Foliar application of silicon sources and shading levels in Peltophorum dubium (Spreng.) Taub.

Abstract Depending on the intensity and ecological successional classification of plants, light availability can become an unfavorable condition for producing high-quality seedlings. We hypothesized that applying silicon sources might contribute to inducing tolerance to different shading levels for Peltophorum dubium (Spreng.) Taub. seedlings. Two independent experiments were developed: I) the application of five doses of silicon oxide (SiO2: 0.0; 1.0; 2.0; 4.0; and 6.0 g L-1); and II) the application of five doses of potassium silicate (K2SiO3: 0.0; 5.0; 10.0; 15.0; and 20.0 mL L-1 of water). Both were associated with three shading levels: 0% (direct sunlight), 30%, and 50%. In experiment I, we observed that seedlings were more responsive to shading levels and had little influence from foliar application of SiO2, with higher growth, biomass, and quality values when grown under direct sunlight (0% shading). In experiment II, the foliar application of 20.0 mL L-1 of K2SiO3 contributed to greater heights under 0% and 30% shading. Meanwhile, under 50% shading, the dose of 5.0 K2SiO3 favored the species' growth. The application of K2SiO3 favored the increase in the dry mass of the aerial part (DMAP). The highest biomass production and seedling quality occurred under 0% and 30% shading. The 50% shaded environment was most unfavorable to the growth and quality of P. dubium seedlings. Even though the seedlings were not very responsive to silicon sources, K2SiO3 provided a greater response than SiO2. High-quality seedling production is favored when the seedlings are grown under direct sunlight (0% shading).


Introduction
The areas destined for growing forest species present low fertility and little soil management practice, which affects the expression of these species' productive potential (Araújo et al., 2020).Thus, these areas become a limiting state of Mato Grosso do Sul.Then, we overcame the seeds' dormancy through thermal scarification and soaked them in hot water at 80 ºC, where they remained for 24 hours (Davide and Silva, 2008).Next, sowing was performed in 120 cm 3 polyethylene tubes filled with Bioplant ® substrate.At 50 days after sowing, when they presented an average height of 8.0 cm, the seedlings were transplanted into 7-liter pots previously filled with a substrate comprising Dystroferric Red Latosol (Oxisols) and coarse sand (2:1, v/v).Then they were placed in the growing environments during the 10-day acclimatization period before the first application of foliar products.
In the two experiments the growing environment comprised a greenhouse with a 150-micron thick, transparent, low-density polyethylene (LDPE) plastic cover.Inside the environment, the seedlings were placed in adapted structures with 2.00 x 1.20 m with 30% and 50% shading.The direct sunlight seedlings were grown only under the environment's plastic cover.The choice of doses of silicon sources was based on preliminary results (unpublished) with other tree species.

Experiment I: shading levels and SiO 2 in P. dubium seedlings
The experimental design was entirely randomized in a 3 x 5 factorial scheme with six repetitions.Each experimental unit comprised one pot with two seedlings each.After acclimatization, the seedlings were conditioned under three light availabilities based on the following shading levels: 0% (direct sunlight), 30%, and 50%.In addition, five doses of powdered silicon oxide (SiO 2 ) with 92% purity were applied: 0.0; 1.0; 2.0; 4.0; and 6.0 g L -1 of water.The growth measurements over time were analyzed in split-plots over time.

Experiment II: shading levels and K 2 SiO 3 in P. dubium seedlings
The design was entirely randomized, and the factors under study were arranged in a 3 x 5 factorial scheme with six repetitions.Each experimental unit comprised a pot with two plants each.Seedlings in each environment received five doses of potassium silicate (K 2 SiO 3 ; 12% Si and 15% K 2 O; density: 1.40 g L -1 ): 0.0; 5.0; 10.0; 15.0; and 20.0 mL L -1 of distilled water, corresponding to 0.0; 0.84; 1.68; and 3.36 g of Si L -1 and 0.0; 1.05; 2.10; and 4.20 g of K 2 O L -1 .The growth measurements over time were analyzed in split-plots over time.

Managing the application of Si sources
The products at the corresponding doses were diluted in distilled water and applied via foliar spraying until drip point (10 mL per plant based in pre-test) using a hand sprayer in the morning period.There were two applications.The first, ten days after transplanting (DAT), and the second, 70 DAT.

