Crop Growth

The experiment was carried out in a polycarbonate greenhouse with automatic temperature control at the Antonio Narro Autonomous Agrarian University, Coahuila, Mexico. Tomato seeds (Solanum lycopersicum L.) of the hybrid var. “Sun 7705”, saladete type and indeterminate growth were used as plant material. The seedlings were transplanted into 12 L capacity black polyethylene bags containing a mixture of Peat most and perlite (1:1) as growth substrate. The crop was handled on a single stem and developed for 150 days after transplantation.

Experimental Design and Plant Nutrition

An experiment with a completely randomized design and factorial arrangement with two factors was established: one factor was the concentration of arsenic (six levels) and another factor was the concentration of SiO₂ NPs (three levels), being a total of 18 treatments. Arsenate (As V) was added using Na₂HAsO₄*7H₂O as a source, it was mixed with the irrigation water whenever the nutritive solution was prepared to obtain the final concentrations of 0, 0.2, 0.4, 0.8, 1.6 and 3.2 mg L^-1 of As (V). These concentrations are defined based on the Official Mexican Standard NOM-001-ECOL-1996, which establishes the maximum permissible limits (MPL) of arsenic in water for agricultural use are 0.2 mg L^-1 monthly average and 0.4 mg L^-1 daily average (^{DOF, 1997}). The SiO₂ NPs were applied via drench from the transplant, every three weeks making a total of six applications (10 ml per application to each plant), the concentrations applied were 0, 250, and 1000 mg L^-1 of SiO₂ NPs. Morphology of SiO₂ NPs is spherical, 10-20 nm size, surface area of 160 m² g^-1, and bulk density of 0.08-0.1 g cm^-3 (SkySpring Nanomaterials Inc., USA).

Arsenic was supplied always in irrigation water to simulate contaminated water according to the maximum permissible concentration of As in irrigation water in Mexico (0.2 mg L^-1) (^{DOF, 1997}). The selection of the SiO₂ NPs was carried out from published studies where it is reported a better understanding of how SiO₂ NPs inhibit the uptake and transport of As and may provide a new strategy for designing effective mechanisms to decrease the accumulation in plants (^{Cui et al., 2019}). In addition, the concentrations of NPs were chosen from a previous study SiO₂ NPs in high concentrations (100, 500 and 2000 mg L^-1) in cotton (^{Le et al., 2014}).

Plant nutrition was managed through a directed irrigation system using Steiner solution (^{Steiner, 1961}). This was applied at different concentrations depending on the phenological stage: 25 % in vegetative stage, 50 % in flowering, 75 % in fruit set, and 100 % in fruit filling and harvesting. The pH of the nutrient solution used was kept between 6.0 and 6.5.

Plant Sampling

At 150 days after transplantation the plants were harvested, and leaf and root samples were taken. The samples were dried in a drying oven at a constant temperature of 80 °C for 72 hours until constant weight was reached. The dried material was ground to a powder for mineral analysis. The dry root and shoot biomass was also determined.

Mineral Nutrient Analysis

The concentration of macro and micro nutrients in roots and leaves was determined by X-ray fluorescence spectroscopy (XRF) in a ThermoScientific Niton FXL instrument (Limit of detection 2 ppm). Three samples were analyzed, and the quality of the results was evaluated in triplicate for quality control and to ensure the reliability of the analytical data, using the reference material NIST 1573a for tomato leaves. Obtaining 90 ± 10 % recovery.

Biomass Production

Biomass production was affected both by the different concentrations of As (V) and by the application of SiO₂ NPs (Figure 1). The dry biomass of plants exposed to low concentrations of As(V) (0.2-0.4 mg L^-1) was slightly increased compared to controls. As (V) concentrations of 0.8-1.6 mg L^-1 decreased root dry weight (RDW), however, the 3.2 mg L^-1 concentration induced 3.3 % more biomass (Figure 1A). In the shoot dry weight (SDW), here the trend shows an increase in biomass as the concentration of As (V) increases (Figure 1C). The application of SiO₂ NPs presented a similar trend in both RDW and SDW, although it is not in the same magnitude, in both cases biomass decreased; RDW decreased by up to 49 % (Figure 1B) and SDW decreased by 18 % (Figure 1D), with 1000 mg L^-1 SiO₂ NPs compared to the control.

