Water shortage reduces silicon uptake in barley leaves

Agricultural Water Management 217 (2019) 47–56

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Water shortage reduces silicon uptake in barley leaves Mateja Grašič , Maja Dobravc, Aleksandra Golob, Katarina Vogel-Mikuš, Alenka Gaberščik ⁎

T

Department of Biology, Biotechnical Faculty, University of Ljubljana, Večna pot 111, SI-1000, Ljubljana, Slovenia

ARTICLE INFO

ABSTRACT

Keywords: Hordeum vulgare L. Water availability UV radiation Element analysis Accumulation

Silicon is an increasingly important element in agriculture due to benefits on plant growth and development under stress conditions. Silicon uptake is facilitated by transpiration flow, while plant resistance to drought and ultraviolet radiation are positively related. Thus, we hypothesised that water shortage in barley (Hordeum vulgare L.) will hinder uptake of silicon, and possibly other elements, and that reduced ambient ultraviolet radiation will worsen these negative effects of water shortage. Barley plants were exposed to favourable and reduced water availability during growth under ambient and reduced ultraviolet exposure. Element composition and morphological, biochemical, physiological, and optical traits of barley leaves growing under these four treatments were investigated. Water shortage affected the element composition of barley plants significantly. Silicon and chlorine levels were the most reduced by water shortage, followed by calcium, phosphorus, and sulphur, while potassium levels were not affected. Ultraviolet radiation did not have any significant effects on uptake of elements. These plants did not undergo water shortage stress, as photochemical efficiency of photosystem II and pigment contents were similar across all treatments. Water shortage affected reflectance of light across the whole spectrum, while ultraviolet radiation affected optical properties of the barley leaves in the UV region only.

1. Introduction Biomineralisation is the process by which minerals are loaded into living organisms (Skinner and Jahren, 2003). This occurs in most plant tissues and across many plant species (Webb, 1999). Both silicon (Si) and calcium (Ca) can form biomineral structures in plants (He et al., 2014). Si-rich species usually have low Ca concentrations, and vice versa (Ma and Takahashi, 2002; Golob et al., 2018). In grasses, Si usually represents up to 4% of dry weight (Lewin and Reimann, 1969), while Ca levels can vary significantly across species (Klančnik et al., 2014a, 2014b). Silicon is one of the most abundant elements in the soil, where it is mainly in the form of silicic acid (Epstein, 1994). Si levels are comparable to those of many important nutrients, such as potassium (K) and Ca, and they even greatly exceed phosphate levels (Epstein and Bloom, 2005). Hydrated silicic acid is taken up via plant roots, while Si is transferred in its unpolymerised form across the plant by the apoplastic pathway through the xylem (Raven, 1983; Epstein, 1994). This is driven by the transpiration flow (Ma and Takahashi, 2002; Faisal et al., 2012), which is under active transport (Liang et al., 2006). Silicon is deposited in plants in the form of solid amorphous Si dioxide (SiO2 × nH2O; silica) (Marschner et al., 1990; Epstein, 1994). The extent of silica deposition depends on the mechanism of Si uptake,

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which varies across species (Ma and Takahashi, 2002; Cooke and Leishman, 2011). Silica is usually deposited in or near the epidermis (Yoshida et al., 1962a; Prychid et al., 2004). In grasses, Si is deposited in long and short epidermal cells, bulliform cells, guard cells, and prickle hairs, in the form of phytoliths (Kaufman et al., 1985). Once it is deposited in the target cells, its relocation is no longer possible (Raven, 1983; Epstein, 1994). Silicon is considered to be a beneficial element for plants because it is important for successful growth of all plants (Tripathi et al., 2016). Its positive influence on growth was reported for a wide variety of crops, including rice, wheat, barley, and cucumber (Ma et al., 2001). The beneficial effects of Si on plants are based on a protective layer of silica that is deposited at the leaf surface, the reactivity of the absorbed Si with metal ions and other compounds, and the metabolic functions of Si in stressed plants (Tubaña and Heckman, 2015). Silicification of leaf tissues prevents herbivory and pathogen infections, and increases the rigidity of plant shoots (Ma and Takahashi, 2002). In grasses, it is a key structural element, as it prevents lodging and shading of leaves (Ma et al., 2001; Ma and Takahashi, 2002). Moreover, Hosseini et al. (2017) showed that Si accumulation in shoots delays osmotic-stress-induced leaf senescence in barley. Studies have also investigated the importance of Si in increased plant tolerance to enhanced ultraviolet (UV) radiation levels (Yao et al., 2011; Shen et al., 2014b; Chen et al., 2016b).

Corresponding author. E-mail address: [email protected] (M. Grašič).

https://doi.org/10.1016/j.agwat.2019.02.030 Received 25 May 2018; Received in revised form 14 February 2019; Accepted 16 February 2019 0378-3774/ © 2019 Elsevier B.V. All rights reserved.

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Silicified epidermal structures have been shown to reduce the need for production of energy-demanding protective phenolic UV-absorbing substances (Schaller et al., 2012, 2013), although this has not been supported by all studies (Klančnik et al., 2014a; Golob et al., 2017). Silicon also mitigates the negative effects of high temperatures on plants, through maintenance of the integrity of cell membranes and increasing the levels of heat-shock proteins and antioxidants (Agarie et al., 1998; Soundararajan et al., 2014). Moreover, Si has an important role in regulation of the nutrient balance in plant tissues and protects plants against toxic metals (Tripathi et al., 2016). The presence of Si and Ca structures in leaves also affects the distribution of light, which can be beneficial if this increases the availability of energy for the lower tissues and prevents photoinhibition in the upper mesophyll (Setoguchi et al., 1989). Multiple benefits of Si fertilisation for cereals are summarised in Artyszak (2018). Studies have also described various Si deficiency symptoms in plants. These include lower shoot dry weight and yield, occurrence of necroses and chloroses, wilting of leaves, susceptibility to various diseases, disturbed development of generative organs and fruits, and accelerated senescence processes (Ma and Takahashi, 2002). Most soils are rich in Si, clayey soils containing from 200 g Si/kg to 300 g Si/kg, and sandy soils 450 g Si/kg (Meena et al., 2014). Unlike many other elements, accumulation of Si in plants largely depends on environmental conditions, such as water availability and the amount of plant-available Si in the soil (Dietrich et al., 2003). Plant-available Si levels in the soil depend on soil type. Generally, young mineral soils contain more Si than weathered acidic soils (Farooq and Dietz, 2015). In spite of the high Si levels in soils, crops such as wheat, barley, soybean, and sugar beet will likely require Si application in the future to maintain maximum yields (Haynes, 2014), since centuries of continuous crop harvesting has resulted in depleted soil pools of bioavailable Si (Vandevenne et al., 2011). For example, more than 40% of the total agricultural land soil in China is Si-deficient (Zhu and Gong, 2014). In recent decades, extreme droughts have been one of the main factors for reduction of cereals production (Farooq et al., 2009; Lesk et al., 2016). One of the reasons for this might also be related to reduced Si uptake. Si accumulation is also affected by UV radiation levels. Golob et al. (2018) reported a decreased Si uptake in hybrid buckwheat under ambient UV radiation, while a similar study with a Si accumulator crop wheat revealed the opposite (Golob et al., 2017). Such UV effects are in line with some recent studies, which have revealed that ambient UV radiation is not only a plant stressor, but also an important regulator of plant growth and development (Björn, 2015). Indeed, plant resistance to drought has been shown to be positively related to plant resistance to UV radiation, and vice versa (Alexieva et al., 2001). Over recent decades, our environment has been changing at a global level. Periods without rain are getting longer, which can result in severe droughts. On the other hand, UV levels are variable, due to ozone depletion (Chipperfield et al., 2015). For crop plants, UV levels can be greatly reduced through different management practices, such as their growth under artificial light and in greenhouses, or while protecting them against hail and herbivores using different covers (Wargent and Jordan, 2013). With its benefits for plant growth and development, Si has become an increasingly important element in agriculture (Cooke and DeGabriel, 2016; Tripathi et al., 2016). Seven out of the 10 most abundant crops in the world are Si accumulators, as their Si tissue content is ˃1% (Guntzer et al., 2012). These also include barley (Hordeum vulgare L.) (Ma and Takahashi, 2002), which is one of the oldest crops, and has been grown for multiple purposes (Ullrich, 2011; Bonafaccia et al., 2016). In the present study, we aimed to analyse the effects of reduced water availability and reduced ambient UV radiation levels on accumulation of Si and some other elements in barley. As Si uptake is facilitated by transpiration flow, we hypothesised that water shortage will hinder the uptake of Si, and possibly of other elements, while

