INTRODUCTION

The radiation which electric field vector is oscillating in an ordered manner is called polarized radiation. Natural sunlight is usually not polarized, but at certain hours during the day, due to various phenomena, it can become polarized. The greatest amount of polarized radiation comes from the sky due to the scattering. It is well known that in the direction perpendicular to the sun radiation the scattered light has the highest polarization degree, which in the absence of clouds can reach 90%. Thus, the time when polarized light comes from the sky zenith point is the morning and evening hours, when the sun is low above the horizon. Thus, plants in addition to non-polarized light daily receive a portion of polarized light since long times ago. Given that nature always uses all the opportunities provided, plant organisms could probably adapt to use polarized radiation. In [1, 2] it was established that when linearly polarized laser radiation passes through the monocotyledons leaves, the radiation ellipticity changes. We assume the change is due to the light interaction with the leaf epidermis layer. The epidermis cells in monocotyledonous plants have a near rectangular shape and are ordered as a two-dimensional lattice. As established by Anderson [3] rapeseed (Brassica napus L.) plants grew better under left-handed circular polarization than under linear polarization. Consequently, it is possible that light with a greater polarization ellipticity is of considerable importance for plants. In that case, the transformation of linearly polarized radiation into elliptically polarized radiation by plants with optically anisotropic leaf epidermis could be one of the mechanisms they use linearly polarized light.

While much is known about the effect of light intensity and wavelength on plants [4], there exists only a small number of works where the polarized light impact on plants is studied. Mostly these works describe circularly polarized light impact on plant growth. It was established [5] that germination rates of lettuce (Lactuca sativa) and Arabidopsis (Arabidopsis thaliana) seeds were higher when grown under red light with right-handed circular polarization, while under white light the plants had higher mass values. Hypocotyl elongation was faster in red light with left-handed circular polarization. In [6] authors found that the lentil (Lens culinaris) and pea (Lathyrus oleraceus) plants develop better under left-handed light. In work [3] author found that the rapeseed plants develop better under left-hand circular polarization. In works, the authors proceeded from the idea of biomolecules homochirality in nature, which is common for proteins and sugars. This means that there is only one of two possible symmetrical mirror versions of their structure. Such a structure could also be common for plants pigments molecules which are responsible for the light energy absorption and play an important role in the photosynthesis. Light-sensitive parts of photoreceptors regulating plant morphogenesis also can have chiral structure. Thus, they can be characterized by circular dichroism [7] meaning the different absorption of left- and right-handed circular polarized light. Consequently, the plants development depending on the radiation polarization state proceeds in different ways, which was demonstrated in works.

The positive impact of polarized laser radiation on various plants seeds and crops was described by many authors [8–15]. Nevertheless, the stimulating mechanism of light radiation remains unclear. Macht [16] conducted the first studies of plant growth under the linearly polarized light at the beginning of the last century. It was shown that wheat, squash, sunflowers, and lupine plants grow better when they are irradiated with linearly polarized light compared to non-polarized light, but the experimental setups then used were far from perfect. Since then, this issue hardly been studied. In [3] author demonstrated that rapeseed plants develop worse under linearly polarized light than under unpolarized light and light with left circular polarization. In work [17] authors observed polarotropism in Vaucheria algae, which revealed itself several days after irradiation. In [18] it was established that the cells of moss grow in the direction perpendicular to the electric field vector.

The purpose of our work was to study the effect of polarized light radiation on plant growth. The other purpose was to examine the assumption that linearly polarized light passed through the leaves cells transforms into elliptically polarized and more effectively interacts with the photosynthetic structures of cells under the epidermis layer. This interaction could positively affect the monocotyledons development.

MATERIALS AND METHODS

Growth Conditions

In our study, warm white LEDs (200 μmol m–2 s–1) were used for plants cultivation. Two treatments for each maize variety were: unpolarized (сontrol) light and linearly polarized light. Polarized irradiation was generated by filtering the LED light through linearly polarizing filters.

To analyze the radiation polarization state the polarimeter (PAX5710 Thorlabs, United States) was used. The results of polarization measurements were processed using the TXP Series Instrumentation software (Starter, Server Control, TPX Polarimeter, United States).

The degree of polarization (DOP) was used as main parameter in the analysis of the polarization state. The DOP equation is given below.

$${\text{DOP}} = \frac{{{{I}_{{{\text{pol}}}}}}}{{{{I}_{{{\text{pol}}}}} + {{I}_{{{\text{unp}}}}}}}.$$
(1)

For absolutely polarized light DOP is 100%, and for absolutely unpolarized light, it is 0%.

The maximum quantum efficiency of photosystem II (Fv/Fm) is a widely used indicator of photosynthetic health in plants [19]. The chlorophyll fluorescence parameter Fv/Fm in maize leaves was estimated using the Hansatech FMS 1+ pulsed fluorimeter. The modulated light source was light with the 594 nm wavelength, the source for actinic light and saturating flashes was light with a maximum intensity of photosynthetically active radiation 4.000 µmol m–2 s–1 for actinic light, and 13.000 µmol m–2 s–1 for saturating flashes.