Evaluated characteristics
In both experiments, growth evaluations in height, diameter, and robustness index were performed at 30, 60, 90, and 120 days after application (DAA) of SiO 2 and K 2 SiO 3 .Meanwhile, biomass production and quality standard evaluations were performed at 120 DAA (190 DAT).
Thus, depending on the ecological successional classification and the phenotypic plasticity potential, each species can adjust to the light environment or be more sensitive to stressful conditions that can impair their early establishment (Santos et al., 2020;Lima et al., 2021).Thus, there has been an increase in the use of nursery management practices that can contribute to inducing tolerance to adverse stressful conditions to increase the quality standard of the plants.Among the products used, we can mention silicon (Si), used as fertilizer in agriculture and forestry, which has several sources on the market.The most commonly used sources include silicon oxide (SiO 2 ) and potassium silicate (K 2 SiO 3 ).
Even though silicon is not an essential element, it is beneficial and acts in the plant metabolism, especially the antioxidant system, alleviating the stressful effects of adverse conditions (Cavalcanti Filho, 2017;Verma et al., 2020;Santos et al., 2022;Santos et al., 2023a).Furthermore, it is involved in hormonal and nutritional regulation (Greger et al., 2018;Bhat et al., 2019) varying according to the source.For example, in K 2 SiO 3 , there is also the effect of Si and K on plant nutrition and metabolism.In contrast, SiO 2 has the individual effect of Si.
Native to the Brazilian Cerrado, Peltophorum dubium (Spreng.)Taub.(Fabaceae) is a tree species popularly known as "canafistula", which presents silvicultural interest (Melotto et al., 2019).The species is classified as early secondary, with a pioneer characteristic (Oliveira et al., 2018).It presents high hardiness and fast growth (Lorenzi, 2020).Usually, this species occurs in several phytophysiognomies in different phytogeographic domains (Lorenzi, 2020;Duarte et al., 2021;Silva et al., 2022).P. dubium seedlings have been indicated for recovering degraded environments.However, information on the species' growth response to light gradients is still scarce.Studies are still aimed at establishing protocols for ex situ cultivation.
Therefore, considering the importance of P. dubium and that insufficients the studies on light availability associated with Si sources, we hypothesized that the species might be sensitive to increasing shading levels and that the foliar application of Si sources may contribute to obtaining high-quality seedlings.Thus, we aimed to evaluate the effect of SiO 2 and K 2 SiO 3 on the growth and quality of seedlings of P. dubium under different shading levels.

Study conditions and seedling production
Two independent experiments were conducted at different times at the School of Agricultural Sciences of the Universidade Federal da Grande Dourados (22°11'53.2"S;54°56'02.3"W).The first, from January to May 2021, and the second, from June to October 2021.According to the Köppen classification, the region's climate is CWa, with hot summers and a rainy season and winters with moderate temperatures and dryness (Fietz et al., 2017).
Thus, P. dubium seeds were collected from random matrices distributed in the city of Nova Andradina in the Foliar application of silicon sources in Peltophorum dubium Initial growth: plant height -PH (cm) was determined by the distance from the collar to the inflection of the uppermost leaf using a ruler graded in centimeters (cm); stem diameter -SD (mm) was measured with a digital pachymeter inserted 1 cm above the substrate level, and we calculated the robustness index based on the height/diameter ratio (HDR).
Biomass production: the seedlings were removed whole from the pots and separated into aerial parts (leaves + stem) and root systems.The aerial part and the roots were weighed on precision analytical scales (0.001g) to determine the fresh mass of the aerial part (FMAP) and the root fresh mass (RFM).Then, the materials were conditioned separately in Kraft paper bags and dried in a forced air drying oven at 60 ºC for 72 hours to obtain the seedlings' root dry mass (RDM), dry mass of the aerial part (DMAP), and total dry mass (TDM).The root/aerial part ratio (RAPR) was calculated based on these data.
Quality standard: It was based on the Dickson Quality Index (DQI) calculated according to Dickson et al. (1960) using the following Equation 1: ( )

Statistical analyses
Data from the two experiments were analyzed individually and subjected to analysis of variance (ANOVA, F-test -P ≤ 0.05).In both experiments, the means were compared using Tukey's test ± standard deviation for shading levels (P ≤ 0.05) and by regression analysis testing the linear and quadratic mathematical models for evaluation periods, SiO 2 and K 2 SiO 3 doses (P ≤ 0.05) and coefficient of determination ≥ 0.60.

