm) light range (somewhere 1–3% of the total intensity). It should also be noted that white LEDs have different conditional values but are often characterized by correlated color temperature (CCT), which is measured in Kelvins (K) and, in a simple sense, shows the ratio of the red and blue wavelength ranges in the spectrum. The most common types of white LEDs are 2700 K (“warm white light”) 4500 K (“medium white light”) 6500 K (“cold white light”) [18]. All these variants of white light differ significantly in the ratio of the main ranges of the visible spectrum BL, GL, and RL. For example, two types of 2700 K and 6200 K white LEDs were used in this work, and as seen from their spectral characteristics, these variants of white light differ by three times in the amount of blue light and almost 2 times in the amount of red light (Figure 2). These differences can cause significant morphological and physiological differences in plants.
It is also necessary to consider color rendering since lighting with a deficiency of GL has low color rendering, which complicates the working conditions of operators. Low color rendering is another reason white LEDs are more commonly used in artificial horticulture than Phyto-LEDs, which lack a GL in their spectra [11].
It should be emphasized that there are few works in which different variants of polyspectral light are compared since, in such works, several spectral regions often change among the variants, and accordingly, the resulting physiological effect is difficult to attribute to the change in a particular range of the spectrum. Due to the frequent occurrence of polychromatic white-LED lamps and special Phyto-LED lamps in crop-production farms, there is a great need to conduct such comparative experiments.
Therefore, the purpose of this study was to determine the effects of different white-LED spectra and Phyto-LED light spectra, as well as their combinations, on the growth and basic photosynthetic parameters of lettuce plants. We assumed that by choosing a certain combination of white light and Phyto-LEDs, we could find light spectra that could lead to an extension of the vegetative stage period and an increase in the rate of plant biomass recruitment. These spectrum options can be considered for practical use in crop production.

2. Materials and Methods

2.1. Plant Growing Conditions and Experimental Design

Lettuce plants of the Batavia variety “lettuce Batavia” (Lactuca sativa L.) were used in the experiments. The study was carried out in a controlled environment in the laboratory of the artificial climate of the Russian State Agrarian University, Moscow.
Each chamber in the phytotron had white walls and a 3.5 m2 area for cultivation. The temperature was maintained at 23/21 °C day/night, and the humidity was 65–80%. Plant growth was carried out with an aero-hydroponic system, which was realized due to the constant supply of the nutrient solution flow (0.8 L/min) to the root zone. For the mineral nutrition of plants, a Knopp–Hoagland solution diluted 2 times was used [19]. The nutrient solution was replaced every time the electrical conductivity of the solution fell below 1.6 mS/cm.
The seeds were soaked in distilled water for 12 h. Then, the seeds were sown in mineral wool cubes soaked in distilled water and placed in growing vessels for two days in the dark. The growing vessels were a 1 L plastic cup filled with expanded clay. After dark germination, 36 plants were installed for each of the five light treatments (the total number of plants in the experiment was 180). Then, a light was turned on in each light cell. The light conditions were PPFD 170 µmol (photons) m−2 s−1 and photoperiod 14 h, and the spectral characteristics were separate for each variant (Figure 1 and Figure 2).
The experiment lasted for 60 days from the day of soaking. A total of 30 days after the date of sowing, 18 plants from each light treatment were selected randomly to determine the fresh and dry mass of leaves and roots, the number of leaves, the length of the largest leaf, the width of the largest leaf, and the total leaf area of each plant. The roots of each plant were removed to measure the fresh and dry mass of the roots. The leaves were separated from the stem. The length of the stem was measured from the base of the stem to the apex. Fresh leaves were counted for each plant, and the fresh weight, total leaf area, length, and width of the largest leaf were measured. The leaves were then dried and weighed to obtain the dry mass of the leaves.
Additionally, the photosynthetic rate, stomatal conductance, transpiration rate, chlorophyll concentration, photochemical activity, and efficiency of photosynthetic water consumption efficiency of the leaves were measured.
Then, on the 60th day of the experiment, 18 plants from each light treatment were analyzed for the presence of peduncle germ, number of leaves per plant, and stem length.