reduced ambient UV radiation will worsen any negative effects of such water shortage. In addition, we examined the reflectance and transmittance spectra of barley leaves, as the presence of Si (and also Ca) can alter the optical properties and energy balance of leaves (Klančnik et al., 2014a, 2014b). 2. Material and methods 2.1. Experimental design Winter barley (Hordeum vulgare L.) cv. Bingo was chosen as the test species for the present study. Barley seeds were sown on 19 October, 2016, in 20 plastic pots (44 cm × 44 cm × 36 cm; 64 seeds/pot) that contained soil from the Ljubljana Botanical Garden (Ljubljana, Slovenia), which was well-stirred and sifted before the experiment was set to achieve as homogeneous initial soil element composition as possible. These pots were positioned in an outdoor research plot (Biotechnical Faculty, University of Ljubljana; 46°03′03.9′' N, 14°28′14.4′' E). By the end of winter 2016–2017, an average of 22 barley plants/pot remained viable. On 4 May, 2017, the pots were positioned under two types of panels for the two treatments: the first was transparent to UV and visible radiation (transmission wavelengths ≥290 nm; UV+); the second was transparent to the visible region of the spectrum only (transmitting wavelengths > 380 nm, with UV radiation reduced to ∼10%; UV–). Until the first barley plants entered the flowering stage, all of these plants were well-watered. On 23 May, 2017, the soil moisture levels were determined, and from 24 May, 2017, water shortage (W–) was simulated for half of the UV+ and half of the UV– pots. These provided the four different treatments (five pots/ treatment): W+UV+ (well-watered, ambient UV; control), W+UV–, W–UV+, and W–UV–. The W+ plants were watered every 2 days or 3 days until the end of the experimental period, and the W– plants received water five times during the experimental period. Overall, the W + plants received nearly 5-fold the water of the W– plants. To maintain these soil moisture levels as required, transparent protective foil was positioned on top of the panels during any precipitation. After 1 week (30/31 May, 2017) and 2 weeks (8 June, 2017), the leaf physiological parameters and soil moisture levels were determined. For the physiological analyses, leaves from five barley plants/pot (subsamples) were used. UV-B radiation and photosynthetically active radiation (PAR) were recorded several times during these 2 weeks (23 May to 8 June, 2017). After 3 weeks, leaves from four barley plants/pot (subsamples) were analysed for their morphological, biochemical, and optical properties. All of these analyses were carried out on the second youngest fully developed barley leaves. Ultimately, all remaining vital barley leaves of different ages were harvested for element analysis. Soil samples were collected from each experimental pot for soil structure analysis and element analysis after the required plant material had been obtained. These soil samples were air-dried in separate unsealed paper bags for ∼45 days and were later sifted through a 0.5 mm × 0.5 mm mesh sieve for the soil element analysis and the analysis of plant-available Si in the soil. For the soil structure analysis, these soil samples were combined for each experimental treatment. 2.2. Leaf analyses 2.2.1. Morphological properties The analysis of leaf morphology was carried out on transverse sections of vital, fully developed barley leaves. The measurements included thickness of leaf and mesophyll (magnification, 100×), thickness of epidermis and cuticle, and length and density of leaf stomata and prickle hairs (magnification, 400×). The leaves were held between two pieces of expanded polystyrene while cutting. Slices (∼20 μm thick) were cut with a razorblade in the central part of the leaf, to the left or right of the main leaf vein. All of the measurements were performed on the central part of the leaves, under light microscopy (CX41; 48

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Olympus, Tokyo, Japan) equipped with a digital camera (XC30; Olympus) and the CellSens software (Olympus). Specific leaf area was calculated and is expressed as leaf area per unit of dry matter (cm2/mg DM).

assurance for the element analysis was performed using standard reference materials: NIST SRM 1573a (tomato leaves as a homogenised powder) in the form of pressed pellets. 2.3. Soil analyses

2.2.2. Biochemical properties The chlorophyll a, chlorophyll b, and carotenoid contents were determined on leaf extracts according to Lichtenthaler and Buschmann (2001a, 2001b), with absorbance measured at 470 nm, 645 nm, and 662 nm using a UV/VIS spectrometer (Lambda 25; Perkin-Elmer, Norwalk, CT, USA). The anthocyanin content was determined on leaf extracts as described by Drumm and Mohr (1978), with absorbance measured at 530 nm. Total methanol-soluble UV-B–absorbing and UVA–absorbing compounds were also extracted from fresh plant material, according to Caldwell (1968), with absorbance measured from 280 nm to 319 nm, and 320 nm to 400 nm, respectively. The extinction values were integrated for each UV region. The biochemical parameters are expressed per leaf area.

2.3.1. Analysis of plant-available Si in the soil Plant-available Si in the soil was extracted from ∼300 mg of dried and powdered soil samples using 0.01 M CaCl2 according to Korndörfer et al. (1999). The samples were further processed using a commercially available kit (Heteropoly Blue Method (1.600 mg/L SiO2; method number, 8186); HACH LANGE GmbH, Düsseldorf, Germany), with absorbance measured at 815 nm using a spectrophotometer (DR 3900; HACH LANGE GmbH, Düsseldorf, Germany). The results are expressed as mg Si per kg of soil dry matter (mg Si/kg soil DM). 2.3.2. Bulk element analysis Total soil Si and Ca levels were measured from ∼300 mg of dried and powdered soil samples, following the methodology already described in section 2.2.5, using the Peduzo T02 X-ray spectrometer equipped with an Rh tube and SDD with a 12-μm-thick beryllium window (Amptek, Inc.; Bedford, MA, USA).

2.2.3. Physiological properties Chlorophyll fluorescence was measured using a portable chlorophyll fluorometer (PAM-2100; Heinz Walz GmbH, Effeltrich, Bavaria, Germany). The potential and effective photochemical efficiency of photosystem (PS) II were evaluated according to Schreiber et al. (1996). Stomatal conductance was measured using a steady-state leaf porometer (Decagon Devices, Inc., Pullman, WA, USA), which measured the rate of water vapour diffusion via the leaf surfaces. All of the leaf physiological parameters were measured in situ.

2.3.3. Soil structure analysis The samples were analysed by the Infrastructural Centre for Pedology and Environmental Protection (Department of Agronomy, Biotechnical Faculty, University of Ljubljana). The soil parameters investigated were soil texture (%), pH in CaCl2, available P and K (mg/ 100 g; measured in the form of P2O5 and K2O, respectively), electrical conductivity (mS/cm), carbonates (%), C (%), N (%), C/N ratio, total C (%), organic matter (%), cation exchange capacity (measured as mmol charge/100 g; i.e., mmolc/100 g), Ca2+ (mmolc/100 g), Mg2+ (mmolc/ 100 g), K+ (mmolc/100 g), Na+ (mmolc/100 g), total exchangeable bases (mmolc/100 g), and base saturation (%). The data are presented as means ± SD of the composite samples from the four treatments.

2.2.4. Optical properties The optical properties were determined in the laboratory on vital, fully developed barley leaves on the day that they were cut. The reflectance spectra were measured from 290 nm to 880 nm, and the transmittance spectra from 290 nm to 800 nm, at a resolution of ∼1.3 nm, using a portable spectrometer (Jaz Modular Optical Sensing Suite; Ocean Optics, Inc., Dunedin, FL, USA; grating, #2; slit size, 25 μm) with an optical fibre (QP600-1-SR-BX; Ocean Optics, Inc.) and an integrating sphere (ISP-30-6-R; Ocean Optics, Inc.). The leaf reflectance spectra were measured for the adaxial leaf surface by illumination with a UV/VIS-near infrared (NIR) light source (DH-2000; Ocean Optics, Inc.). The spectrometer was calibrated to 100% reflectance using a white reference panel with > 99% diffuse reflectance (Spectralon; Labsphere, North Sutton, NH, USA). The leaf transmittance spectra were measured for the abaxial leaf surface by illumination of the adaxial surface with the light source. The spectrometer was calibrated to 100% transmittance with a light beam that passed directly into the interior of the integrating sphere.

2.4. Environmental conditions Soil moisture levels were measured in the morning before watering for five specified equally distant spots per pot, around and in the middle of the barley plants, using a moisture probe meter (MPM-160-B; ICT International Pty Ltd, Armidale, NSW, Australia). UV-B radiation and PAR measurements were carried out at 12:00 h for five specified equally distant spots per treatment, directly above the barley plants. UV-B radiation and air temperature measurements were carried out using a radiometer and a UV-B radiometric sensor (RM-22; Opsytec Dr. Gröbel GmbH, Ettlingen, Baden-Württemberg, Germany). PAR measurements were carried out with a data logger (LI-1000; LI-COR, Inc., Lincoln, NE, USA) and a quantum sensor (LI-190SA; LI-COR, Inc.). Soil temperature was recorded once per hour over the experimental period (23 May to 23 June, 2017) using water temperature data loggers (UTBI001 TidbiT v2; Onset Computer Corporation, Bourne, MA, USA). The sensors were buried ∼3 cm deep between the barley roots, as close as possible to the middle of each of two selected pots per treatment, with the data taken as the means for each treatment. The resulting data were transferred to the computer via a USB connector, using a U-4 HOBO Optic USB Base Station and a 2-D Coupler (Onset Computer Corporation).

