Leaf and stem anatomy of the Stylosanthes guianensis complex (Aubl.) Sw. (Leguminosae, Papilionoideae, Dalbergieae) and its systematic significance

Abstract

Stylosanthes comprises about 50 species, with Brazil being one of the main diversity centers. Stylosanthes guianensis constitutes a problematic taxonomic complex, comprising four varieties of S. guianensis and seven other species. Considering the difficulty of taxon delimitation, this study assessed the potential value of the secretory structures and other anatomical characters to the diagnosis of taxa within the complex. We analyzed the leaflet and stem anatomy and micromorphology of representatives of the S. guianensis complex using light and scanning electron microscopy. Long uniseriate non-glandular hairs occurred on the stems and leaflets of all species. Secretory emergences with a dilated base and a tapered distal portion occurred on both stems and leaflets, except in S. guianensis var. canescens. Polysaccharides, lipids and terpenes were histochemically detected in the secretory exudates. In the stem of S. gracilis, chlorenchyma cells were observed to be radially elongated, while in the other taxa studied they were isodiametric. On leaflets of S. guianensis var. pauciflora, S. acuminata, S. aurea, and S. gracilis, the midrib was eccentric and the veins closer to the leaflet margin had larger fiber caps. Anatomical structures proved to be useful for separating taxa. Stylosanthes guianensis var. canescens and S. guianensis var. pauciflora differ from the other varieties of the species and are possibly worthy of recognition at species rank. Our findings also reinforce the status of S. gracilis as a distinct species.