Shading levels and SiO 2 in P. dubium seedlings
The interaction between evaluation seasons x shading significantly influenced the height of P. dubium seedlings (Figure 1A).In addition, stem diameter (SD) was influenced by a two-way interaction between evaluation seasons and, shading levels for SD (Figure 1B).Overall, all characteristics assessed as a function of evaluation periods and SiO 2 doses showed quadratic fits.Furthermore, we observed that in this first experiment, P. dubium seedlings were more responsive to shading levels than to the foliar application of SiO 2 .
For plant height, there was an increase in height under 50% shading throughout the growing cycle.However, at 120 DAA, the values were close to those observed in the seedlings in direct sunlight (Figure 1A).Regarding the SD, the values between shading levels were not so discrepant until 60 DAA.However, at 120 DAA, the seedlings grown in direct sunlight and 50% shading showed values close to and greater than those of seedlings under 30% shading (Figure 1B).Thus, P. dubium seedlings adapted to light-intensity variations, indicating potential morpho-physiological plasticity (Botterweg-Paredes et al., 2020).In other words, the species can adjust to different light environments through its phenotypic plasticity, which suggests ecological resilience.First, however, we must consider all the evaluated characteristics because, many times, the plant optimizes resources for its vegetative growth but presents a low potential for photoassimilate production.
When under lower light availability conditions, plants tend to invest in height, aiming for canopy exposure.Thus, they provide a competitive advantage for light access with competing vegetation (Grossnickle and MacDonald, 2018), as observed here with P. dubium.
Plant height evaluation generally provides information on the plant's physiological adjustment potential to the growing conditions, especially light availability.Indeed, some species, such as Tabebuia aurea (Silva Manso) Benth.& Hook.f.ex S.Moore (Souza and Freire, 2018) and Cojoba arborea (L.) Britton & Rose (Carvalho et al., 2021) etiolate under higher shading levels due to their hormonal relationships (Pinho et al., 2022).This fact is also associated with the ecological group of each species, which becomes a negative factor, especially for seedlings in the initial phase.
However, for P. dubium seedlings, we cannot say there was no etiolation, especially since the robustness index was not influenced by the factors under study (P> 0.05), presenting a 4.31 average.Thus, height growth was followed by an increase in diameter, promoting balance in the plant's development.Regarding the foliar application of SiO 2 , the only dosage influence was observed for SD in the triple interactiono.At the end of 120 DAA, the highest mean value observed was 5.53 mm for the 2 g dose under direct sunlight.The lowest value was 3.94 mm for the 30% shading at the zero doses (control) (Figure 2).At 30 and 60 DAA, SiO 2 doses had no significant effects regardless of the shading level.At 120 DAA, we observed a statistically significant effect of the 2 g dose in seedlings under direct sunlight and the 4 g dose in seedlings grown under direct sunlight and 50% shading.However, there were no differences between the two doses.
For biomass production and seedling quality, we only found the effects of shading levels (Figure 3).Silva et al. (2018) observed similar results when evaluating different doses of Si in Euterpe precatoria Mart.seedlings.They found no influence of silicon addition on phytomass production and seedling quality.Foliar application of silicon sources in Peltophorum dubium Overall, P. dubium seedlings produced under direct sunlight showed higher values of fresh and dry masses for the aerial part and root than the shaded ones.Larraburu et al. (2018) also observed that fresh and dry root masses of Handroanthus ochraceus seedlings were influenced by light.Those exposed to higher irradiance levels (0% shading) showed higher biomass productiono.The results for A. peregrina seedlings (Santos et al., 2020) were similar to those observed for P. dubium.These same authors describe that A. peregrina seedlings possibly showed the highest fresh and dry mass values due to the higher amount of Photosynthetically Active Radiation (PAR) under direct sunlight, favoring photoassimilate and biomass productiono.Thus, the increased leaf dry mass under direct sunlight may have occurred because the plant obtained a higher concentration of photoassimilates and adaptation to irradiance compared to the shaded environments (Ajalla et al., 2012).
The highest RAPR and DQI values occurred in seedlings grown under direct sunlight (Figure 4), indicating greater exploration in the aerial part than in the root.This response is associated with the fact that the species is classified as early secondary, with a pioneer characteristic (Oliveira et al., 2018).Similarly, Santos et al. (2020), when evaluating the effect of shading levels (direct sunlight, 30%, 50%, and 70%) on the growth and quality of Anadenanthera peregrina L. Speg.seedlings, a pioneer species, observed higher RAPR, biomass, and quality values when produced under direct sunlight.The increase in DQI is desirable because it indicates that the seedlings show greater vigor based on photoassimilate production and growth, contributing to better seedlings to be commercialized or transplanted to field conditions (Santos et al., 2020;Santos et al., 2023b).
From a practical perspective, this information supports that for this species in this experimental period, there is no need for investment in nursery structure, such as shading screen and acquisition of SiO 2 , which reduces production costs.Furthermore, another important factor is that it can reduce the acclimatization time and speed up their shipment to the field since the plants will be hardened under direct sunlight.Therefore, even though it is not our objective, considering that it provided good results under direct sunlight, P. dubium would be a potential species for insertion in areas with greater light exposures.
The little influence of SiO 2 may be related to this experiment's timeframe and the doses used, in addition to intrinsic factors of the species itself.When considering that for P. dubium, there is no specific dose recommendation information yet; perhaps the doses used were low or could have increased the number of applications.Still, little is known about the direct effects of Si on plant growth (Santos et al., 2022;Silva, 2022).Based on this information, we suggest further work with the species aimed at knowing the effects of SiO 2 on leaf metabolism and nutritional aspects.

