*3.1. Experimental Set-Up*

The experiment was conducted in a greenhouse located at the Department of Agricultural, Food, and Agro-Environmental Sciences (DISAAAa) of the University of Pisa (PI), characterized by a controlled temperature system (13–30 ◦C) and a ventilation air temperature of 27 ◦C. The seedlings of *Solanum lycopersicum* var. Kreos were provided in rock wool cubes (GRODAN, ROCKWOOL Group, 75 × 75 × 65 mm) supplied with a nutrient solution for about a week before being transferred to rock wool slabs (1 × 0.15 × 0.75 m).

Plants were grown in a hydroponic system with a density of 3 plants m−<sup>2</sup> . They were trimmed above the second truss. Pollination was performed by mechanical vibration of the inflorescences. The nutrient solution, characterized by E.C. of 2.6 mS cm−<sup>1</sup> and pH value of 5.6, was distributed through a drip irrigation system, three times a day in the early phenological phases of plant growth, and five times a day in the final phases. The composition of the nutrient solution was the following: 14 mM L−<sup>1</sup> N-NO3, 1 mM L−<sup>1</sup> N-NH4, 1 mM L−<sup>1</sup> P, 8 mM L−<sup>1</sup> K, 5 mM L−<sup>1</sup> Ca, 1.6 mM L−<sup>1</sup> Mg, 5.23 mM L−<sup>1</sup> Na, 1.71 mM L−<sup>1</sup> S-SO4, 3.43 mM L−<sup>1</sup> Cl, 15 µM L−<sup>1</sup> Fe, 20 µM L−<sup>1</sup> B, 1 mM L−<sup>1</sup> Cu, 5 µM L−<sup>1</sup> Zn, 10 µM L−<sup>1</sup> Mn, and 1 µM L−<sup>1</sup> Mo.

The experimental scheme was organized in randomized blocks. Selenium was sprayed as sodium selenate solution (Sigma Aldrich, colorless crystalline powder) at concentrations of 0.1 and 1.5 mg Se L−<sup>1</sup> . Each plant was treated with 250 mL of Se-enriched solution, whereas each control plant was sprayed with 250 mL of distilled water. When the solution was supplied, the surrounding plants were shielded with plastic sheets to avoid contamination. For each selenium dosage, half of the plants were treated at flowering (FL) (June), and the other half at the immature green (IG) fruit stage (July). Fruits were harvested for all the analyses at the red ripening stage.

After harvesting, the selenium content in the fruit of both trusses was analyzed. Composition, taste index, micro- and macro element concentrations, and VOC profiles of fruit were determined.

#### *3.2. Selenium and Mineral Content Analyses*

Total selenium and micro- and macro- element concentrations were determined in the fruit of Se-enriched and control plants. Collected fruit samples were oven-dried at 50 ◦C and then ground in a mortar. A total of 0.5 g of powder for each replicate was digested with nitric and perchloric acids. The mineral elements (Cu, Zn, Mn, Fe, K, Ca, Mg) were determined with a fast sequential atomic absorption spectrometer (AA240FS, Agilent). To calculate the selenium content, the digests were reduced by hydrochloric acid, following Zasoski and Burau [37], and the atomic absorption spectrometer was coupled to a hydride generator (Varian VGA 77) [38].
