Nano-silicon and sodium mitigate damage by potassium deficiency in chicory

Abstract

Chicory is a food with high nutritional. The use of beneficial elements in plants, such as sodium (Na) and silicon (Si), may be important to mitigate nutritional disorders, such as potassium (K) deficiency, but research is lacking on this topic. The objective was to evaluate the effects of sodium and nano-silicon on the nutritional, physiological, growth, and quality parameters of chicory under K deficiency and sufficiency. We used a concentration for sufficient K (3.0 mmol L⁻¹), K-deficiency (1.5 mmol L⁻¹), combined with the lack or presence of Na (2.0 mmol L⁻¹) and Si (2.0 mmol L⁻¹). The experiment was carried out in a greenhouse with six treatments corresponding to K sufficiency, K-sufficiency with Na, K-sufficiency with Si, K deficiency, K-deficiency with Na, and K-deficiency with Si, with six replications. The following growth variables were evaluated: (i) plant height, (ii) stem diameter, (iii) number of leaves, (iv) leaf area, and (v) plant biomass. Potassium and Si contents in the above ground part and K utilization efficiency were assessed, and the accumulation of K, Na, and Si was calculated. The efficiency of the quantum yield of photosystem II (Fv/Fm) and the photosynthetic pigments was determined. Electrolyte leakage index and relative water content, as well as phenolic compounds, ascorbic acid, and leaf firmness index were also determined. We found that supplying nano-Si and Na to a K-deficient nutrient solution increased K accumulation by 60% and 50% and K use efficiency by 79% and 62% compared to plants without supply of those elements. Nano-Si reduced electrolyte leakage, being 41% less than Na in K-deficient chicory. However, when Na was added to a nutrient solution with sufficient potassium, the K use efficiency decreased by 48% compared to sufficient potassium without Na. Under the same condition of sufficient supply of potassium and Na, K accumulation decreased by 20% in chicory compared to sufficient potassium without Na, and the photosynthetic pigments—total chlorophyll and carotenoids—were reduced by 5% and 10%, respectively. Our findings contribute to improve cultivation systems with low supply of K as the supply of Na and nano-Si mitigates the damage caused to the metabolism of chicory under K deficiency.

Introduction

Hydroponics is a cultivation system that uses the nutrient solution to nourish plants and allows favouring synergistic interactions between elements, also increasing the use efficiency of nutrients¹. These synergistic interactions can enhance the metabolism of plant species, for instance, by increasing beneficial compounds for both plants and humans, resulting in an increase in crop production².

In this scenario, chicory is an important vegetable for food security because, in addition to its high nutritional value, it can provide antioxidant compounds³, such as phenols, vitamin C and carotenoids, which are provitamin A⁴. Furthermore, such antioxidants are part of the non-enzymatic protection of plants⁵ and act by removing reactive oxygen species (ROS)⁶. Natural antioxidants, such as those from plants consumed by humans, are known and important in disease prevention⁷, as they aid the immune system, which is a capacity already described for vitamin C⁸, and act as important anti-inflammatory agents, such as vitamin A⁹.

A proper development is associated with the ability of the plant to perform vital functions, such as osmotic regulation, maintaining the water content in plants optimal due to K¹⁰, and producing compounds that are associated with plant defence, such as the mechanisms mentioned above.

In K deficiency, plant yield is severely affected because, even though it is not part of the plant’s structure, K performs some functions, such as (i) osmotic regulation and (ii) cell expansion¹⁰. Furthermore, K deficiency reduces the production of phenolic compounds and photosynthetic pigments, increases cellular electrolyte leakage¹¹ and decreases water content in plants¹⁰.

In a situation of K deficiency, chicory (Cichorium endivia L.) presents a reduction in leaf area and number of leaves and consequently a smaller photosynthetic area and less shoot and root dry mass. This set of changes directly affects the yield of leafy vegetables compared to chicory grown without K restriction¹².

The use of beneficial elements for horticultural species in conditions of nutritional disorders can be an alternative to increase the use efficiency of nutrients, for example Si as an attenuator of K deficiency in snap beans¹¹, basil¹³ and corn¹¹, and Na in kale¹⁴.

