Background <p>Blackcurrant (<i>Ribes nigrum L</i>.), which is known for its nutrient richness, faces agricultural challenges, such as environmental stress and limited genetic diversity. Polyploidisation to increase the number of chromosome sets is a promising approach for enhancing plant traits but is underexplored in blackcurrant breeding. This study investigated the effects of chromosome doubling on blackcurrant cultivars ‘Polares‘ and ‘Gofert‘, examining growth, cyto-anatomical structures, and photosynthesis of 5- and 6-year-old bushes, to understand how genome duplication influences plant phenotypes and the potential of autotetraploids in breeding programs.</p> Results <p>Compared to diploids, tetraploid blackcurrant showed reduced growth and delayed flowering, with changes in leaf morphology, such as a larger area, increased stomatal size, lower density, and thicker epidermal and mesophyll layers. The flowers of tetraploid plants were larger, but their number per inflorescence was reduced, and pollen viability was lower. Although the berry size and weight were comparable between ploidy levels, tetraploids were characterised by reduced seed set and fruit yield. Tetraploids had a higher chlorophyll content and increased photosynthetic parameters. The cell and chloroplast ultrastructure observed through transmission electron microscopy differed between ploidy levels. The number and intensity of 5S and 35S rDNA loci also varied, as shown by fluorescence in situ hybridisation.</p> Conclusions <p>Induction of tetraploid in blackcurrant resulted in significant phenotypic changes. Although tetraploidisation reduced plant vigour and fruit yield, it enhanced leaf and floral traits and photosynthetic efficiency. These findings highlight the complexity of the effect of chromosome doubling on blackcurrant performance, emphasising the need for research to optimise the benefits of polyploidisation in crop improvement.</p>