The dual effects of two inorganic selenium forms on the growth, selected physiological parameters and macronutrients accumulation in cucumber plants

The boundary between beneficial and phytotoxic levels of selenium (Se) is narrow, and both induce alteration in plant growth and their physiology. In this study, the influence of two Se forms (selenite or selenate) with different concentrations (2–80 µM) on cucumber plants was investigated. The toxicity threshold for selenate and selenite was determined at the concentrations of 80 and 20 µM, respectively. In the Se-exposed plants, the growth-promoting effect was found at 6 µM of selenite and at 6–20 µM of selenate. The root activity considerably increased with increasing selenite concentrations suggesting the upregulation of mitochondrial dehydrogenases activity. Selenite treatment also impaired photosynthetic pigments accumulation and chlorophyll fluorescence parameters. Moreover, Se exerted a dual effect on lipid peroxidation in roots: at low concentrations it inhibited this process, whereas at high concentrations it enhanced the accumulation of harmful lipid peroxides. Under low Se concentrations (10 µM, the accumulation of Se in shoots was greater in selenate-exposed than selenite-exposed plants. However, in the roots the Se concentrations were always higher after selenite exposure comparing to selenate. The N level in plants was generally maintained constant, while the remaining macronutrients (especially K, P, and S) concentrations were significantly changed depending on the form and concentrations of Se. These results imply that an application of either selenate or selenite at concentrations <10 µM may be potentially used for biofortification of cucumber with Se and changes in plant macronutrient contents are not expected under these conditions.