Si is widely described in the literature as an attenuator of abiotic stresses¹⁵. In K-deficient plants, Si can improve relative water content¹⁰ and increase K use efficiency¹⁶. Despite this, in a situation of adequate K, the effects of Si may not be evident, that is, the beneficial effect may not be significantly perceived in plant metabolism, as occurs in forage species¹⁶ and maize¹¹. Although Si evidences its effects in stress situations through supply by silicate sources, such as K and Na silicatebuchelt^11,16–18, the effects of Si nanoparticles on chicory are not yet known. It is possible that this form of Si can increase its absorption by plants and increase its benefits.

Another beneficial element for plants is Na¹⁹. In conditions of K deficiency, Na can increase the use efficiency of K¹⁴ and act in some functions originally performed by K, such as osmotic regulation¹⁹. However, in situations of adequate K, Na can reduce the dry mass of plants, as occurs in cabbage¹⁴. A still unknown aspect that deserves attention is the comparison of the effects of these two beneficial elements in plants with nutritional stress since most studies focus on Si or Na.

Thus, in this study, the objective is to evaluate the effects of sodium and nano-silicon on the nutritional, physiological, growth, and quality parameters of chicory under K deficiency and sufficiency. The hypotheses are (i) K deficiency can decrease with the supply of Na and nano-Si and (ii) in K sufficiency the use of Na is harmful to plants and the supply of nano-Si in the same condition of adequate K produces no changes in the studied parameters.

If the hypotheses of this research indeed indicate benefits of Na and nano-Si especially in the cultivation of chicory with K deficiency, unveiling the mechanisms involved in it, a sustainable strategy will emerge for the cultivation of chicory in nutrient solution, saving on this macronutrient as hydroponic cultivation of vegetables and the supply of K have increased worldwide.

Material and methods

Plant material and growing conditions

The experiment was carried out in a greenhouse of the Faculty of Agricultural and Veterinary Sciences, Jaboticabal Campus, Brazil. The cultivation system was hydroponic with washed sand as substrate. Polypropylene vases with a volume of 5 L filled with 4.5 L of sand were used. The vases had six holes measuring 16.9 mm for the water to escape and were lined with a blanket made of a fibre alloy and a polypropylene polymer to prevent sand from escaping.

The seeds of chicory, cultivar ‘escarolam lisa,’ were sown. Twelve days after sowing, thinning was performed, remaining three plants per pot. The variables temperature and relative air humidity inside the greenhouse were recorded daily using a thermo-hygrometer sensor (Fig. 1).

Figure 1 [Images not available. See PDF.]

Meteorological variables: maximum (T_max) and minimum (T_min) air temperature (°C) and maximum (H_max) and minimum (H_min) relative air humidity (%) during the experimental period.

Experiment conditions

The experimental design was completely randomized with six treatments and six replications. The position of pots was changed twice a week to avoid a possible influence of shading in the greenhouse. The treatments were control with sufficiency of K (+ K), K-sufficiency with Na (+ K + Na), K-sufficiency with Si (+ K + Si), K-deficiency (−K), K-deficiency with Na (−K + Na), and K-deficiency with Si (−K + Si) (Appendix A).

The K concentration in the treatments with + K (3.0 mmol L⁻¹) was defined considering 50% of²⁰ recommendation, as this concentration provides a good development for snap beans²¹. To induce K deficiency in the -K treatment, in the first 18 days after plant emergence 1.0 mmol L⁻¹ of K was used; after that, the supply was 1.5 mmol L⁻¹. This concentration was defined because it induces K deficiency in kale plants¹⁴. For the treatments that received Na, the concentration was 2 mmol L⁻¹, as this concentration is sufficient to attenuate K deficiency in kale¹⁴. In treatments with Si, the concentration was 2.0 mmol L⁻¹, as this concentration promotes adequate absorption of Si in most species²². Bindzil^® nano-silica was used as a source of Si, with the following properties: Si: 168.3 g L⁻¹, specific surface area: 300 m₂ g⁻¹, pH: 10.5, density: 1.2 g cm⁻³, Na₂O: 0.5%, and viscosity: 7 centi Poise—cP.

The nutrient solution was prepared using distilled and deionized water with a pH between 8 and 9. As soon as the salts were added, the pH was corrected using a hydrochloric acid solution (HCl) and kept between 5.5 and 6.0. The hydrochloric acid (HCl) solution was added carefully so as not to raise the pH. The HCl was used quite diluted (30%) and chosen instead of nitric acid because chlorine can exhibit a wide range of toxicity in leafy vegetables (20,000–30,000 mg kg⁻¹)¹⁰. Therefore, it was not necessary to use sodium hydroxide (NaOH) or potassium hydroxide (KOH), avoiding possible interferences from the accompanying ion (Na and K) solutions that could affect treatments.

