Feasibility of Using a Silicon Preparation to Promote Growth of Forest Seedlings: Application to Pine (Pinus sylvestris) and Oak (Quercus robur)

Abstract

The present study was inspired by scientific reports describing the positive effects of silicon preparations on fruit and vegetables in horticulture. The use of silicon preparations in forest protection represents a new research application, especially in the cases of oak seedlings that are damaged by oak powdery mildew. Additionally, these preparations increase the photosynthetic efficiency of the seedlings, and thus promote increased biomass and better nutritional value of the root systems. The new idea of using silicon formulations in forestry, based on the initial test results, is particularly important for seedlings in the post-planting period in forest plantations. In particular, these preparations increase yield and plant growth, and improve plant health. So far, no comparable studies have been conducted on forest tree species. To this end, one year-old pine (Pinus sylvestris L.) and two year-old English oak (Quercus robur L.) seedlings were sprayed with silicon preparations of different concentrations, and changes in growth parameters and photosynthetic efficiency were observed. The main objective of the study was to determine the effect of the silicon preparation on the seedlings during their first life span, i.e., the first year after planting. In this study, 50 pine and 50 oak seedlings were sprayed three times with the silicon preparation, in five variants (control; concentrations of 1%, 2%, 3% and 5%), as well as the chlorophyll fluorescence, height, thickness and biomass of the roots, were measured. It was found that the treatment with a concentration of 2% was better and statistically, significantly different from the other variants, e.g., in terms of increasing photosynthetic efficiency.

1. Introduction

Recently, research has focused intensively on the potential use of silicon in agriculture and horticulture [1,2,3,4], which will be discussed in the individual sections of this article. In contrast, little work has been done in forestry, and given the many benefits of silicon described below, we sought to test a silicon preparation on two primary forest-forming species. In Poland, pine dominates among conifers, and English oak among hardwoods, in terms of nursery production. The species should be planted in forest plantations, where the soil is rich in humus and the pH is 4.0–4.5 or 5.5–6.5 for the mentioned species [5,6]. In practise, the humus layer is deposited laterally during ploughing for technical reasons, and the plants are planted in mineral-poor soils, so there is a risk of element deficiency, especially in the first years after planting. Therefore, we hypothesised that foliar application of silicon would strengthen the seedlings in the first period after planting in forest plantations.

Silicon (Si) is one of the most abundant elements on Earth, making up 26% of the entire Earth’s crust. When combined with oxygen, it forms the independent mineral quartz (SiO${}_2$), which occurs in granites, sands, sandstones and clays. Silicon is also found in amorphous silica, aluminosilicates, humus and organo-mineral compounds that play an important role in the physiology (metabolism) of plants, animals and humans. Silicon, along with C, N, H, O, S, P, K, Ca, Mg and Na, is an essential component in plants, together with the micronutrients Fe, Mn, Cu, Zn, Mo, B and Cl [7]. It is adsorbed in the form of inorganic ions or acids, and is found in this form in plant cells, or is mainly bound to the hydroxyl groups of sugars to form phosphate, borate or silicate esters [8]. The study by Kulikova et al. [9] confirmed that siliceous preparations improve crop yield and quality. As foresters, we hope to observe the positive properties of silicon, and are inclined to start research on its use in breeding and protecting seedlings of forest species.

In soil, silicon is released slowly, and in small amounts, by the acidification of silicates with carbonic acid, produced by the secretion of enzymes by microorganisms and insect larvae [10,11]. When silicon is released from minerals, some of it is immediately taken up by other free elements that are present in water, soil particles or near plant roots. Some of the silicon is irretrievably lost through the process of leaching. Furthermore, only a small proportion of the remaining silicon can be taken up by plant roots, because it is very unstable as a small molecule and quickly polymerises into a long chain of biologically inactive particles, known collectively as colloidal silicon and silica gel [12,13].

