Comparative morphological characteristics of the chromosomes of two Rheum species

Rheum maximowiczii Losinsk. and Rheum turkestanicum Janisch. are closely related species occurring in the flora of Uzbekistan, yet their cytogenetic characteristics and karyological relationships remain insufficiently documented. In this study, chromosome number, karyotype structure, karyomorphometric characteristics, and selected morphological traits were comparatively investigated. Cytogenetic analyses were performed using metaphase chromosome spreads obtained from root apical meristem cells, and karyotype parameters and asymmetry indices were calculated. Morphological characteristics of the vegetative and reproductive organs were also compared between the two species. The results revealed that Rheum maximowiczii is diploid (2n = 2x = 22 = 22m), whereas Rheum turkestanicum is tetraploid (2n = 4x = 44 = 44m). Both species possess highly symmetrical karyotypes composed exclusively of metacentric chromosomes and were classified within Stebbins’ (1971) 1A karyotype category. The total length of the monoploid chromosome set was 33.53 ± 1.82 μm in R. maximowiczii and 28.82 ± 0.31 μm in R. turkestanicum. Differences were also detected in several karyomorphometric parameters and morphological traits, particularly in stamen number, which ranged from 9 to 14 in R. maximowiczii but remained consistently nine in R. turkestanicum. These findings provide new insights into the cytotaxonomy, karyotype evolution, and systematics of Rheum, and establish a valuable basis for future molecular cytogenetic and phylogenetic investigations of the genus.

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