Figure 1 Relationship between dry biomass production [root (A, B) and shoot (C, D)] of S. lycopersicum with the individual factors concentration of As (V) and concentration of SiO₂ NPs. The regression curve of As (V) refers to the mean values of three replicates. The regression curve of SiO₂ NPs refers to the mean values of six replicates. 

The interaction of As (V) with SiO₂ NPs had a negative effect on biomass production, since the RDW decreased considerably, being up to 61.9 % and 63.4 % with the concentration of 0.8-1.6 mg L^-1 of As (V) with 1000 mg L^-1 of SiO₂ NPs (Figure 2A). The effect of the As (V)-SiO₂ NPs interaction in the production of SDW showed the same decreasing trend although in a lesser proportion than RDW. The concentration of 3.2 mg L^-1 of As (V) with the application of 250 mg L^-1 of SiO₂ NPs decreased the SDW by 3.1 %. However, this same concentration of As (V) together with 1000 mg L^-1 of SiO₂ NPs increased by 6.89 % the production of SDW in relation to the control (Figure 2B).

Figure 2 Effect of the interaction of factors concentration of As (V) and concentration of SiO₂ NPs on the production of root and shoot biomass in plants of S. lycopersicum. N= 6 ± standard error. * indicate significant differences between treatments according to the Fisher´s Least Significant Difference test (α = 0.05). 

Effect of Arsenate and SiO₂ NPs on the Uptake of Micronutrients in Root and Leaves

The concentration of As (V) in nutritive solution affected the concentration of Fe and Cu in the root, since as the concentration of As (V) increased, the concentration of both micronutrents decreased (Figure 3A and 3C). In the concentration of Zn a different behavior was observed, the low concentrations of As (V) (0.4-0.8 mg L^-1) induced a higher concentration of this micronutrient by 4.9 % and 15.5 % respectively; while with the highest concentration (3.2 mg L^-1) the Zn concentration decreased in the root (12.6 %) (Figure 3E). A very similar effect was observed in the concentration of Fe, Cu and Zn in the leaves, at high doses of As (V) (1.6 and 3.2 mg L^-1) the concentration of Fe (9.7 % and 30.6 %), Cu (27.2 % and 41.8 %), and Zn (23.5 % and 22.4 %) decreased (Figure 3E).

Figure 3 Relationship between the concentration of Fe (A, B), Cu (C, D), and Zn (E, F) with the individual factors concentration of As (V) and concentration of SiO₂ NPs in roots and leaves of S. lycopersicum. The regression curve of As (V) refers to the mean values of three replicates. The regression curve of SiO₂ NPs refers to the mean values of six replicates. 

SiO₂ NPs modified the Fe concentration. In the root, a decrease of 14.4 % and 8.7 % of the concentration of this micronutrient was observed due to exposure to 250 and 1000 mg L^-1 respectively. In contrast, the opposite effect was presented in the leaves, since the Fe concentration increased 45.7 % and 27.2 % with the application of 250 and 1000 mg L^-1 of SiO₂ NPs (Figure 3B). The application of 250 and 1000 mg L^-1 SiO₂ NPs induced an increase in Cu (10.8 % and 14.8 %) and Zn (18.5 % and 22.5 %) in the root; in the leaves, no effect was observed (Figure 3D and 3F).

The interaction of As (V) with the SiO₂ NPs presented a negative effect since the Fe concentration of the root decreased above 45.7 % with 3.2 mg L^-1 of As and 1000 mg L^-1 of SiO₂ NPs (Figure 4A). However, in the absence of As (V) it was observed that both concentrations of SiO₂ NPs (250 and 1000 mg L^-1) induced a higher concentration of Fe (8.3 % and 14.1 % more than control) in the leaves. When the concentration of As (V) was 0.2-1.6 mg L^-1, the application of 250 mg L^-1 of SiO₂ NPs induced an increase in the Fe content in the leaves of 13.1 %, 49.6 %, 37.9 % and 20.9 % respectively, in relation to the control (Figure 4B).