Silicon increases the supply of phosphorus to plants by up to 40–60%, without the need to use additional sources of phosphorus, and this increases the efficiency of the application of phosphorus from rock by 100–200% by preventing the conversion of phosphorus into insoluble compounds. Silicon as a soil additive can reduce the leaching of nutrients into sandy soils, especially nitrogen and potassium, by storing them in a form available to plants [14,15]. Dziadowiec [16] points out that, depending on their cation exchange capacity (CEC), soils can adsorb nutrients with electrical colloid charges, which hinders their uptake by plants. The nutrients must first enter the soil solution, where silicon attaches to the colloids, releasing them and making them available to plants. When fertilisers are used as a source of phosphorus, much of the phosphorus is not taken up by plants, and phosphorus-isolating reactions occur in the soil [11].

Nutrients are needed to increase biomass (in forest wood), and foliar fertilisation is often the most effective of the various fertilisation methods for supplementing biomass. Corrective foliar fertilisation aims to compensate for nutrient deficiencies that may occur during the development cycle of the plant. If this supplementation were done through the soil, the plants would respond more slowly, and there would be greater losses in plant production. Foliar fertilisation can also be “preventive”, if it is known that a nutrient (in most cases a micronutrient) has low concentrations within the soil, and its supply via the soil would be ineffective. On the other hand, meeting the nutrient needs of plants through foliage alone, as in floriculture, for example, cannot replace nutrient supply through the soil. For the vast majority of crops, this substitution would be very difficult, especially for macronutrients, as a large number of applications would be required, making the process uneconomical [17].

Availability of the above nutrients must be ensured for optimum seedling growth in the soil after planting, taking into account foliar fertilisation, including that of silicon (Si). The availability of N, P, Ca and Mg depends on pH, water solubility and the presence of microorganisms, while the availability of silicon and its importance have not yet been fully studied. In recent years, the use of liquid fertilisers, either completely dissolved or in aqueous suspensions, has become widespread [18]. Liquid fertilisers are usually easier to transport than solid fertilisers, as they are less cumbersome to handle, process and apply [7].

The test formulation applied to the leaf surface penetrated the epidermis per se, presumably along cracks or imperfections in the epidermis, and/or through stomata. The epidermis is an effective barrier to water loss, but at the same time, it hinders the uptake of products from foliar applications, therefore, substances that lower the surface tension of the working fluid were used. The presence of cracks in the epidermis or the appearance of altered epidermis structures can significantly affect the absorption rate of foliar fertiliser applications [19]. In summary, the basic requirement for effective foliar fertilisation is that the active ingredient penetrates the plant’s surface, so that it can become metabolically active in the target cells where the nutrients are needed [19].

Another advantage of using silicon is that its solution can be successfully combined with fungicides and insecticides. Furthermore, the addition of silicon preparations leads to better hydration, and, thus, better permeability of the plant epidermis (cuticle), which increases nutrient uptake and improves the effect of pesticides and/or foliar fertilisers containing micronutrients [20]. Therefore, it is worthwhile to add a low-percentage aqueous solution of the silicon preparation to the spray broth when applying pesticides, especially to plants that are wilted, diseased or damaged by pests. To protect against further water losses or infection, the plants then thicken the epidermis structure, which reduces the penetration of pathogens into the leaves. Silicon preparations can be applied through the leaves, as they do not cause phytotoxicity when applied correctly (in a solution with a certain concentration).

The presence of silicic acid in plant cell walls prevents pathogens from entering the plant tissue. Silica stimulates and prevents the toxic effects of elements, including manganese, iron, aluminium, zinc and others that are present in higher concentrations in the soil, on plants [20].