Shading levels and K 2 SiO 3 in P. dubium seedlings
We observed that in this second experiment, P. dubium seedlings were responsive to shading levels when the foliar application of K 2 SiO 3 doses occurred.In addition, plant height was significantly influenced by the three-way interaction between evaluation seasons, shading levels, and K 2 SiO 3 doses.Similar to exp.I, all characteristics showed quadratic fits when influenced by evaluation periods or K 2 SiO 3 doses.
Stem diameter was influenced only by the evaluation periods, with a maximum calculated value of 13 mm at 120 DAA (Figure 5A).The increase in diameter favors the distribution of water and photoassimilates to the different organs, contributing to vegetative growth (Santos et al., 2023a).It is worth noting that the increase in diameter should follow the seedlings' height so that the characteristic aspect of etiolation does not occur in the seedlings as an escape mechanism (Quaresma et al., 2021), as observed in the seedlings under 50% shading, represented by the height/diameter ratio (HDR) (Figure 5B).
This ratio indicates the robustness and/or vigor of the seedlings that will be taken to the field.The more balanced this ratio, the greater the chances of survival (Padilha et al., 2018) because they demonstrate photoassimilate distribution among the different organs.Possibly, these seedlings, under field conditions, would face difficulties in development.Moreover, for the criteria of a homogeneous forest stand, they would not be indicated for field expeditions.Plant growth depends on its degree of adaptability to the type of environment.Overall, the application of K 2 SiO 3 contributed to the height of P. dubium seedlings, varying according to the shading level.For example, seedlings grown under 0% and 30% shading showed greater heights when they received an application of 5.0 mL L -1 of K 2 SiO 3 at 120 DAA (Figure 6A and 6B).On the other hand, the highest height values in seedlings under 50% shading occurred when they received 5.0 mL L -1 of K 2 SiO 3 at 120 DAA.However, responses to K 2 SiO 3 vary between species.For example, Coffea arabica L. seedlings do not show increased growth characteristics in height and diameter as a function of the application of this silicon source.
We emphasize that P. dubium seedlings show phenotypic plasticity to different shading levels.However, they show greater potential for expression under direct sunlight, considering their ecological classification as pioneers (Oliveira et al., 2018).Based on our study, it is possible to infer that increasing the shading level begins to promote changes that impair the production and allocation of photoassimilates and that if they had a higher shading level, the responses would be pronounced and stressful.
The 50% shading was a condition in which the P. dubium seedlings allocated most of their metabolic resources to grow in height.In contrast, the seedlings under 0% shading were smaller.The reason for this is that in high irradiance conditions, auxin production and transport to the meristematic region is limited, which leads to the formation of short internodes (Fiorucci and Fankhauser, 2017), influencing the production of smaller seedlings, such as those grown under direct sunlight.Moreover, in seedlings exposed to high irradiance, represented here by direct sunlight, in addition to hormonal relations, lower height values represent an adaptive mechanism aimed at reducing canopy exposure to high temperature, minimizing water loss significantly (Santos et al., 2023b), regulating water relations within limiting conditions.According to Santos et al. (2023b), Alibertia edulis (Rich.) A. Rich.seedlings also showed lower heights but maintained high biomass production.The root/aerial part ratio (RAPR) and Dickson quality index (DQI) were influenced only by shading levels (Figure 9A and 9B).The lowest and highest RPAR and DQI values occurred in the seedlings under 0% and 30% shading, respectively.Enterolobium contortisiliquum (Vell.)Morong seedlings also showed higher RAPR under 50% shading (Santos et al., 2019).