2.2.5. Bulk element analysis The contents of Si, P, sulphur (S), chlorine (Cl), K, and Ca in the barley leaves were determined using X-ray fluorescence spectrometry. From 100 mg to 500 mg of dried and powdered leaves was pressed into pellets using a pellet die and a hydraulic press. 55Fe (25 mCi; Isotope Products Laboratories, Valencia, PA, USA) was used as the primary excitation source for the analysis. The fluorescence radiation emitted was collected by a Si drift diode (SDD) detector (Amptek, Inc.; Bedford, MA, USA) with a 12-μm-thick beryllium window. The energy resolution of the spectrometer at count rates < 1000 cps was 140 eV at 5.9 keV. The X-ray fluorescence spectrometry analysis was performed under vacuum and the samples were irradiated for 5000 s to obtain spectra with sufficient statistics (Nečemer et al., 2008). The analysis of the Xray spectra was performed using an iterative least-squares programme, as included in the quantitative X-ray analysis system software package (Vekemans et al., 1994). Element quantification from the measured spectra was performed using quantitative analysis of environmental samples based on fundamental parameters (Kump et al., 2011). Quality

2.5. Statistical analysis Normal distributions of the data were evaluated using Shapiro-Wilk tests. Homogeneity of variance from the means was analysed using Levene’s tests. One-way analysis of variance (ANOVA) according to Duncanʼs post-hoc multiple range tests was used to assess differences between the four treatments for each measured parameter. Factorial 49

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ANOVA was carried out for the measured parameters to test the significance of water availability (W) and UV radiation (UV) alone, and their interaction (W × UV). To investigate the relationships between the selected leaf traits and environmental conditions or soil parameters, Pearson’s correlation analysis was performed. IBM SPSS statistics 25.0 was used for these statistical calculations, with significance accepted at p ≤ 0.05. The Figure for leaf element contents in barley leaves under the different treatments was drawn in Microsoft Excel 2016. Detrended correspondence analysis was used for the exploratory data analysis using the CANOCO for Windows 4.5 programme package. Due to the gradient lengths obtained (< 3 SD) (ter Braak and Šmilauer, 2002), redundancy analysis was used to determine whether the variations in leaf element contents were related to water availability indicators. The significance of the effects of the variables was determined using Monte

Carlo tests with 999 permutations. Forward selection of the explanatory variables was used to avoid co-linearity. All of the variables used in the analysis were standardised. 3. Results 3.1. Leaf traits and environmental conditions Water availability (W+/W–) and UV radiation (UV+/UV–) affected many of the measured parameters for the barley leaves (Table 1). Among the morphological leaf traits, water shortage (W–) significantly increased lower epidermis thickness. Specific leaf area was significantly decreased by low water availability, as it was significantly higher under the W+UV– treatment compared to both W– treatments (i.e., W–UV+,

Table 1 Barley leaf measures and their treatment interactions, and the environmental factors, as determined in this study. Trait

Parameter

Units

Treatment

Factorial ANOVA

W+UV+ Morphological

Biochemical

Physiological Optical

²Environmental

Leaf thickness Mesophyll thickness Specific leaf area Upper surface Cuticle thickness Epidermis thickness Stomata density Stomata length Prickle hair density Prickle hair length Lower surface Cuticle thickness Epidermis thickness Stomata density Stomata length Prickle hair density Prickle hair length Chlorophyll a Chlorophyll b Carotenoids Anthocyanins UV-B–absorbing compounds UV-A–absorbing compounds ¹Potential photochemical efficiency ¹Effective photochemical efficiency ¹Stomatal conductance Reflectance UV-B UV-A Violet Blue Green Yellow Red Near infrared Transmittance UV-B UV-A Violet Blue Green Yellow Red Near infrared ¹Soil moisture Soil temperature Photosynthetically active radiation UV-B

W+UV– ab

W–UV+ ab

W–UV– b

a

W

UV

W × UV

μm μm cm2/mg

183 ± 11 141.5 ± 12.3b 0.243 ± 0.015ab

182 ± 9 136.3 ± 7.0ab 0.261 ± 0.025b

190 ± 13 145.1 ± 10.8b 0.231 ± 0.017a

170 ± 8 124.0 ± 9.0a 0.224 ± 0.019a

ns ns *

* ** ns

ns ns ns

μm μm /mm2 μm /mm2 μm

3.7 ± 0.2b 13.5 ± 0.8a 67 ± 4a 45.3 ± 1.4b 26 ± 3a 33.4 ± 1.4b

2.6 ± 0.3a 14.3 ± 1.7a 70 ± 7ab 42.6 ± 1.0a 27 ± 5ab 32.9 ± 0.4b

2.7 ± 0.1a 14.7 ± 1.9a 71 ± 4ab 44.7 ± 1.6b 28 ± 2ab 30.8 ± 2.1a

2.8 ± 0.0a 14.4 ± 0.9a 75 ± 6b 41.9 ± 1.5a 32 ± 3b 33.5 ± 1.4b

** ns ns ns ns ns

** ns ns ** ns ns

** ns ns ns ns *

μm μm /mm2 μm /mm2 μm mg/cm2 mg/cm2 mg/cm2 a.u./cm2 a.u./cm2 a.u./cm2 a.u. a.u. mmol/m2s %

3.7 ± 0.1c 13.7 ± 1.1a 70 ± 5a 45.9 ± 2.1b 20 ± 1a 34.2 ± 2.4a 0.025 ± 0.007b 0.017 ± 0.007a 0.007 ± 0.002b 1.084 ± 0.279a 2.96 ± 0.52a 5.95 ± 0.96a 0.72 ± 0.07a 0.21 ± 0.01a 89.6 ± 30.0b

2.5 ± 0.2a 14.5 ± 0.7ab 73 ± 6ab 44.6 ± 1.4a 21 ± 3ab 32.2 ± 2.3a 0.024 ± 0.007b 0.016 ± 0.006a 0.007 ± 0.002b 1.118 ± 0.409a 2.79 ± 0.25a 5.40 ± 0.46a 0.71 ± 0.06a 0.22 ± 0.02a 96.0 ± 29.6b

2.7 ± 0.1b 15.3 ± 0.9b 70 ± 6a 47.9 ± 1.6b 20 ± 2a 31.6 ± 1.9a 0.016 ± 0.003a 0.015 ± 0.005a 0.005 ± 0.001a 1.512 ± 0.829b 2.84 ± 0.41a 5.57 ± 0.80a 0.77 ± 0.06a 0.23 ± 0.03a 41.2 ± 26.5a

2.7 ± 0.1b 15.0 ± 0.6b 79 ± 6b 42.5 ± 1.5a 24 ± 3b 31.4 ± 2.6a 0.028 ± 0.010b 0.025 ± 0.013b 0.008 ± 0.003b 1.255 ± 0.762ab 2.76 ± 0.41a 5.22 ± 0.81a 0.73 ± 0.05a 0.23 ± 0.03a 48.3 ± 23.7a

** * ns ns ns ns ns ns ns * ns ns ns ns **

** ns * ** * ns * ns * ns ns ns ns ns ns

** ns ns ns ns ns ** * ** ns ns ns ns ns ns

5.71 ± 0.16a 7.94 ± 0.33a 6.19 ± 0.27a 7.07 ± 0.24ab 15.62 ± 0.67ab 13.04 ± 0.70a 8.65 ± 0.44a 39.41 ± 0.83ab

5.71 ± 0.33a 8.21 ± 0.61a 6.37 ± 0.23a 6.75 ± 0.36a 15.21 ± 1.08a 12.53 ± 1.10a 8.22 ± 0.69a 38.57 ± 0.67a

5.68 ± 0.22a 8.10 ± 0.31a 6.56 ± 0.48a 7.60 ± 0.49bc 16.30 ± 0.95ab 13.64 ± 1.02a 9.22 ± 0.61b 39.47 ± 0.43ab

6.09 ± 0.30b 8.81 ± 0.44b 7.17 ± 0.67b 7.91 ± 0.75c 16.75 ± 1.38b 14.01 ± 1.30a 9.35 ± 0.93b 40.17 ± 1.16b

ns ns * ** * * * *

* ns ns ns ns ns ns ns

ns ns ns ns ns ns ns ns

0.05 ± 0.04c 0.07 ± 0.07b 0.88 ± 0.10a 2.06 ± 0.22a 13.48 ± 0.88a 11.70 ± 0.85a 6.48 ± 0.49a 55.04 ± 1.32a 24.5 ± 4.5b 22.1 ± 4.4a 1627 ± 4ab 0.091 ± 0.002b

0.13 ± 0.05d 0.19 ± 0.06c 1.06 ± 0.15a 2.37 ± 0.30a 15.64 ± 1.45ab 13.89 ± 1.60ab 7.96 ± 1.10a 54.73 ± 1.41a 22.8 ± 4.4b 22.1 ± 4.7a 1619 ± 7a 0.005 ± 0.001a

−0.09 ± 0.03a −0.15 ± 0.06a 0.76 ± 0.26a 2.13 ± 0.58a 15.86 ± 2.01ab 13.72 ± 2.29ab 7.33 ± 1.82a 55.44 ± 1.96a 7.0 ± 3.4a 23.4 ± 5.7a 1638 ± 17b 0.091 ± 0.002b

−0.02 ± 0.04b 0.00 ± 0.07b 1.01 ± 0.33a 2.45 ± 0.74a 17.04 ± 2.50b 14.81 ± 2.47b 8.25 ± 1.77a 59.12 ± 1.80b 8.3 ± 3.4a 23.6 ± 6.6a 1628 ± 16ab 0.005 ± 0.002a

** ** ns ns * ns ns **

** ** ns ns ns ns ns *

ns ns ns ns ns ns ns *

%

% °C μmol/sm2 mW/cm2

Data are means ± SD; n = 5 for each treatment. Different superscript letters within each row indicate significant differences (p ≤0.05; Duncan tests). For factorial ANOVA: *, p ≤0.05; **, p ≤0.01; ns, not significant. Reflectance and transmittance spectra represent means within 5−nm intervals (p ≤0.05, Duncan tests). a.u., arbitrary units. ¹Means from measures conducted 1 week and 2 weeks after start of the experimental period. ²Mean air temperature, 31.9 °C; mean ambient photosynthetically active radiation, 1865 μmol/sm2; mean ambient UV-B, 0.125 mW/cm2. 50

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a a

3.5 3

a

b

a

b

2.5 2

Si

0.5

a b b c

P

a

b

1.5 1

Ca

a b

S a

Cl K

b

b bc

a

ab a

ab

a

a

0 W+UV+

Fig. 1. Silicon (Si), calcium (Ca), phosphorus (P), sulphur (S), chlorine (Cl), and potassium (K) contents of barley leaves grown under the four treatments. Data are means ± SD. Different letters above columns indicate significant differences within each element (p ≤ 0.05; Duncan tests). Water availability had a significant impact on all of these elements, except for K (p ≤ 0.01; factorial ANOVA). DM, dry matter.

