Leaf architecture in Rhododendron subsection Rhododendron (Ericaceae) from the Alps and Carpathian Mountains: Taxonomic and evolutionary implications

Abstract

The European Rhododendron subsect. Rhododendron is noteworthy due to its unresolved geographical isolation from Asian relatives, yet it is understudied anatomically. This study examines qualitative leaf architecture of the alpine rhododendrons to reveal its taxonomic significance and improve understanding of the evolutionary relationships in these species. Leaf samples of R. ferrugineum, R. hirsutum, and R.×intermedium were obtained from localities in the Alps and those of R. myrtifolium were from the Carpathians. Herbarium material was also used. Leaf architecture was examined under LM and SEM and the traits were clustered to estimate relationships between the species and hybrid entities. Characters such as leaf shape, epicuticular wax morphology, position of stomata, diversity and anatomy of trichomes, occurrence of hypodermis-like cells and mesophyll anatomy varied depending on the species and were unrelated to altitude and slope exposition of plant growth. Epicuticular wax types new for the genus and the family and a hair type new for the subsection were described. Clustering leaf traits separated well the species and hybrid entities, showing a higher level of similarity between R. ferrugineum and R. myrtifolium. Leaf architecture in Rhododendron subsect. Rhododendron proves taxonomically valuable and suggests closer affinity between R. ferrugineum and R. myrtifolium, while a comparatively higher trait variation in hybrids may be regarded contributive to their reported superior fitness under favourable conditions.