Evidence of natural hybridization on Portuguese Digitalis thapsi L.: morphological and chemical characterization

In Flora Europaea [1] two species are described for Portugal, namely Digitalis thapsi L. and D. purpurea L. Until now, and in spite of their economic importance, little information is available concerning the chemical composition of the Portuguese populations of D. purpurea and only some old studies are available on the identification of both purpurea glycosides and lanatosides of D. thapsi [2] and on lipophilic flavonoids [3].

Our main goal with the present work is to prove the existence of natural hybrids between populations of Digitalis in the Northeast region of Portugal where both species exist. Methodology used include microscopic and chemical analysis of the leaves from about 30 specimens of two different populations, one from Barragem da Marateca (BM) where only D. thapsi is described and other from Seia (S) where D. thapsi and D. purpurea exist together.

The morphological analysis was performed on the lower and upper epidermises characterization by determination of the stomata types, trichome types and stomata index (IE). The chemical profile was established by means of TLC and LC/UV analysis of the lipophilic flavonoids and leaves cardenolides.

At naked eye, all specimens are similar to D. thapsi. Botanical microscopic analysis shows that all BM specimens have the same leaf characters, namely the presence of typical glandular trichomes (abundant); scarce protector trichomes (uniseriated, 1 to 5 cells, with smoothly walls and narrow lumen); few paracytic and frequent anomocytic stomata. On Seia samples 4 specimens were found with distinctive characteristics namely few stomata in the upper surface of the leaf. TLC and LC/UV chemical profile of these samples confirm their divergence from those of D. thapsi. These results hint the hybrid nature of these four specimens. The identification of spontaneous hybrids could be of great interest for future plant breeding of these species of pharmacological interest.

References: 1. Flora Europaea, Royal Botanic Garden, Edinburg, http://rbg-web2.rbge.org.uk, 2006–09–13. 2. Graça, J.A.B. (1967) Digitalis thapsi L., Aspecto morfológico, farmacodinâmico e químico de exemplares portugueses, Ed. Gráfica portuguesa, Lisboa. 3. Neutel, M.F. et al. (1992) in: Proceedings of Jornadas Ibéricas de Plantas Medicinales Aromáticas y de Aceites Esenciales, Madrid.