Comparative Evaluation of the Effect of Foliar Nano-Se/Si Supply on the Yield and Nutritional Quality of Iceberg and Romaine Lettuce Grown in Hydroponics

Hydroponic production of vegetables is beneficial for plant yield and quality, especially when utilizing growth stimulators. To evaluate the effect of Se/Si nanoparticle application, two contrasting lettuce types of relatively low (Iceberg) and high (Romaine) nutritional density were foliar-treated with nanoparticles of Se (25 mg L−1) and Si (14 mg L−1), singly or jointly. Only single nano-Se/Si supply showed a beneficial effect on lettuce yield, photosynthetic pigments, and antioxidant parameters. Compared to Romaine, Iceberg displayed a lower yield increase under nano-Se/Si supply, despite the higher photosynthetic pigment rise, while Romaine lettuce was characterized by a significantly higher augmentation of ascorbic acid, polyphenols, and total antioxidant activity (AOA). Nano-Se/Si halved the chlorophyll a/b ratio in Iceberg, with no effect on Romaine lettuce. Iceberg showed more evident mineral profile changes under all Se/Si treatments, with a significant Mn increase and Fe decrease. In both lettuce types, single nano-Se supply reduced Cu accumulation by more than twice, while joint Se/Si treatment significantly diminished both Cu and Zn. Nano-Si supplementation had contrasting effects on Cu, Zn, and Mn accumulation, increasing the assimilation capacity of minerals in Iceberg and decreasing Cu and Mn levels in Romaine, with no significant effect on Zn. Under single Se supply, both lettuce types provided 920–1030 µg Se kg−1 d.w., compared to lower values (611–720 µg kg−1 d.w.) associated with joint Se/Si treatment. The results of this research confirm the existence of significant genetic peculiarities in plant adaptation connected with photosynthetic pigment accumulation and mineral profile changes in the Iceberg genotype and with antioxidant parameters in Romaine lettuce.