Current negative effects of abiotic factors are one of the main world-wide problem in ensuring agricultural production and people nutrition. From the whole acreage of agricultural land in Slovakia even 28 %, i.e. 700 000 ha, is the land with acid soil reaction below pH 5.5. Our research was aimed at selection of alfalfa genotypes tolerant to soil acidity and increased concentration of free aluminium ions. Aluminium belongs to the main parts of the soil, where it is released from aluminium-silicates. In the soil, aluminium is present in the form of hydroxides with different stability. Stability of aluminium compounds is determined especially by environment acidity, i.e. that with increasing acidity also the content of free aluminium ions, which can react with other ions, is also increasing. High concentration of aluminium dissolved in the soil is the main factor limiting the yield of cultivated crops. In our experiments for evaluation of alfalfa genotype tolerance to low pH and higher concentration of aluminium ions, a modified method for testing the effect of irrigation on alfalfa plants was applied. In the stage of 5th leaflet we started to apply an acid rain watering with addition of aluminium salts and nutrients in the sand cultures on evaluated alfalfa genotypes. For artificial acid aluminium watering, the mixture of solution of sulphur acid (H₂SO₄) and nitrogen acid (HNO₃) with pH 4.0 (± 0.2) and with addition of aluminium chloride (AlCl₃.6H₂O) in 20 ppm concentration was used. This solution, together with nutritious KnoppŐ s nitrogen-free solution, was applied 10 times. On the basis of statistical evaluation by variance analysis, a highly significant effect of substrates in selected traits - root weight, plant weight, and number of rhizobium nodules per 1 plant was found out in evaluated experiments. From the aspect of varieties, highly significant interaction between the tested varieties and substrates was revealed in nodule number per plant. Varieties were characterized by a very high variability in nodule number per plant. Evaluating the plant height, the highest average plant height was found in the standard substrate (33.32 cm) and the lowest one in the silicic substrate (22.99 cm). Highly significant correlation between the evaluated traits within particular substrates was also found in the experiment. Strong correlation was between the root weight and plant weight in the sand substrate (0.77++), silicic substrate (0.80++), and soil substrate (0.80++). Biological material, which was tested in individual substrates, was also used for preselection of the plants tolerant to the given negative abiotic factors. Selected tolerant plants of individual varieties were used in the following year for evaluation in the field conditions with the aim of further utilization in hybridization programme and further assessment of the level of tolerance to low pH and Al³⁺ ions in alfalfa varieties and populations.

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