The Influence of Foliar Nutrition of Apple Trees with Silicon on Growth and Yield as Well as Mineral Content in Leaves and Fruits

Świerczyński, Sławomir; Zydlik, Zofia; Kleiber, Tomasz

doi:10.3390/agronomy12071680

Open AccessArticle

The Influence of Foliar Nutrition of Apple Trees with Silicon on Growth and Yield as Well as Mineral Content in Leaves and Fruits

by

Sławomir Świerczyński

^1,*

,

Zofia Zydlik

¹

and

Tomasz Kleiber

²

¹

Department of Ornamental Plants, Dendrology and Pomology, Poznan University of Life Sciences, Dąbrowskiego 159, 60-594 Poznan, Poland

²

Department of Plant Physiology, Poznan University of Life Sciences, Wołyńska 35, 60-637 Poznan, Poland

^*

Author to whom correspondence should be addressed.

Agronomy 2022, 12(7), 1680; https://doi.org/10.3390/agronomy12071680

Submission received: 17 June 2022 / Revised: 4 July 2022 / Accepted: 13 July 2022 / Published: 15 July 2022

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Abstract

:

The technology of producing fruits containing an increased amount of elements essential for the health of the human body, including silicon, has become very important in recent years. Due to the popularity of apple tree cultivation in Europe, appropriate research has been undertaken. The aim of the experiment was to determine the advisability of foliar silicon nutrition in apple tree cultivation. Apple cultivars ‘Gala Schniga’, ‘Ligol’ and ‘Topaz’ were studied and treated with three different doses of silicon (100, 200, 300 mg SiO₂·L⁻¹). The treatments were performed five times during tree growth in the third year of cultivation from early June to late July. Foliar application of silicon did not improve the evaluated tree growth parameters. It significantly influenced, in the highest analysed concentration, a better fruit yield (12%), a higher yield coefficient, depending on the concentration, from 16 to 18%, a better fruit quality (weight and size) and a more extensive red blush. The considered cultivars differed in the growth and yield of trees in the third year of cultivation. ‘Topaz’ grew strongest and ‘Ligol’ yielded the best. The highest yield coefficient was found for the Topaz’ cultivar. The fruit of ‘Topaz’ had the biggest blush and ‘Gala Schniga’ the biggest sugar content. Macro- and micro-nutrient nutritional status of trees were no better, except for iron and copper in leaves, and at some concentrations of applied silicon also zinc and copper in fruit.

Keywords:

fruit trees; additional fertilisation; fruit quality; nutritional status

1. Introduction

Along with oxygen, silicon is the second most abundant element in soil [1]. Although silicon is abundant in soil (50–400 g/kg), it is in the form of oxides and silicates not available to plants [2]. Silicon is taken up by plants as uncharged silicic acid and eventually precipitated irreversibly throughout the plant as silica [3]. Considering mineral nutrition, 14 components are essential for plants, and some of them are considered particularly beneficial as they condition proper plant growth and development [4]. Among them is silicon, which has structural and metabolic functions in plant physiology [5]. Among other things, silicon improves plant tolerance to various abiotic and biotic stresses, among them salinity [6], excessive transpiration [7], pathogenic fungal growth, and low and high temperature stress [8]. In addition, proper silicon nutrition influences the condition of plants, which ensures the uprightness of leaves and thus increases the sunlight interception and photosynthesis efficiency [9]. The benefits of silicates are not always attributed to silicon, among them are the increase in exchangeable Ca and Mg and the increase in P availability [10]. So far, foliar application of silicic acid and boric acid has increased yield and firmness of apple fruit by 17% [11]. The in-soil application of silicon improved vine yield by 13.5% and fruit quality [12]. Lalithya et al. [13,14] observed that foliar application of K₂SiO₃ improved fruit weight of sapota and yield per unit area. More et al. [15] found that silicic acid application improved the yield and quality of mango fruit. The experience of Hanumanthaiah et al. [16] with foliar application of silicon in banana crops showed improvement in fruit quality parameters such as acidity and sugar content. El- Rhman [17] observed that foliar application of kaolin improved pomegranate yield, fruit weight and sugar content. As reported by Gong et al. [18] silicon fertilisation improved water management and dry matter yield of wheat grains under water deficit conditions and reduced micronutrients in rice leaves [19]. Kumbargire et al. [20] found the highest content of macronutrients and some micronutrients (Zn, Cu) in banana leaves after silicon application, which guaranteed better tree growth and fruit yield.

