Difference between Selenite and Selenate in the Regulation of Growth and Physiological Parameters of Nickel-Exposed Lettuce

SIMPLE SUMMARY: Nickel is a trace metal that can cause toxicity in plants. In contrast, selenium, as a beneficial element, can have a favorable effect on plants grown under metal stress. However, the biological activity of selenium largely depends on its chemical form. Therefore, we assessed the effect of two chemical forms of Se (selenite and selenate) on the toxicity and accumulation of Ni in lettuce. We found that Ni phytotoxicity severely increased in the presence of higher doses of selenite. On the other hand, a low dose of selenate stimulated the growth of roots of Ni-exposed plants. We found that selenium can modify Ni activity in lettuce, but this depends not only on the concentration but also on the form of selenium. The strong intensification of Ni toxicity in the presence of selenite was most likely related to the very high selenium accumulation in root tissues. The obtained results indicate that depending on the conditions in the root environment, Se in a given concentration and chemical form can be either beneficial or toxic for plants. Therefore, the use of selenium in metal-stressed plants requires consideration of not only the concentration but also the chemical form of this element. ABSTRACT: Nickel is an essential plant micronutrient; however, even at low concentrations, it may be phytotoxic. Selenium is a beneficial element with an alleviating effect that has been confirmed in the case of many abiotic stresses, including metal toxicity. The aim of this study is to assess the effect of two forms of Se (Se(IV) or Se(VI)) on the phytotoxicity, accumulation, and translocation of Ni in lettuce. Nickel causes a reduction in lettuce growth and vitality of roots, probably through increased lipid peroxidation. The application of Se(IV) to a Ni-contaminated medium resulted in a further reduction of growth, especially in the presence of 6 µM Se(IV). The growth-promoting effect of Se was found only in the 2 µM Se(VI)/10 µM Ni treatment. The application of 6 µM Se, regardless of the Se form, to the Ni-containing substrate caused an increase in shoot Ni concentration. In turn, a decrease in root Ni content was found for all Se treatments. The strong aggravation of Ni phytotoxicity in the presence of 6 µM Se(IV) was most likely related to the accumulation of high Se concentration in the roots, and the combination of high root Ni accumulation caused irreversible dysregulation of cell metabolism.