Introduction

Tomato (Solanum lycopersicum L.) is the most preferred vegetable worldwide (^{Infoagro, 2017}). It has high contents of bioactive compounds such as: folate, ascorbate, polyphenols, carotenoids, vitamins, and other essential nutrients, which is why it is palatable for human health (^{Ahmed et al., 2017}). ^{FIRA (2019)} mentions that, worldwide, the tomato is grown on approximately 5 million hectares (ha), with a production of 170.08 million tonnes (t) of fresh tomato, where China ranks first in the world with a tomato production of 46.2% of the total, while Mexico ranks tenth with a production of 1 878 289 t year^-1.

Salinity affects 1 125 million ha worldwide (Hossain, 2019). This condition generates problems that agriculture faces worldwide, particularly due to the presence of NaCl, which limits the optimal development of plants of commercial interest (^{Durukan and Demirbas 2018}). Mexico has an area of 54% of arid zones throughout the national territory, located mainly in the northern and northeastern regions of the country (^{Briones et al., 2018}), which are affected with a high degree of salinity, where sodium (Na⁺) cations displace calcium (Ca⁺) of the clay-humic complex, thus increasing the osmotic potential of the soil solution (Cisneros et al., 2020).

Currently, various alternatives are being sought to mitigate the negative effects of salinity, such as obtaining tolerant varieties, natural products such as biofertilizers and biostimulants that favor the growth and development of plants and benefit the environment (^{Mazón et al., 2020}).

Si is a mineral element that protects the plant against biotic and abiotic stress, in addition, chemically active Si restores damage and optimizes soil fertility, keeps nutrients available to the plant and improves cation exchange capacity, especially at pH greater than 7 (Meunier et al., 2018). Therefore, the objective was to evaluate the effect of Si as a salinity mitigator on germination and initial growth in seedlings of three tomato varieties.

Materials and methods

The research was conducted in the germplasm laboratory of the Autonomous University of Baja California Sur (UABCS, for its acronym in Spanish). The biological material used was three tomato cultivars: cherry (Solanum lycopersicum var. Cerasiforme), beef (Solanum lycopersicum var. Floradade) and saladette (Solanum lycopersicum var. Río Grande), the beef and saladette varieties are commercial varieties, while cherry is a landrace reproduced at the Autonomous University of Baja California Sur. Germination tests were carried out in sterilized Petri dishes (150 x 15 mm), covered at the bottom with a layer of absorbent paper. Each dish was moistened with 5 ml of the solution resulting from the combination of sodium chloride (NaCl) (0, 25 and 50 mM) and amorphous silicon dioxide (SiO₂) (0, 1 and 2 mM), the application of the solution was made every four days, distilled water was used for the control treatment. The germination tests were performed for 14 days, in a germination chamber (Seedburo, Model 549, NY, USA) at a temperature of 25 °C ±0.5 °C and 80% relative humidity, which were monitored daily if applicable.

Results and discussion

The statistical analysis showed that the effect of NaCl, Si and the interaction between these factors was statistically different (p≤ 0.05) in each tomato variety (Table 1).

For the cherry variety, the concentration of 50 mM NaCl increased germination energy and germination index. Meanwhile, fresh biomass increased at concentrations of 25 and 50 mM NaCl. However, germination percentage, germination energy, germination index and germination speed were affected at the concentration of 25 mM NaCl, with respect to the control treatment (Table 2).

^{Cuartero et al. (2006}) reported that the genus lycopersicum has a heritable salinity tolerance trait. ^{Vítámvá et al. (2007)} mentioned that there is a group of proteins inducible by abscisic acid (ABA), which are associated with salinity tolerance and water deficit, which could be explained to what happened in the present work.

Several studies show that increases in the percentage, energy and index of germination have been found at concentrations of 50 mM NaCl in crops such as: spinach Spinacia oleracea (^{Turhan et al., 2011}), tomato Solanum lycopersicum (^{Doğan et al., 2008}) and eggplant Solanum melongena (Akıncı et al., 2004).