An experiment was conducted to verify these statements and to confirm the usefulness of foliar silicon application in apple trees. The results of this experiment will answer the question of whether foliar nutrition with silicon affects the growth, yield and quality of apple fruit.

2. Material and Methods

2.1. Plant Material and Growth Conditions

The experiment was carried out at the Experimental Station in Poznań belonging to the Poznań University of Life Sciences. Apple tree cultivars ‘Gala Schniga’, ‘Ligol’ and ‘Topaz’ growing on M.9 rootstock were studied and treated with three different doses of silicon (100, 200, 300 mg SiO₂·L⁻¹). The source of silicon was silica sol (Optysil, Intermag Olkusz, Poland). The control treatment consisted of cultivation without additional Si foliar nutrition and with distilled water spraying. The experiment set up was organized as a completely randomized block design with four replications per treatment and five trees per replicate. To avoid any contamination between treatments, replicates on the same row were separated by an interval of three untreated trees, whereas a buffer row was used to separate plots on adjacent rows. Trees were selected according to their uniformity as for flowering and growth, by estimating the number of flowers per tree and measuring trunk circumference at 20 cm from the ground, respectively. Silicon applications to the tree canopy started 50 days after full bloom (DAFB) at the beginning of June and were performed five times at two weekly intervals until the end of July. Tree care consisted of cutting in spring and summer as recommended. Care work included irrigation with drip lines, manual weed control, and performing spraying treatments against diseases and pests. The experiment was carried out on podzolic-type soil. Before the growing season in 2020 began, the soil had been analysed chemically. It contained the following amounts of soluble forms of macronutrients: 93 phosphorus (mg·dm⁻³), 122 potassium (mg·dm⁻³), 375 calcium (mg·dm⁻³) and 75 magnesium (mg·dm⁻³). The total rainfall in 2020 was 438 mm. The amount of rainfall was lower than the annual average for the last twenty years, amounting to 550 mm.

2.2. Growth of Apple Trees, Yield and Quality of Fruit Measurements

All the measurements of the tree growth vigour were conducted in the autumn of 2020. The height of the trees, trunk diameter 30 cm above the soil surface and the crown span in two planes was measured. The vigour of growth was estimated on the basis of trunk cross-sectional area (cm⁻²) calculated from the measurement of the tree trunk circumference. The productivity of the individual trees was calculated on the basis of the yield of fruit per 1 cm⁻² of the trunk cross-sectional area. After ripening, the fruit was harvested from the trees. The weight of 100 fruits, their diameter and degree of blush was determined. Also delivered the following parameters: number of fruits and total yield per tree, average fruit weight. The colorimetric coordinates (L*, a*, and b*) were determined with a colorimeter (Minolta, model CR-400, Tokyo, Japan) by measuring ten fruits randomly selected among those belonging to the same replicate at five different positions around the equatorial side of each fruit. Values are presented as colour index [CI = (1000 * a)/(L * b)], with a higher CI value indicating a more intense red colour in the fruit. The total soluble solids (TSS as °Brix), titratable acidity (TA as g L⁻¹ of malic acid) of ten fruits per replicate (40 per treatment) were determined at harvest.

2.2.1. Chemical Analysis of Soil Samples

Soil samples were collected for chemical analyses in late July. Collected samples were chemically analysed by the universal method. Extraction of macronutrients (N-NH₄, N-NO₃, P, K, Ca, Mg, S-SO₄), Cl and Na was carried out in 0.03 M CH₃COOH with a quantitative 1:10 proportion of substrate to extraction solution. After extraction, the following determinations were made: N-NH₄, N-NO₃—by microdistillation according to Bremer in Starck’s modification; P—calorimetrically with ammonium vanadomolybdate; K, Ca, Na—photometrically; Mg—by atomic absorption spectrometry (ASA); S-SO₄—nephelometrically with BaCl₂; Cl—nephelometrically with AgNO₃. Micronutrients (Fe, Mn, Zn and Cu) were extracted from the soil with Lindsay’s Solution containing in 1 dm³: 5 g EDTA (ethylenediaminetetraacetic acid); 9 cm³ of 25% NH₄ solution, 4 g citric acid; 2 g Ca (CH₃COO)2·2H₂O. Micronutrients were determined by the 150 ASA method. Salinity was identified conductometrically as an electrolytic conductivity (EC in mS∙cm⁻¹) (substrate:water = 1:2), and pH—was determined by a potentiometric method (substrate:water = 1:2).