W+UV–

W–UV+

W–UV–

W–UV–). Reduced UV radiation (UV–) significantly decreased leaf and mesophyll thickness, along with stomata length on both leaf surfaces. However, for the lower leaf surface alone, UV– significantly increased stomata and prickle hair density. The interaction between water availability and UV radiation only resulted in significant differences for cuticle thickness (for both leaf surfaces) and upper surface prickle hair length, which all showed an opposite response to decreasing water availability under the two considered levels of ambient UV radiation. With decreasing water availability, cuticle thickness slightly increased when under reduced UV radiation, and markedly decreased under ambient UV radiation for both leaf surfaces. Both upper and lower cuticle were thickest under the control treatment (W+UV+), while the thinnest cuticles were observed for the well-watered plants under reduced UV radiation (W+UV–). Upper surface prickle hair length also revealed distinct decreases under ambient UV radiation, while it increased under reduced UV radiation with decreasing water availability. Significantly shorter upper surface prickle hairs compared to all the other treatments were found in the water-deprived plants when under ambient UV radiation (W–UV+). The photosynthetic pigments (i.e., chlorophyll a and b, and carotenoids) were all significantly affected by the interaction between water availability and UV radiation in a similar way (Table 1). Namely, with decreasing water availability, their contents increased under reduced ambient UV radiation, while they decreased under ambient UV radiation. They were all lowest for the W–UV+ group and highest for the W–UV– group. Anthocyanins were only significantly affected by water availability, showing higher contents under reduced water availability. The highest anthocyanin contents were observed for the W–UV+ group. On the other hand, UV-absorbing compounds showed no significant differences among the treatment groups. Nevertheless, their contents were highest under the W+UV+ treatment and lowest under the W–UV– treatment. For the potential photochemical efficiency, the lower water availability treatments (i.e., W–UV+, W–UV–) did not significantly affect the barley plant vitality, as also seen for the effective photochemical efficiency. Conversely, under these water shortage conditions, the stomatal conductance of the barley leaves was significantly decreased (Table 1). The reflectance and transmittance spectra of the barley leaves did not show pronounced differences between these four treatments, although some colour regions differed significantly (Table 1). For reflectance, water shortage (W–) resulted in significant increase in the visible and NIR regions, while reduced UV radiation (UV–) significantly increased the UV-B region exclusively. On the other hand, transmittance was significantly decreased by water shortage (W–) and significantly increased by reduced UV radiation (UV–) in both UV regions. Moreover, water shortage alone (W–) significantly increased

transmittance of light in the green region. The interaction between water availability and UV radiation was only significant for transmittance in the NIR region. It showed an increase with decreasing water availability, which was more pronounced in the case of reduced UV radiation. The amount of transmitted light was thus significantly higher in the W–UV– treatment, but did not differ from the control (W+UV+) under the remaining two treatments (i.e., W+UV–, W–UV+). In general, reflectance and transmittance were highest under the most stressful treatment (W–UV–), with the lowest transmittance under the control conditions (W+UV+), and the least light reflected under the W +UV– treatment. Measurements of the plant environmental conditions (i.e., soil, radiation) showed the expected marked decreases in soil moisture from the W+ to W– treatments, as for the UV-B radiation levels from the UV + to UV– treatments (Table 1). However, the soil temperatures and PAR levels were not significantly affected by these two factors, either alone or in terms of their interactions. At the start of the experimental period, the soil moisture levels ranged from 16.7% to 23.7%, with no significant differences among these four treatments. The element analysis of the barley leaves indicated marked, and generally significant, decreases with decreased water availability (i.e., W+ vs. W–) for almost all of the elements examined (i.e., Si, P, S, Cl, Ca), with the exception of K (Fig. 1). These reached significance for Si, Cl, and Ca. Leaf P content was lower for W– in comparison to both W + treatments, although significance was only reached for the W–UV + treatment versus the W+ treatments. For the leaf S content, the decrease from W+ to W– treatments did not reach significance only for W + UV– versus W–UV–. As indicated, contrary to other elements examined, K was not significantly affected by the reduced water availability under these treatments. Redundancy analysis revealed strong positive relationships between almost all of the elements determined in this study, again with the exception of K, and also for soil water content and stomatal conductance (Fig. 2). Here, soil water content alone explained 43% and stomatal conductance alone 36% of the variability of the element composition of these barley leaves. When considered together, soil water content explained 43% (p = 0.002) variability, and stomatal conductance included an additional 8% (p = 0.040) variability. The samples formed two distinct groups according to the water availability. 3.2. Soil element composition and structure, and plant-available Si levels in the soil Total soil Si and Ca levels did not show any significant differences among the treatments (Table 2). Plant-available Si (CaCl2-extractable Si) levels in the soil were significantly lower under the W–UV+ treatment in comparison to all the other treatments. 51

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revealed strong positive relationships between the leaf contents of almost all of the elements examined (i.e., except for K), and the soil moisture level (p ≤ 0.01) and the stomatal conductance (p ≤ 0.05). Almost all of the elements considered in this study (i.e., again, with the exception of K) showed significant positive correlations (p ≤ 0.05) with one another. Leaf Si contents did not correlate with total soil Si levels, while they showed weak positive correlation with plant-available Si (CaCl2‐extractable Si) levels in the soil (p ≤ 0.05). Plant-available Si levels in the soil were significantly positively related to soil moisture levels (p ≤ 0.01).

Cl

Si

soil water content

Ca

K

S P

stomatal conductance

4. Discussion 4.1. Effects of water availability In terms of the effects of the water availability, most were observed in both of the water-deprived groups (i.e., W–UV+, W–UV–). This was expected, because water shortage directly affects plant growth through reduction of transpiration, photosynthetic activity, and cell division and growth rate (Larcher, 2003; Duman, 2012). The present study also revealed a significant decrease in stomatal conductance, although there were no significant changes in the potential and effective photochemical efficiencies of PSII. This thus indicates that stomatal regulation enabled the maintenance of a favourable water regime, and that the photoprotective mechanisms, such as the xanthophyll cycle (Bernal et al., 2015), appeared to prevent photochemical stress. The water shortage (W–) also altered the morphological properties of these barley leaves, with significantly decreased cuticle thickness and significantly increased epidermis thickness, and significantly decreased specific leaf area. The increase in anthocyanin content under reduced water availability, on the other hand, goes well in line with their important role in drought tolerance (Gould, 2004). The thick cuticle and thin epidermal layer in the control (well-watered; W+) plants were probably a consequence of their better Si supply (Faisal et al., 2012). Sisoluble fraction is redox- and pH-dependent (Ma and Takahashi, 2002), ranging from 0.1 to 0.6 mM in soil solutions (Epstein, 2001; Sommer et al., 2006). Miles et al. (2014) examined the relation between CaCl2‐extractable Si levels in the soil and soil pH for 112 soils, and found that these levels ranged from about 10 mg/kg of soil DM at pH 4 to about 100 mg/kg of soil DM at pH 7. However, in our study, CaCl2‐extractable Si levels in the soil ranged from 20.6 mg/kg of soil DM to 23.6 mg/kg of soil DM at soil pH 7.1. These values were positively related to soil moisture levels, which is a consequence of the capillary rise of water containing dissolved Si (SiO2 × nH2O) (Kozlowski, 1987). Comparison of a rainforest site with a sugarcane site in Northern Queensland, Australia showed that CaCl2‐extractable Si levels in the soil were 13.1 mg/kg and 5.3 mg/kg, respectively, indicating that crops deplete Si-supplying ability of the soil (Haynes, 2014). Keller et al. (2012) estimated that under a temperate climate the exportation of Si due to straw removal would result in depletion of the initial phytogenic Si in soils within a few decades. Although the degree of water shortage used in this study did not impose stress on these plants, it significantly decreased the uptake of

Fig. 2. Redundancy analysis plot showing the strength of associations between the water availability indicators (i.e., soil water content, stomatal conductance) and the element accumulation levels per leaf area. Black diamonds, W+UV+; dark grey diamonds, W+UV–; light grey diamonds, W–UV+; white diamonds, W–UV–.