Nutrition solution²⁰, with modifications according to the treatments (Table 1), was applied at 20% of its ionic strength in the first week after plant emergence, 35% in the second week, and 50% from the third week onwards until the end of the experiment.

Table 1. Composition of the nutrient solution and treatments.

Fertilizer	+ K –Na	+ K + Na	−K + Na	−K −Na	+ K + Si	−K + Si
mol L−1	mmol L−1
KH2PO4	0.5	0.5	–	–	0.5	–
MgSO4 7H2O	–	–	1.0	1.0	–	1.0
KCl	2.5	2.5	1.5	1.5	2.5	1.5
NH4H2PO4	–	–	0.5	0.5	–	0.5
NH4NO3	–	–	1.0	1.0	–	1.0
(NH4)2SO4	1.0	1.0	–	–	1.0	–
MgNO3 6H2O	1.0	1.0	–	–	1.0	–
NaCl	–	2.0	2.0	–	–	–
Ca (NO3)2 4H2O	2.25	2.25	2.5	2.5	2.25	2.5
CaCl2 2H2O	0.25	0.25	–	–	0.25	–
SiO2 Nano	–	–	–	–	0.33	0.33
Micronutrientsa	0.5	0.5	0.5	0.5	0.5	0.5
Fe-EDDHAb	1	1	1	1	1	1

^aIn 1 L: 2.86 g H₃BO₃; 1.81 g MnCl₂.₄H₂O; 0.22 g ZnSO₄.₇H₂O; 0.04 g CuCl₂; 0.02 g H₂MoO₄H₂O.

^bFe-EDDHA with 6% of Fe (83.33 g L⁻¹).

Growth assessment

At 90 days after emergence, the growth evaluations were carried out using a digital calliper to measure stem diameter. Number of leaves was obtained by counting. The leaf areas were quantified using a leaf area integrator (model L3100, LiCor, USA).

The plants were collected and, using a precision analytical balance, the shoot fresh mass was obtained. The plants were sequentially washed in water with a detergent solution (0.1%), HCl solution (0.3%), and deionized water¹⁰. Then, the plant material was dried in an oven with forced air circulation (65 °C) until constant mass. The shoot and root dry mass was obtained.

Chemical analysis of plant shoots

The dried shoots of plants were ground in a Wiley-type mill (Model MA 340) (MARCONI, São Paulo, SP, BR) and used to determine K, Na, and Si contents. The K and Na content was obtained following the methodology of²³ and Si was obtained according to²⁴. The accumulation of elements and nutrients was calculated using the shoot contents and shoot dry mass.

Use efficiency of K

The K use efficiency for the shoots was calculated according to the formula below²⁵: $$EUK=\frac{{(\text{shoot dry mass})}^2}{\text{K accumulation in shoot dry mass}}$$

Analysis of electrolyte leakage

Five leaf disks were extracted from living tissues of plants, added to Beckers containing 20 mL of distilled and deionized water. After two hours at room temperature, the initial electrical conductivity (IEC) was measured using a bench conductivity meter. After that, the Beckers were covered with aluminium foil and placed in an autoclave, where they spent 20 min at 120 °C. Then, after this period, they were removed from the autoclave and cooled until they reached 25 °C. Then, the final electrical conductivity (FEC) was measured. The electrolyte leakage rate was obtained using the equation proposed by²⁶: $$\text{ELR}=\frac{\text{IEC}}{\text{FEC}}\times 100$$

Photosystem II quantum efficiency

Between 7 and 8 a.m., a fully developed leaf was used to carry out measurements aiming to quantify the quantum efficiency of the photosystem II (Fv/Fm), given by the ratio between variable fluorescence (Fv) and mean fluorescence (Fm)²⁷, using a portable fluorometer (Os30P+, Opti-Sciences Inc., USA).

