Establishment of the experiment and environmental conditions

The experiment was conducted in the experimental field of the Institute of Agricultural Sciences of the Autonomous University of Baja California located in the common Nuevo Leon, Mexicali in the period from March to June 2017. A rooftop greenhouse with a polycarbonate roof was used. Low technology, no temperature control and no heating. The Amalia tomato saladette cultivar was used. The transplant was performed on March 31 using two plants per 9 L pots. The substrate used was stream sand with granulometry less than 0.5 cm. The application of the nutrient solution was through pressurized irrigation spaghetti type with an expense per dropper of 100 mL min^-1. There were between one and three irrigations per day. The irrigation drainage was adjusted to 30%.

Experimental design

The experimental design used was divided plots randomly distributed with five repetitions. The main plot was two salinity conditions at a value of 4 dS m^-1 (SNS and saline solution induced with NaCl) and the subplots were doses of Si (0, 4, 8 g L^-1 of SiO₂), for a total of twelve treatments As a source of Si, 94% silicon dioxide was used (Diatomix, Bio Agrinor^®, Zapopan, Jalisco Mexico). Each main plot consisted of 12 plants, while the subplot was 4 plants.

Treatment Management

The SNS used was the one recommended by ^{Gomez and Sanchez (2003)} and the following nutrient concentrations N 224, P 47, K 281, Ca 212, Mg 65, Fe 2, Mn 0.55, Zn 0.33, Cu 0.05, were handled. B 0.28 and Mo 0.05 mg L^-1. At the time of transplantation, all SNSs were sufficiently diluted in water to reach an electrical conductivity of 2 dS m^-1.

After 15 days after the transplant, the application of the treatments began. For factor one, in the plots of the SNS treatment, a gradual increase was made for two days in the concentration of the macro and micronutrients until a salinity value of 4 dS m^-1 was reached. The same happened in the salinity treatment with NaCl. The increase in salinity of the solution was achieved by adding in addition to the SNS to an electrical conductivity of 2.0 dS m^-1, the amount of ≈2.6 g L^-1 of NaCl and reaching the value of 4 dS m^-1. On the other hand, the doses of Si were applied daily to each pot manually at 12:00 noon.

The pH of the nutrient solution was maintained between 5.5-6 units. During the time the experiment lasted, the relative humidity of the greenhouse was achieved by wetting the floor with running water at 10:00 and 14:00 h. The maximum temperature inside the greenhouse at 15 hours a day reached 45 °C and the relative humidity at that same time was 14.5% and were monitored using a CEM DT-172 digital thermohygrometer (Twin Light Instruments, Monterrey, Mexico) placed at 20 cm just above the plants.

Variables evaluated

The physiological variables evaluated were leaf temperature, photochemical efficiency and stomatal conductance. The measurements were made at 34 days after the transplant at 9:00, 11:00, 13:00, 15:00 and 17:00 a day. For all variables, the fourth or fifth leaf counted from the apex down was used as a reference. All measurements were made on three plants per previously labeled treatment.

Leaf temperature

The leaf temperature was determined with an infrared digital laser gun thermometer (DAN-tronics model P045440, Mexico). The measurements were made at a distance of 20 cm between the sensor and the leaf.

Photochemical efficiency

The maximum photochemical efficiency of PSII (Fv/Fm), minimum fluorescence (Fo) and maximum fluorescence (Fm) was determined with a portable fluorometer (Chlorophyll Fluorometer OS-30p, OPTI-SCIENCE, USA) using an actinic light intensity of 2 100 µmol photon m^-2 s^-1 intensity for a period of 2 s. Prior to the determination, the leaves were acclimatized for 30 min of darkness with light exclusion clamps to ensure that all reaction centers were open (^{González et al., 2008}).

Stomatic conductance (g_s)

Stomatal conductance was determined with a leaf porometer model SC-1 Decagon Devices, Inc. (SC-1 Leaf Porometer^®, Pullman WA, USA).

Statistical analysis

The data obtained were analyzed using the statistical program Minitab 17. An analysis of variance and comparison of means was performed using the Tukey test (p≤ 0.05).

Leaf temperature

A response in the temperature of the leaves of the tomato crop was not identified by the Si × S interaction (p> 0.05) (Table 1). Likewise, the addition of Si also showed no significant effect on leaf temperature in all treatments (p> 0.05). However, salinity treatments did show response to the increase in leaf temperature (p> 0.05). At 9:00 am, the application of NaCl, increased the leaf temperature by more than 2 °C than plants treated with SNS-induced salinity, while at 1:00 p.m. and 5:00 p.m. was inverted.

In this sense, the plants subjected to SNS-induced salinity had the ability to stay cooler in the morning due to greater water availability or better perspiration (^{Shahenshah and Isoda, 2010}); however, in the afternoon the plants with NaCl remained fresher than those salinized with SNS. The above happens when the crop is subjected to water stress due to lack of water in the substrate or a poor root system (^{Sánchez-Blanco et al., 2014}). In either case, in this study salinity restricted stomatal conductance and consequently foliar temperature increased (^{Ben-Asher et al., 2006}).

Maximum photochemical efficiency of PSII (Fv/Fm)

Table 2 shows the results of maximum photochemical efficiency of PSII (Fv/Fm) due to the application of Si and two forms of salinity. The Si × S interaction was significant between 11:00 am and 3:00 pm (p≤ 0.004). At that same time, the application of Si had a significant effect on the Fv/Fm of the leaves of tomato plants (p≤ 0.005). Throughout the day, the salinity caused by the SNS significantly affected the Fv/Fm (p≤ 0.001) maintaining higher values than with the salinity caused by NaCl.