The texture of the soil was 49.2 ± 2.7% sand, 37.2 ± 2.7% silt, and 13.6 ± 0.3% clay, and thus it was identified as loamy. The further soil parameters were: pH, 7.1 ± 0.1; available P, 75.8 ± 3.4 mg/ 100 g; available K, 39.5 ± 5.0 mg/100 g; electrical conductivity, 0.23 ± 0.06 mS/cm; carbonates, 14.5 ± 2.4%; C, 10.9 ± 1.1%; N, 0.74 ± 0.03%; C/N ratio, 14.9 ± 1.1; organic matter, 18.1 ± 0.4%; total C, 12.63 ± 1.29%; cation exchange capacity, 41.74 ± 1.81 mmolc/100 g; Ca2+, 34.81 ± 2.20 mmolc/100 g; Mg2+, 6.19 ± 0.61 mmolc/100 g; K+, 0.94 ± 0.11 mmolc/100 g; Na+, 0.08 ± 0.02 mmolc/100 g; total exchangeable bases, 42.01 ± 2.71 mmolc/100 g; and base saturation, 99 ± 2%. 3.3. Relationships between certain leaf and soil parameters Pearson's correlation analysis was carried out to obtain a more detailed view of some of the relationships between certain leaf traits, environmental conditions, and some soil parameters. The reflectance of light was significantly negatively related (p ≤ 0.05) to leaf Si content throughout the whole spectrum. However, the reflectance showed a significant, and marked, negative relationship (p ≤ 0.01) with leaf Ca content in the violet and blue regions of the spectrum only. In addition, reflectance was significantly negatively related (p ≤ 0.05) to specific leaf area in the UV-A, UV-B, violet, blue, and NIR regions, and correlated positively (p ≤ 0.05) with upper surface stomata as well as prickle hair densities in both UV regions. For upper surface stomata density, positive correlation (p ≤ 0.05) with reflectance was also found in the violet, blue, and NIR regions. The transmittance of light in both of the UV regions was significantly positively related (p ≤ 0.05) to both leaf Si and Ca contents. Moreover, the transmittance in the NIR region was significantly negatively related (p ≤ 0.05) to leaf Si and Ca contents. Similar to the redundancy analysis, Pearson's correlation analysis

Table 2 Total Si and Ca, and CaCl2-extractable Si levels in the soil under the four treatments. Element

Total Si Total Ca CaCl2-extractable Si

Units

% of DM % of DM mg Si/kg of DM

Treatment W+UV+

W+UV–

W–UV+

W–UV–

7.2 ± 0.9a 3.0 ± 0.1a 23.6 ± 0.9b

6.3 ± 0.6a 3.0 ± 0.3a 23.6 ± 0.8b

6.2 ± 0.7a 3.1 ± 0.2a 20.6 ± 1.1a

7.3 ± 1.0a 2.9 ± 0.3a 22.7 ± 1.3b

Data are means ± SD; n = 5 for each treatment. Different superscript letters within each row indicate significant differences (p ≤ 0.05; Duncan tests). DM, dry matter. 52

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almost all of the elements studied here, with the exception of K. Indeed, K influences a number of physiological and biochemical processes in plants that are involved in their resistance to biotic and abiotic stresses (Wang et al., 2013). K is also an important inorganic osmolyte in plants, and it has a crucial role in osmotic adjustment under drought conditions (Hawkesford et al., 2012). Therefore, the maintenance of the K levels in plants is crucial for plant survival during periods of water shortage (Chen et al., 2016a). Water availability also affected the P content in these barley leaves. The lowest P content was seen for the W–UV+ treatment. Phosphate ions move through the soil by diffusion, and therefore water shortage in the soil (i.e., W–) will decrease their mobility (da Silva et al., 2011). On the other hand, high P availability increases the plant sensitivity to UV radiation (Murali and Teramura, 1985). He and Dijkstra (2014) also reported that drought leads to decreased plant N and P levels, with this effect greater for P. Moreover, leaf P content appears to be related to leaf Si content, as also shown in the present study. Sufficient Si availability can reduce the need of plants for P, as Si fertilisation of soils with insufficient P results in increased plant production (Ma and Takahashi, 2002). Deren (1997) also reported increased P levels following application of Si fertiliser. Silicon was the most affected element in terms of the water availability. The negative effects of drought might be mitigated by Si uptake (Maghsoudi et al., 2015; Ma et al., 2016; Marques et al., 2016). The content of Si in the leaves of the well-watered barley plants (W+) in this study reached ∼2.5%, and when these plants were under water shortage (W–), this remained relatively high (> 1%) in comparison to other plant species (Ma and Takahashi, 2002). In an experiment with barley subjected to osmotic stress under low or high K supply and under two Si regimes, leaf Si contents ranged from less than 1 mg Si/g DM to 6.5 mg Si/g DM (Hosseini et al., 2017). By deposition of Si in different plant parts, the plants generally improve their water retention capacity and optimise their water-use efficiency (Gao et al., 2004; Ma et al., 2004; Ahmed et al., 2011). Water loss by transpiration takes place not only through the stomata, but also through the cuticle (Ma and Takahashi, 2002). Therefore, incrustation of the cuticle together with wax deposition reduces water loss (Yoshida et al., 1962b; Postek, 1981; Ma and Takahashi, 2002). Si also strengthens the cell walls of the root endodermis, thereby preventing water loss from the roots when there is a decrease in soil water potential (Lux et al., 2002). Additionally, Si can improve the hydraulic conductivity of plant roots (Shi et al., 2016). The role of Si is also related to gene regulation, as it can regulate water uptake by promotion of expression of the genes that code for aquaporins in water channels (Liu et al., 2014), while also increasing the expression of certain transcription factors involved in drought acclimatisation (Khattab et al., 2014). Si also has a role in osmotic regulation, through changes in the levels of osmotic regulators such as proline and soluble carbohydrates (Crusciol et al., 2009; Pereira et al., 2013; Mauad et al., 2016). Studies have also revealed interactions between Si and Ca, with changes in Ca and micronutrient levels in leaf tissues reported to be related to Si availability during plant growth (Brackhage et al., 2013). The study of Greger et al. (2018) showed that Si fertilisation increased the uptake of Mg, Ca, S, Mn, and Mo, and decreased the uptake of Fe, Cu, and Zn, while the uptake of K, P, N, Cl, and B was not affected. The effects were similar across different experimental plant species and soil types. The positive relationship between Si and Ca, S, P, and Cl, but not between Si and K, was observed in the present study. It was reported that the amount of phosphate ions released into the soil solution increases with increasing levels of H4SiO4 (Tubaña and Heckman, 2015). In comparison to Si, Ca accumulation was less affected by water availability. As well as Si, Ca is involved in plant drought resistance (Xu et al., 2013), through improved membrane integrity (Duman, 2012) and its involvement in the signalling of drought resistance responses (Shao et al., 2008). Contrary to the positive correlation between Si and Ca in barley, a study by Liang (1999) did not reveal interactions

between these two elements. However, Brackhage et al. (2013) investigated reeds and showed that Ca levels decreased as Si levels increased, while a study of leaf water content of wheat showed that Ca levels were positively related to Si levels (Mali and Aery, 2008), as also shown in the present study. 4.2. Effects of UV radiation An impact of reduced UV radiation was expected, as UV radiation is an important evolutionary factor that has shaped plant growth and development (Björn, 2015). Manipulating UV levels is becoming more and more important in agriculture (Wargent and Jordan, 2013). Barley usually grows in areas with high light intensity. In the present study, the reduced UV radiation (UV–) led to decreased leaf and cuticle thickness and prickle hair density, and to increased chlorophyll a and carotenoid contents, while showing no significant effects on element contents. We did note here a small beneficial impact of ambient UV radiation on Si content in the leaves of the barley plants exposed to water shortage (i.e., W–UV+), although this did not reach significance due to high inter-sample variability. Here, the Si level in water-deprived plants under ambient UV radiation (i.e., W–UV+) was 63% of that in the well-watered plants (i.e., W+UV+), while it was only ∼48% under the reduced UV radiation (W+UV–). A reverse trend was observed for P. Studies using enhanced UV-B radiation have revealed decreased P levels in plants (Yue et al., 1998), while the combination of UV-B radiation and drought can significantly increase the P, K, Ca, and Mg contents in soybean leaves (Shen et al., 2014a), although this is not in line with the results of the present study. The absence of negative effects under the present ambient UV radiation in barley was possibly the consequence of its efficient mechanisms to cope with such UV radiation levels, and even to benefit from them (Jansen et al., 2012; Sen Mandi, 2016). 4.3. Combined effects of water availability and UV radiation High UV radiation levels and water shortage can often occur together in nature, and the responses of plants to these parameters trigger similar changes in their leaf traits (Comont et al., 2012). It has been shown that UV-B radiation mitigates water shortage stress by lowering stomatal conductance and reducing leaf area by inhibition of cell division (Nogués et al., 1998). This was not the case in the barley leaves in the present study, as the reduced UV radiation and degree of water shortage used here did not impose stress on the experimental plants. The only such observed combined effect of reduced water availability and reduced UV radiation was the reduction of leaf cuticle thickness. 4.4. Effects of treatments on the optical properties of barley leaves Reflectance and transmittance showed some small effects under reduced water availability and reduced UV radiation. Water shortage (W–) significantly increased the reflectance in the visible and NIR regions, while reduced UV radiation (UV–) had the same effect in the UVB region only. This appears to be due to the differences in Si levels, as the reflectance of light was negatively related to the leaf Si content across all regions of the spectrum. A negative relationship between reflectance and leaf Ca content was seen for the violet and blue regions only. In previous studies, a protective role against strong light was ascribed to Si structures at the leaf surface (i.e., in the cuticle and prickle hairs) that increased the reflection especially in the UV region, and thereby reduced UV penetration into the deeper leaf tissues (Klančnik et al., 2014a, 2014b). Prickle hairs were also shown to have the same role in the present study. Increased tolerance to UV radiation due to Si has been associated with expression of the Lsi1 gene, which encodes the transporter for Si uptake in plants (Ma and Yamaji, 2008). Expression of this gene also facilitates the expression of some other genes that are crucial for increased plant tolerance to UV radiation, as they are 53

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involved in both repair of photodamage and detoxification mechanisms (Fang et al., 2011; Chen et al., 2016b).