2.2. Spectral and Energy Characteristics of Light

The light intensity and other spectral characteristics were measured using an Upr-Tek PG100N (Zhunan Township, Taiwan). PPFD was equalized for all variants and was at the level of 170 µmol (photons) m−2 s−1. BL refers to light with a wavelength in the range of 400–500 nm, GL refers to light with a wavelength in the range of 500–600 nm, RL refers to light with a wavelength in the range of 600–700 nm, FRL refers to light with a wavelength in the range of 700–800 nm. Upr-Tek PG100N (Taiwan) automatically calculated percentages of different light spectra.
For the manufacture of LED matrices, two types of white LEDs were used (EPISTAR Technology, Zhunan Township Taiwan), which differed from each other in the content of Blue light (BL), Red light (RL), and FRL (far-red light) in the spectrum—cold white light LED—6500 K (BL, GL, RL, FRL: 35%, 42%, 21%, 2%) and warm white light LED—2700 K (BL, GL, RL, FRL: 11%, 49%, 38%, 3%). Also, for the manufacture of LED matrices, full-spectrum plant growth light emitting diodes or a Phyto-LED (EPISTAR Technology, Taiwan) were used, in which the amount of GL in the spectrum is sharply reduced (BL, GL, RL, FRL:26%, 6%, 56%, 12%). From these LEDs, 5 LED matrices with different spectral compositions were assembled, including variants 6500K (BL, GL, RL, FRL:35%, 42%, 21%, 2%), 2700K (BL, GL, LR, FRL:11%, 49%, 38%, 3%), Phyto-LEDs (BL, GL, RL, FRL:26%, 6%, 56%, 12%), as well as options with a combination of white LEDs and Phyto-LEDs—2700 K + Phyto-LED (BL, GL, RL, FRL:31%, 24%, 39%, 7%) and 6500 K + Phyto-LED (BL, GL, RL, FRL:19%, 28%, 47%, 7%).
The consumption of electrical energy in terms of one square meter of the chamber in the phytotron area was calculated based on the volt-ampere characteristics of the matrices. The power (W) of the lamp is calculated according to the formula W = I × U, where I is the current of the power supply of the dimmed power supply that powers the LEDs, and U is the voltage at the plus and minus outputs of the power supply. The intensity of illumination was the same for all the matrices, and the height of the matrix above the plants was 55 cm.

2.3. Leaf Area and Gravimetric Indicators

The measurements were carried out using the LI-3000A device (LI-COR Biosciences, Lincoln, NE, USA), n = 18.
The fresh mass of the organs was determined with analytical scales (Scout Pro SPU123, Ohaus Corporation, Parsippany, NJ, USA). The dry mass was determined after reaching a constant dry mass in a drying chamber (AB54-S, Mettler Toledo, Greifensee, Switzerland), n = 18.

2.4. Photosynthetic and Transpiration Rates

The photosynthesis and transpiration rates were analyzed using the LI-6400XT Portable Photosynthesis System (LI-COR Biosciences, Lincoln, NE, USA) with a standard 2 × 3 cm leaf chamber. During the measurements, the CO2 concentration was maintained at 400 ± 10.0 µmol mol−1, the air temperature was 25 °C, and the humidity was 70 ± 4.0%. Photosynthesis and transpiration were measured directly in the growth chambers under the same conditions as those used for plant growth. The water use efficiency was calculated as the ratio of the photosynthetic and transpiration rates. To determine photosynthesis and transpiration, fully mature leaves at the middle level were used, n = 18.

2.5. Chlorophyll Fluorescence and Total Chlorophyll Content

The fluorescence of Chl a in PSII was measured using a Junior-PAM fluorimeter (Heinz Walz, Effeltrich Germany). The minimum (Fo) and maximum (Fm) fluorescence rates were determined after a 30-min dark adaptation. The maximum quantum output of FS2 Fv − Fm = (Fm − Fo)/Fm was calculated separately [20]. The fluorescence parameters were determined in 3 biological repetitions and 3 analytical replicates.
The total Chl content was determined spectrophotometrically. Formulae and extinction coefficients are used for the determination of chlorophyllous pigments (total chlorophyll) [21].

2.6. Microscopy

On the 60th day of the experiment, the plants were analyzed for the presence of generative formation in the apex of the stem. The apex of the stem was viewed in a microscope in reflected light at 100× magnification stereoscopic microscope Stemi 2000CS (Zeiss, Gottingen, Germany).

2.7. Statistics

The fluorescence and CO2 gas exchange measurements were performed in six biological replicates. For each of these experiments, at least three parallel independent measurements were performed. The significance of the differences among the groups was calculated by one-way analysis of variance (ANOVA) followed by Duncan’s method using SigmaPlot 12.3 (Systat Software Inc, Chicago USA). Letters indicate significant differences between the light treatments (p < 0.05). The data are shown as the means ± standard errors.

3. Results

3.1. Spectral and Energy Characteristics of Light Sources

By combining white LEDs with Phyto-LEDs, intermediate spectral compositions 2700 K + Phyto-LED and 6500 K + Phyto-LED lighting were obtained. These compositions have parameters significantly different from both white lighting and Phyto-LED lighting (Figure 3).
The addition of LEDs with a deficiency of GL to the white LEDs from matrices 6500 K and 2700 K leads to the following:
(1)
A decrease in the proportion of GL by 1.75 times for 6500 K and 2700 K
(2)
An increase in the proportion of RL in the spectrum by an average of 1.85 times for 6500 K and 1.23 times for 2700 K
(3)
An increase in the proportion of FRL in the spectrum by an average of 3.5 times for 6500 K and 2.3 times for 2700 K
The spectral compositions presented in this experiment differed in parameters such as the number of individual wavelength ranges and such an important parameter as the color rendering index (CRI) (Figure 4).
The LED matrices differed in characteristics, such as electrical power per square meter. The 6500 K matrix was the most energy-efficient, consuming 22% less electricity per 1 square meter than the 2700 K, 2700 + Phyto, and Phyto-LUK matrices and 11% less than the 6500 K + Phyto-LED matrices (Figure 5).