Figure 4 Effect of the interaction of factors concentration of As (V) and concentration of SiO₂ NPs on the concentration of Fe, Cu and Zn in root and leaves of S. lycopersicum. N= 6 ± standard error. * indicate significant differences according to the Fisher´s Least Significant Difference test (α = 0.05). 

Cu concentration in the root was positively affected by the application of SiO₂ NPs in the absence of As (V), presenting an increase of 9.7 % and 25.2 % with 250 and 1000 mg L^-1 of SiO₂ NPs respectively compared to the control. The presence of As (V) in general induces a negative effect on the concentration of Cu in the root. However, a positive effect was observed when SiO₂ NPs were applied with respect to dose of As (V) (0.8 and 1.6 mg L^-1), but in relation to the control the dose of 3.2 mg L^-1 in combination with 250 and 1000 mg L^-1 of SiO₂ NPs reduced 19.1 % and 32.4 % the concentration of Cu, respectively (Figure 4C). The Cu concentration in the leaves increased 154.6 % with the combination of 0.4 mg L^-1 As (V) and 250 mg L^-1 SiO₂ NPs only, in the rest of the As (V)-SiO₂ NPs interactions the effect was null or negative (Figure 4D).

The zinc concentration in the root increased by the application of SiO₂ NPs up to 104.5 % in relation to the control in the absence of As (V), while in the presence of As (V) increases of up to 58% approximately were observed (Figure 4E). In the leaves, Zn increased with the presence of 0.4 mg L^-1 of As and in the absence of SiO₂ NPs, but in combination with SiO₂ NPs a lower concentration of Zn was observed (Figure 4F).

The zinc concentration increased 45.75 % with the concentration of 0.4 mg L^-1 As (V) and 250 mg L^-1 of SiO₂ NPs. However, also the combination of 0.2 mg L^-1 As (V) and 1000 mg L^-1 of SiO₂ NPs induced an increase of 55 % in relation to the control (Figure 4F).

Effect of Arsenate and SiO₂ NPs on Macronutrient Uptake in Root and Leaves

The concentration of K in both root and leaves decreased as the concentration of As (V) in the nutritive solution increased; the decreases were up to 17.8 % in root and 22.5 % in leaves at the high dose of 3.2 mg L^{- 1} of As (V) (Figure 5A). However, the application of 250 and 1000 mg L^-1 of SiO₂ NPs induced the opposite effect, since the concentration of K was increased by 46.1 % and 68.2 % in root, and 23.5 % and 33.3 % in leaves respectively (Figure 5B).

Figure 5 Relationship between the concentration of K (A, B), S (C, D), and P (E, F) with the individual factors concentration of As (V) and concentration of SiO₂ NPs in roots and leaves of S. lycopersicum. The regression curve of As (V) refers to the mean values of three replicates. The regression curve of SiO₂ NPs refers to the mean values of six replicates. 

The sulfur concentration in the root was not affected by the different concentrations of As (V), nor by the application of SiO₂ NPs (Figure 5C and 5D). However, the sulfur concentration in the root increased 10 % with the highest concentration of As (V) (3.2 mg L^-1) and 10.5 % with the concentration of 0.8 mg L^-1. The lowest concentration of As (V) (0.2 mg L^-1) induced a decrease of 14.7 % in the leaves sulfur concentration (Figure 5C). On the other hand, the application of 1000 SiO₂ NPs increased the sulfur concentration in the leaves 6.7 % (Figure 5D).

The concentration of P was affected by As (V); however, the effect was different between the organs of the tomato plant. In the leaves, the concentration of P decreased 41.4 %, 40.4 %, 11.8 %, 54.7 % and 31.1 % with the different concentrations of As (V) (0.2-3.2 mg L^-1 respectively). In the root, P concentration increased 24.6 % with the As (V) at a concentration of 1.6 mg L^-1 (Figure 5E). In the case of the applications of 250 and 1000 mg L^-1 of SiO₂ NPs, the application induced an increase of 26.9 % and 54.9 % in the concentration of P in leaves and 40.3 % and 36.2 % in roots respectively (Figure 5F).

