Anatomical and histochemical adaptations of Melpomene peruviana to the xeric environment of high altitudes

Melpomene peruviana is one of the ecologically toughest fern species that prospers under climatically extreme conditions between 3400-5200 meters above sea level and occasionally in dry areas at lower elevations. To gain a better understanding of the adaptations that allow this species to withstand such extreme environmental conditions, we investigated the anatomy and histochemistry of the sporophyte of three populations of M. peruviana. All plant organs exhibited thick cuticles, and vascular bundles surrounded by a ring or cap of sclereids and suberized endodermis. Roots also showed an exodermis with U-shaped thickening. The sclereids and the chlorenchyma of the leaf mesophyll were rich in phenolic compounds. Tannins were detected in the spongy parenchyma close to the vascular bundles of the leaf. Hydathodes revealed a cutinized endodermal layer and the presence of flavonoids, lipidic/terpenoid droplets, and free sugars in the epidermis and parenchyma cells. Rhizome scales had basal idioblasts and apical clavate glands with complex contents rich in phenolic compounds, lipids, terpenes, mucilaginous polysaccharides and proteins. Some of the observed morpho-anatomical characteristics such as microphylly, small stomata in high densities, thick cuticles and sclerenchyma close to the vascular tissue, reduce water loss by evapotranspiration and correspond to typical xeromorphic fern features. Among the histochemical characteristics, mucilaginous polysaccharides may act as gels to absorb and retain water while phenolic compounds are recognized for their antioxidant activity. The three populations exhibited a remarkable uniformity concerning their histochemical assays, stomatal dimensions and densities, and tissue thickness. The anatomy and histochemistry of M. peruviana are described for the first time, highlighting morphoanatomical and histochemical characteristics that act as key adaptations to xeric environmental conditions.