Recently, the molecular mechanisms of resistance reactions in plants and their role in regulating the expression of selected genes have been discovered. It can be assumed that the mechanism of silicon’s impact on plants is similar, and is based on the activation or deactivation of transcription of different genes, with possible hormonal and mitogen-activated protein kinase (MAPK)-mediated regulation. As a micro-nutrient that protects plants from pathogen attack, silicon can enhance the up-regulation of several genes, including the WRKY-11d5 gene, involved in systemic acquired resistance (SAR), as well as the 1-aminocyclopropane-1-carboxylate oxidase (ACCO), jasmonate and ethylene-responsive factor (JERF3), and tomato stress-responsive factor (TSRF1) genes, involved in the ethylene- or jasmonic acid-mediated hypersensitivity response (HR) in the plant cell. Some genes, i.e., phenylalanine ammonia lyase (PAL), cinnamyl alcohol dehydrogenase (CAD) and chalcone synthase (CHS), responsible for the biosynthesis of phenolic, flavonoid and phenylpropanoid compounds, are also actively expressed in plants when biotic stress from Podosphaera pannosa or Magnaporthe oryzae infestation occurred [21,22]. Increased expressions of the pathogenesis-related genes (PR-1) and several peroxidases (POD) were observed in plants coping with pathogen infestation.

Since the early observation that Si has an impact on plant biomass [23], insight at the molecular level has shown that this microelement plays an important role in regulating plant homeostasis under abiotic stress. Of the many mechanisms that may play a role in this area, two are quite well elucidated at the level of Si-mediated gene expression: (1) the first acts by activating the transcription of genes encoding various membrane transporters; (2) the second occurs by regulating gene expression through complex intracellular signalling pathways involving plant hormones and MAPK kinases [24,25,26].

First of all, silicon uptake by plant roots is tightly regulated by low-silicon genes (Lsi1, Lsi2, and Lsi6), which encode membrane transporters such as aquaporins or anion transporters that promote the activity of the proton H+-ATPase pump. This complex interaction between membrane transporters is essential for the uptake of silicon and its distribution within the endodermis, cytoplasm and vacuoles of the roots and shoots of many plants [27,28].

Recent studies have reported that silicon may be involved in the up- or down-regulation of protein-coding genes responsible for the endogenous hormonal response of plants to salt or drought stress. Nevertheless, increasing soil salinity poses an immediate threat to the cultivation of lemongrass, as it is moderately sensitive to salt [29]. For this reason, the above authors used silicon nanoparticles (SiNPs) to stimulate salt tolerance in lemongrass, as SiNPs are of particular importance in stress situations [29]. In this context, Si could act as an up-regulator of abscisic acid biosynthesis in rice, or as a down-regulator of jasmonic acid biosynthesis in rice and legumes [30,31]. A detailed list of housekeeping genes that are up- and down-regulated in rice upon Si supplementation and abiotic stress was presented by Manivannan and Ahn [25]. By studying the transcriptome of Vitis vitifera under drought stress, activation of the gene expression of several proteins of the MAPK kinase family, commonly known as enzymes involved in the signalling network cascade, was detected [32]. The activation of numerous genes encoding MAPK kinases has also been demonstrated in response to powdery mildew, drought and plant hormones, such as salicylic acid and ethanol [33].

It has also been suggested that Si influences gene expression, leading to the attenuation of the negative effects of many other abiotic stresses, e.g., heavy metal pollution (via the regulation of genes, such as POD and superoxide dismutase SOD), oxidative stress caused by reactive oxygen species (SOD and MAPK) and exposure to low or high temperatures (ascorbic peroxidase (APX) and guaiacol peroxidase(GPOX)) [26,34].

In the literature, there are descriptions of the effect of silicon preparations on fruit trees and vegetables or cereals, but there are no studies on the effect of these preparations on forest trees. In the present study, the first trials with a silicon preparation were carried out, in order to determine its effect on the growth and development of pine and oak seedlings, especially concerning the possibilities of controlling oak powdery mildew.

2. Materials and Methods

2.1. Plant Material and Silicon Preparation

In Polish forestry, due to site conditions, pine seedlings are raised in nurseries for only one year and then are planted outside, while deciduous tree species (including oaks) are raised for more than 2–3 years, and are only then planted outside. The idea behind this is that they develop better following planting, and foresters’ experience shows that they then have a higher survival rate. In our trial, we followed these principles and took seedlings from forest nurseries so that the results would be relevant to common practise.