3.2. Dry and Fresh Mass of Organs

Changes in the spectral characteristics significantly changed the parameters of the fresh and dry mass of plants. The leaves of plants grown under white light treatments 6500 K and 2700 K had a similar biomass (53.4 g/plant). The plants grown under combined light 2700 K + Phyto-LED and 6500 K + Phyto-LED had a similar biomass (73.3 g/plant). The plants grown under Phyto-LED light had a biomass of 75.3 g/plant, therefore demonstrating the highest fresh mass of leaves (Table 1).
The leaves of plants grown under the white light options 6500 K and 2700 K had a similar dry biomass (2.56 g/plant). Plants grown under Phyto-LED lighting and plants grown under combined lighting 2700 K + Phyto-LED and 6500 K + Phyto-LED had a similar biomass (3.57 g/plant) (Table 1). According to the obtained results, there was no significant difference between the fresh and dry mass of the roots of plants between the variants. The average fresh mass of the roots of all plants was on average 8.85 g/plant, and the dry mass of the roots was 0.30 g/plant (Table 1).

3.3. CO2 Gas Exchange and Transpiration, Stomatal Conductance, Water Use Efficiency, Fluorescent Parameters and Chl Content

The photosynthetic rate was similar among the plants in all the treatments and was 4.05 µmol/m2 s (Table 2).
The transpiration rate was similar among the plants grown under white light at 6500 K and 2700 K and at 1.72 mmol/m2 s. The transpiration rate under 2700 K + Phyto-LED, 6500 K + Phyto-LED, and Phyto-LED light was 2.57 mmol/m2 s, which was greater than that under white light at 6500 K and 2700 K (Table 2).
The stomatal conductance under white light at 2700 K was the lowest at 0.122 mol/m2 s. The stomatal conductance under white light at 6500 K for 0.149 mol/m2 s was greater than that under 2700 K for 0.122 mol/(m2 s). The stomatal conductance under the 2700 K + Phyto-LED, 6500 K + Phyto-LED, and Phyto-LED treatments was not significantly different and was 0.177 mol/m2 s (Table 2).
The water use efficiency (WUE) of plants grown under 2700 K + Phyto-LED, 6500 K + Phyto-LED, and Phyto-LED lights was on average 1.6 µmol CO2/mmol H2O. The highest photosynthetic water consumption efficiency in plants grown under light at 6500 K and 2700 K occurred at 2.66 µmol CO2/mmol H2O (Table 2).
The total Chl content in the leaves was similar among the plants in the experiment. (Table 2).
In all the experimental treatments, the maximum quantum yield of PSII was the same (0.83) (Table 2).

3.4. Morphometric Parameters

The plants grown under white light at 6500 K had the lowest number of leaves at 30 days of the experiment (13 per plant). The plants grown under 2700 K + Phyto-LED, 6500 K + Phyto-LED, and Phyto-LED light and under 2700 K light had the same average number of leaves for 30 days of the experiment (14.8 per plant) (Table 3).
The length of the largest leaf under light at 6500 K and 2700 K was 14.2 cm. The length of the largest leaf under the 2700 K + Phyto-LED, 6500 K + Phyto-LED, and Phyto-LED treatments was 17.4 cm (Table 3). The width of the largest plant leaf among the lighting options did not change (Table 3).
The smallest total leaf area was obtained under white light (6500 K—805 cm2). The total leaf area of plants grown under 2700 K white light was 938 cm2. The total leaf area of plants grown under combined 2700 K + Phyto-LED, 6500 K + Phyto-LED, and Phyto-LED lighting was 1189 cm2 (Table 3).
There were 52 leaves on the 60th day of the experiment under the 6500 K and 2700 K light treatments and under the 2700 K + Phyto-LED treatment. The number of leaves on the 60th day of the experiment in plants grown under light, 6500 K + Phyto-LED or Phyto-LED, was 62.3 per plant (Table 3).
Plants grown under the 2700 K + Phyto-LED, 6500 K + Phyto-LED, and Phyto-LED lighting conditions had the longest stems on the 60th day of the experiment, and the average length of these plants was 25.6 cm. Plants grown in light at 6500 K and 2700 K had an indicator height of 17.3 cm (Table 3).
On the 60th day of the experiment, plants grown under the light of 6500 K + Phyto-LED and Phyto-LED had the greatest number of leaves—62.3 per plant. There were 52 plants grown under 6500 K + Phyto-LED and Phyto-LED lights (Table 3).
Microscopic analyses of the apexes of plants on the 60th day in the presence of peduncle germ showed that plants grown under 6500 K and 2700 K lighting did not have peduncles at the apex of the stem. Sixty percent of the plants grown under light 6500 K + Phyto-LED had peduncle germ at the apex of the stem, and 15% of the plants grown under light 2700 K + Phyto-LED had peduncle germ at the apex of the stem. The plants grown under Phyto-LED had 100% of the peduncle germ in the apex of the stem (Table 3).