The combination of As (V) with SiO₂ NPs induced changes in potassium concentration both in the root and in the leaves (Figure 6). In both organs when only SiO₂ NPs were applied, a significant increase in potassium concentration was observed, being 70 % in roots and 65.2 % in leaves (Figure 6A and 6B). When SiO₂ NPs were applied in combination with As (V), an increase in potassium was generally observed compared to the application of only As (V), the effect being greater with the concentration of 0.2 mg L^-1 of As (V) and 1000 mg L^-1 of SiO₂ NPs the increase was 106.7 % in root.

Figure 6 Effect of the interaction of factors concentration of As (V) and concentration of SiO₂ NPs on the concentration of K, S and P in root and leaves of S. lycopersicum. N= 6 ± standard error. * indicate significant differences according to the Fisher´s Least Significant Difference test (α = 0.05). 

The sulfur concentration in the leaves increased 32.5 % with the application of 1000 mg L^-1 of SiO₂ NPs in the absence of As (V). However, the concentration of 0.8 mg L^-1 of As (V) without application of SiO₂ NPs also increased the content of this element by 43.1 % (Figure 6C). In the leaves, the concentration of 0.8 mg L^-1 of As (V) without application of SiO₂ NPs increased the sulfur content (44.4 %). Furthermore, the combination of 1000 mg L^-1 of SiO₂ NPs with 1.6 and 3.2 mg L^-1 of As (V) increased the sulfur concentration by 15.6 % and 31.7 % respectively (Figure 6D).

In the absence of As (V), the phosphorus concentration increased significantly in root (37.7 % and 117.6 %) and leaves (45.5 % and 45.1 %) with the application of 250 and 1000 mg L^-1 of SiO₂ NPs (Figure 6E and 6F). With the presence of 1.6 mg L^-1 of As (V) and with the application of 1000 mg L^-1 of SiO₂ NPs, an increase up to 78.5 % was observed in the phosphorus concentration in leaves (Figure 6F). In the root, the interaction of 1.6 and 3.2 mg L^-1 of As (V) with 1000 mg L^-1 of SiO₂ NPs decreased the phosphorus concentration by 12.1 % and 17.3 % respectively (Figure 6E).

Dry Matter Production

The As problem in Mexico is worrisome, high As levels have been detected in drinking water in certain locations of Coahuila (up to 435 μg L^-1) and Sonora (up to 1,004 μg L⁻¹); in continental surficial water in Puebla (up to 780 μg L^-1) and Matehuala, SLP (up to 8,684 μg L^-1); in groundwater in SLP (up to 16,000 μg L^-1) and Morelia, Michoacán (up to 1,506 μg L⁻¹); in soils in Matehuala, SLP (up to 27,945 μg g^-1) and the Xichú mining area, Guanajuato (up to 62,302 μg g^-1); and in sediments in Zimapán, Hidalgo (up to 11,810 μg g⁻¹) and Matehuala, SLP (up to 28,600 μg g⁻¹) (^{Osuna-Martinez et al., 2021}). Has been reported indicated that fertilizers containing macronutrients may also contribute trace amount of heavy metals (HM) and potential non-degradable and non-destroyable pollutants, such as Cd and Pb, which do not have any established biological purpose in plants but are acknowledged to cause physiological, morphological and biochemical dysfunctions in plants (^{Tchounwou et al., 2012}). Moreover, ^{Salem et al. (2020)} reported that the application of phosphate and urea fertilizers in agricultural soils during forty years of fertilization, provide considerable concentrations of Cr, Cd and Ni, and the concentrations of HM in the soil vary significantly with the seasons (winter, spring, summer and autumn). However, the application of agricultural fertilizers does not significantly increase total HM content in the soil above background levels for several years (^{Rutkowska et al., 2009}). High As levels in Mexico are related mainly with geogenic and mining origin.