The pine seedlings were obtained from the forest nursery of the Lomźa forest district, and the oak seedlings were obtained from the Knyszyn forest district. One year-old pine (Pinus sylvestris) and two year-old oak (Quercus robur) seedlings (50 each) were used for the study. The age of the seedlings was typical of forestry practise and they were planted in May 2021 in plastic pots, with a capacity of about 2 L for pines and 5 L for oaks. The substrate was soil from the tree nursery. Five variants of the experiment were prepared, including the control, and four concentrations of the preparation (1%, 2%, 3% and 5%). For each variant, 10 identical seedlings were selected. The pots between the variants were placed 0.5 m apart, while for one particular variant, they were placed every 20 cm apart (Figure 1).

The silicon formulation contained 2% silicon (SiO${}_2$), and fertilisation of the pines was done regularly, every month, starting on 9 June 2021, then 10 July 2021 and then 10 August 2021. Before the first spraying, the seedlings were measured, i.e., the height, thickness and length of the pine needles were measured. These measurements were taken before each subsequent spraying. In the case of the oaks, a total of six measurements of height and diameter were taken at the root necks of the seedlings; these were taken on 2 August 2021, 1 September 2021, 16 May 2022, 1 August 2022. 1 September 2021 and 30 September 2022, at the end of the growing season.

A Venus Super 360 hand sprayer with a capacity of 1 litre and a fogging nozzle was used for spraying. Foliar spraying of the oak seedlings was carried out on 2 August 2021, 16 May 2022 and 1 August 2022. The spraying of the pine seedlings with silicone preparation was carried out regularly every month, with the spraying procedures being postponed in rainy weather. The first spraying was done on 9 June 2021, then it was repeated on 10 July 2021; the last spraying was done on 10 August 2021.

2.2. Testing the Photosynthetic Efficiency

The photosynthetic efficiency of the seedlings was determined by measuring the chlorophyll fluorescence, which is inversely proportional to the photosynthetic efficiency. The analysed photosynthetic performance parameters mainly comprised the index performance total (IP) parameter, which reflects all other measurements of chlorophyll fluorescence (which is inversely proportional to photosynthesis). However, the Fv/Fm ranges (=$\varphi$Po maximum quantum yield of PSII) were also checked.

The measurement was made with a chlorophyll fluorometer (Handy PEA+), on 4 pairs of randomly selected needles and 4 randomly selected leaves of the seedlings, representing each variant of the experiment (100 seedlings). Twenty-five minutes before the measurement, clips were attached to the needles/leaves to silence the photosynthetic processes in the tissues. Chlorophyll fluorescence of the oaks was measured on 16 September 2021 (more than one month after the first treatment). The measurement lasted 1 s and the applied light pulse reached an intensity of 3500 $\mathsf{\mu }$mol/m²/s. In the pine seedlings, immediately after the third and last spraying on 10 August 2021, the chlorophyll fluorescence of the needles was measured using a chlorophyll fluorometer (Handy PEA+).

2.3. Measurement of Fresh and Dry Biomass

The fresh biomass of the selected needles was weighed, then the length was measured with a ruler, and the thickness with a calliper. The measured needles were kept wrapped in paper at room temperature. After a week, they were dried in a drying oven (65 °C) and weighed. The same procedure was applied to oaks, weighing the above-ground and below-ground parts.

The fresh weight of the root system of the seedlings was weighed; then, the length of the main roots was measured with a ruler, and the thickness of the root necks with a calliper. The roots were laid out to dry on paper at room temperature for a fortnight. Then, they were dried to a constant weight (65 °C), and the dry weight was weighed.

Both the dry weight of the leaves and that of the needles were measured, but not all the results are presented, as we have only selected the roots for this work. The reason for this is that we focused on photosynthetic efficiency for needles and leaves. We considered that this would not bring new results to the discussion.