Plant Material

We chose maize (Zea mays L.) for our study according to our previous works [1, 2] and if the polarized light use by monocotyledons is more efficient rather than by dicotyledons. We used three maize cv.: Ranniya Lakomka 121 (RL), Kubanskiy Sakharnyi 210 (KS), and Zolotoy Batam (ZB). Different varieties were used to assess the probable species-specific response to polarized light treatment. Each treatment included seven plants.

The plant samples preparation was following: the seeds were soaked in distilled water for 24 h. Than the germinated seeds were sown in pots (W 9 cm × H 10 cm, Sady Primor’ya, Ussuriysk, Russia) that were filled with the commercial universal growing substrate (N1 : P1 : K1, mg/L: 160–240 : 145–215 : 180–290, organic matter, mg/L: 35, pH 5.5–7, Terra Master, Novosibirsk, Russia). The plants were cultivated for 21 days and in phytoboxes under warm white LEDs (200 μmol m–2 s–1). The relative humidity of the chamber was set at 70 ± 5% and a temperature of 24 ± 2°C. Irrigation was provided one time in 3 days.

GROWTH AND DEVELOPMENT CHARACTERISTICS

To assess polarized radiation impact on plants, following morphometric and weight characteristics were estimated on the 21st day: the number of leaves; leaf length, width and area; plants height, stem height and diameter, fresh and dry shoots and roots weight, and dry matter content. Dry matter content percentage (C) was estimated using the equation:

$${\text{C}} = \frac{{{\text{Wd}}}}{{{\text{Wf}}}} \times {\text{100}}{\text{,}}$$
(2)

where Wd is dry shoot (or root) weight, Wf is fresh shoot (or root) weight.

The results were statistically processed using Microsoft Office (Excel) package. Diagrams were constructed using mean values of the examined parameters with their standard errors.

RESULTS

The morphometric and photosynthetic parameters of maize grown under polarized and unpolarized light on the 21st day shown below (Figs. 1, 2).

Fig. 1.
figure 1

Maize leaves morphometric characteristics (mean values): leaves number (a), total leaf area (b), leaf width (c), and leaf length (d).

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Fig. 2.
figure 2

Maize plants morphometric and photosynthetic characteristics: plant height (a), stem height (b), stem diameter (c), and Fv/Fm parameter (d).

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The maize plants of all varieties developed better under polarized light and have higher mean height and leaves number, stem length and diameter. The differences between control and polarized light treatments were following: RL—14.9% in height and 9.2% in leaves number, KS—6.8 and 8.6%, and ZB—4.8 and 4.7%; RL—12.4% in stem length and 10.2% in diameter, KS—3.2 and 16.0%, and ZB—3.0 and 6.7%.

Total leaf area of all varieties was higher in polarized light treatments. The differences between control and polarized light treatments were following: RL—28.7%, KS—15.5%, and ZB—14.0%.

The weight characteristics of maize grown under polarized and unpolarized light on the 21st day shown below (Figs. 3, 4).

Fig. 3.
figure 3

Maize weight characteristics: fresh shoots (a) and roots (b) weight, and dry shoots (c) and roots (d) weight.

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Fig. 4.
figure 4

Maize weight characteristics: total fresh (a) and dry (b) weight, and dry matter content of shoots (c) and roots (d).

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The dry and fresh weight of shoots and roots were highest under polarized light treatment. The differences in shoots fresh and dry weight between control and polarized treatments were following: RL—37.2 and 35.7%, KS—22.1 and 20.5%, and ZB—17.8 and 17.8%. The differences in roots fresh and dry weight between control and polarized treatments were RL—37.2 and 35.7%, KS—22.1 and 20.5%, and ZB—17.8 and 17.8%. The differences in total fresh and dry weight between control and polarized light treatments for RL variety were 31 and 29%, for KS 26 and 24%, and for ZB 11 and 16%, respectively.

The values of dry matter content show no significant differences between control and polarized treatments.

The results obtained demonstrated that the plants grown under polarized light did not have any distinguishing features externally, except for higher morphometric parameters and weight characteristics compared to plants grown under control light. The fluorescence parameter Fv/Fm (Fig. 2d) of maize leaves grown under polarized and unpolarized light on the 21st day showed that the intensity of photosynthesis was slightly higher in leaves of all three maize varieties grown under polarized light. The Fv/Fm values were 0.79–0.799 for the control and 0.8–0.824 for polarized treatment. The highest difference between control and polarized treatments was in ZB, the lowest in RL.

CONCLUSIONS

The results obtained established that under linearly polarized light maize of all three varieties used developed better in comparison with non-polarized light treatment. The plants grown under polarized light had higher height, dry and fresh weight in comparison with the control treatment. In addition, plants of polarized light treatment had greater leaves number and size.