4. Discussion

The acceleration of plant growth in response to Phyto-LEDs irradiation has also been shown for some plants, including Latuca sativa [22,23,24].
In this work, we demonstrated how Phyto-LED light and different variations in white-LED light affected plant productivity. We observed a clear pattern of the plant leaf mass increase in response to adding Phyto-LEDs to lighting (Table 1). Complete and partial replacement of white-LED with Phyto-LEDs led to a slight increase in productivity; however, at the same time, it stimulated the formation of peduncle germs (Table 3).
Stomatal conductance and transpiration intensity are related and change with increasing BL/GL ratio [25]. In our study, this pattern was found when growing on two different variants of light 2700K (11%, 49%, 38%, 3%) and 6500K (35%, 42%, 21%, 2%); however, this pattern was disrupted when we compared the white light options 6500 K and 2700 K with the combined lighting options 2700 K + Phyto-LED and 6500 K + Phyto-LED (see Table 2). We suggest that the observed effects may be related to the fact that in addition to the effect of the BL/GL ratio on the stomatal aperture, the effect of a phytochrome-dependent decrease in the stomatal conductance of FRL is superimposed, as was shown in studies with Arabidopsis thaliana [26], Cucumis sativus [27] Nicotiana tabacum [28], and Solanum lycopersicum [29], where there was a decrease in stomatal conductance in response to an increase in FRL. There are studies in which GL was shown to have a stimulating effect on plants. The positive or negative effect of plant productivity in the experiment depended on the percentage of BL in the spectrum, as previously shown [30]. When growing plants with a low GL in the spectrum (10%), there was an increase in the fresh biomass of plants by 57% relative to that of control plants grown in fluorescent light, in which there was a large amount of GL in the spectrum (50%); however, in the complete absence of GL, productivity decreased to 42%, which indicates the multidirectional effect of GL on productivity [31]. Previously, it was shown that an increase in the proportion of GL in the spectrum led to an increase in plant mass, which contradicts the assumption that GL stimulates plant growth to a lesser extent [16]. Over time, there have been more similar studies [1,16,32]. Thus, to date, there is no clear answer to the question of whether it is worth using light with a high GL content.
In this work, when the Phyto-LED spectrum was added to the white spectrum (2700 K, 6500 K), we observed an increase in plant leaf mass (by 48% for the 6500 K + Phyto-LED spectrum and by 30% for the 2700 K 6500 K + Phyto-LED spectrum) in response to a decrease in GL in the spectrum (by 43% for the 6500 K + Phyto-LED and 2700 K + Phyto-LED spectra) and a simultaneous increase in BL, RL and FRL (Table 1, Figure 3). A further decrease in GL in the spectrum with a simultaneous increase in BL, RL, and FRL in the spectrum in the case of the Phyto-LED spectrum did not lead to a significant increase in yield but, at the same time, provoked accelerated flowering (Table 3).
The influence of GL on the growth and root development of L. sativa plants has been studied in many studies, but the influence of GL on root development is still controversial. In some studies, a decrease in GL did not lead to changes in root biomass [31], and an increase in GL in the spectrum to 30% caused an increase in the dry biomass of L. sativa roots [32]. Root mass is not the most important indicator in the industrial cultivation of green crops, especially when using hydroponic cultivation methods, where even a small root system always has access to mineral nutrition in solution. This is consistent with the results obtained in our study, where with the same mass of the root system, there were significant differences in the masses of the aboveground parts of plants (Table 1).
Changes in the light spectra cause morphophysiological reactions in plants [33,34]. In our study, plants grown under conditions with a relatively high GL had a reduced leaf area (Table 3), which is consistent with other studies [35]. There are studies in which the opposite effect of GL on the area of L. sativa leaves was observed [16]. It should be taken into account that BL can disable the action of GL; for example, in the work of Kang and coauthors, the length of leaves significantly decreased in response to the addition of 10% GL only in the absence of BL [36].
It is generally believed that the effect of far-red light on plant growth and development is mediated by the phytochrome system. The molecules of the inactive photoreceptor phytochrome B have an absorption peak in the red spectrum region (660 nm). During the absorption of red quanta, the molecule of inactive phytochrome B is activated and becomes active, with a maximum absorption peak in the region of 730 nm. During the absorption of far-red light quanta, the molecule of active phytochrome B returns to an inactive state with maximum absorption in the red spectrum (660 nm). The ratio of red to far-red in the spectrum leads to a dynamic ratio of the active to inactive forms of phytochrome [37]. The ratio of the active and inactive forms of phytochrome determines the photomorphogenesis of the plant. Activated phytochrome stimulates photomorphogenetic reactions, the inactive form of phytochrome ceases to stimulate photomorphogenetic reactions, and the program of shadow avoidance syndrome is initiated in the plant [38].