2.4. Statistical Evaluations

The differences between the investigated experimental variants were analysed, in order to check their statistical significance. Since different concentrations of silicon treatment were applied in the experiments, we analysed the relationship between the concentration and different measured variables. For this purpose, we used linear regression models with two explanatory variables: the first and second power of the silicon concentration. The transformation of silicon concentration to the second power was used, because the observed relationship in the data was not strictly linear. We assumed statistical significance when the p-value < 0.05 for all regression coefficients through the model.

Data processing and statistical analysis of the data were performed using SAS 9.4 software (SAS Institute, Cary, NC, USA) via the SAS Enterprise Guide user interface, as well as the SAS/Stat procedures [35], in particular PROC REG.

3. Results

3.1. P. sylvestris

It was observed that photosynthetic efficiency, as measured by chlorophyll fluorescence, increased with the concentration of silicon preparation (Figure 2).

In pines, supplied silicon reduced the needle length but increased the needle thickness (Figure 3).

The height growth of the seedlings, as well as their diameter in the root necks, was most favourable at the 2% concentration of the silicon preparation (Figure 4).

It was observed that the studied pine seedlings first invested in the expansion of their above-ground parts, and they are expected to expand their root system (e.g., their length) only in the next stage, already showing signs of increased thickness here (Figure 5). However, the biomass decreases significantly with increasing concentrations of the silicon preparation.

3.2. Q. robur

In the oak seedlings, chlorophyll fluorescence measurements showed an increase in the photosynthetic performance at a silicon formulation concentration of 2% (Figure 6), however, the statistical tests have not confirmed the significance of these results.

The application of silicon preparations diminished the degree of oak tissue infection by powdery mildew (Figure 7).

The increase in height was highest at the 2% concentration of the silicon preparation, in contrast to the increase in root neck thickness, which was highest at the 1% concentration (Figure 8).

The highest amount of dry biomass was obtained at a silicon preparation concentration of 2% (Figure 9). Further increases of the preparation concentration did not improve the performance of the plants in this respect.

The highest values, in terms of root length and root dry biomass, were again obtained when the 2% concentration of silicon preparation was applied (Figure 10). Further increases of the preparation concentration even resulted in a decrease in the values of these parameters to below the control. Root width, on the other hand, tended to change positively when compared to the control.

4. Discussion

4.1. Influence of Silicon on Photosynthetic Performance

The efficiency of photosynthesis has now been studied, which may also be an indication of the water balance of plants [36]. This is because silicon reduces the rate of transpiration, thereby regulating water loss and increasing the drought resistance of plants [37], thus also improving the ability of plants to absorb light, which enhances photosynthesis [38]. In the present study, this phenomenon was observed in pines when using concentration of 2% of the formulation, which had a positive effect on biomass growth. The observed trend that photosynthetic efficiency increases with increasing silicon addition (concentration of the preparation) is probably due to the increased availability of the water, which is required for this process (Figure 2). This, in turn, leads to a higher production of assimilates (sugars), increased height and thickness growth and, consequently, to an increased plant biomass, as observed in agriculture and horticulture.

A statistically significant increase in needle thickness was found in pines and oaks (also in biomass); for example, an increase in the seedling height and seedling thickness at the root neck was observed (Figure 8 and Figure 10). The latter improves the quality of the planting material. The increase in average needle thickness in pines with a simultaneous tendency to shorten the needles is perhaps an indication of the trees’ adaptation to the new situation (Figure 3). It is likely that longer but thinner, or shorter but thicker, needles are similarly efficient from the point of view of photosynthetic efficiency. However, thicker and shorter needles may be more resistant to biotic and abiotic damage.