Ascorbic acid determination

Collections of 0.1 g of fully developed chicory leaves were carried out and immediately placed in an ice bath. The samples were macerated in a porcelain mortar, initially adding 5.0 mL of 5% oxalic acid ice-cold at 15 °C after homogenization of the macerated sample, plus 5.0 mL of 5% oxalic acid ice-cold at 15 °C. After this, the sample was filtered in a beaker to obtain the extract. In triplicate, 1.0 mL of the extract was removed and 4.0 mL of 5% oxalic acid were added. Sequentially, titration was performed with DFI—2.6 dichloro—0.02% indophenol phenol (Tilman's solution), considering the turning point a light pink colour²⁸.

Total phenols

Was collected 0.1 g of fully developed leaves and added to a test tube covered with aluminium foil and 2 mL of methanol, remaining for three hours in a place with complete absence of light and room temperature of 25 °C. After this period, another 3 mL of methanol were added. 1 mL of the extract aliquot was removed and added to another test tube with aluminium foil. In that same tube, 10 mL of distilled and deionized water and 0.5 mL of Folin-Ciocalteu (2N) were added, leaving the solution to rest for three minutes. After that, a colorimetric reaction was performed with the addition of 1.5 mL of 20% Na carbonate, resting for two hours in a place without light at average room temperature of 25 °C. Subsequently, the absorbance reading of 765 nm (nm) was performed in the spectrophotometer. The levels obtained were calculated as gallic acid equivalent (GAE)²⁹.

Quantification of chlorophyll and carotenoids

Leaf discs with mass between 0.025 and 0.030 g were collected from the living tissue of plants and placed in an Eppendorf tube filled with 1.5 mL of 80% acetone. After that, the tubes were stored in a dark place at an average temperature of 15 °C. As soon as there was complete depigmentation of disks, readings were taken in a spectrophotometer at an absorbance of 663 nm for chlorophyll a, 647 nm for chlorophyll b, and 470 nm for carotenoids³⁰.

Relative water content

Ten leaf discs were removed. After that, using a precision analytical balance, the fresh mass (MF) was obtained. Sequentially, the disks were added to Beckers with 20 mL of distilled and deionized water, where they were rehydrated for six hours. After that, the turgid mass (MT) values were obtained, and the discs were placed in paper bags in a forced circulation oven at 80 °C for 24 h to obtain dry mass (DM). The relative water content was calculated using the equation proposed by³¹: $$RWC=\frac{FM-DM}{TM-DM}\times 100$$

Leaf firmness index

A fully developed leaf of each plant was used for measurements on three different points distant from the main vein and between the secondary veins³². The equipment used was a digital penetrometer from 5 to 200 Newton (N), with a precision of ± 1 N (Impac, Model IP-90DI, São Paulo, SP, Brazil) with an 8-mm tip.

Plant images

Photos were taken of one repetition per treatment, arranged in front of a black background, using a 5-megapixel camera coupled to a smartphone.

Analysis of results

The results were subjected to Shapiro–Wilk test to verify the normality of data and Levene test to verify homoscedasticity. Given the assumption of normality and homoscedasticity, the data were submitted to t test (STT) for comparison of means (p < 0.05). Statistical analyses were performed using the software R, version 4.1.0. In the figures, means and mean standard error bars were used.

Results

In general, the highest accumulations of K occurred in plants sufficient in this nutrient and the lowest accumulations in plants deficient in K regardless of the use of Na or Si. However, in K sufficiency, the addition of Na resulted in a decrease in K accumulation. On the other hand, in K deficiency, plants that received Na and Si showed an increase in K accumulation (Fig. 2a).

Figure 2 [Images not available. See PDF.]

K accumulation (a), K use efficiency (b), Na accumulation (c) and Si accumulation in shoots (d) of chicory cultivated in K sufficiency (+ K), K deficiency (–K), K plus Na deficiency (–K + Na), K plus Si deficiency (–K + Si), K plus Na sufficiency (+ K + Na) and K plus Si sufficiency (+ K + Si). Different letters indicate differences between treatments by t test (LSD) (p < 0.05). The bars represent mean standard error. n = 5.

K use efficiency decreased in K-deficient chicory. Si and Na in K-deficient chicory resulted in an increase in K use efficiency, with emphasis for Si. Plants with K sufficiency and K sufficiency plus Si did not show differences in relation to the control. However, when Na was added, the K use efficiency decreased (Fig. 2b).

Na had the highest accumulation in plants that received this element (Fig. 2c). Likewise, plants supplemented with Si showed the highest Si accumulation (Fig. 2d).