2.2.2. Chemical Analysis of Leaves and Fruit Samples

In early August of the research, samples of leaves were collected from the middle part of the long shoots of randomly selected apple trees for analyses of the content of macro- and micronutrients. The total nitrogen content was measured with the Kjeldahl method on a Parnas-Wagner apparatus. The phosphorus content was measured with a colorimetric method with ammonium molybdate. The potassium and calcium content were measured with flame photometry, whereas the magnesium content was measured with atomic absorption. The content of macronutrients was expressed as a percentage. The content of the total forms of micronutrients such as iron, zinc, manganese, and copper (ppm) were measured by means of atomic absorption spectrometry (ASA) after wet mineralisation of 2.5-g samples in a mixture of nitric acid and perchloric acid at a volume ratio of 3:1.

2.3. Data Analysis

Data was analysed with the STATISTICA 13.1 software. Two-way analysis of variance with the Duncan test was applied separately for individual characteristics of the growth, yield, and fruit quality of apple trees. The results of the leaf and fruit content of macro- and micronutrients were subjected to the same analysis of variance. The differences were considered significant at p = 0.05.

3. Results

3.1. The Growth Parameters of Apple Trees

The crown volume of apple trees depended on the cultivar under study, as well as on foliar silicon treatment in the case of ‘Gala Schniga’. This variety had a larger crown volume at the lowest silicon concentration than in the control. The average volume of tree crowns for the three studied cultivars did not differ depending on the foliar treatments. Trees of ‘Gala Schniga’ and ‘Topaz’ were characterised by a larger crown volume than the ‘Ligol’ cultivar (Table 1). Only the trees of the ‘Ligol’ variety with two lower silicon concentrations were lower than the control ones. The mean values for the three tested cultivars did not differ depending on the foliar treatments. The trees of ‘Ligol’ were lower than those of ‘Topaz’ (Table 2). Application of differential foliar silicon nutrition had no effect on the most important growth parameter of apple trees i.e., TCSA (trunk cross sectional area). Only in the case of ‘Ligol’, the application of the lowest concentration of this element decreased the strength of tree growth compared to the control (Table 3). The mean value for the three cultivars considered, did not differ among the silicon fertilisation combinations. The strongest tree growth was obtained for ‘Topaz’ and the weakest for ‘Gala Schniga’.

3.2. Apple Tree Yield and Quality of Fruits

Foliar application of silicon, irrespective of the dose, had a positive effect on the fruit weight of ‘Ligol’ apples (Table 4). No such regularity was found for the other two varieties. The average fruit weights of the three cultivars tested were higher at the highest silicon concentration. The ‘Ligol’ variety was characterised by fruit with the highest weight, while ‘Topaz’ had the lowest. Foliar application of the highest dose of silicon resulted in increased fruit size in ‘Ligol’ (Table 5). On the other hand, for ‘Topaz’, two lower silicon concentrations decreased the size of the fruit as compared to the control ones. The average fruit size at 300 mg SiO₂·L⁻¹ concentration was better than the control and worse than the control at lower concentrations. The ‘Ligol’ variety had larger fruit than the other two. Fruit yield per tree was dependent on foliar nutrition applied for two cultivars. An increase in fruit yield of ‘Ligol’ and ‘Topaz’ was observed at the highest concentration of this component, but it was not significantly higher than the control (Table 6). However, the mean value for this treatment was significantly better. The fruit yield for the ‘Ligol’ variety was significantly higher than that for the ‘Gala Schniga’ variety. Tree yield coefficients for the two extreme silicon concentrations in ‘Gala Schniga’ and all applied concentrations in ‘Topaz’ turned out to be higher than the control. The mean values for the silicon treatments were also better than the control. The ‘Topaz’ variety had a higher yield coefficient than the other two cultivars (Table 7).