2015. Quantifying the ozone and ultraviolet benefits already achieved by the Montreal Protocol. Nat. Commun. 6. https://doi.org/10.1038/ncomms8233. Comont, D., Winters, A., Gwynn-Jones, D., 2012. Acclimation and interaction between drought and elevated UV-B in A. thaliana: differences in response over treatment, recovery and reproduction. Ecol. Evol. 2, 2695–2709. https://doi.org/10.1002/ece3. 387. Cooke, J., DeGabriel, J.L., 2016. Editorial: plant silicon interactions between organisms and the implications for ecosystems. Front. Plant Sci. 7, 1–3. https://doi.org/10. 3389/fpls.2016.01001. Cooke, J., Leishman, M.R., 2011. Is plant ecology more siliceous than we realise? Trends Plant Sci. 16, 61–68. https://doi.org/10.1016/j.tplants.2010.10.003. Crusciol, C.A.C., Pulz, A.L., Lemos, L.B., Soratto, R.P., Lima, G.P.P., 2009. Effects of silicon and drought stress on tuber yield and leaf biochemical characteristics in potato. Crop Sci. 49, 949–954. https://doi.org/10.2135/cropsci2008.04.0233. da Silva, E.C., Nogueira, R.J.M.C., da Silva, M.A., de Albuquerque, M.B., 2011. Drought stress and plant nutrition. In: Anjum, N.A., Lopez-Lauri, F. (Eds.), Plant Nutrition and Abiotic Stress Tolerance III: Plant Stress 5. Global Science Books, Ltd., Isleworth, pp. 32–41. Deren, C.W., 1997. Changes in nitrogen and phosphorus concentrations of silicon-fertilized rice grown on organic soil. J. Plant Nutr. 20, 765–771. https://doi.org/10. 1080/01904169709365292. Dietrich, D., Hinke, S., Baumann, W., Fehlhaber, R., Bäucker, E., Rühle, G., Wienhaus, O., Marx, G., 2003. Silica accumulation in Triticum aestivum L. and Dactylis glomerata L. Anal. Bioanal. Chem. 376, 399–404. https://doi.org/10.1007/s00216-003-1847-8. Drumm, H., Mohr, H., 1978. The mode of interaction between blue (UV) light photoreceptor and phytochrome in anthocyanin formation of the Sorghum seedling. Photochem. Photobiol. 27, 241–248. https://doi.org/10.1111/j.1751-1097.1978. tb07595.x. Duman, F., 2012. Uptake of mineral elements during abiotic stress. In: Ahmad, P., Prasad, M.N.V. (Eds.), Abiotic Stress Responses in Plants: Metabolism, Productivity and Sustainability. Springer, New York, pp. 267–281. https://doi.org/10.1007/978-14614-0634-1_15. Epstein, E., 1994. The anomaly of silicon in plant biology. Proc. Natl. Acad. Sci. U. S. A. 91, 11–17. https://doi.org/10.1073/pnas.91.1.11. Epstein, E., 2001. Silicon in plants: facts vs. concepts. In: Datnoff, L.E., Snyder, G.H., Korndörfer, G.H. (Eds.), Silicon in Agriculture. Elsevier Science B.V., Amsterdam, pp. 1–15. https://doi.org/10.1016/S0928-3420(01)80005-7. Epstein, E., Bloom, A.J., 2005. Mineral Nutrition of Plants: Principles and Perspectives, second ed. Sinauer Associates, Inc., Sunderland. Faisal, S., Callis, K.L., Slot, M., Kitajima, K., 2012. Transpiration-dependent passive silica accumulation in cucumber (Cucumis sativus) under varying soil silicon availability. Botany 90, 1058–1064. https://doi.org/10.1139/B2012-072. Fang, C.-X., Wang, Q.-S., Yu, Y., Li, Q.-M., Zhang, H.-L., Wu, X.-C., Chen, T., Lin, W.-X., 2011. Suppression and overexpression of Lsi1 induce differential gene expression in rice under ultraviolet radiation. Plant Growth Regul. 65, 1–10. https://doi.org/10. 1007/s10725-011-9569-y. Farooq, M., Wahid, A., Kobayashi, N., Fujita, D., Basra, S.M.A., 2009. Plant drought stress: effects, mechanisms and management. Agron. Sustain. Dev. 29, 185–212. https://doi. org/10.1051/agro:2008021. Farooq, M.A., Dietz, K.-J., 2015. Silicon as versatile player in plant and human biology: overlooked and poorly understood. Front. Plant Sci. 6, 994. https://doi.org/10.3389/ fpls.2015.00994. Gao, X., Zou, C., Wang, L., Zhang, F., 2004. Silicon improves water use efficiency in maize plants. J. Plant Nutr. 27, 1457–1470. https://doi.org/10.1081/PLN-200025865. Golob, A., Kavčič, J., Stibilj, V., Gaberščik, A., Vogel-Mikuš, K., Germ, M., 2017. The effect of selenium and UV radiation on leaf traits and biomass production in Triticum aestivum L. Ecotox. Environ. Saf. 136, 142–149. https://doi.org/10.1016/j.ecoenv. 2016.11.007. Golob, A., Stibilj, V., Kreft, I., Vogel-Mikuš, K., Gaberščik, A., Germ, M., 2018. Selenium treatment alters the effects of UV radiation on chemical and production parameters in hybrid buckwheat. Acta Agric. Scand. B 68, 5–15. https://doi.org/10.1080/ 09064710.2017.1349172. Gould, K.S., 2004. Nature’s Swiss Army knife: the diverse protective roles of anthocyanins in leaves. J. Biomed. Biotechnol. 2004, 314–320. https://doi.org/10.1155/ S1110724304406147. Greger, M., Landberg, T., Vaculík, M., 2018. Silicon influences soil availability and accumulation of mineral nutrients in various plant species. Plants 7, 41. https://doi. org/10.3390/plants7020041lants. Guntzer, F., Keller, C., Meunier, J.-D., 2012. Benefits of plant silicon for crops: a review. Agron. Sustain. Dev. 32, 201–213. https://doi.org/10.1007/s13593-011-0039-8. Hawkesford, M., Horst, W., Kichey, T., Lambers, H., Schjoerring, J., Skrumsager, I., White, P., 2012. Functions of macronutrients. In: Marschner, P. (Ed.), Marschner’s Mineral Nutrition of Higher Plants, third ed. Academic Press, London, pp. 135–190. https://doi.org/10.1016/C2009-0-63043-9. Haynes, R.J., 2014. A contemporary overview of silicon availability in agricultural soils. J. Plant Nutr. Soil Sci. 177, 831–844. https://doi.org/10.1002/jpln.201400202. He, M., Dijkstra, F.A., 2014. Drought effect on plant nitrogen and phosphorus: a metaanalysis. New Phytol. 204, 924–931. https://doi.org/10.1111/nph.12952. He, H., Veneklaas, E.J., Kuo, J., Lambers, H., 2014. Physiological and ecological significance of biomineralization in plants. Trends Plant Sci. 19, 166–174. https://doi. org/10.1016/j.tplants.2013.11.002. Hosseini, S.A., Maillard, A., Hajirezaei, M.R., Ali, N., Schwarzenberg, A., Jamois, F., Yvin, J.-C., 2017. Induction of barley silicon transporter HvLsi1 and HvLsi2, increased silicon concentration in the shoot and regulated starch and ABA homeostasis under osmotic stress and concomitant potassium deficiency. Front. Plant Sci. 8. https://doi. org/10.3389/fpls.2017.01359.

5. Conclusions The moderate water shortage and reduced UV radiation treatments used in the present study did not stress these barley plants, as revealed by the photochemical efficiency of PSII. However, the photosynthetic pigments and anthocyanins were significantly affected by either one of the two studied factors or their interaction. Other leaf traits were only slightly affected by these two factors, which is also apparent from the minor differences in the leaf optical properties. The differences in the reflectance and transmittance spectra of these barley leaves were related to the Si and Ca levels. Water shortage alone (W–) significantly reduced the uptake of Si, Ca, P, S, and Cl, but not of K, while reduced UV radiation resulted in slightly decreased Si accumulation in the water-deprived plants (i.e., W–UV+ vs. W–UV–). Leaf Si content was positively related to plant-available Si levels in the soil, and these showed positive correlation with soil moisture levels. The present study has thus revealed that barley plants have efficient mechanisms to resist this moderate water shortage (W–) without undergoing stress, and that ambient UV radiation (UV+) contributes to this plant protection, as reduced UV radiation (UV–) decreases Si uptake. Acknowledgements The authors are grateful to Christopher Berrie for revision of the English writing. The authors acknowledge financial support from the Slovenian Research Agency through core research funding for the programme Plant Biology (P1-0212), the Young Researchers project (39096), and the project Optimisation of Barley and Buckwheat Processing for Sustainable Use in High Quality Functional Foods (L47552). References Agarie, S., Hanaoka, N., Ueno, O., Miyazaki, A., Kubota, F., Agata, W., Kaufman, P.B., 1998. Effects of silicon on tolerance to water deficit and heat stress in rice plants (Oryza sativa L.), monitored by electrolyte leakage. Plant Prod. Sci. 1, 96–103. https://doi.org/10.1626/pps.1.96. Ahmed, M., Hassan, F.-U., Qadeer, U., Aslam, M.A., 2011. Silicon application and drought tolerance mechanism of sorghum. Afr. J. Agric. Res. 6, 594–607. https://doi.org/10. 5897/AJAR10.626. Alexieva, V., Sergiev, I., Mapelli, S., Karanov, E., 2001. The effect of drought and ultraviolet radiation on growth and stress markers in pea and wheat. Plant Cell Environ. 24, 1337–1344. https://doi.org/10.1046/j.1365-3040.2001.00778.x. Artyszak, A., 2018. Effect of silicon fertilization on crop yield quantity and quality – a literature review in Europe. Plants 7, 54. https://doi.org/10.3390/plants7030054. Bernal, M., Verdaguer, D., Badosa, J., Abadía, A., Llusià, J., Peñuelas, J., Núñez-Olivera, E., Llorens, L., 2015. Effects of enhanced UV radiation and water availability on performance, biomass production and photoprotective mechanisms of Laurus nobilis seedlings. Environ. Exp. Bot. 109, 264–275. https://doi.org/10.1016/j.envexpbot. 2014.06.016. Björn, L.O., 2015. On the history of phyto-photo UV science (not to be left in skoto toto and silence). Plant Physiol. Biochem. 93, 3–8. https://doi.org/10.1016/j.plaphy. 2014.09.015. Bonafaccia, G., Merendino, N., Bonafaccia, F., Molinari, R., Galli, V., Pravst, I., Škrabanja, V., Luthar, Z., Golob, A., Germ, M., 2016. Concentration of proteins, beta-glucans, total phenols and antioxidant capacity of Slovenian samples of barley. Folia Biol. Geol. 57, 11–18. https://doi.org/10.3986/fbg0015. Brackhage, C., Schaller, J., Bäucker, E., Dudel, E.G., 2013. Silicon availability affects the stoichiometry and content of calcium and micro nutrients in the leaves of common reed. Silicon 5, 199–204. https://doi.org/10.1007/s12633-013-9145-3. Caldwell, M.M., 1968. Solar ultraviolet radiation as an ecological factor for alpine plants. Ecol. Monogr. 38, 243–268. https://doi.org/10.2307/1942430. Chen, D., Cao, B., Wang, S., Liu, P., Deng, X., Yin, L., Zhang, S., 2016a. Silicon moderated the K deficiency by improving the plant-water status in sorghum. Sci. Rep. 6. https:// doi.org/10.1038/srep22882. Chen, J., Zhang, M., Eneji, A.E., Li, J., 2016b. Influence of exogenous silicon on UV-B radiation-induced cyclobutane pyrimidine dimmers in soybean leaves and its alleviation mechanism. J. Plant Physiol. 196–197, 20–27. https://doi.org/10.1016/j. jplph.2016.01.019. Chipperfield, M.P., Dhomse, S.S., Feng, W., McKenzie, R.L., Velders, G.J.M., Pyle, J.A.,