Silicon preparations reduce the effects of stress, both of biological origin and of stress caused by unfavourable weather or substrate conditions. Active silicon has a positive effect on the plant’s nutrient uptake from the soil (e.g., via up-regulation of the transcription of Lsi genes and the activation of membrane transporters), intensifies photosynthesis in low light conditions, causes more intensive growth of root mass and reduces plant stress, caused by too-low or too-high temperatures [27,39]. In fruit crops, silicon treatments are recommended at a very early stage of development. Fruit growers who apply silicon from the green bud stage onwards have found that the intensity of spider mites at a later stage of plant growth is significantly lower than in plots where silicon has not been applied. One of the many mechanisms of action of silicon is promoting the stiffening of the cell walls, which makes the tissue more resistant to mechanical damage, so that strawberries, for example, survive transport better and are more durable when marketed. Even better effects were observed in apples that were sprayed several times during their development; the apples were crunchier and could also be stored better [39].

On the other hand, some nutrients can also be lost during the growing season through rainfall leaching. Ageing leaves are particularly susceptible to the loss of potassium, sodium, chloride, nitrate and phosphate. It is possible that chlorophyll fluorescence measurements have detected potassium deficiencies at concentrations of 1%, which could translate into an increased rate of total energy loss (DIO/RC). The nutrient recycling in the soil–plant cycle described above is controlled by several factors [40]. It depends, in particular, on the intensity of mineralisation of the soil particles, the nature of the parent material of the soil and the leaching rate of nutrients from the upper soil layers [41]. If the leaching rate is high and the mineralisation intensity is low, plant nutrients can be leached from the soil at a higher rate than they are taken up by the plants [7]. However, our study did not investigate this issue in relation to silicon, which perhaps should be done in a follow-up study.

According to Reynolds et al. [42], silicified leaf surfaces also make it more difficult for insects to attack. Silicon quickly damages the mouthparts of insects, leading to high mortality through cannibalism or starvation. The presence of silicon also deters insects that sting the leaf in search of plant juices. The same phenomenon was observed with the incorporation of silicon in root cells, which form a barrier against pathogens in the soil, such as nematodes [42]. This aspect requires the design and implementation of a separate experiment, as well as clarification in terms of the potential protective effects of shoots, sprayed with silicon, on gnawing by forest animals.

4.2. Silicon Influence on Pine and Oak Growth Parameters

The doses required for foliar fertilisation are lower than those for soil fertilisation, which is why our experiment was designed accordingly. However, the residual effect of foliar fertilisation is very low or zero, so regular application is necessary. For iron, soil fertilisation is ineffective unless chelates are used, but these are expensive. Therefore, the best alternative is foliar fertilisation [17]. Foliar fertilisation, unlike soil fertilisation, allows the plants to respond quickly, so that nutrient deficiencies can be corrected as soon as characteristic symptoms appear, especially during the intensive growth phase of the plants.

The disadvantage of foliar fertilisation is the additional cost of repeated applications, which are often necessary because of the lower mobility of micronutrients, unless they can be combined with crop protection treatments [17]. There is concern that solutions with concentrations greater than 3% could cause foliar toxicity, which is why different concentrations of formulation sprays were used in our study. Comparing the above Faquin trials with the results of the present study, it is noticeable that spraying with the silicon preparation at a concentration of 1%, and especially 2%, had a positive effect on the oak treatment group studied, which was not observed with the 3% preparation liquid variant.

4.2.1. Effect of Silicone in Leaf Protection against Oak Powdery Mildew

In our study, we found a positive effect of silicon on the health of the oak seedlings tested: they had fewer leaf infections with the parasitic fungus than the leaves of the control seedlings. According to Ahammed and Yang [43], the silicon-protected leaf surface provides a physical barrier against the fungal infection and makes the leaf surface more resistant to invasion by the germinating spores. The researchers found that even if the spores manage to penetrate the cuticle, they encounter an active chemical defence stimulated by the silicon. Johnson et al. [44] hypothesised that fungal infection triggers a systemic immune response in plants that stimulates the synthesis of phenolic compounds with fungicidal properties, and that the cell walls at the site of infection become locally saturated with silicon. In his paper, he explained that the accumulation of dense organic silicon complexes at the site of invasion forms a barrier that intercepts the developing spores, limiting further penetration into the plant tissue. This description fits well to explain our favourable results in reducing powdery mildew development on the oak seedlings tested. The foliar treatment applied had a favourable effect, as it slowed down the development of the infection. The above studies, which agree with our observations, have proven that spraying the leaves of oak seedlings is quite an effective means of controlling powdery mildew, which is a common disease in nurseries throughout Europe. Similar conclusions were also reached by Blaich and Groundhofer [45] and Gorecki et al. [46], who confirmed that silicon reduces the development of powdery mildew on oaks.