Fruit brightness level (L) did not vary depending on the silicon foliar fertilisation. The fruits of ‘Gala Schniga’ and ‘Ligol’ were paler than those of ‘Topaz’ (Table 8). Fruits of ‘Gala Schniga’ at the highest concentration of silicon applied were redder (parameter a) than the control ones (Table 9). However, in the other two cultivars, the average silicon concentration resulted in less intense red coloration of the fruit compared to the control. On average, for all cultivars considered, the highest concentration of the element resulted in better red coloration of fruit, while the average resulted in less red coloration than the control. ‘Gala Schniga’ had redder fruit than the other two. Only the silicon concentration of 200 mg SiO₂·L⁻¹ resulted in the enhancement of the yellow colour (parameter b) of the fruit of ‘Gala Schniga’ and ‘Ligol’ compared to the control (Table 10). The mean value was also the best at this concentration. ‘Topaz’ fruits were the most yellow and ‘Gala Schniga’ the least.

Sugar content was similar for all treatments relative to the control. Only the average concentration of silicon fertilisation reduced sugar levels for the ‘Ligol’ variety (Table 11). Additionally, lower sugar content in fruits were found in the ‘Ligol’ variety as compared to the other two. Fruit acidity for ‘Gala Schniga’ and the average for the three cultivars at the average concentration of foliar applied silicon was the lowest, and at 300 mg SiO₂·L⁻¹ the highest (Table 12). Acidity of ‘Gala Schniga’ fruits were the lowest, and ‘Ligol’ had the highest.

3.3. The Concentration of Macro and Microelements in Apple Tree Leaves and Fruits

On average, the highest leaf nitrogen content was obtained at the highest concentration of silicon applied, and in the control. Lower nitrogen levels were found for ‘Gala Schniga’ than the other two cultivars (Table 13). On average, the highest phosphorus content was obtained for the second silicon dose and was higher for ‘Gala Schniga’ than for ‘Ligol’ (Table 13). On average, higher potassium levels were found for the silicon concentration of 300 mg SiO₂·L⁻¹ and the control combination, and for the ‘Gala Schniga’ variety (Table 13). Also, in the case of calcium, the two aforementioned combinations had a higher accumulation of this element, as did ‘Topaz’ (Table 13). There was no effect of silicon nutrition on mean leaf magnesium and sodium content. Only ‘Ligol’ and ‘Topaz’ had higher sodium content than the third one considered (Table 13). Regardless of the silicon concentration used, a higher iron content was obtained than the control (Table 14). The first two cultivars considered had more leaf iron than the third one. Only the highest silicon concentration guaranteed similar leaf manganese content as the control. The effect of varieties was the same as for iron. The highest zinc content was found for the control and lower than the other two, for the ‘Ligol’ variety. Copper levels were highest for the two higher silicon concentrations than the control and for the ‘Gala Schniga’ than the other two cultivars (Table 14).

The nitrogen content of the fruit did not differ depending on the level of silicon fertilisation and the cultivar considered (Table 15). Phosphorus and potassium levels in fruit were higher only for ‘Gala Schniga’ than the other two cultivars (Table 15). Better calcium nutrition was obtained for the control fruit and at the highest silicon concentration and for the ‘Topaz’ variety. For magnesium and sodium content, there was no effect of silicon nutrition. The fruit of ‘Topaz’ had more magnesium than ‘Ligol’. The highest iron content in fruit was found at the highest silicon concentration and for the ‘Ligol’ variety (Table 16). Manganese was highest in the control combination and for ‘Gala Schniga’ fruit. Zinc for a medium silicon dose and the ‘Topaz’ variety. Copper for the lowest silicon dose and the ‘Ligol’ variety.