On the other hand, ^{Eitel (2021}) evaluated the effect of salt stress on the production and quality of tomato seeds, in different concentrations, where it was observed that the germination index decreased significantly. For their part, ^{Ruiz et al. (2014}) evaluated the germination of eight genotypes of tomato (Solanum lycopersicum L.), which were exposed to different levels of NaCl. Where they observed that the fresh and dry biomass of the aerial part increased in the presence of NaCl. Similarly, ^{Can et al. (2017}) evaluated the germination and initial growth of beans (Phaseolus vulgaris) in response to salinity induced with NaCl, sodium sulfate (Na₂SO₄) and sodium bicarbonate (NaHCO₃), where they observed that fresh biomass was decreased with the salt type NaHCO₃, while NaCl increased it.

Although the impact of Si was not indicated as the main factor, there was a significant effect in the interaction with NaCl. The presence of Si in the germination rate indicates a quadratic effect in the presence of NaCl (Figure 1A). Similar results were found by ^{Haghighi et al. (2012}), who evaluated the interaction factor of Si + NaCl in the germination of tomato seeds, where they observed that Si stimulated germination variables, especially the germination rate, which increased around 20% with respect to the control treatment. Likewise, ^{Sun et al. (2021}) observed the effect of Si on corn (Zea mays) germination, which significantly increased germination percentage (GP), germination rate (GR), germination index (GI) and vigor index (VI) compared to the control treatment.

The presence of Si in saline conditions (50 mM) generated an increase in root length (Figure 1B). Similar results were reported by ^{Asgari et al. (2018}), who found a greater increase in stem length, fresh and dry biomass of shoots and roots in oat (Avena sativa) plants treated with Si.

However, ^{Gong et al. (2006}) evaluated the interaction of Si + NaCl in rice (Oryza sativa) seedlings. Where they observed that Si did not mitigate the adverse effects of NaCl. A positive effect of Si was observed in the development of the stem (Figure 1C), particularly seedlings in saline conditions (50 mM) increased stem length with the presence of Si. While, in the production of dry biomass (Figure 1D), a positive impact of Si was observed in the non-saline situation, showing a quadratic effect. ^{Haghighi and Pessarakli (2013}) evaluated the interaction factor of Si + NaCl in cherry tomato. They observed that Si mitigated the adverse effects of NaCl and increased stem length, fresh and dry biomass, photosynthetic rate and water content.

Figure 1 Effect of the interaction of sodium chloride (NaCl) and silicon (Si) on germination rate (A); root length (B); stem length (C); and dry biomass (D) of cherry tomato seedlings after 14 days of establishment of the experiment. The asterisks indicate the levels of significance among treatments 0.01^** - 0.05^*. 

^{Wang et al. (2015}) observed that Si promoted the increase in the length of stem, root, and fresh biomass of cucumber (Cucumis sativus) plants subjected to NaCl stress. On the other hand, ^{Carballo et al. (2019}) evaluated the interaction factor of Si + NaCl in the germination of moringa (Moringa oleifera). These results showed a positive effect of Si by increasing biomass, vigor index, total phenols, total flavonoids, and antioxidant capacity.

In the Floradade variety, it was observed that the concentration of 50 mM NaCl reduced the germination rate by 14%. Contrary to the above, the concentration of 25 mM NaCl increased the root length by 19%, while the concentration of 50 mM NaCl increased the dry biomass by 19%, both with respect to the control (Table 3).

In this sense, ^{Camejo and Torres (2000}) observed an increase in dry biomass in tomato plants under saline conditions. Plants, when exposed in a saline medium, activate their defense mechanism, and raise the levels of enzymes, prolines, amino acids, and antioxidants. The presence of Si, regardless of the concentration, increased germination rate and root length (Table 3).