The addition of FRL to the spectrum causes the appearance of shadow avoidance syndrome, such as elongation of the petiole and expansion of the leaf blade in lettuce plants. In the work of Legendre, R., and van Iersel (2021), the addition of FRL led to an increase in the leaf area of lettuce plants of 48%, while a decrease in the rate of photosynthesis of Pmax was always 12%, which ultimately led to an increase in the dry biomass of plants of 37%. Similar results were obtained in many other works with the addition of FRL [39,40]. In our study, replacing some of the white LEDs with Phyto-LEDs, in addition to other spectral changes, led to a decrease in the RL/FRL ratio (Figure 3). Probably because of this, we obtained similar results, namely an increase in the leaf area (45% for 6500 K + Phyto-LEDs compared to 6500 K, 26% for 2700 K + Phyto-LEDs compared to 2700 K). At the same time, there was a slight decrease in the rate of photosynthesis, which eventually led to an increase in dry plant biomass of 50% for 6500 K + Phyto-LEDs compared to 6500 K and 26% for 2700 K + Phyto-LEDs compared to 2700 K. The complete replacement of the white spectrum with the Phyto-LEDs spectrum did not affect the photosynthesis rate, area, or dry weight of the leaves.
Shadow avoidance syndrome can be caused by both a decrease in the RL/FRL ratio and a decrease in the BL/GL ratio [41,42] since photomorphogenesis is also coordinated by the cryptochrome system; according to this, scotomorphological reactions such as the expansion of the leaf blade and the elongation of the petiole can also be caused by an increase in green light or a decrease in blue light in the spectrum [3]. In our study, this pattern was not detected; when white LEDs were replaced with Phyto-LEDs, the BL/GL ratio increased (Figure 3), and there was an increase in the effect of shadow avoidance syndrome (an increase in leaf area and stem length). The absence of the effect described by other researchers may be due to the difficulty of studying the effect of polychromatic spectra since, under these conditions, not one wavelength range changes, but several changes occur at once. For example, phytochrome signaling may determine the photomorphogenesis of a plant by triggering shadow avoidance syndrome in response to a decrease in the spectral RL/FRL ratio while silencing the cryptochrome system, which should turn off shadow avoidance syndrome in response to an increase in the spectral BL/GL ratio.
It was previously shown that WUE is positively associated with plant productivity and is associated with stomatal conductance in L. sativa plants [43]. An increase in BL in the spectrum can affect water use efficiency through the regulation of stomatal conductance. At work [44], stomatal conductance was directly proportional to the concentration of BL, and the WUE decreased. In our work, in all the plants treated with Phyto-LEDs, the WUE decreased, which may indicate that the WUE is influenced not only by the amount of BL but also by the amount of GL, RL, and FRL, which demonstrates a complex mechanism for regulating the physiological parameters of plants in response to light spectral characteristics (Table 2).
WUE is an important parameter under drought conditions because it reduces the growth rate of plants [45]. In our work, there appears to be an inverse relationship between this parameter and the accumulation of plant biomass under hydroponic conditions (Table 1 and Table 2). We assume that such a relationship between productivity and WUE may be because the larger size of the stomatal pore or their aperture causes an increase in the transpiration index and ensures the supply of CO2. On the other hand, the transpiration rate under conditions of different spectral compositions is not always associated with CO2 assimilation [46]. An increase in transpiration in plants grown under 2700 K + Phyto-LED, 6500 K + Phyto-LED, and Phyto-LED light relative to those grown under 6500 K and 3200 K indicates that a decrease in the spectrum of GL can increase the intensity of transpiration (Table 2).
It has been reported that with increasing GL, the length of the stem of L. sativa plants increases [35]. In our study, we measured the stem length on day 60 and found that when Phyto-LEDs were added to the spectrum, the stems of the lettuce plants tended to lengthen (Table 3), which may be due to an increase in the FRL and a decrease in the GL. Additionally, the lengthening of the stem is a characteristic sign of the plant’s transition to the flowering stage [47]. This pattern is also indicated by the data obtained in our study on the number of plants that had passed to flower (Table 3).
In our study, we observed a shortening of the growing season in response to the addition of the Phyto-LED spectrum, which may be due to a decrease in the GL-band spectrum and an increase in the far-red light spectrum, as shown in other studies conducted on a group of long-day plants, including Ageratum houstonianum, Antirrhinum majus and A. thaliana [48]. We also assume that flowering could have been triggered by a large amount of vegetative mass, as indicated by other works [6,49].
Given the significant share of energy costs consumed by lighting fixtures, reducing these costs is an important goal. This problem must be considered using data such as the energy consumption of lighting fixtures and the rate of biomass gain of plants grown under different types of lighting. For example, based on the data presented in Figure 3, we can conclude that compared with Phyto-LED and 2700 K + Phyto-LED lighting, 6500 K lighting is the most energy-efficient, saving approximately 25% of the total energy. However, it should be noted that plants grown under Phyto-LED and 2700 K + Phyto-LED lighting had 50% and 40% greater fresh weight, respectively, than those grown under 6500 K lighting (Table 1). The growth rate of plants grown under 6500 K + Phyto-LED lighting was intermediate between that under 6500 K and 2700 K + Phyto-LED lighting, but plants grown under this lighting had the same biomass growth rate as those grown under Phyto-LED lighting. Based on research data on energy consumption and biomass growth rate, the 6500 K + Phyto-LED option appears to be the best in terms of productivity.
Thus, reducing energy costs when lighting plants requires an integrated approach that takes into account not only the efficiency of energy consumption but also the impact on the morphological and physiological characteristics of plants, such as stem length and the speed of transition to flowering.

