Leaf micromorphology of 19 Mentha taxa

Abstract

Mentha (mint) is a genus in the Lamiaceae with a worldwide distribution. It has a complex classification due to frequent hybridisation at the interspecific level, variation in basic chromosome number and the occurrence of polyploidy (diploid to nonaploid). Although there have been many studies of Mentha leaf micromorphology, usually only a few taxa were described. The aim of this study was to characterise the micromorphology of Mentha leaves. Nineteen Mentha taxa, covering all four sections of the genus, were grown under controlled conditions and adaxial and abaxial leaf surfaces were examined using stereo and scanning electron microscopes. This study included examination of the previously uninvestigated Australian species, M. australis and M. diemenica. The study revealed that average lamina length varied from 3 mm (M. requienii) to 34 mm (M. × niliaca) and leaves were sessile (M. spicata) to where petiole length was 50% of total leaf length (M. requienii). Peltate and capitate glandular trichomes were found on the adaxial and abaxial leaf surfaces of almost all taxa. Most taxa were hypostomatous. A few taxa had amphistomatous leaves which was interesting given that Mentha is a mesophytic genus naturally found in moist environments beside streambanks and lake shores. Average guard cell length varied from 14 µm (M. suaveolens) to 27 µm (M. × piperita f. citrata ‘Basil’) with larger guard cell length correlated with larger DNA content and chromosome number. Two species in section Pulegium (M. requienii and M. pulegium) had small laminas, relatively long petioles and high adaxial stomatal density which distinguished them from taxa in the other three sections. Larger DNA content in plants can be associated with larger cell size. Most studies of Mentha leaf micromorphology make no mention of ploidy. The present study indicates this should be considered when comparing relative cell size between species.