For the present, there is no data in literature about the linearly polarized light impact on the maize development. Moreover, very few works study the linearly polarized light impact on other crops. In [16] Macht stated that polarized light has a positive effect on plant growth. It was established that when growing one day under polarized light and another one under non-polarized for 4 days, wheat, squash, sunflower, and lupine plants grow faster after polarized light treatment. In our work, we cultivated plants for 21 days, and did not alternate between polarized and non-polarized light. Nevertheless, maize plants under polarized light also grew faster than control samples, which correlates with the conclusions of work [16] about the positive effect of polarized light.

In addition, Macht [16] stated that wheat cultivated under polarized light had higher total weight. In our study, maize grown under polarized light also had higher values of morphometric parameters in comparison with the control treatment. The main part of total weight was shooting weight. Despite relatively high difference between the KSC and KSP treatments roots weight, there was no high differences between roots weight of the other treatments. Thus, it can be assumed that polarized light leads to increase in shoots weight more than in mass of roots.

The results of Fv/Fm parameter measuring also demonstrated the advantage of plants developed under polarized light. The data obtained established the higher photosynthetic activity of chlorophyll in the leaves of plants grown under polarized light. This should lead to the formation of the larger organic matter amount. In our experiment, as morphometric data show, there was just such an increase in the amount of organic matter observed. The Fv/Fm parameter values obtained were within the normal range and amounted to 0.79–0.799 for the control treatments and 0.8–0.824 for polarized light treatments. In works [20–23] Fv/Fm parameter under normal conditions was in the range of 0.78–0.81. Consequently, the results also demonstrate that polarized radiation might have statistically significant impact on the PSII maximum efficiency.

In work [24] it was shown that irradiation of plants with polarized light alone, without alternation with unpolarized light, leads to a decrease in their starch content, negative polarotropism, leaf shedding and whole plant aging. Normal plants development requires alternation of polarized and unpolarized light, as it occurs in nature on the bright days. We established those plants grown under constant irradiation with polarized light for 21 days developed better than the control ones, which means polarized light might affect different crops differently.

In plants light energy is absorbed by pigments, and the main pigment is chlorophyll. The features of light propagation in plant leaves depend on the long-range ordering in chloroplasts and spectral characteristics of pigments [25]. Such auxiliary pigments as carotenoids, flavonoids, etc. have different absorption peaks, thus, plants can use the radiation energy of almost the entire visible range. The regulation of plant development by light signals occurs by registering them with photoreceptors. Photoreceptors consist of apoprotein and chromophore, which is attached to it and functioning as antenna. When light is absorbed by a chromophore, structural changes occur in the photoreceptor. These changes lead to a series of signal transduction events, which result in gene expression changes [1]. Photoreceptors are also represented by various classes, allowing plants to sense light in a wide range of wavelengths. And here an important point for the following discussion is that all photosensitive structures can contain chiral chromophores that absorb light with left- and right-handed circular polarization in different ways [26].

In work [3] established that rapeseed plants developed better under circularly polarized light rather than linearly polarized light. Maize belongs to monocotyledonous plants with parallel leaves venation, the epidermis layer cells have a rectangular shape and ordered in the form of a two-dimensional lattice. Previously, authors in works [1, 2] established that the epidermis of plant leaves can change the state of linearly polarized radiation into elliptically polarized. In addition, in work [2] was shown that linearly polarized light passed through epidermal cells changes the light ellipticity depending on the rotation angle of the lattice axis relative to the predominant direction of intensity vector oscillation. The light could transform into elliptically polarized. Thus, it might lead to more efficient light absorption by chiral molecules of pigments and photoreceptors. We suggest that this is the one of the possible mechanisms leading to higher morphometric and photosynthetic parameters of maize grown under polarized light compared to those grown under non-polarized light.

The results obtained showed that monocotyledonous maize plants cv. Ranniya Lakomka 121, Kubanskiy Sakharnyi 210, and Zolotoy Batam grown under linearly polarized light developed better than under non-polarized light. Differences in morphometry and weight between polarized and unpolarized treatments on the 21st day were following: 5–15% in height, 11–31% in total fresh weight, 16–29% in total dry weight, depending on the variety. The dry matter content didn’t change significantly. The effect of polarized light was greatest on cv. RL and least on cv. ZB, which indicates the species-specific plant response to polarized light. PSII maximum efficiency assessed by measuring the Fv/Fm parameter demonstrated that the photosynthesis intensity was higher under polarized light treatments.

We suggested the effect of polarized light on maize characteristics could be explained by the following: maize leaves have ordered structure of the cells epidermal layer forming a two-dimensional lattice, which has the refractive index anisotropy, and thus when passing through this layer light becomes elliptically polarized and, presumably, interacts more efficiently with photosynthetic cells of the underlying tissues.

In our work, we established that maize plants could grow successfully irradiated with polarized light only. Moreover, polarized light in comparison with unpolarized light could efficiently increase maize morphometric parameters. Thus, light polarization should be considered when growing certain crops under artificial lighting. The mechanism of this phenomenon is still not completely clear. Nevertheless, significant polarized light impact on maize confirms proposed theory about the affinity between the epidermal cells structure and the linearly polarized light efficiency.