K deficiency in chicory increased electrolyte leakage in relation to K sufficiency. In the presence of Si and Na with less K, there was a reduction in electrolyte leakage in relation to K-deficient plants without the elements, with emphasis for Si. Na in plants sufficient in K increased electrolyte leakage in relation to the control. This did not occur with Si supplementation at adequate K levels (Fig. 3a).

Figure 3 [Images not available. See PDF.]

Leakage of electrolytes (a), ascorbic acid (b), total phenols (c), relative water content (d), chlorophyll a + b (Chl a + b) (e), carotenoids (f), Fv/Fm (g) and leaf firmness index (b) of chicory cultivated in K sufficiency (+ K), K deficiency (–K), K plus Na deficiency (–K + Na), K plus Si deficiency (–K + Si), K sufficiency plus Na (+ K + Na) and K sufficiency plus Si (+ K + Si). Different letters indicate differences between treatments by t test (STT) (p < 0.05). The bars represent the mean standard error. n = 5.

K deficiency in chicory decreased ascorbic acid production in relation to K-sufficient plants. Si and Na supplied under K deficiency increased ascorbic acid. Sufficient K with the presence of Na resulted in a decrease in ascorbic acid compared to plants sufficient in K. Si had no effects under the condition of adequate K (Fig. 3b). The use of Si under K sufficiency did not affect total phenols in relation to K sufficiency. On the other hand, when supplying Na with adequate K, phenols decreased in relation to the same condition without this element (Fig. 3b,c).

The relative water content decreased in K-deficient chicory. The beneficial elements, Si and Na, with less K increased the relative water content. By supplying Na with sufficient K, the relative water content decreased. On the other hand, Si did not affect the water content in K sufficiency (Fig. 3d).

Total chlorophylls and carotenoids decreased in chicory with K deficiency in relation to K sufficiency. Na and Si with less K increased total chlorophylls and carotenoids. Na in plants with K sufficiency decreased the total content of chlorophylls and carotenoids in relation to the control. On the other hand, Si did not influence the chlorophyll content with adequate K (Fig. 3e,f).

Fv/Fm decreased in chicory with less K. Na increased the Fv/Fm of K-deficient plants, but the greatest increase in Fv/Fm of chicory with K deficiency happened after adding Si. Chicory with K sufficiency and in the presence of Na reduced the Fv/Fm in relation to plants sufficient in K. Si did not interfere in the Fv/Fm of plants in K sufficiency (Fig. 3g).

The firmness index of chicory leaves decreased in K deficiency compared to K sufficiency. Si and Na with less K increased the firmness index of leaves with low supply of K. The presence of Na in adequate K levels decreased the firmness index of leaves. There were no effects of Si on the firmness index of leaves with K sufficiency (Fig. 3h).

In K deficiency, chicory showed a reduction in leaf area, number of leaves, and stem diameter. Supplying Si and Na with less K increased leaf area, number of leaves, and stem diameter in relation to K deficiency without the elements. Na with less K decreased leaf area and stem diameter but did not decrease number of leaves in relation to K sufficiency. There were no effects of Si on K sufficiency as for leaf area, leaf number, and stem diameter in relation to adequate K levels without Si (Fig. 4a–c).

Figure 4 [Images not available. See PDF.]

Leaf area (a), leaf number (b), stem diameter (c), shoot fresh mass (d), shoot dry mass (e), and root dry mass (f) of chicory cultivated in K sufficiency (+ K), K deficiency (–K), K plus Na deficiency (–K + Na), K plus Si deficiency (–K + Si), K plus Na sufficiency (+ K + Na), and K sufficiency plus Si (+ K + Si). Different letters indicate differences between treatments by t test (STT) (p < 0.05). The bars represent the mean standard error. n = 5.

Shoot fresh mass, shoot dry mass and root dry mass of K-deficient chicory decreased. When Si and Na were added in conditions of K deficiency, there was an increase in shoot fresh mass, shoot dry mass and root dry mass. Si stood out as for the increase in shoot fresh and dry mass. Plants cultivated with K sufficiency with Na showed a reduction in shoot fresh mass, shoot dry mass and root dry mass in relation to plants sufficient in K. This did not occur when offering Si with adequate K levels, as there were not differences (Fig. 4d–f).

Chicory cultivated with nutrient solution with less K showed a reduced growth and chlorosis that evolved to necrosis at the leaf apex (−K). However, when offering Na with less K, the plants showed an increase in growth. Si with less K also increased plant growth and reduced the visual symptoms of K deficiency. Na with sufficient K reduced plant growth (Fig. 5).