Arsenic can replace phosphate in respiration processes, interrupting cellular metabolism, generating adenosine diphosphate-arsenate (ADP-As) instead of adenosine triphosphate (ATP) (^{Meharg, 1994}). Within plant tissue, As (V) is reduced to As (III) by the enzyme arsenate reductase which binds with thiol groups of enzymes and proteins leading to the inhibition of cellular functions (^{Finnegan & Chen, 2012}). The exposure of plants to As produces growth inhibition, stops the accumulation of biomass and causes physiological disorders in plants, due to the high toxicity of this metalloid (^{Garg & Singla, 2011}; ^{Stoeva et al., 2005}). A 50 % reduction in dry biomass production has been reported in rice plants grown in nutritive solution with 4.0 mg L^-1 of As (III) (^{Wang et al., 2010}). Sunflower plants grown in soil contaminated with As (V) reduced root dry weight by up to 60.5% and shoot dry weight by up to 49.2% when exposed to concentrations of 40 and 80 mg kg^-1 of As (V) respectively (^{Azeem et al., 2017}). The present study found that the application of different concentrations of As, at low levels there is a slight stimulation of the biomass while at high levels there was no negative effect. An increase of 17% of the dry biomass of shoots has also been reported in grafted tomato plants subjected to arsenic stress (100 µg L^-1 of As (V) in nutritive solution) (^{Stazi et al., 2016}).

There are mechanisms that the plant uses to detoxify itself such as chelation with polypeptides such as glutathione (GSH) and phytochelatins (PC), once chelated, arsenic is stored in the vacuoles of the roots (^{Liu et al., 2010}), and does not cause damage to plants. Regardless of the arsenic accumulation and detoxification mechanisms, most of the arsenic in tissue is probably physiologically inert (^{Santos et al., 2010}). In addition, the hormetic effect could be another response caused by arsenic. This is characterized by stimulation at low concentrations and inhibition at high concentrations of heavy metals that takes the form of concentration-response of U or inverted U (^{Agathokleous et al., 2019}). This inverted U shape can be caused by hormesis triggered by nonessential element ions (^{Poschenrieder et al., 2013}).

Stimulation effect of NPs is generally reported by low doses, regarding to SiO₂ NPs, an increase of 25 % in dry root biomass and 75 % in shoots has been reported in fenugreek plants (Trigonella foenum-graecum) with application of this NPs (^{Nazaralian et al., 2017}). In Pisum sativum plants subjected to chromium stress, the application of 10 μM of Si NPs improves the production of dry biomass (^{Tripathi D. K. et al., 2015}). Si NPs have been reported as beneficial, since they can improve the growth of plants subjected to heavy metal stress (^{Cui et al., 2017}; ^{Tripathi D. K. et al., 2015}). The positive effects of SiO₂ NPs on growth can be attributed to the improvement of some elements of transport in xylem sap, moreover improve of uptake capacity of water and fertilizers (^{Janmohammadi et al., 2016}). Also, another mechanisms of Si NPs is the stimulation of acid exudates through the root such as oxalic acid and polyphenols, which could reduce the toxic effects of aluminum in corn (^{De Sousa et al., 2019}).

The stimulating effect can be physicochemical, and this occurs when the energy and surface charges of the NPs interact with cell walls and membranes, modifying the activity of receptors, transporters and other proteins (^{Zuverza-Mena et al., 2017}). The absorption process of NPs includes cellular mechanism such as signaling, recycling and regulation of the plasma membrane, also is considered as an active transport (^{Tripathi D. K. et al., 2017}). Once NPs come into contact with plants, they can be absorbed and transported, generating various effects such as stimulating antioxidant compounds, improving the growth of plants subjected to abiotic stress, and in the worst case causing toxicity (^{Cox et al., 2016}; ^{Pérez-Labrada et al., 2019}; ^{Zhang et al., 2018}).

