Epiparasitism in Phoradendron durangense and P. falcatum (Viscaceae)

Phoradendron, the largest mistletoe genus in the New World, extends from temperate North America to temperate South America. Most species are parasitic on terrestrial hosts, but a few occur only, or primarily, on other species of Phoradendron. We examined relationships among two obligate epiparasites, P. durangense and P. falcatum, and their parasitic hosts. Fruit and seed of both epiparasites were small compared to those of their parasitic hosts. Seed of epiparasites was established on parasitic-host stems, leaves, and inflorescences. Shoots developed from the plumular region or from buds on the holdfast or subjacent tissue. The developing endophytic system initially consisted of multiple separate strands that widened, merged, and often entirely displaced its parasitic host from the cambial cylinder. During establishment the epiparasite invaded the cortex, vascular cylinder, and pith of its parasitic host and spread aggressively within host tissues, extending between and into individual host parenchyma cells, eventually isolating host cells or cell groups. The parasitic host showed little visual response to the epiparasite. Endophytic system growth of the epiparasite within its parasitic host was compared to that of a parasitic Phoradendron within its terrestrial host. The results indicate growth dynamics similar to those of parasitic species on terrestrial hosts. We conclude that the epiparasite/parasitic-host union should not be regarded as a graft union. The harmonious appearance of the union is a result of the growth of the epiparasite replacing entirely tissues of its parasitic host, with little or no hypertrophy of parasitic-host branches.