Figure 5 [Images not available. See PDF.]

Chicory grown in a hydroponic system with K sufficiency (+ K), K deficiency (–K), K plus Na sufficiency (+ K + Na), K plus Si sufficiency (+ K + Si), K plus Na (–K + Na), and K plus Si deficiency (–K + Si). Scale: 25 cm.

Discussion

Si is beneficial in K deficiency and indifferent in K sufficiency.

The accumulation of K in shoots occurs due to the distribution and transport of this nutrient within the plant¹⁰. In K deficiency, Si-supplemented chicory plants increased K accumulation (Fig. 2a). Thus, we may presume that Si facilitated K transport from roots to shoots³³, that is, it acted by regulating the expression of specific transporters such as SKOR and AKT1, as reported for sorghum³⁴. However, this still requires investigation for non-Si accumulating species such as chicory.

The response of fertilization can be understood and quantified through the use efficiency of nutrients and can be improved in the plant using different strategies: decreasing the amount absorbed, increasing growth with the same amount absorbed, or absorbing more nutrients and growing more³⁵; the latter occurred in our study under K deficiency plus Si supply (Fig. 2b). The initial precursor for Si to increase K use efficiency in deficient plants was the ability of this species to absorb Si, a fact still unknown. The supply of Si in a plant deficient in K in relation to a plant without application of Si resulted in an increase in the accumulation of Si by 86% possibly because the source used is nano-silicon. This occurs because the very small particle size favours absorption³⁶ and may be a strategy for chicory-like species that have restriction in the transport of Si from the root to shoots.

In K deficiency, there is a reduction in carbon dioxide (CO₂), generating excess electrons and increasing the production of reactive oxygen species (ROS)³⁷. Furthermore, the reduction of cytosolic K contents, either by K efflux or reduction of K supply, can trigger mechanisms that stimulate the production of ROS, potentially increasing oxidative stress in cells⁵. In our results, the values of electrolyte extravasation in K-deficient plants were 75% higher. However, when supplemented with Si, there was a 63% reduction in this variable. In this sense, Si may have acted in different ways in the plant to reduce the amount of peroxides and free radicals. One of these ways occurred by increasing the production of antioxidant compounds and reducing the production of ROS¹⁰.

l-ascorbic acid, known as vitamin C, is widely found in plants and protects the plant against stress because it is a non-enzymatic antioxidant compound³⁸. However, most animals, including humans, cannot synthesize it due to the lack of the enzyme l-gulono-1,4-lactone-oxidase, which is essential at the last stage of its production³⁹. Notably, in the presence of Si, K-deficient chicory showed an increase in ascorbic acid (Fig. 3b). This result may be due to the increase in the activity of the enzymes dehydroascorbate reductase and monodehydroascorbate reductase, consequently enabling the efficient recycling of ascorbic acid back to its active form⁴⁰.

As mentioned earlier, K deficiency leads to increased ROS, resulting in a marked damage caused by oxidative stress in cells³⁷. In this scenario, phenolic compounds act as antioxidants, that is, they help to neutralize ROS by donating electrons to molecules, providing stabilization and preventing damage⁶. Furthermore, oxidative stress is also present in humans and is a cause of aging and diseases⁴¹. The increased production of total phenols in chicory with low supply of K was provided by the presence of Si (Fig. 3c). Although the mechanism of action of Si is still not understood metabolically in the biosynthesis of these compounds, there is evidence about its action helping to increase the activity of key enzymes in the synthesis of phenols, such as phenylalanine ammonia-lyase and tyrosine ammonia-lyase, which are enzymes involved in the conversion of amino acids into phenol precursors⁴².

The imbalance of osmotic potential in cells caused by K deficiency leads to dysregulation of stomatal opening and closing, consequently increasing water loss through transpiration¹⁰. In the case of K deficiency in chicory, when we added Si, there was an increase in relative water content (Fig. 3d). A possibility for Si action that may explain this is its deposition in epidermis cells, resulting in water loss through transpiration, which is then reduced, causing water to be retained in these cells⁴³. However, this condition of Si deposition is often reported in species that accumulate Si, and evidence related to such a deposition in species outside this group is still unknown.