The NaCl-Si interaction showed that the presence of Si increased root length and fresh biomass (Figure 2). ^{Khan et al. (2020}) evaluated thermotolerance induced by Si in tomato plants at doses of 1 and 2 mM. The results showed that Si significantly improved plant growth, fresh biomass, and root length, compared to the control treatment. Similarly, ^{Mushinskiy et al. (2018}) found that the application of Si increased the root length of tomato seedlings by 27.8% compared to the control treatment.

Figure 2 Effect of the interaction of sodium chloride (NaCl) and silicon (Si) on root length (A); and fresh biomass (B) of Floradade tomato seedlings after 14 days of establishment of the experiment. The asterisks (*) indicate significance levels among treatments 0.01^** - 0.05^*. 

In the present study, it was observed that the concentration of 50 mM NaCl significantly reduced the germination rate by 31% compared to the control treatment. Contrary to the above, it was observed that doses of 25 and 50 mM stimulated root length by 36.02 and 27.57% compared to the control treatment. In addition, stem length was also increased by 40% and 26% at these NaCl concentrations (Table 4).

Our results coincide with those obtained by ^{Loudari et al. (2020}), who observed the effect of NaCl on tomato, which increased the length and diameter of the root. On the other hand, ^{Batista et al. (2017}) evaluated the effect of NaCl on the germination of three varieties of basil (Ocimum basilicum L.). Where they observed a decrease in the percentage and rate of germination. Similarly, ^{González et al. (2020}) studied the effect of NaCl on the germination of seeds of tomato cv ‘Río Grande’. They observed that the germination rate decreased significantly compared to the control treatment.

The concentrations of 1 and 2 mM Si had a significant impact by increasing root and stem length, both concentrations were higher than the control treatment (Table 4). It has been shown that Si, despite not being an essential element for plants, favors the growth and development of some species. In this sense, ^{Chourasiya et al. (2021}) mention that the effect of Si on the germination and initial growth of wheat (Triticum) has a positive effect on the variables evaluated.

Si significantly increased fresh biomass (Figure 3). Particularly at the concentration of 50 mM NaCl, the presence of Si generated a positive linear effect. ^{Ahmad et al. (2019}) evaluated the interaction factor of Si + NaCl in bean seedlings, which improved the adverse effects of NaCl on growth parameters, fresh and dry biomass, pigment synthesis and relative water content.

Figure 3 Effect of the interaction of sodium chloride (NaCl) and silicon (Si) on the fresh biomass (A) of Río Grande tomato seedlings after 14 days of establishment of the experiment. The asterisks (*) indicate significance levels among treatments 0.01^** - 0.05^*. 

In addition, ^{Torabi et al. (2012}) reported that Si positively impacted on the growth of borage (Borago officinalis) seedlings, thus increasing their fresh and dry biomass. Therefore, Si, once inside the plant, is found in different structures, diffuses through the tissues, and forms a continuous layer between the cuticle and the epidermis composed of two sublayers, one as Si gel and another in a silicon-cellulose complex, which isolates and defends the plant from adverse environmental factors.

In this sense, ^{Emamverdian et al. (2018}) mentions that Si has the ability to activate the Lsi1, Lsi2 and Lsi6 genes in plants, the Lsi2 gene is considered an anion transporter and has the ability to express itself in the root endodermis, while Lsi1 and Lsi6 are genes that belong to the aquaporin family and their function is to transport silicon to the tissues of shoots and roots. In addition, when genes express themselves, multiple enzymes are activated, such as: catalases, peroxidases, superoxide dismutases, ascorbate peroxidases and glutathione reductases, which contribute with the plant to relieve stress (biotic or abiotic).

Conclusions

Tomato cultivars showed different effects regarding salinity. The cherry cultivar showed decreases in its germinative variables with increasing salinity, which indicates that the Floradade (beef) and Río Grande (saladette) cultivars had greater tolerance to salinity. Silicon (Si) had a protective effect in the interaction (NaCl + Si), showing positive effects by increasing the variables evaluated. Therefore, Si could be considered as a salinity mitigating agent.