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.

A subarctic-boreal relict species, Salix lapponum, found within the area of the Łęczna-Włodawa Lakeland (Eastern Poland), is particularly threatened with extinction due to the southern limit of its range and the specific conditions of its occurrence. This conclusion is evidenced by a systematic reduction in the number of its locations, which is accompanied by a decrease in the numbers of individuals making up its populations. The aim of the present study was to determine habitat conditions of the occurrence of Salix lapponum populations on the basis of an analysis of physico-chemical factors of groundwater at the sites associated with various stages of succession taking place in small peatland water bodies. The results of this study confirm the trend that the stands and population numbers of this species are decreasing with the intensification of ecological succession and the degree of its progress and at the same time show that this species exhibits a wide amplitude of many of the investigated factors. Among the groundwater parameters under investigation, the low level of total nitrogen, phosphorus fraction and DOC as well as the high level of Ca, pH and electrolytic conductivity can be considered to be a set of conditions that promote the maintenance of Salix lapponum populations. On the other hand, in the case of nitrates, nitrites, sulphates as well as the Na, K and Mg ions, the Kruskal-Wallis statistical test showed that the distribution of their values was invariant in relation to the studied habitats, which is probably evidence that they do not determine the development or extinction of the investigated populations in these habitats.

A b s t r a c tThe aim of this study was to make an evaluation of the current stands of a relict species, Salix lapponum, within the area of the Polesie National Park, to determine the population size and condition of downy willow as well as to analyse biocenotic conditions in its stands.The study, conducted in the years 2001-2003, allowed to establish that the number of S. lapponum stands had decreased significantly and that its population size in the confirmed stands in the Polesie National Park was substantially lower than 50 years ago.Salix lapponum was a component of different phytocoenoses, but its highest population size and a high degree of development were noted at the sites with the Betulo-Salicetum repentis and Thelypteridi-Phragmitetum communities. The character of the phytocoenoses in which Salix lapponum occurred in greatest numbers may evidence the wide range of tolerance of downy willow with respect to habitat conditions.There is a probability that not only the changes in its habitat conditions, which were noted over the last half-century, had a significant effect on the reduction in the number of stands and in the population size of this species. An essential problem appears to be the fact of the absence of flowering in individuals in many of the studied populations, which may result in an insufficient degree of adaptation of S. lapponum to changing habitat conditions

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A b s t r a c tThis research, carried out in the years 2011-2013, aimed to evaluate Salix lapponum stands in the peat bogs of Polesie Lubelskie Region as well as to determine the condition of the population and the changes that have taken place since the 1950's. An inventory carried out in 25 stands of S. lapponum known from the literature shows that the number of its stands has decreased by 80% in Polesie Lubelskie Region. In all the confirmed locations, a decrease in population numbers was also found in relation to the data known from the literature since the 1950's. In the majority of the population locations that were considered to be extinct, there were no significant changes in habitat conditions, and ecological succession and changes in hydrological conditions could have been the cause of habitat changes only at a few sites. In the light of the study, the preservation of the S. lapponum population in Polesie Lubelskie seems to be impossible if appropriate active conservation measures are not taken immediately. Because area-based conservation, which covers most of the habitats of the studied species, does not bring the expected results, the possibility of ex situ conservation and enlargement of the populations existing in the natural environment in peat bog ecosystems in Polesie should be explored.