Micronutrients

The present study found that application of As in irrigated water decreased the absorption of micronutrients (Fe, Cu, Zn) in tomato plants. The decrease in micronutrients due to exposure to As is consistent with those reported with ^{Carbonell-Barrachina et al. (1997)}. They found that the absorption of B, Cu, Zn and Mg in tomato plants was reduced when they were exposed to arsenite. ^{Gomes et al. (2012)} reported a reduction in Fe that was directly proportional to the increase in As in Anadenanthera peregrina.

As can induce stress in the plant due to the impact that generates on the homeostasis of essential elements, by reducing the absorption of some nutrients (^{Kumar et al., 2015}). In addition, exposure to As generates anatomical deformations in the root, reduces the cells of the parenchyma, and the size of the xylem cells (^{Tripathi P. et al., 2015}), which can directly affect nutrient absorption. It can also be attributed to the formation of metal complexes that cannot be absorbed by the root, thus preventing the absorption of essential elements (^{Khan et al., 2019}).

Root exudates, can be amino acids, organic acids, sugars, phenolic compounds, and other secondary metabolites (^{Haichar et al., 2014}), It has been reported that under cadmium stress the concentration of organic acids (malic, citric, acetic, oxalic, glutamic and formic acids) increases in corn plants as a tolerance mechanism (^{Javed et al., 2017}). It is likely that tomato plants follow this same strategy, since exposure to As induces the exudation of organic acids by the root, and the transport of As to the shoots is limited, increasing accumulation in the roots (^{Carbonell-Barrachina et al., 1997}; ^{Madeira et al., 2012}; ^{Stazi et al., 2016}). Radical exudation may be involved in the stabilization process but could also sequester micronutrients.

Under metallic stress (metallic ions, metallic NPs), plants release more low molecular weight substances such as organic acids (oxalate, acetate and malate) as a defense against stress (^{Shang et al., 2019}). In particular, citric, oxalic and malic acids form complexes with metals, which affects their fixation, mobility and availability to plants (^{Xie et al., 2013}). The positive effect of organic acids on Cd complexation has been reported, showing up to 85 % with citric acid in the soil solution of Lupinus plants (^{Römer et al., 2000}). However, the modification in the exudation of plants under metallic stress can influence the dynamics of soil nutrients and the microbial activity of the rhizosphere (^{Jia et al., 2014}). In addition to the fact that the ability to pump protons by the H⁺-ATPases in the plasmalemma of the plant cell decreases under stress by metals, which can affect the assimilation of nutrients (^{Javed et al., 2017}).

The SiO₂ NPs induced a decrease in Fe content, however a positive trend was observed in the absorption of Cu and Zn. ^{Le et al. (2014)}, reported the concentration of minerals nutrients such as Cu and Mg were affected by SiO₂ NPs in shoots of Bt-transgenic cotton, the negative effect was greater compared to non-transgenic cotton. The adverse effects of nanoparticles include reduction of root elongation, in different species of plants such as corn, cucumber, soybean, cabbage and carrot (^{Lin & Xing, 2007}), this could cause a decrease in the absorption of nutrients. However, the positive effects can due to mesoporous nature of Si NPs, this characteristict may help in agriculture due they act such a nanocarrier for different molecules (^{Rastogi et al., 2019}), and can be useful in plant nutrition. Nanosilica increase soil nutrient, can maintain soil pH, and even stimulates soil bacterial community (^{Karunakaran et al., 2013}). These improvements in the soil system make the availability of nutrients for plants more efficient.

The characteristics and nature of nanomaterials, in addition to plant species, will greatly influence translocation and accumulation in plant tissues. Size seems to be one of the main restrictions for penetration into plant tissues, moreover, in the soil, nanoparticles can interact with microorganisms and compounds, which might facilitate or hamper their absorption (^{Pérez-de-Luque, 2017}).