5. Conclusions

In our study, we investigated the effect of the spectral composition of light on a number of physiological characteristics of lettuce. When the Phyto-LED spectrum was added to the white spectrum, we observed an increase in leaf area and leaf mass, which is a positive effect for industrial cultivation. We also observed an acceleration of plant flowering in the case of adding the Phyto-LED spectrum to the cold white spectrum (66% flowering plants) at 6500 K and using only the Phyto-LED spectrum (100% flowering plants), which has a negative effect on the commercial cultivation of lettuce plants.
By combining white LEDs and Phyto-LEDs, we obtained better light (2700 K light + Phyto-LED) for growing lettuce. The decrease in weight compared to that under full Phyto-LED light was minor (2–3%), but at the same time, 2700 K + Phyto-LED light increased the vegetative period of lettuce. Additionally, the spectrum makes it more comfortable to visually monitor the physiological state of plants, similar to white lights (lights at 6500 K and 2700 K), as demonstrated by the CRI indicators (Figure 4).

Author Contributions

Conceptualization, M.V., P.P. and I.T.; methodology, M.V., P.P. and I.T.; software, M.V., P.P. and I.T.; validation, M.V., P.P. and I.T.; formal analysis, M.V., P.P. and I.T.; investigation, M.V., P.P. and I.T.; resources, M.V., P.P. and I.T.; data curation, M.V., P.P. and I.T.; writing—original draft preparation, M.V., P.P. and I.T.; writing—review and editing, M.V., P.P. and I.T.; visualization, M.V., P.P. and I.T.; supervision, M.V., P.P. and I.T.; project administration, M.V., P.P. and I.T.; funding acquisition, M.V., P.P. and I.T. All authors have read and agreed to the published version of the manuscript.

Funding

I.T. acknowledges the support of the Ministry of Science and Higher Education of the Russian Federation in accordance with agreement № 075-15-2022-317 dated 20 April 2022 on providing a grant in the form of subsidies from the Federal budget of the Russian Federation for state support for the creation and development of a World-class Scientific Center “Agrotechnologies for the Future”. The results in Table 2 were obtained within the state assignment of the Ministry of Science and Higher Education of the Russian Federation, grant numbers 122042700044-6.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

We would like to thank Ivan S. Chuksin for his technical assistance.

Conflicts of Interest

Mikhail Vereshchagin, Pavel Pashkovskiy, and Ivan Tarakanov declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper “Adding Phyto-LED spectrum to white-LED light increases the productivity of lettuce plants”.

