Yield

The weight of the fruits of each plant was harvested using an Ohaus^® portable digital scale and was obtained with an approximation of 0.01 g. The data obtained are reported in kg plant^-1.

Equatorial diameter of the fruit

The longitudinal and transverse diameters were obtained from the fruits, for this a Truper model 14388 digital vernier was used and they were expressed in millimeters (mm).

Soluble solids content

The soluble solids content of bell pepper fruits was determined with an Atago refractometer (Master 2311). Four fruits were taken from each treatment and repetition. The fruits were cut to extract a drop and place it on the lens of the refractometer, this was repeated for each fruit, the results were expressed in °Brix.

Sample preparation

Fresh fruit samples were washed with distilled water for 2 min to remove residues and lyophilized for 10 d. The dried material was then manually crushed (in mortar) and stored in Eppendorf plastic tubes at -18 °C until extracts were obtained.

Phenolic compound extracts

A 100 mg dry sample was mixed with 5 mL of methanol in a plastic screw cap tube. This was placed on a shaker (ATR Inc., USA) for 24 h (20 rpm) at 5 °C. The tubes were centrifuged at 30 000 x g for 5 min and the supernatant was removed for analytical tests.

Determination of total phenols

Total phenolic content was measured using a modification of the Folin-Ciocalteu method (^{Singleton et al., 1999}). 30 μl of extract was mixed with 270 μl of distilled water in a test tube, to then add 1.5 ml of Folin-Ciocalteu reagent (Sigma-Aldrich, St Louis MO, USA) diluted (1:15) and vortexed for 10 sec. After 5 min, 1.2 ml of sodium carbonate (7.5% w/v) was added and the mixture was stirred for 10 s.

The solution was placed in a 45 °C water bath for 15 min, and then allowed to cool to room temperature. The absorbance of the solution was read at 765 nm on a UV spectrophotometer (Genesys 10). To calculate the phenolic content, a calibration curve was performed using gallic acid as standard, and the results were recorded in mg of gallic acid equivalent per 100 g on a dry weight basis (mg AGE/100 g PS).

Antioxidant capacity

The equivalent antioxidant capacity in Trolox was evaluated according to the in vitro ABTS⁺ method published by ^{Esparza-Rivera et al. (2006)}. An ABTS⁺ solution was prepared with 40 mg of ABTS (Aldrich, St. Louis, Missouri, USA) and 1.5 g of manganese dioxide (Fermont, Nuevo León, Mexico) in 15 ml of distilled water. The mixture was vigorously stirred and allowed to stand covered with aluminum foil for 20 minutes. The solution was then filtered on Whatman 40 paper (GE Healthcare UK Limited, Little Chalfont, Buckinghamshire, UK) and the absorbance was adjusted to 0.7 ±0.01 at a wavelength of 734 nm using 5 mM phosphate buffer solution.

For the determination of antioxidant capacity, 100 µl of sample and 1 ml of ABTS⁺ solution were mixed, and after 60 and 90 seconds of reaction the absorbance of the sample was read at 734 nm. A standard curve was prepared with Trolox (Aldrich, St. Louis, Missouri, USA) and the results were reported as equivalent antioxidant capacity in equivalent mM in Trolox per g dry base (mM equiv Trolox/g BS). The analyzes were carried out in triplicate.

Statistical analysis

The data obtained from the results were subjected to an analysis of variance and the simultaneous Tukey test p< 0.05 was used for the separation of means.

Yield

The yield decreases with an EC greater than 2 dS m^-1, the lower yield caused by the increase in the EC of the NS, is due to the decrease in the size and weight of the fruits (Figure 1a), similar results are reported by (^{Fallik et al., 2019}), when indicating that the yield of bell pepper decreases as salinity increases, due to the fact that the pepper is a crop sensitive to salinity (^{Ben-Gal et al., 2008}), with a higher EC at 3 dS m^-1, they exceed the nutrient requirements of the crop (^{Sonneveld and Van der Burg, 1991}); however, not all cultivars respond in a similar way (^{Aktas et al., 2006}).

The reduction in yield due to the smaller size of the fruits is attributable to the fact that they present a greater difficulty in absorbing water, caused by a high concentration of ions in the rhizosphere, thus affecting the expansion of the fruits, in addition to the above as well some nutrients can be reduced, mainly those that are absorbed by mass flow, which can cause problems with blossom end rot (^{Terraza et al., 2008}). It is also possible that the decrease in yield is caused by low photosynthesis, which decreases the availability of CO₂ as a result of diffusion limitations (^{Flexas et al., 2007}) and by a decrease in the conductance of CO₂ in the stomata and mesophyll (^{Ashraf and Harris, 2013}).

Total soluble solids

The soluble solids in pepper fruits increased as the EC increased in the NS (Figure 1c). Similar results were found by ^{Navarro-López et al. (2012)}, finding that an EC of 3 and 4 dS m^-1 significantly increased TSS in pepper fruits. The greater accumulation of soluble solids in the fruit is due to the fact that when there is a high EC in the NS, there is a reduction in the flow of water towards the fruit and an increase in the hydrolysis of sucrose, which would produce fructose and glucose, in response to the high osmotic potential in the nutrient solution (^{Fallik et al., 2019}), which causes an active accumulation of solutes in the fruits as simple sugars (glucose, fructose and sucrose), thereby reducing the osmotic potential , thus facilitating the absorption of water in the fruits (^{Goykovic-Cortes and Saavedra-del Real, 2007}).

Figure 1 Effect of the electrical conductivity of the nutritive solution, on the yield, total soluble solids, phenolic compounds and antioxidant activity of bell pepper fruits. 

Phenolic compounds and antioxidant activity

The consumption of foods rich in bioactive compounds is associated with a lower risk of cancer diseases and prevention of many diseases, hence the importance of increasing the biosynthesis of these compounds in the fruits before harvest and their subsequent consumption. The synthesis and accumulation of bioactive compounds in plants is generally stimulated by biotic or abiotic stress such as salinity, these compounds have the function of protecting lipids, proteins and nucleic acids from serious oxidative damage (^{Navarro et al., 2006}).

As the EC in the NS increases, the content of phenolic compounds (Figure 1b) and the antioxidant activity (Figure 1d) in pepper fruits increases. The total phenol content and total antioxidants increased an average of 46 and 42%, respectively, between the lowest and highest EC. This behavior is caused by the high EC of the NS that induces a saline stress and therefore the plants will produce a higher content of antioxidant substances for their defense against oxidative stress (^{Navarro et al., 2006}; ^{Carmine et al., 2017}).

The increase in bioactive compounds in fruits is desirable because antioxidant compounds protect human cells from free radicals, which cause damage to cells and increase the risk of developing cancer, cardiovascular diseases and other degenerative disorders (^{Carranco et al., 2011}).