*4.5. Selenium*

Selenium (Se) is an essential trace mineral, constituent of the selenoproteins responsible for important enzymatic functions. The function of selenoproteins in the human metabolism is most commonly connected to immunocompetence and cancer prevention, but it is known that Se functions go above that, as this mineral plays an important role in fertility and reproduction, brain functions, mood, thyroid health, and cardiovascular diseases [93]. The RDA of Se is 55 μg day−1, and the UL for adults is set at 400 μg day−<sup>1</sup> [69]. Selenium is not considered a micronutrient, but its appropriated use in plant nutrition can increase growth, stimulate seed germination, and contribute to protect several crops against pathogens and pests [94]. Soil concentration of Se is relatively low and it varies according to the type of rocks, being generally between 0.01 and 7 mg kg−1, with a worldwide average of 0.4 mg kg−<sup>1</sup> [95]. Plants take up Se inorganically both as selenite (SeO3 <sup>2</sup>−) and selenate (SeO4 <sup>2</sup>−) [96]. Plant absorption and transportation of Se are active processes [97]. Into the roots, due to its chemical similarity to sulfur (S), selenate moves through high-affinity sulphate transporters, while selenite movement is partially mediated by phosphate transporters [97,98]. Translocation of Se from root to the aerial parts of the plant is more likely to happen as selenate, since selenite is more prone to be accumulated in roots. Leaf surface can absorb volatile forms of Se from the atmosphere [99]. Foliar application of Se at late growth stages seems to optimize the uptake, translocation, and distribution of Se into the edible portions of plants, whereas selenate is more efficiently accumulated in plant tissues than selenite [100]. The tolerable Se content in most plant species is between 10 and 100 mg kg−<sup>1</sup> DW [101] and phytotoxic effects due to Se excess can compromise plant growth through damages to photosynthetic apparatus, photosynthesis inhibition, and over-production of starch [102]. In addition, secondary accumulators, also called Se-indicator, as some vegetables of the Asteraceae, Brassicaceae, and Fabaceae family, when supplied with exogenous Se can accumulate up to 1 g kg−<sup>1</sup> DW, being a good target for Se biofortification [101]. Skrypnik et al. [103] reported that Se biofortification of basil through foliar application of sodium selenite (Na2SeO3) at 10 μM (4 applications starting from the 7º day after transplanting) enhanced the Se concentration in leaves to up 10.74 mg kg−<sup>1</sup> DW (more than 700-times higher than untreated plants). Moreover, five applications of Na2SeO4 (0.633 mM), as foliar spray, from the six-leaf phase, resulted in lettuce leaves enriched with up to 40 mg Se kg−<sup>1</sup> DW, around 40 times greater than the control [71]. In another study, radish plants sprayed with 5 mg Se per plant 7 days before harvest, as sodium selenate, were able to produce roots with 346.5 mg kg−<sup>1</sup> DW of Se, meaning that the consumption of 1–10 radishes is enough to fulfill the daily human requirement [104]. Meanwhile, da Silva et al. [105] found that fertigation of radish plants could be more efficient than foliar spray, after treating plants with a low dose of Na2SeO4 (3.6 mg of Se per pot). They obtained roots with approximately 50 mg Se kg−<sup>1</sup> DW, while the leaf spray of the same chemical (0.36 mg of Se per pot, 93 mL per pot) resulted in plants with approximately 15 mg Se kg−<sup>1</sup> DW. Lettuce appears to be a good candidate for Se biofortification, as demonstrated by do Nascimento da Silva et al. [106]. In this experiment, plants submitted to fertigation at 25 μM Se L−<sup>1</sup> (as sodium selenate) resulted in lettuce leaves with as much as 39.4 mg Se kg−<sup>1</sup> DW, around 40 times greater than the control. While, higher application rates of both sodium selenate (Na2SeO4) and selenite (Na2SeO3) reached numbers that exceeded the RDA of Se. Similarly, tomato plants fertigated with 5 mg L−<sup>1</sup> of Se as sodium selenate, were enough to obtain a significant increase in Se concentration of fruits (35.8 mg kg−<sup>1</sup> DW), twice the concentration in the untreated plants. At the same time this dose allowed to achieve good physiological responses on plants, such as increased enzyme activity of catalase, glutathione peroxidase, and superoxide dismutase in fruits [107]. Selenium biofortification was successfully implemented in many vegetable

crops, using Na selenate or Na selenite. Besides, possible antioxidant and antisenescence effects of Se can improve shelf-life during postharvest storage [108]. However, because of the high toxicity of Se, especially in the form of selenate, attention must be made regarding agricultural workers and product safety.
