Development and anatomy of the attachment structure of woodrose-producing mistletoes

Abstract

The formation of woodroses in Sclerocarya birrea (A. Rich.) Hochst, and Combretum collinum Fresen. in reaction to the parasitic mistletoes Erianthemum dregei (Eckl. & Zeyh.) Tiegh, and Pedistylis galpinii (Schinz ex Sprague) was investigated. Eight woodroses of varying sizes, and by implication age, were sectioned with the freeze microtome and the sledge microtome. As the mistletoe entered the host tissues it assumed a flask-shape due to mechanical constriction by layers of host cork. There was a clear differentiation between host cells containing tannin and mistletoe cells without tannin throughout the sections. In addition these were separated by a thin wavy black line of gum. The mistletoe occupied a central position as well as producing finger-like projections into the host tissue. Older sections indicated an increase in tannin and sclereids, while the arrangement of axial xylem elements became increasingly disrupted. There appear to be two strategies to prevent a functional connection with the mistletoe: 1) the production of wound periderm at the host surface and 2) internal necrosis and disruption of internal host tissues. The study species showed signs of both but the mistletoes managed to by-pass these strategies and instead lead to the formation of elaborate and intricate woodroses.