Pollen, nutlet and trichome micro-morphology of Satureja from flora of Iran, and their systematic implications

Pollen, nutlet, and trichome micro-morphology of 12 species of Satureja (Lamiaceae) from the flora of Iran were examined by light and scanning electron microscopy (SEM). Our investigations indicated that these species have hexacolpate pollen with a prolate, (exceptionally sub-prolate shape) and microreticulate ornamentation. In most species the periclinal walls of the surface cells of the nutlets are wrinkled, and as a secondary sculpturing, the surface exhibits both spherical-ovoid pits and a tuberculate pattern on one seed, or a regular papillae-tuberculate pattern. In a few species, the nutlets display an undulate-reticulate pattern. S. isophylla and S. khuzistanica differ from the other studied species by the size of nutlets and S. bachtiarica, S. hortensis, and S. kermanica are distinguished by their nutlet sculpturing. Satureja species possess non-glandular trichomes together with two types of glandular trichomes, classified as sub-sessile (peltate) and capitate. Notably, the large distinctive capitate trichomes consisting of a multiseriate stalk with an enlarged, rounded multicellular head as well as the extremely long trichomes that are in two subtypes: acicular trichomes and trichomes with ridges and marked internodes are uniquely exhibits in S. khuzistanica. The characteristics of trichomes and nutlet were constant among different populations of Satureja species. The functions of both glandular and non-glandular trichomes have also been discussed. We indicated that while pollen characteristics among Satureja species show consistency, our findings of the micromorphological characteristics of trichomes and nutlets could provide diagnostic characters at species level and may enhance our understanding of the relationships among these species in future phylogenetic studies