The importance of Si in increasing chlorophylls is well known¹⁰. Chlorophylls are key factors in photosynthesis as they act in light absorption, electron transfer, and phosphorylation. Total chlorophylls (a and b) are mainly involved in the absorption step and in the transfer of electrons⁴⁴. The increase in the amount of total chlorophylls observed in chicory with K deficiency plus Si (Fig. 3e) is due to some factors: (i) preservation of the internal structure of chloroplasts, (ii) improvement of the absorption capacity and K storage, and (iii) lasting stabilization of chlorophyll from increased ROS neutralizing activity⁴⁵.

The decrease in Fv/Fm indicates that the centres of the photosystem II (PSII) are compromised; in other words, it indicates that there may be a difficulty in transferring electrons from the PSII to the photosystem I (PSI)⁴⁶. PSII functionality can be compromised in K-deficient plants¹⁰. The maintenance of adequate levels (0.80)⁴⁷ in chicory with K deficiency can be understood by the fact that Si contributes to the maintenance of the ultrastructure of chloroplasts, which are important for photosynthesis efficiency⁴⁵ and which occurred in this research, according to previously discussed results.

Another benefit of Si in plants is to provide greater rigidity for tissues¹⁰. The firmness of leaves can be understood by the greater average force exerted on the leaf tissue until its point of rupture. The greater the force, possibly the greater the firmness⁴⁸. In K-deficient chicory, our results showed the highest leaf firmness index when Si was added (Fig. 3h). In this scenario, Si helps to increase the turgor pressure of cells, contributing to their structural support, that is, the cell content increases the volume and exerts pressure on the cell wall. This pressure in turn contributes to the stability and integrity of structures, contributing to plant stiffness, which may indirectly increase leaf firmness index³².

Decreased K content within cells acts as a switch to inhibit anabolic reactions and stimulate catabolic processes, resulting in the interruption of plant growth and redirecting available energy to fight damage caused by stress⁵. Si in K-deficient chicory promoted growth and increased dry mass by mitigating stress, increasing chlorophyll contents and antioxidant compounds (Fig. 3b,c,f), improving the relative water content and firmness index of leaves (Fig. 3d,h), resulting in more efficient photosynthesis and a greater biomass production¹⁰.

On the other hand, in terms of K sufficiency, Si was indifferent in all variables studied (except for Si accumulation: the highest accumulations were associated with plants that received Si). This confirms our second hypothesis, according to which this element is indifferent in K adequate levels. A possible explanation is the adequate growth conditions in a complete nutrient solution, where there is no stress to be relieved by the application of Si⁴⁹. Therefore, the addition of Si to the nutrient solution in systems with adequate K is unnecessary, as it does not improve plant growth. However, it can be useful if the interest is quality and aiming Si biofortification, as it is a nutrient for humans.

Na is beneficial in K deficiency and toxic in K sufficiency

Na can interact with other co-transporters or ion channels in roots, modifying the electrochemical balance in the cell environment and allowing the transport of K into the plant⁵⁰, resulting in a greater accumulation and K use efficiency in shoots of K-deficient chicory (Fig. 2a,b). However, the presence of Na in the nutrient solution in plants with adequate K may have generated less accumulation or absorption of K (Fig. 2a). This may be related to the low expression of K transporter genes, such as OsHAK5 (reported in rice), which accumulate greater amounts of K in shoots. When the expression of this gene is low, it may lead to increased Na absorption and limit plant growth⁵¹.

Na can be absorbed by plant roots through the same ion transport systems as for K⁵². This could explain why even at low concentration, plants absorbed Na in K deficiency and sufficiency (Fig. 2c).

Imbalance in ionic balance can lead to increased ROS production in plant cells. This is due to disturbances in metabolic pathways and cellular processes that normally control ROS production and elimination⁴⁰. Na can perform the function of vacuole load balancing in K-deficient plant cells, in other words, it can reduce or contain this ionic imbalance in the cellular environment⁵³. However, the exact mechanism by which this occurs is not yet known.

Na helps to regulate osmotic pressure within cells, thus maintaining water absorption and retention⁵². This occurred in chicory with low supply of K plus Na (Fig. 3d). In our findings, in K deficiency, plants with Na supply increased the content of total chlorophylls and carotenoids (Fig. 3e,f). Na may have helped by replacing K in the vacuoles, thus improving the ionic balance and contributing to reducing the loss of photosynthetic pigments¹³.