In addition, the mass use of fungicides in nurseries means that pathogens are becoming increasingly resistant to their active ingredients. Therefore, there is a risk that new pathogenic fungal strains will emerge, leading to a loss of plant resistance to the disease, a phenomenon that has a genetic basis [42]. For this reason, the use of silicon in forest protection is becoming increasingly important [47].

The different effects of the two substances mean that the best way to control fungal diseases in plants may be to combine silicon with the limited use of fungicides [48], and this research needs to continue in forestry. In horticulture, it has been found that raspberry or strawberry fruits are more durable (less susceptible to fungi). Similarly, oaks have been found to have less-severe infestations by oak powdery mildew (Figure 7), possibly due to its more difficult penetration through the closed stomata. The effective action of silicon in oak leaves to protect against powdery mildew (Figure 7) is probably due to this mode of action. It stimulates the closure of the stomata, and makes it difficult or impossible for the germinating fungal spores to penetrate the leaf cuticle. In forests, the cell wall of the assimilation apparatus of trees is often attacked (and/or weakened by various pathogens or insect pests). Silicon can help form what is known as a physical barrier in the cell when it actively participates in the reactions that form signalling pathways cascade [49]. Wozniak et al. [50] found that the activity of fungal pathogens was reduced after treatment with propolis extract in combination with silicon. They also confirmed that silicon increased cell wall resistance. Helaly et al. [51] found that silicon nanoparticles have specific physiological properties that enable them to better penetrate plants, and influence their metabolic activity. Silicon nanoparticles form a protective layer on the epidermal cell wall called a film. This acts as a reinforcing material and prevents fungal and bacterial infections. García-Gaytán et al. [52] also confirmed that silicon nanoparticles help to reduce stress and protect the plant from its harmful effects. Similar to the present experiment, other researchers [37,53] also confirmed the usefulness of silicon in protecting plants from powdery mildew.

4.2.2. Effect of Silicone on Oak Growth Parameters

After the application of the silicon preparation, increases in the height and thickness of the seedlings were first observed. These peaked at a silicon concentration of 2%, indicating that this is the optimum dose, and further increases in silicon availability no longer had any additional effect (Figure 4). It is likely that the less mycelium-damaged leaves of the oak seedlings produced more assimilates (sugars), which translated into increased height and thickness and, consequently, an increase in biomass. Although three and five concentrations of the product were tested, increasing the dose did not lead to the expected better effect. From an economic point of view, this indicates that cheaper treatments can be formulated, as lower amounts of the product ensure the desired development of the seedlings.

However, these observations only apply to one growing season and should, therefore, be confirmed over a longer observation period. This is especially true for the development of the root system (Figure 5), because only when a larger above-ground biomass is formed can the emerging root systems be better nourished, and this requires at least two to three growing seasons. In nurseries, however, pines are only grown for one year and then planted outside. Oaks stay in the nursery for 2 to 4 years longer, and their root systems are undermined when they are taken out of the ground. The oaks grown in pots had deformed roots that wrapped around the pots. The best concentration of silicon preparation in terms of increased height and root neck is between 1 and 2% (Figure 8). This result is in line with research results in horticulture, where a concentration of 2% is recommended.