The results for P. dubium demonstrated that in this condition, the plant invests more in the aerial part than in the root system as an adjustment mechanism to the environment since, although possibly having a lower rate of evapotranspiration, it has less light availability for photosynthetic metabolism and photoassimilate productiono.
The seedling quality standard was similar to experiment I, but with good results also under 30% shading, reinforcing the classification of early secondary with a pioneer.
In future perspectives, new studies with P. dubium should be developed to increase the information on leaf metabolism and antioxidant and nutritional systems using silicon sources as a function of further doses (even higher and more frequent applications and higher shading levels) to favor the production of high-quality seedlings more tolerant to adverse environmental conditions.
In conclusion, although P. dubium seedlings present phenotypic plasticity in light environments, cultivation under 50% shading is an unfavorable condition for biomass production and quality.Foliar application of SiO 2 and K 2 SiO 3 did not influence seedling quality.However, K 2 SiO 3 at 20 mL L -1 favors the aerial part's growth and biomass productiono.The production of P. dubium seedlings under 0% shading contributes to seedling quality, regardless of silicon sources.
These results corroborate Silveira et al. ( 2020), who, when evaluating the development of Anadenanthera macrocarpa (Benth.)Brenan under different shading levels, observed higher plant height growth under shadier environments, while plants grown under direct sunlight showed the lowest growth.
The fresh mass of the aerial part was influenced by the factors alone, with higher values in shaded seedlings (Figure 7A) and those with 20 mL L -1 of K 2 SiO 3 (Figure 7B).On the other hand, root fresh mass was highest under 30% shading, not statistically different from those under 0% shading (Figure 7C).The dry mass of the aerial part was influenced only by doses of K 2 SiO 3 with a maximum value of 20 mL L -1 (Figure 8A).Meanwhile, root dry mass was highest under 0% and 30% shading, regardless of K 2 SiO 3 application (Figure 8B).
The higher values of the aerial part masses with the higher dose of K 2 SiO 3 were due to the beneficial effect of the Si and K in the product.That is because silicon contributes to plant leaves becoming more erect and rigid (Tubana et al., 2016;Lima et al., 2019;Santos et al., 2022), which favors higher solar interception, reflecting in higher photoassimilate production, leaf increment, and biomass production (Santos et al., 2022).Potassium acts in osmotic adjustment and cell wall structure (Pinto, 2019;Soares, 2022).
Regarding the higher biomass values of the root system under greater light availability, this is because, under these conditions, generally, the environment temperature tends to be higher than under 50% shading, which promotes pronounced evapotranspiration more rapidly.As a result, the plant's water balance is reduced, and consequently, the turgescence decreases due to the lower hydraulic conductivity in the soil because of the lower water status (Oliveira et al., 2021).

Figure 3 .
Figure 3. Fresh mass of the aerial part -FMAP (A); root fresh mass -RFM (B); dry mass of the aerial part -DMAP (C); and root dry mass -RDM (D) in Peltophorum dubium (Spreng.)Taub.seedlings submitted to different light availability conditions.Different letters differ by Tukey's test (P ≤ 0.05).

Figure 7 .
Figure 7. Fresh mass of the aerial part -FMAP (A, B) and root fresh mass -RFM (C) in Peltophorum dubium (Spreng.)Taub.seedlings submitted to different light availability conditions and K 2 SiO 3 doses.Different letters differ by Tukey's test (P ≤ 0.05).