## 3 Results
### Genome Size, Endopolypolidy and Chromosome Number at Different Stages of Plant Culture  
Plants used for in vitro culture were tetraploids (4x) and possessed 2C = 0.829 [\(\pm\)0.028] pg DNA. Similarly, the calli obtained from these plants contained 0.823 [\(\pm\)0.029] pg/2C DNA, which confirmed that the nuclei of the lowest ploidy present in the calli were tetraploid. Nevertheless, at the stage of adventitious shoots after 4-6 weeks of cultivation of the calli on 1 mg L\({}^{-1}\) TDZ, besides tetraploids, plant material with double amount of DNA (about 1.6-1.7 pg/2C; representative for octaploids, 8_x_) was also detected; octapolids also occurred among regenerated acclimatized (both originating from hydroponic and from soil cultures) plants. The frequency of octapolids was high in all examined stages of regeneration (54.2-78%) (Table 2). The mean genome size of 90 octaploid regenerants was  
Figure 1: Successive stages of obtaining _Arabidopsis arenasa_ regenerants. Fragments of seedlings laid out on the callus induction medium (**A**), callus tissue derived from the hypocotyl with cotyledons on the \(\frac{1}{2}\) MS medium supplemented with 2 mg L\({}^{-1}\) 2,4-D and 2 mg L\({}^{-1}\) BAP (**B**), adventitious shoots on the \(\frac{1}{2}\) MS medium supplemented with 1 mg L\({}^{-1}\) TDZ (**C**), rooted plant on the \(\frac{1}{2}\) MS after 10 weeks on a rooting medium (**D**), regenerated plant on \(\frac{1}{2}\) MS ready for transfer to hydroponics (**E**), flowering plants maintained in a hydroponic culture (**F**), and acclimatized to outdoor conditions (**G**). 2,4-D—2,4-Dichlorophenoxyacetic acid; BAP—6-Benzylaminopurine; MS—Murashige and Skoog medium; TDZ—Thidiazuron. 2C = 1.642 [\(\pm\)0.042] pg. Moreover, among the rooted plants after 5-12 weeks of culturing, seven plants possessed DNA content characteristic for 16\(\alpha\) (2C = 3.265 [\(\pm\)0.064] pg) (Table 2). In all analyzed plant materials, nuclei with 2C, 4C, 8C, 16C, and 32C were detected; however, they differed in endopolyploidy level; the highest (\(\Sigma\) > 2C)/2C ratio was observed in 10-15-week-old callus on \(\frac{1}{2}\) MS supplemented with 2 mg L\({}^{-1}\) 2,4-D + 2 mg L\({}^{-1}\) BAP (over 2.5; Figure 3B). The lowest endopolyploidy occurred in young leaves of the seedlings (the (\(\Sigma\) > 2C)/2C ratio almost 0.3); most nuclei (almost 80%) contained 2C DNA, and endopolyploid nuclei (> 4C) constituted only slightly over 4% (Figure 3A). In calli, regardless of the medium/treatment, the frequency of 2C nuclei essentially decreased, and this of endopolyploid nuclei increased to about 16% and 13%, in 10-15-week-old callus and callus treated for 3 weeks with 1 mg L\({}^{-1}\) TDZ, respectively (Figure 3B,C). It must be noted, however, that the calli differed greatly in endopolyploidy level (see high standard deviation). The chromosome number of the initial 4\(x\) plants was 2\(n\) = 32 [20], and 8\(x\) progeny was 2\(n\) = 64 (Figure 4A). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Plant Material** & **4x (\%)** & **8x (\%)** & **16x (\%)** \\ \hline Initial plants & 13 (100) & 0 (0) & 0 (0) \\ \hline Callus obtained on \(\frac{1}{2}\) MS supplemented with 2 mg L\({}^{-1}\) & & & \\
2,4-D + 2 mg L\({}^{-1}\) BAP (8–16 weeks after induction) * & 15 (100) & 0 (0) & 0 (0) \\ \hline Advertitious shoots after 4–6 weeks on \(\frac{1}{2}\) MS & & & \\ supplemented with 1 mg L\({}^{-1}\) TDZ & & & \\ \hline Rooted plants after 5–12 weeks on rooting \(\frac{1}{2}\) MS medium & 14 (23.7) & 38 (64.4) & 7 (11.9) \\ \hline Regenerated plants (from hydroponic culture and soil) & 11 (22) & 39 (78) & 0 (0) \\ \hline \end{tabular}  
* The DNA-ploidy of a callus was estimated according to the ploidy of the first peak present in the flow cytometric histogram. MS—Murashige and Skoog medium; 2,4-D—2,4-Dichlorophenoxyacetic acid; BAP—6–Benzylaminopurine; TDZ—Thidiazuron.  
\end{table} 
Table 2: Number and frequency (%) of 4\(x\), 8\(x\), and 16\(\alpha\) at different stages of _Arabidopsis arenasa_ culture.