4.2.3. Factors That May Affect Silicon Uptake by Above Ground Plant Parts

The foliar uptake of nutrients by plants depends on the interaction with the environment, the plants and the spraying technique [54]. The timing of nutrient uptake into the metabolism of the sprayed plant is crucial for the effectiveness of the treatment [55]. The high relative humidity of the leaf’s surface favours the penetration of nutrients. For this reason, the timing of the treatment should be either very early in the morning or late in the afternoon, depending on regional conditions [56]. In the present study, a morning treatment with a hand spray during a rain-free and sunny period ensured good uptake of silicon by the leaf tissue. The good uptake of the administered silicon preparation is evidenced by the subsequent measurements of height growth, which is related to water retention (higher fresh biomass than in the control), higher photosynthetic efficiency (PI total index) and a, consequently, increased dry biomass. In addition, relative humidity is higher in the morning, which slows the evaporation rate of water applied to the leaves with the solution. Sunlight, relative leaf moisture, and the timing of the application affect the effectiveness of foliar fertilisation in practice [57]. Light plays an important role in photosynthesis, and in order for the plant to convert nutrients into metabolites, the activity of the photosynthetic processes must be maintained, which was ensured in the present experiment by carrying out the treatment in the morning. In addition, special care was taken to spray the undersides of the leaves, where the cuticle is thinner and the stomata predominate, which generally results in greater absorption of the applied solution [58,59,60]. For this reason, the seedlings were arranged in such a way that the foliar treatment, in practise, allowed an even distribution of the solution and also reached the abyssal (deepest) side of the leaves. Both oaks and pines were irrigated, as needed, by controlling the moisture content of the topsoil, as the hydration level of the leaves/needles is very important for nutrient uptake, anc hydrated cuticles are more permeable to water solutions. In contrast, dehydrated cuticles of wilted leaves are completely impermeable to them [17]. No surfactants were used in his experiment.

The seedlings studied were in an intensive phase of growth, and the nutritional state of the plants determined their tendency to absorb mineral nutrients. Alexander [61] assumes that they reach their greatest capacity at the time when they need the mineral substance in question the most, which was probably the case in our experiment, as this is related to the growth and development stages of the plant [60], and, in the case of the present study, potted seedlings were examined.

The foliar treatments were repeated several times, so that nutrient uptake from the solution was incorporated into the development of the young leaves/needles, as it is greater than in the old leaves. Presumably, the penetration of the solution deteriorates with increasing cuticle thickness; moreover, younger tissues have a higher metabolic activity and consume nutrients more rapidly during the synthesis processes, reducing their internal ionic status [17,62].

When the nutrient content or concentration in plant tissues is very low, the growth rate is also low [63]. Repeated treatments with different concentrations showed that the most favourable concentration was 2%, and higher concentrations did not lead to further growth. It is possible that the supply of this element then met the demand. According to Mengel and Kirkby [7], the growth rate and nutrient contents increase with increasing nutrient availability, until a so-called critical value is reached. Further improvements in nutrient availability have no significant effect on the growth rate, while nutrient content increases. For practical purposes, the critical value, above which an increase in nutrient content does not increase yield, is the relevant point. Extremely high nutrient levels result in a high nutrient content, but they also reduce growth [7]. A similar phenomenon was observed when using concentrations of 3% and 5% in the formulation spray.

5. Conclusions

• The silicon preparation has a positive effect on photosynthetic efficiency and, thus, on seedling growth and biomass.
• Even high concentrations of the silicon preparation do not cause phytotoxicity in Scot’s pine needles and English oak leaves.
• The silicon preparation has a positive effect on the needle thickness of pine seedlings.
• Foliar fertilisation with silicon preparations in low concentrations (1–2%) is particularly beneficial, also for economic reasons. The silicon preparation with a concentration of 2% improves photosynthetic efficiency and increases the production of assimilates, which strengthens the seedlings in their first years of life.
• The application of the silicon preparation (spray concentrations of 1% and 2%) reduces the infestation of and damage to oak leaves by powdery mildew (E. alphitoides).
• The duration of the pot trial, one growing season, was too short to fully reveal possible differences between the treatments and the control, especially regarding the effects on root system development.
• In view of the above results, further field trials of at least two to three growing seasons are needed before the preparation can be recommended for silvicultural use on pines and oaks in the first years of their development.