Foliar spray with selenium salts can be used to fortify tomatoes, but the results vary in relation to the Se concentration and the plant developmental stage. The effects of foliar spraying with sodium selenate at concentrations of 0, 1, and 1.5 mg Se L⁻¹ at flowering and fruit immature green stage on Se accumulation and quality traits of tomatoes at ripening were investigated. Selenium accumulated up to 0.95 µg 100 g FW⁻¹, with no significant difference between the two concentrations used in fruit of the first truss. The treatment performed at the flowering stage resulted in a higher selenium concentration compared to the immature green treatment in the fruit of the second truss. Cu, Zn, K, and Ca content was slightly modified by Se application, with no decrease in fruit quality. When applied at the immature green stage, Se reduced the incidence of blossom-end rot. A group of volatile organic compounds (2-phenylethyl alcohol, guaiacol, (E)-2-heptenal, 1-penten-3-one and (E)-2-pentenal), positively correlated with consumer liking and flavor intensity, increased following Se treatment. These findings indicate that foliar spraying, particularly if performed at flowering stage, is an efficient method to enrich tomatoes with Se, also resulting in positive changes in fruit aroma profile.

In the present work, the effects of enriching tomatoes with selenium were studied in terms of physiological, metabolic, and molecular processes in the last stages of fruit development, particularly during ripening. A selenium concentration of 10 mg L^-1 with sodium selenate and selenium nanoparticles was used in the spray treatments on the whole plants. No significant effects of selenium enrichment were detected in terms of ethylene production or color changes in the ripening fruit. However, selenium enrichment had an influence on both the primary and secondary metabolic processes and thus the biochemical composition of ripe tomatoes. Selenium decreased the amount of β-carotene, increased the accumulation of naringenin and chlorogenic acid, and decreased the coumaric acid level. Selenium also affected the volatile organic compound profile, with changes in the level of specific apocarotenoid compounds, such as β-ionone. These metabolomic changes may, to some extent, be due to the impact of selenium treatment on the transcription of genes involved in the metabolism of these compounds. RNA-seq analysis showed that the selenium application mostly impacted the expression of the genes involved in hormonal signaling, secondary metabolism, flavonoid biosynthesis, and glycosaminoglycan degradation.

At appropriate concentrations, selenium (Se) is beneficial for humans. Tomato appears to be one of the best commodities for producing Se-biofortified fruit for dietary supplementation. To assess the efficacy of different enrichment protocols, a total of four on-plant and off-plant trials were conducted. Hydroponically grown tomato plants were sprayed with: (i) chemically synthesized Se nanoparticles (SeNPs) at 0, 1, and 1.5 mg Se L⁻¹ at blooming; (ii) sodium selenate (Na₂SeO₄) or SeNPs solution at 0, 5, and 10 mg Se L⁻¹ when the fruit entered the immature green stage. With regard to the off-plant trials, harvested mature green fruit were immersed in Na₂SeO₄ solution: (iii) at 0, 5, 10, and 20 mg Se L⁻¹ for 15 s under a vacuum; (iv) at 0, 40, and 80 mg Se L⁻¹ for 1 h. Spraying Na₂SeO₄ induced higher Se accumulation in plant tissue than SeNPs: both protocols were effective in enriching tomatoes. Postharvest Se enrichment via vacuum infiltration caused textural damage, whereas passive immersion in solution induced fruit Se accumulation without causing any damage. SeNPs appear to be quantitatively less effective than Na₂SeO₄, but might be environmentally safer. Elemental Se carried by NPs may be more easily incorporated into organic forms, which are more bioavailable for humans. Passive immersion may represent an alternative Se-enrichment strategy, allowing for the biofortification of harvested tomato fruit directly, with lower risks of environmental pollution.