Morphoanatomical and histochemical characterization of Larrea species from Northwestern of Argentina

Larrea divaricata Cav., L. cuneifolia Cav. and L. nitida Cav., Zygophyllaceae, are evergreen xerophytic shrubs occurring in Northwestern Argentina used in traditional medicine. The aim of this work was to characterize the morphology, anatomy and histochemistry of the vegetative organs of three Larrea species by light and scanning electron microscopy in order to provide supporting data for their correct identification and to determine the site of synthesis and accumulation of its main active compounds. The shape, number and percentage of coalescence of leaflets, presence or absence of mucrones and rachis and the shape of the stipules represented the main botanical differences between the studied Larrea species. Anatomically three species presented amphystomatic leaves, with thick resinous slightly striated cuticle with resinous deposits, polygonal epidermal cells with straight anticlinal walls, ciclocytic, brachy-paracytic and paracytic stomatal types, non-glandular trichomes and isolateral mesophyll. The position and abundance of the sclerenchyma at the mid vein and petiole transection allows the differentiation of the three species, been more abundant in L. cuneifolia. Secondary phloem and parenchyma cells presented abundant calcium oxalate druses and solitary rhomboidal crystals. Epidermal cells and cuticle layer of leaflets and stipules of the three species presented amber resin deposits and content which stained positively for polysaccharides, phenolic compounds, flavonoids and tannins, while mesophyll palisade cells showed small refracting droplets stained positively for lipophilic substances.