Effect of Foliar Application of Sodium Selenate on Mineral Relationships in Brassicaceae Crops

The relationships of selenium (Se) with other elements in plants is important for producing functional food with high Se contents and a predicted quality. To unveil the peculiarities of the element interactions, eight botanical varieties of <i>Brassica oleracea</i> L. were grown in similar conditions with or without foliar application of sodium selenate. High varietal differences, elicited by the Se supply, were recorded with regard to the accumulation of the elements examined, except for Mg, P and Si. Cabbage florets (broccoli and cauliflower) were characterized by both the lowest total mineral content and number of elements showing content changes under the Se supply (7–8 out of 25), whereas in Savoy cabbage, the highest number of minerals displayed content changes (13–14 from 25). The Se treatment did not significantly interfere with the high correlation coefficients recorded between Sr–Ca, Co–Ni and Zn–Mg (0.824–0.952). The selenium biofortification value varied from 12 to 138 depending on the species and was inversely correlated with the Si accumulation in the control plants (r = −0.872, <i>p</i> < 0.001). A significant decrease in the correlation coefficients occurred due to the Se supply regarding Zn with P and Co, Ca with Co and Li, Li and V, and Na and Sn, while the V–Pb relationship was significantly enhanced. Among the 25 elements studied, Cr demonstrated the highest number of significant correlation coefficient changes (with K, Na, P, Si, Zn, Cu, Co, I, As, Pb, and V). The results of this research prove the variability of the element interactions under foliar Se treatments in <i>Brassica oleracea</i> plants and reveal, for the first time, an inverse correlation between the Se biofortification level and Si content in untreated plants.