Upon soil application, NPs can enter the root, and then penetrate the cell wall/plasma membrane, reaching the root cortex and entering the xylem vessels, thereby moving upward to aerial plant parts (^{Ma et al., 2010}). Moreover, NPs and NMs have a high density of surface charges capable of unspecific interactions with the surface charges of the cell walls and membranes of plant cells that can cause stimulation (^{Juárez-Maldonado et al., 2019}). The addition of SiO₂ NPs can improve the proportion of living cells by weakening oxidative stress after exposure to As. It can maintain the integrity of the cell, increase the thickness of the cell wall and the proportion of As in pectin. Moreover, the supply of SiO₂ NPs regulates the expression of the genes encoding the As-related transporters (^{Cui et al., 2019}). Probably these modifications that the SiO₂ NPs induce from physiological-morphological, biochemical and molecular could balance or increase the absorption of essential nutrients.

Macronutrients

Our results generally showed a negative effect of the application of arsenic in irrigation water on macronutrients, although P in roots had a positive trend, but in shoots the accumulation decreased. ^{Roy et al. (2012)}, reported that As affects P, K and S uptake in shoot and root of amaranthus plants, although only As and S relationship showed a significant negative effect. It has also been reported that application of 5, 10, 20, and 50 mg L^-1 As as As(III) or As(V) in a nutrient solution, decreased the concentrations of K, Mg and P in the roots and shoots of mesquite plants (^{Mokgalaka-Matlala et al., 2008}).

The decrease in P accumulation probably resulted from phytotoxicity by As (^{Wang et al., 2002}), or because the As can be replaced by P (^{Tu & Ma, 2005}). This is due to their analogy, as they both have similar electronic configurations and chemical properties and compete for the same absorption transporters (^{Meharg & Hartley-Whitaker, 2002}). For this reason, P also strongly influences the absorption of As in plants (^{Anawar et al., 2018}). However, although As can replace P in the plant, it cannot perform its biological functions (^{Tu & Ma, 2005}). In the present study, the K concentration showed an increasing trend in roots and decreased in shoots. Similar results reported by ^{Parson et al. (2008)}, the K content in the plants was found to improve in the roots and decrease shoots, here As reduced 60% K concentration in shoot. The positive relationship between arsenic and some nutrients may be due to a “concentration effect” since high doses of As in solution decrease shoot and roots biomass (^{Melo et al., 2009}). Maintaining sufficient S nutrition may be particularly important in the As contaminated environment, due to detoxification of As through S occurs by complexation of arsenite with thiol rich peptides (^{Praveen et al., 2018}).

As causes different effects on the macronutrient content, the responses will be different in plant species and in each nutrient, even for the same nutrient and species (^{Khan et al., 2019}). Furthermore, it has also been suggested that the stress caused by arsenic regulates the nutritional status of plants (^{Kumar et al., 2015}). The application of SiO₂ NPs induced a positive effect on the absorption and accumulation of macronutrients. ^{Tripathi D. K. et al. (2015)}, reported an increase of K and P in root and leaves of Pisum sativum seedlings with the addition of Si NPs alone, and also improved absorption even when the seedlings were exposed to chromium (VI). ^{Alsaeedi et al. (2019)} reported an increase in the absorption and concentration of K (52 %, 75 % and 41 % in root, stem and leaves respectively) of Cucumis sativus plants with the application of Si NPs. Silicon has the ability to activate H⁺-ATPases located in the plasma membrane, which increases cellular absorption of potassium through electrochemical gradients and K⁺ channels and transporters (^{Liang et al., 2006}).

NPs are capable of inducing signaling reactions in root cells (^{Sosan et al., 2016}). It is known that the exposure of plants to NPs can modify the growth patterns of roots by altering their morphology (^{Dimkpa et al., 2015}), generating effects such as the proliferation of root hairs that could increase the absorption of nutrients (^{Adams et al., 2017}). On the other hand, the increase in macronutrient concentrations in plants by SiO₂ NPs may be related to a “concentration effect”, since shoots and root biomass decreased. ^{Yang et al. (2020)}, reported that the concentration of N and P increases in response to the decrease in the height of Leymus chinensis and Stipa krylovii. The concentration of Mg and Zn in root, and K in leaves of Pfaffia glomerata increases in response to the decrease in dry root and leaf biomass caused by exposure to 50 μM of As (^{Gupta et al., 2013}).