The reduction in the Fv/Fm values in the plants subjected to salinity with NaCl, would indicate a reduction in the photochemical efficiency of the PSII and a disturbance or damage to the photosynthetic apparatus caused by the specific salinity of the NaCl and not by that caused by the SNS (^{Jimenez-Suancha et al., 2015}). The interactive Si × S effect at 11, 13 and 15:00 h, was characterized by high Fv/Fm values when using the higher dose of Si (Figure 1). At 11:00 h, no difference was found between the type of salinity when Si was not applied. However, as used, if the difference in Fv/Fm was smaller in salinity induced by NaCl. At 13:00 the same effect was repeated, but only with the highest dose of Si [(8 g) (Figure 1b)]. Likewise, at 15:00 (Figure 1c) the effect of Si at doses of 4 and 8 g was the same as during the morning of the day.

Figure 1 Silicon versus salinity (Si × S) interactive effect on the maximum photochemical efficiency of tomato cultivation during the day [a) 11:00 h; b) 13:00 h; and c) 15:00 h]. 

In general, lower Fv/Fm values were found at higher doses of Si, especially in the treatment of salinity induced by NaCl. These results agree with those reported by ^{Cao et al. (2015)}, who, when applying Si to tomato seedlings subjected to saline stress, found that as time progressed, the Fv/Fm values were lower than those obtained in plants without the addition of Si.

Minimum fluorescence (Fo)

Table 3 shows the effect of the application of Si and the type of S on the Fo in tomato crop leaves. A significant effect was found in the interaction between Si × S for most of the day (p˂ 0.001). The highest dose of Si (8 g) applied to the NaCl solution was what made the difference from the rest of the treatments (Figure 2). Possibly the combination of the high dose and the NaCl caused that reaction. High values of Fo mean damage to the PSII reaction center or a reduction in the ability to transfer the excitation energy from the antenna to the reaction center (^{Baker, 2008}; ^{Khan et al., 2016}). Contradictory data are those reported by ^{Maghsoudi et al. (2015)} who studied the Fo on the application of Si in wheat under water stress. They explain that the stress caused by the lack of water decreased Fo values.

Figure 2 Interactive effect silicon versus salinity (Si × S) in the minimum fluorescence efficiency of chlorophyll in tomato cultivation during the day [a) 9:00 h; b) 11:00 h; c) 13:00 h; and d) 17:00 h]. 

Maximum fluorescence (Fm)

Significant differences were observed in the Si factor and in the Si × S interaction, over the Fm at 9:00 and 15:00 h (p˂ 0.05) (Table 4). Additionally, the salinity caused by the addition of NaCl increased the Fm reflex values in the reduction of ‘closure’ of the PSII reaction centers (^{González et al., 2008}), which in turn implies a greater activation of the reactions photochemicals (^{Jiménez-Suancha et al., 2015}). This reinforces the hypothesis that the salinity damage presented in crops is the result of the use of NaCl rather than a balanced nutrient solution (SNS).

Stomatic conductance (g_s)

Table 5 shows the Si × S interaction at g_s (p ˂ 0.037) at the end of the day. Figure 3c indicates that when NaCl was applied without Si, the g_s was increased while the salinity produced by the SNS kept low g_s values even when Si was applied. On the other hand, Si significantly decreased GS at 9:00 h. While the salinity caused by the addition of NaCl, it increased at 9:00 and 17:00 in about 120 mmol m^-2 s^-1. Contrary to this study, ^{Savvas et al. (2009)} found that fertilization with Si promoted an increase in g_s in zucchini plants (Cucurbita pepo L. cv. ‘Rival’) when they were subjected to a salinity of 6.2 dS m^-1, with NaCl. On the other hand, ^{Romero-Aranda et al. (2006)} found no differences in the g_s of tomato seedlings when they applied different combinations of NaCl and Si in Hoagland nutrient solutions to the Si application. However, when plants grown under salinity (NaCl) were treated with Si, the water content in them increased by 40%.

Figure 3 Silicon versus salinity (Si × S) interactive effect on the maximum chlorophyll fluorescence (Fm). a): 9:00 h; b) 15:00 h] and stomatal conductance; and c) 17:00 h] in the tomato crop. 

The present investigation showed the response of the plant to two types of salinity and the application of Si. The use of SNS-induced salinity resulted in an increase in Fv/Fm expressed as light absorbed by PSII only early and late in the day. This behavior was the result of high values in Fo and low values in Fm caused by NaCl. ^{Zribi et al. (2008)} found similar results to those of this work. They mention that there is a relationship between the use of NaCl and the concentration of Na⁺ in the leaf tissue of tomato plants. The presence of Na⁺ in the leaf tissue has a negative impact on the amount of electron transport, as well as the photochemical efficiency of PSII.

The behavior of the g_s had the same response to the type of salinity as the Fo. ^{Romero-Aranda et al. (2006)} presented similar values to those found in this study, but without deference for salinity or the addition of Si. In subsequent studies, the role of NaCl and the increase in g_s should be studied in more detail, because the results in this study do not allow obtaining a clear role response of this type of salt. The same should happen with the foliar temperature response to this type of salinity.

Si failed to positively affect leaf temperature and most of the day negatively affected Fv/Fm. The interactive effect of Si and the type of salt made it clear that the dose of 8 g pot^-1 day^-1 impaired Fv/Fm instead of helping it (Figure 1). Something similar happened with the variable Fo. Which was increased by using the combination of Si at a dose of 8g and the salinity induced by NaCl. The opposite case when measuring the Fm (Figure 3a and 3b). More studies will be necessary but using lower doses than those proposed in this experiment, in addition to considering a longer period of time than the one studied. This could make it possible to find more conclusive results.