Comparative fruit morphology of nine Psittacanthus Mart. (Santalales: Loranthaceae) mistletoe species occurring in Mexico

The genus Psittacanthus is distributed from Mexico to Argentina and contains c. 110 species. The large, lipid-rich, one-seeded fruits of Psittacanthus species frequently depend on frugivorous birds for seed dispersal; however, fruit morphology of Psittacanthus (Loranthaceae) mistletoes remain poorly studied. In this study we describe the fruit morphology of nine Psittacanthus species. Fruits were collected from the study sites in which mistletoe plants were inhabiting contrasting habitats and host tree species. The morphological characteristics of fruits by species were described and compared with the use of inclusion and staining techniques and light microscopy. Our results show that fruit size varies among species, with P. macrantherus having the largest fruits. Most fruits have an ellipsoid to ovoid shape, color of fruits transits from green when immature to red or purple before ripening to blackish or purplish black, with color variations observed in different species, and the cupular pedicel length varying among species. Fruit sections indicate the following parts: exocarp, viscin layer, seed coat and a dicotylar to polycotylar embryo, with P. schiedeanus having the highest cotyledon count. No endosperm is present in the studied species. Our results provide valuable information for further species comparisons regarding the lack of endosperm and polycotylar embryo. Additionally, interspecific variation in cotyledon number and seed coat highlights distinct processes, including the potential effects of environmental differences.