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Agricultural Water Management 217 (2019) 47–56

M. Grašič, et al. Jansen, M.A.K., Coffey, A.M., Prinsen, E., 2012. UV-B induced morphogenesis: four players or a quartet? Plant Signal. Behav. 7, 1185–1187. https://doi.org/10.4161/ psb.21260. Kaufman, P.B., Dayanandan, P., Franklin, C.I., Takeoka, Y., 1985. Structure and function of silica bodies in the epidermal system of grass shoots. Ann. Bot. 55, 487–507. https://doi.org/10.1093/oxfordjournals.aob.a086926. Keller, C., Guntzer, F., Barboni, D., Labreuche, J., Meunier, J.‐D., 2012. Impact of agriculture on the Si biogeochemical cycle: input from phytolith studies. C. R. Geosci. 344, 739–746. https://doi.org/10.1016/j.crte.2012.10.004. Khattab, H.I., Emam, M.A., Emam, M.M., Helal, N.M., Mohamed, M.R., 2014. Effect of selenium and silicon on transcription factors NAC5 and DREB2A involved in droughtresponsive gene expression in rice. Biol. Plant. 58, 265–273. https://doi.org/10. 1007/s10535-014-0391-z. Klančnik, K., Vogel-Mikuš, K., Gaberščik, A., 2014a. Silicified structures affect leaf optical properties in grasses and sedge. J. Photochem. Photobiol. B 130, 1–10. https://doi. org/10.1016/j.jphotobiol.2013.10.011. Klančnik, K., Vogel-Mikuš, K., Kelemen, M., Vavpetič, P., Pelicon, P., Kump, P., Jezeršek, D., Gianoncelli, A., Gaberščik, A., 2014b. Leaf optical properties are affected by the location and type of deposited biominerals. J. Photochem. Photobiol. B 140, 276–285. https://doi.org/10.1016/j.jphotobiol.2014.08.010. Korndörfer, G.H., Coelho, M.N., Snyder, G.H., Mizutani, C.T., 1999. Evaluation of soil extractants for silicon availability in upland rice. R. Bras. Ci. Solo 23, 101–106. https://doi.org/10.1590/S0100-06831999000100013. Kozlowski, T.T., 1987. Soil moisture and absorption of water by tree roots. J. Arboric. 13, 39–46. Kump, P., Nečemer, M., Rupnik, Z., Pelicon, P., Ponikvar, D., Vogel-Mikuš, K., Regvar, M., Pongrac, P., 2011. Improvement of the XRF quantification and enhancement of the combined applications by EDXRF and micro-PIXE. Integration of Nuclear Spectrometry Methods as a New Approach to Material Research. IAEA, Vienna, pp. 101–109. Larcher, W., 2003. Physiological Plant Ecology: Ecophysiology and Stress Physiology of Functional Groups, fourth ed. Springer, Berlin. Lesk, C., Rowhani, P., Ramankutty, N., 2016. Influence of extreme weather disasters on global crop production. Nature 529, 84–87. https://doi.org/10.1038/nature16467. Lewin, J., Reimann, B.E.F., 1969. Silicon and plant growth. Annu. Rev. Plant Physiol. 20, 289–304. https://doi.org/10.1146/annurev.pp.20.060169.001445. Liang, Y., 1999. Effects of silicon on enzyme activity and sodium, potassium and calcium concentration in barley under salt stress. Plant Soil 209, 217–224. https://doi.org/10. 1023/A:1004526604913. Liang, Y., Hua, H., Zhu, Y.G., Zhang, J., Cheng, C., Römheld, V., 2006. Importance of plant species and external silicon concentration to active silicon uptake and transport. New Phytol. 172, 63–72. https://doi.org/10.1111/j.1469-8137.2006.01797.x. Lichtenthaler, H.K., Buschmann, C., 2001a. Extraction of photosynthetic tissues: chlorophylls and carotenoids. Curr. Protoc. Food Anal. Chem. 1, 165–170. https://doi.org/ 10.1002/0471709085.ch21. Lichtenthaler, H.K., Buschmann, C., 2001b. Chlorophylls and carotenoids: measurement and characterization by UV-VIS spectroscopy. Curr. Protoc. Food Anal. Chem. 1, 171–178. https://doi.org/10.1002/0471709085.ch21. Liu, P., Yin, L., Deng, X., Wang, S., Tanaka, K., Zhang, S., 2014. Aquaporin-mediated increase in root hydraulic conductance is involved in silicon-induced improved root water uptake under osmotic stress in Sorghum bicolor L. J. Exp. Bot. 65, 4747–4756. https://doi.org/10.1093/jxb/eru220. Lux, A., Luxová, M., Hattori, T., Inanaga, S., Sugimoto, Y., 2002. Silicification in sorghum (Sorghum bicolor) cultivars with different drought tolerance. Physiol. Plant. 115, 87–92. https://doi.org/10.1034/j.1399-3054.2002.1150110.x. Ma, J.F., Takahashi, E., 2002. Soil, Fertilizer, and Plant Silicon Research in Japan, first ed. Elsevier, Amsterdam. Ma, J.F., Yamaji, N., 2008. Functions and transport of silicon in plants. Cell. Mol. Life Sci. 65, 3049–3057. https://doi.org/10.1007/s00018-008-7580-x. Ma, J.F., Miyake, Y., Takahashi, E., 2001. Silicon as a beneficial element for crop plants. In: Datnoff, L.E., Snyder, G.H., Korndörfer, G.H. (Eds.), Silicon in Agriculture. Elsevier Science B.V., Amsterdam, pp. 17–39. https://doi.org/10.1016/S09283420(01)80006-9. Ma, C.C., Li, Q.F., Gao, Y.B., Xin, T.R., 2004. Effects of silicon application on drought resistance of cucumber plants. Soil Sci. Plant Nutr. 50, 623–632. https://doi.org/10. 1080/00380768.2004.10408520. Ma, D., Sun, D., Wang, C., Qin, H., Ding, H., Li, Y., Guo, T., 2016. Silicon application alleviates drought stress in wheat through transcriptional regulation of multiple antioxidant defense pathways. J. Plant Growth Regul. 35, 1–10. https://doi.org/10. 1007/s00344-015-9500-2. Maghsoudi, K., Emam, Y., Ashraf, M., 2015. Influence of foliar application of silicon on chlorophyll fluorescence, photosynthetic pigments, and growth in water-stressed wheat cultivars differing in drought tolerance. Turk. J. Bot. 39, 625–634. https://doi. org/10.3906/bot-1407-11. Mali, M., Aery, N.C., 2008. Influence of silicon on growth, relative water contents and uptake of silicon, calcium and potassium in wheat grown in nutrient solution. J. Plant Nutr. 31, 1867–1876. https://doi.org/10.1080/01904160802402666. Marques, D.J., Ferreira, M.M., da Silva Lobato, A.K., de Freitas, W.A., de Assunção Carvalho, J., Ferreira, E.D., Broetto, F., 2016. Potential of calcium silicate to mitigate water deficiency in maize. Bragantia 75, 275–285. https://doi.org/10.1590/16784499.446. Marschner, H., Oberle, H., Cakmak, I., Römheld, V., 1990. Growth enhancement by silicon in cucumber (Cucumis sativus) plants depends on imbalance in phosphorus and zinc supply. Plant Soil 124, 211–219. https://doi.org/10.1007/BF00009262. Mauad, M., Crusciol, C.A.C., Nascente, A.S., Grassi Filho, H., Pereira Lima, G.P., 2016. Effects of silicon and drought stress on biochemical characteristics of leaves of upland

