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The genome size of 265 plants and the GC content of 126 plants from 63 populations of the Cyanus triumfetti and Cyanus montanus groups, collected across the Carpathians, Pannonia, Bohemian Massif, and Western and Dinaric Alps were determined by PI and DAPI flow cytometry. Variation of the nuclear DNA content among homoploid species, and intraspecific and interpopulation variation were confirmed in simultaneous analyses. The 2 C-value at the diploid level (the C. triumfetti group) varied from 2.53 for Cyanus dominii subsp. sokolensis to 3.06 pg for C. triumfetti s.s. (1.21-fold range). At the tetraploid level (the C. montanus group), the 2 C-value varied from 5.19 for Cyanus mollis to 5.84 pg for C. montanus (1.13-fold range). High intraspecific and interpopulation variation in the amount of nuclear DNA in the C. triumfetti group correlates with the extensive morphological variation found in this group. Significant between-species differences in genome size indicate that this attribute may be used as a supportive taxonomic marker for both of the groups studied. The GC content varied by 2.93 %, from 39.46 % for ' Cyanus axillaris' to 40.61 % for Cyanus adscendens; this character is of no value for taxonomic purposes. Genome size of the studied populations is significantly higher in southern parts of the distribution area and at higher elevations. Plants with smaller genomes tend to occur in dry areas at low altitudes with high diurnal and annual temperature oscillations. The GC content of the populations studied is significantly correlated with longitude, increasing from east to west; and plants with GC-rich genomes are concentrated in the coldest areas with low minimum temperatures. [ABSTRACT FROM AUTHOR]