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Figure 1. The spectra of light sources were used in the experiment (BL, GL, RL, FRL): 6500 K (35%, 42%, 21%, 2%), 2700 K (11%, 49%, 38%, 3%), 2700 K + Phyto-LED (19%, 28%, 47%, 7%), 6500 K + Phyto-LED (31%, 24%, 39%, 7%), and Phyto-LED (26%, 6%, 56%, 12%).
Figure 1. The spectra of light sources were used in the experiment (BL, GL, RL, FRL): 6500 K (35%, 42%, 21%, 2%), 2700 K (11%, 49%, 38%, 3%), 2700 K + Phyto-LED (19%, 28%, 47%, 7%), 6500 K + Phyto-LED (31%, 24%, 39%, 7%), and Phyto-LED (26%, 6%, 56%, 12%).
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Figure 2. The proportions of blue (BL), green (GL), red (RL), and far-red (FRL) in the total light intensity of the two types of white LEDs (2700 K and 6500 K).
Figure 2. The proportions of blue (BL), green (GL), red (RL), and far-red (FRL) in the total light intensity of the two types of white LEDs (2700 K and 6500 K).
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Figure 3. (A)-Diagrams of the percentage of light spectral regions from the total PAR in the light sources used in the experiments. The lighting options presented in the experiment were as follows. (B)-Diagrams of the percentage of light spectral regions from the total PAR in the light sources used in the experiments. The ratio (RU) was calculated by dividing the number of light quanta of a certain wavelength range in each of the lighting options (for BL 400–500 nm, for GL 500–600 nm, and RL 600–700 nm for FRL 700–800 nm).
Figure 3. (A)-Diagrams of the percentage of light spectral regions from the total PAR in the light sources used in the experiments. The lighting options presented in the experiment were as follows. (B)-Diagrams of the percentage of light spectral regions from the total PAR in the light sources used in the experiments. The ratio (RU) was calculated by dividing the number of light quanta of a certain wavelength range in each of the lighting options (for BL 400–500 nm, for GL 500–600 nm, and RL 600–700 nm for FRL 700–800 nm).
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Figure 4. Visual appearance of plants in experimental installations under light with various spectral compositions (BL, GL, RL, FRL): 1. 6500 K (35%, 42%, 21%, 2%), 2. 2700 K (11%, 49%, 38%, 3%), 3. 2700 K + Phyto-LED (19%, 28%, 47%, 7%), 4. 6500 K + Phyto-LED (31%, 24%, 39%, 7%), 5. Phyto-LED (26%, 6%, 56%, 12%), specifying the color rendering index (CRI).
Figure 4. Visual appearance of plants in experimental installations under light with various spectral compositions (BL, GL, RL, FRL): 1. 6500 K (35%, 42%, 21%, 2%), 2. 2700 K (11%, 49%, 38%, 3%), 3. 2700 K + Phyto-LED (19%, 28%, 47%, 7%), 4. 6500 K + Phyto-LED (31%, 24%, 39%, 7%), 5. Phyto-LED (26%, 6%, 56%, 12%), specifying the color rendering index (CRI).
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Figure 5. The electric power of the LED matrices provides a photon flux density of 170 µmol (photons) m−2 s−1 of the chamber in the phytotron for growing plants at a distance of 55 cm from plants.
Figure 5. The electric power of the LED matrices provides a photon flux density of 170 µmol (photons) m−2 s−1 of the chamber in the phytotron for growing plants at a distance of 55 cm from plants.
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Table 1. The effect of light spectra on the dry (DW of the leaves) and fresh (FW of the leaves and roots) biomass of lettuce plants on the 30th day of the experiment. Different letters indicate significant differences (p < 0.05) between the experimental treatments (n = 18).
Table 1. The effect of light spectra on the dry (DW of the leaves) and fresh (FW of the leaves and roots) biomass of lettuce plants on the 30th day of the experiment. Different letters indicate significant differences (p < 0.05) between the experimental treatments (n = 18).
Light
(BL, GL, RL, FRL)		FW Leaves, gDW Leaves, gFW Roots, gDW Roots, g
6500 K
(35%, 42%, 21%, 2%)		50.2 ± 0.8 d2.2 ± 0.1 c9.42 ± 0.7 a0.325 ± 0.043 a
2700 K
(11%, 49%, 38%, 3%)		54.4 ± 0.9 c2.7 ± 0.2 b8.47 ± 1.24 a0.292 ± 0.035 a
2700 K + Phyto-LED
(19%, 28%, 47%, 7%)		70.5 ± 2.1 b3.4 ± 0.2 a9.59 ± 1.58 a0.310 ± 0.016 a
6500 K + Phyto-LED
(31%, 24%, 39%, 7%)		74.0 ± 1.5 ab3.3 ± 0.2 a8.38 ± 0.75 a0.285 ± 0.028 a
Phyto-LED
(26%, 6%, 56%, 12%)		75.3 ± 2.1 a3.2 ± 0.2 a8.39 ± 1.38 a0.295 ± 0.028 a