rice cultivars. Rev. Ciênc. Agron. 47, 532–539. https://doi.org/10.5935/1806-6690. 20160064. Meena, V.D., Dotaniya, M.L., Coumar, V., Rajendiran, S., Ajay, Kundu, S., Subba Rao, A., 2014. A case for silicon fertilization to improve crop yields in tropical soils. P. Natl. A. Sci. India B 84, 505–518. https://doi.org/10.1007/s40011-013-0270-y. Miles, N., Manson, A.D., Rhodes, R., van Antwerpen, R., Weigel, A., 2014. Extractable silicon in soils of the South African sugar industry and relationships with crop uptake. Commun. Soil Sci. Plant Anal. 45, 2949–2958. https://doi.org/10.1080/00103624. 2014.956881. Murali, N.S., Teramura, A.H., 1985. Effects of UV-B irradiance on soybean. VI. Influence of phosphorus nutrition on growth and flavonoid content. Physiol. Plant. 63, 413–416. https://doi.org/10.1111/j.1399-3054.1985.tb02319.x. Nečemer, M., Kump, P., Ščančar, J., Jaćimović, R., Simčič, J., Pelicon, P., Budnar, M., Jeran, Z., Pongrac, P., Regvar, M., Vogel-Mikuš, K., 2008. Application of X-ray fluorescence analytical techniques in phytoremediation and plant biology studies. Spectrochim. Acta Part B At. Spectrosc. 63, 1240–1247. https://doi.org/10.1016/j. sab.2008.07.006. Nogués, S., Allen, D.J., Morison, J.I.L., Baker, N.R., 1998. Ultraviolet-B radiation effects on water relations, leaf development, and photosynthesis in droughted pea plants. Plant Physiol. 117, 173–181. https://doi.org/10.1104/pp.117.1.173. Pereira, T.S., da Silva Lobato, A.K., Tan, D.K.Y., da Costa, D.V., Uchôa, E.B., do Nascimento Ferreira, R., dos Santos Pereira, E., Ávila, F.W., Marques, D.J., Silva Guedes, E.M., 2013. Positive interference of silicon on water relations, nitrogen metabolism, and osmotic adjustment in two pepper (Capsicum annuum) cultivars under water deficit. Aust. J. Crop Sci. 7, 1064–1071. Postek, M.T., 1981. The occurrence of silica in the leaves of Magnolia grandiflora L. Bot. Gaz. 142, 124–134. https://doi.org/10.1086/337202. Prychid, C.J., Rudall, P.J., Gregory, M., 2004. Systematics and biology of silica bodies in monocotyledons. Bot. Rev. 69, 377–440. https://doi.org/10.1663/0006-8101(2004) 069[0377:SABOSB]2.0.CO;2. Raven, J.A., 1983. The transport and function of silicon in plants. Biol. Rev. 58, 179–207. https://doi.org/10.1111/j.1469-185X.1983.tb00385.x. Schaller, J., Brackhage, C., Dudel, E.G., 2012. Silicon availability changes structural carbon ratio and phenol content of grasses. Environ. Exp. Bot. 77, 283–287. https:// doi.org/10.1016/j.envexpbot.2011.12.009. Schaller, J., Brackhage, C., Bäucker, E., Dudel, E.G., 2013. UV-screening of grasses by plant silica layer? J. Biosci. 38, 413–416. https://doi.org/10.1007/s12038-0139303-1. Schreiber, U., Kühl, M., Klimant, I., Reising, H., 1996. Measurement of chlorophyll fluorescence within leaves using a modified PAM fluorometer with a fiber-optic microprobe. Photosyn. Res. 47, 103–109. https://doi.org/10.1007/BF00017758. Sen Mandi, S., 2016. Natural UV Radiation in Enhancing Survival Value and Quality of Plants. Springer, India, New Delhi. https://doi.org/10.1007/978-81-322-2767-0. Setoguchi, H., Okazaki, M., Suga, S., 1989. Calcification in higher plants with special reference to cystoliths. In: Crick, R.E. (Ed.), Origin Evolution and Modern Aspects of Biomineralization in Plants and Animals. Plenum Press, New York, pp. 409–418. https://doi.org/10.1007/978-1-4757-6114-6_32. Shao, H.B., Song, W.Y., Chu, L.Y., 2008. Advances of calcium signals involved in plant anti-drought. C. R. Biol. 331, 587–596. https://doi.org/10.1016/j.crvi.2008.03.012. Shen, X., Li, Z., Duan, L., Eneji, A.E., Li, J., 2014a. Silicon effects on the partitioning of mineral elements in soybean seedlings under drought and ultraviolet-B radiation. J. Plant Nutr. 37, 828–836. https://doi.org/10.1080/01904167.2013.873458. Shen, X., Li, Z., Duan, L., Eneji, A.E., Li, J., 2014b. Silicon mitigates ultraviolet-B radiation stress on soybean by enhancing chlorophyll and photosynthesis and reducing transpiration. J. Plant Nutr. 37, 837–849. https://doi.org/10.1080/01904167.2013. 873459. Shi, Y., Zhang, Y., Han, W., Feng, R., Hu, Y., Guo, J., Gong, H., 2016. Silicon enhances water stress tolerance by improving root hydraulic conductance in Solanum lycopersicum L. Front. Plant Sci. 7, 1–15. https://doi.org/10.3389/fpls.2016.00196. Skinner, H.C.W., Jahren, A.H., 2003. Biomineralization. Treat. Geochem. 8, 117–184. https://doi.org/10.1016/B0-08-043751-6/08128-7. Sommer, M., Kaczorek, D., Kuzyakov, Y., Breuer, J., 2006. Silicon pools and fluxes in soils and landscapes – a review. J. Plant Nutr. Soil Sci. 169, 310–329. https://doi.org/10. 1002/jpln.200521981. Soundararajan, P., Sivanesan, I., Jana, S., Jeong, B.R., 2014. Influence of silicon supplementation on the growth and tolerance to high temperature in Salvia splendens. Hortic. Environ. Biotechnol. 55, 271–279. https://doi.org/10.1007/s13580-0140023-8. ter Braak, C.J.F., Šmilauer, P., 2002. CANOCO Reference Manual and CanoDraw for Windows User’s Guide: Software for Canonical Community Ordination (Version 4.5). Microcomputer Power, Ithaca. Tripathi, D.K., Singh, V.P., Ahmad, P., Chauhan, D.K., Prasad, S.M. (Eds.), 2016. Silicon in Plants: Advances and Future Prospects. CRC Press, Boca Raton. Tubaña, B.S., Heckman, J.R., 2015. Silicon in soils and plants. In: Rodrigues, F.A., Datnoff, L.E. (Eds.), Silicon and Plant Diseases. Springer International Publishing Switzerland, Cham, pp. 7–51. https://doi.org/10.1007/978-3-319-22930-0_2. Ullrich, S.E., 2011. Barley: Production, Improvement, and Uses. Blackwell Publishing Ltd, Oxford. https://doi.org/10.1002/9780470958636. Vandevenne, F., Struyf, E., Clymans, W., Meire, P., 2011. Agricultural silica harvest: have humans created a new loop in the global silica cycle? Front. Ecol. Environ. 10, 243–248. https://doi.org/10.1890/110046. Vekemans, B., Janssens, K., Vincze, L., Adams, F., Vanespen, P., 1994. Analysis of X-ray spectra by iterative least squares (AXIL): new developments. Xray Spectrom. 23, 278–285. https://doi.org/10.1002/xrs.1300230609. Wang, M., Zheng, Q., Shen, Q., Guo, S., 2013. The critical role of potassium in plant stress response. Int. J. Mol. Sci. 14, 7370–7390. https://doi.org/10.3390/ijms14047370.

55

Agricultural Water Management 217 (2019) 47–56

M. Grašič, et al. Wargent, J.J., Jordan, B.R., 2013. From ozone depletion to agriculture: understanding the role of UV radiation in sustainable crop production. New Phytol. 197, 1058–1076. https://doi.org/10.1111/nph.12132. Webb, M.A., 1999. Cell-mediated crystallization of calcium oxalate in plants. Plant Cell 11, 751–761. https://doi.org/10.1105/tpc.11.4.751. Xu, C., Li, X., Zhang, L., 2013. The effect of calcium chloride on growth, photosynthesis, and antioxidant responses of Zoysia japonica under drought conditions. PLoS One 8. https://doi.org/10.1371/journal.pone.0068214. Yao, X., Chu, J., Cai, K., Liu, L., Shi, J., Geng, W., 2011. Silicon improves the tolerance of wheat seedlings to ultraviolet-B stress. Biol. Trace Elem. Res. 143, 507–517. https:// doi.org/10.1007/s12011-010-8859-y.

Yoshida, S., Ohnishi, Y., Kitagishi, K., 1962a. Chemical forms, mobility and deposition of silicon in rice plant. Soil Sci. Plant Nutr. 8, 15–21. https://doi.org/10.1080/ 00380768.1962.10430992. Yoshida, S., Ohnishi, Y., Kitagishi, K., 1962b. Histochemistry of silicon in rice plant. Soil Sci. Plant Nutr. 8, 1–5. https://doi.org/10.1080/00380768.1962.10430992. Yue, M., Li, Y., Wang, X., 1998. Effects of enhanced ultraviolet-B radiation on plant nutrients and decomposition of spring wheat under field conditions. Environ. Exp. Bot. 40, 187–196. https://doi.org/10.1016/S0098-8472(98)00036-7. Zhu, Y., Gong, H., 2014. Beneficial effects of silicon on salt and drought tolerance in plants. Agron. Sustain. Dev. 34, 455–472. https://doi.org/10.1007/s13593-0130194-1.

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