4. Discussion

Differential foliar silicon fertilisation had no significant effect on the studied growth parameters of apple trees. These results are not consistent with the observations of Saleem and Joody [21] reporting that foliar application of K₂SiO₃ increased trunk diameter and number of second order lateral branches in apple trees. Also, All-Hamadani and Joody [22], found that silicon nutrition at a dose of 50 mL L⁻¹ increased the height and number of lateral shoots of peach trees. Similarly, Wang and Galletta [23] with foliar application of potassium silicate obtained higher strawberry plants. On the other hand, the study by Soppels et al. [24] proved no improvement in the growth of apple trees of the ‘Jonathan’ variety after foliar application of Siliforce silicon fertiliser, as assessed by the dynamics of one-year shoot growth. However, the same authors obtained an improvement in the yield of apple trees as shown by the yield and number of fruits per tree after application of the above-mentioned silicon fertiliser. Similarly, in the experiment under consideration, a significant increase in fruit yield per tree was obtained after applying the highest silicon concentration tested (the same as in the aforementioned authors) and improvement in tree yield coefficient at each silicon concentration tested. On this basis, it can be concluded that the silicon dose (300 mg SiO₂·L⁻¹) is the most appropriate and significantly affects the yield of apple trees. This is confirmed by the research of Wang et al. [25] who, after applying sodium silicate to the soil in a dose of 100 g per tree, obtained an increase in the yield of apple trees of the ‘Fuji’ variety by 17%, and at half the dose they did not find a significantly better yield as compared to the control. The experiment showed an increase in yield of apple trees at the highest concentration of foliar silicon. On the other hand, Hussien and Kassem [26] after foliar application of potassium silicate at concentrations of 1 and 2% found improved yield of fig trees, which increased with increased concentration of silicon fertiliser. Similarly, other authors [27,28] found improved yield in tomato and onion, respectively, after soil application of diatomite (source of silicon) at different doses. Also, Salim et al. [29] obtained improved yields of squash, which increased with silicon dose, and also under water stress conditions. On the other hand, Górecki et al. [30] using different forms of silicon (Na₂SiO₃, CaSiO₃, K₂SiO₃) at a dose of 1.5 g dm⁻³ obtained an increase in dry matter in fruit extract, but did not find better yields of tomatoes. Similarly, Segalin et al. [31] with foliar application of silicon did not obtain any improvement in wheat grain yield. This proves that silicon applications don’t necessarily translate into better yields for treated plants. Variable effects of silicon fertilisation on yield can be explained by the variable susceptibility of different plant species to the supply of this element in foliar form. The apple cultivars themselves in the experiment also differed in their response to silicon fertilisation. The most sensitive cultivar was Ligol, which responded with the highest yield increase and improvement in fruit size and weight. This cultivar differed in fruit yield per tree from ‘Gala Schniga’. The ‘Topaz’ variety was characterised by an average fruit yield, which was better than that obtained by other researchers [32] when using standard NPK soil fertilisation (9.79 kg). Sturm [33] studied the yield of five different mutants of the ‘Gala’ variety, among them ‘Gala Schniga’, which in the same year of cultivation (third year), gave a fruit yield 1/3 lower (6.8 kg). However, this was due to climatic conditions, particularly frosts during the flowering period of the trees.

In the experiment conducted, an improvement in fruit quality, weight and size was obtained when the highest concentration of silicon was applied. Also, Abd Al-Rhman et al. [34] observed improvement in fruit weight of ‘Anna’s apple, but without change in fruit size, with foliar application of 3% potassium silicate. Similarly, other researchers [35,36] obtained improvement in mango and strawberry fruit weight with foliar silicon fertilisation. Also, the form of silicon fertilisation had a significant effect on mango fruit quality. This may support the view of Kleiber et al. [37] that the application of silicon fertiliser increases the water content of leaves and the fresh weight of plant organs at the same time. Consistent with this is the study by Marodin et al. [38], who obtained an improvement in tomato fruit size using different doses of silicon. In the absence of silicon fertilisation, the marketable yield was 40.1 t ha⁻¹ and non-marketable yield was 26.6 t ha⁻¹ when calcium silicate was applied at 4000 kg ha⁻¹, (56.0 t ha⁻¹ and 15.1 t ha⁻¹, respectively).

In the experiment in question, foliar silicon fertilisation at the highest dose improved fruit acidity but had no effect on sugar content. In a similar experiment [34], an increase in sugar content and acidity of apple fruit was obtained after silicon fertilisation, but only in one of the two growing seasons considered. In the above-mentioned experiment, out of three applied silicon fertilisation dates, the second one falling on the beginning of June was the most effective. On the other hand, another [36] reported an improvement in sugar content and acidity of strawberry fruits after application of silicon-containing fertiliser (Siliforce). Weerahewa et al. [39] obtained lower acidity of tomatoes at lower silicon doses (50 and 100 mg SiO₂·L⁻¹) regardless of the growth stage of plants. On the other hand, higher doses of silicon resulted in higher levels of sugars, which was not confirmed in the experiment under consideration. Results obtained by Beckles [40] suggest that the effect on plant response to foliar application of minerals depends on many factors including: time and method of application, chemical form of silicon, and plant genotype. However, he stated that the application of silicon fertiliser could improve the sugar content and reduce the acid content of the fruit, which was not confirmed in this experiment. Also, no effect of silicon fertilisation on sugar content in sugar beet was found by other researchers [41].

