Doubled haploid induction is one of the available methods normally used for sugar beet breeding. Gynogenic haploid explants of sugar beet induced with 1 or 2 mg L -1 6-benzylaminopurine were treated with 5 g L -1 colchicine, then subcultured on a solidified MS medium plus 0.2 mg L -1 kinetin. Colchicine doubled the chromosome number of 27.7% of the treated haploid explants. With the aim of increasing the number of doubled haploid explants, the effects of five levels of proline (0.0, 0.1, 0.2,

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... 3, or 0.4 mM) on the explants' proliferation, propagation, and shoot length were compared. With a large effect size (ES), proline at 0.3 mM induced the highest amount of proliferation, while proline-free medium resulted in the lowest amount of it. The highest propagation rates were observed for the explants treated on media with 0.2 and 0.3 mM proline (very large ES). Proline at 0.3 mM induced the shortest shoots (medium ES). A very strong positive correlation between proliferation and propagation, a moderate negative correlation between proliferation and length, and a strong negative correlation between propagation and length were observed. For the first time our results show beneficial effects of proline on in vitro proliferation and propagation of sugar beet.