Morphology and Anatomy of the Haustorium of the Root Hemiparasite Olax phyllanthi (Olacaceae), with Special Reference to the Haustorial Interface

Gross morphology and internal structure of haustoria of Olaxphyllanthi are described in parasitism with a range of hosts,including roots of woody and herbaceous dicotyledons and certainmonocotyledons, and occasional instances of autoparasitism andhaustorial formation on monocotyledon rhizomes. Successful penetrationto xylem occurs on virtually all hosts across broad diameters,ages and anatomies of host root, but anatomical impedimentsto haustorial establishment and penetration are recorded forcertain host taxa. Each haustorium is a comparatively simpleand ephemeral structure. Its developing sucker (endophytic regionof the haustorium) spreads laterally around the surface of thehost xylem, yet never completely encircles the host stele. Damageto hosts is minimal and secondary thickening (of hosts) continueson the side of a host root opposite to a haustorium. The haustorialsucker lacks phloem and its interface with host xylem is comprisedalmost entirely (more than 98.7%) of parenchyma. The few terminatingtracheids at an interface lie in very close proximity to oroccasionally directly against exposed xylem vessels, but lumento lumen continuity between tracheary elements of the partnersis not achieved. Three dimensional reconstructions based onserial transverse sectioning indicate that well defined filesof tracheids connect back from an interface to the core of graniferoustracheary elements in the external body of the haustorium, andthence to the xylem of the parent parasite root. The findingsare discussed in relation to existing studies on haustorialanatomy. Root parasite, Olacaceae, haustorial anatomy, host specificity