The effects of different concentrations of selenite (2-30 lM) and selenate (2-60 lM) on biomass production, leaf area, and concentrations of photosynthetic pigments in lettuce plants were investigated. On the basis of the obtained results, the threshold of toxicity for the selenite and selenate has been designated. The toxicity thresholds for selenite and selenate were determined at concentrations of 15 and 20 lM, respectively. Next, four selenium (Se) concentrations (2, 4, 6 or 15 lM), below or near the toxicity boundary, have been selected for the lettuce biofortification experiment. In the biofortified plants, the oxidant status (levels of lipid peroxidation and H 2 O 2 concentrations), as well as Se and sulphur (S) accumulation were analysed. In the edible parts of the lettuce, the Se concentration was higher for selenate presence compared to selenite; however, this difference was not as obvious as it was noted in the case of the roots, where selenite application caused the high accumulation of Se. An application of 15 lM Se as selenite caused a decline in the biomass and an intensification of prooxidative processes in the plant's tissues and as toxic should be excluded from further biofortification experiments. These results indicate that an application of either selenate or selenite to the nutrient solution at concentrations below 15 lM can be used for biofortification of lettuce with Se, evoking better plant growth and not inducing significant changes in the oxidant status, the concentration of assimilation pigments and S accumulation.

The study was conducted in order to determine the effects of exogenous selenium (Se) supply (5, 10, or 20 μM) on the resistance of cucumber (Cucumis sativus L.) cv. Polan F1 seedlings to salt stress (50 mM NaCl). Plant growth was negatively affected by excessive salinity and dry mass production as well as photosynthetic pigments accumulation severely decreased. Se treatments at 5 and 10 μM significantly improved the growth rate and increased the photosynthetic pigments and proline contents in cucumber leaves subjected to salt stress. Moreover, it is concluded that Se enhanced the salt tolerance of seedlings by protecting the cell membrane against lipid peroxidation. The growth-promoting effect of low Se concentrations (5 and 10 μM) under saline conditions could be due to the antioxidative activity of Se, increase in proline accumulation and/or decrease in content of chloride ions in the shoots tissues. Thus, optimal Se supplementation presents a promising potential for use in conditions of relatively high levels of NaCl in the medium.

No abstract

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 lM) on cucumber plants was investigated. The toxicity threshold for selenate and selenite was determined at the concentrations of 80 and 20 lM, respectively. In the Se-exposed plants, the growthpromoting effect was found at 6 lM of selenite and at 6-20 lM 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 lM), the accumulation of Se in shoots was similar in the presence of selenate and selenite. When Se concentration was [10 lM, the accumulation of Se in shoots was greater in selenate-exposed than seleniteexposed 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 lM may be potentially used for biofortification of cucumber with Se and changes in plant macronutrient contents are not expected under these conditions.

Exposure of sunflower and maize plants to increasing concentrations of lithium (0–50 mg Li dm−3) in a nutrient solution induced changes in biomass, leaf area and photosynthetic pigment accumulation, as well as levels of lipid peroxidation. The highest applied lithium dose (50 mg Li dm−3) evoked a significant reduction in the shoot biomass for both examined species, as well as necrotic spots and a reduction of the leaf area in sunflower plants. An enrichment of a nutrient solution with 5–50 mg Li dm−3 did not significantly affect chlorophylls a and b and the carotenoid content in sunflower plants. However, in maize, a significant decrease in all pigment content under highest used lithium concentration was noted. The levels of lipid peroxidation of the cell membranes in leaves of sunflower plants and the roots of maize increased significantly in the presence of 50 mg Li dm−3, which suggests disturbances of the membrane integrity and pro-oxidant properties of the excess lithium ions. Nonetheless, in maize, an increase of shoot biomass and leaf area in the presence of 5 mg Li dm−3 was found. An analysis of the metal content indicated that lithium accumulated significantly in sunflower and maize shoots in a dose-dependent manner, but differences occurred between species. The sunflower plants accumulated considerably greater amounts of this metal than maize. The potassium content in shoots remained unchanged under lithium treatments, except for a significant increase in the potassium levels for sunflower plants grown in the presence of 50 mg Li dm−3. These results suggest that lithium at 50 mg Li dm−3 is toxic to both plant species, but the symptoms of toxicity are species-specific. Moreover, the lithium influence on plants is dose-dependent and its ions can exert toxicity at high concentrations (50 mg Li dm−3) or stimulate growth at low concentrations (5 mg Li dm−3).