Based on this experiment, the red coloration of apple fruit was improved. Also, Soppelsa et al. [24], using silicon-containing fertiliser found an improvement in the intensity and occurrence of red blush in the ‘Jonathan’ apple cultivar. In this experiment, the anthocyanin content of the fruit peel was increased, indicating a potential effect of this element on the synthesis of secondary metabolites. In our experiment the best red coloration of fruit was obtained in ‘Gala Schniga’, which confirms the observations of other authors [33,42] who report that about 90% of ‘Gala’ mutants have more than one third of the skin surface covered with red blush.

Based on this experiment, there was no improvement in macronutrient nutrition of apple trees. Very similar results were obtained by Zhang et al. [43], who studied the effect of silicon fertilisation and found an increased content of calcium in apple leaves and no change in the content of other macronutrients. Also, Sollplesa et al. [36], on foliar application of silicon fertiliser found an increase in the content of potassium only in strawberry leaves, which were also similar in phosphorus and magnesium, and lower in nitrogen and calcium. Similarly, Saavas et al. [44] in the hydroponic cultivation of gerbera after adding silicon to the nutrient solution obtained increased levels of only calcium in gerbera leaves without observing any change in the levels of other macronutrients. Different results were obtained by Saalem and Joody [21] with increased levels of nitrogen, phosphorus, potassium and calcium in apple leaves, which increased with an increasing silicon dose. Similarly, Cuong et al. [45] found increased accumulation of nitrogen, phosphorus and potassium, in rice leaves, with the same increasing trend. Also, other researchers [21] when fertilising with two concentrations of silicon obtained increased levels of nitrogen, potassium, and calcium in fig fruit, to a lesser extent phosphorus (in only one of the two years of observation). The micronutrient content in the experiment only increased for iron and copper in the leaves after silicon fertilisation. Other researchers [36] using fertilisation with this element in strawberry cultivation obtained an increase in the level of most micronutrients, except for manganese. Also, others Gu et al. [46] only found an increase in Zn content in leaves of rice seedlings after silicon application. Such an increase in zinc accumulation was not found in the experiment conducted. In contrast, Cu content increased at higher silicon doses, which also did not confirm the observations of Li et al. [47], who observed that the level of this component does not change in leaves even after external application of silicon. Diverse relationships were found by Saavas et al. [44], because they obtained a reduction in the content of Zn and Cu in gerbera leaves and no effect of this element on the content of Mn and Fe, which was also not confirmed in the experiment under consideration. In the opinion of some authors [7,48] the role of silicon fertilisation is, among others, to increase the uptake and transport of water and nutrients by stimulating the development of the root system. However, this was not confirmed by the experiment conducted.

5. Conclusions

Silicon fertilisation applied in this experiment did not improve the growth parameters of apple trees. However, it increased the yield of trees as well as the quality of fruit expressed by its weight, size and red colour. Moreover, all applied levels of silicon fertilisation had a positive effect on the yield coefficient of trees. Foliar treatments of apple trees with silicon did not improve their nutrient status in macronutrients, increasing only the levels of some micronutrients (iron and copper). However, the yield and fruit quality parameters, which are the most important from the apple fruit producer’s point of view, improved, which leads us to recommend foliar silicon application in the cultivation of this species at a dose of 300 mg SiO₂·L⁻¹.

Author Contributions

Conceptualization, S.Ś. and T.K.; methodology, S.Ś.; formal analysis, S.Ś.; data curation, Z.Z. and T.K.; writing—original draft preparation, S.Ś. All authors have read and agreed to the published version of the manuscript.

Funding

The publication was co-financed within the framework of the Polish Ministry of Science and Higher Education’s program: ‘Regional Initiative Excellence’ in the year 2019–2022 (No. 005/RID/2018/19)’, financing amount 1,200,000 PLN.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.