Na can improve sunlight harvesting efficiency by increasing the rate of photosynthesis and electron transfer, as well as improving the circulation of CO₂ in leaves⁵⁴. This could explain the increase in Fv/Fm in chicory with K deficiency since, with K limitation, there is a decrease in stomatal conductance, hindering the entry of CO₂ and causing a decrease in the activity of the enzyme ribulose bisphosphate carboxylase (RuBisCO), which is important for the assimilation of CO₂³⁷.

Fertilization with Na in a tree species (Eucalyptus grandis) resulted in an increase in variables related to cell rigidity (modulus of elasticity, turgor pressure at full turgor, and others)⁵⁴. Despite being an indirect measure for leaf stiffness, indicates possible mechanisms that may change firmness of leaves of species with Na fertilization, as this study found (Fig. 4h).

The presence of Na negatively affected the growth and development of chicory with K sufficiency by decreasing the variables studied, such as photosynthetic pigments, antioxidant compounds (Fig. 3), shoot dry mass and root dry mass (Fig. 4). This confirms our second hypothesis, according to which this element could be harmful to plants with adequate K. The finding above is consistent with the study on kale, a species that is also a glycophyte. The authors indicated that the presence of Na with adequate K decreased the amount of K in the plant and the use efficiency of K, and consequently the growth and development of the plant¹⁴.

Si is better than Na in mitigating K deficiency

In K deficiency, Na increased the use efficiency of this nutrient by 67% and Si by 80% compared to K-deficient plants without these elements. The accumulation of K in plants was 60% and 50% higher with supply of Si and Na compared to the same condition of K without Na and Si.

In K-deficient treatments with Si and Na supply, leakage decreased by 65% and 35% and total chlorophylls (a + b) increased by 82% and 76%, respectively, compared to K-deficient plants without the presence of these elements. Furthermore, given this same K scenario and the presence of Si and Na, the leaf firmness index was 56% and 29% higher compared to the same K condition without Si and Na.

The antioxidant compounds (ascorbic acid, phenols, and carotenoids), the Fv/Fm, the relative water content, and the root dry mass did not show differences between the supply of Na or Si under K-deficiency. However, the increase in shoot fresh mass and shoot dry mass were 63% and 65% in Si supply and 57% and 63% in Na supply in relation to K-deficient plants without the presence of these elements. In this context, possibly, the plants treated with Si reached a new metabolic state considered stable; in other words, there was a reorganization in the metabolic processes of leaf cells, and this state was similar to the state K-sufficient plants³³ according to the variables that showed better results for chicory cultivated with K-deficient compared to Na in the same K condition, as discussed above.

Although Si is better than Na for some variables, the damage to the metabolism of K-deficient plants was attenuated when supplying both elements, confirming our first hypothesis.

Conclusion

The results obtained in this study provide improvements in cultivation systems with low K availability by offering Na or nano-Si. The supply of these elements contributes to reduce damage caused by K deficiency and provides an increase in biomass. In addition, the maintenance and increase of compounds such as ascorbic acid, phenols, carotenoids, and leaf firmness index indicate improvements in the quality of the food, in addition to the better nutritional availability of Si or Na, making the food more nutritious. However, according to our findings, it is not necessary to increase fertilization with nano-Si and especially with Na when K levels are adequate. Thus, the beneficial mechanisms of Na and Si in chicory plants were unveiled. It is a sustainable strategy if there is a limitation in the use of K. This finding is important given the advance of hydroponic cultivation of vegetables in the world. The advance of this cropping system occurs because the increasing rate of population and urbanization growth in cities has led to the spread of urban agriculture aiming to increase food security and urban sustainability.

Acknowledgements

The authors would like to thank the Coordination for the Improvement of Higher Education Personnel (CAPES), Brazil, Code 001 and the Study Group on Plant Nutrition (Genplant) and the Paulista State University for their support in the research.

Author contributions

All authors read and approved the final manuscript. R.M.P. and R.F.B. planned and designed the research; D.M.R.A. and L.T.S.C. performed laboratory experiments and analyses. D.M.R.A., performed data analysis and preparation of graphs and figures. D.M.R.A., R.F.B. and R.M.P. wrote the manuscript. All authors reviewed the manuscript.

Funding

This study was funded by Coordination for the Improvement of Higher Education Personnel (CAPES), Brazil, Code 001.

Data availability

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1038/s41598-024-67875-0.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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