Table 2. The effect of light spectra on the transpiration rate (Tr), stomatal conductance (gs), water use efficiency (WUE), net photosynthetic rate (Pn), total chlorophyll concentration (Chl a + b) and maximum quantum yield of PSII (Fv/Fm) in lettuce leaves on the 30th day of the experiment. Different letters indicate significant differences (p < 0.05) between the experimental treatments (n = 18).
Table 2. The effect of light spectra on the transpiration rate (Tr), stomatal conductance (gs), water use efficiency (WUE), net photosynthetic rate (Pn), total chlorophyll concentration (Chl a + b) and maximum quantum yield of PSII (Fv/Fm) in lettuce leaves on the 30th day of the experiment. Different letters indicate significant differences (p < 0.05) between the experimental treatments (n = 18).
Light
(BL, GL, RL, FRL)		Tr,
mmol H2O/m2 s		gs,
mmol/m2 s		WUE,
µmol CO2/mmol H2O		Pn,
µmol CO2/m2 s		Chl a+b,
mg/g FW		Fv/Fm
6500 K
(35%, 42%, 21%, 2%)		1.58 ± 0.15 c0.149 ±0.013 b2.52 ± 0.40 a3.90 ± 0.26 a2.57 ± 0.4 a0.840 ± 0.018 a
2700 K
(11%, 49%, 38%, 3%)		1.75 ± 0.08 c0.122 ±0.007 c2.81 ± 0.18 a4.27 ± 0.50 a2.51 ± 0.3 a0.838 ± 0.014 a
2700 K + Phyto-LED
(19%, 28%, 47%, 7%)		2.39 ± 0.16 b0.184 ±0.007 a1.60 ± 0.05 b3.75 ± 0.26 a2.49 ± 0.3 a0.840 ± 0.022 a
6500 K + Phyto-LED
(31%, 24%, 39%, 7%)		2.7 ± 0.22 ab0.193 ±0.012 a1.64 ± 0.07 b4.1 ± 0.30 a2.54 ± 0.4 a0.827 ± 0.025 a
Phyto-LED
(26%, 6%, 56%, 12%)		2.81 ± 0.21 a0.179 ±0.009 a1.58 ± 0.04 b4.2 ± 0.33 a2.52 ± 0.3 a0.844 ± 0.013 a
Table 3. The effect of different light spectra on the morphometric parameters of lettuce plants on the 30th and 60th days of the experiment. Number of leaves on the 30th and 60th days (per plant), length of the largest leaf on the 30th day, width of the largest leaf on the 30th day, total leaf area of the plant on the 30th day, stem length on the 60th day, and number of flowering plants on 60 days. Different letters indicate significant differences (p < 0.05) between the experimental treatments. The values are the means, n = 18.
Table 3. The effect of different light spectra on the morphometric parameters of lettuce plants on the 30th and 60th days of the experiment. Number of leaves on the 30th and 60th days (per plant), length of the largest leaf on the 30th day, width of the largest leaf on the 30th day, total leaf area of the plant on the 30th day, stem length on the 60th day, and number of flowering plants on 60 days. Different letters indicate significant differences (p < 0.05) between the experimental treatments. The values are the means, n = 18.
Light
(BL, GL, RL, FRL)		Number of Leaves per PlantLength of the Largest Leaf, cmWidth of the Largest Leaf, cmTotal Leaf Area per Plant, cm2Number of Leaves per PlantStem Length, cmFlowering Plants, %
30th Day of Experiment60th Day of Experiment
6500 K
(35%, 42%, 21%, 2%)		13.0 ± 1 ab13.8 ± 1.0 c13.1 ±2 a806 ± 25 c52 ± 1.52 d17.3 ± 1.5 c0
2700 K
(11%, 49%, 38%, 3%)		14.3 ± 0.5 ab14.7 ± 1.6 bc13.4 ± 1 a925 ± 27 b54.0 ± 1.00 c18.3 ± 2.5.c0
2700 K + Phyto-LED
(19%, 28%, 47%, 7%)		15 ± 0.6 ab16.3 ± 1.6 ab15.7 ± 2 a1190 ± 33 a51.3 ± 1.08 d26 ± 1.15 b15 c
6500 K + Phyto-LED
(31%, 24%, 39%, 7%)		14 ± 0.8 ab17.6 ± 1.3 ab16.0 ± 2.8 a1180 ± 30 a63.3 ± 2.51 a24.3 ± 1.5 b60 b
Phyto-LED
(26%, 6%, 56%, 12%)		15.8 ± 0.5 a18.2 ± 1.8 a17.0 ± 2.4 a1210 ± 28 a59.3 ± 1.15 b28 ± 1 a100 a
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MDPI and ACS Style

Vereshchagin, M.; Pashkovskiy, P.; Tarakanov, I. Adding Phyto-LED Spectrum to White-LED Light Increases the Productivity of Lettuce Plants. Horticulturae 2024, 10, 795. https://doi.org/10.3390/horticulturae10080795

AMA Style

Vereshchagin M, Pashkovskiy P, Tarakanov I. Adding Phyto-LED Spectrum to White-LED Light Increases the Productivity of Lettuce Plants. Horticulturae. 2024; 10(8):795. https://doi.org/10.3390/horticulturae10080795

Chicago/Turabian Style

Vereshchagin, Mikhail, Pavel Pashkovskiy, and Ivan Tarakanov. 2024. "Adding Phyto-LED Spectrum to White-LED Light Increases the Productivity of Lettuce Plants" Horticulturae 10, no. 8: 795. https://doi.org/10.3390/horticulturae10080795

APA Style

Vereshchagin, M., Pashkovskiy, P., & Tarakanov, I. (2024). Adding Phyto-LED Spectrum to White-LED Light Increases the Productivity of Lettuce Plants. Horticulturae, 10(8), 795. https://doi.org/10.3390/horticulturae10080795

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